Wednesday 2 August 2017

Eruptions on Mount Sinabung lead to significant ashfalls.

A series of severe ashfalls have been recorded following a series of eruptions on Mount Sinabung in North Sumatra on Wednesday 2 August 2017. The volcano has been relatively inactive for the past few months, but sprang back into life this week with nineteen eruptions recorded on Wednesday morning. The eruptions produced an ash column that rose 4.2 km above the summit of the volcano, as well as ash fall affecting ten villages, up to 4.5 km from the mountain. One school has been shut due to the danger presented by the ash, and concerns have been raised about ash entering and damming the Laborus River, a phenomenon which can lead to ash-laden flash floods called lahars.

Eruption on Mount Sinabung on 2 August 2017, seen from the village of Karo.  Endro Rusharyanto/AP.

Mount Sinabung, a 2460 m stratovolcano (cone shaped volcano made up of layers of ash and lava) in the Karo Regency; it is potentially a very dangerous volcano, as a large number of people live in its immediate vicinity. The last major eruption prior to the twenty-first century happened in about 1600, with small eruptions occurring in 1889 and 1912. However the volcano returned to life in late August 2010, erupting throughout September and causing about 12 000 people to flee their homes.
 The location of Mount Sinabung. Google Maps.
The Indo-Australian Plate, which underlies the Indian Ocean to the west of Sumatra, is being subducted beneath the Sunda Plate, a breakaway part of the Eurasian Plate which underlies Sumatra and neighbouring Java, along the Sunda Trench, passing under Sumatra, where friction between the two plates can cause Earthquakes. As the Indo-Australian Plate sinks further into the Earth it is partially melted and some of the melted material rises through the overlying Sunda Plate as magma, fuelling the volcanoes of Sumatra.
 The Subduction zone beneath Sumatra. NASA/Earth Observatory.
The two plates are not directly impacting one-another, as occurs in the subduction zones along the western margins of North and South America, but at a steeply oblique angle. This means that as well as the subduction of the Indo-Australian plate beneath the Sunda, the two plates are also moving past one-another. This causes rifting within the plates, as parts of each plate become stuck to the other, and are dragged along in the opposing plate's direction. The most obvious example of this is the Sumatran Fault, which runs the length of Sumatra, with the two halves of the island moving independently of one-another. This fault is the cause of most of the quakes on the island, and most of the island's volcanoes lie on it.
 The movement of the tectonic plates around Sumatra. NASA/Earth Observatory.
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