Plate tectonics (from the Greek word for "one who constructs", τεκτων, tekton) is a theory of geology developed to explain the phenomenon of continental drift, and is currently the theory accepted by the vast majority of scientists working in this area. In the theory of plate tectonics the outermost part of the Earth's interior is made up of two layers, the outer lithosphere and the inner asthenosphere. The word theory has a number distinct meanings depending on the context. ... Geology (from Greek γη- (ge-, the earth) and λογος (logos, word, reason)) is the science and study of the Earth, its composition, structure, physical properties, history, and the processes that shape it. ... Portrayal of shifting continents The concept of continental drift was first proposed by Alfred Wegener. ... Earth, also known as the Earth or Terra, is the third planet outward from the Sun. ... The lithosphere (from the Greek for rocky sphere) is the solid outermost shell of a rocky planet. ... The asthenosphere (from an invented Greek a + sthenos without strength) is the region of the Earth between 100-200 km below the surface—but perhaps extending as deep as 400 km—that is is the weak or soft zone in the upper mantle. ...

The lithosphere essentially "floats" on the asthenosphere and is broken-up into ten major plates: African, Antarctic, Australian, Eurasian, North American, South American, Pacific, Cocos, Nazca, and the Indian plates. These plates (and the more numerous minor plates) move in relation to one another at one of three types of plate boundaries: convergent (two plates push against one another), divergent (two plates move away from each other), and transform (two plates slide past one another). Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along plate boundaries (most notably around the so-called "Pacific Ring of Fire"). The African Plate is a continental tectonic plate covering the continent of Africa and extending westward to the Mid-Atlantic Ridge. ... The Antarctic Plate is a continental tectonic plate covering the continent of Antarctica and extending outward under the surrounding oceans. ... Categories: Plate tectonics | Geology stubs ... The North American plate is shown in brown on this map The North American Plate is a continental tectonic plate covering the continent of North America, extending eastward to the Mid-Atlantic Ridge and westward to the Cherskiy Range in East Siberia. ... The South American Plate is a continental tectonic plate covering the continent of South America and extending eastward to the Mid-Atlantic Ridge. ... The Pacific Plate is an oceanic tectonic plate beneath the Pacific Ocean. ... The Cocos Plate is an oceanic tectonic plate beneath the Pacific Ocean off the west coast of Central America. ... The Nazca plate is shown in light blue on this map The Nazca Plate is an oceanic tectonic plate in the eastern Pacific Ocean basin off the west coast of South America. ... Global earthquake epicenters, 1963–1998 An earthquake is a trembling or a shaking movement of the Earths surface. ... A volcano is a geological landform (usually a mountain) where magma (rock of the earths interior made molten or liquid by high pressure and temperature) erupts through the surface of the planet. ... Mount Cook, a mountain in New Zealand A mountain is a landform that extends above the surrounding terrain in a limited area. ... The oceanic trenches are several hundred kilometres long but narrow topographic depressions of the sea floor. ... The Ring of Fire is a zone of frequent earthquakes and volcanic eruptions that encircles the basin of the Pacific Ocean. ...

Plate tectonic theory arose out of two separate geological observations: continental drift, noticed in the early 20th century, and seafloor spreading, noticed in the 1960s. The theory itself was developed during the late 1960s and has since almost universally been accepted by scientists and has revolutionized the Earth sciences (akin to the development of the periodic table for chemistry, the discovery of the genetic code for genetics, or evolution in biology). Portrayal of shifting continents The concept of continental drift was first proposed by Alfred Wegener. ... Age of oceanic crust. ... Events and trends The 1960s was a turbulent decade of change around the world. ... Earth science (also known as geoscience, the geosciences or the Earth Sciences), is an all-embracing term for the sciences related to the planet Earth. ... The periodic table of the chemical elements, also called the Mendeleev periodic table, is a tabular display of the known chemical elements. ... Chemistry (in Greek: χημεία) is the science of matter and its interactions with energy and itself (see physics, biology). ... RNA codons. ... Genetics (from the Greek genno γεννώ= give birth) is the science of genes, heredity, and the variation of organisms. ... Charles Darwin, the father of modern evolutionary theory In the life sciences, evolution is a change in the traits of living organisms over generations, including the emergence of new species. ... Main article: Life There are many universal units and common processes that are fundamental to the known forms of life. ...

The tectonic plates of the world were mapped in the second half of the 20th century.

