Eleven confirmed dead and twelve still missing following eruption on Mount Marapi, Sumatra.

Eleven people have been confirmed dead and another twelve are still missing following an eruption on Mount Marapi, a 2885 m high volcano in West Sumatra Province, Indonesia on Sunday 3 December 2023. The volcano erupted with little warning, producing a 3 km high column of ash as well as ash falls on villages more than 5 km from the summit of the volcano. Seventy five people, including a group of 18 students, were on the volcano at the time of the eruption. Forty nine people of the hikers were able to escape from the mountain promptly, leaving 26 unaccounted for. Over 160 rescue workers were deployed to the mountain, and were able to rescue three of the missing hikers alive, all of who were suffering from burns and one from a broken leg. They also recovered eleven bodies, but were eventually forced to break of the search before accounting for all of the missing persons, due to the ongoing eruption.

A survivor of the 3 December 2023 Mount Marapi eruption being carried off the mountain by rescue workers. Antara Foto/Reuters.

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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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.

See also Eruptions on Krakatau, Large Earthquake to the south of East Java, Seismic activity beneath Mount Tangkubanparahu, Indian Ocean shaken by a series of major Earthquakes and Volcanoes on Sciency Thoughts YouTube.

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