chapter

206 Unit Three Forces Within
when the Arabian Peninsula rifted from Africa, an event
that began about 20 million years ago. Consequently,
the Red Sea provides oceanographers with a view of
how the Atlantic Ocean may have looked in its infancy.
Not all rift valleys develop into full-fledged spreading
centers. Running through the central United States is
a failed rift extending from Lake Superior to Oklahoma.
This once active rift valley is filled with volcanic rock that
was extruded onto the crust more than a billion years ago.
Why one rift valley develops into an active spreading center
while others are abandoned is not yet known.
Convergent Plate Boundaries
EA R T H S C I E N C E
Forces Within
Plate Tectonics
Although new lithosphere is constantly being added at
the oceanic ridges, our planet is not growing larger—its
total surface area remains constant. To accommodate
Mid-Indian Ridge
Southwest Indian Ridge
Java (Sunda)
Trench
Philippine
Trench
Southeast Indian Ridge
the newly created lithosphere, older portions of oceanic
plates return to the mantle along convergent
( con = together,
vergere = to move)
plate boundaries.
(Because lithosphere is “destroyed” at convergent
boundaries, they are also called destructive plate margins.)
As two plates slowly converge, the leading edge
of one is bent downward, allowing it to slide beneath
the other. The surface expression produced by the descending
plate is an ocean trench, like the Peru–Chile
trench (Figure 7.12, on previous page). Trenches formed
in this manner may be thousands of kilometers long, 8
to 12 kilometers deep, and between 50 and 100 kilometers
wide (Figure 7.13). Destructive plate margins where
oceanic crust is being consumed in the mantle are called
subduction ( sub = under,
duct = lead)
zones.
The average angle at which oceanic lithosphere descends
into the mantle is about 45 degrees. However,
depending on its buoyancy, a plate might descend at
an angle as small as a few degrees or it might plunge
vertically (90 degrees) into the mantle. When a spreading
center is located near a subduction zone, the litho-
Japan Trench
Kurile Trench
Mariana Trench
Aleutian Trench
Bougainville Trench
Alpine Fault
Tonga Trench
Kermadec Trench
San Andreas
Fault
Figure 7.13 Distribution of the world’s oceanic trenches, ridge system, and transform faults. Where transform faults offset ridge
segments, they permit the ridge to change direction (curve) as can be seen in the Atlantic Ocean.
Central
America
Trench
East Pacific Rise