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Rhyolite caldera complexes are the most explosive of Earth's volcanoes but often don't even look like volcanoes. They are usually so explosive when they erupt that they end up collapsing in on themselves rather than building any tall structure (George Walker has termed such structures "inverse volcanoes"). The collapsed depressions are large calderas, and they indicate that the magma chambers associated with the eruptions are huge. In fact, layers of ash (either ash falls or ash flows) often extend over thousands of square kilometers in all directions from these calderas. Fortunately we haven't had to live through one of these since 83 AD when Taupo erupted. Many rhyolite caldera complexes, however, are the scenes of small-scale eruptions during the long reposes between big explosive events. The vents for these smaller eruptions sometimes follow the ring faults of the main caldera but most often they don't. The origin of these rhyolite complexes is still not well-understood. Many folks think that Yellowstone, for example, is associated with a hotspot. However, a hotspot origin for most other rhyolite calderas doesn't work; they occur in subduction-related arcs. Examples of rhyolite caldera complexes include Yellowstone, La Primavera, Rabaul, Taupo, Toba, and others.
This is an outcrop in the Los Chocoyos ignimbrite, the product of one of the most powerful eruptions known...