Thursday 4 October 2012

Eruption on Mount Marapi, Sumatra.

Mount Marapi in, western Sumatra, is generally considered to be the island's most active volcano. It rises 2891 m above the sea, and has a broad summit with numerous, overlapping, craters inside a 1.4 km caldera. There have been over 50 eruptions since 1900, often with more than one in a year, though these generally only comprise small explosions and volcanic plumes; no lava flow outside the caldera has been recorded in historic times. Despite this the volcano can be dangerous, an unexpected eruption in 1992 killed a climber and injured a number of tourists.

A tourist inspecting a small eruption on Mount Marapi in August 2011. The localized nature of these events has occasionally lead people to become over-confident around them, leading to fatalities. Johann Angerler/Picasa.

The volcano produced a 600 m plume in May 2012, then was quiet until August, when it began to produce thick ash and smoke. By Tuesday 18 September this was reaching 200 m above the summit, although this was often hidden by mist and cloud. On 26 September this eruptive activity increased sharply, throwing up an ash plume that rose 1.5 km above the summit; the first plume of Marapi to exceed 1 km since August 2011. The alert level on the volcano remains high, but there are no plans for an evacuation since there are no permanent settlements inside what is considered to be the volcano's danger zone.

Maps showing the topography of Mount Marapi (top) and its location on Sumatra (bottom). Google Maps.

Sumatra lies on the Sunda Plate, immediately to the east of the Sunda Trench, along which the Indian Plate is being subducted. The subducting plate passes under Sumatra as it sinks into the Earth, leading to Earthquakes and volcanism. The quakes are caused by the plates (which are not smooth) constantly sticking together and then breaking apart as the pressure builds up. Volcanism is caused by the partial melting of the Indian Plate, due to the friction, pressure, and the heat of the Earth's interior. This leads to the formation of liquid magma, which then rises through the overlying Sunda Plate, feeding the volcanoes of Sumatra.

Diagrammatic representation of the subduction of the Indian Plate beneath Sumatra. Virtual Upper Mantle of the Earth.

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