Pyroclastic flow

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Pyroclastic flow at Merapi in Indonesia
Pyroclastic rock

A pyroclastic current (from ancient Greek πῦρ pyr , German 'fire' and κλαστός klastós , German 'broken' ) is a solid-gas dispersion that can occur in the presence of explosive volcanic eruptions and that moves very quickly down the slope.

Origin and impact

Pyroclastic currents occur in connection with felsic, i.e. quartz - and feldspar-rich , less often intermediate, but in any case gas-rich magmas and ash. The term (based on nuée ardente , which is synonymous with coulée pyroclastique , pyroclastic flow in French ) was first used in connection with the eruption of the Pelée in 1902.

When magma rises in a volcano , the pressure drops and the solubility of gas in the magma decreases. As a result, gas bubbles form, which, however, cannot escape due to the viscosity of the magma. Due to the increasing gas pressure, the magma around the bladder solidifies in the form of a paste and can no longer flow together when the gas escapes, creating a cavity. The viscous magma pushes on top of each other and forms a so-called dome (also known as a lava dome or, with more pointed shapes, as a lava needle ). From a certain height (around 40 meters) the viscous, semi-rigid structure becomes unstable and can collapse.

The gas dissolved in the magma can escape when it exits the chimney. A pyroclastic flow occurs when rocks and magma are torn into particularly fine volcanic ash and slide down the slope together with the escaping gases at speeds of up to 700 km / h, unfolding an enormous destructive force. Even large areas of water (e.g. open sea water areas) can be overcome effortlessly. During the eruption of the Soufrière Hills on Montserrat , currents could be observed for the first time, which spread over the sea. Temperatures between 300 and 800 ° C can prevail inside the stream  , depending on the size of the stream. Pyroclastic currents destroy everything on their way, including buildings. Ashes and dust are deadly even near these currents.

Examples of historical pyroclastic flows

Cooled pyroclastic current on Pinatubo

Pliny the Younger observed the eruption of Vesuvius in 79 and described a Plinian eruption . His depictions of a black cloud plunging into the valley were later identified as a pyroclastic flow. The deposits show that several pyroclastic currents occurred when Vesuvius erupted. One of them reached Herculaneum and killed many people who had taken shelter in boat houses. Another reached Pompeii, further away from Vesuvius, 18 hours after the eruption began . Its temperature of 300 degrees Celsius killed the people, but left their clothing largely undamaged.

After 1812, the Indonesian volcano Tambora became very active and reached its maximum in 1815 ( VEI strength 7). During this outbreak, 160 cubic kilometers of pyroclastics were ejected, which subsequently caused a global climate catastrophe with drastic temperature drops (up to 5.5 ° C) (“ year without summer ”).

On May 8, 1902, on the Montagne Pelée in the Caribbean, the most costly eruption of the 20th century occurred, killing an estimated 29,000 people.

On May 18, 1980, the volcano Mount St. Helens erupted in the USA with a horizontal flank eruption and released a pyroclastic current that devastated a 37 kilometers wide and 30 kilometers long fan-shaped area. In addition to 56 other people, the volcanologist David A. Johnston was killed. Nine people survived seriously injured. The United States Geological Survey had not reckoned with such a huge pyroclastic current, which reached 1080 km / h and possibly even the speed of sound for a short time , and therefore had a protection zone that was too small.

The ounce in Japan is particularly notorious for its pyroclastic currents . During his last active phase (1990–1995) he sent over 175 of them into the valley. On June 3, 1991, 41 other people, the famous volcanologists Katia and Maurice Krafft , died there while filming when a pyroclastic current suddenly fell. The Soufrière on the Caribbean island of Montserrat is also known for it; From June 25, 1997, numerous pyroclastic currents, which occurred until December 1997, led to the destruction of the southern half of the island.

On September 29, 2014, mountain hikers were surprised by a pyroclastic flow on the Ontake-san volcano in Japan. Several injured people were rescued and over 55 dead were recovered.

Delimitation of related terms

Fire avalanche on Mayon ( Philippines )
  • A glow avalanche is a variant of the pyroclastic flow.
  • A lahar is a mudslide triggered by lava. It is significantly colder than a pyroclastic stream and reaches flow speeds of less than 100 km / h.
  • Pyroclastic flows are fundamentally differentiated from lava flows . Pyroclastic flows arise from explosive eruptions or eruption phases of volcanoes, lava flows, however, from effusive eruptions or in effusive eruption phases.
  • In volcanology , a distinction is made between pyroclastic surges and pyroclastic currents (in the narrower sense). Currents and surges differ in their density (gas content), so surges can be referred to as glowing clouds .


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Web links

Commons : Pyroclastic Current  - collection of images, videos and audio files
Wiktionary: pyroclastic  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. Pyroclastic flows. In: Retrieved September 13, 2017 (English).
  2. Pliny: epistulae 6, 16, 6 ; 6, 20, 11 .
  3. Documentation Elmar Bartlmae: The climate catastrophe of 1816. Sent to arte on April 16, 2011.
  4. Report of the Smithsonian to the outbreak of Ontakesan