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Plate tectonics

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   Bridge across the Álfagjá rift valley in southwest Iceland, the
   boundary of the Eurasian and North American continental tectonic
   plates.
   Enlarge
   Bridge across the Álfagjá rift valley in southwest Iceland, the
   boundary of the Eurasian and North American continental tectonic
   plates.

   Plate tectonics (from Greek τέκτων, tektōn "builder" or "mason") is a
   theory of geology which was developed to explain the observed evidence
   for large scale motions within the Earth's crust. The theory
   encompassed and superseded the older theory of continental drift from
   the first half of the 20th century and the concept of sea floor
   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, which
   is a more viscous zone of the mantle. Although solid, the asthenosphere
   has very low shear strength and can flow like a liquid on geological
   time scales. The deeper mantle below the asthenosphere is more rigid
   again.

   The lithosphere essentially floats on the asthenosphere. The
   lithosphere has broken up into what are called tectonic plates—in the
   case of Earth, there are ten major and many minor plates. These plates
   move in relation to one another at one of three types of plate
   boundaries: convergent, divergent, and transform. Earthquakes, volcanic
   activity, mountain-building, and oceanic trench formation occur along
   plate boundaries. The lateral movement of the plates is typically at
   speeds of several centimetres per year.

Synopsis on development

   Plate tectonic theory is currently the theory accepted by the vast
   majority of scientists working in the geosciences. It arose out of and
   was preceded by early hypotheses associated with continental drift, and
   following the development of the mechanism of seafloor spreading, (for
   which the detection of magnetic anomalies distributed by a clear
   pattern of parallel stripes on the seafloor served as impressive
   evidence) plate tectonics quickly became a theory on the brink of
   scientific revolution. Simultaneous advances in early seismic imaging
   techniques in and around wadati-benioff zones collectively with
   numerous other geologic observations soon solidified plate tectonics as
   a theory with extraordinary explanatory and predictive power in
   subsequent decades (and continuing). Plate tectonics was developed
   during the late 1960s and has since been essentially universally
   accepted by scientists as predominant throughout all geoscientific
   disciplines. The theory has revolutionized the earth sciences because
   of its unifying and explanatory power for diverse geological phenomena.

Key principles

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

   The division of the outer parts 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 lithosphere
   contains both crust and some mantle. A given piece of mantle may be
   part of the lithosphere or the asthenosphere at different times,
   depending on its temperature, pressure and shear strength. The key
   principle of plate tectonics is that the lithosphere exists as separate
   and distinct tectonic plates, which float on the fluid-like
   (visco-elastic solid) asthenosphere. The relative fluidity of the
   asthenosphere allows the tectonic plates to undergo motion in different
   directions.

   The plates are around 100 km (60 miles) thick and consist of
   lithospheric mantle overlain by either of two types of crustal
   material: oceanic crust (in older texts called sima from silicon and
   magnesium) and continental crust ( sial from silicon and aluminium).
   The two types of crust differ in thickness, with continental crust
   considerably thicker than oceanic (50 km vs 5 km).

   One plate meets another along a plate boundary, and plate boundaries
   are commonly associated with geological events such as earthquakes and
   the creation of topographic features like mountains, volcanoes and
   oceanic trenches. 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 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 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).

Types of plate boundaries

   Three types of plate boundary.
   Enlarge
   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:
    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 either sinistral (left side
       toward the observer) or dextral (right side toward the observer).
    2. 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)).
    3. Convergent boundaries (or active margins) occur where two plates
       slide towards each other commonly forming either a subduction zone
       (if one plate moves underneath the other) or a continental
       collision (if the two plates contain continental crust). 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 Japanese island arc.

Transform (conservative) boundaries

   The left- or right-lateral motion of one plate against another along
   transform 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 strain threshold of rocks on either side of the fault the
   accumulated potential energy is released as strain. Strain is both
   accumulative and instantaneous depending on the rheology of the rock;
   the ductile lower crust and mantle accumulates deformation gradually
   via shearing whereas the brittle upper crust reacts by fracture, or
   instantaneous stress release to cause motion along the fault. The
   ductile surface of the fault can also release instantaneously when the
   strain rate is too great. The energy released by instantaneous strain
   release is the cause of earthquakes, a common phenomenon along
   transform boundaries.

   A good example of this type of plate boundary is the San Andreas Fault
   which is found in the western coast of North America and is one part of
   a highly complex system of faults in this area. At this location, the
   Pacific and North American plates move relative to each other such that
   the Pacific plate is moving northwest with respect to North America.
   Other examples of transform faults include the Alpine Fault in New
   Zealand and the North Anatolian Fault in Turkey. Transform faults are
   also found offsetting the crests of mid-ocean ridges (for example, the
   Mendocino Fracture Zone offshore northern California).

