Geoscience Reference
In-Depth Information
later loud explosions occurred at 10 min intervals and a dense tephra cloud up to
28 km high rose over the island. The next day, another three explosions occurred.
During the first explosion the 130 m high peak of Perboewetan collapsed to form
a caldera that was immediately filled with seawater. The second explosion led to
the collapse of the 500 m high peak of Danan. This resulted in more sea water
being released into the molten magma chamber. The last explosion formed a
6kmdiametercaldera where the peak of Rakata had been. This caldera was
300 m deep. As the third of these explosions occurred, an 80 km high, 18 km 3
volume, ash cloud formed. Ignimbrite deposits 60 m thick were deposited. Tephra
fell over a total area of 300 000 km 2 .
This eruption generated an atmospheric shock wave that travelled around
theglobe seven times, blocking the sun for two days downwind of the eruption.
Solar radiation to the Earth was reduced by 13% as ash entered the stratosphere,
and remained low for more than two years. Spectacular sunrises and sunsets
could also be seen globally for up to two years as sunlight was reflected off dust
particles. The Sunda Straits were blocked by large rafts of pumice. The collapse
of the peaks, in addition to the pyroclastic surges and the shock waves, produced
several large tsunamis, up to 40 m high, killing approximately 36 000 people.
Tsunami waves were also experienced at various other sites globally such as
theEnglish Channel and Calcutta on the Ganges River. The sound of the last
explosion was also the loudest sound ever heard in historic times and was heard
4800 km away (Bolt et al ., 1977).
The Mount St Helens eruption in Washington in 1980 was preceded by an
earthquake triggering a large landslide. The movement of the landslide down
themountain caused a pressure drop in the gaseous magma. This, together
with very hot ground water, produced an explosion and a pyroclastic flow that
behaved as a low-viscosity fluid. The explosion resulted in the vent of the volcano
opening, exposing the magma chamber and reducing the height of the peak by
500 m. This led to a 20 km high tephra cloud and another pyroclastic flow with
temperatures up to 370 Cthat attained a velocity of more than 100 km h 1 .
Sixty people died during this eruption, mainly due to lahars (Bryant, 2005).
palaeovolcanic eruptions
Volcanic eruptions make an indelible impression upon people and com-
munities. The truly large eruptions can become the focus of myths and legends.
Forthis reason, long historical records of these hazards are preserved, espe-
cially the biggest ones, compared to other hazards. Archaeological evidence of
theeruptions that impacted early civilisations also provides an excellent means
by which to reconstruct the age and extent of prehistoric eruptions. Volcanic
Search WWH ::




Custom Search