5 relief typology and lithologyhydrologie.org/the/durr/5.pdf · as mmts) : 500 – 2000 m of mean...

5 Relief typology and lithology

5.1 Introduction..............................................................................................................................196

5.2 Methodology.............................................................................................................................197

5.3 Generalities on the character of relief types and lithologies ................................................202 5.3.1 General remarks on the different relief types........................................................................202 5.3.2 Lithologic classes – natural grouping and quantities ............................................................207 5.3.3 Global distribution of lithologic classes................................................................................208

5.4 Lithology of the 7 major relief types ......................................................................................210 5.4.1 Global view ...........................................................................................................................210 5.4.2 Lithology of the different relief types – view per continent..................................................214

5.4.2.1 Africa...........................................................................................................................214 5.4.2.2 Asia..............................................................................................................................217 5.4.2.3 Australasia ...................................................................................................................221 5.4.2.4 Europe..........................................................................................................................223 5.4.2.5 North (and Middle) America .......................................................................................226 5.4.2.6 South America .............................................................................................................230

5.5 Relief distribution for the different lithologies......................................................................233 5.5.1 General remarks ....................................................................................................................233 5.5.2 Global picture........................................................................................................................234 5.5.3 Analysis per individual continents ........................................................................................244

5.5.3.1 Africa...........................................................................................................................244 5.5.3.2 Asia..............................................................................................................................246 5.5.3.3 Australasia ...................................................................................................................249 5.5.3.4 Europe..........................................................................................................................251 5.5.3.5 North (and Middle) America .......................................................................................253 5.5.3.6 South America .............................................................................................................254

5.6 Discussion .................................................................................................................................256 5.6.1 Summary of the most important results ................................................................................256 5.6.2 Contrasts between ‘old’ and ‘young’ continents...................................................................259 5.6.3 Contrasts between ‘cratons’ and ‘living orogens’.................................................................261

5.7 Conclusions...............................................................................................................................263

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5.1 Introduction As a first application of correlation studies using our new lithology database we will couple

it with the database of relief types by MEYBECK et al. (2001). These relief classes are in fact, as shown in this section, close to tectonic classes. If a database of tectonic classes had been available at the time of our study, the relationships could have been studied directly using such data, but not being available for us, we used MEYBECK’s (2001) relief typology, developed in the general framework of the research interests of our group, in collaboration with the Water Systems Analysis Group, University of New Hampshire, USA.

Figure 5-1 gives an overview of the relations between different rock types and relief types as well as a representation of the concepts and methodology used in this chapter.

•••

Lithology of major relief types (plains, plateaux, hills etc.) Relief distribution for different lithologies Contrasts

- ‘old’ vs. ‘new’ continents - ‘cratons’ vs. ‘living orogens’

Meybeck et al. 2001

L I T H O L O G Y

Mountains Plateaux Hills Plains

R E L I E F

this work - chapters 3 & 4

New Digital Lithologic World Map in vector format

Relief types (Meybeck et al. 2001) Elevation (altitude)

Morphology (Relief roughness)

Global map Relief Comparison

Lithology ‘dry’ & Relief

Figure 5-1 : Schematic representation of the relationships lithology – relief, studied in this chapter.

196

5.2 Methodology

In this chapter we present and discuss – after some methodological remarks (section 5.2) and some general remarks on the character of relief types and lithologies (section 5.3) – the relations between relief and lithology, globally as well as by continent. Chapter 5.4 concerns the lithology of major relief types (17 lithology types per relief class), chapter 5.5 presents the distribution of the lithologic classes into the different relief types (here 7 major relief types per lithology class).

We try to answer, among others, the following questions :

(i) does the lithology change with the mountain type, i.e. old / low mountains vs. alpine / high mountains ?

(ii) how are old continents organised compared to ‘new’ ones (e.g. Australia vs. Asia) ?

Some of the relationships are obvious, for example dunes are more prevalent in lowlands, especially in endorheic ones. Complex rocks are found, by definition, in mountain chains, alluvial deposits should be prevailing in plains. In general, loose sedimentary rocks are more widespread in the low-lying parts of the continents.

Another question concerns Precambrian basement rocks, they should be found mainly on plateaus and in low-lying plains, but they may also occur in low to medium-high mountains where they are affected by subsequent mountain building processes and thus brought back to the surface ; the rift zone in Eastern Africa is an example of special interest here.

For the following considerations of relief types versus lithology distributions, we regard first the globe as a whole and then the continents (see chapter 2.5), distinguishing only endorheic from exorheic regions – in contrast to chapters 6 and 7 where the focus will be on the landmass organisation by water drainage.

5.2 Methodology We used the relief typology proposed by MEYBECK et al. (2001), based on a combination

of mean elevation (m) and a roughness parameter (‰), see also chapter 2.3.1. We further clustered their 9 main classes into 7 super-classes (class numbers from MEYBECK et al. 2001) and use short names (underlined) for them :

- Plains are : Plains (1) and Mid-altitude plains (2) – new aggregated class (abbreviated as Pls for some parts of this chapter or some figures) : 0 – 500 m of mean elevation and relief roughness < 5 ‰

197


- Lowlands and low plateaus have been adopted (classes (4) and (6)) (abbreviated as Llds) : 0 – 500 m of mean elevation with relief roughness between 5 – 20 ‰

- Low & mid-altitude plateaus are : High altitude plains (3), Low (7) & mid-altitude plateaus (8) – new aggregated class (abbreviated as MPts) : 500 – 2000 m of mean elevation and relief roughness ranging from 0 – 20 ‰

- Rugged lowlands & hills have been adopted (classes (5) and (11)) (abbreviated as Hls) : 0 – 500 m of mean elevation and relief roughness > 20 ‰

- Low & mid-altitude mountains have been adopted (classes (12) and (13)) ‘abbreviated as MMts) : 500 – 2000 m of mean elevation with a relief roughness > 20 ‰

- High & very high plateaus have been adopted (classes (9) and (10)) (abbreviated as HPts) : mean elevation > 2000 m and relief roughness ranging from 0 – 40 ‰

- High & very high mountains have been adopted (classes (14) and (15)) (abbreviated as HMts) : mean elevation > 2000 m with relief roughness > 40 ‰

Table 5-1 : Global relief distribution for the 7 re-aggregated classes (M km2 and % endorheic and total parts) as represented when combining lithology and relief typology.

Endorheic parts (Area) Total (Area in M km2) M km2 % in relief class M km2 % of global total

Plains 4,51 15,6 28,95 22,4 Lowlands and very low plateaus 2,86 8,7 32,76 25,4

Low- & mid altitude plateaus 4,20 22,3 18,83 14,6 Rugged lowlands and hills 0,45 3,2 14,13 11,0

Low- & mid-altitude mountains 3,95 14,5 27,15 21,1 High & very high plateaus 0,62 47,0 1,32 1,0

High & very high mountains 2,11 36,4 5,80 4,5 Total 18,70 14,5 128,94 100,0

The global relief distribution values for the 7 super-classes – taking into account combined and defined / explained areas of the combination of lithology and relief typology – are listed in Table 5-1.

A nomenclature remark has to be made : In order to restrict the term of ‘platform’ to its geologic meaning (see explanation in section 5.3.1 under A), and also for reasons of consistency, we prefer and will use here the term of ‘very low plateaus’ instead of ‘platforms’ for the relief class from 5 – 20 ‰ roughness and 200 – 500 m mean elevation as used by MEYBECK et al. (2001). The term of ‘very low plateaus’ has also been proposed alternatively by the authors : ‘platforms … can be considered as very low altitude plateaus’ (MEYBECK et al. 2001).

198

5.2 Methodology

It has to be reminded that the lithology class of Polar Ice and glaciers (Ig) concerns only glaciated continental landmass without the ice shields of Greenland and Antarctica. Water bodies (Wb) concern, as explained, only major lakes without Caspian and Aral Sea. Their surface has been excluded from the continental landmass, but the river basins discharging into these seas have been counted as part of the endorheic parts of the continents. Subsurface evaporite occurrences, as marked on our lithology maps, can not enter into the calculations established here as we are not able to attribute actual surfaces to them.

The global rock distribution in 17 classes (in M km2), for both endorheic and total shares, as detailed in the preceding chapter, is illustrated in Figure 5-2.

Global Rock distribution

0

5

10

15

20

25

Wb Ig Pb Pa Vb Va Pr Mt Cl Ss Sm Sc Ep Su Ad Lo Ds

Are

a (M

km

2 )

TotalEndorheic

Figure 5-2 : Global rock distribution (M km2 at surface for total and endorheic parts). Lithology codes : Pb – Plutonic basic, Pa – Plutonic acid, Vb – Volcanic basic, Va – Volcanic acid, Pr – Precambrian, Mt – Metamorphic, Cl – Complex lithology, Ss – Siliciclastic sedimentary consolidated, Sm – Mixed sedimentary consolidated, Sc – Carbonates consolidated, Ep – Evaporites, Su – Semi- to unconsolidated sedimentary, Ad – Alluvial deposits, Lo – Loess, Ds – Dunes and shifting sand.

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For the presentation of correlation results in chapters 5.4 and 5.5, we present two different sets of diagrams with absolute values and normalised values for the comparative comprehension of the differences between values for different continents or different relief classes compared to the global average. Here a collection of selected results will be presented, the complete set of absolute and normalised values can be found in Annexe II.

Absolute values (M km2) given concern the values in a given relief class for chapter 5.4, globally or per continent, and values in a given lithology class for chapter 5.5 – again globally or per continent.

For a better understanding of the different values (e.g. rock distribution in plains compared to the global rock distribution) we used a normalisation approach : we divided the % value (e.g. % of a given rock type in plains) by the value we wanted to compare to (e.g. % of a given rock type globally). For individual continents we compared either values per relief class (e.g. % of a given rock type in plains in Africa, divided by the % value of the same rock type in plains globally – in order to observe the differences of a rock type occurrences on a given continent in contrast to their global occurrences) – chapter 5.4 – or values per lithology class (e.g. percentages of the relief types represented in the rock class of basic volcanic rocks in Africa, divided by the % value of the relief types, again for basic volcanic rocks, but globally) – chapter 5.5.

This approach fosters a better appreciation of the over- or under-representation of a given rock type in a given relief class on a given continent with respect to its global occurrences.

The results in the corresponding figures are to be interpreted as follows : a value of 1 means that the distribution is the same for a given continent / relief type / lithology type as its global distribution. A value above 1 indicates a representation above mean values with respect the global occurrences, below 1 the values are under-represented. The normalisation – globally and per continents – concerns the % values of each lithology class. Thus in this representation do not enter into account the sometimes highly differing absolute values of the different lithology classes. This means that globally abundant rocks will possibly not show great differences in normalised view, but that rare rocks, as for example basic Plutonic rocks or Evaporites may seem extremely over-represented where they occur, even if the absolute values of their different occurrences remain lower than the differences in an abundant rock class (but less expressed in the normalised approach).

This approach of using normalised values is represented in Figure 5-3, using 2 examples (complex lithology and Precambrian rocks). The figure also shows in an exemplary way the differentiation into cratons and active orogens which will be explained, applied and used in this chapter.

200

5.2 Methodology

Normalised relief distribution for Complex lithology

0,1

1

10

Plains Lowlands Midplateaus

Hills Midmnts.

Highplateaus

Highmnts.%

out

crop

in r

elie

f typ

e %

out

crop

glo

bal Cratons

Active orogens

Normalised relief distribution for Precambrian (shield) rocks

0,1

1

10

Plains Lowlands Midplateaus

Hills Midmnts.

Highplateaus

Highmnts.%

out

crop

in r

elie

f typ

e %

out

crop

glo

bal Cratons

Active orogens

Figure 5-3 : Example of the normalised representation of the relief distribution for different rock types.

Combining different databases may result in the loss of some areas, not defined in one of the databases used. Concerned here are some areas in coastal regions, defined slightly differently in the lithology database and the relief database, as well as some missing grid cells in the relief typology database. Due to these effects and the limitation – here – to areas south of 78°N, the summation of values may not always result in a global continental landmass area of about 133 M km2, the summation in a given relief or lithology class may then not always be 100 % either – resulting in 128,94 M km2 or 97 % of the global continental landmass being defined here.

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5.3 Generalities on the character of relief types and lithologies First we regard the geologic / lithologic context of the relief classes, beginning with global

observations (global values, then comparison of each relief type with regards to global values), then regarding major continental individual aspects.

5.3.1 General remarks on the different relief types The following general commentaries take up and complement the discussion of

MEYBECK et al. (2001).

The 15 original relief classes, clustered to 9 main and then further aggregated to 7 super-classes, for relief mapping, can be subdivided, for a first order approximate distinction in a geologic / lithologic context, into two groups (Figure 5-4) :

A) ‘An-orogenic’ reliefs,

B) ‘Actively orogenic’ reliefs

Mean elevation (m) Relief roughness

(‰) < 200 200 –

500 500 – 1000

1000 – 2000

2000 – 3000

3000 – 4000

4000 – 5000 > 5000

> 160 80 – 160 40 – 80 20 – 40 10 – 20 5 – 10

< 5

E N D O G E N E O R O G E N I C s e n s u l a t o A C T I V I T Y (B)

A N O R O G E N I C A R E A S (A)

Figure 5-4 : 9 main relief type classes and their colour codes (MEYBECK et al. 2001, slightly modified) as used and shown in chapter 2.3.1. Separation between orogenic s.l. (see text) = dissected (sensu MEYBECK et al. 2001) relief types and an-orogenic = non-dissected, sub-horizontal – flat reliefs. Compare fig Figure 5-5 for a different separation of relief classes, characteristic for cratons in contrast to subactive and active orogens sensu stricto.

202

5.3 Generalities on the character of relief types and lithologies

This fundamental distinction can be made following geological reasoning :

Exogenic forces and processes as weathering, erosion and sedimentation, tend to level continents down to the sea. In contrast, endogenic forces and processes as volcanism, divergent or convergent orogenic movements (both together ‘orogeny sensu lato’, cf. chapter 2 and Annexe I), create relief. Subsequently relief suffers erosive dissection – arid regions partly excepted – therefore elevation / altitude and relief roughness testify of either magmatic or tectonic-orogenic activity, both normally being linked.

A) The ‘lower’ 5 main relief types, with roughness < 20 ‰ and subhorizontal – flat relief, correspond essentially to an-orogenic, cratonic continental platform areas.

N.B. : ‘Platform’ is used here in the broad geologic sense : continental flat ‘raised-level surface’ (raised in comparison to the oceans’ floor) ; identical with the cratonic parts of continents – deeply eroded, more or less peneplained former orogens.

These continental platforms may be covered by younger, post-orogenic, (platform-) sediments, later possibly somewhat eroded, including both terrestrial and epicontinental shallow marine (shelf) areas – the actually marine parts not being considered in our study. These 5 main relief type classes are the subhorizontal (low) plains, the mid-altitude plains (200 – 500 m) – combined later in one super-class together with the plains ; then the very flat to flat lowlands combined with the very low plateaus (200 – 500 m, ‘platforms’ of MEYBECK et al. 2001) ; additionally high altitude plains (500 – 2000 m), combined later into one super-class together with the low- to mid-altitude plateaus (500 – 2000 m of mean elevation, but higher relief roughness).

B) The ‘upper’ 4 main relief types with dissected relief (relief roughness generally > 20 ‰, including small shares of high – very high plateaus with relief roughness of 10 – 20 ‰) correspond essentially to areas of active orogeny (sensu lato) – of divergent (plume / hot spot and taphrogenetic = rifting) as well as convergent (collisional chains = orogens sensu stricto) character. These 4 main ‘dissected’ classes (no further clustering has been achieved here for the super-classes) are the rugged lowlands combined with hills (mean elevation of 0 – 500 m), the low & mid-altitude mountains (500 – 2000 m), the high & very high plateaus and the high & very high mountains (both with 2000 – > 6000 m of mean elevation per 30’ x 30’ grid cell but different relief roughness).

203


Different from separating an-orogenic and actively orogenic areas – with their rather clear morphologic expression – is another fundamental geologic distinction within the continents (Figure 5-5). We have already introduced it in chapter 3 and Annexe I, and used it previously :

I) Cratons as the older, deeply eroded and essentially peneplained parts of the continents

II) Subactive and active young collisional chains (‘living’ orogens sensu stricto)

Comparison of the relief map in chapter 2 (Figure 2–1) with a global geologic / tectonic map showing the distribution of young ‘Alpidic’ chains in contrast to the older cratonic parts of continents, clearly demonstrates :

I) More or less cratonic relief classes are the 5 (3 when clustering to super-classes is taken into account) subhorizontal, very flat to flat classes (with roughness of 0 – 20 ‰ and up to 2000 m of mean elevation) – the (low) plains, clustered with mid-altitude plains, the lowlands (including very low plateaus), and the low and mid-altitude plateaus, clustered with high-altitude plains.

I – II) From the more strongly ‘dissected’ classes, the two lower (< 2000 m) – rugged lowlands & hills, and low- & mid-altitude mountains – are ‘hybrid’ concerning their character. To some extent – estimated one half, with probably a greater part of the low mountains – they characterise cratonic areas, among them many – but not all – of the passive margins of continents (cf. Annexe I on the structuring of continents), affected by i) active uplift through divergent rifting, leading to ocean spreading, ii) isostatic rebound, and also iii) locally by intracontinental hot spot activity – volcanic areas over mantle plumes

II) The remaining half of both classes, strongly dissected and of greater altitude, with probably greater parts of the mid-altitude mountains, clearly belongs to the young – active or subactive – chains, orogens sensu stricto, created by convergence and actual collision of plates. This holds true entirely for the high and very high mountains class as well as for the high and very high plateaus. Many of them are decorated with chains of towering active volcanoes. As most of the orogenic processes are directly connected to the subduction of oceans, the resulting cordilleras mostly occupy margins of the continents, as for example the Circum-Pacific chains and ‘fire belt’ with volcanoes supplied by subduction. High plateaus (Tibet, Andes and the North American Cordilleras) occur in broader belts belonging to continent-continent collision conditions ; sometimes they are relics of former microcontinents (‘terranes’).

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In conclusion it can be stated, regarding the distribution of relief types as depicted in Figure 5-5, that the non-dissected and low to mid altitude relief classes (‘plains’, ‘lowlands’ and ‘mid plateaus’) more or less characterise cratonic parts of continents. The dissected and high altitude ‘high mountains’ and ‘high plateaus’ characterise young active orogens s.str.. Relief types of the remaining two dissected and low to mid altitude classes may belong to either cratons (probably ‘hills’ to > 50 %) or to young orogens s.str. (probably ‘mid-mountains’ to > 50 %).

