Pyroclastic 2 - Pyroclastic Flows and Surges Also known as...

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1 Pyroclastic Flows and Surges Also known as glowing ash clouds, glowing avalanches and nuées ardentes (French). ± These are the most hazardous of all volcanic processes. ± Estimated to have caused about 55,000 deaths since 1600 A.D. (about 48% of all volcano-related fatalities. ± Causes of death are:- ± Asphyxiation by hot ash and dust. ± Burning, boiling and dehydration during brief periods of high temperature (200 - 800 0 C). ± Impact by ash and debris driven at high speeds. What are pyroclastic flows and surges? ² Hot mixture of ash, pumice blocks and gases that form:- ² Gravity-driven clouds that travel at great speed. ² Speeds are typically 20-70 mph, but speeds up to 300 mph have been recorded (Mount St. Helens). ² Temperatures are highly variable from less than 200 to 800 0 C.
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2 Sources of Energy 1. Gravity – remember rock avalanches have sufficient gravitational energy that they can travel across valley floors and move up-slope. 2. Fluidization – buoyancy created by entrained and heated air – plus hot gases released from particles and clasts Pyroclastic Flows – can travel large distances from a volcano, typically about 10 – 15 km, but sometimes up to 100 km. Mostly they follow drainage patterns – but they may also have sufficient kinetic energy to surmount hills and ridges They include some of the world’s largest volcanic deposits:- Age Volume (km 3 ) Yellowstone 600 ka >1000 Long Valley 700 ka >2000 Toba (Sumatra) 75 ka ~2000 Tambora (Indonesia) 1815 AD 35-50 Katmai (Alaska) 1912 AD 10-15 Pinnatubo (Philippines) 1991 AD 4-5
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3 Three ways that they form Soufrière Type – the eruption column can no longer be sustained (due to loss of pressure), so the column collapses forming pyroclastic flows on the flanks of the volcano (St Vincent, 1902). They tend to be cooler than the others. Pelée Type – a dome of viscous magma (rhyolite, dacite) is blocking the conduit. Eventually it explodes under pressure, blasting pyroclastic material down one of the flanks of the volcano (Mont Pelée, 1902). If it is extremely violent it could produce a lateral blast.
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4 Merapi Type – a dome of viscous magma (dacite, rhyolite) grows in the crater. Eventually it gets so large, it collapses under gravity, producing a pyroclastic flow (Merapi, Indonesia, 1951; Unzen, Japan, 1991; Montserrat, West Indies, 1996). Basic Terminology
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Pyroclastic 2 - Pyroclastic Flows and Surges Also known as...

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