Continental collision

Continental collision

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Continental collision is a phenomenon
A phenomenon , plural phenomena, is any observable occurrence. Phenomena are often, but not always, understood as 'appearances' or 'experiences'...

 of the plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...

 of Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 that occurs at convergent boundaries
Convergent boundary
In plate tectonics, a convergent boundary, also known as a destructive plate boundary , is an actively deforming region where two tectonic plates or fragments of lithosphere move toward one another and collide...

. Continental collision is a variation on the fundamental process of subduction
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...

, whereby the subduction zone is destroyed, mountains produced, and two continents sutured together. Continental collision is known only from this planet and is an interesting example of how our different crusts, oceanic and continental, behave during subduction.

Continental collision is not an instantaneous event, like a car crash, but may take several tens of millions of years before the faulting
Geologic fault
In geology, a fault is a planar fracture or discontinuity in a volume of rock, across which there has been significant displacement along the fractures as a result of earth movement. Large faults within the Earth's crust result from the action of tectonic forces...

 and folding
Fold (geology)
The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in...

 caused by collision stop. Collision between India and Asia has been going on for about 50 million years already and shows no signs of abating. Collision between East and West Gondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...

 to form the East African Orogen took about 100 million years from beginning (610 Ma) to end (510 Ma). Collision between Gondwana and Laurasia
In paleogeography, Laurasia was the northernmost of two supercontinents that formed part of the Pangaea supercontinent from approximately...

 to form Pangea occurred in a relatively brief interval, about 50 million years long.

Subduction zone: the collision site

The process begins as two continent
A continent is one of several very large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents—they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...

s (different bits of continental crust
Continental crust
The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...

), separated across a tract of ocean (and oceanic crust
Oceanic crust
Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium...

), approach each other, while the oceanic crust is slowly consumed at a subduction zone. The subduction zone runs along the edge of one of the continents and dips under it, raising volcanic mountain chains at some distance behind it, such as the Andes
The Andes is the world's longest continental mountain range. It is a continual range of highlands along the western coast of South America. This range is about long, about to wide , and of an average height of about .Along its length, the Andes is split into several ranges, which are separated...

 of South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...

 today. Subduction involves the whole lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...

, the density of which is largely controlled by the nature of the crust it carries. Oceanic crust is thin (~6 km thick) and dense (about 3.3 g/cm³), consisting of basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...

, gabbro
Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....

, and peridotite
A peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron...

. Consequently, most oceanic crust is subducted easily at an oceanic trench
Oceanic trench
The oceanic trenches are hemispheric-scale long but narrow topographic depressions of the sea floor. They are also the deepest parts of the ocean floor....

. In contrast, continental crust is thick (~45 km thick) and buoyant, composed mostly of granitic rocks (average density about 2.5 g/cm³). Continental crust is subducted with difficulty, but is subducted to depths of 90-150 km or more, as evidenced by ultra-high pressure
High pressure
High pressure in science and engineering is studying the effects of high pressure on materials and the design and construction of devices, such as a diamond anvil cell, which can create high pressure...

 (UHP) metamorphic suites
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...

. Normal subduction continues as long as the ocean exists, but the subduction system is disrupted as the continent carried by the downgoing plate enters the trench. Because it contains thick continental crust, this lithosphere is less dense than the underlying asthenospheric mantle
The asthenosphere is the highly viscous, mechanically weak and ductilely-deforming region of the upper mantle of the Earth...

 and normal subduction is disrupted. The volcanic arc
Volcanic arc
A volcanic arc is a chain of volcanoes positioned in an arc shape as seen from above. Offshore volcanoes form islands, resulting in a volcanic island arc. Generally they result from the subduction of an oceanic tectonic plate under another tectonic plate, and often parallel an oceanic trench...

 on the upper plate is slowly extinguished. Resisting subduction, the crust buckles up and under, raising mountains where a trench used to be. The position of the trench becomes a zone that marks the suture between the two continental terrane
A terrane in geology is short-hand term for a tectonostratigraphic terrane, which is a fragment of crustal material formed on, or broken off from, one tectonic plate and accreted or "sutured" to crust lying on another plate...

s. Suture zones are often marked by fragments of the pre-existing oceanic crust and mantle rocks, known as ophiolites
An ophiolite is a section of the Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks...


