notes 1 basic geology

PgDip/MSc Energy Programme/Subsurface Basic Geology

The Robert Gordon University 2006 1 campus.rgu.ac.uk

Basic Geology

Review

In this topic the student is introduced to the fundamentals ofthe Earths structure, plate tectonics and rock types.

Content

Earth Structure

Figure 1. The Earths Structure. (From THE DYNAMIC EARTH by B.J. Skinnerand S.C. Porter, copyright 2000 John Wiley and Sons. This material is used bypermission of John Wiley and Sons, Inc.)

Figure 1 illustrates the structure of the Earth. There is a central solid ironcore, surrounded by a liquid iron core, the lower mantle and the uppermantle. The upper mantle consists of a weak, partially moltenasthenosphere and finally there is a strong lithosphere with a surficial



crust of light rock. About 90% of the earths crust is made up of the fourelements: iron, oxygen, silicon and magnesium, which are thefundamental building blocks of most minerals. Iron, being heavy, sinks tothe core, and lighter elements such as silicon, aluminium, calcium,potassium and sodium have risen to the crust.

Plate TectonicsPlate Tectonics was first proposed in the 1960s. The central idea is thedivision of the lithosphere into 12 rigid plates (6 major ones), which eachmove as distinct units (Figure 2). The plates consist of rigid lithosphere(with either thin, dense oceanic crust or thick, less dense continentalcrust), which floats on the partially molten asthenosphere (Figure 3).Convection currents within the asthenosphere are thought to be thedriving force behind the plate movement. Where hot matter rises underthe ocean it flows apart and carries the plates along with it (Figure 4).When this hot matter cools and sinks the plates also begin to sink. Theplates are constantly moving, which explains why the Atlantic Ocean didnot exist 150 Ma (million years ago). At this time it has been establishedthat Eurasia, Africa and the Americas were all one continent calledPangea. It is possible to trace the effects of tectonics back approximately4.6 billion years, although the rock record and hence history becomeshazy after about 1 billion years.

The margins between the 12 plates are Divergent (spreading apart),Convergent (colliding together) or Transform (sliding past each other).Plates are constantly produced and consumed. Volcanic and seismicactivity along plate margins varies depending on type. Trailing edgestend not to be particularly active (most of Europe) whereas leading edgestend to be very active.

Figure 2. Tectonic Plates Today (Peter J Sloss, NOAA-NESDIS-NGDC).



Figure 3. Close-Up of Crust and Asthenosphere. (From UNDERSTANDINGEARTH by Frank Press and Raymond Siever, 1998, 1994 W.H. Freeman and Company.Used with permission.)

Figure 4. Convection Currents and Plate Movement Theories. (FromUNDERSTANDING EARTH by Frank Press and Raymond Siever, 1998, 1994 W.H.Freeman and Company. Used with permission.)



Divergent MarginsFigure 5A illustrates a divergent plate boundary. Related features includelinear Mid Ocean Ridges (the Mid Atlantic Ridge) where the lithospherebreaks and a rift develops. As the lithosphere breaks hot lava rises fromthe asthenosphere. The rift continues to open thus separating the twoplates. This occurred between America and Africa and lead to theformation of the Atlantic Ocean basin. The Mid Ocean Ridge (MOR) ischaracterised by earthquakes and volcanism. Different lavas havedifferent viscosities.This leads to a variation of divergent speeds, and inturn to offsets in the plate margin. The Mid Atlantic ridge shows anaverage speed of 2.5 cm/year whereas 18 cm/year can be found in theSouth Pacific.

Convergent MarginsWhen two plates are being pushed together the denser one will ridebelow the lighter one, creating a subduction zone. Less buoyant oceaniccrust usually sinks below the thicker, lighter continental crust. Featuresassociated with this subduction include mountain building, trenchformation, earthquakes and volcanism. The contact of the Nazca plateand the South American plate led to the formation of the Andes mountainrange and the Chilean deep-sea trench (Figure 5B). The Nazca plate(plate 1) buckles downwards and the overriding South American plate(plate 2) is crumpled and uplifted. As the subducted plate sinks it willmelt, generating a source of hot molten rock that rises into the overlyingcrust, inducing volcanism.

