A caldera forms when a shallow magma chamber empties faster than the overlying rock roof can be held up by the pressure of the magma beneath it. As the chamber drains — through eruption, lateral intrusion, or degassing — the roof loses its buoyant support and eventually founders along a near-vertical ring fault, dropping down into the space vacated by the magma and leaving a broad depression at the summit.
Crater Lake in Oregon and Aniakchak in Alaska both formed this way: the caldera-forming eruption of Mount Mazama around 7,700 years ago emptied so much magma so quickly that roughly 5 cubic miles of summit collapsed downward, leaving the roughly 8 km-wide basin that is now Crater Lake.
A cutaway 3D volcano lets you drain a simulated magma chamber and watch the overlying rock roof lose its support, fracture along a ring fault, and drop inward to form a caldera.
Once chamber fill drops below the roof's strength threshold, subsidence ramps up along a ring fault. Choose between a coherent piston-style collapse or a fragmented piecemeal collapse, both real caldera-forming patterns.
Drag the chamber fill slider to drain magma manually, or switch on auto-drain to watch a full eruption-and-recharge cycle. Widen the cutaway to see deeper inside, and compare collapse styles.
Crater Lake, Oregon, formed when Mount Mazama's climactic eruption 7,700 years ago drained its magma chamber so quickly that the summit collapsed roughly 1,000 metres to form the caldera now filled by the lake.