Relief classes characteristic for cratons (lower left in the figure) can thus be separated from subactive to active (collisional) orogens sensu stricto (upper right in Figure 5-5).

We are now able to quantify these different characteristics with regards to lithology, the differences should be reflected by different rock or relief distributions.

A note of caution, regarding the quantification of typologies : Attributing to certain relief classes – as we have just done – geologic terms of characteristic, but to some extent ‘fuzzy’ meaning, should not be misunderstood as an establishing of precise limits and ‘firm values’.

Mean elevation (m) Relief roughness

(‰) < 200 200 – 500

500 – 1000

1000 – 2000

2000 – 3000

3000 – 4000

4000 – 5000 > 5000

> 160 80 – 160 40 – 80

High Mountains

20 – 40

Hills

Mid Mountains

10 – 20

High Plateaus 5 – 10

Lowlands

< 5 Plains

Mid

Plateaus

C R A - T O N S

A C T I V E O R O G E N S

s e n s u s t r i c t o

Figure 5-5 : 7 relief super-class clusters or types, used for the calculations and resulting diagrams of this chapter. The diagonal line separates classes characterising cratons (lower left) from relief classes characterising active orogens sensu stricto (upper right), within the continental landmass. ‘Hills’ and ‘Mid Mountains’ belong to both, cratons as well as orogens sensu stricto, hills presumably more to cratons and mid mountains more to the parts of orogens sensu stricto.

Concerning the global distribution of relief types as based on the sum data in Table 5-1, we have to bear in mind that all terrains above 2000 m of mean elevation (high mountains and high plateaus) take only 5,5 % of the global land area. Together with the low & mid-altitude mountains (21,1 %), ‘mountains’ occupy about a quarter of the continental land surface as defined for our study (without Greenland and Antarctica).

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Regarding the relief distribution, a geologically ‘aggregating’ statement can be made : when all ‘mountains’ (mid and high) are considered, including 1,0 % of ‘high plateaus’, and joined to the 11,0 % of ‘hills’, a good third (37,6 %) of the continents appear dissected. They are, as already explained, actually affected by (recent) orogenic – sensu lato – activity, i.e. endogenic magmatic and tectonic, divergent – distensive and convergent – compressive – processes. The remaining 62,4 % of subhorizontal – very flat to flat terrains – ‘plains’, ‘lowlands’ and ‘mid plateaus’ – consequently represent the an-orogenic and geologically relatively quiet parts of the continents.

To separate young, Alpidic, active collisional orogens from craton parts of the continents in terms of our relief classes, the ‘dissected’ landscapes up to 2000 m of mean elevation (rugged lowlands, hills, low & mid-altitude mountains), have to be split – as already explained – into two parts, estimated to be about equal in area. It can be assumed – but not be proven within the frame of our study – that the ‘actively orogenic sensu stricto’ – half of the relief classes concerned takes more than 50 % of mid-mountains, the ‘cratonic’ half accordingly more of the hills.

Thus splitting the dissected relief classes up to 2000 m of mean elevation (together taking 32,1 % of the area, splitting them into half leading to 16,1 %) leads to regard globally about 78,4 % of the continental area as cratons, and the remaining 21,6 % as young, active (collisional) orogens sensu stricto.

These considerations, in analogy to the grouping of relief classes in Figure 5-4 and Figure 5-5 (see explanations above), lead to the explained grouping of relief-class columns, illustrated in Figure 5-6.

C R A T O N S 78,4 %

A C T I V E O R O G E N S s e n s u s t r i c t o 21,6 %

A n – o r o g e n i c a r e a s 62,4 % A c t i v e l y o r o g e n i c

( s e n s u l a t o ) a r e a s 37,6 %

Plains Lowlands Mid Plateaus Hills Mid Mountains

High Plateaus

High Mountains

Low & mid-alt. plains

Lowl. & v.low plateaus

Low & mid-alt. plateaus

Rugged lowl. & hills

Low & mid-alt. mountains

High & v.high plateaus

High & v.high mountains

S u b h o r i z o n t a l – f l a t D i s s e c t e d r e l i e f s

Figure 5-6 : Relation of 7 super-class relief types to endogene processes : an-orogenic (tectonically quiet) vs. actively orogenic sensu lato (divergent and convergent movement) areas, and cratons vs. active orogens sensu stricto (convergent movements only). Percentages are given as surface proportions, not volumes.

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5.3.2 Lithologic classes – natural grouping and quantities For a clear representation of relations between relief types and lithology, the different

quantities and their natural groupings (cf. chapter 3, Annexe I) are important, and will be reminded here. The primary aggregation of lithologic classes (see Figure 5-2)is illustrated in Table 5-2 : ‘Hard’ or crystalline rocks embrace magmatic (Plutonic and Volcanic) and metamorphic (including the Precambrian and parts of the complex lithologies) rocks ; ‘Soft’ rocks are the different groups of sediments and sedimentary rocks.

Table 5-2 : Aggregation of rocks and sediments according to their lithologic nature.

‘ H a r d ’ c r y s t a l l i n e r o c k s ‘ S o f t ’ r o c k s W a t e r I c e Magmatic rocks Metam. Complex Sediments

Plutonic Volcanic Lithologies Sedimentary

rocks eolian

Wb – Ig Pb – Pa Vb – Va Pr – Mt Cl Ss – Sm – Sc – Ep Su Ad Lo – Ds

Concerning the quantities or abundances of the different lithologies, attention has to be drawn to the most abundant species on one hand, and really rare rock classes on the other hand (Table 5-3).

Table 5-3 : Aggregation of rocks and sediments according to their frequency (total global areas).

R o c k s

most abundant : Pr Ss Ad (more so if combined with Mt) limit ----- at about 14 M km2 ---------------------------------------- or ca. 10,5 % ----- of total area

rather frequent : Pa Vb Mt Cl Sm Sc Su

limit ----- at about 4 M km2 ---------------------------------------- or ca. 3,0 % ----- of total area

really rare : Pb Va Ep Lo Ds Lithology codes : Pb – Plutonic basic, Pa – Plutonic acid, Vb – Volcanic basic, Va – Volcanic acid, Pr – Precambrian, Mt –

Metamorphic, Cl – Complex lithology, Ss – Siliciclastic sedimentary consolidated, Sm – Mixed sedimentary consolidated, Sc – Carbonates consolidated, Ep – Evaporites, Su – Semi- to unconsolidated sedimentary, Ad – Alluvial deposits, Lo – Loess, Ds – Dunes and shifting sand. Note the variations of the ordinate scales.

The differences in abundances of the lithologic classes have to be taken into account, especially when normalised distributions are considered (compare also Table 5-4).

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5.3.3 Global distribution of lithologic classes It should be reminded here that the distribution quantities for given rock types, as most

figures in our study, concern the lithologic surface picture, quantities of areas, and not volumes of rocks within the crust.

It has already been stated (chapters 3 and 5.3.1, Annexe I) that 21,6 % of the continental landmass is occupied by young, subactive to active orogens, and that the remaining, nearly 80 % of the continents are of cratonic nature.

The ‘old’ cratons on their part – volumetrically – consist mainly of deeply eroded ‘mid-crustal’ metamorphic Precambrian (Pr) and Metamorphic (Mt) rocks with partly large quantities of granites (Pa). Furthermore eroded Paleozoic and Mesozoic orogens belong to the cratons, with Metamorphic rocks (Mt), Complex lithologies (Cl) and also many granites (Pa). The outcropping basement of the cratons thus accounts for the global predominance of Precambrian (Pr), followed by granitic (Pa) and metamorphic (Mt) lithologies – on the one hand.

On the other hand, cratonic platforms receive and store most of the sediments eroded in the mountain reaches of young orogens – a fundamental and well know geologic fact, stated and corroborated repeatedly (e.g. by MILLIMAN & SYVITSKI 1992). The sedimentary cover of cratons takes 2 to 3 times the area of the cratonic basement, with all types of sedimentary rocks occurring. Silici-clastic sedimentary rocks (consolidated, Ss) dominate by far (cf. chapters 3 and 4), with a high proportion of semi- and unconsolidated sediments (Su – Ad). ‘Recycling’ of sediments – i.e. the cyclic repetition of deposition and erosion – explains the general predominance of younger over older sediments, especially the large areas of Alluvial deposits (Ad), which – ‘naturally’ – dominate in plains. In the arid belt from N-Africa and Arabia to near and middle Asia important areas are covered by Dunes (Ds). As considerable parts of the continents, including now dry epicontinental seas, were – during Cenozoic times – or still are situated in warm climate, carbonate sediments (Sc) take a rather great share of sediments globally. Endorheic basins are sediment sinks and contain thus primarily shares of unconsolidated sediments (Su, Ad and Ds).

Young flood basalts in and around rift areas – including certain passive continental margins as ‘half’ rifts, bisected by ocean spreading – account for the high proportions of basic volcanic rocks on cratonic crust as for example in the Paraná region of South America, E-Africa, NW-India, etc..

208


Young subactive or active orogens (sensu stricto), as zones of compressive tectonics, shoving together all types of crustal rocks – mainly the thick sedimentary piles of former passive continental margin basins, show a mixture of different sedimentary rocks (Ss, Sm, Sc, Su), often to some extent of metamorphic nature, with inlays of diverse crystalline rocks, mappable only as complex lithologies (Cl).

In the Alpidic chains along the southern margin of Europe and Asia, ophiolites (mapped as Pb) are a minor, but remarkable constituent, testifying of the subduction of oceans.

Due also to processes initiated by subduction is the orogenic magmatism with occurrence of volcanoes and volcanic rocks, of mainly andesitic (Vb), but to some extent also acidic (Va) nature – the plutonic equivalents and palingenetic granites being exposed rather rarely.

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5.4 Lithology of the 7 major relief types

5.4.1 Global view It is important to note, when comparing the individual relief types in Figure 5-7, that the

scale of the ordinate varies with a maximum factor of > 20, corresponding to the maximum share of one of the rock types and the %-share of the relief type.

The distribution of rocks (both endorheic and total) in each of the 7 aggregated relief types worldwide (global values) is depicted in Figure 5-7 (area in M km2), Figure 5-8 shows the % values as normalised with respect to the global distribution for the total continental landmass (shown for reference also – in the lower right corner – on Figure 5-7).

Both views, absolute values as well as the normalised illustration, will be appreciated and discussed for the global comparisons, regarding the rock distribution.

Many of the statements may reflect common geologic knowledge. The interest of our study is the specific focus – on relief types in this chapter – and the quantification available through use of digital data.

The discussion of the coupled data within this chapter also serves as validation of our lithology map.

Some dominant lithologic features characterise the different relief types :

Plains (low and mid-altitude plains) are occupied mostly by alluvial deposits. All semi- and unconsolidated sediments are over-represented, especially Loess and Dunes / shifting sand. All ‘hard’ rocks are clearly under-represented. Plains thus show characteristics as expected on the global scale.

Lowlands (low and very low plateaus), besides of Alluvial deposits, are covered with sedimentary rocks (Ss and Sc being slightly over-represented), but a considerable part – about one third – is without cover. These uncovered parts reveal the cratonic crystalline basement, Precambrian and Metamorphic rocks (Pr and Mt) are over-represented. Volcanic rocks have rather few occurrences, with Vb being particularly below global average values.

Mid Plateaus (low & mid-altitude plateaus) are similar to lowlands and low plateaus for most of their lithologic characteristics. Semi- to unconsolidated sediments (mainly Tertiary, Su) are slightly, Dunes (Ds, in Inner Africa, the Near East and Inner Asia – to be shown in the parts on the individual continents) greatly over-represented. Volcanic rocks (mainly Vb) occur to some extent, but are under-represented with respect to global values.

210


The main dissected relief type classes, hills (rugged lowlands and hills), mid-mountains (low & mid-altitude mountains), and high mountains (high & very high mountains), show principally similar characteristics.

Young sediments as Su and Ad take certain shares, but are under-represented with respect to the global mean value, like all other unconsolidated sediments. Consolidated sedimentary rocks and crystalline ‘hard’ rocks occur with varying shares, in general they are mostly over-represented – except for Metamorphic rocks (Mt) in mid-mountains, and Precambrian basement (Pr) in high mountains. Precambrian rocks still take the highest shares in mid-mountains, and are slightly over-represented with respect to their global occurrences. These are presumably the uplifted parts of cratons as for example in East Africa (to be shown in the following).

Complex lithologies – as expected, and by definition of the class – characterise both (main) mountain classes and are clearly more abundant in these classes than on global average, especially in the high mountains.

Both classes of volcanic rocks – basic (Vb) rocks being about 6 times more frequent than the acid (Va) rocks – are mainly to be found in hills and both mountain classes, they are over-represented in these classes. These occurrences are characteristic for and represent in clear evidence the magmatic belts of young orogens.

High plateaus (high & very high plateaus) hold only 1 % of the continental landmass and are represented mainly by the Tibetan plateau and some relatively smaller areas in the Andes and the North American cordilleras. They are insignificant at global scale and of only local, specific interest. Tibet accounts for the high share of Mixed sedimentary consolidated (Sm) rocks, due to the presence of different sedimentary rocks as explained in chapters 2 and 3 (in many maps large shares of this area are qualified as Carbonate rocks, but according to DATONG et al. 1985 – which seems a reliable source – important outcrops consist of Triassic Siliciclastic sedimentary rocks leading to an overall classification as Mixed sedimentary rocks). The Andean salars explain the over-representation of Quaternary Evaporites (Ep), the volcanism in this region is responsible for the over-representation of basic Volcanics (Vb).

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Rock distribution in plains (22,4 % = 28,95 M km2 of global area)

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Figure 5-7 : Rock distributions for each of the 7 aggregated relief types (M km2) and global values. Lithology codes : Pb – Plutonic basic, Pa – Plutonic acid, Vb – Volcanic basic, Va – Volcanic acid, Pr – Precambrian, Mt – Metamorphic, Cl – Complex lithology, Ss – Siliciclastic sedimentary consolidated, Sm – Mixed sedimentary consolidated, Sc – Carbonates consolidated, Ep – Evaporites, Su – Semi- to unconsolidated sedimentary, Ad – Alluvial deposits, Lo – Loess, Ds – Dunes and shifting sand. Note the variations of the ordinate scales.

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Figure 5-8 : Normalised rock distribution for the 7 aggregated relief types compared to the global rock distribution. Lithology codes : Pb – Plutonic basic, Pa – Plutonic acid, Vb – Volcanic basic, Va – Volcanic acid, Pr – Precambrian, Mt – Metamorphic, Cl – Complex lithology, Ss – Siliciclastic sedimentary consolidated, Sm – Mixed sedimentary consolidated, Sc – Carbonates consolidated, Ep – Evaporites, Su – Semi- to unconsolidated sedimentary, Ad – Alluvial deposits, Lo – Loess, Ds – Dunes and shifting sand.

Normalised rock distribution in plains

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5.4.2 Lithology of the different relief types – view per continent When discussing specific continents, the definition of their limits used here, especially

concerning the classification of islands e.g. in the SE Asian – Australian realm, follows the explanations given in chapter 2.5.

As stated in the preceding section, absolute values and relative, normalised values have been used for our comparisons. For the individual continents, the absolute numbers can be found in Annexe II, we have restricted the figures here to the normalised values. Normalisation here is achieved according to the global values per relief class (e.g. % of a given rock type in plains in Africa divided by the % of the same rock type in plains globally).

The following discussions try to respect the dimensions i.e. the sometimes very different absolute quantities or occurrences of specific rocks. Focus will be first on the general picture of the specific continent, compared to the global mean, and its main features, then peculiar or – for certain reasons – interesting minor characteristics will be highlighted.

5.4.2.1 Africa

Africa, including Madagascar, takes 22,6 % of the global terrestrial area and is thus the second largest continent.

Morphologically it is rather flat, and less than 1 % of its area has mean elevations exceeding 2000 m. (Low- to) Mid-plateaus dominate with 32,8 %, followed by lowlands (21,9 %) and plains (22,3 %). Mid-mountains (17,7 %) characterise the East-African rift system, the southern margins of the continent and Madagascar – rifted in Mesozoic to early Tertiary times, and the young Maghrebide cordilleras along the north(west)ern edge. Hills (rugged lowlands and hills) take only 4,4 % and high mountains (0,8 %), together with high plateaus (0,1 %), are nearly absent.

The general lithologic picture (Figure 5-9, lower right diagram) shows :

Metamorphic Precambrian rocks (Pr) and granites (Pa) are clearly over-represented in Africa, somewhat enriched are Siliciclastic sedimentary rocks (Ss, mostly Paleozoic sandstones) and semi- to unconsolidated sediments (Su, of Cenozoic age).

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Figure 5-9 : Normalised rock distribution in Africa for the 7 aggregated relief types compared to the global rock distribution for the corresponding relief type. For comparison also the normalised rock distribution for Africa in total compared to the global values is given. Lithology codes : Pb – Plutonic basic, Pa – Plutonic acid, Vb – Volcanic basic, Va – Volcanic acid, Pr – Precambrian, Mt – Metamorphic, Cl – Complex lithology, Ss – Siliciclastic sedimentary consolidated, Sm – Mixed sedimentary consolidated, Sc – Carbonates consolidated, Ep – Evaporites, Su – Semi- to unconsolidated sedimentary, Ad – Alluvial deposits, Lo – Loess, Ds – Dunes and shifting sand.

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Normalised rock distribution in high & very high mountains in Africa

Normalised rock distribution in Africa (total)

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In contrast, pure Metamorphic rocks (Mt), Complex lithologies (Cl) and Mixed sedimentary rocks (Sm) are noticeably under-represented. Basic Plutonics (Pb) and Evaporites (Ep) are very rare, acid Volcanics (Va) and Loess deposits (Lo) are completely absent. As about half of Africa is arid, Alluvial sediments (Ad) are slightly depleted, much of them occurring in the central endorheic basin of Lake Chad. Dunes or shifting sand (Ds) however occupy the largest areas worldwide (more than 1 M km2) and are evidently over-represented.

These observations reflect Africa being the world’s biggest, nearly exclusively Precambrian craton. The craton is a huge platform (in the geological sense), slightly bent into a kind of very flatly dented (or wavy) bowl – with the broad northern arid plains and plateaus (Saharan Paleozoic Siliciclastic sedimentary – Ss, Late Precambrian and Mesozoic Carbonates – Sc and Quaternary Dunes – Ds), and with extensive subhorizontal, flat and low to mid-altitude plateaus in its central southern Kalahari basin, mainly covered by young semi- to unconsolidated sediments (Su).

