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CHAPTER 4 • Plate Tectonics and Long-Term Climate 75

4-7 Initial Evaluation of the BLAG Spreading The uplift weathering hypothesis starts from a dif-
Rate Hypothesis ferent perspective. It asserts that the global mean rate of
chemical weathering is heavily affected by the availabil-
Unfortunately, the predictions of the BLAG hypothesis ity of fresh rock and mineral surfaces for the weathering
cannot be directly tested over most of the geologic past process to attack, and proposes that this exposure effect
because no ocean crust older than 175 Myr exists to use can override the combined effects of the climate-related
for calculating past spreading rates. All older crust has factors both locally and globally.
been subducted in ocean trenches. Half of the crust that A simple example of the importance of rock expo-
formed 50 Myr ago has already been destroyed by sure is shown in Figure 4-20. We start with a large cube
rapid subduction under western South America (see of rock with six surfaces consisting of squares 1 m
Figure 4-17) and by the total disappearance of ocean across, each having a surface area of 1 m . This rock
2
crust in a former tropical seaway (“Tethys”) by subduc- cube has a total surface area of 6 m , calculated from the
2
tion and collision along the southern coast of Asia. total areas of the six sides.
Most reconstructions suggest that the global mean Next we slice this cube into halves along all three of its
spreading rate was faster 100 Myr ago than it is at pre- axes. This slicing creates eight smaller cubes, each 0.5 m
sent, but this issue has recently become a point of on a side, so that each side has a surface area of 0.25 m .
2
contention that will be revisited in the next two chap- The total surface area of these smaller cubes is 12 m :
2
ters. If this conclusion holds up to future scrutiny, the
BLAG theory predicts that the rate of input of CO to (8 cubes) × (6 sides each) × (0.25 m of
2
2
the atmosphere should have been higher 100 Myr ago surface area per side)
than it is today (Table 4-2). This prediction agrees with
geologic evidence of a warmer climate 100 Myr ago, Note that simply cutting the large cube into smaller
including the absence of large polar ice sheets. We will cubes has doubled the surface area of the rock without
revisit this important issue in the next two chapters. changing its volume, at least for the imaginary laser-
sharp cut assumed in this example. The fragmentation
Tectonic Control of CO Removal: has created more exposed surface area for the weather-
2 ing process to attack.
Uplift-Weathering Hypothesis The process can be continued to finer grain sizes

A second hypothesis that attempts to explain how plate with similar results. Ten sequential halvings of the rock’s
tectonic processes control atmospheric CO levels dimensions will produce over 1 billion cubes each 1 mm
2
emerged from work by the marine geologist Maureen on a side, about the same size as grains of sand on a
Raymo and her colleagues in the late 1980s. Parts of beach. Together these tiny cubes have a total surface
3
this concept date back to work by the geologist T. C. area 1000 times larger than the original 1-m block, yet
Chamberlain a century ago. The uplift weathering they still retain the same total volume. Fragmentation to
hypothesis proposes that chemical weathering is the even smaller sizes will expose still more surface area.
active driver of climate change rather than a negative With over 1000 times more surface area to act on,
feedback that moderates climate change. chemical weathering would increase by a factor of 1000
or more. This large an increase of weathering far
4-8 Rock Exposure and Chemical Weathering exceeds the combined changes estimated to result from
changes in temperature, precipitation, and vegetation.
The BLAG hypothesis emphasizes change in CO
2 Clearly the climate-related factors are not the only
delivery to the atmosphere by seafloor spreading, and it processes to consider in evaluating chemical weathering.
assumes that removal of CO by chemical weathering
2
responds only to climate-related changes in tempera- 4-9 Case Study: The Wind River Basin of Wyoming
ture, precipitation, and vegetation. Although these fac-
tors do affect chemical weathering (Chapter 3), they are Direct evidence of the importance of rock exposure in
not the only processes that do. chemical weathering comes from a study of a drainage

TABLE 4-2 Evaluation of the BLAG Spreading Rate (CO ) Input Hypothesis
2
Time (Myr ago) Ice sheets present? Spreading rate? Hypothesis supported?
100 No Faster (?) Yes (?)
0 Yes Slower (?) Yes (?)