Subd ucting oce an ic li t h o s p h e r e
A.
Asthenosphere
B.
Continental crust
Continental crust
Forearc
basin
Continental volcanic arc
Asthenosphere
Former
volcanic
arc
Suture
Ophiolite
(fragments of
oceanic crust)
Continental
crust
Continental
crust
Oceanic
crust
31
Dynamic Earth
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Antarctic plate
Eurasian plate
Philippine
plate
Australian-Indian plate
Pacific
plate
North American
plate
Continental
lithosphere
M
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d
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I
n
d
ia
n
R
id
ge
Baikal Rift
India
Himalayas
Alpine Fault
Japan Arc
Mariana Arc
S o u t h e a s t In d ia n R id ge
Urals
C. Transform fault boundary
Asthenosphere
Lithosphere
Sinking
lithosphere
Upwelling
South
America
Africa
Atlantic
Ocean
Pacific
Ocean
M id - A tl a n ti c R id g e
Convergent Convergent
boundary boundary
Divergent Divergent
boundary boundary
Convergent
boundary
Divergent
boundary
Peru-Chile
Trench
Andes Mts.
FIGURE 1.31 Convergent boundaries occur where two plates move together, as along
the western margin of South America. Divergent boundaries are located where
adjacent plates move away from one another. The Mid-Atlantic Ridge is such a
boundary.
denser oceanic lithosphere sinks into the asthenosphere (Figure 1.31). The classic convergent boundary of this type occurs along the western margin of South
America where the Nazca plate descends beneath the adjacent continental block.
Here subduction along the Peru–Chile trench gave rise to the Andes Mountains,
a linear chain of deformed rocks capped with numerous volcanoes—a number
of which are still active.
The simplest type of convergence occurs where one oceanic plate is thrust
beneath another. At such sites subduction results in the production of magma in
a manner similar to that in the Andes, except volcanoes grow from the floor of
the ocean rather than on a continent. If this activity is sustained, it will eventually build a chain of volcanic structures that emerge from the sea as a volcanic
island arc. Most volcanic island arcs are found in the Pacific Ocean, as exemplified by the Aleutian, Mariana, and Tonga islands.
As we saw earlier, when an oceanic plate is subducted beneath continental
lithosphere, an Andean-type mountain range develops along the margin of the
continent. However, if the subducting plate also contains continental lithosphere, continued subduction eventually brings the two continents together
(FIGURE 1.32). Whereas oceanic lithosphere is relatively dense and sinks into the
FIGURE 1.32 When two plates containing continental lithosphere
collide, complex mountains are formed. The formation of the
Himalayas represents a relatively recent example.
the surface, where it gives rise to explosive volcanic eruptions such as Mount St. Helens in 1980.
However, much of this molten rock never reaches
the surface; rather, it solidifies at depth and acts to
thicken the crust.
Whenever slabs of continental lithosphere and
oceanic lithosphere converge, the continental plate
being less dense remains “floating,” while the
A.
Asthenosphere
B.
Continental crust
Continental crust
Forearc
basin
Continental volcanic arc
Asthenosphere
Former
volcanic
arc
Suture
Ophiolite
(fragments of
oceanic crust)
Continental
crust
Continental
crust
Oceanic
crust
31
Dynamic Earth
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Antarctic plate
Eurasian plate
Philippine
plate
Australian-Indian plate
Pacific
plate
North American
plate
Continental
lithosphere
M
i
d
-
I
n
d
ia
n
R
id
ge
Baikal Rift
India
Himalayas
Alpine Fault
Japan Arc
Mariana Arc
S o u t h e a s t In d ia n R id ge
Urals
C. Transform fault boundary
Asthenosphere
Lithosphere
Sinking
lithosphere
Upwelling
South
America
Africa
Atlantic
Ocean
Pacific
Ocean
M id - A tl a n ti c R id g e
Convergent Convergent
boundary boundary
Divergent Divergent
boundary boundary
Convergent
boundary
Divergent
boundary
Peru-Chile
Trench
Andes Mts.
FIGURE 1.31 Convergent boundaries occur where two plates move together, as along
the western margin of South America. Divergent boundaries are located where
adjacent plates move away from one another. The Mid-Atlantic Ridge is such a
boundary.
denser oceanic lithosphere sinks into the asthenosphere (Figure 1.31). The classic convergent boundary of this type occurs along the western margin of South
America where the Nazca plate descends beneath the adjacent continental block.
Here subduction along the Peru–Chile trench gave rise to the Andes Mountains,
a linear chain of deformed rocks capped with numerous volcanoes—a number
of which are still active.
The simplest type of convergence occurs where one oceanic plate is thrust
beneath another. At such sites subduction results in the production of magma in
a manner similar to that in the Andes, except volcanoes grow from the floor of
the ocean rather than on a continent. If this activity is sustained, it will eventually build a chain of volcanic structures that emerge from the sea as a volcanic
island arc. Most volcanic island arcs are found in the Pacific Ocean, as exemplified by the Aleutian, Mariana, and Tonga islands.
As we saw earlier, when an oceanic plate is subducted beneath continental
lithosphere, an Andean-type mountain range develops along the margin of the
continent. However, if the subducting plate also contains continental lithosphere, continued subduction eventually brings the two continents together
(FIGURE 1.32). Whereas oceanic lithosphere is relatively dense and sinks into the
FIGURE 1.32 When two plates containing continental lithosphere
collide, complex mountains are formed. The formation of the
Himalayas represents a relatively recent example.
the surface, where it gives rise to explosive volcanic eruptions such as Mount St. Helens in 1980.
However, much of this molten rock never reaches
the surface; rather, it solidifies at depth and acts to
thicken the crust.
Whenever slabs of continental lithosphere and
oceanic lithosphere converge, the continental plate
being less dense remains “floating,” while the