369
Divergent Boundaries
The lithosphere, in turn, overlies a
weak region in the mantle known as the
asthenosphere. The temperatures and
pressures in the upper asthenosphere
(100 to 200 kilometers in depth) are such
that the rocks there are very near their
melting temperatures and, hence,
respond to stress by flowing. As a result,
Earth’ s rigid outer shell is effectively
detached from the layers below, which
permits it to move independently.
Earth’ s Major Plates
The lithosphere is composed of about
two dozen segments having irregular
sizes and shapes called lithospheric plates
or tectonic plates that are in constant
motion with respect to one another. As
shown in FIGURE 15.9, seven major lithospheric plates are recognized. These
plates, which account for 94 percent of
Earth’ s surface area, include the North
American, South American, Pacific, African,
Eurasian, Australian-Indian, and Antarctic
plates. The largest is the Pacific plate,
which encompasses a significant portion
of the Pacific Ocean basin. The six other
large plates include an entire continent
plus a significant amount of ocean floor.
Notice in Figure 15.9 that the South
American plate encompasses almost all of
South America and about one half of the
floor of the South Atlantic. This is a
major departure from Wegener’ s continental drift hypothesis, which proposed
that the continents moved through the
ocean floor, not with it. Note also that
none of the plates are defined entirely by
the margins of a single continent.
Intermediate-sized plates include the
Caribbean, Nazca, Philippine, Arabian, Cocos,
Scotia, and Juan de Fuca plates. These plates,
with the exception of the Arabian plate, are
composed mostly of oceanic lithosphere. In
addition, there are several smaller plates
(microplates) that have been identified but
are not shown in Figure 15.9.
Plate Boundaries
One of the main tenets of the plate
tectonics theory is that plates move as
semicoherent units relative to all other
plates. As plates move, the distance
between two locations on different plates,
such as New York and London, gradually
changes whereas the distance between
sites on the same plate—New York and
Denver, for example—remains relatively
constant.
Because plates are in constant
motion relative to each other, most major
interactions among them (and, therefore,
most deformation) occur along their
boundaries. In fact, plate boundaries were
first established by plotting the locations
of earthquakes and volcanoes. Plates
are bounded by three distinct types of
boundaries, which are differentiated by
the type of movement they exhibit.
These boundaries are depicted at the
bottom of Figure 15.9 and are briefly
described here:
1. Divergent boundaries (constructive
margins)—where two plates move
apart, resulting in upwelling of hot
material from the mantle to create
new seafloor (FIGURE 15.9A).
2. Convergent boundaries (destructive
margins)—where two plates move
together, resulting in oceanic
lithosphere descending beneath an
overriding plate, eventually to be
reabsorbed into the mantle or possibly
in the collision of two continental
blocks to create a mountain system
(FIGURE 15.9B).
3. Transform fault boundaries
(conservative margins)—where two
plates grind past each other without
the production or destruction of
lithosphere (FIGURE 15.9C).
Divergent and convergent plate boundaries each account for about 40 percent of
all plate boundaries. Transform faults
account for the remaining 20 percent. In
the following sections we will summarize
the nature of the three types of plate
boundaries.
C O N C E P T C H E C K 1 5 . 4
Compare and contrast the lithosphere and
the asthenosphere.
List the three types of plate boundaries,
and describe the relative motion at each
of them.
2
1
Divergent Boundaries
PLATE TECTONICS
Divergent Boundaries
Most divergent boundaries are located
along the crests of oceanic ridges and can
be thought of as constructive plate margins
because this is where new ocean floor is
generated (FIGURE 15.10). Divergent boundaries are also called spreading centers,
because seafloor spreading occurs at these
boundaries. Here, two adjacent plates are
moving away from each other, producing
long, narrow fractures in the ocean crust.
As a result, hot rock from the mantle below
migrates upward to fill the voids left as the
crust is being ripped apart. This molten
material gradually cools to produce new
slivers of seafloor. In a slow, yet unending
manner, adjacent plates spread apart and
new oceanic lithosphere forms between
them.
Oceanic Ridges
and Seafloor Spreading
Most divergent plate boundaries are associated with oceanic ridges: elevated areas of
the seafloor that are characterized by high
heat flow and volcanism. The global ridge
system is the longest topographic feature
on Earth’ s surface, exceeding 70,000 kilometers (43,000 miles) in length. As shown
in Figure 15.9 various segments of the
global ridge system have been named,
including the Mid-Atlantic Ridge, East
Pacific Rise, and Mid-Indian Ridge.
Representing 20 percent of Earth’ s
surface, the oceanic ridge system winds
through all major ocean basins like the
seam on a baseball. Although the crest
of the oceanic ridge is commonly 2 to
3 kilometers higher than the adjacent ocean
GEODe
ESSENTIALS
OF GEOLOGY
D I D Y O U K N O W ?
An observer on another planet would
notice, after only a few million years,
that all the continents and ocean basins
on Earth are indeed moving. The
Moon, on the other hand, is tectonically
dead, so it would look virtually
unchanged millions of years into the
future.
