Page 132 - Earth's Climate Past and Future

108 PART II • Tectonic-Scale Climate Change

In contrast, no major continental collisions occurred there for more than 100 Myr, a mountain range has
from 100 to 65 Myr ago, and no massive plateaus existed long existed in the western Andes, but much of the high
then or for the preceding 150 Myr. The presence of the topography of the central Altiplano and the eastern
Tibetan Plateau and Himalayan complex (Figure 6-14B Andes was created within the last 15 Myr.
and C) is a strong argument for an unusually massive Subduction has also occurred along western North
amount of high topography at the present time. America for some 200 Myr. Scientists are sharply
Most other high-elevation regions on Earth (see divided about the history of uplift in this region over
Figure 6-13) have been formed by subduction of ocean the last 50 Myr, but it seems unlikely that the high
crust beneath continental margins. Because subduction topography in this region today is unique (Box 6-1).
is an ongoing process, mountain terrain has existed con- Another kind of high terrain is the extensive low
tinuously through time, in contrast to plateaus pro- plateau in eastern and southern Africa at an elevation
duced by sporadic continental collisions. The modern of 1 km (see Figure 6-13). This plateau results from
Andes and narrow central plateau called the Altiplano deep-seated heating that causes a broad upward doming
are the result of subduction along the west coast of and outpouring of volcanic lava. Much of the East
South America. Because subduction has been under way African plateau was built in the last 30 Myr, but similar

BOX 6-1 CLIMATE DEBATE

Timing of Uplift in Western North America

cientists disagree about the age of uplift of high terrain Rocky Mountain West since 70 Myr ago, but they infer
Sacross western North America. All agree that moun- that most of the uplift actually occurred between 70 and
tains of some size have existed in Nevada and eastern 45 Myr ago during an interval of heightened tectonic
California for 200 Myr or more because ongoing subduc- activity and that most of the Rocky Mountain area lost
tion occurred along the west coast until about 30 Myr elevation once that activity ceased.
ago. They also agree that the Rocky Mountain West (the Geophysicist Peter Molnar has offered an explana-
modern High Plains, Colorado Plateau, and both the tion for this surprising difference in opinion. He proposed
U.S. and Canadian Rocky Mountains) was flooded by an that the global cooling trend has increased the extent
inland sea from 100 to 70 Myr ago and has since been of glaciation on high-elevation mountains and plateaus
uplifted to its present height. But just about everything by lowering the freezing line onto the high topography.
else about the timing and amount of uplift in this region is As a result, glacial erosion has caused increases in rates
in dispute. of erosion and major incision of mountain valleys. Both
One group emphasizes recent uplift. In their view, the appearance of glaciers and the increase of erosion
the earlier mountain terrain near Nevada was a series can be (and has been) mistaken for evidence of active
of discontinuous low-elevation peaks, not a major topo- uplift in mountain belts that are actually tectonically
graphic feature. This group infers that broad, large-scale “dead.”
upwarping of the entire West from the Sierra Nevada of Temperature reconstructions based on leaf-margin
California to the High Plains of Colorado and Wyoming types have been used to test these opposing views.
began about 20 Myr ago because of some kind of deep- Temperatures are estimated from deposits of the same age
seated heating process. Visitors to parks in the American both in higher-elevation mountain areas and in nearby
West will notice that this view is widely promoted by the coastal regions. Because temperature decreases with ele-
U.S. Park Service. vation in a known way, the estimated temperature differ-
The other group interprets major uplift as occurring at ences between coastal and high-elevation regions can be
an earlier time, followed by a more recent loss of elevation converted to estimates of past elevation. This method
in many regions. In their view, the mountain belt that suggests a small decrease in elevation of most parts of the
existed in the Far West before 100 Myr ago was a continu- Rocky Mountain West during the last 40 or 50 Myr. One
ous feature at 3–4 km elevation like the modern Andes, exception during the last 10 or 20 Myr is the Yellowstone
and it has subsequently dropped to its modern height of hot spot area, which has been domed upward by shallow
1–2 km. This group acknowledges large-scale uplift of the subsurface heating.