In this state, the size of the mountains can remain stable for millions of years, because the rate of erosion matches the rate of uplift. Localized topography within such a mountain range will change as rocks of different strength are exposed at the surface. Average mountain height, however, may undergo little change, because of the long-term balance between tectonics and climate-driven erosion. Mountain ranges, it appears, often go through three distinct phases.
The first, formative stage begins with the converging of plates or some other tectonic event that thickens crust and causes topography to rise. During this stage, rates of uplift exceed those of erosion. Erosion rates increase dramatically, however, as elevations and relief increase.
Depending on the size of the range and the local climate, uplift may persist until erosion rates or the strength of the crust limits the average elevation of the range from increasing any more. This is the second stage, a steady state that may continue as long as the rates of uplift and erosion remain equal. When uplift diminishes, erosion begins to dominate and the final stage begins. In this final stage, the average elevation of the mountain range begins a long, slow decline. The cycle may be interrupted or complicated at any stage by tectonic or climatic events as well as the feedback among those processes and erosion.
The new model of how mountains develop promises to be as revolutionary as was plate tectonics some four decades ago. Just as plate tectonics managed to explain the worldwide distribution of earthquakes, volcanoes, fossils and many different rocks and minerals, the new understanding of mountain building shows how tectonic forces, Earth's climate and topography interact to create some of Earth's most spectacular landscapes.
Like plate tectonics, the new model also illuminates phenomena that had long puzzled geologists. Computer simulations incorporating many of the models principal precepts, for example, have proved very successful in mimicking the effects of complex tectonic histories, climatic variability and different geologic settings.
Continuing research will provide even more details of how Earth's magnificent mountain ranges grow, evolve and decline, as well as details concerning the importance of mountains in shaping the climate and tectonics of our planet. BRANDON began their collaboration at Yale University in the emerging field of active tectonics, which emphasizes the interactions between tectonic deformation and Earths topography.
Pinter carried out postdoctoral research there and is now professor at Southern Illinois University at Carbondale. His research includes a focus on the topographic expression of tectonic processes and has involved work in California, South America and the peri-Adriatic region.
Brandon is professor of structural geology and tectonics at Yale. His research is focused on understanding the interrelation between tectonic uplift and erosion at subduction zones and collisional mountain ranges. Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue. See Subscription Options. Discover World-Changing Science.
Recent Articles by Mark T. Brandon How Erosion Builds Mountains. Get smart. Sign up for our email newsletter. Sign Up. Support science journalism. Knowledge awaits. See Subscription Options Already a subscriber? Create Account See Subscription Options. Continue reading with a Scientific American subscription. These combined forces break up the rocks and erode the peaks into their stark, sculpted forms.
Falling ice, rocks and gushing water wear away at the mountain slopes. The ice and rock debris accumulates in the valleys and flows downwards as slow moving glaciers. When these melt, piles of rock debris called moraines are left behind. Aprons of rock debris make up the scree slopes alongside the Mueller Valley near Mount Cook.
The magma that rises from below these ridges lifts the ocean floor. Mid-ocean ridges circle the earth like the seams on a giant baseball. If the magma rises similarly beneath a continent, the land will bulge and eventually crack, potentially tearing the continent in half and creating a new ocean, as at the Red Sea.
Although compression creates most uplift, the Basin and Range region of western North America resulted from a combination of collision and then extension. A map of North America illustrates a series of parallel north-south oriented mountain ranges separated by north-south trending valleys basins extending from Nevada and Utah down into Mexico. These mountains formed when first the crust arched from the collision of North America with a piece of ocean floor, and then later the crust began to separate.
As the top of the arches cracked similar to how the top of a bent piece of clay cracks along its upper surface when bent , pieces of the crust dropped down to form the valleys, and other pieces formed the mountain ranges. Finally, when a huge weight is removed from the crust, the crust will slowly rise up in a process called isostatic rebound.
Exhumation during thickening can only occur if rapid denudation accompanies the thickening process. During homogeneous thickening with erosion that is elevation-dependent, the initial depth from which rocks can be exhumed is only determined by the density distribution in the column and is independent of erosion or thickening rates.
Uplift as vertical motion of the surface with respect to a reference level, for example the geoid and exhumation defined as vertical motion of rocks with respect to the surface may follow different patterns in time and that the difference between the two evolutions may be a useful indicator of the exhumation process.
0コメント