"A fearful black cloud rent by forked and
quivering bursts of flame sank to earth ..."
Was Pompeii destroyed by falling pumice and ash, and Herculaneum
by slow-moving flows of boiling mud? Were Pompeii's victims killed
by falling rocks, collapsing roofs, and suffocation inside rooms
buried by the deluge? Did all the people of Herculaneum, on the
other hand, escape?
So it used to be thought, but in the last 25 years
vulcanologists have challenged the traditional view of the eruption.
Combining the evidence of Pliny the Younger's letters, the study of
exposed volcanic strata in the Bay of Naples, and the experience of
other eruptions, such as that of Mount Pelée on Martinique in 1902,
they have developed a new – and far more terrifying – account of
what actually happened on 24-25 August AD 79.
The Romans knew that Vesuvius was a volcano, but they thought it
extinct. They had no record of any previous eruption. (The last –
unknown to them – had in fact occurred in c. 1200 BC.) Then, on 5
February AD 62, a violent earthquake reduced much of Pompeii to
Damage was such that reconstruction work was still unfinished
seventeen years later. Probably, something was already happening to
the mountain: a fracturing of the edifice by the movement of magma
from the main chambers some 5km below the surface. But AD 62 was
followed by a period of relative calm: the occasional quake as magma
shifted further beneath the crust, but nothing on a comparable
On 20 August AD 79, however, major activity resumed with a
series of quakes, increasing in frequency over the succeeding four
days, as magma forced its way upwards into the feeder pipe of the
volcano. Springs dried up around the mountain. Finally, at
approximately one o'clock on the afternoon of 24 August, Vesuvius
The carapace of rock sealing the mountain was blown out, and a
great brown column of pumice and ash rose vertically into the
atmosphere, quickly reaching a height of about 15km. "Its general
appearance," Pliny wrote, "can best be expressed as being like an
umbrella pine, for it rose to a great height on a sort of trunk and
then split off into branches."
The 'umbrella' – the spreading cloud at the top of the column –
was driven south-east by the wind, and about half an hour after the
explosion Pompeii, about 9km away, was shrouded in darkness and
began to be pelted by pumice.
This 'Plinian' phase of the eruption lasted eighteen hours, at
first gaining force as the magma ripped rocks from the side of the
vent and as subsidence collapsed more of the rock carapace, creating
a wide 'caldera' through which far greater masses of material could
reach the surface. At peak intensity, the eruptive column reached a
height of 30km or more as the mountain ejected 150,000 tonnes of
material per second. Altogether, in the Plinian phase, an estimated
2.6 cubic km of rock was blasted into the sky.
The air-fall of ash, pumice and occasional rock fragments,
accumulating at about 15cm an hour, eventually covered Pompeii to a
depth of almost three metres. Many flat roofs caved in under the
weight. Sometimes people sheltering beneath were killed. Outside,
fugitives waded through loose pumice in the darkness, trying to make
their escape, and a few unlucky ones were struck down by rocks,
falling tiles or collapsing masonry. But overall the Plinian
air-fall phase claimed few victims, and many hundreds were still
stumbling around in a grey desert of pumice on the morning of 25