Plate tectonics

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

Plate tectonics (from the Late Latin tectonicus, from the Greek: τεκτονικός "pertaining to building")^[1] is a scientific theory which describes the large scale motions of Earth's lithosphere. The theory builds on the older concepts of continental drift, developed during the first decades of the 20th century (one of the most famous advocates was Alfred Wegener), and was accepted by the majority of the Geoscientific community when the concepts of seafloor spreading were developed in the late 1950s and early 1960s.

According to the theory, the lithosphere is broken up into what are called tectonic plates. In the case of Earth, there are currently seven to eight major (depending on how they are defined) 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 collisional boundaries; divergent boundaries, also called spreading centers; and transform boundaries. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along plate boundaries. The lateral relative movement of the plates varies, though it is typically 0–100 mm annually.^[2]

The tectonic plates are composed of two types of crust (lithosphere): thicker continental and thin oceanic crust. This means that a plate can be of one type, or of both types. One of the main points the theory proposes is that the amount of surface of the (continental and oceanic) plates that disappear in the mantle along the convergent boundaries by subduction is more or less in equilibrium with the new (oceanic) crust that is formed along the divergent margins by seafloor spreading. This is also referred to as the "conveyor belt" principle. In this way, the total surface of the Globe remains the same. This is in contrast with earlier theories advocated before the Plate Tectonics "paradigm" as it is sometimes called, became the main scientific model, that proposed gradual shrinking (contraction) or gradual expansion of the Globe, theories that still exist in science as alternative models.

Lateral density variations in the mantle result in convection. Tectonic plates are able to move because the Earth's lithosphere has a higher strength and lower density than the underlying asthenosphere. Their movement is thought to be driven by the motion of seafloor away from the spreading ridge. The tectonic plate motion is some combination of drag, downward suction at the subduction zones, and variations in topography and density of the crust that result in differences in gravitational forces. The relative importance of each of these factors is unclear.

Key principles

The outer layers of the Earth are divided into lithosphere and asthenosphere. This is based on differences in mechanical properties and in the method for the transfer of heat. Mechanically, the lithosphere is cooler and more rigid, while the asthenosphere is hotter and flows more easily. In terms of heat transfer, 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 these same layers into the mantle (comprising both the asthenosphere and the mantle portion of the lithosphere) and the crust: a given piece of mantle may be part of the lithosphere or the asthenosphere at different times, depending on its temperature and pressure.

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 up to a typical 10–40 mm/a (Mid-Atlantic Ridge; about as fast as fingernails grow), to about 160 mm/a (Nazca Plate; about as fast as hair grows).^[3][4]

Tectonic plates 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). Average oceanic lithosphere is typically 100 km thick^[5]; its thickness is a function of its age: as time passes, it conductively cools and becomes thicker. Because it is formed at mid-ocean ridges and spreads outwards, its thickness is therefore a function of its distance from the mid-ocean ridge where it was formed. For a typical distance oceanic lithosphere must travel before being subducted, the thickness varies ~6 km thick at mid-ocean ridges to greater than 100 km at subduction zones; for shorter or longer distances, the subduction zone (and therefore also the mean) thickness becomes smaller or larger, respectively.^[6] Continental lithosphere is typically ~200 km thick^[5], though this also varies considerably between basins, mountain ranges, and stable cratonic interiors of continents. The two types of crust also differ in thickness, with continental crust being considerably thicker than oceanic (35 km vs. 6 km).^[7]

The location where two plates meet is called a plate boundary, and plate boundaries are commonly associated with geological events such as earthquakes and the creation of topographic features such as mountains, volcanoes, mid-ocean ridges, 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 many plates contain both. For example, the African Plate includes the continent and parts of the floor of the Atlantic and Indian Oceans. The distinction between oceanic crust and continental crust is based on their modes of formation. Oceanic crust is formed at sea-floor spreading centers, and continental crust is formed through arc volcanism and accretion of terranes through tectonic processes; though some of these terranes may contain ophiolite sequences, which are pieces of oceanic crust, these are considered part of the continent when they exit the standard cycle of formation and spreading centers and subduction beneath continents. Oceanic crust is also denser than continental crust owing to their different compositions. Oceanic crust is denser because it has less silicon and more heavier elements ("mafic") than continental crust ("felsic").^[8] As a result of this density stratification, oceanic crust generally lies below sea level (for example most of the Pacific Plate), while the continental crust buoyantly projects above sea level (see isostasy for explanation of this principle).

