(World Geography) Volcanoes, Landforms & Distribution of Volcanoes, Types of Lava
MECHANISM OF VOLCANIC
Some Observation about volcanoes are:
1. Volcanoes along divergent boundaries are location of basaltic
outflows which are less violent and far more fluid.
2. Those along continental margins are always andesitic and very explosive.
3. There are no surface eruptions of granitic magma. It always cools under the surface.
4. Hot Spot volcanoes are some of the quietest type and are not at the plate margins.
5. Not all collision boundaries have volcanoes. Continental-Continental collision zones are entirely a-volcanic.
The above points are best explained by plate tectonic theory.
1. In case of Ocean-Ocean collision the subducting plate melts
creating molten magma that rises to form volcanoes. It is also
associated with island arcs.
2. Ocean-Continent collision creates volcanoes that are associated with Andean type of fold mountains. There volcanoes are explosive types.
3. Along the divergent boundaries volcanoes are a consequence of ascending-convecting limb of magma. The mechanism is part of Sea Floor Spreading.
4. There are also intra plate volcanoes, the mechanism of which is not understood well. All that is known about them is that they are localized plumes sourced form core mantle boundaries. They erupt with ultra basaltic and basic magma.
5. It also explains that due to absence of subducting oceanic slab continental-continental collision is not associated with volcanoes,
1. Vapour and gases: Steam and vapour constitute 60 to 90% of total gases discharged during a volcanic eruption. Steam and vapour include phreatic and magmatic vapour. The gases include CO2,NO,SO2,H2,CO,HCl etc.
2. Magma and lava: Generally molten rock material below the earth surface is called magma while they are called as lava when they come at the earth’s surface. The mineral composition of the magma is the most important factor determining the nature of a volcanic eruption.
3. Pyroclastic or fragmental material: There pyroclastic material, also referred to as tephra, vary in size form volcanic ash, which is sand or smaller, to gravel-sized cinders(2-4 mm), lapilli (4-64), blocks (>64mm). They may also include volcanic “bombs,” which are explosively ejected blobs of incandescent lava. A Nuée ardente, is an incandescent, hot, turbulent gas, ash, and pyroclastic cloud that can jet across the landscape in an explosive eruption.
Types of Lava
1. Basic laves: There are highly fluid. They are dark coloured like basalt, rich in iron and magnesium but poor in silica. They are affect extensive areas, spreading out as thin sheets. The resultant volcano is gently sloping with a wide diameter and form a flattened shield or dome.
2. Acid lavas: There lavas are highly viscous with a melting point. They are light-coloured, of low density, and have a high percentage of silica. They flow light-coloured, of low density, and have a high percentage of silica. They slowly and seldom travel far. The resultant cone is therefore steepsided.
The type of landform that results from a volcanic eruption depends primarily on the explosiveness of that eruption.
The commonest intrusive landforms are sills and dykes. When an intrusion of molten magma is made horizontally along the bedding plains of sedimentary rocks, the resultant intrusions is called a sill. Intrusions when injected vertically as narrow walls igneous rocks within the sedimentary layers are termed as dykes. A laccolith is large blister of igneous mound with a dome-shaped upper surface and a level base fed by a pip-like conduit from below. A lopolith is another variety of igneous intrusions with a saucer shape. A phacolith is a lenses-shaped mass of igneous rocks occupying the crest of an anticline or the bottom of a syncline and being fed by a conduit from beneath. A batholith is a huge mass of igneous rocks, usually granite, which after removal of the overlying rocks forms a massive and resistant upland region.
EXTRUSIVE VOLCANIC LANDFORMS
Cinder cones: Cinder cones are of low height and are formed of volcanic dust and ashes etc pyroclastic material. Falling under the influence of gravity, these particles accumulate around the vent, in a large pile of tephra. The form of a cinder cone is very distinctive, with steep straight sides and a crater (depression) at the top of the hill.
Composite cones: A composite cone results when formative eruptions are sometimes effusive and sometimes explosive. Composite cones are therefore composed of a combination-that is, they represent a composite-of lava flow and pyroclastic materials. They are also called stratovolcanoes because they are constructed of layers (strata) of pyroclastics and lava. They are formed due to deposition of alternate layers lava and fragmental material wherein lava acts as cementing material.
Shield volcanoes: When numerous successive basaltic lava flow occur in a given region they can eventually pile up into the shape of a large mountain called a shield volcano.
Plug Domes: Where extremely viscous silica-rich magma has pushed up into the vent of a volcanic cone without flowing beyond it, it forms a plug dome.
Calderas: A caldera is a large, basin shaped depression formed at the volcanic mouth. It forms when summit material on a volcanic mountain collapses inward after an eruption or other loss of magma. A caldera may fill with rainwater.
WORLD DISTRIBUTION OF VOLCANOES
There is close relationship between volcanic activity, earthquakes and plate tectonics, with many volcanoes located above subduction zones and in axial rifts. Around 80% of volcanic activity is found along subduction boundaries. Mid-ocean spreading centres and continental rifts account for around 15% of volcanic activity. Rest is intra-plate volcanism.
Circum-Pacific ring of fire: Most of the high volcanic cones and volcanis mountains are found in this belt, where there is active subduction of the Pacific, Nazca, Cocos, and juan de fuca plates. One good example is the volcanic of Sumatra and Java, which lies over the subduction zone between the Australian plate and the Eurasian plate.
Mid-Atlantic belt: A few basaltic volcanoes of fissure eruption type also occur along the mid-oceanic ridge, where seafloor spreading is in progress.
Mid-Continental belt: It includes volcanoes of the alpine mountain chains and the Mediterranean Sea and those in the fault zone of Eastern Africa. Here volcanoes are caused due to collision of African, Eurasian and Indian plates.
Intra-plate volcanoes are scattered in the inner parts of plates away from the margins. There are also called Hot spot volcanoes as they occur in the middle of plate boundaries where magma exits from weaknesses in the earth’s surface. The Hawaiian Islands are an example of hot spot volcanoes.