Volcanoes as a Hazard - Natural Hazards - page 238

probably exacerbated by melting of snow and ice by
hot gases. In November 1985, over 20 000 people were
killed by a glacier burst mudflow that swept off the
Nevado del Ruiz volcano in Colombia. This was
the worst such event in the twentieth century.
Jökulhlaups in Iceland are caused by continuous
melting of the Myrdalsjökull or Vatnajökull icecaps,
which lie overtop active volcanoes. The Vatnajökull
glacier caps the 40 km 2 Grimsvötn caldera, which
continually emits hot gases. The thickness of these
glaciers prevents heat escape, while their weight
prevents water from flowing out of the caldera.
Meltwater builds up slowly beneath the ice until
water depths are sufficiently deep to float the glaciers.
Jökulhlaups have dominated Iceland since first
reported in the twelfth century. Until the twentieth
century, they occurred every 5-15 years. During the
twentieth century, large jökulhlaups took place in 1903,
1913, 1922, 1934, 1938 and 1996. Smaller events now
occur 2-3 times per decade. Discharges during a
jökulhlaup create some of the largest floods on Earth.
In 1918, the discharge of a jökulhlaup from the
Myrdalsjökull icecap exceeded three times the dis-
charge of the Amazon, the largest flowing river in
the world. The Katla volcano in Iceland has had
glacier bursts from the Vatnajökull glacier exceeding
92 000 m 3 s -1 , with a total volume in excess of 6 km 3 .
The latter floods over time have formed an outwash
plain 1000 km 2 in area. The last major jökulhlaup
occurred at Grimsvötn on 5 November 1996. Flow
peaked at 45 000 m 3 s -1 as 3 km 3 of heated water
discharged over three days across the southern
outwash plain of Iceland. Blocks of ice 15 m in
diameter and weighing up to 1000 tonnes knocked out
bridges and destroyed the main highway.
Water discharge can also accompany volcanic
eruptions in areas without snow or ice. The 1902 erup-
tions of Mt Pelée and Soufrière in the West Indies were
both accompanied by massive flooding of dry rivers.
The 1947 eruption of Mt Hekla, Iceland, led to a dis-
charge of 3 000 000 m 3 of water, which could not be
attributed solely to melting of snow or ice. These latter
events suggest that water may be expelled from the
watertable by escaping gases during some volcanic
eruptions. Nor do these discharges have to accompany
the eruption. In 1945, the Ruapehu volcano in New
Zealand erupted, emptying its crater lake. This lake
slowly refilled inside a newly formed crater that became
dammed by glacial ice. In 1953, an ice cave formed that

began lowering the crater. On Christmas Eve of that
year the crater wall suddenly collapsed near the cave
causing the lake to drop 6 m in two hours at the rate of
900 m 3 s -1 . The floodwater swept down the Whangaehu
River picking up mud and boulders and forming a lahar.
Two hours later, at 10:00 pm, the debris-laden water
swept away a section of the Tangiwai Rail Bridge, three
minutes before the Wellington-Auckland night train
arrived. The train plunged through the gap, killing
151 people.

VOLCANIC DISASTERS

Volcanoes are visually one of the most spectacular
natural hazards to occur, and probably one of the most
devastating in terms of loss of human life. Even the
Sahelian droughts, or earthquakes that completely
flatten cities, leave more survivors than dead.
Volcanoes have wiped out entire populations. There
were only two survivors out of a population of 30 000
in the town of St Pierre, Martinique, following the
Mt Pelée eruption of 1902. Four volcanic events stand
out in the historic record. Three of these, Mt Vesuvius,
Krakatau, and Mt Pelée, are significant because of
either the enormity of the eruption, or the resulting
death toll. The other, Mt St Helens, stands out because
it represents a major eruption beginning a period of
volcanic activity at the end of the twentieth century.

San torini, around 1470 BC

(Yokoyama, 1978; Pichler & Friedrich, 1980; Kastens & Cita,
1981; Cita et al., 1996; Pararas-Carayannis, 1998)

The prehistoric eruption of Santorini, also known as
Stronghyli - the round island - occurred about
1470 BC off the island of Thera, north of Crete in
the southern Aegean Sea (Figure 11.6). Probably the
biggest volcanic explosion witnessed by humans, it is
also one of the most controversial because legend,
myth, and archaeological fact are frequently inter-
twined and distorted in the interpretation of events.
The eruption has been linked to the lost city of Atlantis
(described by Plato in his Critias ), to destruction of
Minoan civilization on the island of Crete 120 km to
the south (Figure 11.6), and to the exodus described in
the Bible of the Israelites from Egypt. Certainly, Greek
flood myths refer to this or similar events that gener-
ated tsunami in the Aegean. Plato's story of Atlantis is
based on an Egyptian story that has similarities with
Carthaginian and Phoenician legends. There is no