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Volcanic activity
Published in F.G. Bell, Geological Hazards, 1999
A pyroclastic surge is a turbulent, low density cloud of gases and rock debris that hugs the ground over which it moves (Sparks, 1976). Hot pyroclastic surges can originate by explosive disruption of volcanic domes caused by rapidly escaping gases under high pressure or by collapse of the flank of a dome. They can also be caused by lateral explosive blast. Hot pyroclastic surges can occur together with pyroclastic flows. Generally, surges are confined to a narrow valley of a volcano, but they may reach speeds of up to 300 km h , so escape from them is virtually impossible. They give rise to similar hazards to pyroclastic flows. Cold pyroclastic surges are produced by phreatic and phreato-magmatic explosions. Vertical explosions can give rise to a primary surge, which moves away from the volcano in all directions. Subsequently, secondary surges may be formed when volcanic material falls to the ground. The high speed of primary surges is attributable to the explosive force, while that of secondary surges is due to the kinetic energy gained during falling and the speed gathered as they descend the slopes of the volcano. Surges decelerate rapidly and tend not to travel more than 10 km from their source. Fortunately, pyroclastic flows and surges tend to affect limited areas, and approximately twenty occur every 100 years.
A review of lahars; past deposits, historic events and present-day simulations from Mt. Ruapehu and Mt. Taranaki, New Zealand
Published in New Zealand Journal of Geology and Geophysics, 2021
Jonathan Procter, Anke Zernack, Stuart Mead, Michael Morgan, Shane Cronin
Volcanic mass-flow deposits are typically mapped and distinguished as: pyroclastic density current deposits (comprising pyroclastic surge deposits, pumiceous ash-rich pyroclastic flow deposits/ignimbrites and block-and-ash flow deposits), lahar deposits (including debris-flow and hyperconcentrated-flow deposits) or debris-avalanche deposits. Volcanic mass flows are dominantly gravity-driven (although sometimes initially accelerated by volcanic explosions) and involve rock materials as their primary solid component along with either water or gas as the fluidising component. Sediment-water mixture flows are the most common type and they span a wide range of volumes, discharges, velocities, compositions, bulk rheologies, and flow hydraulics (Pierson 1998). However, the nomenclature used to distinguish different types of volcanic mass-flows is often misleading (Pierson and Costa 1987) and variations are mainly a result of the interdisciplinary study of volcanic mass-flow processes.