At the more explosive end of the scale are Plinian eruptions. These include the famous eruption of Mount Vesuvius in 79AD, described by the Roman writer Pliny the Younger, which buried the Roman towns of Pompeii and Herculaneum under metres of ash.

Sicily's Mount Etna put on a fiery show on Monday, sending a cloud of smoke and ash several kilometres into the air. Although the eruption sent some tourists scrambling, officials said the activity posed no danger to the population.