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.
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.
advance
Verb

to move forward or progress.

Noun

layers of gases surrounding a planet or other celestial body.

Noun

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
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.

Noun

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
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.

Noun

body of water surrounded by land.

Noun

the geographic features of a region.

Noun

the fall of rocks, soil, and other materials from a mountain, hill, or slope.

lava dome
Noun

feature formed as lava hardens over a volcanic vent.

lethal
Adjective

deadly.

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.

Noun

large body of salt water that covers most of the Earth.

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.

Noun

current of volcanic ash, lava, and gas that flows from a volcano.

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.

Noun

degree of hotness or coldness measured by a thermometer with a numerical scale.

thrust
Noun

force exerted by a propeller, gas, or other mechanism that propels a vehicle.

toxic
Adjective

poisonous.

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.

Noun

fragments of lava less than 2 millimeters across.

volcanic glass
Noun

hard, brittle substance produced by lava cooling very quickly.

Noun

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

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.