Calderas 

One of the most common geological structures in volcanoes is known as the caldera. These objects form when infrequent, yet violent volcanic activity burst forth from the center of a volcano, leaving a newly formed space. This space eventually collapses downward as a seal is formed in the area where lava and ash had been spewed out. Calderas range in diameters from being smaller than 1 mile (1.6 kilometers) to extending larger than 30 miles (48 kilometers).

There exists different types of Calderas. One of them, known as the resurgent caldera forms in several steps.

1. Gas-rich magma accumulates on the top of a magma chamber underneath older volcanic rocks.
2. After the eruption break through the surface creating a vent for steam and gas, as well as other volcanic material.
3. The upper part of the magma chamber froths, expands and flows up the vent. The velocity decreases as magma from the deeper parts of the chamber begin to flow out and the rocks overlying the magma begin to collapse along the fractures into the now emptied chamber
4. Pyroclastic flows continue as the initial burst of ash and deeper parts of magma flow across the surface covering the caldera and surrounding geographic area.
5. The magma chamber is then depleted in gases and minor volcanic activity can persist along the ring fracture for as much as a million years. The crater is then mostly filled with volcanic ash and pumice and is quickly occupied by a crater lake.
6. 6. This flat-floored crater doesn't stay this way for long; the magma, now depleted in gases, continues to slowly rise. The crater floor is pushed up as much as 1 kilometer (.6 miles), forming a giant "blister."