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 [τεκτονικός] Error: {{Lang-xx}}: text has italic markup (help) "pertaining to building") (Little, Fowler & Coulson 1990) 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. The lithosphere is broken up into what are called "tectonic plates". In the case of the Earth, there are currently seven to eight major (depending on how they are defined) and many minor plates. 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 conservative transform boundaries. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along these plate boundaries. The lateral relative movement of the plates varies, though it is typically 0–100 mm annually (Read & Watson 1975).

The tectonic plates are composed of two types of lithosphere: thicker continental and thin oceanic. The upper part is called the crust, again of two types (continental and oceanic). 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, theories that proposed gradual shrinking (contraction) or gradual expansion of the Globe, and that still exist in science as alternative models.

Regarding the driving mechanism of the plates various models co-exist: Tectonic plates are able to move because the Earth's lithosphere has a higher strength and lower density than the underlying asthenosphere. Lateral density variations in the mantle result in convection. Their movement is thought to be driven by a combination of the motion of seafloor away from the spreading ridge (due to variations in topography and density of the crust that result in differences in gravitational forces) and drag, downward suction, at the subduction zones. A different explanation lies in different forces generated by the rotation of the Globe and tidal forces of the Sun and the Moon. 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) (Zhen Shao 1997; Hancock, Skinner & Dineley 2000). The driving mechanism behind this movement is described below in a separate section.

Tectonic lithosphere plates consist of lithospheric mantle overlain by either or both 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 (Turcotte & Schubert 2002); 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 from about 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 (Turcotte & Schubert 2002). Continental lithosphere is typically ~200 km thick, 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) (Turcotte & Schubert 2002).

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.

As explained above, 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") (Schmidt & Harbert 1998). 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 the page isostasy for explanation of this principle).

Types of plate boundaries

Three types of plate boundary.

Main article: List of tectonic plate interactions

Basically, three types of plate boundaries exist (Meissner 2002, p. 100), with a fourth, mixed type, 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 (Conservative) 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 (Constructive) 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 (Destructive) (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.

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 the motor behind this movement is; 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 (see url ). 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 (van Dijk 1992 , van Dijk & Okkes 1991). 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

Main article: Mantle convection

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 1930s 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 until the major break–throughs 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 (Tanimoto & Lay 2000).

How mantle convection relates directly and indirectly to the motion of the plates is a matter of ongoing study and discussion in geodynamics. Somehow, this energy must be transferred to the lithosphere in order for tectonic plates to move. There are essentially two types of forces that are thought to influence plate motion: friction and gravity.

Basal drag (friction): The plate motion is in this way driven by friction between the convection currents in the asthenosphere and the more rigid overlying 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, 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 1930s 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's crust which then provide basal friction to the lithosphere. This theory is called "surge tectonics" and became quite popular in geophysics and geodynamics during the 1980s and 1990s (Smoot et al. 1996).

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 a spreading ridge: 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.

This force is regarded as a secondary force often referred to as "ridge-push". This is 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:

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.

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: Current scientific opinion is that the asthenosphere is insufficiently competent or rigid to directly cause motion by friction along the base of the lithosphere. Slab pull is therefore most widely thought to be the greatest force acting on the plates. In this current understanding, plate motion is mostly driven by the weight of cold, dense plates sinking into the mantle at trenches (Conrad & Lithgow-Bertelloni 2002). Recent models indicate that trench suction plays an important role as well. However, as the North American Plate is nowhere being subducted, yet it is in motion presents a problem. The same holds for 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, exert a pull on the overlying crust.

Gravitational sliding away from mantle doming: According to older theories one of the driving mechanisms of the plates is the existence of large scale asthenosphere/mantle domes, which cause the gravitational sliding of lithosphere plates away from them. This gravitational sliding represents a secondary phenomenon of this, basically vertically oriented mechanism. This 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 and pole flight Force as main driving mechanisms for continental drift. However, these forces were considered far too small to cause continental motion as the concept then was of continents plowing through oceanic crust (see url ). Therefore, also Wegener in his last edition of his book in 1929 converted to convection currents as the main driving force.