Types of plate boundaries

Three types of plate boundary.

Four 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). The San Andreas Fault in California is an example of a transform boundary exhibiting dextral motion.
2. 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.
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. The subducting slab contains many hydrous minerals, which release their water on heating; this water then causes the mantle to melt, producing volcanism. Examples of this are the Andes mountain range in South America and the Japanese island arc.
4. Plate boundary zones occur where the effects of the interactions are unclear and the broad belt boundaries are not well defined.^[9][10]

Driving forces of plate motion

Plate Tectonics is basically a kinematic phenomenon: Earth Scientists agree upon the observation and deduction that the plates have moved one with respect to the other, and debate and find agreements on how and when. But still a major question remains on what is the motor behind this movement; the geodynamic mechanism, and here science diverges in different theories.

Generally, it is accepted that 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, through convection or large scale upwelling and doming. As a consequence, in the current view, although it is still a matter of some debate, because of the excess density of the oceanic lithosphere sinking in subduction zones a powerful source of plate motion is generated. When the new crust forms at mid-ocean ridges, this oceanic lithosphere is initially less dense than the underlying asthenosphere, but it becomes denser 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.^[11] Although subduction is believed to be the strongest force driving plate motions, it cannot be the only force since there are plates such as the North American Plate which are moving, yet are nowhere being subducted. The same is true for the enormous Eurasian Plate. The sources of plate motion are a matter of intensive research and discussion among earth scientists. One of the main points is that the kinematic pattern of the movements itself should be separated clearly from the possible geodynamic mechanism that is invoked as the driving force of the observed movements, as some patterns may be explained by more than one mechanism.^[12][13] Basically, the driving forces that are advocated at the moment, can be divided in three categories: Mantle Dynamics related, Gravity related (mostly secondary forces), and Earth Rotation related.

Mantle Dynamics related Driving Forces

For a considerable period of around 25 years (last quarter of the twentieth century) the leading theory envisaged large scale convection currents in the upper mantle which are transmitted through the asthenosphere as the main driving force of the tectonic plates. This theory was launched by Arthur Holmes and some forerunners in the thirties of the twentieth century and was immediately recognised as the solution for the acceptance of the theory discussed since its occurrence in the papers of Alfred Wegener in the early years of the century. It was, though, long debated because the leading ("fixist") theory was still envisaging a static Earth without moving continents, up till the major break through in the early sixties.

Two- and three-dimensional imaging of the Earth's interior (seismic tomography) shows that there is a laterally varying 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 varying lateral density is mantle convection from buoyancy forces.^[14]

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.

Basal drag (friction): The plate motion is in this way driven by friction between the convection currents in the asthenosphere and the more rigid floating lithosphere.

Slab suction (gravity): Local convection currents exert a downward frictional pull on plates in subduction zones at ocean trenches. Slab suction may occur in a geodynamic setting wherein 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).

Lately, the convection theory is much debated as modern techniques based on 3D seismic tomography of imaging the internal structure of the Earth's mantle still fail to recognise these predicted large scale convection cells. Therefore, alternative patterns of mantle dynamics have been proposed:

In the theory of Plume tectonics developed during the 1990s of the twentieth century, a modified concept of mantle convection currents is used, related to super plumes rising from the deeper mantle which would be the drivers or the substitutes of the major convection cells. These ideas, which find their roots in the early thirties of the twentieth century with the so-called "fixistic" ideas of the European and Russian Earth Science Schools, find resonance in the modern theories which envisage hot spots/Mantle plumes in the mantle which remain fixed and are overridden by oceanic and continental lithosphere plates during time, and leave their traces in the geological record (though these phenomena are not invoked as real driving mechanisms, but rather as a modulator). The modern theories that continue building on the older mantle doming concepts and see the movements of the plates a secondary phenomena, are beyond the scope of this page and are discussed elsewhere for example on the Plume tectonics page.

