Geoscience Reference
In-Depth Information
volcanoes, supplied in the past by mantle plumes even though they are not at the margin of
a tectonic plate. Many are under more than 1,000 metres of water but carry evidence that
they were once volcanic islands that rose above the sea, were eroded flat by waves, and
subsided either individually or regionally back to the depths. Sometimes the subsidence
was slow enough for coral reefs to build up around the island, leaving, after the volcanic
land is gone, a circular atoll. Sometimes there are chains of the islands, formed as the ocean
floor moves across a mantle plume. The most famous chain makes up the Hawaiian Islands
and the Emperor seamounts to the northwest of Hawaii.
Landslides and tsunamis
The steep margins of the continental shelf and of seamounts means that the slopes can eas-
ily become unstable. There is evidence on the sea bed and on surrounding coasts of vast
underwater landslides in which slope failure sends many cubic kilometres of sediments
cascading down to the abyssal plain. Well-studied examples lie in the Atlantic, west of the
islands of Madeira and the Canaries; off the northwest African coast; and off the coast of
northern Norway. Sometimes the slide can be triggered by an earthquake, at other times
it is simply that sediments have piled up too steeply and the slope fails. Either way, the
underwater slides can generate devastating waves called tsunamis. There is evidence of
three exceptionally large underwater slides during the past 30,000 years in the Norwegian
Sea northwest of Norway. In one, about 7,000 years ago, 1,700 cubic kilometres of debris
slid down the continental slope to the abyssal plain east of Iceland. The resulting tsunami
flooded parts of the Norwegian and Scottish coastline to a depth 10 metres above the sea
level at the time. An even more devastating slide occurred about 105,000 years ago south
of the island of Lanai in Hawaii. That island experienced flooding 360 metres above the
sea level of the time and the tsunami crossed the Pacific to deposit debris 20 metres above
sea level in eastern Australia.
The sediment released by these huge slides and by the more gentle trickle of small slides
down continental margins is buoyed up by water in a turbulent flow that can spread it con-
siderable distances. It produces characteristic sediments called turbidites in which the grain
size is graded in individual flows. The original slide might contain a mixture of grains but,
as the flow fans out, coarse sand falls out more quickly than fine silt and mud, so individual
flow bands will have a grading from coarse to fine within them. Such turbidites are often
found in sequences of deep-water sedimentary rocks today.
Sea level
One of the clearest features on the surface of our planet is the boundary between land and
sea: the coastline. It is one of the most dynamic environments on Earth, with features ran-
