A caldera is a volcanic feature formed by the collapse of a volcano into itself, making it a large, special form of volcanic crater.
A caldera collapse is usually triggered by the emptying of the magma chamber beneath the volcano, as the result of a large volcanic eruption.
If enough magma is erupted, the emptied chamber will not be able to support the weight of the volcanic edifice (the mountain) above.
Fractures will form around the edge of the chamber, usually in a roughly circular shape.
These ring fractures may in fact serve as volcanic vents.
As the magma chamber empties, the center of the volcano within the ring fractures begins to collapse.
The collapse may occur as the result of a single massive eruption, or it may occur in stages as the result of a series of eruptions.
The total amount of collapse may be hundreds or thousands of meters.