A Caldera is sub-circular, polygonal, or elongate subsidence basins formed by the eruption of magma from sub-volcanic reservoirs.
During a volcanic eruption, magma underneath the volcano is expelled, often forcefully.
When the magma chamber empties, the support that the magma had provided inside the chamber is lost and as a result, the sides and top of the volcano collapse inward. Calderas vary in size from one to 100 kilometers in diameter.
Type of Caldera
An explosive Caldera- Explosive caldera eruptions are produced by a magma chamber whose magma is rich in silica. Silica-rich magma has a high viscosity, which enables them to hold gas bubbles under very high pressures. When an eruption occurs the result can be an enormous explosion that blasts away large volumes of rock to form the caldera.
Collapse Caldera- Collapse calderas form when a large magma chamber is erupted by a volcanic eruption or by subsurface magma movement. The unsupported rock that forms the roof of the magma chamber collapses to form a large crater. Crater Lake and some other calderas are thought to have formed by this process.
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