Sunday 24 November 2013

Thousands evacuated following series of eruptions on Mount Sinabung, Sumatra.

Over 6300 people have been evacuated following a series of nine eruptions on Mount Sinabung, northern Sumatra, Indonesia on Saturday 23-Sunday 24 November 2013, according to the Badan Nasional Penangulanggan Bencana (Indonesia's Disaster Mitigation Agency). The volcano at its most active produced an ash column over 8000 m high and rained ash and rock fragments down on the surrounding countryside. Around 600 people had already been evacuated from villages closer to the volcano, following a series of eruptions that began in September this year, but this new surge of activity, combined with the onset of the rainy season and accompanying increased risk of lahars (ash-laden flash floods associated with volcanoes), has led to a widening of the evacuation area.

Eruption on Mount Sinabung. Ulet Ifansasti/Getty Images.

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 neighboring 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, fueling the volcanoes of Sumatra.

The Subduction zone beneath Sumatra. NASA/Earth Observatory.

This does not happen at a 90° angle, 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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