Laboratory Manual for Introductory Geology 4e

■ Continental lithosphere is thicker than oceanic lithosphere (FIG. 2.2) because
continental crust alone (without the mantle component) is 25 to 70 km thick,
whereas oceanic crust is only about 7 km thick.
■ The lithosphere plates rest on the asthenosphere, a zone in the upper mantle
(Fig. 2.2) that, although solid, has such low rigidity that it can flow like soft plastic.
The asthenosphere acts as a lubricant, permitting the plates above it to move. Plates
move relative to one another at 1 to 15 cm/yr, roughly the rate at which fingernails grow.
FIGURE 2.2 Cross section showing activity at convergent, divergent, and transform boundaries. At divergent boundaries
(mid-ocean ridges), new lithosphere is created; at convergent boundaries (subduction zones and areas of continental collision), old
lithosphere is destroyed; and at transform boundaries, lithosphere is neither created nor destroyed.
Volcanic
island arc
Convergent
boundary
Divergent
boundary
(mid-ocean ridge)
Transform
fault
Convergent
boundary
Continental arc
Trench
Trench
Ocean crust
Oceanic
lithosphereh
Subduction zone Oceanic crust
Lithospheric h i mantle
Subduction zone
Lithospheric mantle
Melting
zone
Continental
crust
Continental
lithosphere
Asthenosphere
■ A place where two plates make contact is called a plate boundary, and in some places
three plates come together at a triple junction. There are three different kinds of plate
boundaries, defined by the relative motions of the adjacent plates (see Figs. 2.1 and 2.2):
1. At a divergent boundary, plates move away from one another at the axes of huge
submarine mountain ranges called mid-ocean ridges, or oceanic ridges. Molten
material rises from the asthenosphere to form new oceanic lithosphere at the
ridge axis. The ocean grows wider through this process, called seafloor spreading,
as the new lithosphere moves outward from the axis to the flanks of the ridge.
Continental rifts are places where continental lithosphere is stretched and
pulled apart in the process of breaking apart at a new divergent boundary. If rifting
is “successful,” a continent splits into two pieces separated by a new oceanic
plate, which gradually widens by seafloor spreading.
2. At a convergent boundary, two plates move toward each other, and the oceanic
lithosphere of one plate (the subducting plate) sinks into the mantle below the
other (the overriding plate), forming a subduction zone. The lithosphere of the
overriding plate may be oceanic or continental. The boundary between the two
plates is a deep-sea trench. At depths of 100 to 150 km, gases (mostly steam)
released from the heated subducting plate rise into the lower part of the lithosphere.
These gases help melt the mantle component of the lithosphere, and the
resulting magma rises to the surface to produce volcanoes as either a volcanic
island arc, where the overriding plate is made of oceanic lithosphere (as in the
islands of Japan), or as a continental arc, where the overriding plate is made of
continental lithosphere (as in the Andes of South America).
Continental crust cannot be subducted because its lower density makes it too
buoyant to sink into the mantle. When subduction completely consumes an oceanic
2.2 THE THEORY OF PLATE TECTONICS
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