The geologically most remarkable younger event is the late-Mesozoic to Cenozoic isolation of Africa – former part of the Gondwana super-continent, through birth and spreading of the South Atlantic, the Indian, and the Circum-Antarctic oceans. Morphologically this accounts for the uplift of the western, eastern and southern margin leading to the finding and accordingly over-representation of Precambrian basement (Pr) in hills and mid-mountains.

Concomitantly, within the continent, plume activity under several hot spots – e.g. in the Saharan Tibesti mountains, but mainly in East Africa – led to rifting, upheaval – exposing Precambrian basement (Pr) in mid plateaus and mid mountains – and formation of huge rift valleys with some impressive volcanoes (Vb) – resulting in the apparent over-representation of these rocks in high and very high mountains which take only 0,8 % of Africa’s total area.

A narrow and rather old Phanerozoic orogen along the southern margin of Africa is the Cape belt, mostly mapped as Complex lithologies (Cl).

The only, comparatively very narrow, active Alpidic collisional chains (an orogen sensu stricto) are the Maghrebides, along the western parts of the northern margin of Africa, with mainly divers sedimentary rocks.

Relief-specific features for Africa can be characterised as follows :

African plains and lowlands are most extended in the northern arid belt. Alluvial sediments (Ad) are thus under-represented (except for the endorheic parts – Lake Chad basin as explained above), Dunes and shifting sand (Ds) however maximally enriched. Slightly more exposed than on average are also basement rocks as granites (Pa) and Precambrian metamorphics (Pr), covered mainly by Siliciclastic sedimentary rocks (Ss) and locally Carbonates (Sc) of Mesozoic – Cenozoic age in North Africa.

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The south-central mid-plateaus show extended young semi- to unconsolidated sediments (Su).

In hills and mid-mountains the Precambrian continental basement lays bare, being exposed by the divergent plume – mantle convection, subsequent continental margin and intra-continental rifting activities and isostatic rebound of crust and lithosphere, with Pr being greatly enhanced and Pa to some extent. They are relatively depleted in sediments.

The very few high mountains, and even less the few cells of high plateaus on the Ethiopian and Kenyan highlands are big volcanoes with basic Volcanics (Vb).

The large African endorheic areas, around Lake Chad and in the southern Okavango, Etosha and Kalahari basins, have exclusively non-dissected reliefs (plains, lowlands and mid-plateaus) with some basement (Pr and Pa) and sedimentary cover (Ss, Su, Ad and Ds). Smaller endorheic areas occur within the mountainous East African rift grabens with their volcanoes and impressive lakes.

5.4.2.2 Asia

Asia is the largest continent by far, taking 33,0 % of the global terrestrial areas. Endorheic areas (see later) occupy 22,8 % of the total area of Asia. The continent has a highly heterogeneous nature.

Morphologically, Asia is rather ‘mountainous’. Plains (15,0 %) and lowlands (20,0 %) are slightly below average, compared to the global mean. Mid-plateaus take 10,1 %. However all ‘orogenic’, i.e. dissected, relief classes are over-represented, hills with 15,7 %, mid-mountains with 28,4 % taking nearly one third of the whole Asian area ; high mountains (8,7 %) and high plateaus (2,1 %) are clearly above average global values as well, over-represented by nearly a factor two.

The general lithologic picture (Figure 5-10, lower right diagram) illustrates :

In marked contrast to all other continents, the Precambrian cratons of Arabia, India, Siberia and China have comparatively small shares. Large areas are occupied by Paleozoic – Mesozoic – Cenozoic orogens, the older ones being partially reactivated.

Collisional orogenies are in progress along the southeastern (Indonesian) margin by subduction of the Indian ocean, along the eastern margin by subduction of the Pacific ocean and several marginal seas with ocean characteristics (back-arc basins). Broad cordillera type chains – with volcanoes emitting basic-intermediate and locally also acidic lavas – are thus maintained in activity.

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Figure 5-10 : Normalised rock distribution in Asia for the 7 aggregated relief types compared to the global rock distribution for the corresponding relief type. For comparison also the normalised rock distribution for Asia in total compared to the global values is given. Lithology codes : Pb – Plutonic basic, Pa – Plutonic acid, Vb – Volcanic basic, Va – Volcanic acid, Pr – Precambrian, Mt – Metamorphic, Cl – Complex lithology, Ss – Siliciclastic sedimentary consolidated, Sm – Mixed sedimentary consolidated, Sc – Carbonates consolidated, Ep – Evaporites, Su – Semi- to unconsolidated sedimentary, Ad – Alluvial deposits, Lo – Loess, Ds – Dunes and shifting sand.

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Considerable parts of the southern margin however are characterised by continent-continent collision after disappearance of the Neo-Tethys ocean. The Arabian peninsula – former part of Gondwana – collides with Eastern Anatolia and the Iran, thereby creating the ‘Peri-Arabian’ and Zagros collisional chains. Gondwana-India is being bent and ‘under’-thrusted beneath central Asia. Consequently the Himalayas rise as an accretionary wedge of mainly Indian continental crust, and the Tibetan very high plateau as a south-central Asian floe of crust with doubled thickness (80 km !), with occurrences of ophiolites (Pb). In between, the upper valleys of the Indus and Yarlung / Tsangpo rivers mark the Tethyan suture.

These processes are responsible for the ‘orogenic mix’ of hard and soft rocks. The rather northern position of Asia during the Pleistocene period and its glaciation explain the widespread repartition of Loess covers (Lo). The humid climates reigning actually over the major parts of the continent explain the large repartition of Alluvial deposits (Ad) on the one hand, and the relative scarcity of recent, surficial evaporites (Ep) on the other.

Relief-specific features for Asia can be characterised as follows :

The Asian plains, situated in humid climates (W Siberia, Ganges-foredeep below the Himalayas, eastern China etc.), are almost exclusively covered by Alluvial sediments (Ad) when absolute numbers are considered. However, also basic Volcanics (Vb), Evaporites (Ep), and Dunes (Ds) are over-represented with respect to global occurrences in plains.

Lowlands still show large shares of Alluvial cover, but also more sedimentary cover of Ss, Sc, Sm and Su, and some crystalline rocks (but globally under-represented).

The proportion of crystalline rocks increases in the mid-plateaus, with especially volcanic rocks (Va in particular), but also Complex lithologies (Cl) being over-represented with respect to mid-plateaus globally. Due to the endorheic nature of many of them, Ad and Ds, but also Lo – in China – are over-represented.

Hills and mid-mountains share their principal lithologic features with the mid-plateaus – except for the lower shares of Ad, Lo and Ds (but Lo and Ds still being over-represented in mid-mountains in Asia with respect to global mid-mountains, Ds also being over-represented in hills in Asia). The (minor) cratonic parts of these relief classes (Precambrian in India and Arabia ; eroded older Phanerozoic orogens in central Asia) account for the rather high proportions of Precambrian (Pr) and granitic rocks (Pa) – but Pr still being depleted when compared to the global values in these relief classes. The high shares of Vb are due to the flood basalts in Siberia and northwestern India – letting them appear over-represented in hills, but still under-represented in mid-mountains.

The major part of these relief classes belongs to the young collisional chains and is characterised by the mentioned orogenic mix of hard / crystalline and soft / sedimentary rocks, often mappable only as Complex lithologies (Cl). Furthermore orogenic volcanics, of basic-

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intermediate (Vb) and also acidic nature (Va) are present. Especially the acid Volcanics (Va) appear enriched in hills and mid-mountains in Asia with respect to the same relief classes globally, reflecting the presence of these rocks principally towards the Pacific regions in easternmost Asia with important relief parts being located in the hills and mid-mountains relief category.

The high to very high mountains are to be found in parts of the Near East cordilleras, but concentrate in southern central Asia, mainly around Tibet, together with the high and very high plateaus there. The specific combination of hard crystalline rocks (mainly Cl, but also Mt, Pr and Pa, but also Pb being over-represented in both relief classes) and sedimentary rocks (mainly Sm) is due to the origin of these reliefs by continent-continent collision. With respect to the global occurrences of Loess in these relief categories (where it is in general evidently depleted with respect to global means), it is the only loose sediment being enriched.

Endorheic regions in Asia occupy huge areas, 22,8 % of the total area, – including most of the Caspian (the remaining parts belonging to Europe) and all of the Aral Sea, and also large parts of central Asia with the Tarim, Mongolian and other basins (see chapter 2.5 and chapter 7), comparable perhaps only to Australasia where they also take large shares of the total area of the continent.

Most of these endorheic basins are obviously situated in (moderately) dry climates. They can be found as large basins in plains and lowlands, but also – taking relatively larger proportions – in mid- and high-plateaus. Smaller basins can be found in mid- and high-mountains, but especially in the high mountains (due mostly to the interior Tibetan basins) they take large parts of the total high mountains area of Asia. Due to their nature it is thus not astonishing that they all store relatively high amounts of Semi- to unconsolidated (Su) and Alluvial sediments (Ad), as well as Loess (Lo) and Dunes or shifting sand (Ds) – independent of the absolute heights of the basins (except for Su being under-represented in mid- and high plateaus and also slightly in high mountains with respect to their global occurrences in these relief classes). Evaporites are restricted to the totally arid inner Iranian basins where Cambrian salt diapirs foster recent salt pans – they are thus mainly enhanced in the relatively flat relief classes. Curiously they are also over-represented in the mid-mountains relief class in Asia. This is due to some of the Iranian endorheic basins having large shares of mid-mountains – only the innermost parts belong to the mid-plateaus category here.

Otherwise, no ‘particular endorheic’ lithologies are discernible, they depend on the regional or local geologic situations.

220


5.4.2.3 Australasia

Together with Australia and Tasmania, also New Guinea and the smaller islands to the east and southeast down to New Zealand are counted here as ‘Australasia (see continent definitions in chapter 2.5). With 9,0 M km2 it takes 6,8 % of the continental landmass defined and is the smallest of the six continents considered. It is by far the flattest continent.

Morphologically, plains and lowlands up to 500 m occupy nearly three fourths (74,1 %) of the total area. Together with the 11,1 % of mid-plateaus, non-dissected subhorizontal to flat relief holds 85,2 % of the area.

Hills (7,0 %) and mid-mountains (7,3 %) share nearly all of the remaining area, high mountains take only 0,5 % and high plateaus are completely absent. From the about 15 % of dissected relief classes (‘orogenic sensu lato’), only about one third is to be found as the southeastern (passive) margin of the continent itself, the other two thirds make up the active collisional chains in northern New Guinea and the two islands of New Zealand.

The general lithologic picture (Figure 5-11, lower diagram) shows :

Australia, including Tasmania and also – notably – the southern third of New Guinea, is basically a Precambrian craton with basement rocks (Pr slightly below global average ; Mt and Pa above global average) and some Volcanics (Vb, Va, but both under-represented with respect to global values) now covered over about two thirds of its area by platform sedimentary rocks (Ss and Sc still slightly below the world’s average but Mixed Sedimentary rocks – Sm – making up the difference and being clearly above global average), ranging in age from late Precambrian to Mesozoic. The mid-level eroded Paleozoic Trans-Australian fold belt (basement rocks and also Complex lithologies – Cl, and Mixed Sedimentary rocks – Sm) occurs in the center of the craton, and the Paleozoic to early Mesozoic ‘Tasmanides’ orogen constitutes its eastern margin (mainly with Cl, Pa, Vb, and various sedimentary rocks). Cenozoic semi- to unconsolidated (Su) cover much of the plains between the Gulf of Carpentaria in the north and the Darling-Murray basin in the southeast.

The aridity of Australia – except for the northern, eastern and small parts of the southern coastal areas – explains the comparatively small share of Alluvial sediments (Ad) and the presence of recent Evaporites (Ep). Dunes are too rare here to be shown on our map. Loess is lacking as well, as in Africa.

During latest Cenozoic times, collision of the ‘greater’ Australian continent on its northern edge – now southern New Guinea – with the southwest Pacific realm created the orogenic chains constituting the northern and main part of New Guinea, with Complex lithologies (Cl) and various young sediments (Sc, Su, Ad) and young orogenic Volcanics (Vb).

221


Figure 5-11 : Normalised rock distribution in Australasia for the 7 aggregated relief types compared to the global rock distribution for the corresponding relief type. For comparison also the normalised rock distribution for Australasia in total compared to the global values is given. No high & very high plateau areas in Australasia. Lithology codes : Pb – Plutonic basic, Pa – Plutonic acid, Vb – Volcanic basic, Va – Volcanic acid, Pr – Precambrian, Mt – Metamorphic, Cl – Complex lithology, Ss – Siliciclastic sedimentary consolidated, Sm – Mixed sedimentary consolidated, Sc – Carbonates consolidated, Ep – Evaporites, Su – Semi- to unconsolidated sedimentary, Ad – Alluvial deposits, Lo – Loess, Ds – Dunes and shifting sand.

0,1

1

10

Wb

Ig

Pb

Pa

Vb

Va

Pr

M

t

Cl

Ss

Sm

Sc

Ep

Su

Ad

Lo

D

s

% o

utcr

op in

pla

ins

in A

ustr

alas

ia

% o

utcr

op in

pla

ins

glob

al Total

Endorheic

Normalised rock distribution in plains in Australasia

0,1

1

10

Wb

Ig

Pb

Pa

Vb

Va

Pr

M

t C

l Ss

Sm

Sc

Ep

Su

A

d

Lo

Ds

% o

utcr

. low

l. &

low

pla

teau

s A

ustr

alas

ia

% o

utcr

. in

low

l. an

d lo

w p

late

aus

glob

al

TotalEndorheic

Normalised rock distribution in lowlands and low plateaus in Australasia

0,1

1

10

Wb

Ig

Pb

Pa

Vb

Va

Pr

M

t C

l Ss

Sm

Sc

Ep

Su

A

d

Lo

Ds

% o

utcr

. low

& m

id-a

lt. p

lat.

Aus

tral

asia

%

out

crop

low

& m

id-a

lt. p

late

aus

glob

al

TotalEndorheic

Normalised rock distribution in low & mid-altitude plateaus in Australasia

0,1

1

10

Wb

Ig

Pb

Pa

Vb

Va

Pr

M

t C

l Ss

Sm

Sc

Ep

Su

A

d

Lo

Ds

% o

utcr

. rug

ged

low

l. &

hill

s A

ustr

alas

ia

% o

utcr

. rug

ged

low

land

s &

hill

s gl

obal

TotalEndorheic

Normalised rock distribution in rugged lowlands and hills in Australasia

0,1

1

10

Wb

Ig

Pb

Pa

Vb

Va

Pr

M

t C

l Ss

Sm

Sc

Ep

Su

A

d

Lo

Ds

% o

utcr

. low

& m

id-a

lt. m

ount

. Aus

tral

asia

%

out

cr. l

ow &

mid

-alt.

mou

ntai

ns g

loba

l

TotalEndorheic

Normalised rock distribution in low & mid-altitude mountains in Australasia

0,1

1

10

Wb

Ig

Pb

Pa

Vb

Va

Pr

M

t C

l Ss

Sm

Sc

Ep

Su

A

d

Lo

Ds

% o

utcr

. hig

h &

v. h

igh

mnt

. Aus

tral

asia

%

out

cr. h

igh

& v

. hig

h m

ount

ains

glo

bal

TotalEndorheic

Normalised rock distribution in high & very high mountains in Australasia

Normalised rock distribution in Australasia (total)

0,1

1

10

Wb

Ig

Pb

Pa

Vb

Va

Pr

M

t

Cl

Ss

Sm

Sc

Ep

Su

Ad

Lo

D

s

% o

utcr

op in

Aus

tral

asia

%

out

crop

glo

bal

TotalEndorheic

222


A geologic continuation of these chains is visible in the islands following in southeastern direction (Solomon and Fiji islands, New Caledonia) and then in North and South New Zealand. They are separated from Australia by the Coral and the Tasman Sea and actually evolving by mainly ocean-ocean collision processes characterised by the orogenic mix of Cl, sedimentary rocks and sediments, and orogenics Volcanics (Vb dominating).

Relief-specific features for Australasia can be characterised as follows :

Plains in Australasia – see explanations above – have the highest absolute shares of alluvial sediments on the continent, they are to be found around Lake Eyre and in southern Guinea. Their relative proportion however is low and the normalised diagram shows a strong depletion with respect to the generally elevated shares of Alluvial deposits in plains at the global scale. Other sedimentary rocks (Ss, Sm, Sc and Su) dominate without covering totally the basement (Pr, Pa, Mt) which is thus over-represented except for Mt.

Due to the dry climates in many parts of the central Australian plains Evaporites (Ep) are particularly over-represented with respect to their global appearances, even in plains.

All other ‘cratonic’ relief types, the dominating lowlands, mid-plateaus, and also, to about one third (see morphologic remarks above), hills and mid-mountains, have rather similar lithologic profiles with the already mentioned association of basement rocks and divers cover sediments. Only the proportions of Su and Ad are rather varying.

The active orogens sensu stricto of northern Guinea and of New Zealand contribute to the high shares of Cl and Vb in hills and mid-mountains, and the – quantitatively insignificant – high mountains in New Guinea.

Endorheic basins, namely around Lake Eyre, occupy approximately 25 % of the area in Australasia and take the corresponding proportions, as expected, mostly of plains, lowlands and mid-plateaus.

5.4.2.4 Europe

Europe, taking 9,8 M km2, i.e. 7,4 % of the total continental landmass as defined (see chapter 2.5), is only slightly greater than Australasia and is the western annex of Asia, resembling it to some extent. The main landmass is deeply incised by the North and Baltic Sea and, south of the Alpidic chains, by the Mediterranean and the Black Sea.

223


Figure 5-12 : Normalised rock distribution in Europe for the 7 aggregated relief types compared to the global rock distribution for the corresponding relief type. For comparison also the normalised rock distribution for Europe in total compared to the global values is given. No high & very high plateau areas in Europe. Lithology codes : Pb – Plutonic basic, Pa – Plutonic acid, Vb – Volcanic basic, Va – Volcanic acid, Pr – Precambrian, Mt – Metamorphic, Cl – Complex lithology, Ss – Siliciclastic sedimentary consolidated, Sm – Mixed sedimentary consolidated, Sc – Carbonates consolidated, Ep – Evaporites, Su – Semi- to unconsolidated sedimentary, Ad – Alluvial deposits, Lo – Loess, Ds – Dunes and shifting sand.

