Pyroclastic flows are volcanic phenomena. A pyroclastic flow is a high-density mixture of hot, fragmented solids and expanding gases.

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1 This website would like to remind you: Your browser (Apple Safari 4) is out of date. Update your browser for more security, comfort and the best experience on this site. Encyclopedic Entry flow block-and-ash flow, nuée ardente, pumice flow For the complete encyclopedic entry with media resources, visit: Pyroclastic flows are volcanic phenomena. A flow is a high-density mixture of hot, fragmented solids and expanding gases. These heavier-than-air flows race down the sides of a volcano much like an avalanche. Reaching speeds greater than 100 kilometers per hour (60 miles per hour) and temperatures between 200 and 700 Celsius (392 and 1292 Fahrenheit), flows are considered the most deadly of all volcano hazards. The world pyroclast is derived from the Greek pyr, meaning fire, and klastos, meaning broken in pieces. A flow s broken pieces consist of volcanic glass, crystals, and rocks such as pumice or scoria. These solids have been heated and fragmented by an explosive eruption. Heavier fragments roll downward along the ground, while smaller fragments float in a stream of hot gases. Through the process of convection, the hot gases of a flow expand and rise above the mass of denser and cooler materials on the ground. This rapidly expanding mixture of gas and suspended particles creates dense, clouds of volcanic ash that move fluidly over the landscape. Pyroclastic Surges All flows are incredibly fast-moving and lethally hot. Those that contain more gases and less solid materials are known as surges. A cold surge is one with a slightly lower temperature, usually below 100 Celsius (212 Fahrenheit). Cold surges often form where a volcano s vent is beneath a lake or the ocean. A hot surge is one with a slightly higher temperature, usually above 100 Celsius (212 Fahrenheit). How Flows and Surges Form Pyroclastic flows and surges are composed of different materials, and move in different ways depending on how they are formed. Some forms develop after an eruption collapses a volcano s hardened lava dome, whose dense rock then avalanches down the volcano. Within seconds, a faster-moving cloud of ash expands above and in front of the tumbling blocks of rock. These flows are known as block-and-ash flows because of their dual composition. The French geologist Alfred Lacroix originally created the term nuée ardente ( glowing cloud ) for these flows after the 1902 eruption of Mount Pelée caused its lava dome to collapse and sweep down into the city of St. Pierre, Martinique, killing almost all of its 30,000 residents. Other flows result from the collapse of an eruption column, the vertical mass of debris and gas that jets above an 1 of 5

2 explosive volcano vent. Heavy debris falls rapidly from the sky and flows down the flanks of the volcano, mostly as pumice. In fact, this type of flow is sometimes known as a pumice flow. The higher the volcanic debris is thrust into the air, the further it will fall by force of gravity, gaining momentum along the way. For this reason, pumice flows are able to cover larger areas faster than block-and-ash flows. Like block-and-ash flows, pumice flows are made up of a main body of moving rocks that hugs the ground and an ash cloud that expands above it. Pumice flows, however, also include a ground surge of burning ash that advances ahead of the moving rocks. These jets of hot ash heat the air at the front of the flow. This rapid heating of air causes the flow to increase in size and speed, hurling fragmented materials forward at an even faster rate than before. Pyroclastic flows can even move over water. The 1815 eruption of Mount Tambora, Indonesia, is considered the largest volcanic eruption in recorded history. Its eruption column shot 40 kilometers (25 miles) into the atmosphere. This huge column collapsed into numerous pumice flows that reached more than 160 kilometers per hour (100 miles per hour). These fast, hot flows traveled 40 kilometers (25 miles) across the surface of the Flores Sea, causing the ocean to boil and create steam explosions. Pyroclastic Flow Hazards Pyroclastic flows are so fast and so hot that they can knock down, shatter, bury, or burn anything in their path. Even small flows can destroy buildings, flatten forests, and scorch farmland. Pyroclastic flows leave behind layers of debris anywhere from less than a meter to hundreds of meters thick. The 1991 eruption of Mount Pinatubo, Philippines, filled the Marella River valley with a flow 200 meters (656 feet) deep, more than the height of the Washington Monument. When flows mix with water, they create dangerous liquid landslides called lahars. The 1985 eruption of Nevado del Ruiz in Colombia caused flows to mix with melted snow and flow down into the surrounding river valleys. These lahars gained momentum and size as they traveled the river beds, ultimately destroying more than 5,000 homes and killing more than 23,000 people. A flow s deadly mixture of hot ash and toxic gases is able to kill animals and people. The famous 79 CE eruption of Mount Vesuvius buried the nearby cities of Pompeii and Herculaneum, Italy, in fallout, killing about 13,000 people. While many scientists once thought that the residents of Pompeii and Herculaneum suffocated from the fallout of Mount Vesuvius eruption, new studies suggest that they actually died from extreme heat. Volcanologist Giuseppe Mastrolorenzo and the Italian National Institute for Geophysics and Volcanology recently discovered that the flow that reached Pompeii produced temperatures of up to 300 Celsius (570 Fahrenheit). These extreme temperatures are able to kill people in a fraction of a second, effectively forcing them to spasm in contorted postures, like those found amongst the plaster casts of Vesuvius victims. VOCABULARY Term Part of Speech Definition advance verb to move forward or progress. atmosphere layers of gases surrounding a planet or other celestial body. avalanche large mass of snow and other material suddenly and quickly tumbling down a mountain. boil verb to change from a liquid to a gaseous state. cast impression formed when a liquid substance is poured into a form or mold, and then hardens into that shape. city large settlement with a high population density. collapse verb to fall apart completely. 2 of 5

