Calderas: Volcanoes That Sink Instead of Build, Explosive and Nonexplosive

A caldera is a collapse feature. During a large eruption the magma chamber beneath a volcano is partly emptied, and the ground above sinks into it. Calderas are big, generally more than 0.6 miles (1 km) across, the largest tens of miles, and much wider than the vents inside them.

Aniakchak Caldera, Alaska, formed in a VEI 6 eruption about 3,700 years ago when the top of a composite volcano collapsed. National Park Service.

Volcanoes in reverse

Most volcanoes are constructional: cinder cones, composite volcanoes and shield volcanoes grow as lava and tephra pile up. Calderas are excavational, or inverse: they sit below the surrounding land, because they form by the surface foundering. They are both features of other volcanoes and a volcano type of their own. The largest kind, the resurgent caldera, belongs to no single volcano at all: it is a broad volcanic plateau built of thick ash-fall and pyroclastic-flow deposits (ignimbrite).

Two main kinds

Calderas form in both silicic (dacite to rhyolite) and mafic (basalt to andesite) systems, and differ by whether the eruptions that made them were explosive or not.

Explosive calderas vary in size, eruption magnitude, age and whether they formed on an existing volcano. Nonexplosive calderas crown most large shield volcanoes, such as Kīlauea and Mauna Loa, forming when eruptions drain the shallow magma chambers beneath them. A shield volcano can go through several cycles of caldera collapse and refilling by lava.

Mokuʻāweoweo, Mauna Loa’s summit caldera, from the Hawaiian Volcano Observatory webcam on its north rim. U.S. Geological Survey.

Parks with calderas include Aniakchak, Bandelier, Bering Land Bridge, Big Bend, Chiricahua, Channel Islands and Hawaiʻi Volcanoes.

Sources

• National Park Service, "Calderas"; rewritten in hubnx's own words. Photographs: National Park Service and U.S. Geological Survey.