As silica-rich magma ascends from deep beneath a volcano, dissolved water and carbon dioxide come out of solution violently, fragmenting it into a mixture of hot particles and gas that erupts explosively at the surface. If this mixture is lighter thanair it ascends buoyantly as an eruption plume, often high into the stratosphere. But if denser, it collapses back like a fountain and pours over the landscape as a ground-hugging 'pyroclastic flow'. On page 509 of this issue Dade and Huppert present a new model of the transport and sedimentation of the particularly violent Taupo pyroclastic flow which erupted 1,800 years ago in New Zealand, and covered 20,000 km~2 of the North Island under a blanket of red-hot pumice and ash.
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