• 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 Surges
    All 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 Form
    Pyroclastic 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 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 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. 
    The Hazards of Pyroclastic Flows
    Pyroclastic flows are one of the most dangerous volcanic hazards.
    “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.

    atmosphere Noun

    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.

    cast Noun

    impression formed when a liquid substance is poured into a form or mold, and then hardens into that shape.

    city Noun

    large settlement with a high population density.

    collapse Verb

    to fall apart completely.

    compose Verb

    to be made of.

    composition Noun

    arrangement of the parts of a work or structure in relation to each other and to the whole.

    contort Verb

    to distort or bend out of shape.

    convection Noun

    transfer of heat by the movement of the heated parts of a liquid or gas.

    crystal Noun

    type of mineral that is clear and, when viewed under a microscope, has a repeating pattern of atoms and molecules.

    debris Noun

    remains of something broken or destroyed; waste, or garbage.

    dense Adjective

    having parts or molecules that are packed closely together.

    density Noun

    number of things of one kind in a given area.

    Encyclopedic Entry: density
    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 Noun

    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.

    expand Verb

    to grow or get larger.

    explosion Noun

    violent outburst; rejection, usually of gases or fuel

    extreme Adjective

    unusual or extraordinary.

    farmland Noun

    area used for agriculture.

    flank Noun

    side of something.

    fluid Noun

    material that is able to flow and change shape.

    forest Noun

    ecosystem filled with trees and underbrush.

    form Verb

    to make or take shape.

    fraction Noun

    portion or section.

    fragment Noun

    piece or part.

    gas Noun

    state of matter with no fixed shape that will fill any container uniformly. Gas molecules are in constant, random motion.

    geologist Noun

    person who studies the physical formations of the Earth.

    geophysics Noun

    study of the Earth's physical properties and processes.

    gravity Noun

    physical force by which objects attract, or pull toward, each other.

    lahar Noun

    flow of mud and other wet material from a volcano.

    lake Noun

    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.

    lethal Adjective


    liquid Noun

    state of matter with no fixed shape and molecules that remain loosely bound with each other.

    momentum Noun

    speed, direction, or velocity at which something moves.

    ocean Noun

    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.

    plaster Noun

    paste-like material made of crushed stone (usually lime, gypsum, and sand), water, and fiber.

    pumice Noun

    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: The Hazards of Pyroclastic Flows
    pyroclastic surge Noun

    fluid mass of gas and rock ejected during some explosive volcanic eruptions.

    resident Noun

    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.

    rock Noun

    natural substance composed of solid mineral matter.

    scorch Verb

    to destroy by burning.

    scoria Noun

    type of rough, crusty volcanic rock.

    shatter Verb

    to suddenly break into pieces.

    snow Noun

    precipitation made of ice crystals.

    spasm Verb


    to undergo a sudden, involuntary contraction of a muscle or group of muscles.

    steam Noun

    water vapor.

    stream Noun

    body of flowing fluid.

    suffocate Verb

    to be unable to breathe.

    suspend Verb

    to keep from falling, sinking, or collecting.

    temperature Noun

    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.

    toxic Adjective


    vent Noun

    crack in the Earth's crust that spews hot gases and mineral-rich water.

    vertical Noun

    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: Human and Environmental Impacts of Volcanic Ash
    volcanic glass Noun

    hard, brittle substance produced by lava cooling very quickly.

    volcano Noun

    an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.

    Encyclopedic Entry: Plate Tectonics and Volcanic Activity
    volcano hazard Noun

    specific danger posed by an active volcano: gas, lahar, landslide, lava flow, pyroclastic flow, or tephra.

    volcanologist Noun

    scientist who studies volcanoes.

    volcanology Noun

    the study of volcanoes. Also called vulcanology.