Plate Tectonics
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 The theory encompassed and superseded the older theory of continental drift from the first half of the 20th century and the concept of seafloor spreading developed during the 1960s.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 The theory encompassed and superseded the older theory of continental drift from the first half of the 20th century and the concept of seafloor spreading developed during the 1960s.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).
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.
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 (see isostasy for explanation of this principle, which is essentially a large-scale version of Archimedes' Bath).
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:
Plate boundary zones occur in more complex situations where three or more plates meet and exhibit a mixture of the above three boundary types.
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.
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.
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.
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.
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.
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.
Current Major Plates
The Great Rift Valley Wikipedia
History of Ancient Supercontinent's Breakup Detailed Live Science - April 29, 2008
Mystery Of Ancient Supercontinent's Demise Revealed Science Daily - April 24, 2008
Journey to the center of the Earth -- Scientists explain tectonic plate motions PhysOrg - February 21, 2008
Continental Slope Off Alaska 100 Nautical Miles Further Off Coast Than Assumed Science Daily - February 12, 2008
Is the Pacific splitting in two? New Scientist - January 28, 2008
New Fault Found in Europe; May "Close Up" Adriatic Sea National Geographic - January 26, 2008
Moving and Shaking Plates National Geographic - January 26, 2008
Ancient Cataclysm Rearranged Pacific Map, Study Says National Geographic - October 24, 2007
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.
In the land of death, scientists witness the birth of a new ocean Guardian - November 2, 2006
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."
But Dr Ayele, 37, and his colleagues wanted a scientific explanation. They knew the area was geologically unstable, but the number of earthquakes - 162 measuring more than four on the Richter scale in just two weeks - made them suspect that something extraordinary had happened deep underground. They asked a team of British-based scientists with access to satellite technology for help. When the results came back it seemed as unlikely as birds flying out of the ground.
Here in the Afar desert, one of the hottest and driest places on earth, the tribe had witnessed the birth of a new ocean. Images from the European Space Agency's Envisat satellite showed that a huge rift, 37 miles long and up to eight metres (26ft) wide, had opened deep in the Earth's crust. The tear, the largest observed since the advent of satellite monitoring, was created by a violent lateral rush of molten rock, or magma, along the fault line separating the Nubian and Arabian tectonic plates.
Tim Wright, a geologist at the University of Leeds who interpreted the satellite results, was astonished by the images and what they pointed to. "The process happening here is identical to that which created the Atlantic Ocean," said Dr Wright during a recent research expedition in Afar. "If this continues we believe parts of Eritrea, Ethiopia and Djibouti will sink low enough to allow water to flow in from the Red Sea." The findings caused a stir in the scientific community. This year teams from the UK, France, Italy and the US mounted expeditions to Afar, a region described by the British explorer Wilfred Thesiger as a "veritable land of death".
From above you can see vast black tongues of lava lapping at the desert sands, and rust-coloured volcanos with their lids long blown off. There are so many fissures and faults where the ground has opened and slipped that the Earth's skin looks like elephant hide.
The lunar geography reflects what lies beneath. Afar stands at the junction of three tectonic plates, which form the outer shell of the Earth and meet at unstable fault lines. The Nubian and Somali plates run along the Great Rift Valley, which spreads south from Afar. Branching out like a funnel to the north is the Arabian plate.
Tectonic plates across the globe are constantly shifting - though slowly, usually by a few centimetres a year - with the magma beneath the crust. The plates can collide, forcing the crust upwards and creating mountain ranges - as happened with the Himalayas. They can also slide past one another, as occurs along the San Andreas Fault, in California, a notorious earthquake zone.
The plates can also pull apart causing continents to break up and oceans to form. Early in this process, at the plate margins, the Earth's crust stretches and thins in the manner of toffee. Magma rises up, eventually cracking the crust and helping the plates drift apart. Between the fault lines the crust, now heavy with cooled magma, sinks to form a valley and then allows water from a nearby sea to rush in.
This is how the Atlantic was formed, separating Africa and Eurasia from the Americas. And this is what scientists believe is happening in Afar as the Arabian and Nubian plates pull apart. Parts of the region have already sunk to more than 100 metres below sea level, and only the highlands around the Danakil depression stop the Red Sea from rushing in.
Analysis of the new rift is providing an insight into the role of magma injection in cracking the Earth's crust and the pace at which continental break-up occurs. The last big "ocean spreading" occurred in Krafla, Iceland, in the mid 1970s, along the boundary of the North American and Eurasian plates that forms the Atlantic's mid-ocean ridge. But it took nine years to achieve what has occurred in Afar in a few weeks.
In recent months there has been more instability in Afar. After a series of earthquakes in June the rift widened by a further two metres. Hundreds of Afar nomads are still seeking refuge in a town 25 miles from the main fault zone, too afraid to go home. They may be wise; the scientists say there could be more violent earthquakes and eruptions. The new sea is predicted to be formed within about a million years. The separation of the Nubian and Somali plates along the Great Rift Valley could take 10 times as long. But that will be even more dramatic - for then Africa will eventually lose its horn. "Some people think that extreme natural phenomena happened only in historical times," said Cindy Ebinger, an American geologist leading the research in Afar. "But here we can see them happening right now."
Red Sea Region Parting in Massive Split National Geographic - July 19, 2006
Satellite Captures Creation of New Continental Crust Scientific American - July 20, 2006
Arabian tectonic plate and African plate are moving away from each other
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