Plate tectonics (from Greek τέκτων, tektōn "builder" or "mason") is a theory of geology that has been developed to explain the observed evidence for large scale motions of the Earth's lithosphere. Greek (el ελληνική γλώσσα or simply el ελληνικά — "Hellenic" is an Indo-European language, spoken today by 15-22 million people mainly The word theory has many distinct meanings in different fields of Knowledge, depending on their methodologies and the context of discussion. Geology (from Greek γη gê, "earth" and λόγος Logos, "speech" lit EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 The lithosphere (IPA, from the Greek λίθος for "rocky" + σφαίρα for "sphere" is the solid outermost shell of a rocky Planet. The theory encompassed and superseded the older theory of continental drift from the first half of the 20th century and the concept of seafloor spreading developed during the 1960s. Continental drift is the movement of the Earth 's Continents relative to each other Seafloor spreading occurs at Mid-ocean ridges where new Oceanic crust is formed through volcanic activity and then gradually moves away from the ridge

The outermost part of the Earth's interior is made up of two layers: above is the lithosphere, comprising the crust and the rigid uppermost part of the mantle. The lithosphere (IPA, from the Greek λίθος for "rocky" + σφαίρα for "sphere" is the solid outermost shell of a rocky Planet. In Geology, a crust is the outermost solid shell of a planet or moon The mantle is a part of an Astronomical object. The interior of the Earth, similar to the other Terrestrial planets, is Chemically divided Below the lithosphere lies the asthenosphere. The asthenosphere (from an invented Greek a + ' sthenos "without strength" and Greek word σφαίρα (sphera meaning globe is the Although solid, the asthenosphere has relatively low viscosity and shear strength and can flow like a liquid on geological time scales. Viscosity is a measure of the resistance of a Fluid which is being deformed by either Shear stress or Extensional stress. Shear strength in reference to Soil is a term used to describe the maximum strength of soil at which point significant plastic deformation or yielding The deeper mantle below the asthenosphere is more rigid again. This is, however, not because of cooler temperatures but due to high pressure.

The lithosphere is broken up into what are called tectonic plates —in the case of Earth, there are seven major and many minor plates (see list below). The lithospheric plates ride on the asthenosphere. These plates move in relation to one another at one of three types of plate boundaries: convergent or collision boundaries, divergent or spreading boundaries, and transform boundaries. In Plate tectonics, a convergent boundary – also known as a convergent plate boundary or a destructive plate boundary – is an actively deforming region In Plate tectonics, a divergent boundary or divergent plate boundary (also known as a constructive boundary or an extensional boundary) is a A transform fault is a fault which runs along the boundary of a Tectonic plate. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along plate boundaries. An earthquake is the result of a sudden release of energy in the Earth 's crust that creates Seismic waves Earthquakes are recorded with a Seismometer Plate tectonics and hotspots Divergent plate boundaries At the A mountain is a Landform that extends above the surrounding Terrain in a limited area with a peak The oceanic trenches are hemispheric-scale long but narrow topographic depressions of the sea floor The lateral movement of the plates is typically at speeds of 50—100 mm/a. ^[1]

Contents 1 Synopsis of the development of the theory 2 Key principles 3 Types of plate boundaries 3.1 Transform (conservative) boundaries 3.2 Divergent (constructive) boundaries 3.3 Convergent (destructive) boundaries 4 Driving forces of plate motion 4.1 Friction 4.2 Gravitation 4.3 External forces 4.4 Relative significance of each mechanism 5 Major plates 6 Historical development of the theory 6.1 Continental drift 6.2 Floating continents 6.3 Plate tectonic theory 6.3.1 Explanation of magnetic striping 6.3.2 Subduction discovered 6.3.3 Mapping with earthquakes 6.4 Geological paradigm shift 7 Biogeographic implications on biota 8 Plate tectonics on other planets 8.1 Venus 8.2 Mars 8.3 Galilean satellites 8.4 Titan 9 Metaphoric uses 10 See also 11 References 12 Further reading 13 External links

Synopsis of the development of the theory

In the late 19th and early twentieth centuries, geologists assumed that the Earth's major features were fixed, and that most geologic features such as mountain ranges could be explained by vertical crustal movement, as explained by geosynclinal theory. Geosyncline theory is an obsolete concept involving vertical crustal movement that has been replaced by Plate tectonics to explain crustal movement and geologic features It was observed as early as 1596 that the opposite coasts of the Atlantic Ocean — or, more precisely, the edges of the continental shelves — have similar shapes and seem once to have fitted together. The coast is defined as the part of the land adjoining or near the Ocean. The continental shelf is the extended perimeter of each Continent and associated Coastal plain, which is covered during interglacial periods such ^[2] Since that time many theories were proposed to explain this apparent compatibility, but the assumption of a solid earth made the various proposals difficult to explain. ^[3]

The discovery of radium and its associated heating properties in 1896 prompted a re-examination of the apparent age of the Earth,^[4] since this had been estimated by its cooling rate and assumption the Earth's surface radiated like a black body. Radium (ˈreɪdiəm is a radioactive Chemical element which has the symbol Ra and Atomic number 88 In Thermodynamics, the word exothermic "outside heating" describes a process or reaction that releases Energy usually in the form of Heat, but Modern geologists and Geophysicists consider the age of Earth to be around 4 In Physics, a black body is an object that absorbs all light that falls on it ^[5] Those calculations implied that, even if it started at red heat, the Earth would have dropped to its present temperature in a few tens of millions of years. Thermal radiation is Electromagnetic radiation emitted from the surface of an object which is due to the object's Temperature. Armed with the knowledge of a new heat source, scientists reasoned it was credible that the Earth was much older, and also that its core was still sufficiently hot to be liquid.

Plate tectonic theory arose out of the hypothesis of continental drift proposed by Alfred Wegener in 1912^[6] and expanded in his 1915 book The Origin of Continents and Oceans. Continental drift is the movement of the Earth 's Continents relative to each other Alfred Lothar Wegener ( November 1, 1880 – November 2 or 3 1930 was a German Scientist and Meteorologist. He suggested that the present continents once formed a single land mass which had drifted apart thus releasing the continents from the Earth's core and likening them to "icebergs" of low density granite floating on a sea of more dense basalt. Granite (ˈɡrænɪt is a common and widely occurring type of intrusive, Felsic, igneous rock. Basalt (bəˈsɔːlt ˈbeisɔːlt ˈbæsɔːlt is a common Extrusive Volcanic rock. ^[7][8] But without detailed evidence and calculation of the forces involved, the theory remained sidelined. The Earth might have a solid crust and a liquid core, but there seemed to be no way that portions of the crust could move around. Later science proved theories proposed by English geologist Arthur Holmes in 1920 that their junctions might actually lie beneath the sea and Holmes' 1928 suggestion of convection currents within the mantle as the driving force. Arthur Holmes ( January 14 1890 &ndash September 20 1965) was a British Geologist. This article is about the body of water For other uses see SEA and Seas. ^[9][10][3]

The first evidence that crust plates did move around came with the discovery of variable magnetic field direction in rocks of differing ages, first revealed at a symposium in Tasmania in 1956. In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges Initially theorized as an expansion of the global crust,^[11] later collaborations developed the plate tectonics theory, which accounted for spreading as the consequence of new rock upwelling, but avoided the need for an expanding globe by recognizing subduction zones and conservative translation faults. The Expanding Earth theory is an attempt to explain the position and movement of Continents ( Continental drift) on the surface of the Earth It was at this point that Wegener's theory moved from radical to mainstream, and became accepted by the scientific community. Additional work on the association of seafloor spreading and magnetic field reversals by Harry Hess and Ron G. Mason^{[12][13][14][15]} pinpointed the precise mechanism which accounted for new rock upwelling. Seafloor spreading occurs at Mid-ocean ridges where new Oceanic crust is formed through volcanic activity and then gradually moves away from the ridge A geomagnetic reversal is a change in the orientation of Earth's magnetic field such that the positions of magnetic north and magnetic south become interchanged Harry Hammond Hess ( May 24, 1906 – August 25, 1969) was a Geologist and United States Navy officer in World War Ron G Mason was one of the Oceanographers whose pioneering Cold War Geomagnetic survey work lead to the discovery of magnetic stripes around

