A fast-moving avalanche that occurs when hot volcanic ash and debris mix with air and flow down the side of a volcano.

A dense, hot, chaotic avalanche of rock fragments, gas, and ash that travels rapidly away from an explosive eruption column, down the flanks of the volcano (synonymous with 'ash flow'). Pyroclastic flows move at speeds ranging from 10 to several hundred meters per second and are typically at temperatures between 300 and 800 °C (Blong, 1984). Pyroclastic flows form either by collapse of the eruption column, or by failure of the front of a cooling lava dome. Once these flows are initiated, they may travel distances of several kilometers or more and easily override topographic obstacles in the flow path. A person could not outrun an advancing pyroclastic flow.

rock material thrown from a volcano at high speeds during an eruption.

A dense, hot, dry mixture of gas and incandescent rock fragments that travels rapidly along the ground surface, typically at speeds of hundreds of feet per second.

A rapid, extremely hot, downward stream of pyroclastics, air, gases, and ash ejected from an erupting volcano. A pyroclastic flow may be as hot as 800ºC or more and may move at speeds exceeding 150 kilometers per hour.

A volcanic deposit formed by the action of a lahar or blast or landslide of hot volcanic debris. The resulting consolidated rock is called an ash-flow tuff (if predominantly ash) or ash-flow breccia (if predominantly larger blocks).

a dense, high-concentration gas-pyroclast mixture that typically produces massive, poorly sorted deposits that pond in topographically low areas

a fluidized mass of rock fragments and gases that move rapidly down a volcano -- like hot batter down the sides of a waffle iron

a fluidized mixture of solid to semi-solid fragments and hot, expanding gases that flows down the flank of a volcanic edifice

a mixture of hot but solid lava fragments fluidized by expanding volcanic gases and heated air

a mixture of hot gas and volcanic rock particles

a mixture of hot gas, ash, rock, etc

a mixture of hot steam, ash, rock and dust

an avalanche of ground-hugging hot rock accompanied by a cloud of ash and gas that races down the slope of a volcano

an avalanche of hot ash, rock fragments, and volcanic gases that rush down the side of a volcano sixty miles per hour or more

a rapidly-moving mixture of hot, dry rock fragments, ash, and hot gases which knocks down, buries, or burns everything in its path

Mixtures of hot gas and ash travelling very quickly down the slope of a volcano.

a hot, fast-moving and high-density mixture of fine and coarse particles and gas formed during explosive eruptions or from the collapse of a lava dome.

a rapid flow of hot material consisting of ash, pumice, other rock fragments, and gas ejected by an explosive eruption.

A highly mobile avalanche [flow] of high-temperature volcanic (pyroclastic) debris (lava blocks and volcanic ash) and superheted volcanic gases and air. Pyroclastic flows are common products of eruptions at Montserrat and elsewhere.They are formed by dome collapses and by explosive eruptions - when they often additionally contain pumice blocks.In the latter case such flows are believed to form by the fountaining of high eruption columns and may have very high speeds. Velocities as high as 60 m/s have been measured on historic pyroclastic flows, and some are believed to travel as fast as 250 m/s. Owing to their high velocities and to fluidization by escaping gases, some pyroclastic flows may travel many tens of kilometres from the source vent, and may even travel large distances across the sea, as well documented at Krakatau in 1883. Pyroclastic flows which contain abundant pumice give rise to the deposits often called ignimbrites.

Lateral flowage of a turbulent mixture of hot gases and unsorted pyroclastic material (volcanic fragments, crystals, ash, pumice, and glass shards) that can move at high speed (50 to 100 miles an hour.) The term also can refer to the deposit so formed.

These are the most serious hazardous phenomena so far encountered. Pyroclastic flows have been formed both by avalanches from the dome and explosions. The flows consist of three parts. First the main avalanche of hot boulders and ash is confined to valleys and totally destroys most structures and kills people. Second the flows are forced to mix with air as they move and thus form an expanded turbulent hot cloud of ash and air above the main avalanche. These clouds are usually lethal to most people caught in them (typical mortality rates of 90%). They can move at hurricane speeds (about 100 kph), have measured temperatures of 150 to 250°C and badly damage most buildings. People that survive suffer severe burns. The clouds are up to 200 m high and so can spill out of valleys and more energetic clouds can move uphill. Third a pyroclastic flow forms buoyant plumes of very fine ash and hot air that rise above the flows to heights typically between 1 and 6 km. The ash plumes drift in the wind to deposit fine layers of ash.

A hot, fast moving and high-density (thick like wet concrete) mixture of ash, pumice, rock fragments and gas formed during explosive eruptions.

An extremely hot mixture of gas, ash and pumice fragments, that travels down the flanks of a volcano or along the surface of the ground at speeds of 50 to 100 miles per hour.

Hot, fast-moving avalanche of lava blocks, volcanic ash and gases. They can be formed both by explosions and by collapse of parts of an unstable lava dome.

a ground-hugging avalanche of hot ash, pumice, rock fragments, and volcanic gas that rushes down the side of a volcano as fast as 63 miles/hour or more

a body of ash which moves laterally (not up into the air) away from a volcanic eruption - nuee ardente (see above) being a severe example.

A hot, dry, fast-moving, and high-density mixture of of ash, pumice, rock fragments, and gas that formed during explosive eruptions or from the collapse of a lava dome.

A hot avalanche of ash, rock and gas. Flows often have a thick, dense lower part underneath a billowing, more dilute cloud of ash and hot air

A high-density mass of gases, hot ash, and larger material that flows rapidly down the sides of the volcano. Flows tend to be confined to valleys. Because of the speed at which they travel and the intense heat, pyroclastic flows and surges are one of the most dangerous hazards posed by volcanoes.

a suspension of solid particles in gases, whose density pulls it downward an keeps it somewhat distinct from surrounding air. This term has been applied to describe the vast dust clouds that issued from the collapses of the Twin Towers.

a turbulent flowing mass of ejected fragmental volcanic materials mixed with hot gases and moving downslope at high speed (about 60 kph or more). PF may result from the collapse of tall eruption columns or from spillover of ejected materials from erupting vents.

A hot, fast moving and high-density mixture of ash, pumice, rock fragments and gas formed during an eruption.

These are flows of volcanic fragments similar to avalanches of rock in landslides and snow avalanches. They can be formed both by explosions and by parts of an unstable lava dome avalanching.

A surface-hugging eruption cloud of very hot gas and volcanic particles that moves rapidly (up to 100 km/hr) across the ground surface, away from the vent.

A pyroclastic flow is an avalanche of pyroclastic materials (broken rocks, pumice, and ash) and hot gases that erupts from within a volcano. A pyroclastic flow travels at up to 100 miles per hour. Within the flow, temperatures can reach 500 degrees C.

Pyroclastic flows are a common and devastating result of some volcanic eruptions. They are fast-moving fluidized bodies of hot gas, ash and rock (collectively known as tephra) which can travel away from the vent at up to 150 km/h. The gas is usually at a temperature of 100-800 degrees Celsius.