Wednesday 8 January 2014

Further evacuations on North Sumatra as Mount Sinabung continues to erupt.

The number of people evacuated from villages around Mount Sinabung, an active volcano on North Sumatra, has risen to around 20 000 following a series of about 50 eruptions over the first weekend of 2014, which produced an ash column 4 km high and lava flows reaching 5 km southeast from the volcano's crater. Over 6000 people had already been evacuated from villages closer to the volcano, following a series of eruptions that began in September last 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.

Lava flow on Mount Sinabung on Sunday 5 January 2014. Dedi Sahputra/European Pressphoto Agency.

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 approximate location of Mount Sinabung. Google Maps.

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.

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