Following the recognition of magnetic anomalies defined by symmetric, parallel stripes of similar magnetization on the seafloor on either side of a mid-ocean ridge, plate tectonics quickly became broadly accepted. A geomagnetic reversal is a change in the orientation of Earth's magnetic field such that the positions of magnetic north and magnetic south become interchanged A mid-ocean ridge or mid-oceanic ridge is an underwater Mountain range typically having a valley known as a Rift running along its axis formed by Simultaneous advances in early seismic imaging techniques in and around Wadati-Benioff zones collectively with numerous other geologic observations soon solidified plate tectonics as a theory with extraordinary explanatory and predictive power. Seismology (from Greek grc σεισμός seismos, "earthquake" and grc -λογία -logia) is the scientific study of Earthquakes A Wadati-Benioff zone (also Benioff-Wadati zone or Benioff zone) is a deep active seismic area in a Subduction zone

Study of the deep ocean floor was critical to development of the theory; the field of deep sea marine geology accelerated in the 1960s. An ocean (from Greek, ''Okeanos'' (Oceanus) is a major body of saline water, and a principal component of the Hydrosphere. Marine geology involves Geophysical, geochemical, sedimentological and Paleontological investigations of the Ocean floor and coastal Correspondingly, plate tectonic theory was developed during the late 1960s and has since been accepted all but universally by scientists throughout all geoscientific disciplines. The theory revolutionized the Earth sciences, explaining a diverse range of geological phenomena.

Key principles

The division of the outer parts of the Earth's interior into lithosphere and asthenosphere is based on mechanical differences and in the ways that heat is transferred. Mechanics ( Greek) is the branch of Physics concerned with the behaviour of physical bodies when subjected to Forces or displacements The lithosphere is cooler and more rigid, whilst the asthenosphere is hotter and mechanically weaker. Also, the lithosphere loses heat by conduction whereas the asthenosphere also transfers heat by convection and has a nearly adiabatic temperature gradient. Heat conduction or thermal conduction is the spontaneous transfer of thermal energy through matter from a region of higher Temperature to a region of lower Convection in the most general terms refers to the movement of molecules within Fluids (i This division should not be confused with the chemical subdivision of the Earth into (from innermost to outermost) core, mantle, and crust. The lithosphere contains both crust and some mantle. A given piece of mantle may be part of the lithosphere or the asthenosphere at different times, depending on its temperature, pressure and shear strength. The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which ride on the fluid-like (visco-elastic solid) asthenosphere. This is a list of tectonic plates on Earth Tectonic plates are pieces of the Earth's crust and uppermost mantle together referred to as the Lithosphere Viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Plate motions range up to a typical 10-40 mm/a (Mid-Atlantic Ridge; about as fast as fingernails grow), to about 160 mm/a (Nazca Plate; about as fast as hair grows). The Mid-Atlantic Ridge (MAR is a Mid-ocean ridge, a Divergent tectonic plate boundary located along the floor of the Atlantic Ocean. A nail is a horn -like structure at the end of an animal's Finger or Toe. The Nazca Plate, named after the Nazca region of southern Peru, is an oceanic Tectonic plate in the eastern Pacific Ocean basin off the west Hair is a keratinised protein filament that grows through the epidermis from follicles deep within the Dermis. ^[16][17]

The plates are around 100 km (60 miles) thick and consist of lithospheric mantle overlain by either of two types of crustal material: oceanic crust (in older texts called sima from silicon and magnesium) and continental crust (sial from silicon and aluminium). Oceanic crust is the part of Earth's Lithosphere that surfaces in the Ocean basins Sima is the name for the lower layer of the Earth's crust. This layer is made of rocks rich in silicates and Magnesium minerals Silicon (ˈsɪlɪkən or /ˈsɪlɪkɒn/ silicium is the Chemical element that has the symbol Si and Atomic number 14 Magnesium (mægˈniːziəm is a Chemical element with the symbol Mg, Atomic number 12 Atomic weight 24 Sial or SiAl is also the name for the upper layer of the Earth's crust, which is also known as the Continental crust because WikipediaNaming The two types of crust differ in thickness, with continental crust considerably thicker than oceanic (50 km vs 5 km).

One plate meets another along a plate boundary, and plate boundaries are commonly associated with geological events such as earthquakes and the creation of topographic features like mountains, volcanoes and oceanic trenches. An earthquake is the result of a sudden release of energy in the Earth 's crust that creates Seismic waves Earthquakes are recorded with a Seismometer A mountain is a Landform that extends above the surrounding Terrain in a limited area with a peak Plate tectonics and hotspots Divergent plate boundaries At the The oceanic trenches are hemispheric-scale long but narrow topographic depressions of the sea floor The majority of the world's active volcanoes occur along plate boundaries, with the Pacific Plate's Ring of Fire being most active and most widely known. These boundaries are discussed in further detail below.

Tectonic plates can include continental crust or oceanic crust, and typically, a single plate carries both. For example, the African Plate includes the continent and parts of the floor of the Atlantic and Indian Oceans. The African Plate is a Tectonic plate which includes the Continent of Africa, as well as oceanic crust which lies between the continent and various surrounding The distinction between continental crust and oceanic crust is based on the density of constituent materials; oceanic crust is denser than continental crust owing to their different proportions of various elements, particularly, silicon. Oceanic crust is denser because it has less silicon and more heavier elements ("mafic") than continental crust ("felsic"). Mafic is an adjective describing a Silicate mineral or rock that is rich in magnesium and iron the term was derived by contracting "magnesium" and "ferric" Felsic is a term used in Geology to refer to Silicate minerals, Magma, and rocks which are enriched in the lighter elements such as Silicon ^[18] As a result, oceanic crust generally lies below sea level (for example most of the Pacific Plate), while the continental crust projects above sea level (see isostasy for explanation of this principle). The Pacific Plate is an oceanic Tectonic plate beneath the Pacific Ocean. Isostasy (Greek isos = "equal" stásis = "standstill" is a term used in Geology to refer to the state of gravitational equilibrium between the

Types of plate boundaries

Three types of plate boundaries exist, characterized by the way the plates move relative to each other. They are associated with different types of surface phenomena. The different types of plate boundaries are:

1. Transform boundaries occur where plates slide or, perhaps more accurately, grind past each other along transform faults. A transform fault is a fault which runs along the boundary of a Tectonic plate. The relative motion of the two plates is either sinistral (left side toward the observer) or dextral (right side toward the observer). This article is about the geologic usage for human physiology usage see the articles Laterality. This article is about the geologic usage for human physiology usage see the articles Laterality. The San Andreas Fault in California is one example. The San Andreas Fault is a geologic Transform fault that runs a length of roughly 800 miles (1300 km through California in the United States.
2. Divergent boundaries occur where two plates slide apart from each other. Mid-ocean ridges (e. g. , Mid-Atlantic Ridge) and active zones of rifting (such as Africa's Great Rift Valley) are both examples of divergent boundaries. The Great Rift Valley is a name given in the late 19th century by English explorer John Walter Gregory to the continuous geographic trough approximately in length that runs
3. Convergent boundaries (or active margins) occur where two plates slide towards each other commonly forming either a subduction zone (if one plate moves underneath the other) or a continental collision (if the two plates contain continental crust). Orogeny (Greek for "mountain generating" is the process of natural Mountain building and may be studied as a tectonic structural event as a geographical event and Deep marine trenches are typically associated with subduction zones. The subducting slab contains many hydrous minerals, which release their water on heating; this water then causes the mantle to melt, producing volcanism. Hydrate is a term used in Inorganic chemistry and Organic chemistry to indicate that a substance contains Water. Examples of this are the Andes mountain range in South America and the Japanese island arc. The Andes form the world's longest exposed Mountain range. They lie as a continuous chain of highland along the western coast of South America. For a topic outline on this subject see List of basic Japan topics. A volcanic arc is a chain of volcanic islands or Mountains formed by Plate tectonics as an oceanic Tectonic plate subducts under

