Plate Tectonics: A Unifying Theory - The Changing Earth: Exploring Geology and Evolution - page 45

Arabian Plate

African
Plate

The Red Sea represents a more advanced stage of rifting, in
which two continental blocks (Africa and the Arabian Peninsula are
separated by a narrow sea.

b

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Figure 2.17 East African Rift Valley and the Red Sea—Present-
Day Examples of Divergent Plate Boundaries The East African Rift
Valley and the Red Sea represent different stages in the history of a
divergent plate boundary.

system, resulted from the collision between India and Asia
that began 40 to 50 million years ago and is still continuing
(Figure 2.18c; see Chapter 10).

Recognizing Ancient Convergent Plate Boundaries How
can former subduction zones be recognized in the geologic
record? Igneous rocks provide one clue to ancient subduc-
tion zones. The magma erupted at the surface, forming
island arc volcanoes and continental volcanoes, and is of an-
desitic composition. Another clue is the zone of intensely de-
formed rocks between the deep-sea trench where subduction
is taking place and the area of igneous activity. Here, sedi-
ments and submarine rocks are folded, faulted, and metamor-
phosed into a chaotic mixture of rocks termed a mélange.
During subduction, pieces of oceanic lithosphere are
sometimes incorporated into the mélange and accreted onto
the edge of the continent. Such slices of oceanic crust and
upper mantle are called ophiolites (

The East African Rift Valley is being formed by the separation of
eastern Africa from the rest of the continent along a divergent
plate boundary.

a

than oceanic lithosphere (oceanic crust and upper mantle), it
cannot sink into the asthenosphere. Although one continent
may partially slide under the other, it cannot be pulled or
pushed down into a subduction zone (Figure 2.18c).
When two continents collide, they are welded together
along a zone marking the former site of subduction. At this
continental-continental plate boundary, an interior moun-
tain belt is formed consisting of deformed sediments and
sedimentary rocks, igneous intrusions, metamorphic rocks,
and fragments of oceanic crust. In addition, the entire re-
gion is subjected to numerous earthquakes. The Himalayas
in central Asia, the world's youngest and highest mountain

Figure 2.19). They con-
sist of a layer of deep-sea sediments that include graywackes
(poorly sorted sandstones containing abundant feldspars and
rock fragments, usually in a clay-rich matrix), black shales,
and cherts (see Chapter 6). These deep-sea sediments are un-
derlain by pillow lavas, a sheeted dike complex, massive gab-
bro (a dark intrusive igneous rock), and layered gabbro, all of
which form the oceanic crust. Beneath the gabbro is perido-
tite (a dark intrusive igneous rock composed of the mineral
olivine), which probably represents the upper mantle. The
presence of ophiolite in an outcrop or drilling core is a key
indicator of plate convergence along a subduction zone.

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