Geoscience Reference
In-Depth Information
Statistically, the chances of being hit were not high but, as John Murray says, it did not feel
like that at the time.
19. Some of the principal features of an erupting composite volcano such as Etna.
John Murray and his colleagues have been monitoring Etna for many years and are getting
to know the signs of an impending eruption. They can use surveying techniques and GPS
measurements to spot the slight bulging in the mountainside as magma rises beneath. They
can monitor the earth tremors as cracks are forced open. Gravity surveys reveal the dense
magma as it rises. The surveyors also monitor the hillside as an eruption subsides. In par-
ticular they are concerned about the steep southeast flank above the city of Catania. In the
early 1980s parts of that flank subsided 1.4 metres in a single year, and there were fears
that the slope might collapse, perhaps even taking the pressure off the magma within and
triggering a lateral blast like the Mount St Helens eruption. It is possible that that may have
happened in about 1500
BC
, forcing the Ancient Greeks to abandon eastern Sicily.
Typically, an eruption will begin from the summit crater but then, once the initial gases
have discharged, the magma forces a way out through fissures on the flanks of the moun-
