USGS/Cascades Volcano Observatory, Vancouver, Washington
Pyroclastic Flows and Pyroclastic Surges
- Pyroclastic Flows and Pyroclastic Surges
- 1902 - Mont Pelée, Martinique, West Indies
- 1912 - Novarupta, Katmai, Alaska
- 1980 - Mount St. Helens, Washington
Pyroclastic Flows and Pyroclastic Surges
During the May 18, 1980 eruption, at least 17 separate pyroclastic flows descended the flanks of Mount St. Helens. Pyroclastic flows typically move at speeds of over 60 miles per hour (100 kilometers/hour) and reach temperatures of over 800 Degrees Fahrenheit (400 degrees Celsius). Photographed here, a pyroclastic flow from the August 7, 1980 eruption stretches from Mount St. Helens' crater to the valley floor below.
USGS Photograph taken on August 7, 1980, by Peter W. Lipman.
[medium size] ...
Myers and Brantley, 1995,
Volcano Hazards Fact Sheet: Hazardous Phenomena
at Volcanoes, USGS Open-File Report 95-231
High-speed avalanches of hot ash, rock
fragments, and gas move down the sides of a volcano during
explosive eruptions or when the steep edge of a dome breaks apart
and collapses. These pyroclastic flows,
which can reach 1500 degrees F and
move at 100-150 miles per hour, are capable of knocking down and
burning everything in their paths.
A more energetic and dilute
mixture of searing gas and rock fragments is called a
pyroclastic surge. Surges move easily up and over ridges;
flows tend to follow valleys.
From: Tilling, Topinka, and Swanson, 1990,
Eruptions of Mount St. Helens: Past, Present, and Future:
USGS General Interest Publication
The term "pyroclastic" - derived from the Greek words pyro (fire)
and klastos (broken) - describes materials formed by the fragmentation of
magma and rock by explosive volcanic activity. Most
is basically fine-grained pyroclastic material composed of tiny particles of
explosively disintegrated old volcanic rock or new magma.
Larger sized pyroclastic fragments are called
lapilli, blocks, or bombs.
- sometimes called nuees ardentes (French for "glowing clouds") - are
hot, often incandescent mixtures of volcanic fragments and gases that sweep
along close to the ground. Depending on the volume of material, proportion of
solids to gas, temperature, and slope gradient, the flows can travel at
velocities as great as 450 miles an hour. Pyroclastic flows can be extremely
destructive and deadly because of their high temperature and mobility.
During the 1902 eruption of Mont Pelée (Martinique, West Indies),
for example, a nuee ardente demolished the coastal city of St. Pierre,
killing nearly 30,000 inhabitants.
Pyroclastic flows commonly are produced either by the fallback
and downslope movement of fragments from an
or by the direct frothing over at the vent of magma
undergoing rapid gas loss. Volcanic froth so formed is called
Pyroclastic flows originated in both ways at
Mount St. Helens on May 18, but flows of mappable volume were of
the latter type. The flows were entirely restricted to a
small fan-shaped zone that flares northward from the summit
From: Foxworthy and Hill, 1982,
Volcanic Eruption of 1980 at Mount St. Helens: The First 100 Days,
USGS Professional Paper 1249
Pertaining to fragmented (clastic) rock material formed by a volcanic explosion
or ejection from a volcanic vent.
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 and hour). The term also can refer to the
deposit so formed.
Hoblitt, Miller, and Scott, 1987,
Volcanic Hazards with Regard to Siting Nuclear-Power Plants
in the Pacific Northwest, USGS Open-File Report 87-297
are high-density mixtures of hot, dry rock fragments and hot gases that move
away from their source vents at high speeds.
They may result from the explosive eruption of molten or solid
rock fragments, or both, or from the collapse of
vertical eruption columns of ash and larger rock fragments.
Pyroclastic flows may also result from a
laterally directed explosion,
or the fall of hot rock debris from a
dome or thick
Rock fragments in pyroclastic flows range widely in grain size
and consist of dense rock, pumice,
or both. Individual pyroclastic flows, worldwide,
range in length from less than one to more than 200 kilometers,
cover areas from less than one to more than 20,000 square kilometers,
and have volumes from less than 0.001 to more
than 1000 cubic kilometers.
Pumiceous pyroclastic flows with volumes of 1-10 cubic kilometers can
reach distances of several tens of kilometers
from a vent and travel downslope at speeds of 50 to more than
150 kilometers per hour, their
velocity depending largely on their volume and on the
steepness of slopes over which they travel.
Pyroclastic flows and their deposits commonly contain rock
debris and gases with temperatures of several
hundred degrees Celsius.
Most pyroclastic flows consist of two parts: a basal flow of
coarse fragments that moves along the
ground, and a turbulent cloud of finer particles (ash cloud)
that rises above the basal flow.
