Pyroclastic flows are volcanic phenomena. A pyroclastic 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), pyroclastic 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 pyroclastic 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 pyroclastic 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 SurgesAll pyroclastic flows are incredibly fast-moving and lethally hot. Those that contain more gases and less solid materials are known as pyroclastic 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 FormPyroclastic flows and pyroclastic surges are composed of different materials, and move in different ways depending on how they are formed.Some pyroclastic 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 pyroclastic 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 pyroclastic flows result from the collapse of an eruption column, the vertical mass of debris and gas that jets above an 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 HazardsPyroclastic 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 pyroclastic flow 200 meters (656 feet) deep, more than the height of the Washington Monument.When pyroclastic flows mix with water, they create dangerous liquid landslides called lahars. The 1985 eruption of Nevado del Ruiz in Colombia caused pyroclastic 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 pyroclastic 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 pyroclastic fallout, killing about 13,000 people.While many scientists once thought that the residents of Pompeii and Herculaneum suffocated from the pyroclastic 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 pyroclastic 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.Sandblasting“Pyroclastic flows may look like fluffy clouds, but they are more like sandblasting,” says volcanologist Benjamin Andrews. Andrews simulates pyroclastic flows using baby powder, walnut shells, and glass beads. Lasers allow him to study the dust currents left by the simulated flows, which helps other volcanologists estimate the paths or behavior of pyroclastic flows.
Term Part of Speech Definition Encyclopedic Entry advance Verb
to move forward or progress.
layers of gases surrounding a planet or other celestial body.
Encyclopedic Entry: atmosphere avalanche Noun
large mass of snow and other material suddenly and quickly tumbling down a mountain.
Encyclopedic Entry: avalanche boil Verb
to change from a liquid to a gaseous state.
impression formed when a liquid substance is poured into a form or mold, and then hardens into that shape.
large settlement with a high population density.
to fall apart completely.
to be made of.
arrangement of the parts of a work or structure in relation to each other and to the whole.
to distort or bend out of shape.
transfer of heat by the movement of the heated parts of a liquid or gas.
type of mineral that is clear and, when viewed under a microscope, has a repeating pattern of atoms and molecules.
remains of something broken or destroyed; waste, or garbage.
having parts or molecules that are packed closely together.
number of things of one kind in a given area.
Encyclopedic Entry: density derive Verb
to come from a specific source or origin.
to ruin or make useless.
having to do with two of something.
release of material from an opening in the Earth's crust.
eruption column Noun
cylinder-shaped structure of volcanic ash and gas emitted by an explosive volcanic eruption. Also called a volcanic plume.
to grow or get larger.
violent outburst; rejection, usually of gases or fuel
unusual or extraordinary.
area used for agriculture.
side of something.
material that is able to flow and change shape.
ecosystem filled with trees and underbrush.
to make or take shape.
portion or section.
piece or part.
state of matter with no fixed shape that will fill any container uniformly. Gas molecules are in constant, random motion.
person who studies the physical formations of the Earth.
study of the Earth's physical properties and processes.
physical force by which objects attract, or pull toward, each other.
flow of mud and other wet material from a volcano.
body of water surrounded by land.
Encyclopedic Entry: lake landscape Noun
the geographic features of a region.
Encyclopedic Entry: landscape landslide Noun
the fall of rocks, soil, and other materials from a mountain, hill, or slope.
Encyclopedic Entry: landslide lava dome Noun
feature formed as lava hardens over a volcanic vent.
state of matter with no fixed shape and molecules that remain loosely bound with each other.
speed, direction, or velocity at which something moves.
large body of salt water that covers most of the Earth.
Encyclopedic Entry: ocean particle Noun
small piece of material.
phenomena Plural Noun
(singular: phenomenon) any observable occurrence or feature.
paste-like material made of crushed stone (usually lime, gypsum, and sand), water, and fiber.
type of igneous rock with many pores.
pyroclastic fallout Noun
particles that have been ejected from volcanic vents and have traveled through the atmosphere before falling to earth or into water.
pyroclastic flow Noun
current of volcanic ash, lava, and gas that flows from a volcano.
Encyclopedic Entry: pyroclastic flow pyroclastic surge Noun
fluid mass of gas and rock ejected during some explosive volcanic eruptions.
person who lives in a specific place.
river bed Noun
material at the bottom of a river.
river valley Noun
depression in the earth caused by a river eroding the surrounding soil.
natural substance composed of solid mineral matter.
to destroy by burning.
type of rough, crusty volcanic rock.
to suddenly break into pieces.
precipitation made of ice crystals.
to undergo a sudden, involuntary contraction of a muscle or group of muscles.
body of flowing fluid.
to be unable to breathe.
to keep from falling, sinking, or collecting.
degree of hotness or coldness measured by a thermometer with a numerical scale.
Encyclopedic Entry: temperature thrust Noun
force exerted by a propeller, gas, or other mechanism that propels a vehicle.
crack in the Earth's crust that spews hot gases and mineral-rich water.
up-down direction, or at a right angle to Earth and the horizon.
volcanic ash Noun
fragments of lava less than 2 millimeters across.
Encyclopedic Entry: volcanic ash volcanic glass Noun
hard, brittle substance produced by lava cooling very quickly.
an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.
Encyclopedic Entry: volcano volcano hazard Noun
specific danger posed by an active volcano: gas, lahar, landslide, lava flow, pyroclastic flow, or tephra.
scientist who studies volcanoes.
the study of volcanoes. Also called vulcanology.