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Emperor Seamount chain and the Hawaiian Islands in the northern Pacific Ocean,
as shown in Fig. 2.19). Such island chains apparently form when a plate passes
over a 'hotspot', a localized region where magma is rising from deep in the man-
tle. The depth from which the magma comes can be roughly estimated from the
chemistry of the lava. In addition, the islands can be dated and the history of
the passage of the plate over the magma source determined. This information
was used in the determination of the plate motions in Chapter 2. The aseismic
ridges (such as the Walvis Ridge and the Rio Grande Rise in the South Atlantic
Ocean, and the Iceland-Faeroe Ridge in the North Atlantic Ocean) are subma-
rine volcanic mountain chains, typically elevated some 2-4 km above the seabed
which formed as the plate passed over a hot region in the mantle. They are not
tectonically active (hence their name).
Continental margins
The continental margins, as their name suggests, mark the transition between
the continent and the ocean floor. At a passive margin (Fig. 9.2), such as occurs
off the east coast of the Americas and the matching west coast of Europe and
Africa, the first structure seawards of the land is the gently sloping continental
shelf, which is merely continent covered by shallow water. At the outer edge of
the shelf, the gradient abruptly increases and the seabed deepens rapidly; this is
the continental slope. At the base of the slope, the gradient is again much less;
this region is the continental rise. The transition from continental crust to oceanic
crust occurs beneath the continental slope.
An active margin , such as the west coast of North and South America, is so
called because of the igneous and tectonic activity occurring at the plate boundary.
There the continental shelf is often, but not always, very narrow. Where the plate
boundary is a transform fault, the seabed characteristically drops rapidly from
the shelf to oceanic depths. Where the plate boundary is a subduction zone, there
is usually a trench, typically many kilometres deep.
Oceanic trenches
The oceanic trench marks the surface location of a subduction zone, at which one
plate is overriding another; for example, the continental South American plate
is overriding the oceanic Nazca plate along the west coast of South America.
Here the Peru-Chile Trench extends to depths of 8 km and is considerably less
than 200 km in width (see Fig. 9.1). Many of the trenches around the western
margin of the Pacific plate occur where an oceanic plate is overriding the oceanic
Pacific plate. At such destructive plate boundaries, an island arc runs parallel
to the trench, and frequently seafloor spreading occurs on the consuming plate
behind the island arc, thus forming a back-arc ,or marginal basin . The trenches
of the western Pacific are frequently 10 km or more deep.
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