Page 300 - Ultimate Visual Dictionary (DK)

GEOLOGY, GEOGRAPHY, AND METEOROLOGY

The ocean floor Bedrock Shoreline
CONTINENTAL-SHELF FLOOR
exposed by
tidal scour
THE OCEAN FLOOR COMPRISES TWO SECTIONS: the continental
shelf and slope, and the deep-ocean floor. The continental shelf and
Parallel strips
slope are part of the continental crust, but may extend far into the ocean.
of coarse
Sloping quite gently to a depth of about 460 feet (140 m), the continental material left
shelf is covered in sandy deposits shaped by waves and tidal currents. At by strong
tidal currents
the edge of the continental shelf, the seabed slopes down to the abyssal
plain, which lies at an average depth of about 12,500 feet (3,800 m). On
this deep- ocean floor is a layer of sediment made up of clays, fine oozes
formed from the remains of tiny sea creatures, and occasional mineral-
Sand
rich deposits. Echo-sounding and remote sensing from satellites has
deposited
revealed that the abyssal plain is divided by a system of mountain ranges, in wavy
far bigger than any on land—the mid-ocean ridge. Here, magma pattern by
weaker
(molten rock) wells up from the Earth’s interior and solidifies, currents
widening the ocean floor (see pp. 58-59). As the ocean floor spreads,
volcanoes that have formed over hot spots in the crust move away
from their magma source; they become extinct and are increasingly
submerged and eroded. Volcanoes eroded below sea level remain as
seamounts (underwater mountains). In warm waters, a volcano that
projects above the ocean surface often acquires a fringing coral reef,
which may develop into an atoll as the volcano becomes submerged.
FEATURES OF THE OCEAN FLOOR
Sediment Submarine Continental
canyon shelf Course of Irregular patches of
mud river Continental fine sand deposited
rise by weakest currents

Continental slope Seamount
(underwater
mountain)
Guyot (flat-topped
seamount) Abyssal
plain

Continental Ooze (sediment Layer of Pillow Volcanic Oceanic
crust consisting of remains volcanic lava crystalline crust
of tiny sea creatures) rock rock

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