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Plate Tectonics Plate tectonics tive motion determines the type of boundary; convergent, divergent, or transform. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along these plate boundaries. The lateral relative move- ment of the plates typically varies from zero to 100 mm annually.[2] Tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kind of crust. Along convergent boundaries, subduction carries plates into the mantle; the material lost is roughly balanced by the formation of new (oceanic) crust along divergent margins by seafloor spreading. In this way, the total surface of the globe remains the same. This predic- The tectonic plates of the world were mapped in the second half of the 20th century. tion of plate tectonics is also referred to as the conveyor belt principle. Earlier theories (that still have some sup- porters) propose gradual shrinking (contraction) or grad- ual expansion of the globe.[3] Tectonic plates are able to move because the Earth’s lithosphere has greater strength than the underlying asthenosphere. Lateral density variations in the mantle result in convection. Plate movement is thought to be driven by a combination of the motion of the seafloor away from the spreading ridge (due to variations in topog- raphy and density of the crust, which result in differences in gravitational forces) and drag, with downward suction, at the subduction zones. Another explanation lies in the different forces generated by the rotation of the globe and the tidal forces of the Sun and Moon. The relative im- portance of each of these factors and their relationship to each other is unclear, and still the subject of much debate. Remnants of the Farallon Plate, deep in Earth’s mantle. It is thought that much of the plate initially went under North America (particularly the western United States and southwest Canada) at a very shallow angle, creating much of the mountainous terrain 1 Key principles in the area (particularly the southern Rocky Mountains). The outer layers of the Earth are divided into the Plate tectonics (from the Late Latin tectonicus, from [1] lithosphere and asthenosphere. This is based on differ- the Greek: τεκτονικός “pertaining to building”) is a ences in mechanical properties and in the method for the scientific theory that describes the large-scale motion of transfer of heat. Mechanically, the lithosphere is cooler Earth's lithosphere. This theoretical model builds on and more rigid, while the asthenosphere is hotter and the concept of continental drift which was developed flows more easily. In terms of heat transfer, the litho- during the first few decades of the 20th century. The sphere loses heat by conduction, whereas the astheno- geoscientific community accepted the theory after the sphere also transfers heat by convection and has a nearly concepts of seafloor spreading were later developed in the adiabatic temperature gradient. This division should not late 1950s and early 1960s. be confused with the chemical subdivision of these same The lithosphere, which is the rigid outermost shell of a layers into the mantle (comprising both the asthenosphere planet (on Earth, the crust and upper mantle), is bro- and the mantle portion of the lithosphere) and the crust: ken up into tectonic plates. On Earth, there are seven or a given piece of mantle may be part of the lithosphere eight major plates (depending on how they are defined) or the asthenosphere at different times depending on its and many minor plates. Where plates meet, their rela- temperature and pressure. 1 2 2 TYPES OF PLATE BOUNDARIES The key principle of plate tectonics is that the litho- denser because it has less silicon and more heavier ele- sphere exists as separate and distinct tectonic plates, which ments ("mafic") than continental crust ("felsic").[9] As a ride on the fluid-like (visco-elastic solid) asthenosphere. result of this density stratification, oceanic crust gener- Plate motions range up to a typical 10–40 mm/year (Mid- ally lies below sea level (for example most of the Pacific Atlantic Ridge; about as fast as fingernails grow), to about Plate), while continental crust buoyantly projects above 160 mm/year (Nazca Plate; about as fast as hair grows).