0,1

1

10

Wb

Ig

Pb

Pa

Vb

Va

Pr

M

t

Cl

Ss

Sm

Sc

Ep

Su

Ad

Lo

D

s

% o

utcr

op in

pla

ins

in E

urop

e %

out

crop

in p

lain

s gl

obal

TotalEndorheic

Normalised rock distribution in plains in Europe

0,1

1

10

Wb

Ig

Pb

Pa

Vb

Va

Pr

M

t C

l Ss

Sm

Sc

Ep

Su

A

d

Lo

Ds

% o

utcr

. low

land

s &

low

pla

teau

s Eu

rope

%

out

crop

low

land

s &

low

pla

teua

s gl

obal

TotalEndorheic

Normalised rock distribution in lowlands and low plateaus in Europe

0,1

1

10

Wb

Ig

Pb

Pa

Vb

Va

Pr

M

t C

l Ss

Sm

Sc

Ep

Su

A

d

Lo

Ds

% o

utcr

op lo

w &

mid

-alt.

pla

teau

s Eu

rope

%

out

crop

low

& m

id-a

lt. p

late

aus

glob

al

TotalEndorheic

Normalised rock distribution in low & mid-altitude plateaus in Europe

0,1

1

10

Wb

Ig

Pb

Pa

Vb

Va

Pr

M

t C

l Ss

Sm

Sc

Ep

Su

A

d

Lo

Ds

% o

utcr

. rug

ged

low

land

s &

hill

s Eu

rope

%

out

cr. r

ugge

d lo

wla

nds

& h

ills

glob

al

TotalEndorheic

Normalised rock distribution in rugged lowlands and hills in Europe

0,1

1

10

Wb

Ig

Pb

Pa

Vb

Va

Pr

M

t C

l Ss

Sm

Sc

Ep

Su

A

d

Lo

Ds

% o

utcr

. low

& m

id-a

lt. m

ount

ains

Eur

ope

% o

utcr

. low

& m

id-a

lt. m

ount

ains

glo

bal

TotalEndorheic

Normalised rock distribution in low & mid-altitude mountains in Europe

0,1

1

10W

b

Ig

Pb

Pa

Vb

Va

Pr

Mt

Cl

Ss

Sm

Sc

Ep

Su

Ad

Lo

D

s

% o

utcr

. hig

h &

v. h

igh

mou

ntai

ns E

urop

e %

out

cr. h

igh

& v

. hig

h m

ount

ains

glo

bal

TotalEndorheic

Normalised rock distribution in high & very high mountains in Europe

Normalised rock distribution in Europe (total)

0,1

1

10

Wb

Ig

Pb

Pa

Vb

Va

Pr

M

t

Cl

Ss

Sm

Sc

Ep

Su

Ad

Lo

D

s

% o

utcr

op in

Eur

ope

% o

utcr

op g

loba

l

TotalEndorheic

224


Morphologically, plains take 26,1 % and lowlands including very low plateaus take the major share with 42,9 %. Hills (14,7 %) and mid-mountains (14,9 %) together occupy about one third of the total area. Mid-plateaus (1,1 %) and high mountains (only 0,3 % with very few cells in the Alps and in the Caucasus parts of Europe) are rare and high plateaus lack completely.

The general lithologic picture (Figure 5-12, lower diagram) indicates :

The continent Europe consists (compare chapter 4.2.6) to about 50 % of the Precambrian craton of Fennosarmatia, exhumed – by the Pleistocene glaciations – only in the Scandinavian countries, covered by sedimentary rocks (mainly Ss and Sc) and sediments (Su, much Lo, few alluvial sediments Ad) on the East-European platform.

Out of the remaining parts, about 60 % – i.e. ca. 30 % of the whole European area, are also of cratonic nature, but not of Precambrian age. They consist of eroded Paleozoic orogens – Caledonian and Variscan. One half is laid bare in the West-Scandinavian mountains, the British isles, and in parts of western (France, Spain) and central Europe (Germany, Czech Republic), with mainly Cl, Pa and Paleozoic Ss. The other half is covered by Mesozoic (mainly Sc) and Cenozoic (Su) sediments. Parts of this west- and central-European basement is being reactivated by Cenozoic plume and rifting activities, accounting for the presence of basalts (Vb).

The remaining 20 % of the continent consist of the Alpidic chains around the major (European) parts of the Mediterranean Sea, in the Carpathian and the Balkan peninsula. They are made up of mainly Cl, Ss, Sc and Su. From northern Italy on tho the east, ophiolites (Pb) are a minor, but characteristic constituent – in general over-represented with respect to global occurrences. Some Cenozoic orogenic volcanic rocks of intermediate character (Vb) occur within the inner parts of the Carpathian orocline (mountain bend).

Alluvial deposits – according to the relatively smaller importance of rivers here – are rather rare, the largest occurrences being found in the Caspian depression northwest of the Caspian Sea and minor shares in the lower reaches of the Po, Danube and Rhine rivers. Some Dunes (Ds) occur along the neighbourhood of the Caspian Sea.

Loess (Lo), by eolian transport and partly sedimentation in the periglacial terrains of the huge North-European Pleistocene glaciations, covers large areas (about 0,8 M km2), particularly in eastern Europe. Loess is thus largely over-represented in Europe with respect to global average values.

The humid climates forbid formation of recent Evaporites and rapidly dissolve the many fossil occurrences – they are therefore deeply and well sealed hidden in the underground. No outcrops have thus been mapped on our map.

Relief-specific features for Europe can be described as follows :

225


Plains and lowlands, mainly in northeastern and eastern Europe, show similar lithologic features : moderate proportions of basement rocks (Pr and Pa), as platform sediments Ss (Paleozoic) and Sc (Paleozoic in north-eastern Europe, Mesozoic in the remaining parts) are dominating, Su and almost equal shares of Lo (being largely enriched with respect to the global values for these relief classes), and rather few Alluvial deposits (Ad) – being depleted with respect to global values for plains and lowlands.

Mid-plateaus are insignificant in Europe, taking only 1,1 % of the continent’s area, they exist only in central Spain with Variscan Complex lithologies (Cl) and Tertiary sediments (Su and Ad) dominating.

Hills and mid-mountains both obtain about 15 % of the total European area. They belong to the cratonic Paleozoic (mid- to upper-crustal) basement of western and central Europe as well as to the basement of the Alpidic chains, both with mainly Complex lithologies (Cl). Ss also dominate in the cratonic hills, and Carbonates (Sc) in the per-Mediterranean Alpidic chains.

High mountains (> 2000 m of mean elevation in a 30’ x 30’ grid cell) are only to be found in the Alps and in the Caucasus mountains belonging to Europe, they are very rare and occupy only 0,3 % of the area ; they are occupied by mainly Complex lithologies (Cl), with shares of Mixed sedimentary and Carbonate rocks (Sm and Sc). High plateaus are non-existent in Europe.

5.4.2.5 North (and Middle) America

The North American continent, as defined (see chapter 2.5), including Arctic islands, the actually few non-glaciated parts of Greenland and Middle America, takes 16,8 % (22,3 M km2) of the global continental area.

Morphologically, plains, lowlands and mid-plateaus together occupy nearly 60 % of the total area of the continent. Plains take 19,1 %, lowlands dominate with 27,9 % and mid-plateaus, mainly to be found in the continent’s mid-west regions, take 11,3 %. The plains mark a central N-S axial depression between the Hudson Bay and the Gulf of Mexico. To this an-orogenic cratonic parts has to be joined a small fraction of the continent’s hills and mid-mountains along the eastern and north-eastern margins including the Arctic archipelago and Greenland. They have been rifted during Mesozoic and Cenozoic times.

Yet, most of the hills (12,0 %) and mountains – taking a quarter of the total area (25,3 %), together with some high mountains (2,9 %) and high plateaus (1,5 %) belong to the broad western strip of cordilleras and ‘rocky mountains’, from Alaska down to Panama.

The general lithologic picture (Figure 5-13, lower right diagram) shows :

226


Approximately half of the North American area including Greenland is constituted of the Precambrian craton Laurentia. Its basement rocks (mainly Mt, Pa, and some Pr) are exhumed by the Pleistocene glaciations in the north-eastern quarter of the continent. In this region locally, but entirely in the western and central ‘mid-continent’ areas, the basement is covered by Paleozoic (Sc – Sm) and Mesozoic (Ss) sedimentary rocks. Cenozoic Su occur mainly in the mid-western mid-plateaus, Pleistocene Loess (Lo) more in the central plains and lowlands.

The Precambrian craton is bordered to the east and south by the broad Paleozoic Appalachian-Ouachita orogen with folded sedimentary rocks in the external parts, and eroded – down to mid-crustal – metamorphic rocks (Mt, Cl) in its central parts, now situated along the eastern coast of North America. Its eastern continuation is now to be found in north-western Africa and western Europe (cf. chapter 4). Likewise, the northern frame of Laurentia is constituted of the Paleozoic Inuit-Ellesmere orogen, to be found on the northernmost Arctic islands.

The backbone of North and Middle America are the Rocky mountains and Cordilleras along the western margin. They are part of the peri-Pacific active orogens with their ‘fire belt’ volcanoes. Especially the main, North American parts of these cordilleras are very complex, heterogeneous in age and nature with basement rocks (Pr, Mt, Pa), Complex lithologies (Cl) and, evidently, sedimentary rocks. The whole orogen is still in full development by way of the continuing collision and subduction of older and younger parts of the Pacific ocean. The obliqueness of the collision causes major strike-slip movements and thus contributes to the complexity. Another consequence is the unequal distribution of active andesitic volcanoes in Alaska, the middle to western Cascades range, and in Middle America (exclusively Vb, Va areas being too small to appear on our map). Plume activity, crustal thinning and rifting above subducted oceanic lithosphere was and still is responsible for localised basic (basaltic) volcanism (Vb) as for example on the Columbia river plateau.

The equally complex nature and evolution of the Gulf of Mexico and the Caribbean realm has already been described shortly in chapter 4.2.1. The coastal areas are characterised in particular by Cenozoic (including Quaternary) shallow water and abundant reef carbonate rocks (Sc).

227


Figure 5-13 : Normalised rock distribution in North America for the 7 aggregated relief types compared to the global rock distribution for the corresponding relief type. For comparison also the normalised rock distribution for North America in total compared to the global values is given. Lithology codes : Pb – Plutonic basic, Pa – Plutonic acid, Vb – Volcanic basic, Va – Volcanic acid, Pr – Precambrian, Mt – Metamorphic, Cl – Complex lithology, Ss – Siliciclastic sedimentary consolidated, Sm – Mixed sedimentary consolidated, Sc – Carbonates consolidated, Ep – Evaporites, Su – Semi- to unconsolidated sedimentary, Ad – Alluvial deposits, Lo – Loess, Ds – Dunes and shifting sand.

0,1

1

10

Wb

Ig

Pb

Pa

Vb

Va

Pr

M

t

Cl

Ss

Sm

Sc

Ep

Su

Ad

Lo

D

s

% o

utcr

op in

pla

ins

in N

orth

Am

eric

a %

out

crop

in p

lain

s gl

obal

TotalEndorheic

Normalised rock distribution in plains in North America

0,1

1

10

Wb

Ig

Pb

Pa

Vb

Va

Pr

M

t C

l Ss

Sm

Sc

Ep

Su

A

d

Lo

Ds

% o

utcr

. low

l. &

low

pla

t. N

. Am

eric

a %

out

cr. l

owl.

& lo

w p

lat.

glob

al

TotalEndorheic

Normalised rock distribution in lowlands and low plateaus in North America

0,1

1

10

Wb

Ig

Pb

Pa

Vb

Va

Pr

M

t C

l Ss

Sm

Sc

Ep

Su

A

d

Lo

Ds

% o

utcr

. low

& m

id-a

lt. p

lat.

N. A

mer

ica

% o

utcr

. low

& m

id-a

lt. p

late

aus

glob

al

TotalEndorheic

Normalised rock distribution in low & mid-altitude plateaus in North America

0,1

1

10

Wb

Ig

Pb

Pa

Vb

Va

Pr

M

t C

l Ss

Sm

Sc

Ep

Su

A

d

Lo

Ds

% o

utcr

. rug

ged

low

l. &

hill

s N

. Am

eric

a %

out

cr. r

ugge

d lo

wl.

& h

ills

glob

al

TotalEndorheic

Normalised rock distribution in rugged lowlands and hills in North America

0,1

1

10

Wb

Ig

Pb

Pa

Vb

Va

Pr

M

t C

l Ss

Sm

Sc

Ep

Su

A

d

Lo

Ds

% o

utcr

. low

& m

id-a

lt. m

nt. N

. Am

eric

a%

out

cr. i

n lo

w &

mid

-alt.

mnt

. glo

bal

TotalEndorheic

Normalised rock distribution in low & mid-altitude mountains in North America

0,1

1

10W

b

Ig

Pb

Pa

Vb

Va

Pr

Mt

Cl

Ss

Sm

Sc

Ep

Su

Ad

Lo

D

s

% o

utcr

. hig

h &

v. h

igh

plat

. N. A

mer

ica

% o

utcr

. hig

h &

v. h

igh

plat

eaus

glo

bal

TotalEndorheic

Normalised rock distribution in high & very high plateaus in North America

0,1

1

10

Wb

Ig

Pb

Pa

Vb

Va

Pr

M

t C

l Ss

Sm

Sc

Ep

Su

A

d

Lo

Ds

% o

utcr

. hig

h &

v. h

igh

mnt

. N. A

mer

ica

% o

utcr

. hig

h &

v. h

igh

mnt

. glo

bal

TotalEndorheic

Normalised rock distribution in high & very high mountains in North America

Normalised rock distribution in N. America (total)

0,1

1

10

Wb

Ig

Pb

Pa

Vb

Va

Pr

M

t

Cl

Ss

Sm

Sc

Ep

Su

Ad

Lo

D

s

% o

utcr

op in

Nor

th A

mer

ica

% o

utcr

op g

loba

l

TotalEndorheic

228


Relief-specific features for North and Middle America can be expressed as follows :

Plains, as mentioned above, characterise the central parts of the Laurentian basement areas in the North, the central mid-continent’s Mississippi valley axis as well as the northern coasts of the Mexican Gulf and Florida. Therefore basement lithologies occur aside of sedimentary rocks and comparatively small shares of Alluvial deposits and Loess – but Lo in general is still over-represented due to the Mississippi valley occurrences in contrast to global occurrences in plains. Metamorphic rocks (Mt) are clearly over-represented due to the slightly different nature of the Laurentian craton in contrast to the world’s other major cratons, the Precambrian rocks (Pr) are depleted, as in North America in general. Mixed sedimentary rocks (Sm), and even more Carbonates (Sc) dominate, due to the mid-continental parts and the southern coastal areas.

The North American lowlands, being the dominant relief type here, show similar characteristics with nearly equal portions of basement (Mt being enriched as well as Pa and some Cl) and sedimentary rocks (Sm, Sc, and minor Ss), but are still strongly depleted in loose sedimentary rocks (Su, Ad, Lo).

The cratonic mid-western mid-plateaus are dominated by Mesozoic Ss with some Su and Lo ; Metamorphic rocks (Mt) are still over-represented with respect to global values in mid-plateaus.

Hills and mid-mountains together belong – except for the strips along the continent’s eastern margins – to the sub-active to active Cordilleran orogen, in total they account for about one third of North and Middle America’s area. Both are similar with their mix of basement rocks (Mt still being favoured over Pr, and Pa), sedimentary rocks (Ss, Sm, Sc – distributions comparable to the global distributions in these relief classes) and rather small shares of Su and Ad (but Ad being slightly enriched with respect to global occurrences in mid-mountains). Characteristically the proportion of Complex lithologies (Cl) rises with the elevation from hills to mountains but they are not clearly enriched with respect to global values. Similar behaviour is shown by the shares of volcanic rocks (mostly intermediate – Vb) – they are over-represented mainly in mid-mountains.

High mountains and high plateaus are dominated by Vb which are clearly over-represented. Carbonate rocks (Sc) are enriched as well. High plateaus also shelter relatively large quantities of Su and Ad, both being over-represented with respect to global values for high plateaus.

Endorheic regions with their limited extent in the North American Cordilleras, mainly store Su and Ad, also some Loess (Lo), and volcanic rocks (Vb).

Refering to chapter 6, a special remark has to be made : North America, together with Europe, was most affected by the repeated growth and disappearance of huge inland ice shields

229


during the Pleistocene epoch. Some hydrological aspects of these processes are treated in chapter 6.

5.4.2.6 South America

South America takes 17,9 M km2 of the continental landmass as defined, thus occupying 13,4 %.

Morphologically, the non-dissected, cratonic, relief types with plains (31,4 %) and lowlands (31,0 %), together with mid-plateaus (8,2 %), occupy nearly 3 quarters (70,6 %) of its total area. Hills (7,4 %) and mid-mountains (14,6 %) can be found partially along the eastern coast – a Cretaceous rift margin of the South American craton, and also in the northern Guyana shield with its landscapes between table mountains and plains appearing rather dissected. However, their major shares, together with high mountains (6,8 %) and high plateaus (0,6 %) belong to the Andean cordilleras.

The general lithologic picture (Figure 5-14, lower right diagram) illustrates :

Similar to North America, but geologically structured in a simpler way, the non-Cordilleran parts – about 80 % of the continent – belong to a Precambrian craton, part of the late Precambrian super-continent Rodinia, evolved into the late Paleozoic super-continent Gondwana, split from Africa during Cretaceous times, and later from Antarctica.

About one third of the basement of the craton (mainly Pr and some few Pa) is exposed in the shields of Venezuela, Guyana and Brazil. The remaining two thirds are hidden under

i) late Precambrian and Paleozoic as well as Mesozoic platform sediments (mainly Ss),

ii) Jurassic – Cretaceous flood basalts (Vb), and

iii) Tertiary Su in the foredeep east of the Andes and – dominating as cover – Quaternary alluvial sediments (Ad), mainly in the Amazon and the Paraguay-Paraná basins.

iv) ‘Pampa’ loess covering another 1 M km2, slightly more than 5 % of the whole South American area.

230


Figure 5-14 : Normalised rock distribution in South America for the 7 aggregated relief types compared to the global rock distribution for the corresponding relief type. For comparison also the normalised rock distribution for South America in total compared to the global values is given. Lithology codes : Pb – Plutonic basic, Pa – Plutonic acid, Vb – Volcanic basic, Va – Volcanic acid, Pr – Precambrian, Mt – Metamorphic, Cl – Complex lithology, Ss – Siliciclastic sedimentary consolidated, Sm – Mixed sedimentary consolidated, Sc – Carbonates consolidated, Ep – Evaporites, Su – Semi- to unconsolidated sedimentary, Ad – Alluvial deposits, Lo – Loess, Ds – Dunes and shifting sand.

