There are dozens of smaller plates, the seven largest of which are:
Plate tectonics, from Greek "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. The theory encompassed and superseded the older theory of continental drift from the first half of the 20th century and the concept of seafloor spreading developed during the 1960s.
Plate tectonics is a theory of geology that has been developed to explain the observed evidence for large scale motions of the Earth's lithosphere. The theory encompassed and superseded the older theory of continental drift from the first half of the 20th century and the concept of seafloor spreading developed during the 1960s.
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. Below the lithosphere lies the asthenosphere. Although solid, the asthenosphere has relatively low viscosity and shear strength and can flow like a liquid on geological time scales. The deeper mantle below the asthenosphere is more rigid again. This is, however, not due to 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. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along plate boundaries. The lateral movement of the plates is typically at speeds of 0.66 to 8.50 centimeters per year.
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).
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).
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.
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.
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.
There are three types of plate boundaries, 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:
Plate boundary zones occur in more complex situations where three or more plates meet and exhibit a mixture of the above three boundary types.
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.
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.
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.
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 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).
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.
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.
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.
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.
Plate Tectonics Wikipedia
List of Tectonic Plates
The Great Rift Valley Wikipedia
Hidden magma layer: Scientists discover 'lubricant' for Earth's tectonic plates PhysOrg - March 20, 2013
Scientists at Scripps Institution of Oceanography at UC San Diego have found a layer of liquefied molten rock in Earth's mantle that may be acting as a lubricant for the sliding motions of the planet's massive tectonic plates. The discovery may carry far-reaching implications, from solving basic geological functions of the planet to a better understanding of volcanism and earthquakes.
New Look at Earth's Mysterious Layer Live Science - March 20, 2013
A mysterious layer lies beneath Earth's massive tectonic plates. Sandwiched between two rock layers - the rigid lithosphere and the more pliable asthenosphere - this thin boundary is like the jelly in a peanut butter sandwich. Scientists think it could be very wet rock, or even partially melted rock, but no one knows for sure.
Under California: An ancient tectonic plate PhysOrg - March 18, 2013
Mostly gone, not forgotten: the Isabella anomaly (IA), is the seismic signature of a "fossil" slab of the Farallon oceanic plate that was pushed under North American starting around 100 million years ago. The anomaly occurs at the same depth as known Farallon slabs under Washington and Oregon, and lines up due east of a known Farallon fragment off the coast. Most of the Farallon plate was driven deep into the Earth's mantle as the Pacific and North American plates began converging, eventually coming together to form the San Andreas fault. The findings could force scientists to re-examine the tectonic history of western North America. In particular, it forces a rethinking of the delamination of the Sierra Nevada, which had been used to explain the Isabella anomaly.
Fragments of ancient continent buried under Indian Ocean BBC - February 25, 2013
Fragments of an ancient continent are buried beneath the floor of the Indian Ocean, a study suggests. Researchers have found evidence for a landmass that would have existed between 2,000 and 85 million years ago. The strip of land, which scientists have called Mauritia, eventually fragmented and vanished beneath the waves as the modern world started to take shape. Until about 750 million years ago, the Earth's landmass was gathered into a vast single continent called Rodinia. And although they are now separated by thousands of kilometres of ocean, India was once located next to Madagascar. Now researchers believe they have found evidence of a sliver of continent - known as a microcontinent - that was once tucked between the two.
Ancient 'Micro-Continent' Found Under Indian Ocean Live Science - February 25, 2013
The remains of a micro-continent scientist call Mauritia might be preserved under huge amounts of ancient lava beneath the Indian Ocean, a new analysis of island sands in the area suggests. These findings hint that such micro-continents may have occurred more frequently than previously thought. Researchers analyzed sands from the isle of Mauritius in the western Indian Ocean. Mauritius is part of a volcanic chain that, strangely, exists far from the edges of its tectonic plate. In contrast, most volcanoes are found at the borders of the tectonic plates that make up the surface of the Earth.
Cypress Trees Saw Rupturing of Earth's Supercontinents BBC - May 5, 2012
Early members of an ancient family of trees, the cypresses, grew on the supercontinent Pangaea, and when this giant continent split apart, it shaped the future of these trees, according to research that examined the evolution of these trees, which today include giant redwoods and sequoias. More than 200 million years ago, Pangaea contained all the modern continents, squished up against one another. The separation of these continents isolated populations of living things, putting them on different evolutionary paths. Scientists have already found evidence of the separation of the continents in the family histories of reptiles, amphibians and mammals.
New Mexico Is Stretching, Slowly but Surely TIME - January 23, 2012
The driving distance between Phoenix and Dallas is getting farther. It's a minuscule difference -- not even a millimeter a year -- but it's a tangible phenomenon, and you can blame on the middleman: New Mexico. The Rio Grande Rift, fault line that bisects the state, is bursting at the seams, pushing apart New Mexico's borders and stretching the land around it. The stretching of the Earth's surface is easier to see at the edges of tectonic plates, where there are typically volcanoes or mountains, but movement on an continental rift is more mysterious. Fortunately, at the paltry rate it's happening, scientists will have centuries, if not millennia, to come up with a game plan for controlling it.
