MAJOR LANDFORMS IN VOLCANIC REGIONS

Volcanism is not randomly distributed over the world. It is concen-trated near plate boundaries where plate subduction or seafloorspreading takes place. Other occurrences are linked to deep mantleplumes that reach the Earth’s surface at distinct ‘hotspots’.Figure 1 shows the geographic distribution of major volcanicregions.

Landforms in volcanic regions are strongly influenced by thechemical and mineralogical composition of the materials that weredeposited during eruptive phases. Volcanic rocks and magmas aregrouped according to their silica contents in three main categories,viz. ‘Rhyolite’ (65-75% SiO2), ‘Andesite’ (65-55% SiO2) and‘Basalt’ (55-45% SiO2). Individual rock types are distinguished onbasis of mineralogical properties or contents of certain constituents(K2O, Na2O and CaO). See figure 2.

The broad division of volcanic rocks and magmas according to SiO2 content makes sense because thesilica content correlates with the viscosity of magmas and hence with the type of volcanism. A rule ofthumb: the higher the silica content of magma is, the more acid and viscous it is and the more explosiveare volcanic eruptions. Evidently this influences the character and morphology of volcanic phenomena.

In the following, major landforms of volcanic regions will be discussed taking magma composition asa reference point. We shall distinguish between regions with basaltic, andesitic and rhyoliticvolcanism.

Figure 1. Major volcanic regionsof the world (after Robinson, 1975).

(Click to enlarge)

Figure 2. Chemical land mineralogical properties of major volcanic rock types.

Major landforms in regions with basaltic volcanism

Basaltic volcanism occurs where basic mantle material reaches the surface, notably1 at divergent plate margins (sea floor spreading),2 in 'hot-spot' areas, and3 in continental rift valleys.

1: The best-known divergent plate margin is the mid-oceanic ridge or rise. The highest parts of the ridgemay reach the surface of the ocean and form islands, e.g. Iceland and the Canary Islands. It is notsurprising that, like all ocean floors, Iceland consists mainly of basaltic rock.

2: A fine example of basaltic hot-spot volcanism isHawaii, which constitutes the top of the largest'shield volcano' in the world, with a diameter of 250km at the base (on the ocean floor) and a total heightof 9 km. Basaltic magma is little viscous and gasesescape easily. Eruptions are therefore relativelyquiet and produce low-viscosity lava flows, lavalakes and lava fountains, but little ash. The fluidmagma can flow over large distances and the result-ing shield volcanoes are comparatively flat. Mosteruptions are ‘fissure eruptions’ that take placealong extensional cracks in the Earth’s crust. Thefissures may be several kilometers in length; thehistorical ‘Laki’ eruption on Iceland happened along a 24-km long fissure. Much bigger fissureeruptions have taken place in the past. They produced enormous masses of ‘flood basalt’ that coveredhundreds of square kilometers. The Paraña plateau in South America is made up of 1 million km3 basalt,which was extruded within 10 million years. Other examples of large occurrences of flood basalt are inEthiopia, Siberia, Greenland, Antarctica, India (the ‘Deccan Traps’) and in the western USA (ColombiaRiver). See figure 4.

Figure 3. Fissure eruption.

3: Hotspots situated below a continental crust are likely to have mantle plumes that push the crust up(‘updoming’) and cause large-scale dilation cracks in the Earth’s crust. The cracks become manifest aselongated tectonic depressions: the rift valleys. Both basic (SiO2 poor) and acid (SiO2 rich) volcanismoccur in and along rift valleys. Basaltic volcanism in continental rift valleys (e.g. the East-African riftvalley, the Baikal graben, or the Rhine - Rhone graben) is associated with 'strombolian' scoria conesand with ‘maar’ craters (i.e. steam-explosion craters now filled with water). Here too, ash depositsseldom extend beyond the volcanic areas themselves. Where ash blankets are extensive, as in some riftvalleys, they are usually more acidic.The comparatively fluid basaltic lava flows tend to follow river valleys and can flow over considerabledistances into the rift valley. Subsequent erosion of soft sediments adjacent to the lava bodies results in'relief inversion', with the former basaltic valley fills extending as elongated plateaus in the erodedlandscape.

Figure 4. Worldwide occurrence of flood basalts.

Landforms in regions with andesitic volcanism

Andesitic volcanism is a characteristic element at convergent plate boundaries where plate subductiontakes place. Typical settings are:• 'Cordillera'-type mountain belts (like the Andes), and• island arcs (e.g. the Philippines and Japan).

The classic volcano type associated with andesitic volcanism is the 'stratovolcano'. Literally, the termmeans 'stratified' volcano, which is misleading in the sense that all volcanoes are built up of layers, beit of basalt flows, as in the Hawaiian shield volcanoes, or of pyroclastics, as the scoria cones of the Eifel.What the term indicates, actually, is that this type of volcano is composed of alternating layers of lavaand pyroclastic rock, mostly of andesitic composition. Stratovolcanoes are much larger than scoriacones and usually have a long (Ma) history of alternating lava and pyroclastic rock eruptions.

