Volcanoes produce distinctive land-forms, through extrusion of lava. The most obvious landform created by volcanism is a volcano i.e. a conical hill composed of materials erupted during volcanism.
The volcanic topography includes both positive as well as negative relief features. The positive relief features consist of the hills, cones, mountains, lava plateaues etc ,while the negative relief features include the depressed forms like craters, calderas, volcanic-tectonic depressions etc.
Positive – Relief features Some of the important positive relief features produced by volcanism are as follows:
(a) Hornitos
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These are the terms applied to very small lava flows.
(b) Driblet cones
These are small conelets produced by the most acid and viscous lava.
(c) Cinder cones
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These cones are formed due to the accumulation of loose rock fragments ejected during volcanism immediately round the vent. Since such volcanoes erupt only a small quantity of lava, the cones are built up principally of pyroclastic materials as cinder, bombs, ash etc.
The sides of the cones have a slope that is the angle of repose of the pyroclastic debris. These cones commonly have slopes of 30° to 40° and do not exceed a height of 500 metres.
As these cones grow around a central vent, with the accumulation of pyroclastic debris they are also known as Ring crater or as composed dominantly of finer materials.
(d) Lava cone
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These are built up of lava flows, due to heaping of lava during quiet-type of eruption. Such cones are usually dome-shaped but not conical because of the fluidity of lava.
The form of these domes varies depending on the nature of the lava, whether acidic or basic. They have gently sloping sides and very broad base.
These are also known as lava-or plug dome. Highly viscous silica-rich lavas form steep sided cones called cumulo-domes or tholloids.
(e) Composite cone
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The composite cones are made up alternately of pyroclastic material and lava flows. As indicated by its name, a composite volcanic cone is formed partly by explosive eruptions (which are chiefly responsible for increasing its height and steeping its summit slopes) and partly by lava that flows forth quietly and widens its base.
Thus they are intermediate in nature and composition between the cinder cones on the one hand and lava-cones on the other. These cones are built up over long spans of time. Due to the rude stratification of explosive materials and lava flows in such volcanoes they are also known as ‘Strato-volcanoes’.
The important examples of these cones are shown by Fujiyama in Japan, Mayon in the Phillipines, and Vesuvius in Itlay. The slopes of these volcanoes are sometimes found to be covered with deep radially diverging grooves called barrancos.
(f) Shield-Volcano
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These are made up of lava alone and are broad, gently sloping cones constructed of solidified lava flows. These are formed by the piling up of flow after flow of fluid lava; thus a rounded gently sloping domelike mass is produced.
The slopes rarely exceed 8°. These are the lava cones formed chiefly of basalt flows and are frequently the sites of fissure eruptions. The volcano Mauna Loa of Hawaiian Islands is an excellent example of shield-volcano.
(g) Spatter Cone
Sometimes small, steep-sided cones are developed on the surface of a solidified lava flow, at the spots where breaks occur in the cooled surface of the flow due to the pressure of the entrapped gases.
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Thus hot lava and gases blow out forming steep sided cones which rarely exceed 10 metres in height.
(h) Volcanic -plateaus
These are built by extensive extrusions of lavas, notably basalts, and are associated with the fissure eruptions. There is piling of lava flows and each layer ranges in thickness between 15 to 100 metres and hundreds of square kilometres in extent.
The Deccan plateau of the Peninsular India represents the best example.