Mountain belts

Mountain belts are products of plate tectonics, produced by the convergence of crustal plates. Topographic mountains are only the surficial expression of processes that profoundly deform and modify the crust. Long after the mountains themselves have been worn away, their former existence is recognizable from the structures that mountain building forms within the rocks of the crust.

Orogeny

Mountains have many origins. They can be volcanic, like Mount Vesuvius, or form by vertical movements along faults, as the Sierra Nevada or the Ruwenzori of central Africa were. Some mountains result from relatively gentle epeirogenic (deformational) uplift of the crust—for example, the Black Hills or the Adirondack Mountains. The causes of epeirogeny are poorly understood. The great mountain chains of the Earth, such as the Andes, the Rocky Mountains, or the Himalayas, however, formed not only from uplift but also from internal deformation of the crust, volcanic activity, metamorphism, and the intrusion of vast quantities of molten rock into the crust, especially granite and related rocks. These processes are collectively called orogeny, and mountain chains that form from such processes are called orogenic belts. Orogeny is one of the most important consequences of plate tectonics. It occurs when plates collide, and one plate overrides the other—a process known as subduction—in response to compressional forces and heating generated by the plate collision.

The Earth's crust consists of two types of plates that geologists call continental and oceanic. The continents and the adjacent continental shelves are underlain by granitic crust, which averages about forty kilometers thick. The ocean floors are made of gabbro and basalt, which average about five kilometers thick. The true edge of a continent is not the shoreline, which is constantly changing, but the boundary between continental and oceanic crust. The edge of the continental shelf coincides closely with this boundary.

Many mountain belts form at subduction zones with a continental overriding plate and an oceanic descending plate. The downward bending of the plate creates a deep, narrow trench on the ocean floor, sometimes more than ten kilometers below sea level. The descending oceanic plate sinks into the Earth's interior, eventually to be reabsorbed. The overriding plate experiences orogeny. All orogenic belts differ in detail, but most have certain major features in common. A typical orogenic belt consists of parallel zones, which may be defined by distinctive rock type, type of metamorphism, level of igneous activity, or type of deformation of the rocks. The zones, generally parallel to the boundary where the two plates collide, are the result of different crustal conditions and processes at different distances from the plate boundary. It is useful to regard orogenic belts as having an “outer” side, adjacent to the plate boundary, and an “inner” side within the overriding plate.

The first zones recognized in orogenic belts were those defined by environment of deposition: an outer zone of thick, deep-water sedimentary rocks and volcanic rocks and an inner zone of thinner, shallow-water sedimentary rocks without abundant volcanic rocks. The nineteenth-century American geologist James Hall first described these zones, which he envisioned as parallel troughs formed by downward folding of the crust. Because these troughs were viewed as immense versions of ordinary downward folds, or synclines, James Dana later called the troughs “geosynclines.” The outer trough was called the eugeosyncline, and the inner trough was called the miogeosyncline.

The original concept of the geosyncline disturbed many geologists because it did not quite match the structure of active mountain belts. In 1964, Robert Dietz reexamined the geosyncline concept and showed that the rocks need not have accumulated in troughs. With this insight, it became clear that the rocks of miogeosynclines corresponded closely to those of the continental shelves, while eugeosynclines were a good match to the rocks of many volcanic island chains. It eventually became clear that the rocks of the eugeosyncline often formed separately from the miogeosyncline and were later juxtaposed by plate motions.

Because of these revisions of the original geosyncline concept, many geologists have abandoned the original terms and prefer the terms geocline, eugeocline, and miogeocline instead. The eugeocline is the outer belt of deep-water sedimentary rocks and volcanic rocks. The miogeocline is the inner belt of shallow-water sedimentary rocks. Beyond the miogeocline is the platform, where thin shallow-water or terrestrial rocks were deposited on the stable interior of the overriding plate. Additionally, most orogenic belts have an inner belt of coarse sedimentary rocks deposited late in the history of the orogenic belt. This belt, called the Molasse Basin or the North Alpine Foreland Basin,  consists of debris eroded from the mountains and deposited at their base. Molasse basins consist mostly of rocks deposited in shallow-water or land environments.

Mountain Belt Structure

Much of the structure of a mountain belt is related to processes in the descending plate. As the descending plate reaches a depth of about 100 kilometers, it begins to melt, and molten rock, or magma, invades the overriding plate. Generally, volcanic rocks in orogenic belts become progressively richer in silica with increasing distance from the plate boundary because the rising magma has more time to react with silica-rich continental crust. Also, volcanic and intrusive rocks in mountain belts tend to become more silica-rich over time. Most orogenic belts have a main axis of igneous activity, the igneous arc, where volcanic and intrusive activity are concentrated. The igneous arc is generally on the inner side of the eugeocline. In deeply eroded orogenic belts, intrusive rocks of the igneous arc, usually granitic in composition, are exposed as great masses known as batholiths.

