Sunday 24 July 2011

Recent eruptions on Mount Etna.

On the 4th of July 2011 loud explosions were heard from a pit crater on the eastern flank of Mount Etna's southeastern cone (Etna has four cones, the southeast cone in the newest having only formed in 1978). That evening the cone was seen to be glowing, and over the next two days a series of small Strombolian eruptions (eruptions in which liquid lava is thrown into the air) were photographed by scientists from the Sezione di Cantania - Osservatorio Etneo. On the 7th these Strombolian eruptions became more intense, a number of volcanic tremors were felt and several small scoria (ash) cones started to form on the crater floor. Then, suddenly, all activity stopped.

On the morning of the 9th Strombolian eruptions and tremors started up again, then, at about noon, lava started to flow out of the crater and into the upper western part of the Valle de Bove, a horseshoe shaped caldera opening to the east. Later in the afternoon the Strombolian eruptions were replaced by a continual lava eruption. This was followed by the eruption of an ash plume that several kilometers into the air and rained ash down on several local towns, briefly closing Fontanarossa International Airport. Then, again, all activity abruptly ceased.

Strombolian eruption in Etna's southeast crater on the 9th of July 2011, by photographer Martin Rietz.

On the 11th July Strombolian activity started up again, this time on the westernmost cone, known as 'Bocca Nuova', the first lava eruptions from this cone since 2002. For the next two days these eruptions continued; lava bombs were observed being thrown high into the air and falling back into the crater, which remained incandescent throughout the nights. On the 13th a team of volcanologists visited Bocca Nuova, where they discovered a large vent had opened up in the floor of the crater, from which lava bombs were being thrown several tens of meters into the air.
Video footage taken at Bocca Nuovo by volcanologist Boris Behncke of the Institutio Nazionale di Geofisica e Vulcanologia Sezione di Catania.

On the fifteenth the volcanologists re-visited the crater. When they first arrived they found that activity was at a lower level than on the 13th, but within two hours it had risen higher than ever. To the west of the vent lava was issuing in a steady stream from beneath a sheet of solidified lava, and flowing into a depression in the western part of the crater.

On the 16th a series of loud explosions were heard from the southeastern crater, and ash was thrown from the crater on its eastern flank. This was followed on the 18th & 19th by a resumption of Strombolian eruptions in the crater, with lava being thrown 200-250 m in the air, and a number of lava flows were observed. A cloud of ash and gas was produced, and drifted to the east.

This all seems fairly dramatic, and has appeared in a number of news reports around the world. However for Etna this is all quite normal. There have been eruptions of one sort or another in every month since November 2010, when the volcano took a brief rest, having not erupted for the whole month of October (it erupted in September).

Etna first erupted about half a million years ago, beneath the sea off the east coast of Sicily, and has been going strong ever since. It now stands 3330 m above sea level, and covers 1200 km³. It is responsible for fertile soils across eastern Sicily. Records of eruptions on Etna go back to 1500 BC. It is Europe's second largest volcano, after Teide in the Canary Islands, and is one of the most active volcanoes in the world.

The location of Etna, in eastern Sicily.

Despite all this Etna has only ever caused 77 recorded deaths (the most recent being two tourists caught in a summit explosion in 1987) and relatively little destruction. In 1928 it destroyed the village of Mascali on its northeastern flank, though there were no reported casualties, the village being slowly overrun by a lava flow. In 1669 a much larger lava flow destroyed at least 10 villages, reaching the walls of the city of Catania, 40 km to the south, but again without loss of life. In 122 BC a heavy ash fall covered much of the region, causing several buildings to collapse in Catania. The destruction was deemed so severe by the Roman authorities that they granted the city a 10 year tax holiday. In about 6000 BC a landslide on the eastern flank of the volcano is thought to have caused a tsunami that caused destruction around much of the eastern Mediterranean.

Etna is located on the border of the African and European Plates, specifically where Africa is being subducted beneath the European Plate. As it is drawn into the Earth's interior material from the African Plate melts, and the lighter portions rise up through the overlying European Plate, causing a number of volcanoes including Etna and Vesuvius.

The subduction of the African Plate beneath the European Plate.