When volcanoes do produce lava flows they are classified as either Pahoehoe or Aa. The word pyroclastic comes from a Greek word that means "Rock broken by fire". Some volcanoes eject pyroclasts, which are fragmented or broken rock. Some volcanoes produce little or no lava. A pyroclastic flow killed couple Katia and Maurice Krafft, French volcanologists on Mount Unzen, in Japan, on June 3 1991, and in June 1997 flows killed twenty people on the Caribbean island of Montserrat.Lava is melted rock that has reached the Earth's surface through a volcano's main vent or through side vents and fissures. For example, the towns of Pompeii and Herculaneum in Italy were famously engulfed by pyroclastic flows in 79 AD with heavy loss of life. The hot gases and high speed make them particularly lethal. While moving, the kinetic energy of the boulders will flatten trees and buildings in their path. Flows usually consist of two parts: the basal flow hugs the ground and contains larger, coarse boulders and rock fragments, while an ash plume lofts above it because of the turbulence between the flow and the overlying air, admixes and heats cold atmospheric air causing expansion and convection. Most pyroclastic flows are around one to ten cubic kilometres and travel for several kilometres. Volumes range from a few hundred cubic meters to more than a thousand cubic kilometers, and the larger ones can travel for hundreds of kilometres although none on that scale have occurred for several hundred thousand years. Helens) This rapidly transforms into a gravity driven current with distance from the volcano. The directional blast (or jet) when part of a volcano explodes or collapses (e.g.Gravitational collapse of a lava dome or spine, with subsequent avalanching and flow down a steep slope (e.g. This occurred during the eruption of Mount Katmai in 1912 which produced the largest flows to be generated during recorded history. This can lead to the production of a type of igneous rock called ignimbrite.
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