(Redirected from Tectonic plates)
The tectonic plates of the world were mapped in the second half of the 20th century.
'Plate tectonics' (from
Greek Ï„Îκτων, ''tektÅn'' "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 The theory encompassed and superseded the older theory of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.
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 speed at which human nails grow).
Synopsis of the development of the theory
In the late ninteenth and early twentieth centuries, geologists assumed that the Earth's major features were fixed, and that most geologic features such as mountain ranges could be explained by vertical crustal movement, as explained by
geosynclinal theory. The discovery of the
Americas showed that the opposite
coasts of the
Atlantic Ocean — or, more precisely, the edges of the
continental shelves — have similar shapes and seem once to have fitted together. Since that time many theories were proposed to explain this apparent coincidence, but the assumption of a solid earth made the various proposals difficult to explain.
The discovery of
radium and its associated heating properties in 1896 prompted a re-examination of the apparent
age of the Earth, since this had been estimated by taking its temperature and assuming that it radiated like a
black body. Such calculations assumed that, even if it started at red heat, the Earth would have dropped to its present temperature in a few tens of millions of years. With this new heat source, it was credible that the Earth was much older, and also that its core was still sufficiently hot to be liquid.
Plate tectonic theory arose out of the hypothesis of
continental drift first proposed by
Alfred Wegener in 1912 and expanded in his 1915 book ''The Origin of Continents and Oceans'', which suggested that the present continents once formed a single land mass which had drifted apart, floating on the molten rocks of the
core. But without detailed evidence and calculation of the forces involved, the theory remained sidelined. The Earth might have a solid crust and a liquid core, but there seemed to be no way that portions of the crust could move around -- although later science proved theories proposed by English geologist
Arthur Holmes in 1920 that their junctions might actually lie beneath the
sea.
The first evidence that crust plates did move around came with the discovery of variable
magnetic field direction in rocks of differing ages, first revealed at a symposium in Tasmania in
1956. Initially theorized as an
expansion of the global crust,
[1] later collaborations developed the plate tectonics theory, which accounted for spreading as the consequence of new rock upwelling, but avoided the need for an expanding globe by recognizing subduction zones and conservative translation faults. It was at this point that Wegener's theory moved from radical to mainstream, and became accepted by the scientific community. Additional work on the association of
seafloor spreading and magnetic field reversals by
Harry Hess and
Ron G. Mason[2][3] pinpointed the precise mechanism which accounted for new rock upwelling.
Following the recognition of
magnetic anomalies defined by symmetric, parallel stripes of similar magnetization on the seafloor on either side of a
mid-ocean ridge, plate tectonics quickly became broadly accepted. 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.
Study of the deep
ocean floor was critical to development of the theory; the field of deep sea
marine geology accelerated in the 1960s. Correspondingly, plate tectonic theory was developed during the late 1960s and has since been accepted all but universally by scientists throughout all geoscientific disciplines. The theory revolutionized the Earth sciences, explaining a diverse range geological phenomena.
Key principles
The division of the outer parts of the Earth's interior into lithosphere and asthenosphere is based on their
mechanical differences and in the ways that heat is transferred. The lithosphere is cooler and more rigid, whilst the asthenosphere is hotter and mechanically weaker. Also, the lithosphere loses heat by
conduction whereas the asthenosphere also transfers heat by
convection and has a nearly
adiabatic temperature gradient. 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 ride on the fluid-like (
visco-elastic solid) asthenosphere. Plate motions range from a few millimeters per year (mm yr
-1), to a more typical 10-40 mm yr
-1 (
Mid-Atlantic Ridge; about as fast as
fingernails grow), to about 160 mm yr
-1 (
Nazca Plate; about as fast as
hair grows).
[4][5]
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 most widely known. 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 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).
Types of plate boundaries
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 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). The
San Andreas Fault in California is one example.
# '
Divergent boundaries' occur where two plates slide apart from each other. Mid-ocean ridges (e.g., Mid-Atlantic Ridge) and active zones of rifting (such as Africa's
Great Rift Valley) are both examples of divergent 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
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
Main articles: Transform boundary
John Tuzo Wilson recognized that 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/or 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
Main articles: 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 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 year.
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 (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 by
Lawrence W. Morley,
Frederick John Vine and
Drummond Hoyle Matthews in the
Morley-Vine-Matthews hypothesis. 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 articles: 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
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 |  Continental / Continental |  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 northeastern Siberia.
Driving forces of plate motion
Tectonic plates are able to move because of the relative density of oceanic lithosphere and the relative weakness of the asthenosphere. Dissipation of heat from the mantle is acknowledged to be the original source of energy driving plate tectonics, but it is no longer thought that the plates ride passively on asthenospheric convection currents. Instead, it is accepted that the excess density of the oceanic lithosphere sinking in subduction zones drives plate motions. When it forms at mid-ocean ridges, the oceanic lithosphere is initially less dense than the underlying asthenosphere, but it becomes more dense with age, as it conductively cools and thickens. The greater
density of old lithosphere relative to the underlying asthenosphere allows it to sink into the deep mantle at subduction zones, providing most of the driving force for plate motions. The weakness of the asthenosphere allows the tectonic plates to move easily towards a subduction zone.