Another suggestion is that the mantle flows neither in cells nor large plumes, but rather as a series of channels just below the Earth cust which than provide basal friction to the lithosphere. This theory is called "surge tectonics" and became quite popular in geophysics and geodynamics during the eighties and nineties of the 20th century.

Gravity related Driving Forces

Gravity related forces are usually invoked as secondary phenomena within the framework of a more general driving mechanism such as the various forms of mantle dynamics described above.

Gravitational sliding away from spreading ridges

According to many authors, 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.

It must be understood that this force is regarded as a secondary force; 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" horizontally 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. Other mechanisms generating this gravitational secondary force are 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 affect the influence of topographical ocean ridges.

2. Mantle plumes and hot spots impinging on the underside of tectonic plates can drastically alter the topography of the ocean floor. Some of these, on a larger scale, are seen as the major driving force of the plates (see below).

Slab-pull

Most scientists working today believe that the asthenosphere is not strong enough to directly cause motion by the friction of basal forces; Slab pull is therefore most widely thought to be the greatest force acting on the plates. In this current believe, plate motion is mostly driven by the weight of cold, dense plates sinking into the mantle at trenches ^[15]. 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. Slab pull is especially invoked in areas where remnants of older lithosphere become trapped along convergence zones e.g. as relicts in collisional belts, which, sinking into the mantle and rolling backwards, excert a pull on the overlying crust.

Gravitational sliding away from mantle doming

According to older theories, which are though still much debated in the literature, one of the driving mechanisms of the plates is the existence of large scale asthenosphere/mantle domes, which cause the gravitational sliding away from them of lithosphere plates, as a secondary phenomena of this, basically verticalistic, mechanism, which can act on various scales, from the small scale of one island arc up to the larger scale of an entire ocean basin.

Earth Rotation related Driving Forces

Alfred Wegener, being a meteorologist, had proposed tidal forces as a mechanism for continental drift (Coriolis effect/Buys Ballot, and other forces). However, the forces were considered far too small to cause continental motion as the concept then was of continents plowing through oceanic crust.^[16] In the Plate Tectonics context (accepted since the proposals of Heezen, Hess, Dietz, Morley, Vine and Matthews -see below- during the early 1960s), though, oceanic crust in motion with the continents which made other proposals related to Earth Rotation to be reconsidered, also in more recent literature, these are:

1. Tidal drag due to the gravitational force the Moon (and the Sun) exerts on the crust of the Earth.
2. Shear strain of the Earth Globe due to N-S compression related to the rotation and modulations of it.
3. Equatorial drift due to rotation: tendency of the Plates to move from the poles to the equator
4. Coriolis effect the Plates suffer when they move around the Globe
5. Global deformation of the Geoid due to small displacements of rotational Pole with respect to the Earth crust.
6. Other smaller deformation effects of the crust due to Wobbles and Spin movements of the Earth rotation on a smaller time scale.

In order for these mechanisms to be overall valid, systematic relationships should exist all over the Globe between the orientation and kinematics of deformation, and the geographical latitude and longitudinal grid of the Earth itself. Ironically, these systematic relations studies in the second half of the nineteenth century and the first half of the twentieth century to underline exactly the opposite: That the Plates had not moved in time, that the deformation grid was fixed with respect to the Earth equator and axes, and that gravitational driving forces where generally acting vertically and caused only locally horizontal movements (the so-called pre-Plate Tectonic, "fixist theories"). Later studies (discussed below on this page) therefore invoked many of the relationships recognised during this pre-plate tectonics period, to support their theories (see the anticipations and reviews in the work of van Dijk and collaborators^[12][17]).

