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. |
Plate tectonics (from
Greek τέκτων,
tektōn "builder" or "mason") is a
theory of
geology developed to explain the observed evidence for large scale motions within the Earth's
crust. The theory encompassed and superceded the older theory of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: the
lithosphere comprising the
crust and the solidified uppermost part of the
mantle. Below the lithosphere lies the
asthenosphere which comprises the inner viscous part of the mantle. The mantle behaves like a superheated and extremely viscous liquid.
The lithosphere essentially
floats on the asthenosphere. The lithosphere is 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.
Plate tectonic theory is currently the theory accepted by the vast majority of scientists working in this area. It 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 been universally accepted by virtually all scientists. It has revolutionized the
earth sciences and, because of its unifying and explanatory power for diverse geological phenomena, can be compared to other successful theories such as the
periodic table in
chemistry, the
genetic code in
biology, and
quantum mechanics in
physics.
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The tectonic plates of the world were mapped in the second half of the 20th century. Click the image to see a larger version. |
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 (visco-elastic liquid) 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 two principal types of material: oceanic crust (also called
sima from
silicon and
magnesium) and continental crust (
sial from silicon and
aluminium). The two types of crust also differ in thickness, with continental crust considerably thicker than oceanic.
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).
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Three types of plate boundary. |
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:#
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).#
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 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. Due to 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 a long
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 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 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 nothern 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 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. 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 due to 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 due to 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.
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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.
As noted above, the plates are able to move because of the relative weakness of the asthenosphere. Dissipation of heat from the mantle is acknowledged to be the 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 heterogenous 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 heterogeny is mantle convection due to 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.
Frictional torques
;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).Gravitational torques
;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 torque 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 buldging 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 torques. Slab pull is most widely thought to be the greatest torque force acting on the plates.
External forces
;Lunar tidal drag : 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 argue that the westward component of plates is due to 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, controvertially) 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. [
1] This is not a new argument, however. 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. One might note that many plates are moving north and eastward (not west), and that the dominantly westward motion of the the Pacific ocean basins is simply due to the eastward bias of the Pacific spreading center (which is not a predicted manifestation of such lunar torques). 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 torque mechanism
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Plate motion based on Global Positioning System (GPS) satellite data from NASA JPL. Vectors show direction and magnitude of motion. |
Because of basic physical laws, all of these forces must be acting on the plates. However, therein remains the question of to what degree of influence each process contributes to the motion of the plates. One method of dealing with this problem is to consider the relative rate at which each plate is moving, since 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.For this reason geoscientists have characterized such differences of the plates in an attempt to find correlations in relative plate velocity -- and such correlations are found, indeed. 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
Pacific 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 big players that 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.
The main plates are
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African Plate, covering
Africa - Continental plate
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Antarctic Plate, covering
Antarctica - Continental plate
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Australian Plate, covering
Australia (fused with
Indian Plate between 50 and 55 million years ago) - Continental plate
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Eurasian Plate covering
Asia - Continental plate
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North American Plate covering
North America and north-east
Siberia - Continental plate
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South American Plate covering
South America - Continental plate
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Pacific Plate, covering the
Pacific Ocean - Oceanic plate
Notable minor plates include the
Indian Plate, the
Arabian Plate, the
Caribbean Plate, and the
Scotia Plate.
The movement of plates has caused the formation and breakup of continents over time, including occasional formation of a supercontinent that contains most or all of the continents.The supercontinent
Rodinia is thought to have formed about 1000 million years agoand 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 platesContinental 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 with the first edition of
The Origin of Continents and Oceans. In that book he noted how the east coast of
South America and the west coast of
Africa looked as if they were once attached. Wegener wasn't the first to note this (
Francis Bacon,
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. 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'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. [
2]
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 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
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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 accross the 50,000 km-long system of mid-ocean ridges. This hypothesis was supported by several lines of evidence:# at or near the crest of the ridge, the rocks are very young, and they become progressively older away from the ridge crest;# the youngest rocks at the ridge crest always have present-day (normal) polarity;# 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 favor with some scientists who claimed that the shifting of the continents can be simply explained by a large increase in size of the Earth since its formation. However, this so-called "
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 honor of the seismologists who first recognized them,
Kiyoo Wadati of
Japan and
Hugo Benioff of the
United States. The study of global seismicity greatly advanced in the 1960s with the establishment of the
Worldwide Standardized Seismograph Network (WWSSN) to monitor the compliance of the 1963 treaty banning above-ground testing of nuclear weapons. The much-improved data from the WWSSN instruments allowed seismologists to map precisely the zones of earthquake concentration worldwide.