Key principles

The division of the Earth's interior into lithospheric and asthenospheric components is based on their mechanical differences. The lithosphere is cooler and more rigid, whilst the asthenosphere is hotter and mechanically weaker. This division should not be confused with the chemical subdivision of the Earth into (from innermost to outermost) core, mantle, and crust. The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which "float" on the fluid-like asthenosphere. The relative fluidity of the asthenosphere allows the tectonic plates to undergo motion in different directions. The lithosphere (from the Greek for rocky sphere) is the solid outermost shell of a rocky planet. ... The asthenosphere (from an invented Greek a + sthenos without strength) is the region of the Earth between 100-200 km below the surface—but perhaps extending as deep as 400 km—that is is the weak or soft zone in the upper mantle. ... Mechanics refers to: a craft relating to machinery (from the Latin mechanicus, from the Greek mechanikos, meaning one skilled in machines), or a range of disciplines in science and engineering. ... Earth, also known as the Earth or Terra, is the third planet outward from the Sun. ... The mantle is the layer in the structure of the Earth that lies directly under the Earths crust. ... Earth cutaway from core to exosphere. ... Categories: Geology stubs | Plate tectonics ...

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. The majority of the world's active volcanoes occur along plate boundaries, with the Pacific Plate's Ring of Fire being most active and famous. These boundaries are discussed in further detail below. Global earthquake epicenters, 1963–1998 An earthquake is a trembling or a shaking movement of the Earths surface. ... Mount Cook, a mountain in New Zealand A mountain is a landform that extends above the surrounding terrain in a limited area. ... A volcano is a geological landform (usually a mountain) where magma (rock of the earths interior made molten or liquid by high pressure and temperature) erupts through the surface of the planet. ... The oceanic trenches are several hundred kilometres long but narrow topographic depressions of the sea floor. ... The Ring of Fire is a zone of frequent earthquakes and volcanic eruptions that encircles the basin of the Pacific Ocean. ...

Tectonic plates are comprised of two types of lithosphere: continental and oceanic lithospheres; for example, the African Plate includes the continent and parts of the floor of the Atlantic and Indian Oceans. The distinction is based on the density of constituent materials; oceanic lithospheres are denser than continental ones due to their greater mafic mineral content. As a result, the oceanic lithospheres generally lie below sea level (for example the entire Pacific Plate, which carries no continent), while the continental ones project above sea level (see isostasy for explanation of this principle, which is essentially a large-scale version of Archimedes' Bath). The Continental Crust is the layer of granitic and sedimentary rock which forms the continents and the areas of shallow seabed close to their shores, known as continental shelves. ... Oceanic crust is the part of Earths lithosphere which underlies the ocean basins. ... The African Plate is a continental tectonic plate covering the continent of Africa and extending westward to the Mid-Atlantic Ridge. ... In geology, mafic minerals are silicate minerals, magmas, and volcanic and intrusive igneous rocks that have relatively high concentrations of the heavier elements. ... The Pacific Plate is an oceanic tectonic plate beneath the Pacific Ocean. ... Isostasy is a term used in Geology to refer to the state of gravitational equilibrium between the Earths lithosphere and asthenosphere such that the tectonic plates float at an elevation which depends on their thickness and density. ... Archimedes (Greek: ΑΡΧΙΜΗΔΗΣ, Arkhim&ecircas a Greek mathematician, astronomer, philosopher, physicist and engineer. ...

Types of plate boundaries

Three types of plate boundary.

There are three types of plate boundaries, characterised 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: Illustration by Jose F. Vigil. ... Illustration by Jose F. Vigil. ...

1. Transform boundaries occur where plates slide, or perhaps more accurately grind, past each other along transform faults. The relative motion of the two plates is therefore either sinistral or dextral.
2. Divergent boundaries occur where two plates slide apart from each other.
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 an orogenic belt (if the two simply collide and compress).

Plate boundary zones occur in more complex situations where three or more plates meet and exhibit a mixture of the above three boundary types. In plate tectonics, a transform boundary fault (also known as transform fault boundary, transform plate boundary, transform plate margin or conservative plate boundary) is said to occur when continental plates slide and grind against each other. ... A transform fault is a geological fault that is a special case of strike-slip faulting which terminates abruptly, at both ends, at a major transverse geological feature. ... A sinistral is a horizontal movement of blocks either side of a geological fault. ... A dextral is a horizontal movement of blocks either side of a geological fault. ... In plate tectonics, a divergent boundary (divergent fault boundary or divergent plate boundary) is a linear feature that exists between two tectonic plates where the plates are moving away from each other. ... In plate tectonics, a convergent boundary (convergent fault boundary, convergent plate boundary, or active margin) is where two tectonic plates slide towards each other and usually collide forming either a subduction zone with its associated island arc or an orogenic belt and associated mountain range. ... Subduction zones mark sites of convective downwelling of the Earths lithosphere. ... In geology, orogeny is the process of mountain building. ...