Of the many forces discussed in this paragraph, tidal force is still highly debated and defended as a possible principle driving force, whereas the other forces are used or in global geodynamic models not using the Plate Tectonics concepts (therefore beyond the discussions treated in this chapter), or proposed as minor modulations within the overall Plate Tectonics model.

In 1973 George W. Moore ^[18] of the USGS and R.C. Bostrom ^[19] presented evidence for a general westward drift of the Earth's lithosphere with respect to the mantle, and, therefore, tidal forces or tidal lag or "friction" due to the Earth's rotation and the forces acting upon it by the Moon being a driving force for plate tectonics: As the Earth spins eastward beneath the moon, the moon's gravity ever so slightly pulls the Earth's surface layer back westward. In a more recent 2006 study (Scoppola et al., 2006) ^[20], scientists rediscussed these earlier proposed ideas. It has also been suggested recently in Lovett et al. (2006) ^[21] 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. In a recent paper by Torsvik et al. (2010) ^[22] it was suggested that, on the other hand, it can easily be observed that many plates are moving north and eastward, and that the dominantly westward motion of the Pacific ocean basins derives simply from the eastward bias of the Pacific spreading center (which is not a predicted manifestation of such lunar forces). In the same paper the authors admit, however, that relative to the lower mantle, there is a slight westward component in the motions of all the plates. They demonstrated that the westward drift, seen only for the past 30 Ma, is attributed to the increased dominance of the steadily growing and accelerating Pacific plate. We may safely state that the debate is still open.

Relative significance of each Driving Force 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 regarding 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.^[23]

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, except for those plates which are not being subducted.^[15]

The driving forces of plate motion continue to be active subjects of on-going research within geophysics.

Historical context - The Development of the Theory

Plate tectonics is the main current theory in Earth Sciences regarding the development of our planet Earth. It is, therefore, appropriate to dedicate some space to explain how the Earth Science community, step by step, has built this theory, from early speculations, through the gathering of proof and severe debates, up to the refinement and quantification, and still ongoing confrontations with alternative ideas.

A summary

Detailed map showing the tectonic plates with their movement vectors.

In the late 19th and early 20th 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, through geosynclinal theory. Generally, this was placed in the context of a Planet Earth, which was contracting due to the heat loss in the course of a relatively short geological time.

Since it was observed as early as 1596 that the opposite coasts of the Atlantic Ocean—or, more precisely, the edges of the continental shelves—have similar shapes and seem to have once fitted together.^[24] Since that time many theories were proposed to explain this apparent complementarity, but the assumption of a solid earth made these various proposals difficult to explain.^[25][26]

The discovery of radioactivity and its associated heating properties in 1895 prompted a re-examination of the apparent age of the Earth,^[27] since this had previously been estimated by its cooling rate and assumption the Earth's surface radiated like a black body.^[28] Those calculations had implied 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. Armed with the knowledge of a new heat source, scientists realized that the Earth would be much older, and that its core was still sufficiently hot to be liquid.

In line with other previous and contemporaneous proposals, in 1912 the meteorologist Alfred Wegener amply described what he called Continental Drift and the scientific debate started that would end up 50 years later in the theory of Plate Tectonics.^[29][30] and expanded in his 1915 book The Origin of Continents and Oceans.^[31] Starting from the idea (also expressed by his forerunners) that the present continents once formed a single land mass that drifted apart, thus releasing the continents from the Earth's core and likening them to "icebergs" of low density granite floating on a sea of denser basalt.^[32][33] But without detailed evidence and a force sufficient to drive the movement, the theory was not generally accepted: 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. Later science supported theories proposed by English geologist Arthur Holmes in 1920 that plate junctions might lie beneath the sea and Holmes' 1928 suggestion of convection currents within the mantle as the driving force.^[26][34][35]

One of the first pieces of geophysical evidence that was used to support the movement of lithospheric plates came from paleomagnetism. This is based on the fact that rocks of different ages show a variable magnetic field direction, evidenced by studies since halfway the nineteenth century. The magnetic North and South reverse through time, and, especially important in paleotectonic studies, the relative position of the magnetic north varies through time. Initially, during the first half of the twentieth century, the latter phenomena was explained by introducing what was called "polar wander"(see Apparent polar wander), i.e., it was assumed that the north pole had been shifting through time. An alternative explanation, though, was that the continents had moved (shifted and rotated) relative to the north pole, and each continent, in fact, shows its own "polar wander path". The first time this evidence was used to support the movements of continents was at a symposium in Tasmania in March 1956.^[36][37] Initially theorized as an expansion of the global crust,^[36] later work would show that the evidence was equally in support of continental drift on a Globe with a stable radius.