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.
[6] 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.
[7] 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 center (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.
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 1 billion 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
Historical development of the theory
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 in 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 (
Abraham Ortelius,
Francis Bacon,
Benjamin Franklin,
Snider-Pellegrini and
Frank Bursley Taylor 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 that propelled 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 the granite which is the main constituent of continents. They also found that the oceanic crust was much thinner than continental crust. All these new findings raised important and intriguing questions.
[8]
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 observed early 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.
In 1958 the Tasmanian geologist
Samuel Warren Carey published an essay ''The tectonic approach to continental drift'' in support of the expanding earth model.
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.
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:
# 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 reversed 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, most notably S. Warren Carey, 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 "
Expanding earth theory" hypothesis was unsatisfactory because its supporters could offer no convincing mechanism to produce a significant expansion of the Earth. Certainly there is no evidence that the moon has expanded in the past 3 billion years. Still, the question remained: 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 spreads 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 is consumed in the trenches, new magma rises and erupts along the spreading ridges to form new crust. In effect, the ocean basins are 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 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 was "The Earth moves" in astronomy.
Biogeographic implications on fauna and flora
Continental drift theory helps biogeographers to explain the disjunct
biogeographic distribution of present day plants and animals found on different continents but having similar ancestors.
[9] In particular, it explains the
Gondwanan distribution of
ratites and the
Antarctic flora.
Plate tectonics on other planets
Mars
As a result of observations made of the
magnetic fields on Mars by the ''
Mars Global Surveyor'' spacecraft in 1999, it has been proposed that the mechanisms of plate tectonics may once have been active on the planet - see
Geology of Mars. Further data from the ''
Mars Express'' orbiter's ''High Resolution Stereo Camera'' in 2007 clearly showed an example in the
Aeolis Mensae region.
[10]
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 (since there are thus far no 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.
See also
★
List of plate tectonics topics
★
List of tectonic plates
★
List of tectonic plate interactions
★
Geosyncline theory, obsolete explanation of mountain-building
★
Plume tectonics, an extension of plate tectonics that attempts to explain other aspects of the field
References
1. 1958: The tectonic approach to continental drift. In: S. W. Carey (ed.): Continental Drift – A Symposium. University of Tasmania, Hobart, 177-363 (expanding Earth from p. 311 to p. 349)
2. Ron Mason's key work
3. [1]
4. Speed of the Continental Plates
5.
6. http://www.pnas.org/cgi/content/full/97/23/12409 Toshiro Tanimoto and Thorne Lay, ''Mantle dynamics and seismic tomography'', PNAS, November 7, 2000, vol. 97 no. 23 pp. 12409–12410
7. Richard A. Lovett, ''Moon Is Dragging Continents West, Scientist Says'', National Geographic News January 24, 2006 http://news.nationalgeographic.com/news/2006/01/0124_060124_moon.html
8. Maurice Ewing and the Lamont-Doherty Earth Observatory Living Legacies, Laurence Lippsett. Retrieved 14 October 2006.
9. S. J. Moss and M. E. J. Wilson, 1998, ''Biogeographic implications of the Tertiary palaeogeographic evolution of Sulawesi and Borneo,'' Biogeography and geological evolution of SE Asia
10. http://www.esa.int/SPECIALS/Mars_Express/SEMF399OY2F_0.html ''Tectonic signatures at Aeolis Mensae'', European Space Agency, 28 June 2007
★ 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 0-8133-4132-9
★ G. Schubert, DL Turcotte, and P. Olson (2001) ''Mantle Convection in the Earth and Planets'', Cambridge University Press, Cambridge, ISBN 0-521-35367-X
★ Stanley, Steven M. (1999) ''Earth System History'', W.H. Freeman and Company; pages 211–228 ISBN 0-7167-2882-6
★ 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.
★ Thompson, Graham R. and Turk, Jonathan, (1991) ''Modern Physical Geology'', Saunders College Publishing ISBN 0-03-025398-5
★ Turcotte, DL and Schubert, G. (2002) ''
Geodynamics: Second Edition'', John Wiley & Sons, New York, ISBN 0-521-66624-4
★ Winchester, Simon (2003) ''
Krakatoa: The Day the World Exploded:
August 27,
1883'', HarperCollins ISBN 0-06-621285-5
External links
★
Movie showing 750 million years of global tectonic activity
★
More movies over smaller regions and shorter time scales
★
Easy-to-draw illustrations for teaching plate tectonics
★
An explanation of tectonic forces
★
Bird, P. (2003) An updated digital model of plate boundaries, also available as a
large (13 mb) PDF file
★
Map of tectonic plates
★
MantlePlumes.org, a website debating the existence of deep mantle plumes
★
USGS site on plate motions
★
The geodynamics of the North-American/Eurasian/African plate boundaries
★
Cenozoic dynamics of the African plate with emphasis on the Africa-Eurasia collision
★
Speed of the continental plates
★
ImpactTectonics.org, examines tectonic effects associated with hypervelocity bolide impacts on terrestrial planets
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