Undersea mountains march into the abyss BBC - December 6, 2011
Startling new images from the depths of the Pacific Ocean reveal one of Earth's most violent processes: the destruction of massive underwater mountains. The pictures were created by sonar in waters up to 6km (4mi) deep. They expose how tectonic action is dragging giant volcanoes into a chasm in the seabed. The volcanoes are strung across several thousand kilometres of ocean floor and are moving westward on the Pacific tectonic plate at up to 6cm per year. The extraordinary scene was captured along the Tonga Trench during a research expedition last summer.
Europe's future lies under Africa, scientists suggest BBC - April 11, 2011
Europe may be starting to burrow its way under Africa, geologists suggest. The continents are converging; and for many millions of years, the northern edge of the African tectonic plate has descended under Europe. But this process has stalled; and at the European Geosciences Union (EGU) meeting last week, scientists said we may be seeing Europe taking a turn. If they are correct, this would signal the start of a new subduction zone - a rare event, scientifically fascinating. Beneath the Mediterranean Sea, the cold, dense rock at the extreme north of the African plate has virtually all sunk under the Eurasian plate on which Europe sits. But the African landmass is too light to follow suit and descend.
The biggest crash on Earth: India slides under Tibet, but how? PhysOrg - September 16, 2010
During the collision of India with the Eurasian continent, the Indian plate is pushed about 500 kilometers under Tibet, reaching a depth of 250 kilometers. The result of this largest collision in the world is the world's highest mountain range, but the tsunami in the Indian Ocean from 2004 was also created by earthquakes generated by this collision.
New View of Tectonic Plates: Computer Modeling of Earth's Mantle Flow, Plate Motions, and Fault Zones Science Daily - August 30, 2010
Computational scientists and geophysicists at the University of Texas at Austin and the California Institute of Technology (Caltech) have developed new computer algorithms that for the first time allow for the simultaneous modeling of Earth's mantle flow, large-scale tectonic plate motions, and the behavior of individual fault zones, to produce an unprecedented view of plate tectonics and the forces that drive it.
Breakthrough achieved in explaining why tectonic plates move the way they do PhysOrg - July 16, 2010
The new theory extends the theory of plate tectonics - a kinematic description of plate motion without reference to the forces behind it - with a dynamical theory that provides a physical explanation for both the motions of tectonic plates as well as motion of plate boundaries. The new findings have implications for how scientists understand the geological evolution of Earth, and in particular, the tectonic evolution of western North America, in the past 50 million years.
Britain cut off from Europe by 'super-river' as little as 30,000 years ago Telegraph.co.uk - November 30, 2009
Researchers have found sediment on the ocean floor off France which originated in the north of the channel which must have been transported by the river originally fed by the Thames and the Rhine. The samples, taken from the Atlantic sea bed, have provided scientists with the final piece in a geological jigsaw, enabling them to reconstruct the story of the Fleuve Manche (Channel River) – a giant waterway that flowed through the area now occupied by the English Channel. Earlier studies have already suggested that the river existed during a sequence of ice ages that began 450,000 years ago. It formed when a huge glacial lake in the North Sea overflowed, causing a prehistoric “mega-flood”, which sent water surging into the basin between Britain and France and gouging through the hills of chalky rock connecting them. To date, however, the timings and nature of the river have been based on a mixture of evidence from the English Channel and sedimentary deposits in coastal Europe, many of which are incomplete due to erosion.
Team first to record key event that breaks continents apart PhysOrg - December 10, 2008
An international research team led by Eric Calais, a Purdue University professor of geophysics, was able to measure ground displacements as two tectonic plates in Africa moved apart and molten rock pushed its way toward the surface during the first so-called "dyking event" ever recorded within the planet's continental crust.
Supercontinent Pangaea Pushed, Not Sucked, Into Place National Geographic - September 5, 2008
Supercontinents can form when a huge plume of hot rock from deep inside Earth wells up between the continental plates, pushing them apart until all Earth's landmasses collide.
Pangaea Google Videos
History of Ancient Supercontinent's Breakup Detailed Live Science - April 29, 2008
The breakup of the supercontinent Gondwana eventually formed the continents in the Southern Hemisphere. Exactly how this happened has been debated by geologists for years. Most theories say Gondwana broke into many different pieces, but new research suggests the large land mass simply split in two. Researcher Graeme Eagles of the University of London said he was suspicious of the theory that Gondwana had divided into many smaller continents because it was inconsistent with what is known about all other supercontinent breakups, including the breakup of Pangea into Gondwana and Laurasia.
Mystery Of Ancient Supercontinent's Demise Revealed Science Daily - April 24, 2008
Gondwana was a ‘supercontinent’ that existed between 500 and 180 million years ago. For the past four decades, geologists have debated how Gondwana eventually broke up, developing a multitude of scenarios which can be loosely grouped into two schools of thought – one theory claiming the continent separated into many small plates, and a second theory claiming it broke into just a few large pieces.
