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Calderas are remarkable volcanic structures. They are depressions, kilometres to tens of kilometres across, formed as near-surface crust subsides into magma reservoirs during eruption. Although some of the smaller examples have developed during the effusion of lava flows, most calderas are associated with explosive volcanism, from plinian eruptions that expel 1–10 km3 of magma to ignimbrite-forming events that erupt at least a hundred times as much. This Special Publication focuses on the evolution of large calderas, from their formation to post-collapse behaviour. It illustrates the advances that have been made during the past two decades in understanding such calderas and highlights the goals that still need to be achieved in order to mitigate the hazard from their activity.
Larger calderas are associated with larger eruptions and these, because of the greater energy required, occur less frequently than smaller events. As a result, the explosive calderaforming eruptions in historical time represent only the smaller end of the size range. For example, the 1991 eruption of Pinatubo, in the Philippines, expelled about 5 km3 (volumes quoted as dense rock equivalents) of magma to produce a caldera 2.5 km across; the 1912 Katmai eruption (Alaska, USA) involved 12 km3 of magma and left a caldera 2.5 by 4 km across; and, in 1883, the eruption of 10 km3 of magma from Krakatau (Indonesia) produced a caldera 8 km wide (Lipman 2000). Events of such magnitude occur a few times each century and rank among the largest explosive eruptions in history (Simkin & Siebert
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