389
Chapter in Review
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North
America
Antarctica
Eurasia
Australia
Africa
South
America
D I D Y O U K N O W ?
Researchers have estimated that the continents join to
form supercontinents roughly every 500 million years.
Since it has been about 200 million years since Pangaea
broke up, we have only 300 million years to wait before
the next supercontinent is completed.
North
America
Antarctica
Eurasia
Australia
Africa
South
America
FIGURE 15.32 Reconstruction of Earth as it may
appear 250 million years into the future.
A few geologists have even speculated on the nature of
the globe 250 million years into the future. As shown in
FIGURE 15.32, the next supercontinent may form as a result of
subduction of the floor of the Atlantic Ocean, resulting in the
collision of the Americas with the Eurasian–African landmass.
Support for the possible closing of the Atlantic comes from a similar
event when the proto-Atlantic closed to form the Appalachian and
Caledonian mountains. During the next 250 million years, Australia
is also projected to collide with Southeast Asia. If this scenario is
accurate, the dispersal of Pangaea will end when the continents
reorganize into the next supercontinent.
Such projections, although interesting, must be viewed
with considerable skepticism because many assumptions
must be correct for these events to unfold as just
described. Nevertheless, changes in the shapes and positions of continents that are equally profound will undoubtedly occur for many hundreds of millions of years to come.
Only after much more of Earth’ s internal heat has been lost
will the engine that drives plate motions cease.
C O N C E P T C H E C K 1 5 . 1 2
Briefly describe some major changes to the globe when we extrapolate present-day plate
movements 50 million years into the future.
1
C H A P T E R
F I F T E E N
Plate Tectonics
in Review
In the early 1900s Alfred Wegener set forth his continental
drift hypothesis. One of its major tenets was that a supercontinent
called Pangaea began breaking apart about 200 million years ago.
The rifted continental fragments then “drifted” to their present
positions. To support his hypothesis, Wegener used the fit of South
America and Africa, fossil evidence, rock types and structures, and
ancient climates. One of the main objections to the continental
drift hypothesis was its inability to provide an acceptable
mechanism for the movement of continents.
By 1968, continental drift was replaced by a far more encompassing theory known as plate tectonics. According to plate
tectonics, Earth’ s rigid outer layer (lithosphere) overlies a weaker
region called the asthenosphere. Further, the lithosphere is broken
into several large and numerous smaller segments, called plates,
that are in motion and continually changing in shape and size.
Plates move as relatively coherent units and are deformed mainly
along their boundaries.
Divergent plate boundaries occur where plates move apart,
resulting in upwelling of material from the mantle to create new
seafloor. Most divergent boundaries occur along the axis of the
oceanic ridge system and are associated with seafloor spreading.
New divergent boundaries may form within a continent (for
example, the East African Rift Valleys), where they may fragment
a landmass and develop a new ocean basin.
FIGURE 15.31 The world as it may look 50 million years from now. (Modified
after Robert S. Dietz, John C. Holden, C. Scotese, and others)
Chapter in Review
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▲
North
America
Antarctica
Eurasia
Australia
Africa
South
America
D I D Y O U K N O W ?
Researchers have estimated that the continents join to
form supercontinents roughly every 500 million years.
Since it has been about 200 million years since Pangaea
broke up, we have only 300 million years to wait before
the next supercontinent is completed.
North
America
Antarctica
Eurasia
Australia
Africa
South
America
FIGURE 15.32 Reconstruction of Earth as it may
appear 250 million years into the future.
A few geologists have even speculated on the nature of
the globe 250 million years into the future. As shown in
FIGURE 15.32, the next supercontinent may form as a result of
subduction of the floor of the Atlantic Ocean, resulting in the
collision of the Americas with the Eurasian–African landmass.
Support for the possible closing of the Atlantic comes from a similar
event when the proto-Atlantic closed to form the Appalachian and
Caledonian mountains. During the next 250 million years, Australia
is also projected to collide with Southeast Asia. If this scenario is
accurate, the dispersal of Pangaea will end when the continents
reorganize into the next supercontinent.
Such projections, although interesting, must be viewed
with considerable skepticism because many assumptions
must be correct for these events to unfold as just
described. Nevertheless, changes in the shapes and positions of continents that are equally profound will undoubtedly occur for many hundreds of millions of years to come.
Only after much more of Earth’ s internal heat has been lost
will the engine that drives plate motions cease.
C O N C E P T C H E C K 1 5 . 1 2
Briefly describe some major changes to the globe when we extrapolate present-day plate
movements 50 million years into the future.
1
C H A P T E R
F I F T E E N
Plate Tectonics
in Review
In the early 1900s Alfred Wegener set forth his continental
drift hypothesis. One of its major tenets was that a supercontinent
called Pangaea began breaking apart about 200 million years ago.
The rifted continental fragments then “drifted” to their present
positions. To support his hypothesis, Wegener used the fit of South
America and Africa, fossil evidence, rock types and structures, and
ancient climates. One of the main objections to the continental
drift hypothesis was its inability to provide an acceptable
mechanism for the movement of continents.
By 1968, continental drift was replaced by a far more encompassing theory known as plate tectonics. According to plate
tectonics, Earth’ s rigid outer layer (lithosphere) overlies a weaker
region called the asthenosphere. Further, the lithosphere is broken
into several large and numerous smaller segments, called plates,
that are in motion and continually changing in shape and size.
Plates move as relatively coherent units and are deformed mainly
along their boundaries.
Divergent plate boundaries occur where plates move apart,
resulting in upwelling of material from the mantle to create new
seafloor. Most divergent boundaries occur along the axis of the
oceanic ridge system and are associated with seafloor spreading.
New divergent boundaries may form within a continent (for
example, the East African Rift Valleys), where they may fragment
a landmass and develop a new ocean basin.
FIGURE 15.31 The world as it may look 50 million years from now. (Modified
after Robert S. Dietz, John C. Holden, C. Scotese, and others)