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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).
DESCRIPTION: Rhyolite is the aphanitic equivalent of granite -- i.e., rhyolite has the same general composition as granite but the individual grains of rhyolite are so small they cannot be distinguished by the naked eye or even with the aid of a handlens.