Journey to the center of the Earth -- Scientists explain tectonic plate motions PhysOrg - February 21, 2008
The first direct evidence of how and when tectonic plates move into the deepest reaches of the Earth is published in Nature today. Scientists hope their description of how plates collide with one sliding below the other into the rocky mantle could potentially improve their ability to assess earthquake risks. The mantle is a zone underneath the Earth's crust encompassing its super hot molten core. It is divided into an upper and lower area, and is made up of a 2,900 km circumference of churning, viscous rock. It is constantly fed with new material from parts of tectonic plates which slide down from the surface into it.
Continental Slope Off Alaska 100 Nautical Miles Further Off Coast Than Assumed Science Daily - February 11, 2008
New Arctic sea floor data just released by the University of New Hampshire and the National Oceanic and Atmospheric Administration suggests that the foot of the continental slope off Alaska is more than 100 nautical miles farther from the U.S. coast than previously assumed. The data, gathered during a recent mapping expedition to the Chukchi Cap some 600 nautical miles north of Alaska, could support U.S. rights to natural resources of the sea floor beyond 200 nautical miles* from the coast.
New Fault Found in Europe; May "Close Up" Adriatic Sea National Geographic - January 26, 2008
A newly identified fault running under the Adriatic Sea is building more of Croatia's Dalmatian Islands and bulking up the Dinaric Alps, a new study says. Both the islands and the mountain chain - which runs along the upper western coast of the Balkan Peninsula - were believed to have stopped growing 20 million to 30 million years ago. But scientists found that at the new fault the leading edge of the Eurasian tectonic plate is sliding over the South Adria microplate. If this process continues, eventually the seafloor will be completely consumed, bringing Italy into contact with Croatia.
Moving and Shaking Plates National Geographic - January 26, 2008
There are a few handfuls of major plates and dozens of smaller, or minor, plates. Six of the majors are named for the continents embedded within them, such as the North American, African, and Antarctic plates. Though smaller in size, the minors are no less important when it comes to shaping the Earth. The tiny Juan de Fuca plate is largely responsible for the volcanoes that dot the Pacific Northwest of the United States. The plates make up Earth's outer shell, called the lithosphere. (This includes the crust and uppermost part of the mantle.) Churning currents in the molten rocks below propel them along like a jumble of conveyor belts in disrepair. Most geologic activity stems from the interplay where the plates meet or divide.
Ancient Cataclysm Rearranged Pacific Map, Study Says National Geographic - October 24, 2007
A cataclysm 50 million years ago changed the face of the planet from the Hawaiian Islands to Antarctica, according to new research. The collapse of an underwater mountain range in the Pacific Ocean turned Australia into a warm and sunny continent instead of a snowbound wasteland and created some of the islands that dot the South Pacific today.
Slimmer Indian Continent Drifted Ten Times Faster National Geographic - October 17, 2007
The thinness of the Indian continent allowed it to speed northward at ten times the rate of other tectonic plates, a new study suggests. The oldest parts of India are about a half to a third as deep as similarly old portions of Australia, Africa, and Antarctica, geophysicists measured using a new technique. This allowed India to far outstrip the speed of the other sluggish and thicker tectonic plates, also known as lithosphere, after the breakup of a massive supercontinent 150 million years ago.
Tectonic Plates Pulling Apart: In the land of death, scientists witness the birth of a new ocean Guardian - November 2, 2006
Fissures have opened in the Earth's surface in Afar as the Arabian and Nubian tectonic plates pull apart. Scientists say the process is the same as that which created the Atlantic. In Ethiopia's arid Afar region eruptions and earthquakes have created an open-air laboratory. The nomads were terrified. For a week the ground had shuddered violently. Cracks opened up in the soil swallowing goats and camels. Sulphur-laced smoke rose out of the dark slits. After retreating to the hills, the nomads saw chunks of obsidian rock burst through the Earth's crust "like huge black birds" and fly 30 metres into the air. A mushroom cloud of ash dimmed the sun for three days. At night the new crater breathed flashes of fire. "They had experienced earthquakes before but never anything like this," said Atalay Ayele, a seismologist at Addis Ababa University, who interviewed the Afar tribespeople soon after the volcanic eruption 13 months ago in this remote corner of north-eastern Ethiopia. "They said that Allah must have been angry with them."
Red Sea Region Parting in Massive Split National Geographic - July 19, 2006
n a new study, scientists have determined that a recent tear in Earth's continental crust near the sea is the largest single rip seen since satellite monitoring began. For the past 30 million years the Arabian tectonic plate has been moving away from the African (Nubian) plate at the Red Sea. But the rift, in which Earth's crust is being stretched and thinned, is not happening smoothly.
Satellite Captures Creation of New Continental Crust Scientific American - July 20, 2006
Arabian tectonic plate and African plate are moving away from each other. A new sea is forming in the desert of northeastern Ethiopia. Millions of years from now, the pulling apart of the Arabian and Nubian tectonic plates will allow waters to rush in and widen the Red Sea. And thanks to the availability of satellite imagery, scientists have been able to get an unprecedented glimpse of the workings of stretching plates, the rock crust moving across Earth's surface at up to 12 centimeters per year.
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