FIGURE 2.15 Links among tectonics, climate, erosion, and topography at convergent plate boundaries illustrated with a hypothetical cross section of convergent plates. The resulting mountain range in (a) is located near the boundary between the plates. A finite-element numerical model (b) assumes stronger rainfall on the left (windward) side of the mountain, which leads to faster erosion there, and general flow of crustal rock toward the region of rapid erosion (curved black arrow). Warmer colors correspond to higher strain rates, the magenta line is the topographic surface, and the gray portion of the mesh shows the eroded mass. (c) Simplified plot of exhumation, elevation, and precipitation for the model. Figure modified from Willett (1999). SOURCE: Dietrich and Perron (2006). Reprinted by permission from Macmillan Publishers Ltd: Nature, copyright 2006.

comprehensive theory that explains how plate tectonics arises naturally from thermal convection. Establishing the criteria for plate tectonic convection is a fundamental research goal for geologists and doing so will require better models for rock deformation properties and improved approaches to representing those properties in numerical models of planetary convection. Other clues will almost certainly come from the history of plate tectonics on Earth, studies of modern plate boundaries, and comparisons with other planets.

The origin of continents can be plausibly attributed to the existence of plate tectonics, in particular to the existence of subduction zones. However, the apparent silica-rich composition of the continental crust indicates that the continents are not made in a simple process like that which produces oceanic crust from magma. Neither the mechanism of producing continental crust nor the process of destroying it and returning it to the mantle is well understood. Nor do we know whether the continents were smaller or larger in Earth’s past or whether the processes that produce and shape them were the same. The contribution of mantle plumes to continental formation has gained particular attention, as has the origin of the mantle roots under the oldest parts of the continents.

The past decade has seen a new understanding of the roles of erosion and climate in controlling the structure and shape of mountain ranges. This knowledge has become central to understanding the processes that affect continents and the changes that must be made to plate tectonics paradigms as applied to continental collisions. This search has intensified the desire to



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