MAJOR LANDFORMS IN VOLCANIC REGIONS

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1 MAJOR LANDFORMS IN VOLCANIC REGIONS Volcanism is not randomly distributed over the world. It is concentrated near plate boundaries where plate subduction or seafloor spreading takes place. Other occurrences are linked to deep mantle plumes that reach the Earth s surface at distinct hotspots. Figure 1 shows the geographic distribution of major volcanic regions. (Click to enlarge) Landforms in volcanic regions are strongly influenced by the chemical and mineralogical composition of the materials that were deposited during eruptive phases. Volcanic rocks and magmas are grouped according to their silica contents in three main categories, viz. Rhyolite (65-75% SiO 2 ), Andesite (65-55% SiO 2 ) and Basalt (55-45% SiO 2 ). Individual rock types are distinguished on basis of mineralogical properties or contents of certain constituents (K 2 O, Na 2 O and CaO). See figure 2. Figure 1. Major volcanic regions of the world (after Robinson, 1975). The broad division of volcanic rocks and magmas according to SiO 2 content makes sense because the silica content correlates with the viscosity of magmas and hence with the type of volcanism. A rule of thumb: the higher the silica content of magma is, the more acid and viscous it is and the more explosive are 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 as a reference point. We shall distinguish between regions with basaltic, andesitic and rhyolitic volcanism.

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

3 Major landforms in regions with basaltic volcanism Basaltic volcanism occurs where basic mantle material reaches the surface, notably 1 at divergent plate margins (sea floor spreading), 2 in 'hot-spot' areas, and 3 in continental rift valleys. 1: The best-known divergent plate margin is the mid-oceanic ridge or rise. The highest parts of the ridge may reach the surface of the ocean and form islands, e.g. Iceland and the Canary Islands. It is not surprising that, like all ocean floors, Iceland consists mainly of basaltic rock. 2: A fine example of basaltic hot-spot volcanism is Hawaii, which constitutes the top of the largest 'shield volcano' in the world, with a diameter of 250 km at the base (on the ocean floor) and a total height of 9 km. Basaltic magma is little viscous and gases escape easily. Eruptions are therefore relatively quiet and produce low-viscosity lava flows, lava lakes and lava fountains, but little ash. The fluid magma can flow over large distances and the resulting shield volcanoes are comparatively flat. Most eruptions are fissure eruptions that take place along extensional cracks in the Earth s crust. The Figure 3. Fissure eruption. fissures may be several kilometers in length; the historical Laki eruption on Iceland happened along a 24-km long fissure. Much bigger fissure eruptions have taken place in the past. They produced enormous masses of flood basalt that covered hundreds of square kilometers. The Paraña plateau in South America is made up of 1 million km 3 basalt, which was extruded within 10 million years. Other examples of large occurrences of flood basalt are in Ethiopia, Siberia, Greenland, Antarctica, India (the Deccan Traps ) and in the western USA (Colombia River). See figure 4.

4 Figure 4. Worldwide occurrence of flood basalts. 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 as elongated tectonic depressions: the rift valleys. Both basic (SiO 2 poor) and acid (SiO 2 rich) volcanism occur in and along rift valleys. Basaltic volcanism in continental rift valleys (e.g. the East-African rift valley, the Baikal graben, or the Rhine - Rhone graben) is associated with 'strombolian' scoria cones and with maar craters (i.e. steam-explosion craters now filled with water). Here too, ash deposits seldom extend beyond the volcanic areas themselves. Where ash blankets are extensive, as in some rift valleys, they are usually more acidic. The comparatively fluid basaltic lava flows tend to follow river valleys and can flow over considerable distances 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 eroded landscape.

5 Landforms in regions with andesitic volcanism Andesitic volcanism is a characteristic element at convergent plate boundaries where plate subduction takes 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 term means 'stratified' volcano, which is misleading in the sense that all volcanoes are built up of layers, be it 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 lava and pyroclastic rock, mostly of andesitic composition. Stratovolcanoes are much larger than scoria cones 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 to their SiO 2 content, viscosity and gas content. Whereas basaltic, low viscosity magmas hardly produce pyroclastics ('tephra'), and high-viscosity rhyolites hardly produce lavas, andesitic magmas will normally produce both. Because of the greater viscosity of the magma, a higher pressure must build up before 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 do not extend as far from the point of emission, usually only a few kilometers. This explains why stratovolcanoes have steeper slopes than shield volcanoes and the 'classical' cone shape.

6 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, Philippines 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. Figure 5. Stratovolcanoes in Indonesia. Pyroclastic flows are frothy masses of ash and pumice. They evolve when an extrusive dome collapses and generates a fast moving glowing avalanche of gas, ash and pumice. The resulting rocks are known as 'ignimbrites' and can have a variety of structures depending on the flow conditions during emplacement and on the degree of post-depositional welding.

7 'Volcanic ash fall-out often spreads far beyond the direct vicinity of the erupting volcano. Whereas lava and pyroclastic flows are confined to the immediate vicinity of volcanoes, ashes might be blown into the troposphere and stratosphere, and can travel hundreds of kilometers. The thickness of the ash deposits decreases with increasing distance from the point of origin. It may be difficult to recognize the presence of volcanic ash in soils because it is incorporated in the solum, overgrown by vegetation and it weathers rapidly. Nonetheless, 'rejuvenation' of soil material with fresh volcanic ash is often of great importance as it restores or improves soil fertility and promotes physical soil stability. Figure 6. Distribution of the volcanic deposits after the explosion of the Mt. Pinatubo in 1991.

8 Landforms in regions with rhyolitic volcanism Partial melting of the continental crust in cordilleran mountain ranges and rift valleys may produce acid rhyolitic 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 below a stratovolcano, tremendous gas pressures build up so that, once a vent for eruption is opened, the magma chamber empties itself completely, leaving a cavity in the Earth's crust in which the entire stratovolcano 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 Germany). Only occasionally do more quiet eruptions take place. The high viscosity of the lava precludes lava flow; a lava dome is formed instead (e.g. Obsidian Dome, USA). Figure 7. Pyroclastic flow. 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.

9 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 up less than 20 percent of the ash. The only historic ignimbrite-forming eruption was that of the Katmai in Alaska in The largest eruption in comparatively recent times took place some 40,000 years ago and led to the formation of Lake Toba on Sumatra, Indonesia. Volcanic rocks, especially pyroclastic rocks, contain volcanic glass that weathers easily and accounts for the remarkable properties that soils in most volcanic regions 0ave in common. Translocation of weathering products and accumulation of short-range-order minerals and of stable organo-mineral complexes are essential processes in the formation of the characteristic soils of volcanic regions: the Andosols. Figure 8. Animation of the sequence of events that formed Crater Lake in Oregon. After H. Williams. Click on the picture to start the animation.