Wednesday 23 September 2020

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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