Where two plates converge at thick continental crust edges, subduction islow and an ever growing mountain range is formed, termed a collisionboundary (Figure 5C). The Himalayas are formed due to collision of theAsian and Indian plates for example.

Transform FaultsTransform faults occur where two plates slide past each other (Figure5D). The movement is generally not regular and uniform but occursabruptly as a series of sudden slip faults. The San Andreas Fault inAmerica where the Pacific plate slides past the North American plate is anexample. The sudden slip movements produce a series of damagingearthquakes along the fault.

In summary, divergent zones are sources of new lithosphere andsubduction zones are sinks. Material is created and consumed in equalamounts. If this were not true, the Earth would change in size.



Figure 5. Types of Plate Margin. (From THE DYNAMIC EARTH by B.J. Skinnerand S.C. Porter, copyright 2000 John Wiley and Sons. This material is used bypermission of John Wiley and Sons, Inc.)

MagnetismMotions in the fluid iron core of the Earth set up a dynamo action thusgenerating the Earths magnetic field (Figure 6). Rocks are magnetised inthe direction of the magnetic field at the time of their formation. Therocks can be dated radiometrically and thus the history of the magneticfield recorded. Such studies have shown that the field reverses direction(the reason for which is unexplained) with such reversals evident on theseafloor. Figure 7 illustrates the symmetrical pattern of magnetised rockseither side of a MOR.

Figure 6. Magnetic Field Lines. (From THE DYNAMIC EARTH by B.J. Skinnerand S.C. Porter, copyright 2000 John Wiley and Sons. This material is used bypermission of John Wiley and Sons, Inc.)



Figure 7. Magnetised Rocks Either Side of a MOR. (FromUNDERSTANDING EARTH by Frank Press and Raymond Siever, 1998, 1994 W.H.Freeman and Company. Used with permission.)

Minerals and CrystalsA mineral is defined as any naturally formed, solid, chemical substancehaving a specific compostion and characteristic crystal strucure. Diamondis a mineral as it has a defined composition (pure carbon) and crystalstructure (the atoms are packed in a three dimensional array). Graphiteis also a mineral of pure carbon, but with a sheet like crystallographicstrucure. Coal is not a mineral as it is composed of many differentcompounds (although mainly carbon), the proportion of which variesfrom one place to another, and has no defined structure. Coal is a rock,which is an aggregate of minerals. Most minerals are made up of severalelements.

Table 1 shows the percentage of different elements in the Earthscontinental crust. These elements combine to form molecules, which inturn combine to form minerals. Silicates form the majority of the Earthsminerals. Figure 8 shows the evolution of rock. Crystals take on sevenbasic shapes or structures (Figure 9). Some elements and compounds arepolymorphic, ie, they can take on more than one crystal strucure (carbonforms both diamond and graphite). Examination of the crystallographicstrucure of a particular rock mineral can tell us a lot about its history andformation. If a crystal is allowed to grow unhindered space wise, it willtake on a perfect shape (Figure 10). Salt for example forms cubiccrystals. Commonly however in rock formation, crystal growth is haltedby growth of neighbouring crystals, or the crystals are abraded andfractured. Although there are many hundreds of minerals, there are20-30 major rock forming minerals.



Table 1. Most Abundant Elements in the Earths Crust.

Element % by weight Element % by weight

O 45.2 Na 2.32

Si 27.2 K 1.68

Al 8 Ti 0.86

Fe 5.8 H 0.14

Ca 5.06 Mn 0.1

Mg 2.77 P 0.1

All Other 0.77

Figure 8. Evolution of Rock.



Figure 9. Basic Crystal Shapes.

Figure 10. Example of Quartz Crystal in Rock Matrix Pore Space(approximately 10m across).

Mineral PropertiesEach mineral has properties dependant on composition and structure.Once we know which properties are characteristic of which minerals itmay not be necessary to carry out a chemical analysis. Various tests canbe used to identify the type of structure, and to indicate the mineralpresent. Properties such as crystal shape, colour & streak, luster,hardness (Mohs scale), cleavage, specific gravity and opticalcharacteristics can be used for identification (Figure 11).