Transform (conservative) boundaries

The left- or right-lateral motion of one plate against another along transform or strike slip faults can cause highly visible surface effects. Because of friction, the plates cannot simply glide past each other. Rather, stress builds up in both plates and when it reaches a level that exceeds the slipping-point of rocks on either side of the transform-faults the accumulated potential energy is released as strain, or motion along the fault. The massive amounts of energy that are released are the cause of earthquakes, a common phenomenon along transform boundaries. Old fault exposed by roadcut near Harrisburg, Pennsylvania. ... This article is about the resistive force. ... Stress tensor In physics, stress is the internal distribution of forces within a body that balance and react to the loads applied to it. ... Potential energy (U, or Ep), a kind of scalar potential, is energy by virtue of matter being able to move to a lower-energy state, releasing energy in some form. ... Strain, in any branch of science dealing with materials and their behaviour, is the geometrical expression of deformation caused by the action of stress on a physical body. ... Global earthquake epicenters, 1963–1998 An earthquake is a trembling or a shaking movement of the Earths surface. ...

A good example of this type of plate boundary is the San Andreas Fault complex, which is found in the western coast of North America and is one part of a highly complex system of faults in this area. At this location, the Pacific and North American plates move relative to each other such that the Pacific plate is moving north with respect to North America. View of the San Andreas Fault in central California The San Andreas Fault is a geological fault, known as a right-lateral strike-slip fault, that spans a length of roughly 800 miles (1287 kilometers) through California. ... World map showing location of North America A satellite composite image of North America North America is the third largest continent in area and in population after Eurasia and Africa. ... Compass rose with north highlighted and at top North is one of the four cardinal directions, specifically the direction that, in Western culture, is treated as the primary direction: north is used (explicitly or implicitly) to define all other directions; the (visual) top edges of maps usually correspond to the...

Divergent (constructive) boundaries

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. The genesis of divergent boundaries is sometimes thought to be associated with the phenomenon known as hotspots. 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 hot spot believed to have created the Mid-Atlantic Ridge system currently underlies Iceland which is widening at a rate of a few centimetres per century. Such hot spots can be very productive of geothermal power and Iceland is actively developing this resource and is expected to be the world's first hydrogen economy within twenty years. This article is about the type of molten rock. ... In geology, a hotspot is a location on the Earths surface that has experienced active vulcanism for a long period of time. ... Kinetic energy (also called vis viva, or living force) is energy possessed by a body by virtue of its motion. ... Geothermal power is electricity generated by utilizing naturally occurring geological heat sources. ... A hydrogen economy is a hypothetical future economy in which the primary form of stored energy for mobile applications and load balancing is hydrogen. ...

Divergent boundaries are typified in the oceanic lithosphere by the rifts of the oceanic ridge system, including the Mid-Atlantic Ridge, and in the continental lithosphere by rift valleys such as the famous East African Great Rift Valley. 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 sea floor map will show a rather strange pattern of blocky structures that are separated by linear features (http://pubs.usgs.gov/publications/text/baseball.html) 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 (due to thermal contraction and subsidence). An oceanic ridge is an underwater mountain range, usually formed by plate tectonics. ... Courtesy USGS The Mid-Atlantic Ridge is a mostly underwater mountain range of the Atlantic Ocean that runs from 87°N (about 333 km South of the North Pole) to subantarctic Bouvet Island, where it turns into Atlantic-Indian-Ridge and continues further East through Crozet Plateau to the Southwest... Northern section of the Great Rift Valley. ... The result of the action of a transform fault. ... Subsidence is a term used in geology, engineering and surveying to denote the motion of a surface (usually, the earths surface) downwards relative to a datum such as sea-level. ...

