STEPHEN & DONNA O'MEARA / SCIENCE PHOTO LIBRARY STEPHEN & DONNA O'MEARA / SCIENCE PHOTO LIBRARY
A rare close-up of a swelling secondary pyroclastic flow avalanching into the Chambo River Valley on the western slope of Tungurahua volcano in Ecuador. Secondary pyroclastic flows are created when hot, thick pyroclastic flow deposits in valleys down-slope from volcanoes are remobilised into hot debris avalanches. In this case, a swelling Chambo River after a heavy rainstorm sliced into the base of a hot and thick pyroclastic flow deposit in the Chambo River Valley. This view is from directly above the pyroclastic flow on the River Valley wall. Secondary pyroclastic flows are a dangerous post-eruption volcanic hazard; they were not recognised as such until they were observed in 1991 at Mount Pinatubo in the Philippines. This view shows the onset of one such hot debris avalanche.
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