Lecture #30: Mountain Belts of the World
Orogenesis or the formation of mountain ranges is one of the key means by which the continental crust maintains its average elevation, relief, and aerial extent. From the earliest history of its development, Earth's surface has apparently been characterized by mountain ranges. It has become clear that mountains go through long cycles, during which they are uplifted, then gradually waste by erosion, and finally cease to exist in the traditional sense of the word. Their former presence is recorded in the eroded remnants of gigantic batholiths and belts of highly deformed metamorphic rocks. Perhaps all continental rocks were parts of mountains at one time or another.
Mountains differ is age, history, origin, and size. Although in a loose sense, we can define "mountains", it is difficult to classify them in a systematic way because they display a great variety of rocks and structures; no two mountain ranges are identical. About the only think mountains have in common is the fact that they are higher than the surrounding terrain. There are certain characteristics of mountain groups which are of importance in understanding the process of orogenesis.
Mountains are characterized by a hinterland and a foreland. The hinterland is often found closer to the edge of the continent where deformation is has been more severe. The foreland is found next to the craton (interior) of continents. The most severe deformation occurs where continent-continent collision has taken place.
Obvious features of most continental and oceanic mountain systems:
- Mountains tend to occur in elongate or "sub-linear" associations. When a group of mountains which are so aligned are clearly related with respect to composition and origin, they constitute a mountain range or system. The latter term generally applies to a grouping of several ranges which appear to be similar in form, structure, and alignment. The Cascade Range, Sierra Nevada, Rocky Mountains, and Canadian Rockies, among many other ranges, are all members of the American Cordillera system.
- "Young" mountains tend to occur around the margins of continental areas. It has been recognized by geologists for over a century that this basic association is important to the formation of mountains and the evolution and development of continents. Mountain ranges are the consequence of the collision of lithospheric plates.
Types of mountains -- Although mountains show a great diversity in form and rock-types, there are several basic types:
- Valley and Ridge range. This type of range is characterized by a series of broad alternating synclinal and anticlinal folds which involve deformation of only the cover rocks. This type of mountain range is characteristic of deformation in the foreland where the cover has slid like a rug over a polished floor. The valleys have developed by erosion within the least resistant strata, such as limestones and shale. In contrast, the ridges are formed by very resistant rocks. The Valley and Ridge of central Pennsylvania is an example of this type of range.
- Crystalline core ranges. In the hinterland of mountain ranges the crystalline basement is cut up into thrust sheets which have been pushed toward the foreland by plate-tectonic forces. The core of the hinterland areas is characterized by metamorphic rocks which have been folded into great crystalline sheets because the rocks are not able to resist the great forces of orogenesis. Other these thrust sheets are soft and ductile so they are more easily folded particularly at depths of burial greater than 10 km. The Blue Ridge Mountains of the southern Appalachians are an example of this type. Other areas of the core of mountain belts are characterized by numerous intrusions of igneous rocks. The White Mountains of New Hampshire and the Green Mountains of Vermont are examples of this type of mountain core.
- Crystalline upthrust ranges. In some areas of the world, continental crust is compressed to the point where fragments of crystalline basement pops up in the form of brittle block uplifts. The sedimentary cover rocks are folded over the brittle basement in the form of a draping sheet. The Wyoming ranges are characteristic of this type of crystalline block upthrust with monoclines forming over the uplifted crystalline basement.
- Plateau Uplifts. The largest pieces of crustal rock to lift relatively high relative to sea level are called plateaus. If one is standing on a plateau, it is not immediately apparent that one is standing on a mountain range. Topography seems to be relatively subtle. Yet, the edge of the plateau regions appear as mountains. The world's largest plateau is the Tibet Plateau. In the United States, the Colorado Plateau is the largest.
- Fault Block Mountain Ranges. In places where the crust is being extended or pulled apart horsts and grabens form with the uplifted horsts becoming the mountain ranges. These mountains are separated from the intervening valley floors by normal faults of great displacement. The Basin and Range of Nevada consists of a system of ranges which formed during the Tertiary as a consequence of the stretching of the continental crust. These types of mountains are also found along the mid-ocean rifts where oceanic crust is just forming. The Sierra Nevada mountains are a gigantic tilted normal fault-block structure.
- Volcanic Mountain range. Over subduction zones, chains of volcanoes pile up as a consequence of the eruption of magma. Frequent explosive eruptions also contribute to the steep and high slopes of volcanic mountain ranges. The Cascade Range of California and Oregon form a Volcanic Mountain Range.