Geological paradigm shift
The acceptance of the theories of continental drift and sea floor spreading (the two key elements of plate tectonics) can be compared to the Copernican revolution in
astronomy (see
Nicolaus Copernicus). Within a matter of only several years
geophysics and geology in particular were revolutionized. The parallel is striking: just as pre-Copernican astronomy was highly descriptive but still unable to provide explanations for the motions of celestial objects, pre-tectonic plate geological theories described what was observed but struggled to provide any fundamental mechanisms. The problem lay in the question "How?". Before acceptance of plate tectonics, geology in particular was trapped in a "pre-Copernican" box.
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 sea floor into thin atmospheres. The cause of marine trenches oddly placed just off island arcs or continents and their associated volcanoes became clear when the processes of subduction at converging plates were understood.
Mysteries were no longer mysteries. Forests of complex and obtuse answers were swept away. Why were there striking parallels in the geology of parts of Africa and South America? Why did Africa and South America look strangely like two pieces that should fit to anyone having done a jigsaw puzzle? Look at some pre-tectonics explanations for complexity. For simplicity and one that explained a great deal more look at plate tectonics. A great rift, similar to the
Great Rift Valley in northeastern
Africa, had split apart a single continent, eventually forming the Atlantic Ocean, and the forces were still at work in the
Mid-Atlantic Ridge.
We have inherited some of the old terminology, but the underlying concept is as radical and simple as "The Earth moves" was in astronomy.
*
MarsAs 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.
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VenusOtherwise considered to be a "twin" of the Earth, Venus shows no evidence of plate tectonics - see
Geology of Venus.
*
Galilean satellitesSome 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.
*
List of plate tectonics topics*
list of tectonic plates*
list of tectonic plate interactions*
Geosyncline theory, obsolete explanation of mountain-building
Sometimes the idea moving tectonic plates is used metaphorically, e.g. "the tectonic plates have moved" in a
BBC TV news program describing the political effects of
Ariel Sharon's illness on 4 January 2005.
* McKnight, Tom (2004)
Geographica: The complete illustrated Atlas of the world, Barnes and Noble Books; New York ISBN 0-7607-5974-X
* Oreskes, Naomi ed. (2003)
Plate Tectonics : An Insider's History of the Modern Theory of the Earth, Westview Press ISBN 0813341329
* Stanley, Steven M. (1999)
Earth System History, W.H. Freeman and Company; pages 211-228 ISBN 0-7167-2882-6
* Thompson, Graham R. and Turk, Jonathan, (1991)
Modern Physical Geology, Saunders College Publishing ISBN 0-03-025398-5
* Winchester, Simon (2003)
Krakatoa: The Day the World Exploded: August 27, 1883, HarperCollins ISBN 0-0662-1285-5
* Tanimoto, Toshiro and Thorne Lay (2000)
Mantle dynamics and seismic tomography, Proc. Natl. Acad. Sci. USA, 10.1073/pnas.210382197 http://www.pnas.org/cgi/content/full/97/23/12409 Accessed 03/29/06.
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Interactive movie showing 750 myr (million years) of global tectonic activity.
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More movies over smaller regions and smaller time scales.
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Easy-to-draw illustrations for teaching plate tectonics*
Bird, P. (2003) An updated digital model of plate boundaries also available as a large (13 mb) PDF file [
3]
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Map of tectonic plates