Transform (conservative) boundaries

Main article: Transform boundary

John Tuzo Wilson recognized that because of friction, the plates cannot simply glide past each other. A transform fault is a fault which runs along the boundary of a Tectonic plate. John Tuzo Wilson PhD, CC, OBE, DSc, FRS, FRSC, FRSE ( October 24, 1908 &ndash Friction is the Force resisting the relative motion of two Surfaces in contact or a surface in contact with a fluid (e Rather, stress builds up in both plates and when it reaches a level that exceeds the strain threshold of rocks on either side of the fault the accumulated potential energy is released as strain. Stress is a measure of the average amount of Force exerted per unit Area. Potential energy can be thought of as Energy stored within a physical system Strain is both accumulative and/or instantaneous depending on the rheology of the rock; the ductile lower crust and mantle accumulates deformation gradually via shearing whereas the brittle upper crust reacts by fracture, or instantaneous stress release to cause motion along the fault. Rheology is the study of the flow of matter mainly liquids but also soft solids or solids under conditions in which they flow rather than deform elastically Study of geological shear is related to the study of Structural geology, Rock microstructure or rock texture and fault mechanics. The ductile surface of the fault can also release instantaneously when the strain rate is too great. The energy released by instantaneous strain release is the cause of earthquakes, a common phenomenon along transform boundaries. An earthquake is the result of a sudden release of energy in the Earth 's crust that creates Seismic waves Earthquakes are recorded with a Seismometer

A good example of this type of plate boundary is the San Andreas Fault which is found in the western coast of North America and is one part of a highly complex system of faults in this area. The San Andreas Fault is a geologic Transform fault that runs a length of roughly 800 miles (1300 km through California in the United States. At this location, the Pacific and North American plates move relative to each other such that the Pacific plate is moving northwest with respect to North America. Other examples of transform faults include the Alpine Fault in New Zealand and the North Anatolian Fault in Turkey. The Alpine Fault is a Geological fault, known as a right-lateral strike-slip fault that runs almost the entire length of New Zealand 's South Island. New Zealand is an Island country in the south-western Pacific Ocean comprising two main landmasses (the North Island and the South Island The North Anatolian Fault (NAF ( Kuzey Anadolu Fay Hattı is a major active right lateral-moving Geologic fault in northern Anatolia which runs along the Tectonic Turkey (Türkiye known officially as the Republic of Turkey ( is a Eurasian Country that stretches Transform faults are also found offsetting the crests of mid-ocean ridges (for example, the Mendocino Fracture Zone offshore northern California). A mid-ocean ridge or mid-oceanic ridge is an underwater Mountain range typically having a valley known as a Rift running along its axis formed by The Mendocino Fracture Zone is a Fracture zone and Transform boundary off the coast of Cape Mendocino in far northern California.

Divergent (constructive) boundaries

Main article: Divergent boundary

At divergent boundaries, two plates move apart from each other and the space that this creates is filled with new crustal material sourced from molten magma that forms below. In Plate tectonics, a divergent boundary or divergent plate boundary (also known as a constructive boundary or an extensional boundary) is a Magma (Plurals magmas and magmata) is molten rock that sometimes forms beneath the surface of the Earth (or any other Terrestrial planet The origin of new divergent boundaries at triple junctions is sometimes thought to be associated with the phenomenon known as hotspots. A triple junction is the point where the boundaries of three Tectonic plates meet In Geology, a hotspot is a location on the Earth's surface that has experienced active volcanism for a long period of time Here, exceedingly large convective cells bring very large quantities of hot asthenospheric material near the surface and the kinetic energy is thought to be sufficient to break apart the lithosphere. The kinetic energy of an object is the extra Energy which it possesses due to its motion The hot spot which may have initiated the Mid-Atlantic Ridge system currently underlies Iceland which is widening at a rate of a few centimeters per year.

Divergent boundaries are typified in the oceanic lithosphere by the rifts of the oceanic ridge system, including the Mid-Atlantic Ridge and the East Pacific Rise, and in the continental lithosphere by rift valleys such as the famous East African Great Rift Valley. The East Pacific Rise is a Mid-oceanic ridge, a divergent tectonic plate boundary located along the floor of the Pacific Ocean. Divergent boundaries can create massive fault zones in the oceanic ridge system. Spreading is generally not uniform, so where spreading rates of adjacent ridge blocks are different, massive transform faults occur. These are the fracture zones, many bearing names, that are a major source of submarine earthquakes. A fracture zone is a linear oceanic feature--often hundreds even thousands of Kilometers long--resulting from the action of offset Mid-ocean ridge axis segments A submarine is a Watercraft that can operate independently below water as distinct from a Submersible that has only limited underwater capability A sea floor map will show a rather strange pattern of blocky structures that are separated by linear features perpendicular to the ridge axis. If one views the sea floor between the fracture zones as conveyor belts carrying the ridge on each side of the rift away from the spreading center the action becomes clear. Crest depths of the old ridges, parallel to the current spreading center, will be older and deeper (from thermal contraction and subsidence). In Geology, Engineering, and Surveying, subsidence is the motion of a surface (usually the Earth's surface as it shifts downward relative to

It is at mid-ocean ridges that one of the key pieces of evidence forcing acceptance of the sea-floor spreading hypothesis was found. Airborne geomagnetic surveys showed a strange pattern of symmetrical magnetic reversals on opposite sides of ridge centers. Earth 's magnetic field (and the surface magnetic field) is approximately a Magnetic dipole, with one pole near the North pole (see A geomagnetic reversal is a change in the orientation of Earth's magnetic field such that the positions of magnetic north and magnetic south become interchanged The pattern was far too regular to be coincidental as the widths of the opposing bands were too closely matched. Scientists had been studying polar reversals and the link was made by Lawrence W. Morley, Frederick John Vine and Drummond Hoyle Matthews in the Morley-Vine-Matthews hypothesis. A geomagnetic reversal is a change in the orientation of Earth's magnetic field such that the positions of magnetic north and magnetic south become interchanged Lawrence (Whitaker Morley PhD (1920- is a Canadian Geophysicist. Frederick John Vine (born June 17, 1939) is a marine geologist and Geophysicist and was a key contributor to the theory of Plate tectonics. Drummond Hoyle Matthews ( 5 February, 1931 &ndash 20 July, 1997) was a British marine geologist and geophysicist and a key contributor The Morley-Vine-Matthews hypothesis, also known as the Vine-Matthews-Morley hypothesis was the first key scientific test of the Seafloor spreading The magnetic banding directly corresponds with the Earth's polar reversals. This was confirmed by measuring the ages of the rocks within each band. The banding furnishes a map in time and space of both spreading rate and polar reversals.

Convergent (destructive) boundaries

Main article: Convergent boundary

The nature of a convergent boundary depends on the type of lithosphere in the plates that are colliding. In Plate tectonics, a convergent boundary – also known as a convergent plate boundary or a destructive plate boundary – is an actively deforming region Where a dense oceanic plate collides with a less-dense continental plate, the oceanic plate is typically thrust underneath because of the greater buoyancy of the continental lithosphere, forming a subduction zone. At the surface, the topographic expression is commonly an oceanic trench on the ocean side and a mountain range on the continental side. The oceanic trenches are hemispheric-scale long but narrow topographic depressions of the sea floor An example of a continental-oceanic subduction zone is the area along the western coast of South America where the oceanic Nazca Plate is being subducted beneath the continental South American Plate. South America is a Continent of the Americas, situated entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a The South American Plate is a Tectonic plate covering the Continent of South America and extending eastward to the Mid-Atlantic Ridge.

