371
Divergent Boundaries
▲ ▲
C. Transform fault boundary
North American
plate
Juan de Fuca
plate
Pacific plate
Nazca
plate
Cocos
plate
Antarctic plate
Scotia plate
South American
plate
Caribbean plate
African plate
M i d -
A t l a n t i c R id g e
E
as
t
Pacific R is e
Eurasian plate
A n d e s M
o
u
n
t
ai
ns
R
o
c
k
y
M
o
u
n
t
a
in
s
A p p a l a c h i a n M t s .
A lp s
Canadian Shield
Basin
and
Range
San Andreas
Fault
Galapagos
Ridge
Chile Ridge
Antilles
Arc
Iceland
B. Convergent boundary
Oceanic
crust
Trench
Continental
lithosphere
Continental
volcanic arc
Subductin g o c e a n ic li t h o s p h e r e
Oceanic
lithosphere
Stated another way, the thickness of oceanic
lithosphere is age-dependent. The older
(cooler) it is, the greater its thickness.
Oceanic lithosphere that exceeds 80 million
years in age is about 100 kilometers thick:
about its maximum thickness.
Continental
Rifting
Divergent boundaries can
also develop within a
continent, in which case
the landmass may split
into two or more smaller
segments separated by an
ocean basin. Continental
rifting occurs where
opposing tectonic forces
act to pull the lithosphere apart. The initial
stage of rifting tends to
include mantle upwelling
that is associated with
broad upwarping of the
overlying lithosphere
(FIGURE 15.11A). As a
result, the lithosphere is
stretched, causing the
brittle crustal rocks to
break into large slabs. As
the tectonic forces continue to pull the crust
apart, these crustal fragments sink, generating an
elongated depression
called a continental rift
(FIGURE 15.11B).
A modern example of
an active continental rift is
the East African Rift (see
Figure 15.9, left). Whether
this rift will eventually
result in the break-up of
Africa is a topic of continued research. Nevertheless,
the East African Rift is an excellent model of the initial stage in the
break-up of a continent. Here, tensional forces have stretched and
thinned the crust, allowing molten
rock to ascend from the mantle. Evidence
for recent volcanic activity includes several
large volcanic mountains including Mount
Kilimanjaro and Mount Kenya, the tallest
Divergent Boundaries
▲ ▲
C. Transform fault boundary
North American
plate
Juan de Fuca
plate
Pacific plate
Nazca
plate
Cocos
plate
Antarctic plate
Scotia plate
South American
plate
Caribbean plate
African plate
M i d -
A t l a n t i c R id g e
E
as
t
Pacific R is e
Eurasian plate
A n d e s M
o
u
n
t
ai
ns
R
o
c
k
y
M
o
u
n
t
a
in
s
A p p a l a c h i a n M t s .
A lp s
Canadian Shield
Basin
and
Range
San Andreas
Fault
Galapagos
Ridge
Chile Ridge
Antilles
Arc
Iceland
B. Convergent boundary
Oceanic
crust
Trench
Continental
lithosphere
Continental
volcanic arc
Subductin g o c e a n ic li t h o s p h e r e
Oceanic
lithosphere
Stated another way, the thickness of oceanic
lithosphere is age-dependent. The older
(cooler) it is, the greater its thickness.
Oceanic lithosphere that exceeds 80 million
years in age is about 100 kilometers thick:
about its maximum thickness.
Continental
Rifting
Divergent boundaries can
also develop within a
continent, in which case
the landmass may split
into two or more smaller
segments separated by an
ocean basin. Continental
rifting occurs where
opposing tectonic forces
act to pull the lithosphere apart. The initial
stage of rifting tends to
include mantle upwelling
that is associated with
broad upwarping of the
overlying lithosphere
(FIGURE 15.11A). As a
result, the lithosphere is
stretched, causing the
brittle crustal rocks to
break into large slabs. As
the tectonic forces continue to pull the crust
apart, these crustal fragments sink, generating an
elongated depression
called a continental rift
(FIGURE 15.11B).
A modern example of
an active continental rift is
the East African Rift (see
Figure 15.9, left). Whether
this rift will eventually
result in the break-up of
Africa is a topic of continued research. Nevertheless,
the East African Rift is an excellent model of the initial stage in the
break-up of a continent. Here, tensional forces have stretched and
thinned the crust, allowing molten
rock to ascend from the mantle. Evidence
for recent volcanic activity includes several
large volcanic mountains including Mount
Kilimanjaro and Mount Kenya, the tallest