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 origin of new divergent
   boundaries at triple junctions 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 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 oceanic ridge system, including the Mid-Atlantic Ridge and
   the East Pacific Rise, and in the continental lithosphere by rift
   valleys such as the famous East African Great Rift Valley. 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 centre, 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 geomagnetic surveys showed a strange pattern of symmetrical
   magnetic reversals 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

   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 volcanoes inland from the
   continental shelf and parallel to it. The continental spine of western
   South America is dense with this type of volcanic mountain building
   from the subduction of the Nazca plate. 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 buckle and
   compress or one plate delves under or (in some cases) overrides the
   other. Either action will create extensive mountain ranges. The most
   dramatic effect seen is where the northern margin of the Indian Plate
   is being thrust under a portion of the Eurasian plate, lifting it and
   creating the Himalayas and the Tibetan Plateau beyond. 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.

       Oceanic / Continental
              Enlarge
       Oceanic / Continental

                            Continental / Continental
                                     Enlarge
                            Continental / Continental

                                                     Oceanic / Oceanic
                                                          Enlarge
                                                     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
   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

   Plates are able to move because of the relative weakness of the
   asthenosphere. Dissipation of heat from the mantle is acknowledged to
   be the original source of energy driving plate tectonics.

   Two and three-dimensional imaging of the Earth's interior ( seismic
   tomography) shows that there is a laterally heterogeneous density
   distribution throughout the mantle. Such density variations can be
   material (from rock chemistry), mineral (from variations in mineral
   structures), or thermal (through thermal expansion and contraction from
   heat energy). The manifestation of this lateral density heterogeneity
   is [[mantle convection] from buoyancy forces. Tanimoto 2000. How mantle
   convection relates directly and indirectly to the motion of the plates
   is a matter of ongoing study and discussion in geodynamics. Somehow,
   this energy must be transferred to the lithosphere in order for
   tectonic plates to move. There are essentially two types of forces that
   are thought to influence plate motion: friction and gravity.

Friction

   Basal drag
          Large scale convection currents in the upper mantle are
          transmitted through the asthenosphere; motion is driven by
          friction between the asthenosphere and the lithosphere.

   Slab suction
          Local convection currents exert a downward frictional pull on
          plates in subduction zones at ocean trenches. Although, one
          could in effect argue that Slab-suction is actually merely a
          unique geodynamic setting wherein which basal tractions continue
          to act on the plate as it dives into the mantle (although
          perhaps to a greater extent -- acting on both the under and
          upper side of the slab).

Gravitation

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

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

          1. Flexural bulging of the lithosphere before it dives
          underneath an adjacent plate, for instance, produces a clear
          topographical feature that can offset or at least effect the
          influence of topographical ocean ridges.
          2. Mantle plumes impinging on the underside of tectonic plates
          can drastically alter the topography of the ocean floor.

   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 the friction of such basal forces. Slab
          pull is most widely thought to be the greatest force acting on
          the plates. Recent models indicate that trench suction plays an
          important role as well. However, it should be noted that the
          North American Plate, for instance, is nowhere being subducted,
          yet it is in motion. Likewise the African, Eurasian and
          Antarctic Plates. The over-all driving force for plate motion
          and its energy source remain subjects of on-going research.

External forces

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

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

Relative significance of each mechanism

   Plate motion based on Global Positioning System (GPS) satellite data
   from NASA JPL. Vectors show direction and magnitude of motion.
   Enlarge
   Plate motion based on Global Positioning System (GPS) satellite data
   from NASA JPL. Vectors show direction and magnitude of motion.

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

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

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

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

Major plates

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

   Notable minor plates include the Indian Plate, the Arabian Plate, the
   Caribbean Plate, the Juan de Fuca Plate, the Nazca Plate, the
   Philippine Plate and the Scotia Plate.

   The movement of plates has caused the formation and break-up of
   continents over time, including occasional formation of a
   supercontinent that contains most or all of the continents. The
   supercontinent Rodinia is thought to have formed about 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 remaining
   continents).

   Related article

     * List of tectonic plates

   Plate tectonics map

Continental drift

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

   By 1915, Alfred Wegener was making serious arguments for the idea of
   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, Benjamin Franklin 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 (and was supported in this by researchers such as Alex du
   Toit). However, his ideas were not taken seriously by many geologists,
   who pointed out that there was no apparent mechanism for continental
   drift. Specifically they did not see how continental rock could plow
   through the much denser rock that makes up oceanic crust. Wegener could
   not explain the force of continental drift.

   Wegener's vindication did not come until after his death in 1930. In
   1947, a team of scientists led by Maurice Ewing utilizing the Woods
   Hole Oceanographic Institution’s research vessel Atlantis and an array
   of instruments, confirmed the existence of a rise in the central
   Atlantic Ocean, and found that the floor of the seabed beneath the
   layer of sediments consisted of basalt, not granite which was common on
   the continents. They also found that the oceanic crust was much thinner
   than continental crust. All these new findings raised important and
   intriguing questions.

   Beginning in the 1950s, scientists including Harry Hess, using magnetic
   instruments ( magnetometers) adapted from airborne devices developed
   during World War II to detect submarines, began recognizing odd
   magnetic variations across the ocean floor. 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.

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

   When the rock strata of the tips of separate continents are very
   similar it suggests that these rocks were formed in the same way
   implying that they were joined initially. For instance, some parts of
   Scotland and Ireland contain rocks very similar to those found in
   Newfoundland and New Brunswick. Furthermore, the Caledonian Mountains
   of Europe and parts of the Appalachian Mountains of North America are
   very similar in structure and lithology.