While the processes directly associated with the production of melts directly above downgoing plates producing surface volcanism is the subject of some debate in the geologic community, the general consensus from ongoing research suggests that the release of volatiles is the primary contributor. As the subducting plate descends, its temperature rises driving off volatiles (most importantly water) encased in the porous oceanic crust. As this water rises into the mantle of the overriding plate, it lowers the melting temperature of surrounding mantle, producing melts (magma) with large amounts of dissolved gases. Magma (Plurals magmas and magmata) is molten rock that sometimes forms beneath the surface of the Earth (or any other Terrestrial planet These melts rise to the surface and are the source of some of the most explosive volcanism on Earth because of their high volumes of extremely pressurized gases (consider Mount St. Helens). Mount St Helens is an active Stratovolcano located in Skamania County, Washington, in the Pacific Northwest region of the United States The melts rise to the surface and cool forming long chains of volcanoes inland from the continental shelf and parallel to it. Plate tectonics and hotspots Divergent plate boundaries At the The continental spine of western South America is dense with this type of volcanic mountain building from the subduction of the Nazca plate. South America is a Continent of the Americas, situated entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a Orogeny (Greek for "mountain generating" is the process of natural Mountain building and may be studied as a tectonic structural event as a geographical event and The Nazca Plate, named after the Nazca region of southern Peru, is an oceanic Tectonic plate in the eastern Pacific Ocean basin off the west In North America the Cascade mountain range, extending north from California's Sierra Nevada, is also of this type. Such volcanoes are characterized by alternating periods of quiet and episodic eruptions that start with explosive gas expulsion with fine particles of glassy volcanic ash and spongy cinders, followed by a rebuilding phase with hot magma. A cinder is a pyroclastic material Cinders are extrusive Igneous rocks Cinders are similar to Pumice, which has so many cavities and is such low-density The entire Pacific Ocean boundary is surrounded by long stretches of volcanoes and is known collectively as The Ring of Fire.

Where two continental plates collide the plates either buckle and compress or one plate delves under or (in some cases) overrides the other. Either action will create extensive mountain ranges. The most dramatic effect seen is where the northern margin of the Indian Plate is being thrust under a portion of the Eurasian plate, lifting it and creating the Himalayas and the Tibetan Plateau beyond. The Tibetan Plateau, also known as the Qinghai-Tibetan (Qingzang Plateau is a vast elevated Plateau in Central Asia covering most of the Tibet Autonomous It may have also pushed nearby parts of the Asian continent aside to the east. ^[19]

When two plates with oceanic crust converge they typically create an island arc as one plate is subducted below the other. The arc is formed from volcanoes which erupt through the overriding plate as the descending plate melts below it. The arc shape occurs because of the spherical surface of the earth (nick the peel of an orange with a knife and note the arc formed by the straight-edge of the knife). A deep undersea trench is located in front of such arcs where the descending slab dips downward. Good examples of this type of plate convergence would be Japan and the Aleutian Islands in Alaska. For a topic outline on this subject see List of basic Japan topics. The Aleutian Islands (possibly from Chukchi aliat, " Island " are a chain of more than 300 small volcanic islands forming a Volcanic

Oceanic / Continental

Continental / Continental

Oceanic / Oceanic

Plates may collide at an oblique angle rather than head-on to each other (e. g. one plate moving north, the other moving south-east), and this may cause strike-slip faulting along the collision zone, in addition to subduction or compression. In Geology a fault, or fault line, is a planar rock fracture which shows evidence of relative movement

Not all plate boundaries are easily defined. Some are broad belts whose movements are unclear to scientists. One example would be the Mediterranean-Alpine boundary, which involves two major plates and several micro plates. The boundaries of the plates do not necessarily coincide with those of the continents. For instance, the North American Plate covers not only North America, but also far northeastern Siberia, plus a substantial portion of the Atlantic Ocean.

Driving forces of plate motion

Tectonic plates are able to move because of the relative density of oceanic lithosphere and the relative weakness of the asthenosphere. Dissipation of heat from the mantle is acknowledged to be the original source of energy driving plate tectonics. The current view, although it is still a matter of some debate, is that excess density of the oceanic lithosphere sinking in subduction zones is the most powerful source of plate motion. When it forms at mid-ocean ridges, the oceanic lithosphere is initially less dense than the underlying asthenosphere, but it becomes more dense with age, as it conductively cools and thickens. The greater density of old lithosphere relative to the underlying asthenosphere allows it to sink into the deep mantle at subduction zones, providing most of the driving force for plate motions. The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different The weakness of the asthenosphere allows the tectonic plates to move easily towards a subduction zone. ^[20] Although subduction is believed to be the strongest force driving plate motions, it cannot be the only force since there are plates such as the North American Plate which are moving, yet are nowhere being subducted. The same is true for the enormous Eurasian Plate. The sources of plate motion are a matter of intensive research and discussion among earth scientists.

Two and three-dimensional imaging of the Earth's interior (seismic tomography) shows that there is a laterally heterogeneous density distribution throughout the mantle. Seismic Tomography is a methodology for estimating the earth's properties Such density variations can be material (from rock chemistry), mineral (from variations in mineral structures), or thermal (through thermal expansion and contraction from heat energy). The manifestation of this lateral density heterogeneity is mantle convection from buoyancy forces. Mantle convection is the slow creeping motion of Earth's rocky mantle in response to perpetual gravitationally unstable variations in its density ^[21] How mantle convection relates directly and indirectly to the motion of the plates is a matter of ongoing study and discussion in geodynamics. Somehow, this energy must be transferred to the lithosphere in order for tectonic plates to move. In Physics and other Sciences energy (from the Greek grc ἐνέργεια - Energeia, "activity operation" from grc ἐνεργός There are essentially two types of forces that are thought to influence plate motion: friction and gravity. Friction is the Force resisting the relative motion of two Surfaces in contact or a surface in contact with a fluid (e Gravitation is a natural Phenomenon by which objects with Mass attract one another

Friction

Basal drag

Large scale convection currents in the upper mantle are transmitted through the asthenosphere; motion is driven by friction between the asthenosphere and the lithosphere. Convection in the most general terms refers to the movement of molecules within Fluids (i

Slab suction

Local convection currents exert a downward frictional pull on plates in subduction zones at ocean trenches. Slab suction may occur in a geodynamic setting wherein basal tractions continue to act on the plate as it dives into the mantle (although perhaps to a greater extent acting on both the under and upper side of the slab).

Gravitation

Gravitational sliding: Plate motion is driven by the higher elevation of plates at ocean ridges. As oceanic lithosphere is formed at spreading ridges from hot mantle material it gradually cools and thickens with age (and thus distance from the ridge). Cool oceanic lithosphere is significantly denser than the hot mantle material from which it is derived and so with increasing thickness it gradually subsides into the mantle to compensate the greater load. The result is a slight lateral incline with distance from the ridge axis.

Casually in the geophysical community and more typically in the geological literature in lower education this process is often referred to as "ridge-push". This is, in fact, a misnomer as nothing is "pushing" and tensional features are dominant along ridges. It is more accurate to refer to this mechanism as gravitational sliding as variable topography across the totality of the plate can vary considerably and the topography of spreading ridges is only the most prominent feature. For example:

1. Flexural bulging of the lithosphere before it dives underneath an adjacent plate, for instance, produces a clear topographical feature that can offset or at least affect the influence of topographical ocean ridges.