Divergent Boundaries
The lithosphere, in turn, overlies a
weak region in the mantle known as the
asthenosphere. The temperatures and
pressures in the upper asthenosphere
(100 to 200 kilometers in depth) are such
that the rocks there are very near their
melting temperatures and, hence,
respond to stress by flowing. As a result,
Earth’ s rigid outer shell is effectively
detached from the layers below, which
permits it to move independently.
Earth’ s Major Plates
The lithosphere is composed of about
two dozen segments having irregular
sizes and shapes called lithospheric plates
or tectonic plates that are in constant
motion with respect to one another. As
shown in FIGURE 15.9, seven major lithospheric plates are recognized. These
plates, which account for 94 percent of
Earth’ s surface area, include the North
American, South American, Pacific, African,
Eurasian, Australian-Indian, and Antarctic
plates. The largest is the Pacific plate,
which encompasses a significant portion
of the Pacific Ocean basin. The six other
large plates include an entire continent
plus a significant amount of ocean floor.
Notice in Figure 15.9 that the South
American plate encompasses almost all of
South America and about one half of the
floor of the South Atlantic. This is a
major departure from Wegener’ s continental drift hypothesis, which proposed
that the continents moved through the
ocean floor, not with it. Note also that
none of the plates are defined entirely by
the margins of a single continent.
Intermediate-sized plates include the
Caribbean, Nazca, Philippine, Arabian, Cocos,
Scotia, and Juan de Fuca plates. These plates,
with the exception of the Arabian plate, are
composed mostly of oceanic lithosphere. In
addition, there are several smaller plates
(microplates) that have been identified but
are not shown in Figure 15.9.
Plate Boundaries
One of the main tenets of the plate
tectonics theory is that plates move as
semicoherent units relative to all other
plates. As plates move, the distance
between two locations on different plates,
such as New York and London, gradually
changes whereas the distance between
sites on the same plate—New York and
Denver, for example—remains relatively
constant.
Because plates are in constant
motion relative to each other, most major
interactions among them (and, therefore,
most deformation) occur along their
boundaries. In fact, plate boundaries were
first established by plotting the locations
of earthquakes and volcanoes. Plates
are bounded by three distinct types of
boundaries, which are differentiated by
the type of movement they exhibit.
These boundaries are depicted at the
bottom of Figure 15.9 and are briefly
described here:
1. Divergent boundaries (constructive
margins)—where two plates move
apart, resulting in upwelling of hot
material from the mantle to create
new seafloor (FIGURE 15.9A).
2. Convergent boundaries (destructive
margins)—where two plates move
together, resulting in oceanic
lithosphere descending beneath an
overriding plate, eventually to be
reabsorbed into the mantle or possibly
in the collision of two continental
blocks to create a mountain system
(FIGURE 15.9B).
3. Transform fault boundaries
(conservative margins)—where two
plates grind past each other without
the production or destruction of
lithosphere (FIGURE 15.9C).
Divergent and convergent plate boundaries each account for about 40 percent of
all plate boundaries. Transform faults
account for the remaining 20 percent. In
the following sections we will summarize
the nature of the three types of plate
boundaries.
C O N C E P T C H E C K 1 5 . 4
Compare and contrast the lithosphere and
the asthenosphere.
List the three types of plate boundaries,
and describe the relative motion at each
of them.
2
1
Divergent Boundaries
PLATE TECTONICS
Divergent Boundaries
Most divergent boundaries are located
along the crests of oceanic ridges and can
be thought of as constructive plate margins
because this is where new ocean floor is
generated (FIGURE 15.10). Divergent boundaries are also called spreading centers,
because seafloor spreading occurs at these
boundaries. Here, two adjacent plates are
moving away from each other, producing
long, narrow fractures in the ocean crust.
As a result, hot rock from the mantle below
migrates upward to fill the voids left as the
crust is being ripped apart. This molten
material gradually cools to produce new
slivers of seafloor. In a slow, yet unending
manner, adjacent plates spread apart and
new oceanic lithosphere forms between
them.
Oceanic Ridges
and Seafloor Spreading
Most divergent plate boundaries are associated with oceanic ridges: elevated areas of
the seafloor that are characterized by high
heat flow and volcanism. The global ridge
system is the longest topographic feature
on Earth’ s surface, exceeding 70,000 kilometers (43,000 miles) in length. As shown
in Figure 15.9 various segments of the
global ridge system have been named,
including the Mid-Atlantic Ridge, East
Pacific Rise, and Mid-Indian Ridge.
Representing 20 percent of Earth’ s
surface, the oceanic ridge system winds
through all major ocean basins like the
seam on a baseball. Although the crest
of the oceanic ridge is commonly 2 to
3 kilometers higher than the adjacent ocean
GEODe
ESSENTIALS
OF GEOLOGY
D I D Y O U K N O W ?
An observer on another planet would
notice, after only a few million years,
that all the continents and ocean basins
on Earth are indeed moving. The
Moon, on the other hand, is tectonically
dead, so it would look virtually
unchanged millions of years into the
future.