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.

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

Plate tectonic theory

   Significant progress was made in the 1960s, and was prompted by a
   number of discoveries, most notably the Mid-Atlantic ridge. The most
   notable was the 1962 publication of a paper by American geologist Harry
   Hess ( Robert S. Dietz published the same idea one year earlier in
   Nature. However, priority belongs to Hess, since he distributed an
   unpublished manuscript of his 1962 article already in 1960). Hess
   suggested that instead of continents moving through oceanic crust (as
   was suggested by continental drift) that an ocean basin and its
   adjoining continent moved together on the same crustal unit, or plate.
   In the same year, Robert R. Coats of the U.S. Geological Survey
   described the main features of island arc subduction in the Aleutian
   Islands. His paper, though little-noted (and even ridiculed) at the
   time, has since been called "seminal" and "prescient". In 1967, W.
   Jason Morgan proposed that the Earth's surface consists of 12 rigid
   plates that move relative to each other. Two months later, in 1968,
   Xavier Le Pichon published a complete model based on 6 major plates
   with their relative motions.

Explanation of magnetic striping

   Seafloor magnetic striping.
   Enlarge
   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 theorise that mid-ocean ridges mark
   structurally weak zones where the ocean floor was being ripped in two
   lengthwise along the ridge crest. New magma from deep within the Earth
   rises easily through these weak zones and eventually erupts along the
   crest of the ridges to create new oceanic crust. This process, later
   called seafloor spreading, operating over many millions of years
   continues to form new ocean floor all across the 50,000 km-long system
   of mid-ocean ridges. This hypothesis was supported by several lines of
   evidence:
    1. at or near the crest of the ridge, the rocks are very young, and
       they become progressively older away from the ridge crest;
    2. the youngest rocks at the ridge crest always have present-day
       (normal) polarity;
    3. stripes of rock parallel to the ridge crest alternated in magnetic
       polarity (normal-reversed-normal, etc.), suggesting that the
       Earth's magnetic field has 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.

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 favour 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 " Expanded
   earth theory" 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 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.

Mapping with earthquakes

   During the 20th century, improvements in and greater use of seismic
   instruments such as seismographs enabled scientists to learn that
   earthquakes tend to be concentrated in certain areas, most notably
   along the oceanic trenches and spreading ridges. 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 honour 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 world wide.

Geological paradigm shift

   The acceptance of the theories of continental drift and sea floor
   spreading (the two key elements of plate tectonics) may be compared to
   the Copernican revolution in astronomy (see Nicolaus Copernicus).
   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.

   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.

   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.

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

   Mysteries were no longer mysteries. Forests of complex and obtuse
   answers were swept away. Why were there striking parallels in the
   geology of parts of Africa and South America? Why did Africa and South
   America look strangely like two pieces that should fit to anyone having
   done a jigsaw puzzle? Look at some pre-tectonics explanations for
   complexity. For simplicity and one that explained a great deal more
   look at plate tectonics. A great rift, similar to the Great Rift Valley
   in northeastern Africa, had split apart a single continent, eventually
   forming the Atlantic Ocean, and the forces were still at work in the
   Mid-Atlantic Ridge.

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

Biogeographic implications on fauna and flora

   Contentinental drift theory helps biogeographers to explain on the
   disjunct biogeographic distribution of present day plants and animals
   found on different continents but having similar ancestors (Moss and
   Wilson 1998).

Plate tectonics on other planets

     * 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.
     * Venus

   Venus shows no evidence of active plate tectonics. There is debatable
   evidence of active tectonics in the planet's distant past; however,
   events taking place since then (such as the plausible and generally
   accepted hypothesis that the Venusian lithosphere has thickened greatly
   over the course of several hundred million years) has made constraining
   the course of its geologic record difficult. However, the numerous
   well-preserved impact craters has been utilized as a dating method to
   approximately date the Venusian surface (as there are as of yet any
   known samples of Venusian rock to be dated by more reliable methods).
   Dates derived are the dominantly in the range ~500 Mya - 750Mya,
   although ages of up to ~1.2 Gya have been calculated. This research has
   led to the fairly well accepted hypothesis that Venus has undergone an
   essentially complete volcanic resurfacing at least once in its distant
   past, with the last event taking place approximately within the range
   of estimated surface ages. While the mechanism of such an
   impressionable thermal event remains a debated issue in Venusian
   geosciences, some scientists are advocates of processes involving plate
   motion to some extent.
     * Galilean satellites

   Some of the satellites of Jupiter have features that may be related to
   plate-tectonic style deformation, although the materials and specific
   mechanisms may be different from plate-tectonic activity on Earth.

Metaphoric uses

   Sometimes the idea of moving tectonic plates is used metaphorically,
   e.g. "a tectonic shift" in a BBC TV news program describing the
   political effects of Ariel Sharon's illness on 4 January 2005.

   In the late 1980s, Québec theatre director Robert Lepage created a
   large international production called Tectonic Plates, which used this
   image to illustrate the rifts between Europe and America and the
   drifting of various destinies, relative to one another.

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