Figure 11. Examples of Common Mineral Properties. (Photos top tobottom: Breck P Kent, Ed Deggenger & Bruce Coleman, Chip Clark, Chip Clark)

Some Chemical Classes of MineralsClass Defining Atoms ExampleNative elements None: no charged atoms Copper (Cu)Oxides &hydroxides

Oxygen ion (O2-)Hydroxyl ion (OH-)

Hematite (Fe2O3)

Halides Chloride (Cl-), fluoride (F-),bromide (Br-), iodide (I-)

Brucite (Mg[OH]2)Halite (NaCl)

Carbonates Carbonate ion (CO32-) Calcite (CaCO3)Sulphates Sulphate ion (SO42-) Anhydrite

(CaSO4)Silicates Silicate ion (SiO44-) Olivine (Mg2SiO4)

Physical Properties of Minerals

Property Relation to Composition & Crystal Structure

Hardness Strong chemical bonds give high hardness. Covalentlybonded minerals are generally harder than ionicallybonded minerals.

Cleavage Cleavage is poor if bond strength in crystal is high andis good if bond strength is low. Covalent bonds generallygive poor or no cleavage; ionic bonds are weak and sogive excellent cleavage.

Fracture Type is related to distribution of bond strengths acrossirregular surfaces other than cleavage planes.

Lustre Tends to be glassy for ionic bonds, more variablecovalent bonds.

Colour Determined by kinds of atoms and trace impurities.Many ionic crystals are colourless. Iron tends to colourstrongly.

Streak Colour of fine powder is more characteristic than that ofmassive mineral because of uniformly small grain size.

Density Depends on atomic weight of atoms and their closenessof packing in crystal. Iron minerals and metals havehigh density. Covalent minerals have more openpacking, hence lower densities.

Mineral Lustre

Metallic Strong reflections produced byopaque substances

Vitreous Bright, as in glassResinous Characteristic of resins, such

as amberGreasy The appearance of being

coated with an oily substancePearly The whitish iridescence of

materials such as pearlSilky The sheen of fibrous materials

such as silkAdamantine The brilliant lustre of diamond

and similar minerals

Moh's Scale of Hardness

Mineral ScaleNumber

CommonObject

Talc 1Gypsum 2 FingernailCalcite 3 Copper coinFluorite 4Apatite 5 Knife bladeOrthoclase 6 Window glassQuartz 7 Steel fileTopaz 8Corundum 9Diamond 10



Table 2 shows commonly occurring minerals in different rock types.

Table 2. Common Minerals in Rock.

Igneous Sedimentary Metamorphic

Quartz * Quartz * Quartz *

Feldspar * Clay minerals * Feldspar *

Mica * Feldspar * Mica *

Pyroxene * Calcite Garnet *

Amphibole * Dolomite Pyroxene *

Olivine * Gypsum Staurolite *

Halite Kyanite*

* Indicates mineral is a silicate.

Basic Rock Types (Rock Clans)The rock cycle (Figure 12) illustrates the relationship between the threemain rock types or clans: Igneous, Metamorphic and Sedimentary.

Figure 12. The Rock Cycle. (From UNDERSTANDING EARTH by Frank Press andRaymond Siever, 1998, 1994 W.H. Freeman and Company. Used with permission.)



IgneousThe cooling and solidification of hot molten magma from the mantleforms igneous rock. Igneous rock can be classified as intrusive (intrinsic,plutonic) or extrusive (extrinsic, volcanic). Intrusive igneous rocks formas magma pushes its way up through cracks and fissures intosurrounding rocks. Intrusives cool relatively slowly and crystals thereforehave time to develop. They are characterised by large crystal growth.Extrusive igneous rocks form when magma reaches the Earths surface,for example as lava flows from volcanic eruptions. These rocks are cooledrapidly and are characterised by fine crystals that have not had time todevelop (Figure 13). If the lava is cooled extremely rapidly, the atomshave no time to rearrange into crystalline structures, and glass typestructures or minaraloids are formed, obsidian for example.

