Before plate tectonics concept have become mounted, geologists were just undeniable stressed approximately how mountains fashioned. In the context of the new concept, but, the numerous techniques driving mountain building became clean: mountains shape often in response to convergent-boundary deformation, continental collisions, and rifting. Since collision zones, rifts, and plate limitations are linear, mountain belts are linear. Below, we examine these extraordinary settings and the varieties of mountains and geologic systems that increase in every one.
Mountains Related to Subduction
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| Characteristics of convergent-margin orogens. |
At some convergent plate boundaries, compressional stresses develop, and these cause crustal shortening and uplift in the overriding plate. Such shortening may produce a fold-thrust belt, in which a thrust system develops (figure above a). As a consequence of this faulting, thrust slices (sheets of rock above a thrust fault) push up and over their neighbours, and rocks within thrust slices bend and become folded. The thrust faults merge with a sub-horizontal fault, called a detachment, at depth. The Andes orogen of western South America displays the rugged topography that can develop in compressional convergent-margin orogens (figure above b).
If subduction continues over a long time, offshore volcanic arcs, oceanic plateaus, and micro-continents may drift into the convergent margin (see figure above a). Such crustal blocks are too buoyant to subduct and sink back into the mantle, so instead they collide with the overriding plate and “suture” (attach) to the edge of the overriding plate. Geologists refer to this process as accretion; the buoyant crustal block is called an exotic terrane when it is offshore, and an accreted terrane once it has attached to the overriding plate. Once accretion occurs, the convergent plate boundary may jump to the seaward side of the accreted terrane, so that subduction can continue. The process of accretion can add substantial new crust to a convergent-margin orogen. For example, the western half of the North American Cordillera, a region that is up to 500 km wide, consists of accreted terranes (figure above c). Orogens that grew laterally by the attachment of exotic terranes have come to be known as accretionary orogens.
Mountains Related to Continental Collision
Once the oceanic lithosphere among two continents completely subducts, the continents themselves collide with each other. Continental collision effects inside the formation of huge mountain tiers together with the existing-day Himalayas or the Alps and the Paleozoic Appalachian Mountains. The ?Nal level in the growth of the Appalachians came about whilst Africa and North America collided.
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| Characteristics of collisional orogens. |
During collision, intense compression generates fold-thrust belts on the margins of the orogen (figure above a–c). In the interior of the orogen, where one continent overrides the edge of the other, high-grade metamorphism occurs, accompanied by formation of passive-flow folds and tectonic foliation. During this process, the crust below the orogen thickens to as much as twice its normal thickness. During such crustal thickening, rocks squeeze upward in the hanging walls of large thrust faults.
Mountains Related to Continental Rifting
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| Rift-related mountains. |
Continental rifts are places where continents are splitting in two. During rifting, stretching causes normal faulting in the brittle crust (figure above a). Movement on the normal faults drops down blocks of crust, producing deep, sediment-filled basins separated by narrow, elongate mountain ranges that contain tilted rocks. These ranges are sometimes called fault-block mountains. Stretching thins the lithosphere, allowing hot asthenosphere to rise and undergo decompression melting. This process produces magmas that rise to form volcanoes within the rift. Today, the East African Rift clearly shows the configuration of rift-related mountains and volcanoes. And in North America, rifting yielded the broad Basin and Range Province of Utah, Nevada, and Arizona (figure above b).
Forming Rocks in and Near Mountains
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| Various rocks shape in the course of orogeny. |
The method of orogeny establishes geologic situations appropriate for the formation of a awesome form of rocks. We?Ll recall examples from all three rock classes (discern above):
- Igneous activity during orogeny: In convergent plate boundaries, melting takes place in the mantle above the subducting plate. In rifts, stretching and thinning of lithosphere causes decompression melting of the underlying mantle. And during continental collision, melting may take place where deep portions of the crust undergo heating. All of these melting regimes produce magma, which rises and freezes to form igneous rocks in the overlying mountains.
- Sedimentation during orogeny: Weathering and erosion in mountain belts generate vast quantities of sediment. This sediment tumbles down slopes and gets carried away by glaciers or streams that transport it to low areas where it accumulates in alluvial fans or deltas. In some locations, the weight of mountain belts pushes down the surface of the lithosphere, thereby producing a deep sedimentary basin at the border of the range.
- Metamorphism during orogeny: Contact metamorphic aureoles form adjacent to igneous intrusions in orogens. And regional metamorphism occurs where mountain building thrusts one part of the crust over another; when this happens, rock of the footwall ends up at great depth and thus can be subjected to high temperature and pressure. Because deformation accompanies this process, the resulting metamorphic rocks contain tectonic foliation.
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| GPS measurements of shortening within the Andes. The lines imply the speed of the crimson dots relative to the indoors of South America. The line on the yellow dot indicates relative plate movement. |
Not all mountains are simply ?Vintage monuments,? As John Muir mused. The rumblings of earthquakes and the eruptions of volcanoes attest to give-day, persevering with movements in some stages. Geologists can measure the costs of those moves through ?Eld studies and satellite tv for pc era. For example, geologists can decide in which coastal regions were growing relative to the ocean stage by means of finding ancient beaches that now lie excessive above the water. And they can inform in which the land surface has risen relative to a river via figuring out places in which a river has recently carved a brand new valley. In addition, geologists now use the worldwide positioning system (GPS) to measure quotes of uplift and horizontal shortening in orogens. Though fashionable hand held GPS gadgets provide locations with accuracies of best approximately -2 m, studies-fine GPS structures can specify locations to inside -2 mm. By evaluating the position of a area within an orogen to a area outside an orogen over a time period of a few years, it's far viable to detect crustal motion. Thus, we can ?See? The Andes shorten horizontally at a fee of multiple centimetres in keeping with yr (figure above), and we can ?Watch? As mountains along this convergent boundary rise with the aid of more than one millimetres per 12 months.
Credits: Stephen Marshak (Essentials of Geology)
