Geoscience Reference
In-Depth Information
thickness in deposits on any of the adjacent islands,
including Crete. The largest thickness of ash measured
in marine cores appears to originate from pumice that
floated into the eastern Mediterranean. It is possible
at this stage that ocean water made contact with the
magma chamber and produced large explosions that
generated tsunami. The final phase of the eruption was
associated with the collapse of the caldera in its south-
west corner. The volcano sank over an area of 83 km
2
to a depth of 600-800 m. This final collapse produced
the largest tsunami, directed westward (Figure 11.6).
It is estimated that the original height of the tsunami
was 46-68 m, and maybe as high as 90 m. The average
period between the dozen or more peaks in the wave
train was fifteen minutes.
Evidence of the tsunami is found in deposits close to
Santorini. On the island of Anapi to the east, sea-borne
pumice was deposited to an altitude of 40-50 m above
present sea level. Considering that sea levels at the
time of the eruption may have been 10 m lower, this
represents run-up heights greater than those produced
by Krakatau in the Sunda Strait. On the island of
Crete, the wave arrived within thirty minutes with a
height of approximately 11 m. Refraction focused wave
energy on the north-east corner of Crete, where run-
up heights reached 40 m above sea level. In the region
of Knossos, the tsunami swept across a 3 km wide
coastal plain reaching the mountains behind. The
backwash concentrated in valleys and watercourses,
and was highly erosive. Evidence for the tsunami is also
found in the eastern Mediterranean on the western
side of Cyprus, and further away at Jaffa-Tel Aviv in
Israel. At the latter location, pumice has been found on
a terrace lying 7 m above sea level at the time of the
eruption. However, the tsunami wave here had already
undergone substantial defocusing because of wave
refraction as it passed between the islands of Crete and
Rhodes. The greatest tsunami wave heights occurred
west of Santorini. The wave in the central Mediter-
ranean Sea was 17 m high; while closer to Italy over the
submarine Calabrian Ridge it was 7 m high. Bottom
current velocities under the wave crest in these regions
ranged from 20 to 50 cm s
-1
- great enough to entrain
clay- to gravel-sized particles. The maximum pressure
pulse produced on the seabed by the passage of
the wave ranged from 350 to 850 kdynes cm
-2
.
Spontaneous liquefaction and flow of water-saturated
muds is known to occur under pressure pulses of
280 kdynes cm
-2
and greater.
Vesu
vius (25 August 79 AD)
(Bolt et al., 1975; Whittow, 1980; Blong, 1984; Sigurdsson
et al., 1985)
Mt Vesuvius lies on the south-east corner of the bay of
Naples on the west coast of Italy (Figure 11.7). The
Roman towns of Pompeii and Herculaneum were
situated at the base of the south-western slopes, 5 km
from the mountaintop. Both towns were prosperous
regional centers serving as summer retreats for wealthy
Romans. Mt Vesuvius, prior to its eruption, consisted
of a single, flat-topped cone with a small crater. The
Romans recognized it as a volcano, but there is no
Roman or Etruscan record of its ever being active. In
63 AD, a major earthquake struck the region with its
epicenter at Pompeii. Both the towns of Pompeii and
Herculaneum were severely damaged, and by the time
of the eruption in 79 AD, only minor rebuilding had
been completed in Pompeii.
The description of the eruption comes from the eye-
witness account of Pliny the Younger, who recounts the
death of his father during the eruption. The writings of
Cassius provide descriptions of the destruction of the
two cities. Pliny the Elder was the commander of
the Roman fleet at Misenum, 30 km to the west at the
entrance to the Bay of Naples. On 24 August, Vesuvius
began to erupt, accompanied by violent earthquakes.
Pliny sailed to the scene to rescue inhabitants at the
base of the mountain. He could not get near the shore
because of a sudden retreat of the shoreline, and his
boats were covered in a continuous ash fall. He sailed
that night to Stabiae to stay with a friend. Unfortu-
nately, Stabiae was downwind from Pompeii and,
during the night, the ash fall became so deep that
people realized they would have to flee for their own
safety. Pliny the Elder reached the coast during the
daylight hours of 25 August, but the ash fall was so
heavy that it still appeared to be night. As the magma
chamber emptied, the top of the volcano collapsed
inward, consuming 50 per cent of its volume and pro-
ducing a caldera 3 km in diameter. In the early hours
of 25 August, a series of six surges and pyroclastic flows
swept the area. The first surge overwhelmed Hercula-
neum. All remaining inhabitants met a grisly death
from asphyxiation as they sheltered on the beach. The
fourth surge reached Pompeii at about 7:00 am. The
last two surges were the largest, and reached the towns
of Stabiae and Misenum. At Stabiae, Pliny the Elder
died on the beach, apparently from a heart attack. It