The second piece of evidence in support of Continental Drift came during the late 50s and early 60s from data on the deep ocean floors and the development of marine geology which gave evidence for the association of seafloor spreading and magnetic field reversals, published by e.g. Harry Hess and Ron G. Mason^{[38][39][40][41]} pinpointed the precise mechanism which accounted for new rock upwelling.

Simultaneous advances in early seismic imaging techniques in and around Wadati-Benioff zones together with many other geologic observations soon made plate tectonics a theory with extraordinary explanatory and predictive power. The theory revolutionized the Earth sciences, explaining a diverse range of geological phenomena and their implications in other studies such as paleogeography and paleobiology.

Continental drift

Continental drift was one of many ideas about tectonics proposed in the late 19th and early 20th centuries. 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.^[31] 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, Snider-Pellegrini, Roberto Mantovani 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). Furthermore, 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. 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 drove continental drift, and his vindication did not come until after his death in 1930.

Floating continents

The prevailing concept during the first half of the twentieth Century 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 upon it like an iceberg.

During this timespan, numerous milestones were reached that would eventually lead to the development of Plate Tectonics. These are the works of Vening-Meinesz, Holmes, Umbgrove, and numerous others. Often, these milestones are forgotten for various reasons:

1. During this timespan, continental drift was not accepted.
2. Some of these were discussed in the context of abandoned fixistic ideas of a deforming globe without continental drift or an expanding Earth.
3. They were published during an episode of extreem political and economic instability and scientific communication was obviously hampered by this.
4. Many of these were published by European Scientists and at first not mentioned in the papers published by the American researchers which during the 1960s proposed the Plate Tectonics theory.

An example of an important milestone was the work of the Australian geologist Samuel Warren Carey who published an essay The tectonic approach to continental drift in support of the Expanding Earth model in 1958.^[36]

Mapping of 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 specific 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) ^[42] 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.

Mid Oceanic Ridge spreading and Convection

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.^[43][44]

The new data that had been collected on the Ocean Basins also showed particular characteristics regarding the bathymetry. One of the major outcomes of these datasets was that along the mid-oceanic ridges, new ocean floor was being created. This led to the crucial paper of Bruce Heezen(1960)^[45]which would trigger a real revolution in thinking. A profound consequence of seafloor spreading is that new crust was, and is now, being continually created along the oceanic ridges. Therefore, Heezen advocated the expanding Earth hypothesis of e.g 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" 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? In reality, this question had been solved already by numerous scientists during the forties and the fifties, like Arthur Holmes, Vening-Meinesz, Coates and many others: The crust in excess disappeared along what were called the oceanic trenches where so-called "subduction" occurred. Therefore, when various scientists during the early sixties started to reason on the data at their disposal regarding ocean floor, the pieces of the theory fell quickly at its place: The question particularly intrigued Harry Hammond 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 (the former published the same idea one year earlier in Nature, but priority belongs to Hess who had already distributed an unpublished manuscript of his 1962 article by 1960) were among the small handful who really understood the broad implications of sea floor spreading and how it would eventually agree with the, at that time, unconventional and unaccepted ideas of continental drift and the elegant and mobilistic models proposed by previous workers like Holmes.

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 reality, it actually shows that the work by the European scientists on island arcs and mountain belts performed and published dring the 1930s up till the 1950s was applied and appreciated also in the United States.