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 centres. 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. The magnetic banding directly corresponds with the Earth's polar reversals. This was confirmed by measuring the ages of the rocks within each band. In reality the banding furnishes a map in time and space of both spreading rate and polar reversals. Earths magnetic field (the surface magnetic field) is approximately a magnetic dipole, with one pole near the geographic north pole and the other near the geographic south pole. ... Earths magnetic field (the surface magnetic field) is approximately a magnetic dipole, with one pole near the geographic north pole and the other near the geographic south pole. ...

Convergent (destructive) boundaries

The nature of a convergent boundary depends on the type of lithosphere in the plates that are colliding. Where a dense oceanic plate collides with a less-dense continental plate, the oceanic plate is typically thrust underneath, 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. 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. As organic material from the ocean bottom is transformed and heated by friction a liquid magma with a great amount of dissolved gasses will be created. This can erupt to the surface, forming long chains of volcanoes inland from the continental shelf and parallel to it. The continental spine of South America is dense with this type of volcano. 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. The entire Pacific ocean boundary is surrounded by long stretches of volcanoes and is known collectively as The Ring of Fire. Categories: Geology stubs | Plate tectonics ... The oceanic trenches are several hundred kilometres long but narrow topographic depressions of the sea floor. ... South America South America is a continent crossed by the equator, with most of its area in the Southern Hemisphere. ... The Nazca plate is shown in light blue on this map The Nazca Plate is an oceanic tectonic plate in the eastern Pacific Ocean basin off the west coast of South America. ... The South American Plate is a continental tectonic plate covering the continent of South America and extending eastward to the Mid-Atlantic Ridge. ... A volcano is a geological landform (usually a mountain) where magma (rock of the earths interior made molten or liquid by high pressure and temperature) erupts through the surface of the planet. ... South America South America is a continent crossed by the equator, with most of its area in the Southern Hemisphere. ... World map showing location of North America A satellite composite image of North America North America is the third largest continent in area and in population after Eurasia and Africa. ...

Where two continental plates collide the plates either crumple and compress or one plate burrows under or (potentially) overrides the other. Either action will create extensive mountain ranges. The most dramatic effect seen is where the northern margins of the Indian subcontinental plate is being thrust under a portion of the Eurasian plate, lifting it and creating the Himalaya. The Himalaya is a mountain range in Asia, separating the Indian subcontinent from the Tibetan Plateau. ...

When two oceanic plates converge they form an island arc as one oceanic plate is subducted below the other. A good example of this type of plate convergence would be Japan. An island arc is a type of archipelago formed by plate tectonics as one oceanic tectonic plate subducts under another and produces magma. ... Categories: Geology stubs | Plate tectonics ...

Sources of plate motion

As noted above, the plates are able to move because of the relative weakness of the asthenosphere. Dissipation of heat from the mantle is acknowledged to be the source of energy driving plate tectonics. Somehow, this energy must be converted into force in order for the plates to move. There are essentially two forces that could be driving plate motion: friction and gravity. These are further subdivided below. This article is about the resistive force. ... Gravitation is the tendency of masses to move toward each other. ...

Friction

Mantle drag 

Convection currents in the mantle are transmitted through the asthenosphere; motion is driven by friction between the asthenosphere and the lithosphere.

Trench suction 

Local convection currents exert a downward frictional pull on plates in subduction zones at ocean trenches.

Convection is the transfer of heat by the motion of or within a fluid. ...

Gravity

Ridge-push 

Plate motion is driven by the higher elevation of plates at mid-ocean ridges. Essentially stuff slides downhill. The higher elevation is caused by the relatively low density of hot material upwelling in the mantle. The real motion producing force is the upwelling and the energy source that runs it. This is a mis-nomer as nothing is pushing and tensional features are dominant along ridges. Also, it is difficult to explain continental break-up with this.

Slab-pull 

Plate motion is driven by the weight of cold, dense plates sinking into the mantle at trenches.

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 friction. Slab pull is widely believed to be the strongest force directly operating on plates. Recent models indicate that trench suction plays an important role as well. The over-all driving force and its energy source are still debatable subjects of on-going research.

Major plates

The main plates are Download high resolution version (1280x838, 359 KB) --216. ...