0,1

1

10

Wb

Ig

Pb

Pa

Vb

Va

Pr

M

t C

l Ss

Sm

Sc

Ep

Su

A

d

Lo

Ds

% o

utcr

op in

pla

ins

in S

outh

Am

eric

a %

out

crop

in p

lain

s gl

obal

TotalEndorheic

Normalised rock distribution in plains in South America

0,1

1

10

Wb

Ig

Pb

Pa

Vb

Va

Pr

M

t C

l Ss

Sm

Sc

Ep

Su

A

d

Lo

Ds

% o

utcr

. low

l. &

low

pla

teau

s S.

Am

eric

a %

out

cr. l

owl.

& lo

w p

late

aus

glob

al

TotalEndorheic

Normalised rock distribution in lowlands and low plateaus in South America

0,1

1

10

Wb

Ig

Pb

Pa

Vb

Va

Pr

M

t C

l Ss

Sm

Sc

Ep

Su

A

d

Lo

Ds

% o

utcr

. low

& m

id-a

lt. p

lat.

S. A

mer

ica

% o

utcr

. low

& m

id-a

lt. p

lat.

glob

al

TotalEndorheic

Normalised rock distribution in low & mid-altitude plateaus in South America

0,1

1

10

Wb

Ig

Pb

Pa

Vb

Va

Pr

M

t C

l Ss

Sm

Sc

Ep

Su

A

d

Lo

Ds

% o

utcr

. rug

ged

low

l. &

hill

s S.

Am

eric

a %

out

cr. r

ugge

d lo

wla

nds

& h

ills

glob

al

TotalEndorheic

Normalised rock distribution in rugged lowlands and hills in South America

0,1

1

10

Wb

Ig

Pb

Pa

Vb

Va

Pr

M

t C

l Ss

Sm

Sc

Ep

Su

A

d

Lo

Ds

% o

utcr

. low

& m

id-a

lt. m

nt. S

. Am

eric

a%

out

cr. l

ow &

mid

-alt.

mnt

. glo

bal

TotalEndorheic

Normalised rock distribution in low & mid-altitude mountains in South America

0,1

1

10W

b

Ig

Pb

Pa

Vb

Va

Pr

Mt

Cl

Ss

Sm

Sc

Ep

Su

Ad

Lo

D

s

% o

utcr

. hig

h &

v. h

igh

plat

. S. A

mer

ica

% o

utcr

. hig

h &

v. h

igh

plat

. glo

bal

TotalEndorheic

Normalised rock distribution in high & very high plateaus in South America

0,1

1

10

Wb

Ig

Pb

Pa

Vb

Va

Pr

M

t C

l Ss

Sm

Sc

Ep

Su

A

d

Lo

Ds

% o

utcr

. hig

h &

v. h

igh

mnt

. S. A

mer

ica

% o

utcr

. hig

h &

v. h

igh

mnt

. glo

bal

TotalEndorheic

Normalised rock distribution in high & very high mountains in South America

Normalised rock distribution in S. America (total)

0,1

1

10

Wb

Ig

Pb

Pa

Vb

Va

Pr

M

t

Cl

Ss

Sm

Sc

Ep

Su

Ad

Lo

D

s

% o

utcr

op in

Sou

th A

mer

ica

% o

utcr

op g

loba

l

TotalEndorheic

231


The Andes – as the western backbone, including the Venezuelan coastal chains – continue the North and Middle American as well as the circum-Caribbean island chains. Compared to their North American counterpart, they occupy less area and are of somewhat less complex nature. Their ‘basement’ consists of mainly Complex lithologies (Cl) and dioritic to granitic intrusive rocks (Pa). Sedimentary rocks are mixed, with Paleozoic as well as Mesozoic Ss – Sm – Sc, Tertiary Su and Quaternary Ad.

Comparatively large areas are covered by intermediate – the prototypical ‘andesitic’ – volcanic rocks (Vb) and also rather high shares of rhyolitic ignimbrites (Va).

Relief-specific features for South America can be characterised as follows :

Plains in South America are largely identical with foredeeps east of the Andes and with the Amazon and the Paraguay-Paraná basins. They are therefore dominated by Ad with smaller shares of Su and Lo. Compared to plains globally, alluvial deposits still appear enriched, consolidated sedimentary rocks are greatly depleted ; of the ‘hard’ rocks only volcanic rocks and Precambrian rocks appear over-represented.

Lowlands and mid-plateaus correspond to the shield areas of the craton with Pr basement (Pr being over-represented) and the platform sedimentary (mainly Ss, some Sc in mid-plateaus, practically no Sm, some Su) and volcanic (mainly flood basalt – Vb being enriched, but also Va of Paleozoic or older origin in lowlands and low plateaus, here on the Brazilian shield) cover described just above. Alluvial sediments take some shares, more in lowlands than in mid-plateaus – a characteristic to be expected.

Hills and mid-mountains constitute, with some portions as mentioned, parts of the Venezuela-Guyana shield and the east Brazilian coastal margin. This accounts for the enriched presence of Precambrian metamorphics (Pr) and parts of the granites (Pa, they are nearly equally distributed here as in hills and mid-mountains globally). The majority of the hills and mid-mountains however belongs to the Andes and is constituted of the characteristic ‘Cordilleran’ mix of Complex lithologies (Cl – but under-represented with respect to global values), various sedimentary rocks (but in general they are under-represented as well with respect to global occurrences in these relief classes – except for Sm being slightly over-represented in mid-mountains) and sediments (especially Su and Ad are over-represented) as well as volcanic rocks.

The absolute distribution values are somewhat similar for the Andean high mountains and high plateaus except for the Precambrian rocks being practically non-existent. Especially rhyolitic ignimbrites (Va) occur particularly over-represented. Next to the sediments Su and Ad which are also over-represented, especially Evaporites (Ep) appear greatly over-represented in both relief classes, due to the salt lakes of the altiplano. Some of the cells concerned at 30’ scale may be in the ‘high mountain’ category accounting for the over-representation of Evaporites in this relief class as well.

232

5.5 Relief distribution for the different lithologies

Endorheic areas in South America occur to a very minor extent in mid-mountains, take a higher share of the high mountains, and occupy the largest surfaces on the high plateaus where they take more than 3 quarters (79 %) of the total area – however it has to be reminded that high plateaus in total occupy only one tenth of the high mountain’s area. The endorheic parts, as in general, accommodate preferentially young sediments (Su, Ad, and also Lo), and in the arid parts of the Andes, also recent Evaporites – especially in the high plateaus.

5.5 Relief distribution for the different lithologies In contrast to chapter 5.4 where the mutual shares of the different lithologies within each

of the 7 super-classes of relief has been considered, the mutual shares of the different relief classes for each of the lithology classes however have only been mentioned but not visualised. Thus the question of the distribution of the different lithologies on the 7 relief super-classes will be discussed here inversely to the arguments developed in the previous section. The values are thus repeated and some observations have already been remarked, but some characteristics are not apparent when taking into account proportions of lithologies for relief classes, and become visible only by applying the inverse procedure as done in this section.

5.5.1 General remarks Again global values per lithology class will be considered first, then comparisons per relief

class with respect to global values will be achieved. Secondly, a differentiation for some characteristically striking or particularly interesting distribution following the major continents will be accomplished.

Each diagram, valid for one of the lithologic classes, shows the relief classes proportionally correct, but their absolute values vary greatly with the different importance of the respective lithologies. The absolute values should thus be kept in mind for quantitative comparisons.

233


Table 5-4 reminds the global values for the different lithologies as considered when combined with the relief typology, in quantitative order and separated for sediments, sedimentary rocks, metamorphic rocks and magmatic rocks.

Table 5-4 : Lithologic classes, ranked in order of their global appearances. Shown are values as obtained when combining lithology with relief typology (compare Table 5-3).

Sediments Sedimentary rocks

Metamorphic rocks

Magmatic rocks

Water / Ice % M km2

Ss 16,3 21,1

abun

- da

nt

freq

uent

r

a r

e

5.5.2 GlThe

changing remainly the Ds, Ep, anThese procflat lands. Ain ‘orogenic

Ad 15,5 19,7 Pr (+ Mt) 11,6 (15,6) 15,0 (20,1) Sc 10,4 13,2

Su 10,1 12,7 Sm 7,8 9,9 Pa 7,2 9,3 Vb 5,8 7,3 Cl 5,4 6,9 Mt 4,0 5,1

Lo 2,6 3,4 Ds 1,5 2,0

Va 1,0 1,3 Pb 0,2 0,2 Ep 0,1 0,2 Wb 0,5 0,7 Ig 0,02 0,03

obal picture different rock types will be treated following the clarity of their relationships with the lief. ‘Soft rocks’ (Figure 5-15 and Figure 5-16) are most obviously related to the relief, relatively straightforward cases of youngest, geologically ‘recent’ sediments (Ad, Lo, d partially also Su). Their distribution reflects directly the processes of sedimentation. esses take place preferably – regarding the areas occupied – in non-orogenic, low and s a correlate rule all these sediments are more or less under-represented or depleted ’, dissected and elevated relief classes.

234


Alluvial sediments (Ad) (15,5 % of the total global terrestrial area) are transported and sedimented by surface water. They evidently obey the law of gravity – their repartition is inversely correlated with absolute elevation and roughness of the relief types (exponentially decreasing absolute values in Figure 5-15). High plateaus, even more when endorheic, serve as intermediate storage places, Ad are thus slightly over-represented concerning their normalised distribution for high plateaus (see Figure 5-16). The same seems to hold true for endorheic parts of hilly relief types.

Loess (Lo, global area 2,6 %) as the eolian dust sediment has been deposited worldwide during Pleistocene glaciation periods in the – during those times – huge tundras and cold-temperate steppes of North America, Europe and Asia, and also in the Pampean forefields of the Andes in South America. It shows a behaviour principally similar to that of alluvial sediments. However, as even dust-loaded air can flow upwards, loess in over-represented to some degree in endorheic high mountains. Hills are relatively depleted in loess, probably due to their loess covers being mostly too thin to appear on our map, possibly also because of their mostly very young erosive dissection.

Shifting sand grains, deposited in Dunes (Ds, only 1,5 % of the global area), are much heavier than dust. Sand occurrences require sources – deeply weathered quartz-rich rocks, recycling of sandstones and other sandy sediments, but also arid conditions and rather strong winds to be transported. Normally only short distances are covered by eolian transport of sand, and flat ground is needed for deposition. The geologic movement of Dunes is comparatively slow, and many, if not most of them date back to Pleistocene times. It is therefore not astonishing that nearly 50 % of the Dunes and shifting sand occur in plains, another 20 % in lowlands and the remaining nearly 30 % in the arid mid-plateaus of Africa and Asia. In both plains and mid-plateaus, Ds are over-represented by a factor two. Correspondingly they are greatly depleted in hills and mid-mountains and completely absent in high plateaus and high mountains.

Evaporites (Ep, globally only 0,1 % of area) on our map are, as explained in chapter 3, exclusively recent Evaporites in arid climates. The few occurrences of remarkable hydrologically important hidden subsurface deposits can not be taken into account here. About one third of the recent Evaporites occurs in exorheic coastal plains and lowlands (N-Africa, Near East), some in endorheic plains and lowlands. The majority however is to be found in nearly exclusively endorheic mid-plateaus, and entirely endorheic mid-mountains, high mountains and high plateaus – mainly in Iran, parts of central Asia, in Australia, and in some parts of Africa and the South American Andes.

235


18,0

30,6

19,7

11,6

17,9

0,3 1,80

5

10

15

20

25

30

35

Plains Lowlands& low

plateaus

Low &mid-alt.plateaus

Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

f roc

k in

rel

ief t

ype

TotalEndorheic

Relief distribution for Siliciclastic sedimentary consolidated rocks (16,3 % = 21,1 M km2 of global rocks)

17,3

23,0

11,1 11,6

20,4

5,2

11,6

0

5

10

15

20

25


plateaus


Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

f roc

k in

rel

ief t

ype Total

Endorheic

Relief distribution for Mixed sedimentary consolidated rocks (7,8 % = 9,9 M km2 of global rocks)

20,0

31,1

12,910,3

21,4

0,83,6

0

5

10

15

20

25

30

35


plateaus


Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

f roc

k in

rel

ief t

ype

TotalEndorheic

Relief distribution for Carbonate rocks (10,4 % = 13,2 M km2 of global rocks)

34,9

17,2 18,8

0,0

19,8

7,4

1,805

10152025303540


plateaus


Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

f roc

k in

rel

ief t

ype Total

Endorheic

Relief distribution for Evaporites (0,1 % = 0,2 M km2 of global rocks)

35,7

21,3 22,0

8,110,7

0,7 1,505

10152025303540


plateaus


Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

f roc

k in

rel

ief t

ype Total

Endorheic

Relief distribution for Semi- to unconsolidated sedimentary rocks (10,1 % = 12,7 M km2 of global rocks)

47,5

22,5

9,0 8,4 9,6

1,1 1,90

10

20

30

40

50


plateaus


Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

f roc

k in

rel

ief t

ype

TotalEndorheic

Relief distribution for Alluvial deposits (15,5 % = 19,7 M km2 of global rocks)

50,7

22,9

10,02,9

9,3

0,6 3,6

0

10

20

30

40

50

60


plateaus


Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

f roc

k in

rel

ief t

ype

TotalEndorheic

Relief distribution for Loess (2,6 % = 3,4 M km2 of global rocks)

48,8

20,0

29,1

0,6 1,6 0,0 0,00

10

20

30

40

50

60


plateaus


Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

f roc

k in

rel

ief t

ype

TotalEndorheic

Relief distribution for Dunes and shifting sand (1,5 % = 2,0 M km2 of global rocks)

Figure 5-15 : Relief distribution on the globe for different lithology classes – ‘soft’ / sedimentary rocks.

236


Figure 5-16 : Normalised relief distribution on the globe for the different rock types – ‘soft’ / sedimentary rocks.

Normalised relief distribution for Siliciclastic sedimentary consolidated rocks

0,1

1

10


plateaus


Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

op in

rel

ief t

ype

% o

utcr

op g

loba

l

TotalEndorheic

Normalised relief distribution for Mixed sedimentary consolidated rocks

0,1

1

10


plateaus


Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

op in

rel

ief t

ype

% o

utcr

op g

loba

l

TotalEndorheic

Normalised relief distribution for Carbonate rocks

0,1

1

10


plateaus


Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

op in

rel

ief t

ype

% o

utcr

op g

loba

l

TotalEndorheic

Normalised relief distribution for Evaporites

0,1

1

10


plateaus


Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

op in

rel

ief t

ype

% o

utcr

op g

loba

l

TotalEndorheic

Normalised relief distribution for Semi- to unconsolidated sedimentary rocks

0,1

1

10


plateaus


Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

op in

rel

ief t

ype

% o

utcr

op g

loba

l

TotalEndorheic

Normalised relief distribution for Alluvial deposits

0,1

1

10


plateaus


Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

op in

rel

ief t

ype

% o

utcr

op g

loba

l

TotalEndorheic

Normalised relief distribution for Loess

0,1

1

10


plateaus


Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

op in

rel

ief t

ype

% o

utcr

op g

loba

l

TotalEndorheic

Normalised relief distribution for Dunes and shifting sand

0,1

1

10


plateaus


Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

op in

rel

ief t

ype

% o

utcr

op g

loba

l

TotalEndorheic

237


An intermediate situation, from multiple points of view, between young sediments and older lithified sedimentary rocks, is encountered for the lithologic class of Semi- to unconsolidated sediments (Su). Taking 10,1 % of the global area, they are – as defined – completely of Tertiary age. They show some striking similarities to the alluvial deposits regarding the exponentially decreasing total amounts for plains, lowlands and hills ; a similar curve, with smaller values however, can be traced through the columns for mid-plateaus, mid-mountains, high plateaus and high mountains (Figure 5-15). An explanation seems that

a) quite a lot of Tertiary sediments are of terrestrial aquatic nature, thus they have been alluvial sediments at the time of their formation, deposited in flatlands, mainly plains (accounting for the over-representation of Su here) and lowlands,

b) parts of Tertiary plains and lowlands were later lifted by tectonic en-bloc movements and thus became continental mid-plateaus (Su again over-represented) and mid-mountains.

The next and most important group are the normally older, consolidated Sedimentary rocks.

Siliciclastics (Ss, 16,3 %), Mixed Siliciclastic-carbonate sedimentary rocks (Sm, 7,8 %) and Carbonates (Sc, 10,4 %) together occupy about one third (34,5 %) of the global terrestrial area. Combined with the 29,8 % of all young sediments just discussed (Su 10,1 % ; Ad 15,5 % ; Lo 2,6 % ; Ds 1,5 % ; Ep 0,1 %) they take nearly two thirds (64,3 %) of the continents.

As the majority of the world’s big cratons – all of the former Gondwana continents and Laurentia, the exception being Asia – date back to early – middle Precambrian times, platform sediments have been preserved from these times on. Furthermore, all continents have migrated ‘freely’ around the globe during the last and past 500 Ma of Phanerozoic time, changing latitudes and climates geologically fast. In addition, parts of the cratons have experienced tectonic uplifts. This explains why sedimentary rocks – as distributed on the globe today – generally don’t show any marked particularity or preference for certain relief classes. The same, for the same reasons, had been stated concerning their latitudinal distribution (cf. chapter 3.4.3).

Taking into account (cf. section 5.3.1) that about three quarters of the continents (78,4 %) can be regarded as cratons, the remaining 21,6 % being subactive to active orogenic (sensu stricto, collisional) chains, it can be stated as an interesting, general, consequence : Approximately one quarter of the global area is occupied by ‘cratonic’ sedimentary rocks (34,5 % • 0,784 = 27,0 % of the total global area), they belong to cratonic platforms. 7,4 % (= 34,5 % • 0,216) of the global area is occupied by ‘orogenic’ sedimentary rocks belonging to subactive and active orogens sensu stricto.

238


In orogens, sedimentary rocks are shoved together and are thus mixed in a largely arbitrary way.

In summary, no striking features can be remarked with respect to the distribution of sedimentary rocks across the different relief classes. Plains are slightly depleted in consolidated sediments, young sediments being the main cover. Ss are slightly over-represented in lowlands and mid-plateaus, Sc only in lowlands. The only notable exception are high mountains and high plateaus, strongly depleted in Ss and similarly enriched in Sm. This characteristic seems due to the globally large shares – both relief classes and their correlated Sm – in southern central Asia, in and around Tibet, where large areas of Sm appear in our map. A differentiation like in other parts of the world was not possible here (cf. chapter 4.2.6) as our sources (mainly DATONG et al. 1985) show Carbonate rocks as well as Siliciclastic rocks justifying a classification as Mixed sedimentary rocks.