Deep subduction of continental crust

The continental crust
Continental crust
The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...

 on the downgoing plate is deeply subducted as part of the downgoing plate during collision, defined as buoyant crust entering a subduction zone. An unknown proportion of subducted continental crust returns to the surface as ultra-high pressure
High pressure
High pressure in science and engineering is studying the effects of high pressure on materials and the design and construction of devices, such as a diamond anvil cell, which can create high pressure...

 (UHP) metamorphic terranes, which contain metamorphic coesite
Coesite[p] is a form of silicon dioxide SiO2 that is formed when very high pressure , and moderately high temperature , are applied to quartz. Coesite was first synthesized by Loring Coes, Jr., a chemist at the Norton Company, in 1953. In 1960, coesite was found by Edward C. T...

 and/or diamond
In mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions...

 plus or minus unusual silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

-rich garnet
The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" may come from either the Middle English word gernet meaning 'dark red', or the Latin granatus , possibly a reference to the Punica granatum , a plant with red seeds...

s and/or potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...

-bearing pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...

s. The presence of these minerals demonstrate subduction of continental crust to at least 90–140 km deep. Examples of UHP terranes are known from the Dabie–Sulu belt of east-central China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...

, the Western Alps
The Alps is one of the great mountain range systems of Europe, stretching from Austria and Slovenia in the east through Italy, Switzerland, Liechtenstein and Germany to France in the west....

, the Himalaya of India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...

, the Kokchetav Massif of Kazakhstan
Kazakhstan , officially the Republic of Kazakhstan, is a transcontinental country in Central Asia and Eastern Europe. Ranked as the ninth largest country in the world, it is also the world's largest landlocked country; its territory of is greater than Western Europe...

, the Bohemian Massif
Bohemian Massif
The Bohemian Massif; or Český masiv; is in the geology of Central Europe a large massif stretching over central Czech republic, eastern Germany, southern Poland and northern Austria...

 of Europe, the Western Gneiss Region of Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...

, and Mali
Mali , officially the Republic of Mali , is a landlocked country in Western Africa. Mali borders Algeria on the north, Niger on the east, Burkina Faso and the Côte d'Ivoire on the south, Guinea on the south-west, and Senegal and Mauritania on the west. Its size is just over 1,240,000 km² with...

. Most UHP terranes consist of an imbricated sheets or nappe
In geology, a nappe is a large sheetlike body of rock that has been moved more than or 5 km from its original position. Nappes form during continental plate collisions, when folds are sheared so much that they fold back over on themselves and break apart. The resulting structure is a...

s. The fact that most UHP terranes consist of thin sheets suggests that much thicker, volumetrically dominant tracts of continental crust are more deeply subducted.

Orogeny and collapse

An orogeny
Orogeny refers to forces and events leading to a severe structural deformation of the Earth's crust due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts...

 is underway when mountains begin to grow in the collision zone. There are other modes of mountain formation and orogeny but certainly continental collision is one of the most important. Rain
Rain is liquid precipitation, as opposed to non-liquid kinds of precipitation such as snow, hail and sleet. Rain requires the presence of a thick layer of the atmosphere to have temperatures above the melting point of water near and above the Earth's surface...

fall and snow
Snow is a form of precipitation within the Earth's atmosphere in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by...

fall increase on the mountains as these rise, perhaps at a rate of a few millimeters per year (at a growth rate of 1 mm/year, a 5,000 m tall mountain can form in 5 million years, a time period that is less than 10% of the life of a typical collision zone). River
A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...

 systems form, and glaciers may grow on the highest peaks. Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

 accelerates as the mountains rise, and great volumes of sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....

 are shed into the rivers, which carry sediment away from the mountains to be deposited in sedimentary basins in the surrounding lowlands. Crustal rocks are thrust fault
Thrust fault
A thrust fault is a type of fault, or break in the Earth's crust across which there has been relative movement, in which rocks of lower stratigraphic position are pushed up and over higher strata. They are often recognized because they place older rocks above younger...

ed over the sediments and the mountain belt broadens as it rises in height. A crustal root also develops, as required by isostasy
Isostasy is a term used in geology to refer to the state of gravitational equilibrium between the earth's lithosphere and asthenosphere such that the tectonic plates "float" at an elevation which depends on their thickness and density. This concept is invoked to explain how different topographic...

