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Tectonic plates can include [[continental crust]] or [[oceanic crust]], and typically, a single plate carries both. For example, the [[African Plate]] includes the continent and parts of the floor of the Atlantic and Indian Oceans. The distinction between continental crust and oceanic crust is based on the density of constituent materials; oceanic crust is denser than continental crust owing to their different proportions of various elements, particularly, silicon. Oceanic crust is denser because it has less silicon and more heavier elements ("[[mafic]]") than continental crust ("[[felsic]]"). As a result, oceanic crust generally lies below sea level (for example most of the [[Pacific Plate]]), while the continental crust projects above sea level (see [[isostasy]] for explanation of this principle).
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==Types of plate boundaries==
[[Image:Tectonic plate boundaries.png|thumb|right|350px|Three types of plate boundary.]]
Three types of plate boundaries exist, characterized by the way the plates move relative to each other. They are associated with different types of surface phenomena. The different types of plate boundaries are:
# '''[[#Transform (conservative) boundaries|Transform boundaries]]''' occur where plates slide or, perhaps more accurately, grind past each other along [[transform fault]]s. The relative motion of the two plates is either [[sinistral]] (left side toward the observer) or [[dextral]] (right side toward the observer).
# '''[[#Divergent (constructive) boundaries|Divergent boundaries]]''' occur where two plates slide apart from each other (examples of which can be seen at mid-ocean ridges and active zones of rifting (such as with the East Africa rift)).
# '''[[#Convergent (destructive) boundaries|Convergent boundaries]]''' (or ''active margins'') occur where two plates slide towards each other commonly forming either a [[subduction]] zone (if one plate moves underneath the other) or a [[orogeny|continental collision]] (if the two plates contain continental crust). Deep marine trenches are typically associated with subduction zones. Because of friction and heating of the subducting slab, volcanism is almost always closely linked. Examples of this are the [[Andes]] mountain range in South America and the [[Japan]]ese [[island arc]].
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===Divergent (constructive) boundaries===
{{main|Divergent boundary}}
At divergent boundaries, two plates move apart from each other and the space that this creates is filled with new crustal material sourced from molten [[magma]] that forms below. The origin of new divergent boundaries at [[triple junction]]s is sometimes thought to be associated with the phenomenon known as [[hotspot (geology)|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 which may have initiated the Mid-Atlantic Ridge system currently underlies [[Iceland]] which is widening at a rate of a few centimeters per century.
Divergent boundaries are typified in the oceanic lithosphere by the rifts of the [[Mid-ocean ridge|oceanic ridge]] system, including the [[Mid-Atlantic Ridge]] and the [[East Pacific Rise]], and in the continental lithosphere by rift valleys such as the famous [[Great Rift Valley|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 fault]]s occur. These are the [[fracture zone]]s, 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 [http://pubs.usgs.gov/publications/text/baseball.html linear features] perpendicular to the ridge axis. If one views the sea floor between the fracture zones as conveyor belts carrying the ridge on each side of the rift away from the spreading center the action becomes clear. Crest depths of the old ridges, parallel to the current spreading center, will be older and deeper (from thermal contraction and [[subsidence]]).
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 [[Earth's magnetic field|geomagnetic]] surveys showed a strange pattern of symmetrical [[magnetic reversal]]s on opposite sides of ridge centers. 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. The banding furnishes a map in time and space of both spreading rate and polar reversals.
===Convergent (destructive) boundaries===
{{main|Convergent boundary}}
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 because of the greater buoyancy of the continental lithosphere, forming a [[subduction zone]]. At the surface, the topographic expression is commonly an [[oceanic trench]] on the ocean side and a mountain range on the continental side. 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]].
While the processes directly associated with the production of melts directly above downgoing plates producing surface volcanism is the subject of some debate in the geologic community, the general consensus from ongoing research suggests that the release of volatiles is the primary contributor. As the subducting plate descends, its temperature rises driving off volatiles (most importantly water) encased in the porous oceanic crust. As this water rises into the mantle of the overriding plate, it lowers the melting temperature of surrounding mantle, producing melts ([[magma]]) with large amounts of dissolved gases. These melts rise to the surface and are the source of some of the most explosive volcanism on earth because of their high volumes of extremely pressurized gases (consider [[Mount St. Helens]]). The melts rise to the surface and cool forming long chains of [[volcano|volcanoes]] inland from the continental shelf and parallel to it. The continental spine of western [[South America]] is dense with this type of volcanic [[orogeny|mountain building]] from the subduction of the [[Nazca plate]]. In [[North America]] the [[Cascade Range|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 buckle and compress or one plate delves under or (in some cases) overrides the other. Either action will create extensive mountain ranges. The most dramatic effect seen is where the northern margin of the Indian Plate is being thrust under a portion of the Eurasian plate, lifting it and creating the [[Himalaya]]s and the [[Tibetan Plateau]] beyond. It has also caused parts of the Asian continent to deform westward and eastward on either side of the collision.
When two plates with oceanic crust converge they typically create an [[island arc]] as one plate is [[subducted]] below the other. The arc is formed from volcanoes which erupt through the overriding plate as the descending plate melts below it. The arc shape occurs because of the spherical surface of the earth (nick the peel of an orange with a knife and note the arc formed by the straight-edge of the knife). A deep undersea trench is located in front of such arcs where the descending slab dips downward. Good examples of this type of plate convergence would be [[Japan]] and the [[Aleutian Islands]] in Alaska.
{|align="center"
|[[Image:Oceanic-continental convergence Fig21oceancont.svg|thumb|Oceanic / Continental]]
|[[Image:Continental-continental convergence Fig21contcont.gif|thumb|Continental / Continental]]
|[[Image:Oceanic-oceanic convergence Fig21oceanocean.gif|thumb|Oceanic / Oceanic]]
|}
Plates may collide at an oblique angle rather than head-on (e.g. one plate moving north, the other moving south-east), and this may cause [[Geologic fault#Strike-slip faults|strike-slip faulting]] along the collision zone, in addition to subduction.
Not all plate boundaries are easily defined. Some are broad belts whose movements are unclear to scientists. One example would be the Mediterranean-Alpine boundary, which involves two major plates and several micro plates. The boundaries of the plates do not necessarily coincide with those of the continents. For instance, the North American Plate covers not only North America, but also far eastern Siberia and northern Japan.
==Driving forces of plate motion==
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