Origins

Vesuvius is located on a line running parallel to the west coast of Italy and extending north from Naples through Rome to Siena. This line follows a fracture caused by one of the tectonic plates that make up the Earth's crust (the African plate) pushing beneath another (the Eurasian plate).

This process is also closing up the Mediterranean and causing the uplift of the Apennine mountains of Italy. The fracture line is marked by more or less evenly spaced volcanic vents, of which Mount Vesuvius is only one. Vesuvius probably first began as a submarine volcano in the Bay of Naples, then emerged as an island and was finally joined to the land as the products of its eruptions piled up.

The first of these is believed to have occurred after the end of the last Ice Age. Thus Vesuvius is about 10,000 years old, making it a relatively young volcano. At the time of the AD 79 disaster, Vesuvius had been dormant for centuries. From a distance, it appeared as a broad, flat-topped mountain, and its deep crater - intact except for a narrow opening on one side - was visible only at the summit.

During its dormancy, its slopes had become overgrown with trees and other vegetation. So it is not surprising that Vesuvius was not recognised as a volcano except by a few scholars of the time.

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Magma changes

To discover exactly what happened during the eruption of AD 79, researchers have married the results of scientific investigation to the eyewitness account written by Pliny the Younger - see Timetable of AD 79.

Three kilometres (1.9 miles) under Vesuvius is a magma chamber, an underground 'tank' of molten rock. It grows over time, and this growth can create ground 'uplift' in the region, earthquakes and heating of the ground water. We know that earthquakes occurred around Vesuvius in the years prior to AD 79. In addition, researchers reckon that the earthquakes during the volcano's eruption - described by Pliny the Younger as 'so violent that one might think that the world was not being merely shaken but turned topsy-turvy' - were of a magnitude of 4 or 5 on the Richter scale. The rock above the magma chamber eventually splits.

A fissure appears and a thin column of magma makes its way to the surface. If it leaks out slowly, it results in a lava flow, as it did in 1944 - Vesuvius's last eruption. But in AD 79, the column of magma couldn't squeeze out to the surface because of the dense rock plug that had formed during the volcano's dormant period - a period when the pressure continued to build up inside.

If a volcano experiences no activity for a long time, the magma accumulating in the chamber undergoes certain chemical changes that lead to a build-up of gases in the magma. As the magma moves higher up the fissure, the gases leave the liquid rock and form bubbles. The bubbles increase in size and become more numerous, and the magma turns into a foam with gas cavities filled with steam.

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The eruption begins 

At about 1pm on 24 August AD 79, an event occurred at Vesuvius that only happens every 2,000 to 5,000 years - a massive explosive eruption. As the pressure of the magma cracked the volcano's cone, the trapped molten rock shot 17 kilometres (10.6 miles) into the air and solidified in the upper atmosphere.

The magma surrounding the bubbles turned into volcanic glass and the result was pumice - a very light-weight material that floats on water but is still true rock. Within minutes, the first of millions of tons of pumice, driven by high-altitude winds, rained down on Pompeii. But this isn't what killed the vast majority of people in the region. The rare and complex phenomenon that killed them was a surge cloud - also known as a pyroclastic flow or nuée ardente ('glowing cloud').

At Vesuvius, the power that the eruption generated (like that of a gigantic jet engine) kept the column of hot pumice, dust and ash miles up in the air. But then the volcano's crater began to cave in, interrupting the flow of power. As the column briefly collapsed, it hit the ground at over 500km (310 miles) an hour. What Pliny the Younger described as a 'black and dreadful cloud' spread over a vast area like a dust-filled hurricane, destroying everything in its path. But as the column rose again, only a very thin layer of ash was left behind on the ground, which was then buried by further pumice falls.