Andesitic magmas hold an intermediate position between basaltic and rhyolitic magmas with respect totheir SiO2 content, viscosity and gas content. Whereas basaltic, low viscosity magmas hardly producepyroclastics ('tephra'), and high-viscosity rhyolites hardly produce lavas, andesitic magmas willnormally produce both. Because of the greater viscosity of the magma, a higher pressure must build upbefore an eruption can occur; eruptions are less frequent and more violent than in basaltic volcanism.

Lava flows emitted by stratovolcanoes are more viscous than those of basaltic shield volcanoes, and donot extend as far from the point of emission, usually only a few kilometers. This explains why strato-volcanoes have steeper slopes than shield volcanoes and the 'classical' cone shape.

Active, large and high stratovolcanoes are likely to produce devastating volcanic mudflows (also called‘lahars’). Lahars can form in several ways:• because the wall of a crater lake collapses during an eruption, or• because condensation nuclei in the air (volcanic ash) generate heavy rains (e.g. Pinatubo, Philip-

pines 1992), or• because the volcano was covered with snow or glaciers before the eruption (e.g. Nevado del Ruiz,

Colombia, 1985).• because heavy rainfall following an eruption washes fresh ash deposits away.

Pyroclastic flows are frothy masses of ash and pumice. They evolve when an extrusive dome collapsesand generates a fast moving glowing avalanche of gas, ash and pumice. The resulting rocks are knownas 'ignimbrites' and can have a variety of structures depending on the flow conditions during emplace-ment and on the degree of post-depositional welding.

Figure 5. Stratovolcanoes in Indonesia.

'Volcanic ash’ fall-out often spreads far beyond the direct vicinity of the erupting volcano. Whereaslava and pyroclastic flows are confined to the immediate vicinity of volcanoes, ashes might be blowninto the troposphere and stratosphere, and can travel hundreds of kilometers. The thickness of the ashdeposits decreases with increasing distance from the point of origin. It may be difficult to recognize thepresence of volcanic ash in soils because it is incorporated in the solum, overgrown by vegetation andit weathers rapidly. Nonetheless, 'rejuvenation' of soil material with fresh volcanic ash is often of greatimportance as it restores or improves soil fertility and promotes physical soil stability.

Figure 6. Distribution of the volcanic deposits after theexplosion of the Mt. Pinatubo in 1991.

Landforms in regions with rhyolitic volcanism

Partial melting of the continental crust in cordilleran mountain ranges and rift valleys may produce acidrhyolitic magma. Rhyolitic magmas are very viscous and withstand very high gas pressures. As a result,rhyolitic eruptions are rare, but also extremely violent. If a rhyolitic magma chamber is present belowa stratovolcano, tremendous gas pressures build up so that, once a vent for eruption is opened, the mag-ma chamber empties itself completely, leaving a cavity in the Earth's crust in which the entire strato-volcano collapses. Craters of several kilometres in diameter are formed in this way: the 'calderas' (e.g.Krakatoa in Indonesia; Ngorongoro in Tanzania; Crater lake in the USA and the Laachersee in Germa-ny). Only occasionally do more quiet eruptions take place. The high viscosity of the lava precludes lavaflow; a lava dome is formed instead (e.g. Obsidian Dome, USA).

The main extrusive products are:• ashes, in astonishing quantities and spread over vast areas, and• ignimbrites that stem from pyroclastic flows extending over several tens of kilometers and filling

in depressions and valleys of tens or even hundreds of meters depth. In contrast with the irregular surfaces of lava flows and lahars, ignimorite surfaces are completely flat and featureless. White, porous and fibrous pumice inclusions are common.

Figure 7. Pyroclastic flow.

Both ashes and ignimbrites consist for the greater part of volcanic glass and weather easily. Crystals of(mainly) quartz and/or feldspars, biotite and hornblende ( 'phenocrysts'; Dutch.: 'eerstelingen') make upless than 20 percent of the ash. The only historic ignimbrite-forming eruption was that of the Katmai inAlaska in 1912. The largest eruption in comparatively recent times took place some 40,000 years agoand led to the formation of Lake Toba on Sumatra, Indonesia.

Volcanic rocks, especially pyroclastic rocks, contain volcanic glass that weathers easily and accountsfor the remarkable properties that soils in most volcanic regions 0ave in common. Translocation ofweathering products and accumulation of short-range-order minerals and of stable organo-mineralcomplexes are essential processes in the formation of the characteristic soils of volcanic regions: theAndosols.

Figure 8. Animation of the sequence of events that formedCrater Lake in Oregon. After H. Williams. Click on the pictureto start the animation.