Introduction
Ash-flow tuffs (ignimbrites) erupted during caldera-forming eruptions or parts of caldera walls are present in several national park areas. These rocks are testaments to some of the largest volcanic eruptions in the history of North America. The eruptions of these calderas probably ranged from Volcanic Explosivity Index (VEI) 6 to 8 (Colossal to Apocalyptic), producing eruptive columns which extended tens of miles (kms) into the stratosphere.
Calderas are collapse structures that form during especially large eruptions that partially drain shallow magma reservoirs causing the land surface over them to subside. Caldera-forming eruptions also emit pyroclastic flows that can travel tens of miles (kms) from the vents. The presence of thick ash-flow tuffs (ignimbrites) and the presence of megabreccias formed during collapse are two of the main testaments to these violent eruptions in the geologic past.
Later volcanic eruptions (including the formation of younger calderas), faulting, erosion, and other volcanic processes can obscure or remove much of the structural evidence for older caldera complexes.
Source: U.S. Department of the Interior, National Park Service