The basement of sediments and sedimentary rocks in general is constituted of ‘hard’ crystalline rocks (global relief distributions for the different types of ‘hard’ rocks shown in Figure 5-17). Thus quite evidently all hard rocks are more or less strongly under-represented in plains (normalised values in Figure 5-18).

Volcanic rocks however, erupted as more or less liquid lavas out of fissures or chimneys of volcanoes, spread upon the surface covering sediments as well as other hard rocks. When older, they may become covered themselves or deformed tectonically or be metamorphosed.

Basic volcanic rocks (Vb, 5,8 % of global area) are more than five times more frequent than Acid Volcanics.

About 40 % – a rough estimate, between one third and one half – are flood basalts covering whole provinces in rifted areas, in particular over plumes / hot spots. They are rather weathering-resistant rocks except under warm-humid climates. The most prominent, youngest – late Cenozoic to Quaternary – and also most elevated – mid- to locally high mountains – example are the Trap Basalt plateaus in the Ethiopian part of the East African Rift. Following are, with increasing age and – on average – decreasing elevation, the late Tertiary Columbia river plateau in western North America (mid-mountains relief category), the Indian Deccan Traps from around the Cretaceous-Tertiary turn (mid-plateaus to lowlands), the slightly older North Atlantic provinces of East Greenland and Iceland (hills), the Jurassic-Cretaceous Paraná basalts in southern Brazil (mainly lowlands), and the Permian-Triassic central Siberian basalts (mid-mountains, not following the general rule).

239


0,0

14,1

5,7

27,8

47,9

1,1 3,30

10

20

30

40

50

60


plateaus


Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

f roc

k in

rel

ief t

ype Total

Endorheic

Relief distribution for Basic-ultrabasic Plutonic rocks (0,2 % = 0,2 M km2 of global rocks)

10,0

21,6

15,913,0

33,3

0,65,6

05

10152025303540


plateaus


Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

f roc

k in

rel

ief t

ype Total

Endorheic

Relief distribution for Acid Plutonic rocks (7,2 % = 9,3 M km2 of global rocks)

2,1

13,0 10,715,5

46,4

1,8

10,6

0

10

20

30

40

50


plateaus


Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

f roc

k in

rel

ief t

ype Total

Endorheic

Relief distribution for Basic Volcanic rocks (5,8 % = 7,3 M km2 of global rocks)

2,4

17,9

6,7

23,9

39,7

0,5

9,1

05

1015202530354045


plateaus


Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

f roc

k in

rel

ief t

ype Total

Endorheic

Relief distribution for Acid Volcanic rocks (1,0 % = 1,3 M km2 of global rocks)

9,3

30,7

19,8

10,4

26,7

0,32,9

0

5

10

15

20

25

30

35


plateaus


Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

f roc

k in

rel

ief t

ype Total

Endorheic

Relief distribution for Precambrian (shield) rocks (11,6 % = 15,0 M km2 of global rocks)

16,8

44,4

11,4 11,6 9,8

0,35,7

0

10

20

30

40

50


plateaus


Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

f roc

k in

relie

f typ

e TotalEndorheic

Relief distribution for Metamorphic rocks (4,0 % = 5,1 M km2 of global rocks)

4,0

14,7

5,5

15,0

45,9

0,8

14,3

0

10

20

30

40

50


plateaus


Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

f roc

k in

rel

ief t

ype Total

Endorheic

Relief distribution for Complex lithology (5,4 % = 6,9 M km2 of global rocks)

Figure 5-17 : Relief distribution on the globe for different lithology classes – ‘hard’ / crystalline rocks.

240


Normalised relief distribution for Basic-Ultrabasic Plutonic rocks

0,1

1

10


plateaus


Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

op in

rel

ief t

ype

% o

utcr

op g

loba

l

TotalEndorheic

Normalised relief distribution for Acid Plutonic rocks

0,1

1

10


plateaus


Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

op in

rel

ief t

ype

% o

utcr

op g

loba

l

TotalEndorheic

Normalised relief distribution for Basic Volcanic rocks

0,1

1

10


plateaus


Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

op in

rel

ief t

ype

% o

utcr

op g

loba

l

TotalEndorheic

Normalised relief distribution for Acid Volcanic rocks

0,1

1

10


plateaus


Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

op in

rel

ief t

ype

% o

utcr

op g

loba

l

TotalEndorheic

Normalised relief distribution for Precambrian (shield) rocks

0,1

1

10


plateaus


Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

op in

rel

ief t

ype

% o

utcr

op g

loba

l

TotalEndorheic

Normalised relief distribution for Metamorphic rocks

0,1

1

10


plateaus


Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

op in

rel

ief t

ype

% o

utcr

op g

loba

l

TotalEndorheic

Normalised relief distribution for Complex lithology

0,1

1

10


plateaus


Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

op in

rel

ief t

ype

% o

utcr

op g

loba

l

TotalEndorheic

Figure 5-18 : Normalised relief distribution on the globe for the different rock types – ‘hard’ / crystalline rocks.

241


The remaining approximately 60 % (more than one half) of basic Volcanics consist – in global view – of basic to intermediate rocks, generated by subduction of oceanic lithosphere in orogenic (sensu stricto) settings. Most of them– at the present surface – belong to volcanoes in the active ‘fire belt’ around the Pacific, in Australasia, eastern Asia, North and South America. In general these volcanoes are strato-volcanoes, built of lava sheets as well as of pyroclastics, with dimensions from hills to mid-mountains to high mountains.

These characteristics explain the relief distribution of basic Volcanic rocks. They are rare and greatly under-represented in plains, somewhat under-represented in lowlands and also in mid-plateaus. They are enriched in hills, mid-mountains – where nearly half of Vb are concentrated, high mountains, and also in high plateaus – there but with barely significant total amounts.

Acid volcanic rocks (Va, globally 1,0 %) are rare because of the high viscosity of the corresponding magmas. The rather high volume of H2O contained in these magmas explains their often explosive eruption and deposition as extended Ignimbrite sheets (cf. chapter 3 and Annexe I) – they represent nearly all of the occurrences depicted on our map. The majority of them belong to the subactive to active collisional chains and are to be found nearly exclusively in eastern Asia and in the South American Andes. Minor (late Precambrian) exceptions occur in the Guyana shield in South America and in a small area in southern Australia.

The relief distribution is similar to that of Vb, acid Volcanics are over-represented in hills, mid- and high mountains, they are depleted in all other relief classes, particularly in plains.

Complex lithologies (Cl, taking 5,4 % globally) by definition embrace sedimentary as well as diverse crystalline rocks and were mapped mainly in the complex inner zones of younger and of active orogens sensu stricto. Consequently they are over-represented, with factors from 1,5 to 2 in hills, mid- and high mountains with highest absolute surfaces in mid-mountains but highest enrichment in high mountains. They are depleted in the non-dissected relief classes, especially in plains.

Precambrian (shield) rocks – Pr (11,6 %) and Metamorphic rocks – Mt (4,0 %) have a comparable – medium to high grade metamorphic – lithologic character. They are nearly identical to some extent ; the Precambrian basement of Laurentia for example is only marginally different from Pr but has been mapped as Mt. The great majority of both rock types characterises the – essentially Precambrian – cratonic basement of all continents, where mid-crustal levels are exposed.

242


Therefore the relief distribution is rather similar : Pr and Mt occur mainly in lowlands where they are over-represented, in plains with their sedimentary cover they are under-represented. Even more than in plains they are depleted in the high plateaus and high mountains of active orogens (except for Mt in high mountains). They also occur to some extent in hills, mid-plateaus and mid-mountains. No obvious explanations can be given for the slight differences in the distribution profiles.

A rather special case are Plutonic rocks.

Acid Plutonic rocks – Pa (7,2 %) dominate. They are of dioritic – granodioritic – granitic nature and are typically mid-crustal rocks, often intimately interwoven with medium- to high-grade metamorphic rocks (Para-metamorphics of sedimentary origin). Such granitoid – granitic rocks characterise large areas of the deeply eroded, mainly Precambrian, basement of the continents and show a similar relief distribution (e.g. depleted in plains and high plateaus).

Alternatively, the magmas at the origin of these rocks can rise into the upper crust and form intrusions within low or very low metamorphic sedimentary rocks. As a result they also appear in the inner zones of younger and youngest active orogens sensu stricto. Subsequently they are over-represented especially in mid-mountains.

Basic-ultrabasic Plutonic rocks – Pb (0,2 %) altogether play a very minor role. A small part occurs as intra-continental intrusions in basement areas of northern Europe, southern Africa and northern America. The majority are ophiolites, slices of oceanic lithosphere – crust and parts of the underlying mantle, isolated and obducted during collisional processes and occurring mostly in structurally rather high thrust sheets in orogenic chains. There are Paleozoic examples (e.g. in New Foundland / North America, in the Urals), but most ophiolites can be found in the Alpidic chains between the Gondwana continents and Eurasia, from the Balkan peninsula eastwards (see Anatolia and the Near East etc.), and in the middle and southern parts of the very complex marginal continental and island chains bordering the West Pacific ocean. These characteristic occurrences explain the depletion in lowlands and mid-plateaus and the over-representation of Pb in hills and mid-mountains.

243


5.5.3 Analysis per individual continents As stated before, absolute values and relative, normalised values have been used for our

comparisons. For the individual continents, the absolute numbers as well as the complete set of normalised diagrams can be found in Annexe II, we have restricted the figures here to some particularly interesting elements using normalised values as explained. In the following sections, only striking features and deviations from the mean global picture as shown in the normalised diagrams are discussed. The setting are the lithologic-geologic particularities of each continent and their geomorphologic expression – again often younger elements being more clearly expressed. Rarely occurring rock classes (e.g. Va and Pb in Australasia) and relief types (e.g. high mountains and high plateaus in Africa) are mentioned only for conspicuous observations.

5.5.3.1 Africa

Shown in Figure 5-19 are the characteristic relief distributions for Su, Ds, Ss, Sc, Vb, Pa and Pr as well as the normalised overall relief distribution in Africa with respect to the global relief type values (lower right diagram).

Ad (13,1 % in Africa) occur less in plains but are over-represented in the endorheic Chad basin. They are also slightly enriched in mid-plateaus with respect to global occurrences of alluvial sediments in this relief class. No occurrences of Loess (Lo) can be found on our map for Africa. Dunes and shifting sand (Ds, 4,0 %) are over-represented in lowlands and hills, they are slightly depleted in mid-plateaus – which are much rarer in the arid belt of Africa than in Asia. Evaporites (Ep, only 0,1 %) are slightly over-represented in hills and mid-plateaus – the absolute maximum being in this relief class with most of it in endorheic basins, but lack totally in the dissected relief classes – mostly being more humid in Africa.

Su (14,6 %) occupy more surface than on global average and are over-represented in plains, lowlands and mid-plateaus, with a good third being endorheic.

Ss hold 19,1 %, thus being close to the most widespread type of rocks in Africa – Pr. They are over-represented in plains and lowlands, mainly of northern Africa. Sc (11,0 %) are slightly over-represented in lowlands and hills. Sm are comparatively rare (2,5 %) and generally under-represented.

No occurrences of Va have been mapped in Africa. Areas of Vb (4,35 %) are mostly related to rifting processes and occur principally in the East African mid-mountains and mid-plateaus. They clearly dominate the high mountains.

244


Figure 5-19 : Normalised relief distribution in Africa for some rock types showing particular characteristics. For comparison also the normalised overall relief distribution in Africa with respect to the global relief type values is given.

Normalised relief distribution for Semi- to unconsolidated sedimentary rocks in Africa

0,1

1

10

Plains Lowlandsand lowplateaus


Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

op S

u in

rel

ief t

ype

Afr

ica

% o

utcr

op S

u in

rel

ief t

ype

glob

al

TotalEndorheic

Normalised relief distribution for Dunes and shifting sand in Africa

0,1

1

10



Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

op D

s in

rel

ief t

ype

Afr

ica

% o

utcr

op D

s in

rel

ief t

ype

glob

al

TotalEndorheic

Normalised relief distribution for Siliciclastic sedimentary consolidated rocks in Africa

0,1

1

10



Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

op S

s in

rel

ief t

ype

Afr

ica

% o

utcr

op S

s in

rel

ief t

ype

glob

al

TotalEndorheic

Normalised relief distribution for Carbonate rocks in Africa

0,1

1

10



Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.%

out

crop

Sc

in r

elie

f typ

e A

fric

a %

out

crop

Sc

in r

elie

f typ

e gl

obal

TotalEndorheic

Normalised relief distribution for Basic Volcanic rocks in Africa

0,1

1

10



Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

op V

b in

rel

ief t

ype

Afr

ica

% o

utcr

op V

b in

rel

ief t

ype

glob

al Total

Endorheic

Normalised relief distribution for Acid Plutonic rocks in Africa

0,1

1

10



Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

op P

a in

rel

ief t

ype

Afr

ica

% o

utcr

op P

a in

rel

ief t

ype

glob

al

TotalEndorheic

Normalised relief distribution for Precambrian (shield) rocks in Africa

0,1

1

10



Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.%

out

crop

Pr

in r

elie

f typ

e A

fric

a %

out

crop

Pr

in r

elie

f typ

e gl

obal

TotalEndorheic

Normalised relief distribution for Africa (compared to the global relief distribution)

0,1

1

10



Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

f rel

ief t

ype

in A

fric

a %

of r

elie

f typ

e gl

obal

TotalEndorheic

245


Cl (only 0,9 %) are restricted to the Paleozoic Mauritanides at the north-western margin, the younger Cape belt in the very South, and towards the Maghrebides (Atlas) in the North.

Pr (19,4 %) and Pa (9,8 %) together, with some Mt (0,7 %) constitute the main parts of the Precambrian craton which is exposed mainly in the uplifted and partially rifted marginal parts around the huge basins in the northern and the southern part of the continent. Especially Pr are thus maximally over-represented in hills and mountains, quite a unique and remarkable feature not encountered to such extent on the other continents ; this characteristic even influences the global picture letting Precambrian rocks being enriched not only on plateaus but also in mid-mountains. They are also somewhat enriched in mid-plateaus and lowlands.

Pb (0,1 %) are restricted to the mid-plateaus and mid-mountains of the great Dyke in Zimbabwe and the South-African Bushveld massif nearby, the world’s best known basic-ultrabasic bowl-shaped (lopolithic) intrusion.

5.5.3.2 Asia

Asia is not only the largest continent, with arctic characteristic in the North and reaching beyond the equator with its southernmost islands. Asia also contains the globally largest endorheic areas. This seems partially related to its tectonic activity : there are high proportions of orogenic chains, the majority among them is subactive to active.

Illustrated in Figure 5-20 are the characteristic relief distributions for Ds, Lo, Sm, Cl, Vb, Va and Pr as well as the normalised overall relief distribution in Asia with respect to the global relief type values (lower right diagram).

Ad (20,6 % of the continental area) are distributed quite normally – over-represented in flat relief types – with however marked intermediate storages in – mainly endorheic – mid-plateaus and mid-mountain regions. The same holds true for Ds (1,9 %), concentrated in endorheic basins of plains, lowlands, hills and mid-plateaus (maximally, 45,0 % of the total Dunes area of Asia here). Lo (2,4 % of the continent) is only bound to endorheic areas to a small degree, as it is older and less dependent on morphologic criteria. It is clearly over-represented in mid-plateaus, mid-mountains and high mountains as well as high plateaus. Ep (0,2 %) are over-represented in (coastal) plains and lowlands, and in largely endorheic mid-plateau basins.

Su (only 6,2 % in Asia) are under-represented in all relief types, except for lowlands. About one third occurs in endorheic basins.

Sedimentary rocks in Asia (Ss – 14,1 %, Sm – 11,4 % – they are depicted in Figure 5-20 due to their importance here, and Sc – 9,6 %) are distributed rather normally, i.e. similar to the global mean. They are clearly under-represented only in plains – except for Sm which are over-

246


represented generally and in particular in the endorheic basins, due to the difficulties mentioned concerning the differentiation in central Asia and China.

Vb (6,4 %) – with about one third of flood basalts in India and Siberia, the rest belonging to subduction-fed volcanoes – show a rather normal distribution, they are somewhat enriched in plains, lowlands, hills and mid-plateaus, and have their absolute maximum (38,4 % of all Vb in Asia) in mid-mountains, but are nonetheless under-represented in this relief class. A difference to the global mean exists concerning high mountains and high plateaus in Asia. As they belong mainly to continent-continent collision orogens, they are strongly depleted in Vb.

Va (1,9 %) are over-represented in mid-mountains, hills and mid-plateaus, mainly in the Mesozoic orogenic chains near the Pacific margins of eastern and north-eastern Asia.

Cl (8,6 %) – in accordance with the important role of Phanerozoic, mainly young orogens in Asia – are enriched in nearly all relief types, with absolute maxima in mid- and high-mountains. This is corroborated by the similar over-representation of Pb (0,4 %) – all of them being obducted oceanic ophiolites.

With all these rock types being generally over-represented in Asia, the natural counterpart concerns all basement rocks : Pr (6,9 %) – depicted in Figure 5-20 as they show characteristic behaviour as the other basement rocks, Pa (7,4 %) and Mt (2,0 %) are clearly under-represented in Asia. They occur preferably in mid-mountains, but also in mid-plateaus, hills and lowlands.

Endorheic Asia :

For situation and extent of the endorheic areas – around the Caspian and Aral Sea, in Near East and Central Asia, see the maps in chapter 2.5 and chapter 7.

Except for plains, endorheic parts of all relief types are over-represented in Asia, with absolute maxima in mid-mountains, mid-plateaus and high mountains, and relative maxima (about one half of each class) in mid-plateaus, high mountains and high plateaus.

