Friday 3 August 2012

Increased activity on Mount Yasur, Vanuatu.

Mount Yasur is located on the southeast of Tanna Island in the Republic of Vanuatu, close to the volcanically named Sulphur Bay. It is one of the world's most active volcanoes, having been erupting more-or-less continuously for 800 years, though these eruptions are seldom very large, which has enabled the island to develop a thriving tourist industry based around the volcano. Yasur is a stratovolcano (cone-shaped volcano made up of layers of ash and lava), rising 631 m above sea-level, with a summit crater nearly 400 m across. It is sacred to worshipers of John Frum.

An ash-cloud over Mount Yasur. Air Taxi-Vanuatu.

The regular eruptions on Mount Yasur are considered to be 'Strombolian' by geologists. Such eruptions consist of regular explosions throwing lava bombs, incandescent cinders and small stones and ash a few hundred meters in the air; Stombolian eruptions get their name from Mount Stromboli in Italy, which behaves in a similar way. Eruptions of this kind are spectacular, but seldom dangerous as long as a safe distance is kept.

The emptying of the postbox on top of Mount Yasur, with a Strombolian eruption in the crater to the right. The postbox is publicized as the only postbox on top of an active volcano in the world, and caters for tourists who visit the summit. Vanuatu Post.

Over the period 7-12 July 2012, the level of activity on Mount Yasur increased sharply, with lava bombs falling onto the tourist trail and car park, making the summit inaccessible. On 13 July the Vanuatu Geohazards Observatory concluded that the volcano was moving from a Strombolian phase to a sub-Plinian one, and officially closed off the area around the summit.

Plinian eruptions get their name from the eruption on Mount Vesuvius that destroyed Pompeii in AD 79. They are named after either (depending on source) the Roman author Pliny the Younger, who was the first person to describe an eruption of this sort, or his uncle Pliny the Elder, who is recorded to have been the first person killed while trying to investigate such an eruption, and is consequently regarded as the father of volcanology. 

Eruptions of this kind produce clouds of gas and ash that climb many kilometers into the stratosphere, and can prove extremely dangerous, with widespread ashfalls and pyroclastic flows (avalanches of hot ash and gas). Such eruptions can happen several times in a decade on Mount Yasur, though this frequency makes the volcano less hazardous; Vesuvius erupts infrequently, leading to the building of the cities of Pompeii and Herculanium on what appeared to be safe areas of the volcanoes flanks (modern Naples also lies on the flanks of Vesuvius). Because such eruptions are a certainty on Mount Yasur, no similar cities have been built on its flanks, and those settlements that are nearby are prepared for evacuation if this becomes necessary. Consequently major loss of life on Yasur is far less likely.

Mount Yasur, and Vanuatu in general, lies on the margin between the Pacific and Australian Plates. This is a convergent margin, with the Australian Plate being subducted beneath the Pacific. As is sinks into the Earth's interior the Australian Plate is partially melted by a combination of heat from the friction with the overlying Pacific Plate, and that from the Earth's interior. Some of the melted material then rises up through the overlying Pacific Plate as liquid magma, fueling the volcanoes of Vanuatu.

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