Page 131 - Earth's Climate Past and Future

CHAPTER 6 • From Greenhouse to Icehouse: The Last 50 Million Years 107

To determine whether or not these requirements are Although much of today’s high topography is geolog-
met, we have to compare the present time with some ically youthful, this evidence alone does not prove that
interval in the past. The last half of the Cretaceous the modern elevations are uniquely high or extensive.
interval, from 100 to 65 Myr ago, is a useful basis for Plate tectonic processes continually cause uplift in many
comparison for two reasons: (1) abundant evidence is regions throughout geologic time, while erosion contin-
still left in the geologic record and (2) it is an interval of ually attacks the highest topography and wears it down.
full greenhouse climate (Chapter 5). As a result, the highest topography during any interval of
Prediction 1: Extensive High Terrain At first geologic time is always recent in origin, just as it is today.
glance it might seem obvious that uplift has been The strongest evidence that the amount of high ter-
unusually active in most mountain ranges during the rain is indeed more massive today than it was in earlier
last few tens of millions of years. Marine sediments geologic eras is the existence of the Tibetan Plateau,
2
2
deposited at or below sea level 100 to 65 Myr ago are some 2.5 million km (1 million mi ) in area at an aver-
now found at high elevations in Tibet and the Himalaya age elevation above 5 km. This plateau has slowly risen
of Asia, the South American Andes, the North Ameri- since the initial collision of India and Asia 55 Myr ago
can Rocky Mountains, and the European Alps (Figure (Figure 6-14A and B). Uplift occurred earliest in the
6-13). These sediments have been uplifted from sea south-central parts of the plateau and later in the north-
level to their present heights in the last 70 Myr or less. eastern and southeastern sectors.

60˚E 80˚E 100˚E 80˚E 100˚E
0 > 2 km
10
19 0
20˚N 10 40˚N
35
19 Tibet
49
35
0˚ 58 Myr 49 Myr
20˚N
India
Bay of
68 Myr 58 Myr Arabian
Sea Bengal
20˚S
68 Myr
0˚

A B

FIGURE 6-14 India-Asia collision
and Tibet (A) Collision of India and
Asia produced (B) the Tibetan
Plateau, the largest high-elevation
rock feature on Earth today. (C) The
Himalaya Mountains tower over the
Indian subcontinent to the south (in
the foreground). Behind the
Himalayas to the north lies the vast
Tibetan Plateau at an average
elevation above 5000 m. (A and B:
Adapted from P. Molnar et al.,
“Mantle Dynamics, the Uplift of the
Tibetan Plateau, and the Indian
Monsoon,” Review of Geophysics 31
[1993]: 357–96. C: Emil
Muench/Photo Researchers.)