How Erosion Builds Mountains

massive than all human structures combined and are sculpted in greater detail than a Baroque palace. The world’s tallest pinnacle—Mount Everest in the Himalayas— reaches 8,848 meters, or about 15 times higher than anything people have ever built. Not surprisingly, such spectacular topography has evoked awe and inspired artists and adventurers throughout human existence. Recent research has led to important new insights into how this most magnificent of the earth’s relief comes to be. Mountains are created and shaped, it appears, not only by the movements of the vast tectonic plates that make up the earth’s 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 sister of tectonics, many geologists now believe erosion actually may be the head of 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 the earth, these findings have significant implications for earth science. To a geologist, the earth’s plains, canyons and, especially, mountains reveal the outline of the planet’s development over hundreds of millions of years. In this sprawling history, mountains indicate where events in or just below the 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 recent model builds on a lengthy 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 plateHow Erosion Builds Mountains