2. Mantle plumes impinging on the underside of tectonic plates can drastically alter the topography of the ocean floor. A mantle plume is an upwelling of abnormally hot rock within the Earth's mantle.

Slab-pull 

Plate motion is partly driven by the weight of cold, dense plates sinking into the mantle at trenches. ^[22] There is considerable evidence that convection is occurring in the mantle at some scale. The upwelling of material at mid-ocean ridges is almost certainly part of this convection. Some early models of plate tectonics envisioned the plates riding on top of convection cells like conveyor belts. However, most scientists working today believe that the asthenosphere is not strong enough to directly cause motion by the friction of such basal forces. Slab pull is most widely thought to be the greatest force acting on the plates. Recent models indicate that trench suction plays an important role as well. However, it should be noted that the North American Plate, for instance, is nowhere being subducted, yet it is in motion. Likewise the African, Eurasian and Antarctic Plates. The overall driving force for plate motion and its energy source remain subjects of ongoing research.

External forces

In a study published in the January-February 2006 issue of the Geological Society of America Bulletin, a team of Italian and U. S. scientists argued that the westward component of plates is from Earth's rotation and consequent tidal friction of the moon. As the Earth spins eastward beneath the moon, they say, the moon's gravity ever so slightly pulls the Earth's surface layer back westward. It has also been suggested (albeit, controversially) that this observation may also explain why Venus and Mars have no plate tectonics since Venus has no moon, and Mars' moons are too small to have significant tidal effects on Mars. ^[23] This is not, however, a new argument.

It was originally raised by the "father" of the plate tectonics hypothesis, Alfred Wegener. It was challenged by the physicist Harold Jeffreys who calculated that the magnitude of tidal friction required would have quickly brought the Earth's rotation to a halt long ago. Sir Harold Jeffreys ( 22 April 1891 &ndash 18 March 1989) was a mathematician statistician geophysicist and astronomer Many plates are moving north and eastward, and the dominantly westward motion of the Pacific ocean basins is simply from the eastward bias of the Pacific spreading center (which is not a predicted manifestation of such lunar forces). It is argued, however, that relative to the lower mantle, there is a slight westward component in the motions of all the plates.

Relative significance of each mechanism

The actual vector of a plate's motion must necessarily be a function of all the forces acting upon the plate. However, therein remains the problem of to what degree each process contributes to the motion of each tectonic plate.

The diversity of geodynamic settings and properties of each plate must clearly result in differences in the degree to which such processes are actively driving the plates. One method of dealing with this problem is to consider the relative rate at which each plate is moving and to consider the available evidence of each driving force upon the plate as far as possible.

One of the most significant correlations found is that lithospheric plates attached to downgoing (subducting) plates move much faster than plates not attached to subducting plates. The Pacific plate, for instance, is essentially surrounded by zones of subduction (the so-called Ring of Fire) and moves much faster than the plates of the Atlantic basin, which are attached (perhaps one could say 'welded') to adjacent continents instead of subducting plates. It is thus thought that forces associated with the downgoing plate (slab pull and slab suction) are the driving forces which determine the motion of plates, except for those plates which are not being subducted.

The driving forces of plate motion are, nevertheless, still very active subjects of on-going discussion and research in the geophysical community.

Major plates

The main plates are

• African Plate, covering Africa - Continental plate
• Antarctic Plate, covering Antarctica - Continental plate
• Australian Plate, covering Australia (fused with Indian Plate between 50 and 55 million years ago) - Continental plate
• Eurasian Plate covering Asia and Europe - Continental plate
• North American Plate covering North America and north-east Siberia - Continental plate
• South American Plate covering South America - Continental plate
• Pacific Plate, covering the Pacific Ocean - Oceanic plate

Notable minor plates include the Indian Plate, the Arabian Plate, the Caribbean Plate, the Juan de Fuca Plate, the Cocos Plate, the Nazca Plate, the Philippine Plate and the Scotia Plate. The African Plate is a Tectonic plate which includes the Continent of Africa, as well as oceanic crust which lies between the continent and various surrounding The Antarctic Plate is a Tectonic plate covering the Continent of Antarctica and extending outward under the surrounding Oceans The Antarctic The Indo-Australian Plate is a major Tectonic plate that includes the continent of Australia and surrounding Ocean, and extends northwest to include the For a topic outline on this subject see List of basic Australia topics. The India or Indian Plate is a Tectonic plate that was originally a part of the ancient continent of Gondwanaland from which it split off eventually The Eurasian Plate is a Tectonic plate which includes most of the continent of Eurasia (a landmass consisting of the traditional Continents of Europe The North American Plate is a Tectonic plate covering most of North America, Greenland and part of Siberia. Siberia (Сиби́рь Sibir) is the name given to the vast region constituting almost all of Northern Asia and for the most part currently serving The South American Plate is a Tectonic plate covering the Continent of South America and extending eastward to the Mid-Atlantic Ridge. South America is a Continent of the Americas, situated entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a The Pacific Plate is an oceanic Tectonic plate beneath the Pacific Ocean. The Pacific Ocean is the largest of the Earth 's Oceanic divisions The India or Indian Plate is a Tectonic plate that was originally a part of the ancient continent of Gondwanaland from which it split off eventually The Arabian Plate is one of three Tectonic plates (the African Arabian and Indian crustal plates) which have been moving northward over millions of The Caribbean Plate is a mostly oceanic Tectonic plate underlying Central America and the Caribbean Sea off the north coast of South The Juan de Fuca Plate, named after the explorer, is a Tectonic plate arising from the Juan de Fuca Ridge, and subducting under the northerly The Cocos Plate is an oceanic Tectonic plate beneath the Pacific Ocean off the west coast of Central America, named for Cocos Island, which rides The Nazca Plate, named after the Nazca region of southern Peru, is an oceanic Tectonic plate in the eastern Pacific Ocean basin off the west The Philippine Plate is a Tectonic plate beneath the Pacific Ocean to the east of the Philippines. The Scotia Plate is an oceanic Tectonic plate bordering the South American Plate on the north the South Sandwich Plate to the east and the Antarctic

The movement of plates has caused the formation and break-up of continents over time, including occasional formation of a supercontinent that contains most or all of the continents. The supercontinent Rodinia is thought to have formed about 1 billion years ago and to have embodied most or all of Earth's continents, and broken up into eight continents around 600 million years ago. For the Genus of Metalmark butterflies, see Rodinia (butterfly. The eight continents later re-assembled into another supercontinent called Pangaea; Pangaea eventually broke up into Laurasia (which became North America and Eurasia) and Gondwana (which became the remaining continents). Pangaea, Pangæa or Pangea (pænˈdʒiːə from παν pan, meaning entire, and Γαῖα Gaea, meaning Earth in Laurasia (lɔˈreɪʃiə lɔˈreɪʒə was a Supercontinent that most recently existed as a part of the split of the Pangaean supercontinent in the late Mesozoic Gondwana (ɡɒnˈdwɑːnə originally Gondwanaland) was a southern Supercontinent that existed about 500 to 200 Ma ago

Related article

• List of tectonic plates

Historical development of the theory

Continental drift

For more details on this topic, see Continental drift. This is a list of tectonic plates on Earth Tectonic plates are pieces of the Earth's crust and uppermost mantle together referred to as the Lithosphere Continental drift is the movement of the Earth 's Continents relative to each other

Continental drift was one of many ideas about tectonics proposed in the late 19th and early 20th centuries. The theory has been superseded and the concepts and data have been incorporated within plate tectonics.

