The Ocean Basin
The ocean basin is not the featureless, underwater expanse people once
thought it was. It has a remarkable and interesting terrain that, like all of
Earth’s environments, changes over time.
Abyssal Plain and Abyssal Hills
One feature of the deep sea floor is the abyssal plain, the flattest of all
Earth’s surface areas. The abyssal plain off Argentina’s coast, for example,
rises fewer than 3 meters over 1300 kilometers (10 feet over 800 miles).
Abyssal plains are composed of sediments, most of which came from
continents. How could material from continents reach the deep sea floor?
The answer lies in turbidity currents. During periods when the sea level is
low, such as ice ages, rivers deposit sediment at the edges of continental
shelves. Turbidity currents then carry the material down the continental
slopes when the sea level rises again and spread it over the continental rise
and abyssal plain. The sediments in an abyssal plain can be more than one
kilometer thick.
Although they occur in all oceans, abyssal plains are especially well
developed and widespread in the Atlantic Ocean, where the continental
margins have few trenches that can trap continental sediment.
Abyssal hills are another part of the deep ocean basin. These small,
rolling hills often occur in groups next to oceanic ridge systems. In the
North Atlantic, abyssal hills form two strips that run parallel to the midAtlantic Ridge for almost its entire length.
Individual hills are typically several hundred meters to tens of kilometers
across and rise no more than a few hundred meters above the abyssal plain.
Interestingly, scientists have found entire systems of abyssal hills beneath the
layers of sediment that blankets the abyssal plains. These sediment-covered
hills represent the original sea-floor surface that formed at mid-ocean ridges.
23.3
KEY IDEA
The ocean basin has a wide range
of topographical features. Natural
forces change these features
over time.
KEY VOCABULARY
• abyssal plain
• abyssal hill
• island arc
• fracture zone
• seamount
• guyot
• coral atoll
Abyss comes from the ancient
Greek word for “bottomless,”
abussos.
Abyssal
plain
Abyssal
hills
ABYSSAL PLAIN AND HILLS Generated from satellite and ship-board
measurements of sea floor and land elevations, this map uses color variation to
represent changes in elevation. The uniformity of the abyssal plain indicates little
change in elevation while slight changes can be detected for the abyssal hills.
Chapter 23 The Ocean Floor
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