If the Earth's crust was expanding along the oceanic ridges, Hess and Dietz reasoned like Holmes and others before them, it must be shrinking elsewhere. Hess followed Heeze suggesting that new oceanic crust continuously spreads away from the ridges in a conveyor belt-like motion. And, using the mobilistic concepts developed before, he correctly concluded that many millions of years later, the oceanic crust eventually descends along the continental margins where oceanic trench— very deep, narrow canyons are present e.g. along the rim of the Pacific Ocean basin - were formed. The important step Hess made was that convection currents would be the driving force in this process, arriving at the same conclusions as Holmes decades before with the only difference that the thinning of the ocean crust was performed using the mechanism of Heeze of spreading along the ridges. Hess therefore concluded that Atlantic Ocean was expanding while the Pacific Ocean was shrinking. As old oceanic crust is "consumed" in the trenches, (like Holmes and others, he believed this was done by thickening of the continental lithosphere, not, a nowadays believed, by underthrusting at a larger scale of the oceanic crust itself into the mantle) 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, in a way like what later would be called the Wilson cycle (see below). Thus, the new mobilistic concepts 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.

The final proof: Magnetic Striping

Seafloor magnetic striping.

A demonstration of magnetic striping. (The darker the color is the closer it is to normal polarity)

Beginning in the 1950s, scientists like Victor Vacquier, 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, and was published by R.G. Mason and co-workers in 1961, who didn't find, though, an explanation for these data in terms of sea floor spreading, like Vine, Matthews and Morley a few years later.

The discovery of magnetic striping and the stripes being symmetrical around the crests of the mid-ocean ridges suggested a relationship. In 1961, scientists such as Hess and Dietz had begun 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, at first denominated the "conveyer belt hypothesis" and 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. Only four years after the maps with the "zebra pattern" of magnetic stripes where published, the link between sea floor spreading and these patterns was correctly placed, independently by Morley and by Vine and Matthews, now called the Vine-Matthews-Morley hypothesis. 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 reversed many times.^[46]

By explaining both the zebra-like 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 Geomagnetic Field Reversals (GMFR) of the Earth's magnetic field.

The Definition and Refining of the Theory - from New Global Tectonics to Plate Tectonics

After all these considerations, Plate Tectonics became quickly accepted in the scientific world, and numerous papers followed that defined the concepts:

• In 1965, Tuzo Wilson who had been a promotor of the sea floor spreading hypothesis and continental drift from the very beginning (e.g. Wilson, 1963^[47]) added the concept of transform faults to the model, completing the classes of fault types necessary to make the mobility of the plates on the globe work out.^[48]
• A symposium on continental drift was held at the Royal Society of London in 1965 which must be regarded as the official start of the acceptance of Plate Tectonics by the Scientific community, and which abstracts are issued as Blacket et al. (1965, Eds.^[49]). In this symposium, Edward Bullard and co-workers showed with a computer calculation how the continents along both sides of the Atlantic would best fit to close the ocean, which became knwon as the famous "Bullard's Fit".
• In 1966 Tuzo Wilson published the paper that referred to previous plate tectonic reconstructions, introducing the concept of what is now known as the "Wilson Cycle".^[50]
• In 1967, at the American Geophysical Union's meeting, W. Jason Morgan proposed that the Earth's surface consists of 12 rigid plates that move relative to each other.^[51]
• Two months later, Xavier Le Pichon published a complete model based on 6 major plates with their relative motions, and we may say that this parks the final acceptance of the Scientific Community of Plate Tectonics.^[52]

Implications for biogeography

Continental drift theory helps biogeographers to explain the disjunct biogeographic distribution of present day life found on different continents but having similar ancestors.^[53] In particular, it explains the Gondwanan distribution of ratites and the Antarctic flora..

Plate reconstruction

Reconstruction of plate configurations for the whole Phanerozoic

Reconstruction is used to establish past (and future) plate configurations, helping determine the shape and make-up of ancient supercontinents and providing a basis for paleogeography.