Different thermal conditions in different parts of the overriding plate give rise to two distinct zones of metamorphism. Adjacent to the descending plate, rocks are carried downward to great depths, often twenty kilometers or more, but because the rocks are in contact with the still-cool descending plate, they remain unusually cool. Temperatures in this zone generally average 200 to 300 degrees Celsius, instead of the 500 to 600 degrees Celsius that might be expected at twenty-kilometer depth. This low-temperature, high-pressure metamorphism is known as blueschist metamorphism because many minerals that form often impart a bluish color to the rocks.

Generally coinciding with the igneous arc is an inner belt of metamorphism where temperatures are high but pressures are moderate. Peak temperatures commonly exceed 600 degrees Celsius, with pressures typically reflecting depths of five to ten kilometers. Such conditions are called amphibolite metamorphism. Adjacent to the region of highest temperature is a region of lower-temperature metamorphism, where temperatures of 400 to 500 degrees Celsius prevail. This type of metamorphism is called greenschist metamorphism, from the greenish color of many of the minerals formed. The outer zone of blueschist metamorphism generally occupies the outer part of the geocline. Amphibolite metamorphism generally coincides with the igneous arc and the inner part of the eugeocline. Greenschist metamorphism commonly extends into the miogeocline.

The deformation of rocks in the overriding plate depends on the nature of the rocks, stress, temperature, and confining pressure. Orogenic belts display several zones of distinctive structures. The most important of these belts are the accretionary prism, zone of basement mobilization, and the foreland fold-and-thrust belt. The eugeocline, in general, is a region of intense deformation, while deformation in the miogeocline is less intense. The outermost edge of the orogenic belt is occupied by the accretionary prism, and it often forms much of the eugeocline. Where the colliding plates meet, sediment is scraped off the descending plate. Other sediment is eroded from the continent and pours into the trench. The sediment from the continent is deposited rapidly, with little weathering or sorting, to form impure sandstone called graywacke.

Submarine landslides and slumps are common in the unstable setting of the trench. The resulting complex of chaotically deposited greywacke is called flysch. A wedge of intensely deformed sediment accumulates on the edge of the continent, much the way a wedge of snow accumulates ahead of a snowplow. This wedge of sediment is the accretionary prism. Frequently, fragments of oceanic crust break off the descending plate and are incorporated into the accretionary prism. These slices of oceanic crust, called ophiolites, are of enormous geologic value. Not only do they mark the location of former subduction zones, but they also provide otherwise unobtainable cross sections of oceanic crust exposed on dry land. The actual contact between the two plates is marked by mélange, a chaotic mixture of broken rock with fragments ranging from microscopic to kilometers in size.

The high temperatures in the igneous arc and amphibolite zone of metamorphism can make the rocks of the crust plastic—the rocks flow like stiff fluids, even though they do not melt. Because the hot rocks are less dense than the cooler crust around them, they rise upward. A mass of rock that flows upward in this manner is called a diapir. The process of heating deep crust (or “basement”) so that it rises is called basement mobilization, and evidence of it occurs in many mountain belts. The mobilized crust is an intensely deformed and highly metamorphosed rock called gneiss. The rising mass of gneiss often appears to have shouldered the overlying rocks aside, so the rocks are arched upward with a central core of gneiss. Such a structure is called a gneiss dome.

Compressional forces arising from plate convergence thicken the crust of the overriding plate in many ways. Within the accretionary prism, sheets of sedimentary rock are thrust downward beneath the overriding plate, resulting in a stack of faulted slices of rock. These slices may thicken internally by fracturing along small faults and stacking the resulting small slices one above the other, a process called duplexing. Within the igneous arc, crustal thickening occurs when magma invades the crust, increasing its volume. Magma can also be added to the base of the crust, a process called underplating. Heating of the crust within the igneous arc makes much of the lower crust plastic, permitting the plastic crust to be squeezed upward by compression. This process probably assists the upward movement of gneiss domes.

The thickening of the crust results in the uplift of the surface and the formation of topographic mountains. In the foreland, which basically coincides with the miogeocline, deformation is largely a response to events in the active core of the mountain belt. Some of the deformation in the foreland seems to be driven by rising masses of mobilized basement. Rocks of the miogeocline and some of the underlying crust fracture into sheets shoved over the rocks beneath. Fractures or faults where one mass of rock overrides another are called thrust faults. These rocks may also be buckled into folds by compressive forces. Rocks nearer the surface often slide off the rising mountain belt. The rocks may break into thin sheets called nappes that stack one atop the other or may crumple into folds. Often, the folded rocks have detached from the rocks beneath, much like a carpet slides and folds when a piece of furniture is pushed over it. This process of detachment is called décollement. Because this deformation involves only the surface layers of rock and not the underlying basement rocks, it is called thin-skinned deformation.

