Before plate tectonics theory became established, geologists were just plain confused about how mountains formed. In the context of the new theory, however, the many processes driving mountain building became clear: mountains form primarily in response to convergent-boundary deformation, continental collisions, and rifting. Since collision zones, rifts, and plate boundaries are linear, mountain belts are linear. Below, we look at these different settings and the types of mountains and geologic structures that develop in each one.
Mountains Related to Subduction
|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 between two continents completely subducts, the continents themselves collide with each other. Continental collision results in the formation of large mountain ranges such as the present-day Himalayas or the Alps and the Paleozoic Appalachian Mountains. The ﬁnal stage in the growth of the Appalachians happened when Africa and North America collided.
|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
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
|Various rocks form during orogeny.|
The process of orogeny establishes geologic conditions appropriate for the formation of a great variety of rocks. We’ll consider examples from all three rock categories (figure 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.
Measuring Mountain Building in Progress
|GPS measurements of shortening in the Andes. The lines indicate the velocity of the red dots relative to the interior of South America. The line at the yellow dot indicates relative plate motion.|
Not all mountains are just “old monuments,” as John Muir mused. The rumblings of earthquakes and the eruptions of volcanoes attest to present-day, continuing movements in some ranges. Geologists can measure the rates of these movements through ﬁeld studies and satellite technology. For example, geologists can determine where coastal areas have been rising relative to the sea level by locating ancient beaches that now lie high above the water. And they can tell where the land surface has risen relative to a river by identifying places where a river has recently carved a new valley. In addition, geologists now use the global positioning system (GPS) to measure rates of uplift and horizontal shortening in orogens. Though standard hand-held GPS devices provide locations with accuracies of only about +-2 m, research-quality GPS systems can specify locations to within +-2 mm. By comparing the position of a location within an orogen to a location outside an orogen over a time period of a few years, it is possible to detect crustal motion. Thus, we can “see” the Andes shorten horizontally at a rate of a couple of centimetres per year (figure above), and we can “watch” as mountains along this convergent boundary rise by a couple of millimetres per year.
Credits: Stephen Marshak (Essentials of Geology)