In the plate tectonics context (accepted since the seafloor spreading 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 the 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. Pole flight force: equatorial drift due to rotation and centrifugal effects: tendency of the plates to move from the poles to the equator ("Polflucht")
4. Coriolis effect the plates suffer when they move around the globe (coriolis effect/law of Buys Ballot)
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 latitudinal 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 do 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 axis, and that gravitational driving forces were 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 ).

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 section), or proposed as minor modulations within the overall plate tectonics model.

In 1973 George W. Moore of the USGS and R. C. Bostrom 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, just like proposed by Alfred Wegener (see above). In a more recent 2006 study (Scoppola et al. 2006), scientists rediscussed and advocated these earlier proposed ideas. It has also been suggested recently in Lovett (2006) 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) 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 though 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. The debate is still open.

Relative significance of each driving force mechanism

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.

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 (Conrad & Lithgow-Bertelloni 2002). The driving forces of plate motion continue to be active subjects of on-going research within geophysics and tectonophysics.

Historical context - development of the theory

Main article: Timeline of the development of tectonophysics

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.

Summary

Detailed map showing the tectonic plates with their movement vectors.

In line with other previous and contemporaneous proposals, in 1912 the meteorologist Alfred Wegener amply described what he called continental drift, expanded in his 1915 book The Origin of Continents and Oceans and the scientific debate started that would end up fifty years later in the theory of plate tectonics (Hughes 2001a). Starting from the idea (also expressed by his forerunners) that the present continents once formed a single land mass (which was called Pangea later on) that drifted apart, thus releasing the continents from the Earth's mantle and likening them to "icebergs" of low density granite floating on a sea of denser basalt (Wegener 1966; Hughes 2001b).

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 mantle and a liquid core, but there seemed to be no way that portions of the crust could move around.

Notwithstanding much opposition, the view of continental drift gained support and a lively debate started between "drifters" or "mobilists" (proponents of the theory) and "fixists" (opponents). During the 1920s, 1930s and 1940s, the former reached important milestones proposing that convection currents might have driven the plate movements, and that spreading may have occurred below the sea within the oceanic crust. Concepts close to the elements now incorporated in plate tectonics were proposed by geophysisists and geologists (both fixists and mobilists) like Vening-Meinesz, Holmes, and Umbgrove.

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 the mid–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 location 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". During the late 1950s in was shown with success that these data could show the validity of continental drift in two occasions: by Keith Runcorn in a paper in 1956, and by Warren Carey in a symposium held in March 1956.

The second piece of evidence in support of continental drift came during the late 1950s and early 60s from data on the bathymetry of the deep ocean floors and the nature of the oceanic crust such as magnetic properties and, more generally, with the development of marine geology (see e.g. the milestone paper of Lyman & Fleming 1940) which gave evidence for the association of seafloor spreading along the mid-oceanic ridges and magnetic field reversals, published between 1959 and 1963 by Heezen, Dietz, Hess, Mason, Vine & Matthews, and Morley (Korgen 1995; Spiess & Kuperman 2003).

Simultaneous advances in early seismic imaging techniques in and around Wadati-Benioff zones along the trenches bounding many continental margins, together with many other geophysical (e.g. gravimetric) and geological observations, showed how the oceanic crust could disappear into the mantle, providing the mechanism to balance the extension of the ocean basins with shortening along its margins.

All these evidences, both from the ocean floor and from the continental margins made clear around 1965 that continental drift was feasible and the theory of plate tectonics, which was defined in a series of papers between 1965 and 1967, was born, with all its 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

Further information: Continental drift

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 basin development and mountain ranges could be explained by vertical crustal movement, described in what is called the geosynclinal theory. Generally, this was placed in the context of a contracting planet Earth due to heat loss in the course of a relatively short geological time.

