Geoscience Reference
In-Depth Information
Aleutian Arc
Caledonian Mts.
Urals
Rocky
Mountains
Caucasus
Range
Carpathians
Alps
Alaska Range
Pyrenees
Tien Shan Mts.
Kuril Arc
B.C. Coast Range
Appalachians
Atlas
Range
Honshu Arc
Cascade Range
Antilles Arc
Zagros Range
Himalayas
Sierra Madre
Middle America Arc
Great
Dividing
Range
Andes
Sumatra-Java Arc
Drakensberg
Plateau
Southern Alps
Figure 13.15 Location of major alpine chains on Earth.
Many of the Earth's major mountain chains have resulted from com-
pression along crustal plates. Others have developed due to other processes, such as volcanism. The Andes is the longest moun-
tain chain, at 7200 km (4500 mi); the Rockies are not far behind, at 6000 km (3700 mi). However, the tallest mountains are in the
Himalayas.
A
synclinal valley
lies immediately to the right of the hog-
back ridge-and-valley network at point
A
and is what you
would expect since it lies below the adjacent ridge. To the
right of that valley is another hogback ridge, which borders
yet another anticlinal valley. See how the valley is cut into
rocks that arch up?
Now look at point
B
in Figure 13.14b. A ridge exists at the
surface here, but it is actually underlain by a syncline. See the
U-shape of the rock layer that caps the ridge? This is the same
rock body that forms the hogback ridge to the left and now is
on top of the ridge here. Such a pattern is interesting because
it means that the topography at point
B
has been
inverted
. In
other words, what was once the valley floor (point
B
, upper
image) is now the crest of a ridge (point
B
, lower image).
Such complex relationships between the underlying geologic
structure and the way the landscape now appears demonstrate
that landscapes are sometimes more complex than they ap-
pear. They also reflect how much
time
must be involved for
this kind of landscape to evolve. First, there had to be uplift
and then
substantial
amounts of erosion. Overall, it took about
250 million years!
Although the Appalachian Mountains are a great exam-
ple of how collision produced a landscape, it is certainly not
the only such example on Earth. Crustal collision continues
to uplift mountains in several prominent places around the
world, creating many of Earth's distinct mountain chains
shown in Figure 13.15. The famous Swiss Alps, for example,
formed by compression. This mountain range forms the bor-
der between Austria and Italy and began to develop during
the Cretaceous Period (144 my to 65 my ago) when the con-
tinental plate of Africa collided with the southern side of
the European plate. A second orogeny occurred during the
Tertiary Period (65 my to 2 my ago). An interesting aspect of
the Alps geology is that part of the mountain range consists
of rock material from the African plate. In other words, part
of the European landmass is actually rock from Africa. An-
other mountain range that has formed largely through con-
tinental collision is the Himalaya Mountains (Figure 13.16),
which border India and China. Most people probably know
that Mount Everest is the highest mountain in the world.
What you may not have known is that this peak, as well as
the entire Himalayan chain, has been uplifted because the
northeastern part of the Indian plate has been colliding into
the southern side of the Eurasian plate for about the past
30 million years. As a result of this collision, the mountains
have shot out of the ground, at least as far as geologic time is
concerned. In so doing, metamorphic and sedimentary rocks
(including marine fossils) have been uplifted thousands of
meters above sea level.
Synclinal valley
A topographic valley that occurs along the
axis of a structural syncline.
