Guide to Stratovolcanoes of the World
Domes and Pyroclastic flows
Pyroclastic flow is the term used to describe a combination of volcanic gases and fine ash particles that move down slope on the volcano flanks rather than rising like smoke as the ash cloud usually does. Pyroclastic flows can be made up of super-heated gases called nuées ardentes (French for "glowing cloud") and glowing avalanches, composed mainly of tiny ash fragments. The nuées ardentes reach temperatures between 300-800 °C (570-1470 °F) and often jump ridges, burning everything in its path. The heavier glowing avalanche typically follows canyons and drainage patterns. Together the gases and ash make up a "pyroclastic flow."
Pyroclastic flows move at speeds of 80 to 240 km/hour (50 to 150 mi/hour), although nuées ardentes may travel at twice that speed. Pyroclastic flows are extremely dangerous because of their speed of travel and very high temperatures. In 1902, Mont Pelée, a volcano on the island of Martinique in the West Indies, erupted. Within minutes of the eruption, nuées ardentes destroyed the city of St. Pierre, killing almost 30,000 inhabitants.
Nearly 3,000 persons were killed by the pyroclastic flows that accompanied the explosion of Lamington. At the end of the 1951 eruptive period, a 560-m (1848-ft) high lava dome partially filled the summit crater. A period of repeated lava dome building and collapse continued from 1951 to 1956. A dome slowly extruded from the vent like toothpaste. When the dome collapsed it sent pyroclastic flows speeding down the side of the volcano. The tallest dome grew 580 m (1,900 ft) above the floor of the crater before it collapsed.
Several different conditions at a volcano may result in pyroclastic flows. In the Lamington type, a lava dome grows in the vent after an eruption. When the dome reaches a certain height, part of it collapses into glowing rock particles and gases that speed down slope. Other types of pyroclastic flows include those in which the vent is open and a part of the ascending ash cloud collapses and flows down the sides of the volcano. In yet a third type of pyroclastic flow, a dome may occupy the vent but gases may escape past the dome plug and push out and down the side of the volcano.