; mountains can be high if underlain by thicker crust. Crustal thickening may happen as a result of crustal shortening or when one crust overthrusts the other. Thickening is accompanied by heating, so the crust becomes weaker as it thickens. The lower crust begins to flow and collapse under the growing mountain mass, forming rifts near the crest of the mountain range. The lower crust may partially melt
Melt can refer to:* Melting, in physics, the process of heating a solid substance to a liquid*Melt , the semi-liquid material used in steelmaking and glassblowing*Melt inclusions, a feature of igneous rock...

, forming anatectic granites which then rise into the overlying units, forming granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...

An intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly...

s. Crustal thickening provides one of two negative feedbacks on mountain growth in collision zones, the other being erosion. The popular notion that erosion is responsible for destroying mountains is only half correct - viscous flow of weak lower mantle also reduces relief with time, especially once the collision is complete and the two continents are completely sutured. Convergence between the continents continues because the crust is still being pulled down by oceanic lithosphere sinking in the subduction zone to either side of the collision as well as beneath the impinging continent.

The pace of mountain building associated with the collision is measured by radiometric dating
Radiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...

 of igneous rocks or units that have been metamorphosed during the collision and by examining the record of sediments shed from the rising mountains into the surrounding basins. The pace of ancient convergence can be determined with paleomagnetic
Paleomagnetism is the study of the record of the Earth's magnetic field in rocks. Certain minerals in rocks lock-in a record of the direction and intensity of the magnetic field when they form. This record provides information on the past behavior of Earth's magnetic field and the past location of...

 measurements, while the present rate of convergence can be measured with GPS.

Far-field effects

The effects of the collision are felt far beyond the immediate site of collision and mountain-building. As convergence between the two continents continues, the region of crustal thickening and elevation will become broader. If there is an oceanic free face, the adjacent crustal blocks may move towards it. As an example of this, the collision of India with Asia forced large regions of crust to move south to form modern Southeast Asia
Southeast Asia
Southeast Asia, South-East Asia, South East Asia or Southeastern Asia is a subregion of Asia, consisting of the countries that are geographically south of China, east of India, west of New Guinea and north of Australia. The region lies on the intersection of geological plates, with heavy seismic...

. Another example is the collision of Arabia with Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...

, which is squeezing the Anatolian Plate
Anatolian Plate
The Anatolian Plate is a continental tectonic plate consisting primarily of the country of Turkey.The easterly side is a boundary with the Arabian Plate, the East Anatolian Fault, a left lateral transform fault....

 (present day Turkey
Turkey , known officially as the Republic of Turkey , is a Eurasian country located in Western Asia and in East Thrace in Southeastern Europe...

). As a result, Turkey is moving west and south into the Mediterranean Sea
Mediterranean Sea
The Mediterranean Sea is a sea connected to the Atlantic Ocean surrounded by the Mediterranean region and almost completely enclosed by land: on the north by Anatolia and Europe, on the south by North Africa, and on the east by the Levant...

 and away from the collision zone. These far-field effects may result in the formation of rifts, and rift valley
Rift valley
A rift valley is a linear-shaped lowland between highlands or mountain ranges created by the action of a geologic rift or fault. This action is manifest as crustal extension, a spreading apart of the surface which is subsequently further deepened by the forces of erosion...

s such as that occupied by Lake Baikal
Lake Baikal
Lake Baikal is the world's oldest at 30 million years old and deepest lake with an average depth of 744.4 metres.Located in the south of the Russian region of Siberia, between Irkutsk Oblast to the northwest and the Buryat Republic to the southeast, it is the most voluminous freshwater lake in the...

, the deepest lake on Earth.

Fossil collision zones

Continental collisions are a critical part of the Supercontinent cycle
Supercontinent cycle
The supercontinent cycle describes the quasi-periodic aggregation and dispersal of Earth's continental crust. There are varying opinions as to whether the amount of continental crust is increasing, decreasing, or staying about the same, but it is agreed that the Earth's crust is constantly being...

and have happened many times in the past. Ancient collision zones are deeply eroded but may still be recognized because these mark sites of intense deformation, metamorphism, and plutonic activity that separate tracts of continental crust having different geologic histories prior to the collision. Old collision zones are commonly called "suture zones" by geologists, because this is where two previous continents are joined or sutured together.

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