Page 182 - (DK) Ocean - The Definitive Visual Guide

180 THE OPEN OCEAN AND OCEAN FLOOR

Ocean Floor Sediments

OVER VAST AREAS OF THE SEABED, THE UNDERLYING landforms are hidden beneath
deep layers of sediments. Made up of silts, muds, or sands that have built up over
200 million years, they now form a blanket that is several miles thick in places. The
sediments have various origins. One group, terrigenous sediments, come from land,
mainly from fragments of eroded rock that are carried down rivers into the sea, then
down the continental slope to form the continental rise and abyssal plain beyond.
Other sediments are biogenic, formed from the hard remains of dead animals and
plants. A few, called authigenic sediments, are made up of chemicals precipitated
from seawater. There are even cosmogenic sediments, which come from outer space
as particles in space dust and meteors. All accumulate to form extensive, flat plains.
Various animals feed here and burrow into the sediments for shelter.

Deep-sea Sediments
The average thickness of sediments on the
ocean floor is 1,500 ft (450 m), but in the
Atlantic Ocean and around Antarctica,
sediments can be up to 3,300 ft (1,000 m)
deep. Closer to the continents —along the
continental rise—sediments washed from
the land accumulate more rapidly, and can
be up to 9 miles (15 km) deep. In the open
ocean, further from the source of terrigenous
sediments, the buildup rate is very slow: from a
fraction of an inch to a few inches in a thousand
years. That is slower than the rate at which dust builds
SEDIMENT THICKNESS
up on furniture in an average house. The accumulated
0 1,650 ft 3,300 ft 3 miles 6 miles 12 miles sediments tell scientists a great deal about the last 200
(500 m) (1,000 m) (5 km) (10 km) (20 km)
million years of Earth’s history. Their form
MAPPING SEDIMENTS and arrangement provide a vivid snapshot of
Ocean sediment depths sea-floor spreading, the evolving varieties of
can be measured and ocean life, alterations in Earth’s magnetic field,
mapped using echo-
sounding. Some areas and changes in ocean currents and climate.
(white on the map above)
are still unsurveyed. WHITE CLIFFS OF DOVER
Sediment is thickest These chalk cliffs originated on the sea
near land. Glaciers also bed from a biogenic ooze, formed from
carry many sediments algal scales (coccoliths) that built up
into the oceans. to form layers hundreds of yards thick.
They are now raised above sea level.

PTEROPOD OOZE Sediments Derived from the Land
Most terrigenous sediments come from the weathering of rock
Pteropods are small winged snails on land and are swept into the oceans, mainly by rivers but also
that float in midwater. When by glaciers, ice sheets, and wind. Coastal erosion adds to these
they die, their internal shells of
aragonite (calcium carbonate) sediments. Often, they are washed down through submarine
sink to the seabed, contributing canyons to the deeper ocean. Sometimes, the route from land
OCEAN ENVIRONMENTS water temperatures and sea levels. the main sediment is red clay, composed mostly of fine-grained
to sea is more indirect: volcanic eruptions eject material into
to biogenic oozes. The presence
of pteropod remains in samples
the upper atmosphere before it falls as “rain” into the ocean.
collected from deep in the ooze
In the deepest ocean floors, below about 13,000 ft (4,000 m),
reveal changes over millennia in
silts that have washed off the continents and accumulated

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incredibly slowly—about /32 in or 1 mm per thousand years.
These clays may include up to 30 percent
of fine, biogenic particles and have four
main mineral components —chlorite,
DUST STORM
illite, kaolinite, and montmorillonite.
RESULTS IN SILT
Clay types depend on origin and climate.
Winds from arid regions,
such as North Africa
For example, chlorite dominates in polar
(shown in this satellite
regions, kaolinite in the tropics, and
image) carry dust far out
montmorillonite is produced by
to sea, where it sinks to
volcanic activity.
form silts.