Figure 13. Intrusive and Extrusive Igneous Rock Sources andTerms. (From THE DYNAMIC EARTH by B.J. Skinner and S.C. Porter, copyright 2000John Wiley and Sons. This material is used by permission of John Wiley and Sons, Inc.)



Igneous rocks are the most abundant type of rock found in the Earthtoday, about 70%. Minerals such as quartz, feldspar, mica and olivine areimportant building blocks of igneous rocks (Figure 14). Characteristically,the mineral crystals in igneous rocks have been restricted in growth bysurrounding crystals, so their edges are amorphous in appearance(Figure 15). Igneous rocks of the same composition can be classified asdifferent rocks depending on cooling rate and resultant texture. Forexample, granite (intrusive) is coarse grained, but when the samecompositional lava is cooled rapidly it forms fine grained rhyolite(extrusive).

Lavas vary from extremely fluid basalts to viscous and explosivelyeruptive rhyolites, depending on composition. Basalts are the mostcommon fortunately as all major volcanic disasters around the Worldhave been related to rhyolitic eruptions.

Figure 14. Minerals in Common Igneous Rocks. (From UNDERSTANDINGEARTH by Frank Press and Raymond Siever, 1998, 1994 W.H. Freeman and Company.Used with permission.)



Figure 15. Thin Section Depicting Igneous Rock and CrystalStructure

SedimentarySedimentary rocks form when igneous, metamorphic or pre-existingsedimentary rocks are subjected to erosive forces (glaciation, wind, rain,and snow) (Figure 16). The rocks are broken down, and the individualgrains and rock particles (detrital or clastic sediment) are transportedaway from the source area and redeposited in low-lying areas. It is withinsuch low lying basin areas that the majority of petroleum is found.Stratification of sedimentary rocks results from the arrangement ofsedimentary particles in distinct layers known as beds.

The conversion of unconsolidated sediment to rock is termed lithification.Diagenesis is a term used to describe all the chemical, biological andphysical processes involved in a rocks formation during and afterlithification.

Clastic particles can be defined by size (Table 3) which in turn formdifferent types of rock (Figure 17). Crystals within sedimentary rocks thathave been formed by mechanical erosion of source rocks tend to berounded in appearance due to abrasion.



Figure 16. Erosion and Sources of Sedimentation. (FromUNDERSTANDING EARTH by Frank Press and Raymond Siever, 1998, 1994 W.H.Freeman and Company. Used with permission.)

Table 3. Clastic Particle Definitions.

Name ofParticle

Range Limitsof Diameter

(mm)

Name of LooseSediment

Name of ConsolidatedRock

Boulder > 256 Boulder gravel Boulder conglomerate (b)

Cobble 64 - 256 Cobble gravel Cobble conglomerate (b)

Pebble 2 - 64 Pebble gravel Pebble conglomerate (b)

Sand 1/16 2 Sand Sandstone

Silt 1/256 1/16 Silt Siltstone

Clay (a) < 1/256 Clay Mudstone & Shale

Notes: (a) Refers to particle size only and not to clay minerals. (b) Ifclasts are angular, rock is termed Breccia rather than conglomerate.

Sediments may also be chemical in origin. Chemical sediments are theresult of dissolution of the source material, rather than erosion, andsubsequent precipitation at another location. Biogenic chemical(bioclastic) sediments are formed from the accumulation and fossilisationof the remains of plants and animals. Calcium carbonate based rocks forexample may be formed from remains of marine shells. Organicsubstances within biogenic sediments may also be transformed into fossilfuels if composition and conditions are correct.

It is also possible to have a rock that is half way between igneous andsedimentary. This occurs as lava is thrown rather than flows from avolcano, and covers the surrounding area. Grains are usually angular dueto rapid solidification and termed Breccia.



Figure 17. Rocks from Sedimentary Particle Types. (From THE DYNAMICEARTH by B.J. Skinner and S.C. Porter, copyright 2000 John Wiley and Sons. Thismaterial is used by permission of John Wiley and Sons, Inc.)

Sedimentary rocks are the primary rocks involved in oil and gasformation and will be covered in greater detail in Topic 2.

