The negative relief features in a volcanic terrain commonly consist of the following:-
(a) Crarer
This is a depression located at the summit of the volcanic cone. It is usually a funnnel-shaped hollow that marks the top of the volcanic-vent. These craters may be formed as follows:
(i) Sometimes pyroclastic materials ejected out during a volcanic eruption, accumulate around the opening i.e. at the upper end of the vent. Thus a ring-shaped mound is formed, which is known as the Crater-ring .
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The crater is situated, in such cases, at the centre of the ring. Craters of such origin are called ‘Ubehebes’, a term applied by Cotton after the Ubehebe Craters at the north end of Death Valley.
(ii) Craters may also form by subsidence of the apex of the cone due to collapse of the structure because of with- drawl of the underlying support.
(iii) In majority of the cases, craters are formed by the explosion that occurs during a volcanic eruption, whereby the apex of the volcano is blown out.
Sometimes due to explosive eruption small craters are formed without the building of cones. Such small craters are known as explosion pits. The explosion-pits, when occupied by small water-bodies form miniature crater-lakes. Such small crater lakes are seen in the Eifel district of Germany, where they are called maare or maars.
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(b) Caldera
A caldera is generally a volcanic crater, which-is enlarged to a diametre of several kilometres. These circular depressions commonly have steep inner wall s and a flat floor. The calderas may have different origins and therefore, are of different types as follows:
(i) Explosion Calderas
Sometimes due to violent volcanic explosion the entire central portion of the volcano is destroyed and a great central depression remains. This great central depression is called a Caldera. Bandai-San in Japan represents an example of this type of caldera.
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(ii) Callapse Caldera
These are formed when the top of a volcano collapscs or subsides into a vacated magma chamber beneath the volcano, because of the withdrawal of the underlying support It is believed that the rapid eruption of lava and pyroclastic materials etc. lower the level of magma in the main reservoir to such an extent that a potential void is left.
Thus the later-floor subsides, since there is no support from beneath. The caldera of the volcano Krakatau in Indonesia is an example.
(iii) Resurgent Caldera
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According to M.P. Billings. (‘Structural Geology’, Prentice Hall of India, New Delhi), these are formed where collapse is followed by the doming of the central block. Example-Valles Caldera in New Mexico.
(iv) Erosion Caldera
These calderas are formed by the en- largment of craters of volcanoes by various eroding agents.
The Buldir-caldera is the largest known caldera located between the islands of Kiska and Buldir in the Aleutian chain.
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(c) Volcanic-tectonic depressions
Due to tectonic reasons sometimes depressions are formed on the slopes of the volcanic cones. These are volcanic-tectonic depressions.