247


Normalised relief distribution for Dunes and shifting sand in Asia

0,1

1

10



Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

op D

s in

rel

ief t

ype

Asi

a

% o

utcr

op D

s in

rel

ief t

ype

glob

al

TotalEndorheic

Normalised relief distribution for Loess in Asia

0,1

1

10



Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.%

out

crop

Lo

in r

elie

f typ

e A

sia

%

out

crop

Lo

in r

elie

f typ

e gl

obal

TotalEndorheic

Normalised relief distribution for Mixed sedimentary consolidated rocks in Asia

0,1

1

10



Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

op S

m in

rel

ief t

ype

Asi

a

% o

utcr

op S

m in

rel

ief t

ype

glob

al

TotalEndorheic

Normalised relief distribution for Complex lithology in Asia

0,1

1

10



Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

op C

l in

relie

f typ

e A

sia

%

out

crop

Cl i

n re

lief t

ype

glob

al

TotalEndorheic

Normalised relief distribution for Basic Volcanic rocks in Asia

0,1

1

10



Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

op V

b in

rel

ief t

ype

Asi

a

% o

utcr

op V

b in

rel

ief t

ype

glob

al

TotalEndorheic

Normalised relief distribution for Acid Volcanic rocks in Asia

0,1

1

10



Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

op V

a in

rel

ief t

ype

Asi

a

% o

utcr

op V

a in

rel

ief t

ype

glob

al

TotalEndorheic

Normalised relief distribution for Precambrian (shield) rocks in Asia

0,1

1

10



Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.% o

utcr

op P

r in

rel

ief t

ype

Asi

a

% o

utcr

op P

r in

rel

ief t

ype

glob

al

TotalEndorheic

Normalised relief distribution for Asia (compared to the global relief distribution)

0,1

1

10



Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

f rel

ief t

ype

in A

sia

% o

f rel

ief t

ype

glob

al

TotalEndorheic

Figure 5-20 : Normalised relief distribution in Asia for some rock types showing particular characteristics. For comparison also the normalised overall relief distribution in Asia with respect to the global relief type values is given.

248


The highest shares are taken quite naturally by Evaporites (Ep) and Dunes (Ds). Su and Sm (again, in Tibet) follow with some distance. Ad in plains, lowlands and hills are mainly exorheic. In mid-plateaus and mid-mountains however, slightly > 50 % of endorheic Ad are to be found, in high mountains about three quarters, in high plateaus all of them are endorheic.

From the sedimentary rocks, only Sm and partly Sc are over-represented to some extent.

Nearly all Vb and Va in endorheic regions of Asia are to be found in mid-plateaus, mid- and high-mountains, holding there about a fifth of the area.

Concerning ‘hard’ rocks, Cl are most clearly over-represented in endorheic regions. Va and Pb (ophiolites) are enriched in mid-plateaus and mid-mountains, basement rocks (Pa, Pr and Mt) only in the high mountains and high plateaus of Inner Asia, where the mid-crustal rocks are lifted to important elevations by continent-continent collision.

5.5.3.3 Australasia

Illustrated in Figure 5-21 are the characteristic relief distributions for Ss, Sc and Pr as well as the normalised overall relief distribution in Australasia with respect to the global relief type values (lower right diagram).

Australasia is the driest continent and has in general less elevated and less dissected landscapes than on global average, high plateaus are completely missing.

Consequently, Ad (8,2 % of the total surface in Australasia) occur less frequently than normal, they are concentrated in the plains in the North and in the (endorheic) centre. Ep (0,4 %) are greatly over-represented and concentrated in plains – mostly in their endorheic areas, and in lowlands where the whole area occupied is endorheic. Lo and Ds are absent (on our map).

Su occupy 17,8 % of Australasia’s area and are enriched in plains, lowlands, hills and to some extent in mid-plateaus.

249


Figure 5-21 : Normalised relief distribution in Australasia for some rock types showing particular characteristics. For comparison also the normalised overall relief distribution in Australasia with respect to the global relief type values is given.

Ss (14,3 %), Sm (17,9 %) and Sc (6,8 %) together hold the lion’s share of area in Australasia. Interestingly, Ss is mainly depleted in most of the relief classes in Australasia, it is enriched only in plains. This is probably due to the distinct relief distribution on the continent. Sc are over-represented to an important extent only in the high mountains of New Guinea, but probably due to the small occurrences very few cells at 30’ scale can produce this effect. More importantly, the huge Nullarbor plain, entirely made up of Cenozoic Carbonates in south-western Australia, does not significantly influence the normalised appearance of Sc in plains in Australasia with respect to global occurrences of this rock type in plains. This is probably due to the general shares of Carbonates in plains (about 20 % globally), but also to the fact that next to the Nullarbor plain Carbonates the remaining shares on the continent are less important.

0,1

1

10



Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.%

out

crop

Ss

in r

elie

f typ

e A

ustr

alas

ia

% o

utcr

op S

s in

rel

ief t

ype

glob

al

TotalEndorheic

Normalised relief distribution for Siliciclastic sedimentary consolidated rocks in Australasia

Normalised relief distribution for Carbonate rocks in Australasia

0,1

1

10



Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

. Sc

in r

el. t

ype

Aus

tral

asia

%

out

crop

Sc

in r

elie

f typ

e gl

obal

TotalEndorheic

Normalised relief distribution for Precambrian (shield) rocks in Australasia

0,1

1

10



Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

. Pr

in r

elie

f typ

e A

ustr

alas

ia

% o

utcr

op P

r in

rel

ief t

ype

glob

al

TotalEndorheic

Normalised relief distribution for Australasia (compared to the global relief distribution)

0,1

1

10



Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

f rel

ief t

ype

in A

ustr

alas

ia

% o

f rel

ief t

ype

glob

al

TotalEndorheic

Cl (5,4 % and generally over-represented except in plains) have been mapped in the mid-Australian and the eastern Tasmanide Phanerozoic fold belts, and they also characterise, together with the (andesitic) Vb, the active orogens from northern New Guinea down to New Zealand – therefore being distributed preferably in dissected relief types.

250


Pa (10,3 %), Pr (9,8 %) and Mt (5,2 %) constitute the old cratonic basement of Australia, outcropping over about a quarter of its area. They are over-represented in plains, lowlands, and partially also in mid-plateaus.

5.5.3.4 Europe

Represented in Figure 5-22 are the characteristic relief distributions for Lo, Sc and Cl as well as the normalised overall relief distribution in Europe with respect to the global relief type values (lower right diagram).

It should be reminded that Europe (see also Figure 5-22, lower right diagram) has only 1,1 % of mid-plateaus, 0,3 % of high mountains, and no high plateaus are present.

Ad (6,9 % of area in Europe) take a much smaller share in Europe than globally (15,5 %). The distribution shows the expected and normal exponential decrease from plains over lowlands to hills. The alluvial plains are endorheic to more than their half due to their concentration around the Caspian Sea.

Figure 5-22 : Normalised relief distribution in Europe for some rock types showing particular characteristics. For comparison also the normalised overall relief distribution in Europe with respect to the global relief type values is given.

251

Normalised relief distribution for Loess in Europe

0,1

1

10



Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

op L

o in

rel

ief t

ype

Euro

pe

% o

utcr

op L

o in

rel

ief t

ype

glob

al

TotalEndorheic

Normalised relief distribution for Carbonate rocks in Europe

0,1

1

10



Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

op S

c in

rel

ief t

ype

Euro

pe

% o

utcr

op S

c in

rel

ief t

ype

glob

al

TotalEndorheic

Normalised relief distribution for Complex lithology in Europe

0,1

1

10



Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

op C

l in

relie

f typ

e Eu

rope

%

out

crop

Cl i

n re

lief t

ype

glob

al

TotalEndorheic

Normalised relief distribution for Europe (compared to the global relief distribution)

0,1

1

10



Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.%

of r

elie

f typ

e in

Eur

ope

% o

f rel

ief t

ype

glob

al

TotalEndorheic


Recent Evaporites (Ep) are lacking in Europe. Ds (0,2 % only, a seventh of the global mean) occur mainly in the endorheic plains around the Caspian Sea. Lo however (8,5 %) is strongly over-represented in central and east European plains (41,6 % of the total Lo occurrences of Europe) and lowlands (52,9 % of the total Lo surface of the continent) ; to some extent also hills are concerned (4,4 % of total Lo – still being enriched). It is under-represented – compared with mainly the occurrences in Asia – in mid-mountains (1,0 % of Lo area in Europe only).

Su (12,8 %) occur mainly in plains (30,9 % of the occurrences in Europe) and lowlands (34,9 %), the remaining third is divided between hills and mid-mountains. They are over-represented, with rising proportions, in lowlands, hills, mid-plateaus (but only 2,8 % of the total absolute share of Su in Europe) and mid-mountains (1,0 % only when considering absolute values).

Ss (26,1 %) and Sc (19,9 % of area in Europe) are over-represented, Ss in plains, lowlands and hills, Sc in all relief classes in Europe existing on the continent – even in mid-plateaus where only 0,6 % of the total Carbonates in Europe can be found. Principal shares of Sc are taken by lowlands (46,5 %) and plains (24,2 % of the continent). Sm are insignificant in Europe.

Vb (only 1,3 % of area in Europe) are strongly under-represented in Europe and to be found mainly in the hills and (low- to) mid-mountains of Iceland – a mid-ocean ridge segment over a mantle plume, and in central and south-western Europe. Va are nearly absent.

Concerning the hard rocks, it can be stated : Cl (8,8 %) are over-represented, mainly in hills and mid-mountains concerning their absolute shares (32,1 % and 46,9 % respectively), due to their occurrences in the inner parts of the Caledonides, Variscan and Alpidic chains. The strong over-representation of Cl in mid-plateaus in Europe, even if only 2,9 % of Cl in Europe can be found in this relief class is due to the globally low shares of Cl in this relief class (only 5,5 %) whereas Cl here takes 22,5 % of all mid-plateaus (the normalised value thus being elevated). The eastern parts of the Alpidic chains, mainly on the Balkan peninsula, are furthermore characterised by Pb – ophiolites (0,6 %), strongly over-represented – with respect to the global mean – in lowlands, hills and mid-mountains.

Pr (7,3 %) and Pa (6,2 %) occupy the mid-crustal basement of the Fennosarmatian shield in northern Europe. Mainly present in lowlands, but also in plains and hills, they are relatively under-represented (even more in mid-plateaus and mid-mountains) because the outcropping parts of the shield are comparatively small – similarly to the neighbouring Asia. Pa also occur in mid-mountains of the Scandinavian Caledonides.

252


5.5.3.5 North (and Middle) America

Represented in Figure 5-23 are the characteristic relief distributions for Ad, Vb and Mt as well as the normalised overall relief distribution in North and Middle America with respect to the global relief type values (lower right diagram).

Ad (6,8 % of total area for the continent) in North America are under-represented, with decreasing shares in plains, lowlands, hills and mid-plateaus. Endorheic hills however hold a small additional share of Ad. The astonishing feature is that– considering absolute values – the highest fraction (37,0 % of all Ad here) occurs in mid-mountains, with about one eighth of it being endorheic. They are over-represented here, but also in high plateaus (4,6 % of Ad in North America) and in the endorheic parts of high mountains (2,3 % of Ad in high mountains here). These three relief classes with the mentioned feature are situated mainly in the southern half of the North American cordilleras.

Ds and Ep (as in Europe) are not to be found (as mapped). Lo holds 3,1 % and is over-represented in plains (42,1 % of the total occurrences in North America), mid-plateaus (30,7 %) and in high mountains (2,5 %).

Figure 5-23 : Normalised relief distribution in North and Middle America for some rock types showing particular characteristics. For comparison also the normalised overall relief distribution in North (and Middle) America with respect to the global relief type values is given.

253

Normalised relief distribution for Alluvial deposits in North America

0,1

1

10



Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

. Ad

in r

el. t

ype

N. A

mer

ica

% o

utcr

op A

d in

rel

ief t

ype

glob

al

TotalEndorheic

Normalised relief distribution for Basic Volcanic rocks in North America

0,1

1

10



Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

. Vb

in r

el. t

ype

N. A

mer

ica

% o

utcr

op V

b in

rel

ief t

ype

glob

al

TotalEndorheic

Normalised relief distribution for Metamorphic rocks in North America

0,1

1

10



Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

. Mt i

n re

l. ty

pe N

. Am

eric

a%

out

crop

Mt i

n re

lief t

ype

glob

al

TotalEndorheic

Normalised relief distribution for North America (compared to the global relief distribution)

0,1

1

10



Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

f rel

ief t

ype

in N

orth

Am

eric

a %

of r

elie

f typ

e gl

obal

TotalEndorheic


Su (6,0 % of total area) are under-represented in plains, lowlands and in mid-mountains (where yet 18,0 % of Su remain). They are over-represented in hills, mid-plateaus (30,7 % of the total Su in North America here), high plateaus and high mountains.

Ss (14,2 %) are clearly over-represented with Mesozoic sandstones in the (mid-western) mid-plateaus. Sm (10,6 %) and Sc (15,3 %) have their absolute maxima in the lowlands and plains – taking together 60,2 % and 65,7 % respectively of the total area of the rock type in North America – of the southern mid-continent and the coastal areas there.

Vb (taking 8,4 % here, sensibly more than the global mean of 5,7 %) are over-represented in hills, mid-mountains, high mountains and high plateaus. The maximum absolute value is taken by mid-mountains with 64,0 % of the total area of Vb in North America. The occurrences belong to all parts of the subactive to active western ‘backbone’ Cordilleras. Va are absent.

Cl (6,6 %) have been mapped in North America as parts of the young Rocky Mountains and Cordilleras system, of the Paleozoic Appalachians, and also as parts of the Laurentian shield. About half (51,5 %) of the Complex lithologies in North America is to be found in the western young orogenic mid-mountains, the remaining parts in the eastern and northern lowlands, hills and plains.

Similarly Pa (6,2 %) occur with about one third (36,2 %) in the Cordilleran mid-mountains, the other shares mainly in the Laurentian shield. The same picture holds true for Pr (in total only 4,5 % of the continent). 39,2 % of the occurrences of Pr in North America have been included into the build-up of the Rocky Mountains, the remainder forming certain units within the Laurentian basement.

The major part of the Laurentian shield however has been mapped as Mt. In North America therefore Mt – with 16,8 % - occupies about four times the area of the global mean (4,0 %). Mt are strongly over-represented in plains, lowlands, hills and mid-plateaus, the main part to be found in lowlands (53,6 % of the total area of the continent occupied by this rock type) and plains (21,4 %) – in accordance with the shield’s morphology.

5.5.3.6 South America

Shown in Figure 5-24 are the characteristic relief distributions for Ss, Sc and Va as well as the normalised overall relief distribution in South America with respect to the global relief type values (lower right diagram).

254


In contrast to North America, Ad (taking 26,3 % of the continent) are clearly over-represented in South America, mainly in the huge fluvial plains of the Amazon, Orinoco and Paraguay-Paraná basins. The absolute values show the usual exponential decrease from plains to lowlands, hills and mid-plateaus. Intermediate storage areas are to be found in the mid- and high mountains of the Andes, a small part of them being endorheic. In central Brazil some areas of Ad are usually thick soils instead of alluvial sediments proper. Ds are inexistent in South America. Ep (only 0,1 %) occur exclusively in endorheic high-plateau and high-mountain basins of the Andes. Lo (taking a relatively high 5,2 %) occupies the plains of the lower Paraná and Uruguay rivers.

The distribution of Su, as in general, resembles that of Ad, with the difference of a double share preserved in mid-mountains.

Ss (15,2 %) are somewhat, Sm (3,2 %) and especially Sc (1,9 % of the total area in South America only) strongly under-represented. Ss occur mainly as upper Paleozoic and Mesozoic platform sediments upon the South American craton, in lowlands and mid-plateaus where they are enriched. Small shares – concerning absolute values – of Ss (but relatively being high – thus strongly over-represented) constitute the mid- and high mountains of the Andes.

Figure 5-24 : Normalised relief distribution in South America for some rock types showing particular characteristics. For comparison also the normalised overall relief distribution in South America with respect to the global relief type values is given.

255

0,1

1

10



Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

op S

s in

rel

. typ

e S.

Am

eric

a %

out

crop

Ss

in r

elie

f typ

e gl

obal

TotalEndorheic

Normalised relief distribution for Siliciclastic sedimentary consolidated rocks in South America

Normalised relief distribution for Carbonate rocks in South America

0,1

1

10



Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

. Sc

in r

el. t

ype

S. A

mer

ica

% o

utcr

. Sc

in r

elie

f typ

e gl

obal

TotalEndorheic

Normalised relief distribution for Acid Volcanic rocks in South America

0,1

1

10



Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.

% o

utcr

. Va

in r

el. t

ype

S. A

mer

ica

% o

utcr

op V

a in

rel

ief t

ype

glob

al

TotalEndorheic

Normalised relief distribution for South America (compared to the global relief distribution)

0,1

1

10



Ruggedlowl. &

hills

Low &mid-alt.mnts.

High &v.high

plateaus

High &v.highmnts.%

of r

elie

f typ

e in

Sou

th A

mer

ica

% o

f rel

ief t

ype

glob

al

TotalEndorheic


Sc are certain late Precambrian platform series in mid-plateaus upon the East-Brazilian shield. They are thus, despite of their generally low shares, over-represented in this relief category. Sm have been mapped nearly exclusively in the Andes and are therefore concentrated in mid- and high mountains.

Presumably a good third of the South American Vb are the Paraná flood basalts in southern Brazil and Uruguay, occupying lowlands and mid-plateaus. The major remaining share belongs to the – largely – ‘andesitic’ volcanoes of the Andean ocean-continent collision orogen. Consequently 34,2 % of Vb in South America are to be found in mid-mountains and 21,3 % in high mountains. Va (2,3 %) – mainly rhyolitic Ignimbrites – however are divided and thus show particular characteristics here. Approximately half occur in the Proterozoic of the South American craton (39,7 % of the total Va in South America in lowlands), the other half within the Mesozoic-Cenozoic Andes (26,0 % alone belonging to high mountains).

Cl, with 2,1 % rather depleted, occur as another important constituent of the Andes. The same is valid, in contrast to the other continents, for Pa – with 3,3 % under-represented as well. Granitic rocks in South America occur almost exclusively in the Andean chain – except for some minor shares in the Guyana shield – and therefore preferably in mid-mountains (51,6 % of the total Pa occurrences in South America), high mountains (16,2 %) and hills (18,7 %).

The outcropping basement of the South American craton is composed almost entirely of Pr (with 21,5 % of the total continental area over-represented with respect to the global mean of 11,6 %). 51,1 % occur in lowlands, the remaining shares are nearly uniformly distributed in plains, hills, mid-plateaus and mid-mountains, the mid-mountains belonging to the Guyana Shield and along the eastern margin of Brazil. Mt have insignificant shares (0,6 %), the participation of Va in parts of the shield has already been mentioned.

5.6 Discussion

5.6.1 Summary of the most important results This chapter has evaluated the combination of a relief typology with our new lithology

database. Concerning the 7 super-classes of relief as defined, some summarising remarks can be made :

Plains, in general – and as expected, are depleted in hard rocks (Pr, Pa, Vb, Va, Cl etc.), they are enriched in loose sediments (Ep to Ds). Lowlands are well different from plains, they are not significantly enriched or depleted except for endorheic parts being very depleted in Vb. Hills and mid-mountains are similar but show very individualised characteristics. They are enriched in crystalline rocks, and depleted in Su, Ad, Loess and Dunes. Mid plateaus are similar to lowlands

256

5.6 Discussion

in general, with high shares of Pr, Ss, Su and Ds. High plateaus are also quite individualised but are – due to the distribution in Asia – enriched in Sm, but also in Ep. They are depleted in Va, Pr, Mt and Ss. Practically no Ds occur in high plateaus. High and very high mountains are enriched in Volcanics and Cl, they are – as expected – depleted in sediments and sedimentary rocks (e.g. Ss, Su, Ad).

