© Copyright, 1997 by R.A.Kanen, All Rights Reserved
Volcanism is a term used to describe the process by which extrusive igneous rocks form. These rocks are named lava. Lava originates as a silica rich magma in the mantle of the Earth. Do to very high temperatures the magma is very fluid and since it has a low density it rises to the Earth’s surface. Once the magma reaches the crust it may rise through cracks in the country rocks called fissures. As the magma rises up the fissures it leaves spaces behind which must be filled. The country rocks fills some of these spaces. As a result the country rocks fractures supplying additional areas of weakness where the magma may rise. The ejected magma solidifies and accumulates around the place it is ejected to form a dome like structure. The place of emergence of the magma is called the vent. Different types of magma may be ejected from the vent.
If the magma is of felsic composition it is called rhyolite, if it is of mafic composition it is called basalt and if it is of intermediate composition it is named andesite. Other substances may also be ejected from the event. Scoria is a solidified version of spatter, which is liquid drops of magma. If some of the solidified material surrounding the vent falls back into the vent it will be emitted as ash. Consolidated ash is called tuff. Tuff which has formed from falling ash is ash fall tuff and it has good size sorting. Tuff which has formed from ash flows or nuee ardente explosions is called sorted. All material blown] out of the vent is called tephra. Tephra may be ash, fragments of rocks or volcanic bombs. Gasses may also be emitted. These include water vapor, CO2, HCL and H2S. The oxidation of H2S produces sulfataras or sulfur deposits. Condensation of water vapor may cause ground water to heat up and form springs and geysers. In tropical climates where there is lots of moisture lahars or mudflows may occur.
The shape of the cone which forms around the vent of the volcano depends upon the viscosity of the magma and the topography of the land. The viscosity of the magma depends on the composition of the magma. Magma of felsic and intermediate composition is more viscous than magma of basic composition. Viscous magma which is of felsic composition does not flow very far before it solidifies. As a result the magma solidifies around the vent producing steep sided volcanoes called composite or strata volcanoes. They consist both of tephra and lava flows. Mayon volcano in the Philippines and Mt Erebus are examples of strata volcanoes. Very fluid magma of basaltic composition may travel many miles before it solidifies. As a result volcanoes with gently sloping sides form. These are called shield volcanoes. Mauna Loa and most of the other Hawaiian volcanoes are shield volcanoes. If lava flows from these volcanoes will undergo a change in slope the surface of the lava would become ropy and form a ropy lava. In viscous lava the surface of the lava would break up and become block lava or AA lava. Another type of volcano consists entirely of tephra. It is called a basaltic cinder cone. It is the smallest volcano and lava flows from it will travel for many miles before solidification (due to low viscosity). Many basaltic lava’s erupt under the ocean in hot spots orl along the mid oceanic ridge. The surface of these lava’s cool rapidly due to the water surrounding them, resulting in pillow structures.
Viscosity and composition of magma may also explain the explosives of volcanoes. Viscous, felsic magma does not allow gasses to escape very easily. This results in an increasing pressure in the volcano over a period of time. When the pressure becomes too great for the volcano to withhold violent explosions of the peleeean, plinean, vesuvian and vulcanian types occur, peleean being most violent and vulcanian least violent. Nuee ardentes, lahars and the formation of calderas (collapse of the cone do to circular faults forms a crater or caldera five to 10 kilometres in diameter) are commonly associated with eruptions of these types. Krakatoa volcano in Indonesia is a good example of a caldera. Taal volcano in the Philippines consists of a caldera filled with water, creating a crater lake. Since such violent eruptions require viscous, felsic magma they occur in composite volcanoes.
In very fluid basaltic magma gasses can escape much more easily and frequently. Therefore, large pressure buildups which are associated with violent eruptions do not occur and frequent small explosions do occur. As a result shield volcanoes and basaltic cinder cones have eruptions or are formed by eruptions of the strombolian, hawaiian and fissure types, strombolian being most violent and fissure least . In conclusion, it is evident that composition and viscosity of a magma can go a long way in explaining the origin, shape and processes of a volcano.