By 1915, Alfred Wegener was making serious arguments for the idea in the first edition of The Origin of Continents and Oceans. In that book, he noted how the east coast of South America and the west coast of Africa looked as if they were once attached. South America is a Continent of the Americas, situated entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a Wegener wasn't the first to note this (Abraham Ortelius, Francis Bacon, Benjamin Franklin, Snider-Pellegrini, Roberto Mantovani and Frank Bursley Taylor preceded him), but he was the first to marshal significant fossil and paleo-topographical and climatological evidence to support this simple observation (and was supported in this by researchers such as Alex du Toit). Abraham Ortelius ( Abraham Ortels) ( April 2, 1527 – June 28, 1598) was a Belgian Cartographer and Francis Bacon 1st Viscount St Alban KC QC (22 January 1561 – 9 April 1626 was an English Philosopher, Statesman, and author Benjamin Franklin ( April 17 1790 was one of the Founding Fathers of the United States of America. Antonio Snider-Pellegrini (1802-1885 was a French Geographer and scientist who theorized about the possibility of Continental drift, anticipating Roberto Mantovani ( March 25 1854 &ndash January 10 1933) was an Italian Geologist and Violinist. Frank Bursley Taylor (1860 &ndash 1938 was a wealthy amateur American Geologist, a specialist in the Glacial geology of the Great Lakes, and proposed FOSSIL is a standard protocol for allowing serial communication for Telecommunications programs under the DOS Operating system. Alexander Logie du Toit ("DOO-TOY" ( 14 March 1878 &ndash 25 February 1948) was a Geologist from South Africa However, his ideas were not taken seriously by many geologists, who pointed out that there was no apparent mechanism for continental drift. Specifically, they did not see how continental rock could plow through the much denser rock that makes up oceanic crust. Wegener could not explain the force that propelled continental drift.

Wegener's vindication did not come until after his death in 1930. In 1947, a team of scientists led by Maurice Ewing utilizing the Woods Hole Oceanographic Institution’s research vessel Atlantis and an array of instruments, confirmed the existence of a rise in the central Atlantic Ocean, and found that the floor of the seabed beneath the layer of sediments consisted of basalt, not the granite which is the main constituent of continents. William Maurice "Doc" Ewing ( May 12 1906 &ndash May 4 1974) was an American geophysicist and oceanographer The Woods Hole Oceanographic Institution (WHOI is a private nonprofit research and higher education facility dedicated to the study of all aspects of marine science and engineering and They also found that the oceanic crust was much thinner than continental crust. All these new findings raised important and intriguing questions. ^[24]

Beginning in the 1950s, scientists including Harry Hess, using magnetic instruments (magnetometers) adapted from airborne devices developed during World War II to detect submarines, began recognizing odd magnetic variations across the ocean floor. A magnetometer is a scientific instrument used to measure the strength and/or direction of the Magnetic field in the vicinity of the instrument World War II, or the Second World War, (often abbreviated WWII) was a global military conflict which involved a majority of the world's nations, including A submarine is a Watercraft that can operate independently below water as distinct from a Submersible that has only limited underwater capability This finding, though unexpected, was not entirely surprising because it was known that basalt—the iron-rich, volcanic rock making up the ocean floor—contains a strongly magnetic mineral (magnetite) and can locally distort compass readings. Basalt (bəˈsɔːlt ˈbeisɔːlt ˈbæsɔːlt is a common Extrusive Volcanic rock. Magnetite is not to be confused with Magnesite or Maghemite. Magnetite is a ferrimagnetic Mineral with chemical This distortion was recognized by Icelandic mariners as early as the late 18th century. More important, because the presence of magnetite gives the basalt measurable magnetic properties, these newly discovered magnetic variations provided another means to study the deep ocean floor. When newly formed rock cools, such magnetic materials recorded the Earth's magnetic field at the time. Earth 's magnetic field (and the surface magnetic field) is approximately a Magnetic dipole, with one pole near the North pole (see

As more and more of the seafloor was mapped during the 1950s, the magnetic variations turned out not to be random or isolated occurrences, but instead revealed recognizable patterns. When these magnetic patterns were mapped over a wide region, the ocean floor showed a zebra-like pattern. Alternating stripes of magnetically different rock were laid out in rows on either side of the mid-ocean ridge: one stripe with normal polarity and the adjoining stripe with reversed polarity. The overall pattern, defined by these alternating bands of normally and reversely polarized rock, became known as magnetic striping.

When the rock strata of the tips of separate continents are very similar it suggests that these rocks were formed in the same way implying that they were joined initially. In Geology and related fields a stratum (plural strata) is a layer of rock or Soil with internally consistent characteristics that distinguishes For instance, some parts of Scotland and Ireland contain rocks very similar to those found in Newfoundland and New Brunswick. Scotland ( Gaelic: Alba) is a Country in northwest Europethat occupies the northern third of the island of Great Britain. Ireland (pronounced /ˈaɾlənd/ Éire) is the third largest island in Europe, and the twentieth-largest island in the world Newfoundland and Labrador (ˈnuːfɨn(dlənd ən(d ˈlæbrəˌdɔr (Terre-Neuve-et-Labrador is a province of Canada, the tenth and latest to join the Confederation New Brunswick ( French: Nouveau-Brunswick /nuvobʁɔnzwik/ is one of Canada 's three Maritime provinces and is the only constitutionally Furthermore, the Caledonian Mountains of Europe and parts of the Appalachian Mountains of North America are very similar in structure and lithology. The Caledonian orogeny is a mountain building event recorded in the Mountains and Hills of northern Scotland, Ireland, England, The Appalachian Mountains ( often called the Appalachians, are a vast system of mountains in eastern North America. Structural geology is the study of the three dimensional distribution of rock bodies and their planar or folded surfaces and their internal fabrics In Geology, petrology (from Greek πέτρα petra, rock and λόγος logos, knowledge is the study of rocks and the conditions on which

Floating continents

The prevailing concept was that there were static shells of strata under the continents. It was observed early that although granite existed on continents, seafloor seemed to be composed of denser basalt. It was apparent that a layer of basalt underlies continental rocks.

However, based upon abnormalities in plumb line deflection by the Andes in Peru, Pierre Bouguer deduced that less-dense mountains must have a downward projection into the denser layer underneath. Pierre Bouguer ( February 16, 1698 &ndash August 15, 1758) was a French Mathematician and astronomer The concept that mountains had "roots" was confirmed by George B. Airy a hundred years later during study of Himalayan gravitation, and seismic studies detected corresponding density variations. Sir George Biddell Airy FRS (27 July 1801&ndash2 January 1892 was an English Mathematician and Astronomer, Astronomer Royal

By the mid-1950s the question remained unresolved of whether mountain roots were clenched in surrounding basalt or were floating like an iceberg.

In 1958 the Tasmanian geologist Samuel Warren Carey published an essay The tectonic approach to continental drift in support of the expanding earth model. Samuel Warren Carey AO (* November 1, 1911 in Campbelltown; † March 20, 2002 in Hobart) was an Australian

Plate tectonic theory

Significant progress was made in the 1960s, and was prompted by a number of discoveries, most notably the Mid-Atlantic ridge. The most notable was the 1962 publication of a paper by American geologist Harry Hess (Robert S. Dietz published the same idea one year earlier in Nature. Harry Hess (Born July 5, 1968 in Oshawa Ontario, Canada) is a Canadian Record producer, Singer and Guitarist Robert Sinclair Dietz ( September 14, 1914 &ndash May 19, 1995) was Professor of Geology at Arizona State University. However, priority belongs to Hess, since he distributed an unpublished manuscript of his 1962 article already in 1960). Hess suggested that instead of continents moving through oceanic crust (as was suggested by continental drift) that an ocean basin and its adjoining continent moved together on the same crustal unit, or plate. In the same year, Robert R. Coats of the U. Robert Roy Coats (1910-1995 was born in Toronto Canada, and grew up in Marshalltown Iowa and Seattle Washington. S. Geological Survey described the main features of island arc subduction in the Aleutian Islands. His paper, though little-noted (and even ridiculed) at the time, has since been called "seminal" and "prescient". In 1967, W. Jason Morgan proposed that the Earth's surface consists of 12 rigid plates that move relative to each other. William Jason Morgan (born October 10, 1935 in Savannah, Georgia) is an American Geophysicist who has made seminal contributions Two months later, in 1968, Xavier Le Pichon published a complete model based on 6 major plates with their relative motions. Xavier Le Pichon (born June 18, 1937) is a French geophysicist.

