Geology Reference
In-Depth Information
Oceanic Trenches
Long, steep-sided depressions on the seafl oor near conver-
gent plate boundaries, called oceanic trenches , constitute
no more than 2% of the seafl oor, but they are important
features because it is here that oceanic lithosphere is con-
sumed by subduction (see Chapter 2). Because oceanic
trenches are found along active continental margins, they
are common in the Pacifi c Ocean basin, but largely lack-
ing in the Atlantic, those in the Caribbean being notable
exceptions (Figure 9.10). On the landward sides of oceanic
trenches, the continental slope descends into them at up
to 25 degrees, and many have thick accumulations of sedi-
ments. The greatest oceanic depths are found in trenches;
the Challenger Deep of the Marianas Trench in the Pacifi c
is more than 11,000 m deep!
Sensitive instruments can detect the amount of heat es-
caping from Earth's interior by the phenomenon of heat fl ow.
As one might expect, heat fl ow is greatest in areas of active or
recently active volcanism. For instance, higher-than-average
heat fl ow takes place at spreading ridges, but at subduction
zones heat fl ow values are less than the average for Earth as a
whole. Oceanic crust at oceanic trenches is cooler and slightly
denser than elsewhere.
Seismic activity also takes place at or near oceanic
trenches along planes dipping at about 45 degrees. In Chap-
ter 8, we discussed these inclined seismic zones called Benioff
zones (see Figure 8.5), where most of Earth's intermediate
and deep earthquakes occur. Volcanism does not take place
in trenches, but because these are zones where oceanic lith-
osphere is subducted beneath either oceanic or continental
lithosphere, an arcuate chain of volcanoes is found on the
overriding plate (Figure 9.9). The Aleutian Islands and the
volcanoes along the western margin of South America are
good examples of such chains.
composed almost entirely of basalt and gabbro and possess
features produced by tensional forces. Mountain ranges on
land, in contrast, consist of all rock types, and they formed
when rocks were folded and fractured by compressive forces
(see Chapter 10).
Oceanic ridges are mostly below sea level, but they
rise above the sea in Iceland, the Azores, Easter Island, and
several other places. Of course, oceanic ridges are the sites
where new oceanic crust is generated and plates diverge.
The rate of plate divergence is important because it deter-
mines the cross-sectional profile of a ridge. For example,
the Mid-Atlantic Ridge has a comparatively steep profile
because divergence is slow, allowing the new oceanic crust
to cool, shrink, and subside closer to the ridge crest than
it does in areas of faster divergence such as the East Pacifi c
Rise. A ridge may also have a rift along its crest that opens
in response to tension (
Figure 9.12a). A rift is particularly
obvious along the Mid-Atlantic Ridge, but it is absent along
parts of the East Pacifi c Rise. These rifts are commonly 1 to
2 km deep and several kilometers wide. They open as sea-
fl oor spreading takes place (discussed in Chapter 2) and are
characterized by shallow-focus earthquakes, basaltic volca-
nism, and high heat fl ow.
Scientists have been making direct observations of
oceanic ridges and their rifts since 1974. As part of Project
FAMOUS (French-American Mid-Ocean Undersea Study),
submersibles have descended to the ridges and into their rifts
in several areas. Researchers have not seen any active volca-
nism, but they did see pillow lavas (see Figure 5.7), lava tubes,
and sheet lava fl ows, some of which formed very recently. In
fact, on return visits to a site, they have seen the effects of
volcanism that occurred since their previous visit. And on
January 25, 1998, a submarine volcano began erupting along
the Juan de Fuca Ridge west of Oregon. Researchers aboard
submersibles have also observed hot water being discharged
from the seafl oor at or near ridges in submarine hydrother-
mal vents.
Oceanic Ridges
When the first submarine cable was laid between North
America and Europe during the late 1800s, a feature called
the Telegraph Plateau was discovered in the North Atlantic.
Using this data and data from a 1925-27 voyage of the
German research vessel Meteor , scientists proposed that the
plateau was actually a continuous ridge extending the length
of the Atlantic Ocean basin. Subsequent investigations
revealed that this conjecture was correct, and we now call
this feature the Mid-Atlantic Ridge (Figure 9.10).
The Mid-Atlantic Ridge is more than 2000 km wide and
rises 2 to 2.5 km above the adjacent seafl oor. Furthermore, it
is part of a much larger oceanic ridge system of mostly sub-
marine mountainous topography. This system runs from the
Arctic Ocean through the middle of the Atlantic and curves
around South Africa, where the Indian Ridge continues into
the Indian Ocean; the Atlantic-Pacifi c Ridge extends east-
ward and a branch of this, the East Pacifi c Rise, trends north-
east until it reaches the Gulf of California (Figure 9.10). The
entire system is at least 65,000 km long, far exceeding the
length of any mountain system on land. Oceanic ridges are
Submarine Hydrothermal Vents
Scientists fi rst saw submarine hydrothermal vents on the
seafl oor in 1979 when they descended about 2500 m to the
Galapagos Rift in the eastern Pacifi c Ocean. Since 1979, they
have seen similar vents in several other areas in the Pacifi c
(Figure 9.12b), Atlantic, the Indian Ocean, and the Sea of
Japan. The vents are at or near spreading ridges where cold
seawater seeps through oceanic crust, is heated by the hot
rocks at depth, and then rises and discharges into the sea-
water as plumes of hot water with temperatures as high as
400°C. Many of the plumes are black because dissolved min-
erals giving them the appearance of black smoke—hence the
name black smoker (Figure 9.12c).
Submarine hydrothermal vents are interesting from
the biologic, geologic, and economic points of view. Near
the vents live communities of organisms, such as bacteria,
crabs, mussels, starfi sh, and tube worms, many of which had
never been seen before. No sunlight is available, so these
 
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