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 (Kious & Tilling 1996).

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 accept (Frankel 1987).

The discovery of radioactivity and its associated heating properties in 1895 prompted a re-examination of the apparent age of the Earth (Joly 1909) since this had previously been estimated by its cooling rate and assumption the Earth's surface radiated like a black body (Thomson 1863).

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.

By 1915, after having published a first article in 1912 (Wegener 1912). Alfred Wegener was making serious arguments for the idea of continental drift in the first edition of The Origin of Continents and Oceans. In that book (re-issued in four successive editions up to the final one in 1936), 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 just to mention a few), 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 margins 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 - paleomagnetism - seismicity zones

As it was observed early that although granite existed on continents, seafloor seemed to be composed of denser basalt, the prevailing concept during the first half of the twentieth century was that there were two types of crust, named "sial" (continental type crust), and "sima" (oceanic type crust). Furthermore, it was supposed that a static shells of strata was present under the continents. It therefore looked apparent that a layer of basalt (sial) underlies the continental rocks.

However, based upon abnormalities in plumb line deflection by the Andes in Peru, Pierre Bouguer had 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. Therefore, 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 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) 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.

Meanwhile, debates developed around the phenomena of polar Wander (see apparent polar wander). Since the early debates of continental drift, scientists had discussed and used evidence that polar drift had occurred due to the fact that continents seemed to have moved through different climatic zones during the past. Furthermore, paleomagnetic data had shown that the magnetic pole had also shifted during time. Reasoning in an opposite way, the continents might have shifted and rotated, while the pole remained relatively fixed. The first time the evidence of magnetic polar wander was used to support the movements of continents was in a paper by Keith Runcorn in 1956, and successive papers by him and his students Ted Irving (who was actually the first to be convinced of the fact that paleomagnetism supported continental drift) and Ken Creer.

This was immediately followed by a symposium in Tasmania in March 1956 (Carey 1956; see also Quilty 2003). In this symposium, the evidence was used in the theory of an expansion of the global crust. In this hypothesis the shifting of the continents can be simply explained by a large increase in size of the Earth since its formation. However, this 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; other work would soon show that the evidence was equally in support of continental drift on a globe with a stable radius.

During the thirties up to the late fifties, 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, in which concepts close or near identical to modern plate tectonics theory where defined and outlined. The most important milestone was reached when the English geologist Arthur Holmes proposed in 1920 that plate junctions might lie beneath the sea, and in 1928 that convection currents within the mantle might be the driving force (Holmes 1928; see also Holmes 1978; Frankel 1978).

Often, all these milestones are forgotten for various reasons:

1. During this timespan, continental drift was not accepted.
2. Some of these ideas 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 extreme 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 or given little credit in the papers published by the American researchers which during the 1960s presented evidence for sea floor spreading.

Mid oceanic ridge spreading and convection

Further information: Seafloor spreading

Further information: Mid-ocean_ridge

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 (Lippsett 2001 ; Lippsett 2006 ).

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 all along the globe, a system of mid-oceanic ridges was detected. An important conclusion was that along this system, new ocean floor was being created, which led to the concept of the "Great Global Rift". This was described in the crucial paper of Bruce Heezen (1960) 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 so-called "expanding Earth" hypothesis of S. Warren Carey (see above). So, 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 during the 1930s up until 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 Heezen 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 trenches – 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 had decades before with the only difference that the thinning of the ocean crust was performed using the mechanism of Heezen of spreading along the ridges. Hess therefore concluded that the 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, as 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

Further information: Vine-Matthews-Morley_hypothesis

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: 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 Ron 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 (Mason & Raff 1961); (Raff & Mason 1961).

