Page 100 - Earth's Climate Past and Future

76 PART II • Tectonic-Scale Climate Change

Volume = l × w × h
Surface area = (l × w) × number of faces 70
Number of particles 50
60

40
1m 30
0.5 m 20
0.25 m
10
0.5
1 m m 1
0.25 m
6 9 12 15 18 21 24
2
1 cube, 6 faces 8 cubes, 48 faces 64 cubes, 384 faces Total surface area (m )
Volume = 1 m 3 Total volume = 1 m 3 Total volume = 1 m 3
Surface area = 6 m 2 Surface area of 1 cube = 1.5 m 2 Surface area of 1 cube = 0.375 m 2
Total surface area = 12 m 2 Total surface area = 24 m 2
FIGURE 4-20 Fragmentation of rock Each time a cube-shaped rock is sliced into smaller
cubes (with each side half as long as before), the total surface area of rock doubles, even though
the volume remains the same. (D. Merritts et al., Environmental Geology, © 1997 by W. H. Freeman
and Company.)

basin in the Wind River Mountains of Wyoming. later on. Why would younger glacial deposits weather
Because all the bedrock in this basin consists of granite, so much faster?
the kind of silicate rock most typical of continental One explanation is that freshly ground rock has more
crust, this watershed is reasonably representative of the weatherable material—the kinds of fresh, unweathered
average response of continental rocks to weathering. silicate grains that are most vulnerable to the weathering
The Wind River Mountains have been glaciated process. These vulnerable minerals are removed through
repeatedly over the last several hundred thousand years, time. Once only the more resistant minerals are left, rates
and each glaciation has left deposits of unsorted debris of weathering are slower.
(moraines) in the foothills of the valleys below. Because Another part of the explanation relates to the effect
some of the older deposits have not been overridden by of grain sizes on weathering (see Figure 4-20). Finer
later glacial advances, undeformed moraines of various
ages (from 200 to 130,000 years) can be found in the
same valley.
The Wind River moraines provide an opportunity
Soils on glacial moraines
to quantify the amount of weathering of ground-up
Very old weathered soils
debris that is identical in composition but differs widely
in age. The extent of weathering is determined by ana-
lyzing soils that have subsequently developed on the
moraines. The soils gradually lose their major cations
+1
+1
+2
+2
(Mg , Na , K , Ca , and others) during the chemical
weathering process. The cumulative amount of chemi- Rate of weathering
cal weathering that has occurred since each moraine
was deposited can be determined by measuring the total
loss of these cations. Dividing this total amount of
weathering by the time elapsed since the moraine was
deposited yields the average rate of chemical weathering 0 100,000 200,000
Time of moraine exposure (years)
over that entire interval.
The Wind River deposits show a rapid (exponential) FIGURE 4-21 Weathering and exposure time Glacially
decrease in the mean rate of weathering versus time of eroded and fragmented granite weathers quickly soon after
exposure (Figure 4-21). The younger moraines have deposition but much more slowly 100,000 years later.
average rates of weathering that are at least a factor of (Adapted from J. D. Blum, “The Effect of Late Cenozoic
100 faster than the older ones. The older moraines also Glaciation and Tectonic Uplift on Silicate Weathering Rates,” in
weathered much faster during a brief interval after their Tectonic Uplift and Climate Change, ed. W. F. Ruddiman [New York:
deposition, but they then weathered much more slowly Plenum Press, 1997].)