MetamorphicMetamorphic rocks form when igneous, sedimentary or pre-existingmetamorphic rocks are altered by heat and pressure due to their deepburial in the Earth or due to a hot molten rock intrusion. For example, inthe subduction zone the pressure, temperature and deformation whichrocks are subjected to will lead to the formation of new mineral grains,textural changes and thus new metamorphic rocks.

Metamorhpic rocks can be characterised by both grade and type ofmetamorphism. Figure 18 illustrates the grades of metamorphismdepending on pressure and temperature. The end result is controlled byfactors such as chemical reactivity of inter-granular fluids, pressure,temperature, differential stress across the zone of metamorphism and ofcourse the time span involved.



Figure 18. Metamorphic Grades. (From THE DYNAMIC EARTH by B.J. Skinnerand S.C. Porter, copyright 2000 John Wiley and Sons. This material is used bypermission of John Wiley and Sons, Inc.)

The types of metamorphism are defined relative to the physicalconditions that are present during metamorphism.

Regional is most common in the continental crust and may occur overtens of thousands of square kilometres. Regional metamorphism involveshigh differential stress levels and a considerable amount of mechanicaldeformation, along with chemical recrystallisation. Low grade, regionalmetamorphism of shale or mudstone forms slate. The slaty cleavageplanes are formed perpendicular to the direction of maximum stressduring metamorphism. Regional metamorphism is a consequence of platetectonics.

Contact metamorphism occurs more locally adjacent to bodies orintrusions of magma, due mainly to chemical recrysatallisation. The zoneaffected is termed an aureole. Mechanical deformation tends to be minordue to generally homogenous stresses around the magma intrusion.

Cataclastic, or dynamic, metamorphism may be found along faults wheretectonic movement leads to high differential stresses, and rockdeformation. The rocks may be fractured and ground almost to a pasteresulting in a pulverised texture. Cataclastic rocks are often foundalongside regionally metmorphosed rocks in narrow zones along faultperimeters. These rocks often act as a major fluid barrier between rocks.

Burial metamorphism genarally occurs in deeply buried sedimentarybasin rocks where temperatures may be as high as 300 Celsius. Thepresence of water within the sedimentary rock speeds up chemicalrecrystallisation processes. As with contact metamorphism, there is littlemechanical deformation. The resultant rock may appear physically verysimilar to the original sedimentary rock, but will differ in its mineralcontent.

Hydrothermal metamorphism occurs due to chemical reactions betweenfluids and heated rocks, and is often associated with mid ocean ridges.



Figure 19 shows examples of the rock types formed duringmetamorphism dependant on pressure and temperature zones, termedfacies.

Figure 19. Metamorphic Facies with Common Tectonic SettingsSuperimposed. (From UNDERSTANDING EARTH by Frank Press and RaymondSiever, 1998, 1994 W.H. Freeman and Company. Used with permission.)

Figure 20 illustrates the minerals present during metamorphosis ofshales. Quartz seen all way through, but changes in character. Muscoviteis an index for low and intermediate grade metamorphism, Biotite forintermediate and Garnet for high grade metamorphosis.

Figure 21 illustrates areas of metamorphism related to plate tectonics.



Figure 20. Metamorphism of Shales. (From THE DYNAMIC EARTH by B.J.Skinner and S.C. Porter, copyright 2000 John Wiley and Sons. This material is used bypermission of John Wiley and Sons, Inc.)

Figure 21. Plate Tectonics and Metamorphosis Examples. (FromUNDERSTANDING EARTH by Frank Press and Raymond Siever, 1998, 1994 W.H.Freeman and Company. Used with permission.)



SummaryFigure 22 summarises rock types and Earth processes involved in theirdevelopment.

Figure 22. Interaction of the Water, Rock and Tectonic Cycles.(From THE DYNAMIC EARTH by B.J. Skinner and S.C. Porter, copyright 2000 JohnWiley and Sons. This material is used by permission of John Wiley and Sons, Inc.)

notes 1 basic geology

Documents

rigid plates

robert gordon university

divergent plate

plate margins

tectonic plates

earths crust

plate tectonicsplate

molten asthenosphere