• African Plate, covering Africa
• Antarctic Plate, covering Antarctica
• Australian Plate, covering Australia
• Eurasian Plate covering Eurasia
• North American Plate covering North America and north-east Siberia
• South American Plate covering South America
• Pacific Plate, covering the Pacific Ocean

Notable minor plates include the Indian Plate and the Arabian Plate. The African Plate is a continental tectonic plate covering the continent of Africa and extending westward to the Mid-Atlantic Ridge. ... A satellite composite image of Africa Africa is the worlds second-largest continent in both area and population, after Asia. ... The Antarctic Plate is a continental tectonic plate covering the continent of Antarctica and extending outward under the surrounding oceans. ... Categories: Plate tectonics | Geology stubs ... Categories: Plate tectonics | Geology stubs ... African-Eurasian aspect of Earth Eurasia is the landmass composed of the continents of Europe and Asia. ... The North American plate is shown in brown on this map The North American Plate is a continental tectonic plate covering the continent of North America, extending eastward to the Mid-Atlantic Ridge and westward to the Cherskiy Range in East Siberia. ... World map showing location of North America A satellite composite image of North America North America is the third largest continent in area and in population after Eurasia and Africa. ... Siberia Siberia (Russian: Сиби́рь, common English transliterations: Sibir, Sibir; possibly from the Mongolian for the calm land) is a vast region of Russia and northern Kazakhstan constituting almost all of northern Asia. ... The South American Plate is a continental tectonic plate covering the continent of South America and extending eastward to the Mid-Atlantic Ridge. ... South America South America is a continent crossed by the equator, with most of its area in the Southern Hemisphere. ... The Pacific Plate is an oceanic tectonic plate beneath the Pacific Ocean. ... The India or Indian Plate is a minor tectonic plate. ... The Arabian Plate is a continental tectonic plate covering the Arabian peninsula and extending northward to Turkey. ...

The movement of plates has caused the formation and breakup 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 1000 million years ago and to have embodied most or all of Earth's continents, and broken up into eight continents around 600 million years ago. The eight continents later re-assembled into another supercontinent called Pangaea; Pangea eventually broke up into Laurasia (which became North America and Eurasia) and Gondwana (which became the rest). Categories: Geology stubs | Historical continents | Plate tectonics ... Map of Pangaea Pangaea (Greek for all lands) is the supercontinent that existed during the Mesozoic era, before the process of plate tectonics separated the component continents. ... Laurasia was a supercontinent that broke off from the Pangaean supercontinent in the late Mesozoic era. ... Pangea broke into the two supercontinents, Laurasia and Gondwana The southern supercontinent Gondwana (originally Gondwanaland) included most of the landmasses which make up todays continents of the southern hemisphere, including Antarctica, South America, Africa, Madagascar, India, Arabia, Australia-New Guinea and New Zealand. ...

Related article

• List of tectonic plates

Global earthquake epicenters, 1963-1998. ...

History and impact

Continental drift

Continental drift was one of many ideas about tectonics proposed in the late 19th and early 20th century. The theory has been superceded by and the concepts and data have been incorporated within plate tectonics. Alternative meaning: Nineteenth Century (periodical) (18th century — 19th century — 20th century — more centuries) As a means of recording the passage of time, the 19th century was that century which lasted from 1801-1900 in the sense of the Gregorian calendar. ... (19th century - 20th century - 21st century - more centuries) Decades: 1900s 1910s 1920s 1930s 1940s 1950s 1960s 1970s 1980s 1990s As a means of recording the passage of time, the 20th century was that century which lasted from 1901–2000 in the sense of the Gregorian calendar (1900–1999 in the...

By 1915 Alfred Wegener was making serious arguments for the idea with 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. Wegener wasn't the first to note this (Francis Bacon and Snider-Pellegrini preceded him), but he was the first to marshal significant fossil and paleo-topographical and climatological evidence to support this simple observation. 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. Alfred Wegeners theory of continental drift was widely ridiculed in his day. ... South America South America is a continent crossed by the equator, with most of its area in the Southern Hemisphere. ... A satellite composite image of Africa Africa is the worlds second-largest continent in both area and population, after Asia. ... Sir Francis Bacon Francis Bacon, 1st Viscount St Albans (January 22, 1561 – April 9, 1626) was an English philosopher, statesman, spy, freemason and essayist. ... FOSSIL is a standard for allowing serial communication for telecommunications programs under DOS. FOSSIL stands for Fido Opus Seadog Standard Interface Layer and was made by a group of Fidonet sysops to make their software work on different machines. ...

