Page 101 - Earth's Climate Past and Future

CHAPTER 4 • Plate Tectonics and Long-Term Climate 77

grain sizes expose more surface area and cause faster accompanied by earthquakes that generate large amounts
weathering early in the process, but the finer sizes also of energy, shake the ground, and dislodge debris. Even
disappear earlier as weathering consumes them. The more fresh rock is exposed as a result.
coarser grain sizes that remain weather more slowly A third important characteristic of steep slopes is
because they expose less surface area per unit of volume. that they are focal points for precipitation (companion
Coarser fragments may also develop an outer coating or Web site, p. 21). When warm air is forced up and over
“rind” of weathering-resistant material that protects high terrain and cooled, water vapor condenses and
fresher material in their interiors and slows the weath- precipitation occurs. High but narrow mountain belts
ering attack. in the tropics and mid-latitudes capture much of the
moisture carried by winds. In addition, large plateaus
such as the Tibetan Plateau create their own wet
4-10 Uplift and Chemical Weathering
(monsoonal) circulations by pulling moisture in from
The uplift weathering hypothesis begins with the evi- adjacent oceans. Heavy precipitation favors chemical
dence that exposure of fragmented and unweathered weathering.
rock is an important factor in the intensity of chemical Glacial ice also enhances chemical weathering in
weathering. It then links this evidence to the fact that high terrain. Uplift can elevate rock surfaces to altitudes
exposure of freshly fragmented rock is enhanced in where temperatures are cold enough for mountain glac-
regions of tectonic uplift. iers to form. Mountain glaciers pulverize blocks of
Several factors increase rates of exposure of fresh underlying bedrock and deposit the debris in moraines
rock in uplifting areas. Mountains and plateaus have at lower elevations. As we saw in the case study of the
steep slopes both on their margins and in valleys Wind River Range, glacial grinding greatly enhances
between high peaks. Erosional processes known as rates of chemical weathering.
mass wasting are unusually active on such slopes. All these factors (steep slopes, mass wasting, earth-
Mass-wasting processes include rock slides and falls, quakes, heavy precipitation, and glaciers) are present
flows of water-saturated debris, and a host of other in high mountains and plateaus. The uplift weather-
processes that dislodge everything from huge slabs of ing hypothesis proposes that uplift accelerates chemi-
rock to loose boulders, pebbles, and soil. Every event cal weathering through the combined action of these
that removes overlying debris exposes fresh bedrock processes (Figure 4–23). Faster weathering draws more
and unweathered material. Many high-mountain slopes CO out of the atmosphere and cools global climate
2
consist almost entirely of fresh debris moving down- toward icehouse conditions. Conversely, during times
slope (Figure 4-22). when uplift is less prevalent, chemical weathering is
Another important factor is earthquakes. Mountains slower, and CO stays in the atmosphere and warms the
2
and high plateaus are built by tectonic forces that push climate, producing greenhouse conditions.
together and stack huge slivers of faulted rock at the The two major kinds of plate tectonic processes that
margins of converging plates. This stacking process is cause uplift have different implications for the uplift

FIGURE 4-22 Debris on steep
slopes Steep slopes of actively
eroding mountains consist of highly
fragmented debris periodically
dislodged downslope. (Photosphere
Images/Picture Quest.)