3 compose verb to be made of. composition arrangement of the parts of a work of art in relation to each other and to the whole. contort verb to distort or bend out of shape. convection transfer of heat by the movement of the heated parts of a liquid or gas. crystal type of mineral that is clear and, when viewed under a microscope, has a repeating pattern of atoms and molecules. debris remains of something broken or destroyed; waste, or garbage. dense adjective having parts or molecules that are packed closely together. density number of things of one kind in a given area. derive verb to come from a specific source or origin. destroy verb to ruin or make useless. dual adjective having to do with two of something. eruption release of material from an opening in the Earth's crust. eruption column cylinder-shaped structure of volcanic ash and gas emitted by an explosive volcanic eruption. expand verb to grow or get larger. explosion violent outburst; rejection, usually of gases or fuel extreme adjective unusual or extraordinary. farmland area used for agriculture. flank side of something. fluid material that is able to flow and change shape. forest ecosystem filled with trees and underbrush. form verb to make or take shape. fraction portion or section. fragment piece or part. gas state of matter with no fixed shape that will fill any container uniformly. Gas molecules are in constant, random motion. geologist person who studies the physical formations of the Earth. geophysics study of the Earth's physical properties and processes. gravity physical force by which objects attract, or pull toward, each other. lahar flow of mud and other wet material from a volcano. lake body of water surrounded by land. landscape the geographic features of a region. landslide the fall of rocks, soil, and other materials from a mountain, hill, or slope. lava dome feature formed as lava hardens over a volcanic vent. 3 of 5

4 lethal adjective deadly. liquid state of matter with no fixed shape and molecules that remain loosely bound with each other. momentum speed, direction, or velocity at which something moves. ocean large body of salt water that covers most of the Earth. particle small piece of material. phenomena plural (singular: phenomenon) any observable occurrence or feature. plaster paste-like material made of crushed stone (usually lime, gypsum, and sand), water, and fiber. pumice type of igneous rock with many pores. fallout flow surge particles that have been ejected from volcanic vents and have traveled through the atmosphere before falling to earth or into water. current of volcanic ash, lava, and gas that flows from a volcano. fluid mass of gas and rock ejected during some explosive volcanic eruptions. resident person who lives in a specific place. river bed material at the bottom of a river. river valley depression in the earth caused by a river eroding the surrounding soil. rock natural substance composed of solid mineral matter. scorch verb to destroy by burning. scoria type of rough, crusty volcanic rock. shatter verb to suddenly break into pieces. snow precipitation made of ice crystals. spasm verb to undergo a sudden, involuntary contraction of a muscle or group of muscles. steam water vapor. stream body of flowing fluid. suffocate verb to be unable to breathe. suspend verb to keep from falling, sinking, or collecting. temperature degree of hotness or coldness measured by a thermometer with a numerical scale. thrust force exerted by a propeller, gas, or other mechanism that propels a vehicle. toxic adjective poisonous. vent crack in the Earth's crust that spews hot gases and mineral-rich water. vertical up-down direction, or at a right angle to Earth and the horizon. volcanic ash fragments of lava less than 2 millimeters across. 4 of 5

5 volcanic glass hard, brittle substance produced by lava cooling very quickly. volcano an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions. volcano hazard specific danger posed by an active volcano: gas, lahar, landslide, lava flow, flow, or tephra. volcanologist scientist who studies volcanoes. volcanology the study of volcanoes. Also called vulcanology. For Further Exploration Articles & Profiles National Geographic Kids: Ten Freaky Forces of Nature USGS: Pyroclastic Flows and their Effects San Diego State University: How Volcanoes Work Pyroclastic Flows Video Geoff Mackley: Pyroclastic flows on Mt Merapi Java Indonesia National Geographic Society. All rights reserved. 5 of 5

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