Etna's eruptions are low-key compared with other volcanoes

 

The lava flows now channelling down the slopes of Mount Etna in Sicily have provided dramatic pictures. Locals, however, are less frightened than annoyed about what the volcano might do.

Lava eruptions at Etna are generally low key. It throws up beautiful fiery-red fountains but lacks the explosive force of a Vesuvius or Mount St Helens in the US, stated Dr Chris Stillman, a vulcanologist in the department of geology at Trinity College Dublin.

Etna has been active on a regular basis for at least two decades with slow lava flows breaking through fissures in the mountainside every few years. "This is the largest in a long time," he said.

Etna behaves much differently than explosive volcanoes because of the type of lava it releases, he added. "Mount Etna is a basaltic eruptive volcano" which releases the same type of rock as seen on the Giant's Causeway in Co Antrim. Basaltic lava does not usually hold much dissolved gas or water so Etna's flows are slow and regular, similar to those seen in the Hawaiian islands.

Vesuvius and St Helens release a very different form of lava, andesite or rhyolite, both a granite-like material that can dissolve large volumes of both water and gas. The volcano dome holds back the tremendous pressures caused by this volatile mix unless a fissure or crack allows a sudden release, Dr Stillman said.

Should the pressure find an outlet the results are a dramatic eruption. Escaping gas and water vapour carries large amounts of rock and ash with it. The result can be a spectacular explosion of devastating power. Rock and ash can be ejected 30 kilometres into the sky and these eruptions are often accompanied by "pyroclastic flows", waves of fiercely hot gas and ash that rush along the ground at speeds of up to 160 kilometres per hour.

Vesuvius blew up in AD 79, wiping out the cities of Pompeii, Herculaneum and Stabiae. The explosion released six cubic kilometres of ash and rock in the process, blanketing the inhabitants and smothering them where they stood.

So while Mount Etna can produce impressive lava flows and striking pictures, it doesn't pack the punch or the destructive potential of a Vesuvius, Dr Stillman said. Italian officials are therefore much more fearful of an awakening Vesuvius, something that tends to happen every 30 years or so.

Vesuvius is now overdue, he said, its last significant release occurring in 1944. "The worrying thing with Vesuvius is there is plenty of evidence that something is going on," Dr Stillman said. Unfortunately, scientists do not know how to interpret what these signs mean.

The greatest worry is that about two million people live within Vesuvius's shadow and would be at risk should an eruption as powerful as that of 79 take place. Etna is more a nuisance, affecting a relatively small number of people in nearby villages.

Etna will continue to ooze lava until the pressure in its "magma chamber" lessens or the fissures through which the molten rock emerges eventually clog, Dr Stillman said. Vesuvius will remain a threat however, with no way to predict when it might come to life. AFP adds:

Volcanic ash from Mount Etna forced the temporary closure of the airport in nearby Catania yesterday, because of the risk it caused to aircraft taking off and landing. The airport was scheduled to reopen later yesterday after the runway had been cleared of ash, the management said. It has been forced to close several times since Etna became active this month.