Volcanic Eruptions - Tsunami - page 156

Basal surges or lateral blasts are formed when a volcano
erupts sideways (Latter 1981 ). Taal Volcano in Lake
Bombon, Philippines, has been subject to at least five basal
surge events since 1749. All of the resulting tsunami took
lives. Avalanches of hot rock can generate tsunami when
ejecta, piled up on the side of an erupting volcano, collapses
into the sea. These events occur frequently on Stromboli
volcano in Italy. The resulting tsunami are usually localized
and have not been associated with any deaths. Lahars are
ash deposits that fail after becoming saturated with water
(Blong 1984 ). Failures occur during an eruption because of
the displacement of ground water, mixing of ash with water
in a crater lake, or melting of snow and ice at the crest of the
volcano. When the lahar reaches the ocean, it can generate
significant tsunami; however, these are usually localized
because the lahar tends to flow down valleys. For example,
on May 5, 1902, a 35 m high lahar from Mt. Pelée swept
down Rivière Blanche north of the nearby town of St. Pierre
(Latter 1981 ). When it reached the sea, it generated a 4.5 m
high tsunami that only affected the lower part of the town,
killing one hundred people. Large explosive volcanoes
generate a pressure pulse through the atmosphere. Krakatau,
in 1883, generated tsunami in the Pacific Ocean, and in
Lake Taupo in the middle of the North Island of New
Zealand, via this mechanism (Choi et al. 2003 ). However,
nowhere did the tsunami exceed more than 0.5 m in height.
The 1955-1956 eruption of Bezymianny on the Kamchatka
Peninsula in Russia also generated a global pressure wave,
but the resulting tsunami in the Pacific Ocean did not reach
more than 0.3 m in height (Latter 1981 ). Finally, if lava
reaches the ocean en masse it can generate tsunami. These
events are rare and have only been noted at one or two
locations, with no widespread destruction being produced.
For example, the lava from the eruption of Matavanu
Volcano on Savaii, Samoa in 1907 generated a tsunami
3.0-3.6 m high when it poured into the sea. No deaths were
reported.

the volcano underwent increased earthquake activity. In
May 1883, one vent became active, throwing ash 10 km
into the air. By the beginning of August, a dozen Vesuvian-
type eruptions had occurred across the island. On August
26th, loud explosions recurred at intervals of 10 min, and a
dense tephra cloud rose 25 km above the island (Verbeek
1884 ). The explosions could be heard throughout the islands
of Java and Sumatra. In the morning and later that evening,
small tsunami waves 1-2 m in height swept the strait,
striking the towns of Telok Betong on Sumatra's Lampong
Bay, Tjaringin on the Java coast north of Pepper Bay, and
Merak (Fig. 8.3 ) (Myles 1985 ). On the morning of August
27th, three horrific explosions occurred (Verbeek 1884 ;
Myles 1985 ). The first explosion at 5:28 AM destroyed the
130 m peak of Perboewatan, forming a caldera that
immediately infilled with seawater and generated a tsunami.
At 6:36 AM, the 500 m high peak of Danan exploded and
collapsed, sending more seawater into the molten magma
chamber of the eruption and producing another tsunami.
The third blast, at 9:58 AM, tore the remaining island of
Rakata apart. Including ejecta, 9-10 km 3 of solid rock was
blown out of the volcano. About 18-21 km 3 of pyroclastic
deposits spread out over 300 km 2 to an average depth of 40
meters (Self and Rampino 1981 ). Fine ash spread over an
area of 2.8 9 10 6 km 2 , and thick pumice rafts impeded
navigation in the region up to 5 months afterwards. A cal-
dera 6 km in diameter and 270 m deep formed where the
central island had once stood. This third blast was the
largest sound ever heard by humanity, and was recorded
4,800 km away on the island of Rodriguez in the Indian
Ocean and 3,200 km away at Elsey Creek, Northern Ter-
ritory, Australia (Latter 1981 ). Windows 150 km away were
shattered. The atmospheric shock wave travelled around the
world seven times. Barometers in Europe and the United
States measured significant oscillations in pressure over
9 days following the blast. The total energy released by the
third eruption was equivalent to 200 megatons of TNT.
(Kinetic energy for volcanic eruptions and asteroids
exploding in the atmosphere or impacting with the ocean is
expressed in megatons of TNT. One megaton of TNT is
equivalent to 4.185 9 10 15 J.)
The two pre-dawn blasts each generated tsunami that
drowned thousands in Sunda Strait. The third blast-induced
wave was cataclysmic and devastated the adjacent coastline
of Java and Sumatra within 30-60 min (Fig. 8.3 ). The
coastline north of the eruption was struck by waves with a
maximum run-up height of 42 m (Verbeek 1884 ; Blong
1984 ; Myles 1985 ). The tsunami penetrated 5 km inland
over low-lying areas. The largest wave struck the town of
Merak. Here, the 15 m high tsunami was increased to 40 m
because of the funnel-shaped nature of the bay. The town of
Anjer Lor was swamped by an 11 m high wave (Fig. 8.1 ),
the town of Tjaringin by one 23 m in height, and the towns

8.3 Krakatau, August 26-27, 1883

The eruption of Krakatau on August 26-27, 1883 was one
of the largest explosive eruptions known to humanity. It is
the only eruption for which detailed historical information
exists on volcano-induced tsunami. To date, the mecha-
nisms generating tsunami during its eruption are still
debated. The volcano lies in the Sunda Strait between
Sumatra and Java, Indonesia (Fig. 8.3 ). The Javanese Book
of Kings describes an earlier eruption, referring to Krakatau
as Mount Kapi (Keys 1999 ). Then, the volcano exploded
and created a sea wave that inundated the land and killed
many people throughout the northern part of Sunda Strait.
Krakatau had last been active in 1681, and during the 1870s