Page 133 - Earth's Climate Past and Future

CHAPTER 6 • From Greenhouse to Icehouse: The Last 50 Million Years 109

amounts of plateau construction may have occurred Prediction 2: Unusual Physical Weathering The
on Africa near 100 Myr ago as the giant continent of second test of the uplift weathering hypothesis is
Pangaea broke up. As a result, it is hard to argue that whether or not today’s high topography is causing
the present plateau exceeds features that may have higher rates of physical weathering and rock fragmenta-
existed during earlier intervals. tion than occurred in the past. The concentrations of
suspended particles carried southward and eastward
from the Himalaya and Tibet by Asian rivers are larger
IN SUMMARY, the existence of the massive Tibetan
Plateau makes modern topography unusual, than any on Earth (Figure 6-15). The youthful topogra-
consistent with the uplift weathering hypothesis. phy of the eastern Andes drained by the Amazon River is
Regions of high youthful terrain also exist along another region of high particle concentrations. These
subducting plate margins and elsewhere, but they measurements clearly indicate intense physical weather-
may be similar to features that existed in the past. ing at the present time in the two regions with highest
terrain.

BOX 6-1 CLIMATE DEBATE

CONTINUED

Rockies

High plains
Basin and range

Sierra
Rockies
Colorado
Plateau

Topography in the American West A broad bulge of high topography reaches from the Sierra in
the far western United States to the Rocky Mountains and High Plains farther east, with the
Colorado Plateau and Basin and Range lying in between. Low-elevation regions a few hundred
meters above sea level are shown in green, with progressively higher elevations in yellow, brown,
pale orange, and white. (Courtesy of Peter Schloss, National Geophysical Data Center, Boulder, CO.)