Defining plate boundaries

Current plate boundaries are defined by their seismicity.^[54] Past plate boundaries within existing plates are identified from evidence of vanished oceans, such as ophiolites.^[55]

Past plate motions

Various types of quantitative and semi-quantitative information are available to constrain past plate motions. The geometric fit between continents, such as between west Africa and South America is still an important part of plate reconstruction.^[56] Magnetic stripe patterns provide a reliable guide to relative plate motions going back into the Jurassic period.^[56] The tracks of hotspots give absolute reconstructions but these are only available back to the Cretaceous.^[57] Older reconstructions rely mainly on paleomagnetic pole data, although these only constrain the latitude and rotation, but not the longitude. Combining poles of different ages in a particular plate to produce apparent polar wander paths provides a method for comparing the motions of different plates through time.^[58] Additional evidence comes from the distribution of certain sedimentary rock types,^[59] faunal provinces shown by particular fossil groups and the position of orogenic belts.^[56]

Current plates

Major plates

Depending on how they are defined, there are usually seven or eight "major" plates:

• African Plate
• Antarctic Plate
• Indo-Australian Plate, sometimes subdivided into:
  • Indian Plate
  • Australian Plate
• Eurasian Plate
• North American Plate
• South American Plate
• Pacific Plate

Minor plates

There are dozens of smaller plates, the seven largest of which are:

• Arabian Plate
• Caribbean Plate
• Juan de Fuca Plate
• Cocos Plate
• Nazca Plate
• Philippine Sea Plate
• Scotia Plate

Movement

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; Pangaea broke up into Laurasia (which became North America and Eurasia) and Gondwana (which became the remaining continents).

Plate tectonics on other planets

The appearance of plate tectonics on terrestrial planets is related to planetary mass, with more massive planets than Earth expected to exhibit plate tectonics. Earth may be a borderline case, owing its tectonic activity to abundant water.^[60] (Silica and water form a deep eutectic.)

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 have 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 dominantly in the range ~500 to 750 Ma, although ages of up to ~1.2 Ga 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 impressive thermal event remains a debated issue in Venusian geosciences, some scientists are advocates of processes involving plate motion to some extent.

One explanation for Venus' lack of plate tectonics is that on Venus temperatures are too high for significant water to be present.^[61][62] The Earth's crust is soaked with water, and water plays an important role in the development of shear zones. Plate tectonics requires weak surfaces in the crust along which crustal slices can move, and it may well be that such weakening never took place on Venus because of the absence of water. However, some researchers remain convinced that plate tectonics is or was once active on this planet.

Mars

Mars is considerably smaller than Earth and Venus, and there is evidence for ice on its surface and in its crust.

In the 1990s, it was proposed that Martian Crustal Dichotomy was created by plate tectonic processes.^[63] Scientists today disagree, and believe that it was created either by upwelling within the Martian mantle that thickened the crust of the Southern Highlands and formed Tharsis,^[64] or by a giant impact that excavated the Northern Lowlands.^[65]

Observations made of the magnetic field of Mars by the Mars Global Surveyor spacecraft in 1999 showed patterns of magnetic striping discovered on this planet. Some scientists interpreted these as requiring plate tectonic processes, such as seafloor spreading.^[66][67] However, their data fail a "magnetic reversal test", which is used to see if they were formed by flipping polarities of a global magnetic field.^[68]

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.

Titan

Titan, the largest moon of Saturn, was reported to show tectonic activity in images taken by the Huygens Probe, which landed on Titan on January 14, 2005.^[69]

Exoplanets

It is believed that many planets around other stars will have plate tectonics. On Earth-sized planets, plate tectonics is more likely if there are oceans of water,^[70] but on larger super-earths plate tectonics is very likely even if the planet is dry.^[60]

See also

• Geosyncline theory, obsolete explanation of the cycle of basin development and mountain-building
• Mantle convection One of the main driving mechanisms of Plate tectonics
• Orogeny The process of mouintain building
• Plume tectonics, an extension of plate tectonics that attempts to explain other aspects of the field
• Tectonophysics The science occupied with the quantitative analyses of geodynamic processes
• Supercontinent The assembly of continental elements of the tectonic plates into one large continent, which happened several times during the past
• Supercontinent cycle The cycle of assembly and dispersal of continental crust fragments

