Geography Reference
In-Depth Information
Box 5.1
Volcanic hazards at Mount Vesuvius
Located east of Naples in Campania region, southern Italy,
Mount Vesuvius is a 1281 m high strato-volcano composed
of layers of basaltic tephrite that are up to 300,000 years
old. This complex volcanic edifice sits astride a subduction
zone and has a history of occasionally violent eruptive and
seismic activity. Major eruptions occurred in 5960 BC, 3580
BC, AD 79 and 1631. The AD 79 event involved the
emission of 4 km
3
of magmatic products in 19 hours and
the creation of a plinian column of tephra and gas that was
32 km high. About 18,000 people may have died in this
event, which deposited 3 m of ash on the city of Pompeii
and, by pyroclastic flow, 23 m on Herculaneum.
The last eruption occurred in March 1944, and since
then, the volcano has been dormant. The time length of its
repose is likely to be correlated positively with the strength
of the next eruption. The probability of this has
been calculated per ten-year period as 0.099 for a VEI-
3 event (a volcanic explosivity index of 3, classified as a
'violent strombolian eruption'), 0.017 for VEI-4
('subplinian') and 0.003 for VEI-5 ('plinian'). According to
vulcanological simulations, the last of these would
generate a blast column 11-16 km high, which would
consist of 5-10 per cent gas and 20 per cent tephra by
weight. It would have an initial temperature of 1000°C, a
duration of 3-12 hours and a diffusion rate of 3000 m
2
/s
-1
.
About 700,000 people now live within a 15 km radius
of the summit of Mount Vesuvius, mostly in the arc of
towns on the southern side, which stretches from Torre
Annunziata in the east to the Barra district of Naples in
the west. Among these settlements, the urban area of
Portici (1990 population 67,824) has a density of more
than 17,000 people per km
2
, five times that of central
Milan. Yet in 1631 4000 people died there in a pyroclastic
flow that marked the onset of three centuries of
intermittent eruptions.
Figure 5.3
Volcanic hazards in
the circum-Vesuvian area of
southern Italy.
Source:
Compiled from various
sources - see text and bibliography.
Risk levels depend on the geographical pattern of eruptive
effects and human settlement (Figure 5.3). High-velocity
winds at altitudes of 8-15 km would blow ash from a
vertical column predominantly eastwards. Most buildings
would collapse only under weights of deposited ash in
excess of 100 kg/m
2
, which would be the case in areas
that are very limited yet would still encompass several
towns with a combined population of at least 51,000. The
less populated areas to the north and northwest are
shielded by a rampart, which is the remnant of the proto
Vesuvian Somma caldera. On the other hand, the coastal
settlements to the south and southeast would bear the
full brunt of lava flows, pyroclastic flows, faulting and
localised tephra deposition. It has been estimated that an
unexpected eruption might take 15,000-20,000 lives,
especially as road congestion would probably be
immediate and total. However, the volcano is intensively
monitored, and detailed plans have been made to
evacuate up to 1 million people by every available means,
including sea transport.
