Authorities on the Indonesian island of Sumatra have begun to evacuate people from four villages to the south and southeast of Mount Sinabung, an active volcano on the north of the island, following warnings from the Badan Nasional Penangulanggan Bencana (Indonesia's Disaster Mitigation Agency) after a sharp increase in volcanic activity on Thursday 4 June 2015, including clouds of hot and potentially lethal ash which were passing over the villages. 963 people have been evacuated from the village of Tiga Pancur, 1108 from Gurukinayan, 256 from Pintu Besi and 400 from Berastepu. Since the onset of the current bout of volcanic activity on Mount Sinabung in August 2010, around 10 000 people have been forced to evacuate homes in surrounding villages, though many of these have later been able to return when periods of intense activity have subsided.
An ash-flow on Mount Sinabung with villages in the foreground. Anka Haber.
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 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.
See also...
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