What is a caldera? How are they formed?
A caldera is the landform that forms after the structural support for a volcano has weakened and the surface of the volcano has collapsed.
A caldera is the landform that exists after the structural support for a volcano has weakened and the surface of the volcano has collapsed. If enough magma is expelled from a volcano, the walls of the volcano collapse in on themselves and what remains is the caldera, a sort-of sinkhole.
Just because a volcano has collapsed and formed a caldera does not mean that all activity has stopped. There may still be magma present under the caldera.
Crater Lake National Park in Oregon, US is an ancient caldera. This site is no longer active and all magma was expelled.
Note: a crater forms when a volcano explodes outward but a caldera forms when a volcano collapses inwards.