FIGURE 2.14 Satellite photo of the central Himalaya and Tibetan plateau. The strongly asymmetric distribution of rainfall is reflected in the vegetation pattern and distribution of glaciers. The regions with dark green color, on the south side of the mountains, have the highest rainfall and also have the youngest metamorphic rocks exposed at the surface due to the rapid erosion. SOURCE: National Aeronautics and Space Administration, <http://rapidfire.sci.gsfc.nasa.gov/gallery/?2002348-1214/Tibet2.A2002348.0505.1km.jpg>.

temporal patterns of erosion and uplift with varying patterns of precipitation (e.g., Burbank et al., 2003). These studies, in turn, require the latest techniques of measuring erosion rates and crustal movement and imaging the deeper parts of the continental crust and upper mantle. The large scale of tectonic systems has made them challenging to study, a task recently made easier by satellite sensors and systems like Interferometric Synthetic Aperture Radar and the Global Positioning System.

Critical to understanding the coupling of climate and tectonics are empirical models that relate rainfall and topography to the production and transport of sediment and the erosion of bedrock. These geomorphic transport laws are still in their infancy. Experimentally tested, field parameterized, and theoretically sound expressions for most surface processes, especially as they apply to geological temporal and spatial scales, do not yet exist. This gap in erosion process theory presents a great opportunity for scientific advance—and a challenging one because most relevant processes cannot be easily simulated in controlled laboratory settings. Furthermore, the heterogeneity of Earth materials presents challenges, especially for threshold-dependent processes such as landsliding. New tools, especially cosmogenic radionuclide dating and thermochronology, are now enabling us to determine the rates of processes through space and time, but others will be needed, for example to incorporate biotic effects (Question 8).

The other critical component of models for mountain building, as well as for plate tectonics, is the rheology (deformation behavior) of rocks deep in the continental crust and in the upper mantle beneath the mountains. As Figure 2.15 implies, deep crustal rock flows laterally when pressure is decreased by erosion. The rate of flow depends on the rock properties, which in turn depend on mineralogy, temperature, pressure, stress, and the flow rate itself. Although it is possible to determine the deformation behavior by laboratory measurements, these measurements do not appear to replicate deformation of most rocks under natural conditions. On average, the strength of rocks determined from laboratory measurements is much greater than the strength inferred from the study of regional geological systems (Question 6). This discrepancy is probably a matter of scaling, since natural systems are many orders of magnitude larger, and deform many orders of magnitude more slowly, than laboratory samples. Some large-scale mechanisms of deformation, like faulting, are not reproducible in small-scale experimental samples. Also, fault systems within the crust may self-organize to create high fluid pressure along zones of active deformation, further lowering the stresses needed for continued large-scale deformation (Sleep, 2002). In such cases the strength of deforming rock masses is inversely related to their spatial dimension. Although hypotheses like this one can qualitatively account for field observations, a fundamental theory for the rheology of rocks under planetary conditions and scales awaits development (Questions 4 and 6).

Summary

Although plate tectonics theory explains many of Earth’s surface features, fundamental questions remain. There is increasing evidence that the existence of plate tectonics on Earth is related to the presence of abundant water, both at the surface and within Earth’s interior, and that water plays a major role in the creation and destruction of continents. However, there is still no



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