Ash may fall from the cloud over a wide
area downwind from the basal flow.
are turbulent, low-density clouds of
rock debris and air or other gases that move over the ground
surface at high speeds. They typically hug the ground
and depending on their density and speed, may or may not be
controlled by the underlying topography.
are of two types: "hot" pyroclastic surges that consist of "dry"
clouds of rock debris and gases that have temperatures
appreciably above 100 degrees C, and "cold" pyroclastic surges, also
called base surges, that consist of rock debris and steam
or water at or below a temperature of 100 degrees C.
Both hot and cold pyroclastic surges damage or destroy structures and
vegetation by impact of rock fragments moving at
high speeds and may bury the ground surface with a layer of
ash and coarser debris tens of centimeters or more thick.
Because of their high temperatures,
hot pyroclastic surges may start fires and kill or burn
people and animals. Both types of surges can extend as far
as 10 km from their source vents and devastate life and
property within their paths. During an eruption of
Mont Pelee on Martinique in 1902, a cloud of hot ash and gases swept
into the town of St. Pierre at an estimated speed of
160 kilometers per hour or more. About 30,000 people died within
minutes, most from inhalation of hot ash and gases.
Pyroclastic surges have occurred at volcanoes in the Cascade Range in
the past and can be expected to occur again.
Future cold surges (base surges) are most likely to occur where magma can
contact water at volcanic vents near lakes,
those that have crater lakes, and at vents in areas with a shallow water table.
Wright and Pierson, 1992,
Living With Volcanoes, The
U. S. Geological Survey's Volcano Hazards Program:
USGS Circular 1973, p.39
Pyroclastic flows and pyroclastic surges:
Mixtures of hot rock fragments and gases can sweep away from their source vents
at hurricane velocity. Pyroclastic flows are dense and most are confined to
valleys around a volcano; the largest ones can travel tens or even hundreds of
kilometers beyond a volcano. Pyroclastic surges are turbulent, low-density
variants of pyroclastic flows. Some unusually rapid pyroclastic flows or surges
originate from laterally directed explosions from a vent. Because of their high
speed and high temperature, pyroclastic flows and surges kill or destroy
virtually all that is in their path.
Potential Hazards from Future Volcanic Eruptions in California:
U. S. Geological Survey Bulletin 1847
Turbulent, low-density cloud of rock debris and water and (or) steam that moves
over the ground surface at high speed. Base surges are generated by explosions.
1902 - Mont Pelée, Martinique, West Indies
Tilling, 1985, Volcanoes:
USGS General Interest Publication
Mount Pelée in Martinique, West Indies, and Lassen Peak and Mono domes in California
are examples of lava domes. An extremely
destructive eruption accompanied the growth of a dome at Mount Pelée in 1902.
The coastal town of St. Pierre, about 4 miles downslope to
the south, was demolished and nearly 30,000 inhabitants were killed by an
incandescent, high-velocity ash flow and associated hot gases and
Only two men survived; one because he was in a poorly ventilated,
dungeon-like jail cell and the other who somehow made his
way safely through the burning city.
Mont Pelée Menu
1912 - Novarupta, Katmai, Alaska
U. S. National Park Service Website, Geology Fieldnotes -
Katmai National Park and Preserve, Alaska, April 2000
The June 1912 eruption of Novarupta Volcano altered the Katmai area dramatically.
Severe earthquakes rocked the area for a week before
Novarupta exploded with cataclysmic force.
Enormous quantities of hot, glowing pumice and ash were ejected from Novarupta and nearby
fissures. This material flowed over the terrain, destroying all life in its path.
Trees up slope were snapped off and carbonized by the blasts of hot
wind and gas. For several days ash, pumice, and gas were ejected and a
haze darkened the sky over most of the Northern Hemisphere. When
it was over, more than 40 square miles of lush green land lay buried
beneath volcanic deposits as much as 700 feet deep. At nearby Kodiak,
for two days a person could not see a lantern held at arm's length.
Acid rain caused clothes to disintegrate on clotheslines in distant Vancouver,
Canada. The eruption was ten times more forceful than the 1980 eruption of Mount St. Helens.
In the valleys of Knife Creek and the Ukak
River, innumerable small holes and cracks developed in the volcanic ash deposits,
permitting gas and steam from the heated groundwater to
escape. It was an apparently unnamed valley whehn the 20th century's
most dramatic volcanic episode took place. Robert Griggs, exploring the
volcano's aftermath for the National Geographic Society in 1916,
stared awestruck off Katmai Pass across the valley's roaring landscape
riddled by thousands of steam vents. The Valley of Ten Thousand Smokes, Griggs named. it.
1912 Novarupta Eruption
1980 - Mount St. Helens, Washington
Mount St. Helens Pyroclastic Flows Menu
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09/22/04, Lyn Topinka