In the early 1940s, Maurice Ewing seismically tested the Atlantic edge of the North American continental shelf, and found a granitic layer dropped down to the basaltic ocean floor. If the continent had been torn from Europe and was plowing through the ocean bottom, the edge of the continental shelf should have marked the end of granitic rocks. Later studies aboard the Atlantis found that ocean bottom was not smooth, which suggested it was much stronger than if continents could push it aside. Centuries: 19th century - 20th century - 21st century Decades: 1890s 1900s 1910s 1920s 1930s - 1940s - 1950s 1960s 1970s 1980s 1990s Years: 1940 1941 1942 1943 1944 1945 1946 1947 1948 1949 Events and trends Technology First nuclear bomb First cruise missile, the V1 flying bomb and the first ballistic missile, the... Dr. W. Maurice Ewing (May 12, 1906 – May 4, 1974) has been described as a pioneering geophysicist who worked on the research of seismic reflection and refraction in ocean basins, ocean bottom photography, submarine sound transmission, deep sea coring of the ocean bottom, theory and observation of earthquake surface waves...

Beginning in the 1950s, scientists, 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. 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. 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. Millennia: 1st millennium - 2nd millennium - 3rd millennium Events and trends Technology United States tests the first fusion bomb. ... A magnetometer is a scientific instrument used to measure the strength of magnetic fields. ... Mushroom cloud from the nuclear explosion over Nagasaki rising 18 km into the air. ... USS Los Angeles A submarine is a specialized watercraft that can operate underwater. ... Basalt Basalt is an extrusive igneous rock, sometimes porphyritic, and is often both fine-grained and dense. ... Magnetite is a magnetic mineral form of both iron(II) oxide and iron(III) oxide or (iron(II,III) oxide), with chemical formula , one of several iron oxides and a member of the spinel group. ... (17th century - 18th century - 19th century - more centuries) As a means of recording the passage of time, the 18th century refers to the century that lasted from 1701 through 1800. ... Earths magnetic field (the surface magnetic field) is approximately a magnetic dipole, with one pole near the geographic north pole and the other near the geographic south pole. ...

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. Millennia: 1st millennium - 2nd millennium - 3rd millennium Events and trends Technology United States tests the first fusion bomb. ...

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. For instance, some parts of Scotland contain rocks very similar to those found in eastern North America. Furthermore, the Caledonian Mountains of Europe and parts of the Appalachian Mountains of North America are very similar in structure and lithology. Interstate road cut through limestone and shale strata in eastern Tennessee In geology and related fields, a stratum (plural: strata) is a layer of rock or soil with internally consistent characteristics that distinguishes it from contiguous layers. ... Scotland (Scottish Gaelic: Alba) is a country in northwest Europe, occupying the northern third of the island of Great Britain. ... The Appalachian Mountains are a system of North American mountains running from Newfoundland and Labrador, Canada to Alabama in the United States, although the northernmost mainland portion ends at the Gaspé Peninsula of Quebec. ... Structural geology is the study of deformation of rock including breaking (fracturing and faulting) and bending or folding. ... Petrology is a field of geology which focuses on the study of rocks and the conditions by which they form. ...

Floating continents

The prevailing concept was that there were static shells of strata under the continents. It was early observed 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. 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. Pierre Bouguer (February 16, 1698 – August 15, 1758) was a French mathematician. ...

By the mid-1950s the question remained unresolved of whether mountain roots were clenched in surrounding basalt or were floating like an iceberg. Millennia: 1st millennium - 2nd millennium - 3rd millennium Events and trends Technology United States tests the first fusion bomb. ...

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. 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 1967, Jason Morgan proposed that the Earth's surface consists of 12 rigid plates that move relative to each other. Two months later, in 1968, Xavier Le Pichon published a complete model based on 6 major plates with their relative motions. Events and trends The 1960s was a turbulent decade of change around the world. ... Courtesy USGS The Mid-Atlantic Ridge is a mostly underwater mountain range of the Atlantic Ocean that runs from 87°N (about 333 km South of the North Pole) to subantarctic Bouvet Island, where it turns into Atlantic-Indian-Ridge and continues further East through Crozet Plateau to the Southwest... 1962 was a common year starting on Monday (link will take you to calendar). ... Harry Hammond Hess (1906-1969) was an American geologist. ... 1968 was a leap year starting on Monday (the link is to a full 1968 calendar). ... Xavier Le Pichon (born June 18, 1937) is a French geophysicist. ...

Explanation of magnetic striping

Seafloor 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 theorize 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. This process, later called seafloor spreading, operating over many millions of years has built the 50,000 km-long system of mid-ocean ridges. This hypothesis was supported by several lines of evidence: A theoretical model of the formation of magnetic striping. ... A theoretical model of the formation of magnetic striping. ... 1961 (As MAD Magazine pointed out on its first cover for the year) was the first upside-down year—i. ... This article is about the type of molten rock. ... Oceanic crust is the part of Earths lithosphere which underlies the ocean basins. ...