Explanation of magnetic striping

The discovery of magnetic striping and the stripes being symmetrical around the crests of the mid-ocean ridges suggested a relationship. In 1961, scientists began to theorise that mid-ocean ridges mark structurally weak zones where the ocean floor was being ripped in two lengthwise along the ridge crest. New magma from deep within the Earth rises easily through these weak zones and eventually erupts along the crest of the ridges to create new oceanic crust. Magma (Plurals magmas and magmata) is molten rock that sometimes forms beneath the surface of the Earth (or any other Terrestrial planet Oceanic crust is the part of Earth's Lithosphere that surfaces in the Ocean basins This process, later called seafloor spreading, operating over many millions of years continues to form new ocean floor all across the 50,000 km-long system of mid-ocean ridges. This hypothesis was supported by several lines of evidence:

1. at or near the crest of the ridge, the rocks are very young, and they become progressively older away from the ridge crest;
2. the youngest rocks at the ridge crest always have present-day (normal) polarity;
3. stripes of rock parallel to the ridge crest alternated in magnetic polarity (normal-reversed-normal, etc. ), suggesting that the Earth's magnetic field has reversed many times.

By explaining both the zebralike magnetic striping and the construction of the mid-ocean ridge system, the seafloor spreading hypothesis quickly gained converts and represented another major advance in the development of the plate-tectonics theory. Furthermore, the oceanic crust now came to be appreciated as a natural "tape recording" of the history of the reversals in the Earth's magnetic field. Earth 's magnetic field (and the surface magnetic field) is approximately a Magnetic dipole, with one pole near the North pole (see

Subduction discovered

A profound consequence of seafloor spreading is that new crust was, and is now, being continually created along the oceanic ridges. This idea found great favor with some scientists, most notably S. Warren Carey, who claimed that the shifting of the continents can be simply explained by a large increase in size of the Earth since its formation. However, this so-called "Expanding Earth theory" hypothesis was unsatisfactory because its supporters could offer no convincing mechanism to produce a significant expansion of the Earth. The Expanding Earth theory is an attempt to explain the position and movement of Continents ( Continental drift) on the surface of the Earth Certainly there is no evidence that the moon has expanded in the past 3 billion years. Still, the question remained: how can new crust be continuously added along the oceanic ridges without increasing the size of the Earth?

This question particularly intrigued Harry Hess, a Princeton University geologist and a Naval Reserve Rear Admiral, and Robert S. Dietz, a scientist with the U.S. Coast and Geodetic Survey who first coined the term seafloor spreading. Princeton University is a private Coeducational research university located in Princeton, New Jersey. Robert Sinclair Dietz ( September 14, 1914 &ndash May 19, 1995) was Professor of Geology at Arizona State University. The National Geodetic Survey and the Office of Coast Survey are the two successor agencies in the United States to the U Dietz and Hess were among the small handful who really understood the broad implications of sea floor spreading. If the Earth's crust was expanding along the oceanic ridges, Hess reasoned, it must be shrinking elsewhere. He suggested that new oceanic crust continuously spreads away from the ridges in a conveyor belt-like motion. Many millions of years later, the oceanic crust eventually descends into the oceanic trenches — very deep, narrow canyons along the rim of the Pacific Ocean basin. The oceanic trenches are hemispheric-scale long but narrow topographic depressions of the sea floor According to Hess, the Atlantic Ocean was expanding while the Pacific Ocean was shrinking. As old oceanic crust is consumed in the trenches, new magma rises and erupts along the spreading ridges to form new crust. In effect, the ocean basins are perpetually being "recycled," with the creation of new crust and the destruction of old oceanic lithosphere occurring simultaneously. Thus, Hess' ideas neatly explained why the Earth does not get bigger with sea floor spreading, why there is so little sediment accumulation on the ocean floor, and why oceanic rocks are much younger than continental rocks.

Mapping with earthquakes

During the 20th century, improvements in and greater use of seismic instruments such as seismographs enabled scientists to learn that earthquakes tend to be concentrated in certain areas, most notably along the oceanic trenches and spreading ridges. Seismometers (from Greek Seism - "the shakes" - and Metro - "I measure" are instruments that measure and record motions of the ground including An earthquake is the result of a sudden release of energy in the Earth 's crust that creates Seismic waves Earthquakes are recorded with a Seismometer By the late 1920s, seismologists were beginning to identify several prominent earthquake zones parallel to the trenches that typically were inclined 40–60° from the horizontal and extended several hundred kilometers into the Earth. These zones later became known as Wadati-Benioff zones, or simply Benioff zones, in honor of the seismologists who first recognized them, Kiyoo Wadati of Japan and Hugo Benioff of the United States. A Wadati-Benioff zone (also Benioff-Wadati zone or Benioff zone) is a deep active seismic area in a Subduction zone A Wadati-Benioff zone (also Benioff-Wadati zone or Benioff zone) is a deep active seismic area in a Subduction zone Professor Kiyoo Wadati (和達清夫 was an early Seismologist at the Central Meteorological Observatory of Japan, researching deep ( Subduction zone For a topic outline on this subject see List of basic Japan topics. Hugo Benioff (1899-1968 was a Seismologist and a Professor at the California Institute of Technology. The United States of America —commonly referred to as the The study of global seismicity greatly advanced in the 1960s with the establishment of the Worldwide Standardized Seismograph Network (WWSSN) to monitor the compliance of the 1963 treaty banning above-ground testing of nuclear weapons. The much-improved data from the WWSSN instruments allowed seismologists to map precisely the zones of earthquake concentration world wide.

Geological paradigm shift

The acceptance of the theories of continental drift and sea floor spreading (the two key elements of plate tectonics) may be compared to the Copernican revolution in astronomy (see Nicolaus Copernicus). Astronomy (from the Greek words astron (ἄστρον "star" and nomos (νόμος "law" is the scientific study Within a matter of only several years geophysics and geology in particular were revolutionized. Geophysics, a major discipline of Earth sciences, is the study of the Earth by quantitative physical methods especially by seismic, electromagnetic The parallel is striking: just as pre-Copernican astronomy was highly descriptive but still unable to provide explanations for the motions of celestial objects, pre-tectonic plate geological theories described what was observed but struggled to provide any fundamental mechanisms. The problem lay in the question "How?". Before acceptance of plate tectonics, geology in particular was trapped in a "pre-Copernican" box.

However, by comparison to astronomy the geological revolution was much more sudden. What had been rejected for decades by any respectable scientific journal was eagerly accepted within a few short years in the 1960s and 1970s. For a broader class of publications which include scientific journals see Academic journal. Any geological description before this had been highly descriptive. All the rocks were described and assorted reasons, sometimes in excruciating detail, were given for why they were where they are. The descriptions are still valid. The reasons, however, today sound much like pre-Copernican astronomy.

One simply has to read the pre-plate descriptions of why the Alps or Himalaya exist to see the difference. In an attempt to answer "how" questions like "How can rocks that are clearly marine in origin exist thousands of meters above sea-level in the Dolomites?", or "How did the convex and concave margins of the Alpine chain form?", any true insight was hidden by complexity that boiled down to technical jargon without much fundamental insight as to the underlying mechanics. The Dolomites (Dolomiti Dolomiten Dolomitis are a section of the Alps.

