A Caldera is a large depression or a hollow area that forms when a volcano erupts and the magma chamber below it empties. This results in the collapse of side and top of the volcano.Without any structural support below the area, the erupting volcano vent collapses inwardly due to gravity creating a bowl shaped Caldera. There is a difference between a Caldera and crater. Craters form by the outward explosion of materials from a volcano. Craters are more circular than a caldera because in the latter the land collapses unevenly.
It is a large pool of molten rock or magma below the surface of earth or under the existing volcano . The density of magma is less than that of the surrounding hard rocks so it starts to rise through cracks beneath the crust. When the magma is not able to find its way to the surface of the earth ,it is collected and pools into a magma chamber . Over the passage of time , this magma chamber grows in size due to the successive injections from horizontal and vertical magma and thus the pressure inside the chamber increases.
The magma chamber bears the weight of the volcano above it due to the buoyant forces which are acted upon the chamber by the surrounding rocks . The buoyant force gives a resultant upward push to the pool of magma chamber .
Sometimes when a volcano erupts, the whole pool of magma beneath it empties and this results in a void . Now there is nothing to support the weight of volcano. This causes volcano to collapse and form a caldera . Calderas are rare ; only 7-8 of them have known to occur in the last century. They are more than 25 kms in diameter and several kms deep.
Types of calderas
Explosive caldera forms when large magma chambers filled with silica (highly viscous)and gas move upwards. The high viscosity silica holds the gas bubbles under high pressures but as the material rises to the surface, due to decrease in the pressure the gas expands . This causes an enormous explosion which throws away rocks and magma to form a caldera . eg. Yellowstone caldera ,Toba caldera.
When the matter is composed of basalt , which is much less viscous, the magma chamber is gradually drained by large lava flows rather than a single explosive eruption. eg. caldera atop Fernandina Island and Mauna Loa caldera.
Calderas are not limited to Earth . Other planets like Venus and mars or our moon also has caldera formations.