Lahar

A lahar is a muddy casting of volcanic origin . It is mainly made of Eau, of volcanic ash and of Tephra S and thus meets generally on the slopes of the “gray volcanos” emitting of the Lave S andesitic. “Lahar” is a word of origin Indonesia.

The lahars are generally formed when important rains fall down on volcanic deposits. These deposits not being consolidated, they are easily eroded and carried in the rivers which they make overflow. When the volcanic deposits are recent and hot, the lahar can be extreme (up to 90 °C). It can contrary being very cold when water comes from the fusion of Glace or of Neige or when cold rains carry cooled deposits.

These castings, very dense and very doors, carry all that is on their passage: trees, bridges, cars, buildings, etc and can cart rock blocks several tens of tons. The lahars can traverse tens of kilometers at an high speed. The strong material density of the lahars makes that the beginning of cast can be appeared as a face compact creating a true wall of several meters in height and made of blocks, tree trunks and remains pushed by the flood. The composition of the lahars and their strong content of materials also confers a strong erosive capacity to them what makes them all the more dangerous because they then carry quickly grounds on which dwellings can be.

When casting is immobilized, it can leave important layers of deposits. It happens sometimes that while drying, the volcanic ash cements and forms a compact stone.

By their mode of formation, the lahars can affect an area of the years after the end of an volcanic eruption if the deposits are still not consolidated. The lahars are thus very dangerous and constitute the volcanic phenomenon most fatal in front of castings of Lave or the volcanic clouds.

In 1985 in Colombia the town of Armero and: 23000 of its inhabitants were buried under a lahar of eight meters height party of the slopes of the Nevado del Ruiz. These lahars was caused by the cast iron of the Icecap under the heat of the magma.

volcanologists developed a system detection of the lahars. Indeed the lahars, at the time of their progression, emit basic vibrations Fréquence generally ranging between 30 and 80 Hertz. These low frequencies are detected by an apparatus based on the same principle as the Sismographe S.

See too

Related articles

External bond

Short vidéos:
  • a short film of Maurice and Katia Kraft on the lahars
  • Sequence filmed of Maurice Kraft and Katia Kraft on the lahar of the Nevado del Ruiz of 1985.

Sources

  • Geography site
  • USGS

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