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 (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 C.E. 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.