The most destructive waves in the ocean are
tsunamis, often wrongly
called tidal waves. They are not caused by tides or even by the wind,
but by underwater earthquakes, landslides or volcanic eruptions. These
disturbances cause the sea bed to move very quickly, which shifts a
large amount of water and disrupts the sea surface. A train of waves is
set in motion which travel away from the of disturbance.
Tsunamis travel extremely fast - up to
750km/h. They have a long
wavelength, their crests often being 150km apart, so that a crest
passes only every 15 minutes or so.
In the open ocean they are often less than a
meter high and usually
pass ships unnoticed. In shallow water, however, tsunamis change and as
each wave reduces its velocity, the wave height builds up to tens of
meters until it breaks.
In some places they do little damage, but
where the shape of the sea
floor concentrates the energy of the wave, tsunamis can cause
devastation on the shore.
The eruption of a volcano in 1883 on the
island of Krakatoa caused
destructive tsunamis. Waves 30m high crashed into the nearby islands
and even swept a boat about 3km inland.
The shores of the Pacific more than those of
any other ocean have
been damaged by tsunamis because volcanic eruptions and earthquakes
occur frequently in that area.