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 flip-flopped 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. Earths magnetic field (the surface magnetic field) is approximately a magnetic dipole, with one pole near the geographic north pole and the other near the geographic south pole. ...

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 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" hypothesis was unsatisfactory because its supporters could offer no convincing geologic mechanism to produce such a huge, sudden expansion. Most geologists believe that the Earth has changed little, if at all, in size since its formation 4.6 billion years ago, raising a key question: how can new crust be continuously added along the oceanic ridges without increasing the size of the Earth?

This question particularly intrigued Harry H. 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. 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 spread 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. According to Hess, the Atlantic Ocean was expanding while the Pacific Ocean was shrinking. As old oceanic crust was consumed in the trenches, new magma rose and erupted along the spreading ridges to form new crust. In effect, the ocean basins were 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. Robert Sinclair Dietz (September 14, 1914 - May 19, 1995) was Professor of Geology at Arizona State University. ... The U.S. Coast and Geodetic Survey was established by President Thomas Jefferson in 1807 as the Survey of the Coast. ... The oceanic trenches are several hundred kilometres long but narrow topographic depressions of the sea floor. ...

Mapping with earthquakes

During the 20th century, improvements in seismic instrumentation and greater use of earthquake-recording instruments (seismographs) worldwide enabled scientists to learn that earthquakes tend to be concentrated in certain areas, most notably along the oceanic trenches and spreading ridges. 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. 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 worldwide. (19th century - 20th century - 21st century - more centuries) Decades: 1900s 1910s 1920s 1930s 1940s 1950s 1960s 1970s 1980s 1990s As a means of recording the passage of time, the 20th century was that century which lasted from 1901–2000 in the sense of the Gregorian calendar (1900–1999 in the... Seismographs (in Greek seismos = earthquake and graphein = write) are used by seismologists to record seismic waves. ... Sometimes referred to as the Roaring Twenties or the Jazz Age. ... Events and trends The 1960s was a turbulent decade of change around the world. ... 1963 was a common year starting on Tuesday (link will take you to calendar). ...

Geological paradigm shift

The acceptance of the theories of continental drift and sea floor spreading (the two key elements of plate tectonics) can be compared to the Copernican revolution in astronomy (see Nicolaus Copernicus). Within a matter of only several years geophysics and geology in particular were revolutionized. 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. Astronomy is one of the few sciences where amateurs can still play an active role, especially in the discovery and monitoring of transient phenomena. ... Nicolaus Copernicus (in Latin; Polish Mikołaj Kopernik, German Nikolaus Kopernikus; February 19, 1473 – May 24, 1543) was a Polish astronomer, mathematician and economist who developed the heliocentric (Sun-centered) theory of the solar system in a form detailed enough to make it scientifically useful. ... Geophysics, the study of the earth by quantitative physical methods, especially by seismic reflection and refraction, gravity, magnetic, electrical, electromagnetic, and radioactivity methods. ...

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. 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. 1960 was a leap year starting on Friday (link will take you to calendar). ... 1970 was a common year starting on Thursday. ...

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 Alps is the collective name for one of the great mountain range systems of Europe, stretching from Austria in the east, Slovenia, Italy, Switzerland, Liechtenstein and Germany, through to France in the west. ... The Himalaya is a mountain range in Asia, separating the Indian subcontinent from the Tibetan Plateau. ... 360° panoramic view from Marmolada (Photo: Mg-k) The Dolomites 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 sea floor into thin atmospheres. 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. Northern section of the Great Rift Valley. ... A satellite composite image of Africa Africa is the worlds second-largest continent in both area and population, after Asia. ... Courtesy USGS The Mid-Atlantic Ridge is a mostly underwater mountain range of the Atlantic Ocean that runs from 87°N (about 333 km South of the North Pole) to subantarctic Bouvet Island, where it turns into Atlantic-Indian-Ridge and continues further East through Crozet Plateau to the Southwest...