In a generalising view, lowlands and mid-plateaus are very similar in their lithologic distribution (except for Su being higher in mid-plateaus). Hills and mid-mountains also show comparable characteristics. If – for the framework of other studies – further clustering of relief classes is required, these classes could be aggregated to form 5 encompassing relief categories for lithology : Plains, lowlands and mid-plateaus, hills and mid-mountains, high plateaus and high mountains.

Concerning the individual continents, some interesting remarks can be summarised :

The whole continent of Africa is depleted in Pb, Mt and Cl, it is enriched in Pr and Ds. Africa’s plains are depleted in Mt, Sm and Ep, lowlands are depleted in Mt and Cl. Mid-plateaus are depleted in Cl whereas high plateaus and high mountains are very enriched in Vb due to the rifting processes in East Africa. Asia’s plains are depleted in Pa, Pr and Mt. The high plateaus are, in contrast to Africa, depleted in Vb and Su. The mid-mountains approximately correspond to the world’s average. Australasia shows no high plateaus, high mountains are only to be found in New Guinea, hills are depleted in Va, Pr and Mt. Europe in general is depleted in Vb, Va, Mt, Sm and Ds, it is enriched in Pb, Lo and Sc (but probably less than expected considering the many – but not very extended – karst regions of Europe). North America is relatively enriched in Mt due to the slightly different nature of the Laurentian shield in contrast to the world’s remaining cratons. Many endorheic parts consist of Volcanic rocks (e.g. Great Basin) ; the continent is depleted in Pb. In general, North America has nonetheless presence of all major rock types, and shows characteristics closest to mean values, compared to the other continents.

For the different rock types, some striking characteristics can be concluded as follows :

Concerning ‘hard’ rocks, Pb are depleted in lowlands and mid-plateaus, they are unlike Pa which are depleted in plains and high plateaus. Vb and Va are nearly absent (< 0,1 when normalised) in plains. Cl are extremely depleted in plains, very depleted in mid-plateaus, depleted in lowlands but very enriched in high mountains. Pb and Va are enriched in hills. Mt are somewhat depleted in mid-mountains, they are very depleted in high plateaus.

As for ‘soft’ rocks, recent Evaporites are clearly linked to relief roughness, they need flat terrains. Alluvial sediments, and to a lesser degree, Loess, produce flat terrains (plains) when accumulated in sufficient amounts over longer geologic times. As expected, low/mid- and high mountains show much less unconsolidated rock types. Interestingly, Carbonates (Sc) are not preferentially distributed and may be found in any relief class, due to many older occurrences. As

257


already shown with the latitudinal distribution of Sc, recent Carbonates (as e.g. in Florida’s plains) have mainly local importance and older occurrences have to be taken into account.

Wb and Ig have not been treated in detail due to their occurrences and somewhat arbitrary mapping being limited to major lakes and only some glaciers, Greenland and Antarctica being excluded. Nonetheless they show the characteristics expected, e.g. Wb occurring over flat terrains ; plains are enriched in Wb, North America is enriched both in Water bodies and glaciers, South America is rather depleted in Wb, but enriched in ice.

Similar to the distributions achieved in chapters 6 and 7 (but applied there in terms of fluxes distribution), a classification could theoretically be feasible for the normalised representations (relative scale % / %):

- > 10 extremely enriched - 4 – 10 very enriched - 2 – 4 highly enriched - 1,3 – 2 enriched - 0,8 – 1,3 barely significant - 0,5 – 0,8 depleted with respect to world average - 0,25 – 0,5 very depleted - 0,1 – 0,25 extremely depleted - < 0,1 absent

Extremely enriched would be Vb in high plateaus in Africa, depletion examples are Cl and Mt in Africa (plains, lowlands, mid-mountains). Dunes in general are to be found in the arid belt ; high plateaus and high mountains are most important in Asia etc..

For several reasons, such a distribution classification has not been adopted here. The relative scale is probably not appropriate as occurrences of rather insignificant rock types (e.g. Va) can appear greatly under- or over-represented whereas more important differences of abundant rock types (e.g. Ad) may not appear that important. Some features are also due to our mapping strategy, as for example Mt in North America. Individual judgement is thus needed to evaluate the importance of the different under- or over-representations.

The typology achieved by coupling of lithology and relief serves as a strong validation of our new lithology map. Preferential distributions have been clearly shown and other prominent features have been pointed out.

Certain lithologies are preferentially distributed in certain relief classes i.e. well outside a 0,8 – 1,3 range for % / % (normalised) comparisons : unconsolidated sediments, young volcanics (flood basalts, orogenic s.str. Vb and Va) etc.. This clearly shows the direct link (as shown by

258

5.6 Discussion

many authors, e.g. VEIZER & JANSEN 1979 or VEIZER 1988) between tectonic, lithology and relief.

5.6.2 Contrasts between ‘old’ and ‘young’ continents One of the most characteristic features of continents are the morphologic differences

between their old and flat, mostly cratonic parts and the mountain chains of more or less active orogens. In this sense, Africa – except for the Maghrebides fringe adjacent to the western Mediterranean Sea, and even more so Australia – without the New-Guinean and other ‘Australasian’ young mountain chains, can be characterised as ‘old’ continents, in contrast to Asia and Europe, both ‘young’ continents with approximately two thirds of their areas consisting of ‘young’ parts.

Figure 5-25 illustrates and compares the rock distribution of Africa and Australasia on the one hand, and the lithologies of Europe and Asia, on the other hand.

Comparison shows what has been presented and explained in chapters 3 and 4, concerning the geologic and lithologic constitution of continents.

Most parts of all cratons globally have Precambrian age and many parts of them are eroded down to mid-crustal levels ; therefore the basement of these cratons consists mainly of Precambrian shield rocks (Pr) and / or Metamorphic rocks (Mt), interwoven with granitoid intrusions (Pa). More or less marked relative predominance – over-representation in normalised graphs – of these rock types characterises ‘old’ continents, as far as the basement is visible at the surface.

The distribution of the sedimentary cover of the basement, on the contrary, is governed by many different regional factors (mainly by climate), changing in time. As a result, no global uniform ‘facies’ can be distinguished, neither for platform sedimentary rocks nor for alluvial and other youngest sediments on old continents. An exception can however be observed : preservation of Semi- to unconsolidated sediments (Su) seems slightly favoured when they occupy cratonic areas.

Thus, little remains to characterise the expected contrasts to young and more or less active orogens which constitute ‘young’ continents. Not astonishing, the class of ‘complex lithologies’ (Cl), created and adopted for this reason, really and reliably serve that purpose. In addition and rather unexpected the systematic occurrence (slight over-representation) of Acid volcanics (Va) as well as ophiolites (Pb) can be noticed in the young chains of Eurasia.

259


Figure 5-25 : Contrasting rock distribution in ‘old’ (Africa, Australasia) and ‘young’ (Europe, Asia) continents. Absolute and normalised values as used and explained before. Lithology codes : Pb – Plutonic basic, Pa – Plutonic acid, Vb – Volcanic basic, Va – Volcanic acid, Pr – Precambrian, Mt – Metamorphic, Cl – Complex lithology, Ss – Siliciclastic sedimentary consolidated, Sm – Mixed sedimentary consolidated, Sc – Carbonates consolidated, Ep – Evaporites, Su – Semi- to unconsolidated sedimentary, Ad – Alluvial deposits, Lo – Loess, Ds – Dunes and shifting sand.

Rock distribution in Africa (22,64 % = 30,07 M km2 of global area)

0

1

2

3

4

5

6

7


Are

a (M

km

2 )

TotalEndorheic

Normalised rock distribution in Africa (total)

0,1

1

10

Wb

Ig

Pb

Pa

Vb

Va

Pr

Mt

Cl

Ss

Sm

Sc

Ep

Su

Ad

Lo

Ds

% o

utcr

op in

Afr

ica

% o

utcr

op g

loba

l

TotalEndorheic

Rock distribution in Europe (7,39 % = 9,82 M km2 of global area)

0

0,5

1

1,5

2

2,5

3


Are

a (M

km

2 )

TotalEndorheic

Normalised rock distribution in Europe (total)

0,1

1

10

Wb

Ig

Pb

Pa

Vb

Va

Pr

M

t

Cl

Ss

Sm

Sc

Ep

Su

Ad

Lo

D

s

% o

utcr

op in

Eur

ope

% o

utcr

op g

loba

l

TotalEndorheic

Rock distribution in Australasia (6,78 % = 9,00 M km2 of global area)

00,20,40,60,8

11,21,41,61,8


Are

a (M

km

2 )

TotalEndorheic

Normalised rock distribution in Australasia (total)

0,1

1

10

Wb

Ig

Pb

Pa

Vb

Va

Pr

M

t

Cl

Ss

Sm

Sc

Ep

Su

Ad

Lo

D

s

% o

utcr

op in

Aus

tral

asia

%

out

crop

glo

bal

TotalEndorheic

Rock distribution in Asia (33,0 % = 43,85 M km2 of global area)

0123456789

10


Are

a (M

km

2 )

TotalEndorheic

Normalised rock distribution in Asia (total)

0,1

1

10

Wb

Ig

Pb

Pa

Vb

Va

Pr

M

t

Cl

Ss

Sm

Sc

Ep

Su

Ad

Lo

D

s

% o

utcr

op in

Asi

a %

out

crop

glo

bal

TotalEndorheic

260

5.6 Discussion

5.6.3 Contrasts between ‘cratons’ and ‘living orogens’ In order to verify the discussion on contrasting continents, we can compare directly the

rock distribution in cratonic relief types with the rock distribution in orogenic (sensu stricto) reliefs (Figure 5-26).

In section 5.2 has been substantiated the – grosso modo – belonging of all non-dissected low-altitude relief types (plains, lowlands, mid-plateaus) to the cratonic parts of continents, whereas the high-altitude mountains and also high plateaus clearly characterise young, active collisional orogenic chains. Hills together with mid-mountains can be divided between cratons and active orogens, hills with > 50 % belonging to cratons and mid-mountains with > 50 % to active orogens.

Figure 5-26 : Contrasting rock distribution in ‘cratons’ (= ‘cratonic’ relief types : Plains, Lowlands, Mid-Plateaus, 70 % Hills and 30 % Mid-Mountains combined) and active orogens sensu stricto (= ‘orogenic’ s.str. relief types : 30 % Hills, 70 % Mid-Mountains, High Plateaus, High Mountains combined). Lithology codes : Pb – Plutonic basic, Pa – Plutonic acid, Vb – Volcanic basic, Va – Volcanic acid, Pr – Precambrian, Mt – Metamorphic, Cl – Complex lithology, Ss – Siliciclastic sedimentary consolidated, Sm – Mixed sedimentary consolidated, Sc – Carbonates consolidated, Ep – Evaporites, Su – Semi- to unconsolidated sedimentary, Ad – Alluvial deposits, Lo – Loess, Ds – Dunes and shifting sand.

0,6 0,0 0,1

6,2

3,7

0,7

11,3

4,33,3

17,2

6,5

10,2

0,1

11,2

17,3

3,02,0

02468

101214161820


Surf

ace

area

(M k

m2 )

TotalEndorheic

Rock distribution in cratons (76,4% of global area) (Plains, Lowlands, Mid-Plateaus, 70% Hills, 30% Mid-Mountains)

Normalised rock distribution in cratons (Plains, Lowl., Mid-Plts., 70% Hills, 30% Mid-Mnts.)

0,1

1

10


% o

utcr

op in

cra

tons

%

out

crop

glo

bal

TotalEndorheic

0,1 0,0 0,1

3,13,6

0,6

3,7

0,8

3,53,8

3,43,0

0,0

1,5

2,4

0,40,0

00,5

11,5

22,5

33,5

44,5


Surf

ace

area

(M k

m2 )

TotalEndorheic

Rock distribution in active orogens (s.str.)(23,6% of global area) (30% Hills, 70% Mid-Mountains, High Plateaus, High Mountains)

0,1

1

10


% o

utcr

op in

act

ive

orog

ens

(s.s

tr.)

% o

utcr

op g

loba

l

TotalEndorheic

Normalised rock distribution in active orogens (s.str.) (30% Hills, 70% Mid-Mountains, High Plateaus, High Mountains)

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We have thus attributed, as proposed, a 70 % share of hills and 30 % share of mid-mountains to ‘cratons’, whereas the orogens sensu stricto have been attributed, inversely, 30 % of the hills and 70 % of the mid-mountains. Thus delimited, ‘cratons’ take 76,4 % of the continents total area, and ‘young orogens s.str.’ take 23,6 % (cf. Table 5-1).

The diagrams (Figure 5-26) show approximately, but clearer, what has already been observed before (Figure 5-25) :

Cratons (cratonic relief types) are characterised by absolute high proportions of Pr + Mt (but normalised, Pr astonishingly not being over-represented, though Mt – on the contrary – is enriched) as basement rocks. Ss, Su, Ad, Lo and Ds are the characteristic platform sedimentary cover, all being slightly (factors 1 – 1,5) over-represented in normalised view.

Young active to subactive orogens s.str. (areas with their relief types in our map) in contrast, and as expected, have absolute high proportions of Cl, being maximally over-represented. Vb – the orogenic types in contrast to flood basalts forming plateaus – hold similar high shares, as well as Va and Pb (here in particular ophiolites are represented). All four rock classes (Cl, Vb, Va, Pb) are over-represented with factors 2,5 – 3,5 ; therefore they characterise remarkably well young orogens, or – more exactly – the upper storeys of orogens (cf. Annexe I.1.2.3 with Figure I–8).

Rather unexpected is the observation that neither Pr nor Pa particularly characterise cratonic landscapes as they are constituted to about two thirds by platform sediments. The strong participation of Pr in young orogens seems to be responsible for this observation, due to the fact that the majority of young mountain chains consists of continent-continent collision orogens. These always involve the old basement of the continents concerned. Somewhat more difficult to explain is the over-representation (factor 1,62) of Pa in young orogens. It may be argued that granites often ascend into upper crustal levels, but also, that the ‘orogenic s.str.’ relief types on our map comprise, especially in Asia and Europe, not only truly young, but also older Phanerozoic orogens – more or less activated – and that in these preferably the middle storeys are exposed and internal parts (‘Internides’) preserved.

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5.7 Conclusions

5.7 Conclusions Some general conclusions, concerning the relations between lithologies and relief types, can

be made. The relations are ‘naturally’ rather complex. However, some principles and processes are of fundamental character and play a steering role.

The most important characteristics have already been mentioned (cf. section 5.3.1) : if only exogenic processes (weathering – erosion – sedimentation) were to be at work on the Earth’s crust, no remarkable relief would exist and finally no continental landmass, but a global sea reign. Thus relief, and the distinctive existence of oceans and continents, is due to endogenic forces and magmatic – tectonic – metamorphic processes.

I ‘Primary’ positive relief can be created by magmatic processes. Upward and downward movements within the asthenosphere may warp the crust, e.g. the rise of mantle plumes create large circular or elongated swells. Smaller ‘bumps’ (hills, small mountains) can be caused by localised (e.g. lakkolithic) intrusions within the crust, but are rare. The shapes of the ‘ubiquitous’ volcanic relief depend directly on the viscosity – itself a function of the SiO2-content, the basic or acidic character – of the extruding or violently erupting and outflowing magmas, which cool and solidify rapidly – immediately in geological terms. Resulting are either plateaus, or ‘individual’ hills to – sometimes even very high – mountains, often grouped in volcanic chains.

II Large-scale tectonic processes – mostly combined with magmatic processes – involve the whole lithosphere (divided into plates) and are correlated with movements within the rocks in fractal order down to microscopic scales. These tectonic processes are dominantly either divergent or convergent. Correspondingly they create primary relief, positive and negative, either by lifting, faulting and graben- (rift-) formation (hollows / ‘negative’ mountains), or by folding and thrusting – to build chains of hills or (mostly) mid-altitude to high & very high mountains and plateaus.

III ‘Secondary’, negative, rather small-scale relief is the result of erosive dissection. This process depends on a) the primary relief, b) the type and intensity of erosion (water and high temperatures being the most effective agents) and c) the capability of resistance of the rocks concerned.

IV The relief eventually disappears as a result of weathering, erosion and sedimentation filling all cavities. a) A particular case of sedimentation is the formation of eolian sand sediments (mostly dunes) and of eolian dust sediments, especially Pleistocene loess. Both can be blown upwards and cover / mask existing relief without making it disappear entirely. b) Water follows the most direct path of gravity down to the deepest hollows

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available and fills them. Associated on the same path follow all aquatic sediments, finally creating horizontal flats and plains. All alluvial sediments, and in geological perspective also all lake sediments, are only temporarily stored on their way to global seas, and are thus mobile, travelling material. Consequently it can be assumed that most horizontal areas on Earth, continental ‘plains’, in the usual sense of the word, in whatever absolute elevation above sea level, are covered by mobile sediments, are in fact the result or product of ‘normal’ and recent aquatic continental sedimentation.

V Older, lithified sediments and volcanic rocks experience and document ‘geologic history’, the longer and more complicated the older they are. The same holds true for plutonic and all kinds of tectonised and metamorphic rocks from within the Earth that needed uplift and exhumation to appear at the surface. Ultimately the relation of all these rocks to relief classes is governed by the plate-tectonic fate of the various crustal segments – with all corollaries, e.g. regarding the ensuing and changing climatic conditions at the surface etc..

In summary it can be stated :

The overall relief morphology of the globe, as depicted in the relief type map by MEYBECK et al. (2001), directly reflects the endogenic activity of the Earth, with mainly tectonic, but also volcanic processes.

Erosion by (mostly) running water, depending on local climatic conditions, causes ‘dissection’ in various, normally smaller (meter to kilometre) scales.

Recent sedimentation adapts itself to the present relief and creates plains in geologically rather short (< 1 Ma) times.

Distribution of older sedimentary and crystalline (plutonic and metamorphic) rocks is the result of their geologic history, from which only the youngest events bear on the actual morphologic situations.

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5 relief typology and lithologyhydrologie.org/the/durr/5.pdf · as mmts) : 500 – 2000 m of mean...

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