With plate tectonics answers quickly fell into place or a path to the answer became clear. Collisions of converging plates had the force to lift the sea floor to great heights. The cause of marine trenches oddly placed just off island arcs or continents and their associated volcanoes became clear when the processes of subduction at converging plates were understood.

Mysteries were no longer mysteries. Forests of complex and obtuse answers were swept away. Why were there striking parallels in the geology of parts of Africa and South America? Why did Africa and South America look strangely like two pieces that should fit to anyone having done a jigsaw puzzle? Look at some pre-tectonics explanations for complexity. For simplicity and one that explained a great deal more look at plate tectonics. A great rift, similar to the Great Rift Valley in northeastern Africa, had split apart a single continent, eventually forming the Atlantic Ocean, and the forces were still at work in the Mid-Atlantic Ridge. The Great Rift Valley is a name given in the late 19th century by English explorer John Walter Gregory to the continuous geographic trough approximately in length that runs The Mid-Atlantic Ridge (MAR is a Mid-ocean ridge, a Divergent tectonic plate boundary located along the floor of the Atlantic Ocean.

We have inherited some of the old terminology, but the underlying concept is as radical and simple as was "The Earth moves" in astronomy.

Biogeographic implications on biota

Continental drift theory helps biogeographers to explain the disjunct biogeographic distribution of present day life found on different continents but having similar ancestors. Biogeography is the study of the distribution of Biodiversity over Space and Time. A group of organisms is said to have common descent if they have a common Ancestor. ^[25] In particular, it explains the Gondwanan distribution of ratites and the Antarctic flora. Gondwana (ɡɒnˈdwɑːnə originally Gondwanaland) was a southern Supercontinent that existed about 500 to 200 Ma ago A ratite is any of a diverse group of large Flightless birds of Gondwanan origin most of them now extinct Antarctic flora is a distinct community of Vascular plants which evolved millions of years ago on the supercontinent of Gondwana, and is now found on several separate

Plate tectonics on other planets

The appearance of plate tectonics on terrestrial planets is related to planetary mass, with more massive planets than Earth expected to exhibit plate tectonics. A terrestrial planet, telluric planet or rocky planet is a Planet that is primarily composed of Silicate rocks Within our A Super-Earth is an extrasolar Terrestrial planet more massive than the Earth Earth may be a borderline case, owing its tectonic activity to abundant water. ^[26]

Venus

See also: Geology of Venus

Venus shows no evidence of active plate tectonics. Venus has striking surface characteristics which are as beautiful as they are unusual There is debatable evidence of active tectonics in the planet's distant past; however, events taking place since then (such as the plausible and generally accepted hypothesis that the Venusian lithosphere has thickened greatly over the course of several hundred million years) has made constraining the course of its geologic record difficult. However, the numerous well-preserved impact craters have been utilized as a dating method to approximately date the Venusian surface (since there are thus far no known samples of Venusian rock to be dated by more reliable methods). In the broadest sense the term impact crater can be applied to any depression natural or manmade resulting from the high velocity impact of a projectile with larger body In the Natural sciences under the umbrella of Natural history, Geochronology is the Science of determining the absolute age of rocks, Fossils Dates derived are the dominantly in the range ~500 to 750 Ma, although ages of up to ~1. 2 Ga have been calculated. This research has led to the fairly well accepted hypothesis that Venus has undergone an essentially complete volcanic resurfacing at least once in its distant past, with the last event taking place approximately within the range of estimated surface ages. While the mechanism of such an impressionable thermal event remains a debated issue in Venusian geosciences, some scientists are advocates of processes involving plate motion to some extent.

One explanation for Venus' lack of plate tectonics is that on Venus temperatures are too high for significant water to be present. ^[27][28] The Earth's crust is soaked with water, and water plays an important role in the development of shear zones. Study of geological shear is related to the study of Structural geology, Rock microstructure or rock texture and fault mechanics. Plate tectonics requires weak surfaces in the crust along which crustal slices can move, and it may well be that such weakening never took place on Venus because of the absence of water. However, some researchers remain convinced that plate tectonics is or was once active on this planet.

Mars

See also: Geology of Mars

Unlike Venus, the crust of Mars has water in it and on it (mostly in the form of ice). The geology of Mars, also known as areology (from Greek grc Ἂρης Arēs, " Ares " and grc -λογία -logia This planet is considerably smaller than the Earth, but shows some indications that could suggest a similar style of tectonics. The gigantic volcanoes in the Tharsis area are linearly aligned like volcanic arcs on Earth; the enormous canyon Valles Marineris could have been formed by some form of crustal spreading. The Tharsis region on Mars is an enormous Volcanic plateau located on Mars' equator at the western end of Valles Marineris. Valles Marineris ( Latin for Mariner Valleys, named after the Mariner 9 Mars orbiter

As a result of observations made of the magnetic field of Mars by the Mars Global Surveyor spacecraft in 1999, large scale patterns of magnetic striping were discovered on this planet. In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges The Mars Global Surveyor ( MGS) was a US Spacecraft developed by NASA and the Jet Propulsion Laboratory and launched To explain these magnetisation patterns in the Martian crust it has been proposed that a mechanism similar to plate tectonics may once have been active on the planet. ^[29][30] Further data from the Mars Express orbiter's High Resolution Stereo Camera in 2007 clearly showed an example in the Aeolis Mensae region. Mars Express is a Space exploration mission being conducted by the European Space Agency (ESA ^[31]

Galilean satellites

Some of the satellites of Jupiter have features that may be related to plate-tectonic style deformation, although the materials and specific mechanisms may be different from plate-tectonic activity on Earth. The Galilean moons are the four moons of Jupiter discovered by Galileo Galilei.

Titan

Titan, the largest moon of Saturn, was reported to show tectonic activity in images taken by the Huygens Probe, which landed on Titan on January 14, 2005. ^[32]

Metaphoric uses

Sometimes the idea of moving tectonic plates is used metaphorically, e. g. "a tectonic shift" in a BBC TV news program describing the political effects of Ariel Sharon's illness on 4 January 2005. (אריאל Events 46 BC - Titus Labienus defeats Julius Caesar in the Battle of Ruspina. Year 2005 ( MMV) was a Common year starting on Saturday (link displays full calendar of the Gregorian calendar.

In the late 1980s, Québec theatre director Robert Lepage created a large international production called Tectonic Plates, which used this image to illustrate the rifts between Europe and America and the drifting of various destinies, relative to one another. Robert Lepage, OQ (born December 12, 1957 in Quebec City) is a playwright actor and film

See also

• List of plate tectonics topics
• List of tectonic plates
• List of tectonic plate interactions
• Geosyncline theory, obsolete explanation of mountain-building
• Plume tectonics, an extension of plate tectonics that attempts to explain other aspects of the field

References

External links

• The PLATES Project, a comprehensive resource with reconstructions, movies, images, list of publications, and teaching resources, from the University of Texas Institute for Geophysics at the Jackson School of Geosciences.
• The Paleomap Project, Christopher Scotese's website with reconstructions in the past and future, paleogeographies, teaching material etc. Christopher Scotese is geologist at the University of Texas at Arlington. Palaeogeography (sometimes spelled paleogeography) is the study of what the Geography was in times past
• Movie showing 750 million years of global tectonic activity
• Easy-to-draw illustrations for teaching plate tectonics
• An explanation of tectonic forces
• Bird, P. (2003) An updated digital model of plate boundaries, also available as a large (13 mb) PDF file
• Map of tectonic plates
• MantlePlumes.org, a website debating the existence of deep mantle plumes
• USGS site on plate motions
• The geodynamics of the North-American/Eurasian/African plate boundaries
• Cenozoic dynamics of the African plate with emphasis on the Africa-Eurasia collision
• Speed of the Continental Plates. The Physics Factbook (1997).
• ImpactTectonics.org, examines tectonic effects associated with hypervelocity bolide impacts on terrestrial planets

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