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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Eruptions on Mount Sangay, Ecuador.

The Instituto Geofísico-Escuela Politécnica Nacional recorded an increase in seismic activity (Earthquakes) beneath Mount Sangay, a 5300 m stratovolcano (cone-shaped volcano made up of layers of ash and lava) in the central part of the country, on 2 September 2020. This was followed by a number of explosive eruptions, producimg ash columns that rose to as much as 1500 m above the summit of the volcano, triggering a warning to aviation from the Washington Volcanic Ash Advisory Center. A pyroclastic flow (avalanch of hot gas and ash) was observed on the southeastern flank of the volcano. Since then the volcano has undergone a series of explosive eruptions, which have produced ash columns that have reached up to 12 200 m above sealevel, resulting in ashfalls across six of Ecaudor's twenty four provinces, and a number of lahars (violent mudflows composed of a slurry of pyroclastic material).

 

An eruption on Mount Sangay on 16 September 2020. Instituto Geofísico-Escuela Politécnica Nacional.

Mount Sangay is located on the eastern margin of the Andean Cordillera, and is the southernmost and most active of of Ecuador's volcanoes, having been more-or-less constant active since 1934. Due to this high level of activity nobody lives close to the volcano, which is surrounded by the Sangay National Park. This makes the volcano less immediately dangerous than less active volcanoes which people are more inclined to live close to. However, the current Mount Sangay, which is about 14 000 years old, sits on the site of at least two previous volcanoes, which are thought to have formed around 500 000 and 250 000 years ago, each of which was destroyed in a massive Krakatoa-type explosion, which would have caused devastation over a far wider area (though such an event would be unlikely to happen today without significant fore-warning).

 

Streets covered in volcanic ash from Mount Sangay in the town of Alausí in Chimborazo Province, Ecuador, on 20 September 2020. William Briggs/Twitter.

Like all South American volcanoes Mount Sangay owes its existence to the subduction of the Nazca Plate (which underlies the southeast Pacific) beneath South America. The Nazca Plate is being pushed from the east and forced down into the Earth's interior beneath South America. As it sinks rocks in the crust melt, and the lighter portions of it rise up through the overlying South American Plate to form volcanoes at the surface. These are dotted throughout the Andes Mountains; a range of mountains that is formed by a mixture of volcanism and crumpling of the South American Plate where is is forced against the Nazca Plate.

 

The subduction of the Nazca Plate beneath the South American Plate. Marot et al. (2012).

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Eruptions on Mount Fuego, Guatemala.

The Instituto Nacional de Sismologia, Vulcanologia, Meteorologia, e Hidrologia in Guatemala has reported a series of eruptions on Mount Fuego, a stratovolcano (cone shaped volcano made up of layers of ash and lava) that forms part of La Horqueta volcanic complex in the southern part of the country. Between 26 August and 1 September 2020, the volcano produced small explosions 6-12 times per hour, and generated a series of ash columns that rose to as high as 1.1 km above its summit, and drifted for up to 20 km, combined with incandescent material being thrown 100-300 m into the air. Avalanches of blocky debris were observed on the west and southwest sides of the volcano, some reaching as far as the vegetation line. Communities to the west, southwest, and northwest of the volcano reported ashfalls at distances of up to 12 km. On 10 September a lava flow was observed on the southwest flank of the volcano, which by 12 September had elongated to 700 m in length. This was again accompanied by explosive eruptions producing ash columns up to 1.1 km high, though the activity subsequently subsided, and by 15 September the active part of the lava flow was only about 100 m in length.

An ash plume over Mount Fuego, Guatemala, on 1 September 2020. Instituto Nacional de Sismologia, Vulcanologia, Meteorologia, e Hidrologia.

Fuego has been more-or-less constantly active at some level since records in the area began (circa 1524). It forms part of La Horqueta volcanic complex, which also includes the Acatenango volcano to the north, a complex volcano with at least five separate vents, the complex siting on the site of the ancient Meseta volcano, which is thought to have collapsed following a major volcanic episode about 8500 years ago, causing a debris flow that reached the sea, 50 km away.

 

The approximate location of Mount Fuego. Google Maps.

 The volcanoes of Guatemala, and Central America in general, are fed by the subduction of the Cocos Plate beneath the Caribbean Plate along the Middle American Trench, which runs roughly parallel to the southwest coast of the isthmus. As the Cocos Plate sinks into the Earth, it passes under Central America, which lies on the western margin of the Caribbean Plate. As this happens it is heated by the friction and the heat of the planet's interior, causing the sinking plate to partially melt. Some of the melted material then rises through the overlying Caribbean Plate as magma, fuelling the volcanoes of Central America.

 

Diagrammatic representation of the subduction of the Cocos Plate beneath the Caribbean Plate along the Middle American Trench. VCS Mining.

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Eruption on Mount Sinabung produces 5 km high ash column and leads to ash falls 20 km from its summit.

Mount Sinabung, a 2460 m stratovolcano (cone shaped volcano made up of layers of ash and lava) in North Sumatra, Indonesia, erupted on Monday 10 August 2020, producing an ash column that rose 5 km into the atmosphere, and producing ash falls up to 5 km deep in (previously evacuated) villages up to 20 km from the summit of the volcano. Nobody was injured in this incident, largely due o an existing evacuation order that prevents people from approaching the volcano, with nobidy allowed to approach within 5 km of the volcano, and villages evacuated over a wider zone. 

An ash column over Mount Sinabung, North Sumatra, on Monday 10 August 2020. Sugeng Nuryono/AP

Sinabung is considered to be a potentially 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, and has been erupting intermittently ever since.

 

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 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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