[4] sea level (see the page isostasy for explanation of this The driving mechanism behind this movement is de- principle). scribed below. Tectonic lithosphere plates consist of lithospheric mantle overlain by either or both of two types of crustal mate- 2 Types of plate boundaries rial: oceanic crust (in older texts called sima from silicon and magnesium) and continental crust (sial from silicon Main article: List of tectonic plate interactions and aluminium). Average oceanic lithosphere is typically 100 km (62 mi) thick;[5] its thickness is a function of its Three types of plate boundaries exist,[10] with a fourth, age: as time passes, it conductively cools and subjacent mixed type, characterized by the way the plates move rel- cooling mantle is added to its base. Because it is formed ative to each other. They are associated with different at mid-ocean ridges and spreads outwards, its thickness types of surface phenomena. The different types of plate is therefore a function of its distance from the mid-ocean boundaries are:[11][12] ridge where it was formed. For a typical distance that oceanic lithosphere must travel before being subducted, the thickness varies from about 6 km (4 mi) thick at mid- 1. Transform boundaries (Conservative) occur where ocean ridges to greater than 100 km (62 mi) at subduction two lithospheric plates slide, or perhaps more accu- zones; for shorter or longer distances, the subduction zone rately, grind past each other along transform faults, (and therefore also the mean) thickness becomes smaller where plates are neither created nor destroyed. The or larger, respectively.[6] Continental lithosphere is typi- relative motion of the two plates is either sinistral cally ~200 km thick, though this varies considerably be- (left side toward the observer) or dextral (right side tween basins, mountain ranges, and stable cratonic inte- toward the observer). Transform faults occur across riors of continents. The two types of crust also differ a spreading center. Strong earthquakes can occur in thickness, with continental crust being considerably along a fault. The San Andreas Fault in California thicker than oceanic (35 km vs. 6 km).[7] is an example of a transform boundary exhibiting dextral motion. The location where two plates meet is called a plate boundary. Plate boundaries are commonly associ- 2. Divergent boundaries (Constructive) occur where ated with geological events such as earthquakes and two plates slide apart from each other. At zones of the creation of topographic features such as mountains, ocean-to-ocean rifting, divergent boundaries form volcanoes, mid-ocean ridges, and oceanic trenches. The by seafloor spreading, allowing for the formation of majority of the world’s active volcanoes occur along plate new ocean basin. As the continent splits, the ridge boundaries, with the Pacific Plate’s Ring of Fire being the forms at the spreading center, the ocean basin ex- most active and widely known today. These boundaries pands, and finally, the plate area increases causing are discussed in further detail below. Some volcanoes many small volcanoes and/or shallow earthquakes. occur in the interiors of plates, and these have been var- At zones of continent-to-continent rifting, divergent iously attributed to internal plate deformation[8] and to boundaries may cause new ocean basin to form as mantle plumes. the continent splits, spreads, the central rift col- As explained above, tectonic plates may include conti- lapses, and ocean fills the basin. Active zones nental crust or oceanic crust, and most plates contain of Mid-ocean ridges (e.g., Mid-Atlantic Ridge and both. For example, the African Plate includes the con- East Pacific Rise), and continent-to-continent rifting tinent and parts of the floor of the Atlantic and Indian (such as Africa’s East African Rift and Valley, Red Oceans. The distinction between oceanic crust and con- Sea) are examples of divergent boundaries. tinental crust is based on their modes of formation. 3. Convergent boundaries (Destructive) (or active mar- Oceanic crust is formed at sea-floor spreading centers, gins) occur where two plates slide toward each other and continental crust is formed through arc volcanism and to form either a subduction zone (one plate mov- accretion of terranes through tectonic processes, though ing underneath the other) or a continental colli- some of these terranes may contain ophiolite sequences, sion. At zones of ocean-to-continent subduction which are pieces of oceanic crust considered to be part of (e.g., Western South America, and Cascade Moun- the continent when they exit the standard cycle of forma- tains in Western United States), the dense oceanic tion and spreading centers and subduction beneath conti- lithosphere plunges beneath the less dense continent. nents. Oceanic crust is also denser than continental crust Earthquakes then trace the path of the downward- owing to their different compositions. Oceanic crust is moving plate as it descends into asthenosphere, a 3.1 Driving forces related to mantle dynamics 3 trench forms, and as the subducted plate partially melts, magma rises to form continental volcanoes. At zones of ocean-to-ocean subduction (e.g., the Andes mountain range in South America, Aleutian islands, Mariana islands, and the Japanese island arc), older, cooler, denser crust slips beneath less dense crust. This causes earthquakes and a deep trench to form in an arc shape. The upper mantle of the subducted plate then heats and magma rises to form curving chains of volcanic islands. Deep marine trenches are typically associated with sub- duction zones, and the basins that develop along the active boundary are often called “foreland basins”.
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