Introduction
Calderas are collapse features that form during large-volume volcanic eruptions when the underlying magma chamber is partially emptied and the ground above it subsides into it. Calderas may form in both silicic (dacitic to rhyolitic) and mafic (basaltic to andesitic) volcanic systems, leading to two main types of calderas. In addition to the composition of magma erupted, these calderas are also differentiated on the basis of whether the eruptions that produced them were effusive (nonexplosive) or explosive.
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Explosive calderas form during eruptions of silicic magmas that emit large-volume ash-flow tuffs and form Ultra-Plinian eruptive columns.
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Nonexplosive calderas form on the summit of shield volcanoes during especially large eruptions of lava flows at either the summit or along flank rift zones.
Calderas are large, generally with a diameter greater than 0.6 miles (1 km). The largest calderas are tens of miles (kms) wide. A defining characteristic of calderas is that they have diameters that are much wider than their included vents.
Calderas are both landforms that are parts of other volcanoes and a type of volcano in their own right. Resurgent calderas, the largest type of caldera, are not associated with any individual volcanic edifice, but are characterized by broad volcanic plateaus with voluminous ash-fall and pyroclastic-flow deposits (ignimbrites).
Calderas are excavational (or inverse) volcanoes with relief below the general land surface because they form from subsidence (or foundering) of the surface. In this way, they are the opposite of constructional volcanoes like cinder cones, composite volcanoes, and shield volcanoes that form from the accumulation of lava and tephra.
Source: U.S. Department of the Interior, National Park Service