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

Plate tectonics on Mars

As a result of 1999 observations of the magnetic fields on Mars by the Mars Global Surveyor spacecraft, it has been proposed that the mechanisms of plate tectonics may once have been active on the planet - see Geology of Mars. 1999 is a common year starting on Friday of the Common Era, and was designated the International Year of Older Persons by the United Nations. ... In physics, a magnetic field is an entity produced by moving electric charges (electric currents) that exerts a force on other moving charges. ... Computer generated image of Global Surveyor spacecraft (NASA) The launch of the Mars Global Surveyor (MGS) spacecraft in November 1996 by NASA and the Jet Propulsion Laboratory began Americas return to Mars after a 20-year absence. ... Mars is the fourth planet from the Sun in the solar system, named after the Roman god of war (the counterpart of the Greek Ares), on account of its blood red color as viewed in the night sky. ...

See also

• List of plate tectonics topics
• List of Tectonic Plates
• List of Tectonic Plate Interactions

This is a list of articles related to plate tectonics and tectonic plates. ... Global earthquake epicenters, 1963-1998. ... See plate tectonics for a more complete discussion Tectonic plate interactions are of three different basic types: Divergent boundaries are areas where plates move away from each other, forming either mid-oceanic ridges or rift valleys. ...

References

• Earth System History, Steven M. Stanley, (W.H. Freeman and Company; 1999) pages 211-228 ISBN 0-7167-2882-6
• Geographica: The complete illustrated Atlas of the world, Editors of James Mills-Hicks (Barnes and Noble Books; New York; 2004) ISBN 0-7607-5974-X

External links

• U.S. Geological Survey Web Page Links
  • This Dynamic Earth (http://pubs.usgs.gov/publications/text/dynamic.html) provides an excellent overview of the subject.
  • Understanding plate motions (http://pubs.usgs.gov/publications/text/understanding.html)
  • plate map (http://pubs.usgs.gov/publications/text/slabs.html)
  • Artist's cross section illustrating the main types of plate boundaries (http://pubs.usgs.gov/publications/text/Vigil.html)
  • "Ring of Fire", Plate Tectonics, Sea-Floor Spreading, Subduction Zones, "Hot Spots" (http://vulcan.wr.usgs.gov/Glossary/PlateTectonics/description_plate_tectonics.html)
  • J. Tuzo Wilson: Discovering transforms and hotspots (http://pubs.usgs.gov/publications/text/Wilson.html)
  • Active volcanoes (http://vulcan.wr.usgs.gov/Glossary/PlateTectonics/Maps/map_plate_tectonics_world.html)
• San Andreas fault information (http://sepwww.stanford.edu/oldsep/joe/fault_images/BayAreaSanAndreasFault.html)
• Academic research (http://www-sst.unil.ch/research/plate_tecto/links.htm) sites.
• Interactive movie (http://www.ucmp.berkeley.edu/geology/tecall1_4.mov) showing 750 myr (million years) of global tectonic activity.
• More movies (http://www.ucmp.berkeley.edu/geology/tectonics.html) over smaller regions and smaller time scales.
• The Paleomap Project: (http://www.scotese.com/) numerous maps and movies.
• Web Dogs (http://www.uky.edu/ArtsSciences/Geology/webdogs/plates/reconstructions.html) tectonic reconstructions and interactive movies.
• Exceptionally detailed tectonic history (http://my.execpc.com/~acmelasr/mountains/wghnf.html) of Wisconsin.
• Illustration of ridge-push and slab-pull (http://www.windows.ucar.edu/tour/link=/earth/interior/how_plates_move.html).
• Maps of the Earth back to 620 million years ago (http://www2.nature.nps.gov/geology/usgsnps/pltec/scplseqai.html)
• See what happens when you move tectonic plates (http://www.pbs.org/wgbh/aso/tryit/tectonics/) - An interactive guide
• The Origin and the Mechanics of the Forces Responsible for Tectonic Plate Movements (http://www.tectonic-forces.org)
• Evidence (but not proof) for tectonics on Mars (http://www.sciencenews.org/pages/sn_arc99/5_1_99/bob2.htm)
• Plate Tectonics on Venus (https://www3.imperial.ac.uk/portal/page?_pageid=46,73409&_dad=portallive&_schema=PORTALLIVE)"The mapping and interpretion of the regional tectonic features of Venus over the past ten years or so has led to a qualitative picture of buoyant plate tectonics."
• The Story of Plate Tectonics (http://www.platetectonics.com/)
• Plate Tectonics and Climate (http://www.djburnette.com/projects/climate.html)
• Plate Tectonics...and Your Community (http://www.agiweb.org/earthcomm/resources/platetectonics.html)