Geoscience Reference
In-Depth Information
5˚
Borneo
0˚
Coastline inundated
by tsunami
Height of run-up (m)
15
-5˚
Telok Betong
Kelimbang
24
22
42
-10˚
Merak
3
24
105˚
110˚
115˚
36
30
Vlakken Hoek
11
Anjer Lor
Tjaringin
15
Batavia
Krakatau
23
15
0
20
40 km
Welcome Bay
Coastline in the Sunda Strait affected by tsunami following the eruption of Krakatau on 26-27 August 1883 (based on Verbeek, 1884; Blong,
1984 and Myles, 1985).
Fig. 11.8
north of Pepper Bay and Merak. On the morning of
27 August, four horrific explosions occurred. The first
explosion, at 5:28 am, destroyed the 130 m peak of Per-
boewatan, forming a caldera that immediately infilled
with seawater and generated a tsunami. At 6:44 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. A third explosion occurred at 8:20 am. The
fourth blast, at 9:58 am, tore apart the remaining island
of Rakata. Including ejecta, 9-10 km
3
of solid rock was
blown out of the volcano. About 18-21 km
3
of pyro-
clastic deposits spread out over 300 km
2
to an average
depth of 40 m. Fine ash spread over an area of
2.8
The blasts were accompanied by a cloud of ash that
rose to a height of 30 km. The debris from this cloud
formed deposits with a volume of 13 km
3
. On the
island remnants, 60 m of ignimbrite were deposited on
top of 15 m of tephra. Large rafts of pumice ejected by
the volcano blocked the Sunda Straits. Tephra fell over
an area of 300 000 km
2
and turned day into night
200 km downwind for forty-eight hours. Dust
encircled the globe within two weeks of the eruption,
mainly within the zone of tropical easterlies. Within
three months, dust had spread into the northern
hemisphere, covering most of the United States and
Europe. In France, solar radiation dropped 20 per cent
below normal and, three years afterwards, was still
10 per cent below normal at the ground surface. Spec-
tacular, prolonged sunsets were observed everywhere,
caused by high altitude dust scattering incoming solar
radiation as it entered the Earth's atmosphere at low
angles. The first occurrence of such sunsets sparked
false alarms to fire departments in the eastern United
States as residents reported what appeared to be
distant, but horrific, fires.
The two pre-dawn blasts each generated tsunami
that drowned thousands in the Sunda Strait. The
fourth blast-induced wave was cataclysmic and devas-
tated the adjacent coastlines of Java and Sumatra
within 30 to 60 minutes. The coastline north of the
eruption was struck by waves with a maximum run-up
10
6
km
2
and thick pumice rafts impeded naviga-
tion in the region up to five months afterwards. A
caldera formed, 6 km in diameter and 270 m deep,
where the central island had once stood. This final blast
was the largest sound ever heard by modern humans,
and was recorded 4800 km away on the island of
Rodriguez in the Indian Ocean, and 3200 km away at
Elsey Creek in the Northern Territory of Australia.
Windows were shattered 150 km away. The atmos-
pheric shock wave traveled around the world seven
times. Barometers in Europe and the United States
measured significant oscillations in pressure over nine
days following the blast. The total energy released by
the third eruption was equivalent to 840
10
15
joules.