Other Types of Plate Collision

Other kinds of plate collision result in different combinations of structures. Oceanic-oceanic subduction zones have somewhat simpler orogenic belts. When the descending plate begins to melt, magma rises and breaks through the surface to create a volcanic island arc such as those found in the Aleutian Islands or the Lesser Antilles. Since both plates are oceanic crust, made largely of basalt, the magma also is basalt. Erosion strips sediment off the volcanic islands and dumps much of it in the trench to form an accretionary prism. Over a very long time, the island arc may be built up into a continuous belt of intensely deformed volcanic rocks and sedimentary rocks derived from them. The Greater Antilles and the Isthmus of Panama probably formed this way. Such orogenic belts consist essentially of a eugeocline and igneous arc, with the associated metamorphic zones and deformation structures.

Continent-continent collisions start out as continent-ocean subduction zones, but eventually, the convergence of plates brings two continents together, one of which is pushed beneath the other. Continent-continent collisions include the Himalayas, where India is being pushed beneath Tibet, and the Persian Gulf, where the Arabian Peninsula is being pushed beneath Iran. Because continental crust is thick and relatively light, the descending plate cannot be subducted. Instead, one continent rides onto the other, creating a double thickness of crust. Eventually, resistance to further movement may cause plate motions to change on a regional or global scale.

Usually the overriding continent has had a long history of orogeny before the collision, whereas the other continent may have had none. Orogeny results in such a wide range of structures that it is usually immediately obvious which of the continents is or was the overriding plate. The collision boundary between the two continents, called a suture, may display relics of the former accretionary prism, including melange, fragments of ophiolites, or evidence of blueschist metamorphism.

Often, a small block of crust, called a terrane, collides with a larger plate. The terrane may be a volcanic island chain or a small fragment of continental crust. The northern coast of New Guinea is an area where terranes (in this case, volcanic island chains) are colliding with a continent. The addition of terranes to a larger plate is called accretion. Terranes are recognizable as distinct blocks of crust separated from adjacent rocks by major faults. In many cases, eugeoclines did not form near their corresponding miogeoclines but as separate terranes. Repeated accretion of terranes can add large areas to a continent. Roughly 1,000 kilometers of the western United States was accreted to North America in the last 500 million years.

After orogeny ceases, it is common for mountain belts to experience a period of crustal extension and faulting. Once the compressional forces that uplifted the mountains subside, many mountain ranges simply cannot support their own mass and begin to spread under their own weight. It requires about twenty million years for erosion to level a mountain range. Nevertheless, long after the topographic mountains are gone, the structures created by orogeny remain. The most conspicuous markers of ancient orogenies are usually the eugeocline and miogeocline, igneous arc, and molasse deposits.

If mountains are worn away in a few tens of millions of years, it follows that modern Urals and Appalachians—the products of continental collisions over 200 million years ago—cannot be remains of the original mountains. In the case of the Appalachians, this point is clear because rivers such as the Potomac flow across the structures in the mountain belt. The Appalachians were once level enough (or buried) that rivers could flow across them. The Appalachians, the Urals, and even the Alps resulted from recent epeirogenic uplift after erosion had largely or entirely leveled the original mountains. Why mountain belts sometimes experience renewed periods of uplift long after orogeny ceases is unknown.

Principal Terms

continental shelf: the submerged offshore portion of a continent, ending where water depths increase rapidly from a few hundred to thousands of meters

epeirogeny: uplift or subsidence of the crust within a region, without the internal disturbances characteristic of orogeny

gabbro: a silica-poor intrusive igneous rock consisting mostly of calcium-rich feldspar and iron and magnesium silicates; its volcanic equivalent is basalt

granite: a silica-rich intrusive igneous rock consisting mostly of quartz, potassium- and sodium-bearing feldspar, and biotite or hornblende

igneous: from the Latin ignis (“fire”), a term referring to rocks formed from the molten state or to processes that form such rocks

metamorphism: the change in the mineral composition or texture of a rock because of heat, pressure, or the chemical action of fluids in the Earth

orogeny: the profound disturbance of the Earth's crust, characterized by crustal compression, metamorphism, volcanism, intrusions, and mountain formation

plate tectonics: the theory that the crust of the Earth consists of large moving plates; orogeny occurs where plates converge and one plate overrides the other

sedimentary rocks: rocks that form by surface transport and deposition of mineral grains or chemicals

subduction: the sinking of a crustal plate into the interior of the Earth; subduction occurs at subduction zones, where plates converge

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