The following statements summarize and describe many of the key terms and concepts presented
in the chapter.
- Oceanography is an interdisciplinary science that draws on the methods
and knowledge of geology, chemistry, physics, and biology to study all aspects of the world
ocean.
- Earth is a planet dominated by oceans. Seventy-one percent of Earth's
area consists of oceans and marginal seas. In the Southern Hemisphere, often called the
water hemisphere, about 81 percent of the surface is water. The world ocean
can be divided into four main ocean basins: the Pacific Ocean (largest and
deepest ocean), the Atlantic Ocean (about half the size of the Pacific), the
Indian Ocean (slightly smaller than the Atlantic and mostly in the Southern
Hemisphere), and the Arctic Ocean (smallest and shallowest ocean). The average
depth of the oceans is 3729 meters (12,234 feet).
- Ocean bathymetry is determined using echo sounders and multibeam
sonars, which bounce sonic signals off the ocean floor. Ship-based receivers record
the reflected echoes and accurately measure the time interval of the signals. With this
information, ocean depths are calculated and plotted to produce maps of ocean-floor
topography. Recently, satellite measurements of the shape of the ocean surface
have added data for mapping ocean-floor features.
- The zones that collectively make up a passive continental margin
include the continental shelf (a gently sloping, submerged surface extending
from the shoreline toward the deep-ocean basin); the continental slope (the
true edge of the continent, which has a steep slope that leads from the continental shelf
into deep water); and in regions where trenches do not exist, the steep continental slope
merges into a more gradual incline known as the continental rise (which
consists of sediments that have moved downslope from the continental shelf to the deep-ocean
floor).
- Submarine canyons are deep, steep-sided valleys that originate on the
continental slope and may extend to the deep-ocean basin. Many submarine canyons have been
excavated by turbidity currents, which are downslope movements of dense,
sediment-laden water.
- Active continental margins are located primarily around the Pacific
Rim in areas where the leading edge of a continent is overrunning oceanic lithosphere. Here
sediment scraped from the descending oceanic plate is plastered against the continent to
form a collection of sediments called an accretionary wedge. An active
continental margin generally has a narrow continental shelf, which grades into a steep
continental slope and deep-ocean trench.
- The ocean basin floor lies between the continental margin and the
oceanic ridge system. The features of the ocean basin floor include deep-ocean
trenches (the deepest parts of the ocean, where moving crustal plates descend into
the mantle), abyssal plains (the most level places on Earth, consisting of
thick accumulations of sediments that were deposited atop the low, rough portions of the
ocean floor), seamounts and guyots (isolated volcanic peaks on
the ocean floor that originate near the mid-ocean ridge or in association with volcanic hot
spots) and oceanic plateaus (vast accumulations of basaltic lava flows).
- Atolls form from corals that grow on the flanks of sinking volcanic
islands, where the corals continue to build the reef complex upward as the island sinks.
- The oceanic (mid-ocean) ridge winds through the middle of most ocean
basins. Seafloor spreading occurs along this broad feature, which is characterized by an
elevated position, extensive faulting, and volcanic structures that have developed on newly
formed oceanic crust. Most of the geologic activity associated with ridges occurs along a
narrow region on the ridge crest, called the rift valley, where magma moves
upward to create new slivers of oceanic crust.
- There are three broad categories of seafloor sediments. Terrigenous sediment
consists primarily of mineral grains that were weathered from continental rocks and
transported to the ocean; biogenous sediment consists of shells and skeletons
of marine animals and plants; and hydrogenous sediment includes minerals that
crystallize directly from seawater through various chemical reactions. The global
distribution of marine sediments is affected by proximity to source areas and water
temperatures that favor the growth of certain marine organisms.
- Seafloor sediments are helpful when studying worldwide climate changes
because they often contain the remains of organisms that once lived near the sea surface.
The numbers and types of these organisms change as the climate changes, and their remains in
seafloor sediments record these changes.
- Energy resources from the seafloor include oil and natural gas and
large untapped deposits of gas hydrates. Other seafloor resources include
sand and gravel, evaporative salts, and metals within manganese
nodules.
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