Plate Tectonics: A Unifying Theory - The Changing Earth: Exploring Geology and Evolution - page 43

TABLE 2.1 Types of Plate Boundaries

Type Example Landforms Volcanism

Divergent
Oceanic

Mid-Atlantic Ridge Mid-oceanic ridge with axial Basalt

rift valley

Continental East African Rift Valley Rift valley Basalt and rhyolite,

no andesite

Convergent
Oceanic-oceanic

Aleutian Islands Volcanic island arc, offshore Andesite

oceanic trench

Oceanic-continental Andes Offshore oceanic trench, Andesite

volcanic mountain

chain, mountain belt

Continental-continental Himalayas Mountain belt Minor

Transform San Andreas fault Fault valley Minor

surface area of Earth to remain the same. Otherwise, we
would have an expanding Earth. Such plate destruction oc-
curs at convergent plate boundaries (

resulting in the formation of a back-arc basin. This back-
arc basin may grow by spreading if magma breaks through
the thin crust and forms new oceanic crust (Figure 2.18a).
A good example of a back-arc basin associated with an
oceanic-oceanic plate boundary is the Sea of Japan between
the Asian continent and the islands of Japan.
Most present-day active volcanic island arcs are in the
Pacific Ocean basin and include the Aleutian Islands, the
Kermadec-Tonga arc, and the Japanese (Figure 2.18a) and
Philippine Islands. The Scotia and Antillean (Caribbean)
island arcs are in the Atlantic Ocean basin.

Figure 2.18), where
two plates collide and the leading edge of one plate is sub-
ducted beneath the margin of the other plate and eventually
incorporated into the asthenosphere. A dipping plane of
earthquake foci, called a Benioff zone, defines subduction
zones (see Figure 8.5). Most of these planes dip from oceanic
trenches beneath adjacent island arcs or continents, marking
the surface of slippage between the converging plates.
Deformation, volcanism, mountain building, metamor-
phism, earthquake activity, and deposits of valuable min-
erals characterize convergent boundaries. Three types of
convergent plate boundaries are recognized: oceanic-oceanic,
oceanic-continental, and continental-continental.

◗

Oceanic-Continental Boundaries When an oceanic and a
continental plate converge, the denser oceanic plate is subducted
under the continental plate along an oceanic-continental
plate boundary (Figure 2.18b). Just as at oceanic-oceanic plate
boundaries, the descending oceanic plate forms the outer wall
of an oceanic trench.
The magma generated by subduction rises beneath the
continent and either crystallizes as large intrusive bodies,
called plutons , before reaching the surface or erupts at the sur-
face to produce a chain of andesitic volcanoes, also called a
volcanic arc . An excellent example of an oceanic-continental
plate boundary is the Pacifi c Coast of South America where
the oceanic Nazca plate is currently being subducted under
South America (Figure 2.18b; see also Chapter 10). The Peru-
Chile Trench marks the site of subduction, and the Andes
Mountains are the resulting volcanic mountain chain on the
nonsubducting plate.

Oceanic-Oceanic Boundaries When two oceanic plates
converge, one is subducted beneath the other along an
oceanic-oceanic plate boundary (Figure 2.18a). The sub-
ducting plate bends downward to form the outer wall of an
oceanic trench. A subduction complex , composed of wedge-
shaped slices of highly folded and faulted marine sediments
and oceanic lithosphere scraped off the descending plate,
forms along the inner wall of the oceanic trench. As the
subducting plate descends into the mantle, it is heated and
partially melted, generating magma commonly of andes-
itic composition (see Chapter 4). This magma is less dense
than the surrounding mantle rocks and rises to the surface
of the nonsubducted plate to form a curved chain of volca-
nic islands called a volcanic island arc (any plane intersect-
ing a sphere makes an arc). This arc is nearly parallel to the
oceanic trench and is separated from it by a distance of up
to several hundred kilometers—the distance depends on the
angle of dip of the subducting plate (Figure 2.18a).
In those areas where the rate of subduction is faster than
the forward movement of the overriding plate, the litho-
sphere on the landward side of the volcanic island arc may
be subjected to tensional stress and stretched and thinned,

Continental-Continental Boundaries Two continents
approaching each other are initially separated by an ocean
floor that is being subducted under one continent. The
edge of that continent displays the features characteristic of
oceanic-continental convergence. As the ocean fl oor continues
to be subducted, the two continents come closer together until
they eventually collide. Because continental lithosphere, which
consists of continental crust and the upper mantle, is less dense