• List of plate tectonics topics
• List of tectonic plates
• List of tectonic plate interactions

References

Bibliography

• McKnight Tom (2004). Geographica: The complete illustrated Atlas of the world. New York: Barnes and Noble Books. ISBN 076075974X. 
• Oreskes, Naomi (ed) (2003). Plate Tectonics: An Insider's History of the Modern Theory of the Earth. Westview. ISBN 0813341329. 
• Schubert, Gerald; Turcotte, Donald L.; Olson, Peter (2001). Mantle Convection in the Earth and Planets. Cambridge: Cambridge University Press. ISBN 052135367X. 
• Stanley Steven M (1999). Earth System History. W.H. Freeman. pp. 211–228. ISBN 0716728826. 
• Tanimoto, Toshiro; Lay, Thorne (2000). "Mantle dynamics and seismic tomography". Proceedings of the National Academy of Science 97 (23): 12409. doi:10.1073/pnas.210382197. PMID 11035784. 
• Thompson Graham R, Turk Jonathan (1991). Modern Physical Geology. Saunders College Publishing. ISBN 0030253985. 
• Turcotte DL, Schubert G (2002). Geodynamics: Second Edition. New York: John Wiley & Sons. ISBN 0521666244. 
• Winchester, Simon (2003). Krakatoa: The Day the World Exploded: August 27, 1883. HarperCollins. ISBN 0066212855. 
• Atkinson, Larry; Sancetta, Connie (1993). "Hail and Farewell". Oceanography (The Oceanography Society) 6 (34). 
• Lyman, J; Fleming, RH (1940). "Composition of Seawater". Journal of Marine Research 3: 134–146. 
• Sverdrup HU, Johnson MW, Fleming RH (1942). The Oceans: Their physics, chemistry and general biology. Englewood Cliffs: Prentice-Hall. pp. 1087. 
• Vine, FJ; Matthews, DH (1963). "Magnetic anomalies over oceanic ridges". Nature 199: 947–949. doi:10.1038/199947a0. 

Notes

External links

• The PLATES Project, a comprehensive resource with reconstructions, movies, images, list of publications, and teaching resources, from the University of Texas Institute for Geophysics at the Jackson School of Geosciences
• The Paleomap Project, Christopher Scotese's website with reconstructions in the past and future, paleogeographies, teaching material etc.
• Movie showing 750 million years of global tectonic activity
• 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 A short Abstract; the article is for sale
• Huang Zhen Shao (1997). "Speed of the Continental Plates". The Physics Factbook. http://hypertextbook.com/facts/ZhenHuang.shtml. 
• BBC Radio 4 In Our Time - Plate Tectonics
• GPlates, desktop software for the interactive visualisation of plate-tectonics
• MORVEL plate velocity estimates and information C. DeMets, D. Argus, & R. Gordon

Tectonic plates Major African · Antarctic · Eurasian · Indo-Australian · North American · Pacific · South American Minor Arabian · Caribbean · Cocos · Juan de Fuca · Nazca · Philippine · Scotia Other Aegean Sea · Altiplano · Amurian · Anatolian · Apulian · Australian · Balmoral Reef · Banda Sea · Bird's Head · Burma · Capricorn · Caroline · Conway Reef · Easter · Explorer · Falklands · Futuna · Galapagos · Gonâve · Gorda · Greenland · Halmahera · Iberian · Indian · Iranian · Juan Fernandez · Kerguelen · Kermadec · Kula · Madagascar · Manus · Maoke · Mariana · Molucca Sea · New Hebrides · Niuafo'ou · North Andes · North Bismarck · North Galapagos · Nubian · Okhotsk · Okinawa · Panama · Pelso · Philippine Mobile Belt · Rivera · Sangihe · Seychelles · Shetland · Solomon Sea · Somali · South Bismarck · South Sandwich · Sri Lanka · Sunda · Timor · Tisza · Tonga · Woodlark · Yangtze All List of Tectonic Plates