How Erosion Builds Mountains [Preview]

An understanding of how tectonic, erosional and climatic forces interact to shape mountains permits clearer insights into Earth's history

Mountains have evoked awe and inspired artists and adventurers throughout human existence. Recent research has led to important new insights into how these most magnificent of Earths formations came to be. Mountains are created and shaped, it appears, not only by the movements of the vast tectonic plates that make up Earths exterior but also by climate and erosion. In particular, the interactions between tectonic, climatic and erosional processes exert strong control over the shape and maximum height of mountains as well as the amount of time necessary to build--or destroy--a mountain range. Paradoxically, the shaping of mountains seems to depend as much on the destructive forces of erosion as on the constructive power of tectonics. In fact, after 100 years of viewing erosion as the weak sibling of tectonics, many geologists now believe erosion actually may be the strong one in the family. In the words of one research group, "Savor the irony should mountains owe their [muscles] to the drumbeat of tiny raindrops."

Because of the importance of mountain building in the evolution of Earth, these findings have significant implications for earth science. To a geologist, Earth's plains, canyons and, especially, mountains reveal the outline of the planets development over hundreds of millions of years. In this sprawling history, mountains indicate where events in or just below Earth's crust, such as the collisions of the tectonic plates, have thrust this surface layer skyward. Thus, mountains are the most visible manifestation of the powerful tectonic forces at work and the vast time spans over which those forces have operated.

The effort to understand mountain building has a long history. One of the first comprehensive models of how mountains evolve over time was the Geographic Cycle, published in 1899. This model proposed a hypothetical life cycle for mountain ranges, from a violent birth caused by a brief but powerful spasm of tectonic uplift to a gradual slide into "old age" caused by slow but persistent erosion. The beauty and logic of the Geographic Cycle persuaded nearly a century of geologists to overlook its overwhelming limitations.

In the 1960s the plate tectonics revolution explained how mountain building is driven by the horizontal movements of vast blocks of the lithosphere--the relatively cool and brittle part of Earth's exterior. According to this broad framework, internal heat energy shapes the planet's surface by compressing, heating and breaking the lithosphere, which varies in thickness from 100 kilometers or less below the oceans to 200 kilometers or more below the continents. The lithosphere is not a solid shell but is subdivided into dozens of plates. Driven by heat from below, these plates move with respect to one another, accounting for most of our worlds familiar surface features and phenomena, such as earthquakes, ocean basins and mountains.

Earth scientists have by no means discarded plate tectonics as a force in mountain building. Over the past few decades, however, they have come to the conclusion that mountains are best described not as the result of tectonics alone but rather as the products of a system that encompasses erosional and climatic processes in addition to tectonic ones and that has many complex linkages and feedbacks among those three components.

The role of tectonics
PLATE TECTONICS still provides the basic framework that accounts for the distribution of mountains across Earths surface. Mountain building is still explained as the addition of mass, heat or some combination of the two to an area of Earths crust (the crust is the upper part of the lithosphere). Thicker or hotter crust rises upward, forming mountains, because the crust is essentially floating on the mantle under it, and crust that is either thicker or hotter (less dense) floats higher. Plate tectonics contributes to the thickening of the crust by either lateral convergence between adjacent plates or through the upward flow of heat and magma (molten rock).

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