A caldera forms when withdrawal of magma allows the ground above a reservoir to subside. Aniakchak’s broad depression makes the difference between building a volcanic mountain and collapsing part of one especially clear.
Do not identify every crater as a caldera: the formation process matters more than a circular outline. Calderas can develop in both explosive and effusive volcanic systems, and later vents may grow inside them. Which clue now helps you read this landscape? Compare the mountain landscapes next.
Source / photo: NPS / M. William · Public domain. Resized. No claim to original U.S. Government works.