Pyroclastic flow
A pyroclastic flow (from the Greek πῦρ pyr 'fire' and κλαστός klastós, broken ') is a solid-gas dispersion that can occur, accompanied by explosive volcanic eruptions and moves very quickly downhill.
Emergence and impact
Pyroclastic flows occur in connection with felsic, ie quartz and feldspar, rare intermediate, but gas-rich magmas in each case and ashes. The term (starting from nuée ardente, which in French is synonymous with coulée pyroclastique = pyroclastic flow is ) was first used in connection with the outbreak of Pelé in 1902.
When magma in a volcano, then the pressure decreases, and the gas solubility in the magma thus decreases. As a result, gas bubbles, which, however, due to the toughness of the magma for the time being can not escape. By the rising gas pressure, the magma lying around the bladder paste- solidified and can no longer converge at an outlet of the gas, whereby a cavity is formed. The thick magma pushes up on each other, forming a so-called dome (also called lava dome or, in her forms, as referred Lavanadel ). At a certain height (from about 40 meters ) the viscous, semi-rigid structure is unstable and may collapse.
Upon exiting the vent can escape the gas dissolved in the magma. A pyroclastic flow occurs when doing rocks and the magma will be torn to very fine volcanic ash and they along with the exiting gases up to 700 km / h glide slope down, with an enormous destructive power is deployed. Even large bodies of water (eg, open sea water areas ) are easily overcome. At the outbreak of the Soufriere Hills on Montserrat currents could first be observed that spread across the sea. Inside the flow temperatures of between 300 and 800 ° C may be encountered, depending on the magnitude of the current. Pyroclastic flows pulverize everything storm tracks crossing; Explode or implode or building such as through the air in a whirlwind of unimaginable power. Ash and dust are a deadly danger in the vicinity of these streams.
Examples of historic pyroclastic flows
Pliny the Younger observed in 79 the eruption of Vesuvius and described a Plinian eruption. His depictions of themselves falling into the valley black cloud were not identified until late than pyroclastic flow. The deposits show that the eruption of Vesuvius were several pyroclastic flows. One of them reached Herculaneum and killed many people who had sought protection in boathouses. Another reached 18 hours after start of the outbreak, the further away from Mount Vesuvius Pompeii. Although his temperature of 300 ° Celsius killed people, but left their clothes largely undamaged.
After 1812, the Indonesian volcano Tambora was very active and reached its maximum in 1815 ( VEI - strength 7). In this outbreak 160 cubic kilometers of tephra were ejected, which provided in order for a global climate catastrophe with drastic decreases in temperature (up to 5.5 ° C).
On May 8, 1902, came at the Montagne Pelée for costliest eruption of the 20th century, which killed an estimated 29,000 people.
On 18 May 1980, the volcano Mount St. Helens broke out with a horizontal edge departure and unleashed a pyroclastic flow that devastated a 37 km wide and 30 km long fan-shaped area. In addition to 56 other people and the volcanologist David A. Johnston was killed. 9 people survived seriously injured. The United States Geological Survey had not reckoned with a direct, so massive pyroclastic flow, which has 1080 km / h and possibly even briefly reached the speed of sound, and therefore can identify too small a protection zone.
Especially notorious for its pyroclastic flows is the ounces in Japan. During his last active phase (1990-1995), he sent more than 175 of them in the valley. On 3 June 1991, the famous volcanologists Katia and Maurice Krafft died there beside 41 other people when shooting movies, as surprisingly went down a pyroclastic flow. Also the Soufriere on the Caribbean island of Montserrat is known for; as of 25 June 1997, numerous pyroclastic flows, which occurred up to the December 1997 destruction of the southern half of the island.
Definition of related terms
- A pyroclastic flow is a variant of the pyroclastic flow.
- A lahar is a mudflow triggered by lava. It is much colder than a pyroclastic flow and achieved flow rates of less than 100 km / h
- Pyroclastic flows are fundamentally distinguished from the lava flows. Pyroclastic flows generated by explosive eruptions and eruptive phases of volcanoes, lava flows, however, by effusive eruptions or effusive eruption phases.
- In volcanology pyroclastic surges and pyroclastic flows are differentiated (in the narrow sense). Flows and surges are distinguished by their density ( gas content ), in Surges can therefore speak of glowing clouds.