Page 184 - (DK) Ocean - The Definitive Visual Guide

182 THE OPEN OCEAN AND OCEAN FLOOR

Abyssal Plains, Trenches, mouth
barbel

and Mid-ocean Ridges body

OVER VAST AREAS, THE SEABED IS COVERED BY a flat expanse of accumulated
sediments. The sparse life here relies on food falling from above. In places, the abyssal
plains are disrupted by more dramatic features, created by tectonic shifts. Where SEA-BED SCAVENGERS
Hagfish feed on animal corpses that fall
tectonic plates diverge, magma wells up through the gap to create mid-ocean ridges, to the abyssal plain. Blind and jawless,
these primitive fish are attracted by smell.
at which new seabed is constantly being formed. At the other extreme, where plates They bore into corpses, using their horny
collide, one plate is dragged downward, opening up a trench. teeth, and secrete clouds of mucus to
deter other scavengers.

Abyssal Plains
Over large areas of the ocean floor, sediments have built up a blanket several
miles thick, obscuring the underlying topography. This produces vast flat or
gently undulating abyssal plains at a typical depth of 14,800 ft (4,500 m).
These are most common in the Atlantic, where the Sohm Plain alone
covers 350,000 square miles (900,000 square km). Abyssal plains lie at
different depths, with barriers between them, and this leads to submarine
waterfalls, where water spills over the barrier and down into the plain below,
at rates of up to 5 mph (8 kph). Occasional abyssal storms also occur,
MANGANESE NODULE
stimulated, in a way not yet fully understood, by instabilities at the ocean In places, the abyssal
surface resulting from atmospheric conditions. Originally thought to be a plain is littered with
world without seasons, recent studies have shown that life here responds to potato-sized nodules
pulses of food from above, for instance when the summer bloom of plankton of manganese, often
contaminated with other
dies and sinks. Most animals in this zone are scavengers with a body valuable metals such as
temperature close to that of the surrounding water. They move and grow nickel, cooper, and cobalt.
slowly, reproduce infrequently, and live longer than their relatives at the surface.
ABYSSAL FLOOR
A recent study off the east coast of North America
revealed 798 species buried in a small sediment
sample from the seabed. ocean currents carve a deep when a volcanic island sinks, it
gorge, called a submarine eventually becomes a flat-topped
canyon, in the continental slope seamount, or “guyot”
the continental
shelf is the at the mid-ocean ridge two
flooded edge silt carried down a plates pull apart and magma
of a continent, canyon spreads out rises up between them,
which was once at the bottom as a cooling then solidifying,
dry land fan-shaped deposit direction of making a new tectonic plate
plate movement

OCEAN ENVIRONMENTS Earth’s outer layer of the steep continental the gently sloping an underwater plateau is a each tectonic plate melted rock is called

large, flat-topped mound
magma when it occurs
continental rise is a
beneath Earth’s surface,
region that extends
is made of crust
slope goes down
rock in continental
caused by a few million
and lava when it is found
to about 10,000 ft
areas is called
down from the
years of underwater
and the top layer
of the mantle
continental slope
above Earth’s surface
continental crust
volcanic eruptions
(3,000 m)