Volcanic Eruptions - Tsunami - page 154

Fig. 8.1 An artist's impression
of the tsunami from the third
explosion of Krakatau hitting the
coast of Anjer Lor at about 10:30
AM on August 27, 1883.
Lynette Cook

Submarine eruptions within 500 m of the ocean surface
can disturb the water column enough to generate a surface
tsunami wave (Latter 1981 ). Below this depth, the weight
and volume of the water suppress surface wave formation.
Tsunami from this cause rarely propagate more than
150 km from the site of the eruption. One of the largest
such tsunami occurred during the eruption of Sakurajima,
Japan, on September 9, 1780, when a 6 m high wave was
generated. More significant are submarine explosions that
occur when ocean water meets the magma chamber. This
water is converted instantly to steam. In the process, it
produces a violent explosion. Krakatau during its third
explosive eruption in 1883 produced a tsunami 40 m high in
this manner.
Formation of a caldera during the final stages of an
explosive eruption near the sea can permit water to flow
rapidly into the depression. This sets up a wave train that
can propagate away from the caldera within 5 min. Strato-
volcanoes are particularly prone to collapse. In the Ring-of-
Fire subduction zone around the Pacific Ocean there are
hundreds of these types of volcanoes (Lockridge 1988 ).
Krakatau's numerous eruptions produced calderas that may
have been responsible for some of the tsunami observed in
the Sunda Strait (Latter 1981 ). A comparable event
occurred on Ritter Island, Papua New Guinea, on March
13, 1888. Here, the formation of a caldera 2.5 km in
diameter produced a 12-15 m high tsunami. While the
initial wave heights radiating out from the caldera can be
large, the actual volume of water displaced may be small.
In addition, because the height of the wave decays inver-
sely to the square root of the distance travelled, the effect

of the tsunami diminishes rapidly away from the point
source.
The slopes of a volcano are inherently unstable during
eruptions because of earthquakes, inflation, or collapse
operating on essentially landforms that are complex rubbish
piles of stacked lava flows, weathered soils, and prior debris
flows (Keating et al. 1987 ). Collapsing material can form a
debris avalanche that can travel at speeds of 100 m s -1 .
Horseshoe-shaped scars are left behind as evidence of the
failure. The extent of debris avalanching was described in
the previous chapter (Whelan and Kelletat 2003 ). If there
have been an estimated 100 mega-tsunami producing events
over the past 2 million years, then hundreds of smaller
events must have occurred. For small avalanches of
0.1-1.0 km 3 in volume, the travel distance ranges between
six and eleven times the elevation of the initial avalanche.
For higher volumes, the travel distance can increase to 8-20
times the vertical drop. For example, the collapse of a
2,000 m high volcano could generate a debris avalanche
that can travel 16-22 km from its source. Avalanching is a
significant hazard associated with volcanoes in Alaska,
Kamchatka, Japan, the Philippines, Indonesia, Papua New
Guinea, the West Indies, and the Mediterranean Sea. In
most cases, the resulting landslide is localized enough to
generate small tsunami that are highly directional in their
propagation away from the volcano. However, some of the
greatest death tolls have been caused by such events. For
example, during the eruption on May 21, 1792 of Unzen
Volcano, in Japan, 0.34 km 3 of material sloughed off its
flank (Blong 1984 ). The landslide travelled 6.5 km before
sweeping into the Ariake Sea, where it generated a tsunami