Geology Reference
In-Depth Information
used as an energy source. Additionally, their contribution to
global warming must be assessed because a volume of meth-
ane 3000 times greater than in the atmosphere is present in
seafl oor deposits—and methane is 10 times more effective
than carbon dioxide as a greenhouse gas.
The manganese nodules previously discussed are an-
other potential seafloor resource (Figure 9.15). These
spherical objects are composed mostly of manganese and
iron oxides, but also contain copper, nickel, and cobalt. The
United States, which must import most of the manganese
and cobalt it uses, is particularly interested in these nodules
as a potential resource.
Other seafl oor resources of interest include massive sul-
fi de deposits that form by submarine hydrothermal activity
at spreading ridges. These deposits, containing iron, copper,
zinc, and other metals, have been identifi ed within the EEZ
at the Gorda Ridge off the coasts of California and Oregon;
similar deposits occur at the Juan de Fuca Ridge within the
Canadian EEZ.
Within the EEZ, manganese nodules are found near
Johnston Island in the Pacifi c Ocean and on the Blake Plateau
off the east coast of South Carolina and Georgia. In addition,
seamounts and seamount chains within the EEZ in the Pacifi c
are known to have metalliferous oxide crusts several centime-
ters thick from which cobalt and manganese could be mined.
Another important resource found in shallow ma-
rine deposits is phosphate-rich sedimentary rock known as
phosphorite
(see Geo-Focus on page 240).
Chapter Summary
Scientifi c investigations of the oceans began more than
200 years ago, but much of our knowledge comes from
studies done during the last few decades.
Present-day research vessels are equipped to investigate
the seafl oor by sampling and drilling, echo sounding, and
seismic profi ling. Scientists also use submersibles in their
studies.
Deep-sea drilling and observations on land and on
the seafl oor confi rm that oceanic crust is made up, in
descending order, of pillow lava/sheet lava fl ows, sheeted
dikes, and gabbro.
Continental margins consist of a gently sloping continen-
tal shelf, a more steeply inclined continental slope, and, in
some cases, a continental rise.
The width of continental shelves varies considerably.
They slope seaward to the shelf-slope break at a depth
averaging 135 m, where the seafl oor slope increases
abruptly.
Submarine canyons, mostly on continental slopes, carry
huge quantities of sediment by turbidity currents into
deeper water, where it is deposited as overlapping subma-
rine fans that make up a large part of the continental rise.
Active continental margins at the leading edge of a tec-
tonic plate have a narrow shelf and a slope that descends
directly into an oceanic trench. Volcanism and seismic
activity also characterize these margins.
Passive continental margins lie within a tectonic plate and
have wide continental shelves, and the slope merges with
a continental rise that grades into an abyssal plain. These
margins show little seismic activity and no volcanism.
Long, narrow oceanic trenches are found where oceanic
lithosphere is subducted beneath either oceanic lithosphere
or continental lithosphere. The trenches are the sites of
the greatest oceanic depths and low heat fl ow.
Oceanic ridges are composed of volcanic rocks, and many
have a central rift caused by tensional forces. Basaltic
volcanism, hydrothermal vents, and shallow-focus earth-
quakes occur at ridges, which are offset by fractures that
cut across them.
Seamounts, guyots, and abyssal hills rising from the sea-
fl oor are common features that differ mostly in scale and
shape. Many aseismic ridges on the seafl oor consist of
chains of seamounts, guyots, or both.
Submarine hydrothermal vents known as black smok-
ers found at or near spreading ridges support biologic
communities and are potential sources of several
resources.
Moundlike, wave-resistant structures called reefs, consisting
of animal skeletons, are found in a variety of shapes, but
most are classifi ed as fringing reefs, barrier reefs, or atolls.
Sediments called pelagic clay and ooze cover vast areas of
the seafl oor.
The United States claims rights to all resources within
200 nautical miles of its shorelines. Resources including
sand and gravel, as well as metals, are found within this
Exclusive Economic Zone (EEZ).
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