The discovery of magnetic striping called for an explanation. In the early 1960s scientists such as Heezen, 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 (see the previous paragraph). 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 were published, the link between sea floor spreading and these patterns was correctly placed, independently by Lawrence Morley, and by Fred Vine and Drummond Matthews, in 1963 (Vine & Matthews 1963) now called the Vine-Matthews-Morley hypothesis. This hypothesis linked these patterns to geomagnetic reversals and was supported by several lines of evidence (see summary in Heirzler, Le Pichon & Baron 1966):

1. the stripes are symmetrical around the crests of the mid-ocean ridges; 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 alternate in magnetic polarity (normal-reversed-normal, etc.), suggesting that they were formed during different epochs documenting the (already known from independent studies) normal and reversal episodes of the Earth's magnetic field.

By explaining both the zebra-like magnetic striping and the construction of the mid-ocean ridge system, the seafloor spreading hypothesis (SFS) 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. Nowadays, extensive studies are dedicated to the calibration of the normal-reversal patterns in the oceanic crust on one hand and known timescales derived from the dating of basalt layers in sedimentary sequences (magnetostratigraphy) on the other, to arrive at estimates of past spreading rates and plate reconstructions.

Definition and refining of the theory - from new global tectonics to plate tectonics

After all these considerations, Plate Tectonics (or, as it was initially called "New Global 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) 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 (Wilson 1965).
• 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, Bullard & Runcorn (1965). 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 known 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" (Wilson 1966).
• 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 (Morgan 1968).
• 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 (Le Pichon 1967).
• In the same year, McKenzie and Parker independently presented a model similar to Morgan's using translations and rotations on a sphere to define the plate motions (McKenzie & Parker 1967).

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 (Moss & Wilson 1998). In particular, it explains the Gondwanan distribution of ratites and the Antarctic flora.

Plate reconstruction

Main article: Plate reconstruction

File:TectonicReconstructionGlobal2.gif

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 (Condie 1997). Past plate boundaries within existing plates are identified from evidence of vanished oceans, such as ophiolites (Lliboutry 2000).

Past plate motions

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).

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. Magnetic stripe patterns provide a reliable guide to relative plate motions going back into the Jurassic period (see url ). The tracks of hotspots give absolute reconstructions but these are only available back to the Cretaceous (Torsvik 2008). 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 (Butler 1992). Additional evidence comes from the distribution of certain sedimentary rock types (see url ), faunal provinces shown by particular fossil groups, and the position of orogenic belts (Torsvik 2008).

Current plates

Main article: List of tectonic plates

Plate tectonics map

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

Current Motion

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

The current motion of the tectonic plates is nowadays revealed from remote sensing satellite data sets, calibrated with ground station measurements.

Plate tectonics on other celestial bodies (Planets, Moons)

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 (Valencia, O'Connell & Sasselov 2007)

(Silica and water form a deep eutectic.)

Venus

See also: Geology of 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 c. 500 to 750 Ma, although ages of up to c. 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 (Kasting 1988). 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

See also: Geology of 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 (Sleep 1994). 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 (Zhong & Zuber 2001) or by a giant impact that excavated the Northern Lowlands (Andrews-Hanna, Zuber & Banerdt 2008).

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 (Connerney et al. 1999, Connerney et al. 2005)). 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 (Harrison 2000).

Galilean satellites of Jupiter

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, moon of Saturn

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 (Soderblom et al. 2007).

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, but on larger super-earths plate tectonics is very likely even if the planet is dry (Valencia, O'Connell & Sasselov 2007).

See also

These are links to other Wikipedia Pages with related arguments. See for others in the links in the text itself

• Geosyncline theory An obsolete model for the 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
• 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

• Geological history of Earth The history of the Earth in geological terms

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

• Tectonophysics The science occupied with the quantitative analyses of geodynamic processes

Further reading

Listed in chronological order

• Sverdrup, H. U., Johnson, M. W. and Fleming, R. H. (1942). The Oceans: Their physics, chemistry and general biology. Englewood Cliffs: Prentice-Hall. p. 1087.{{cite book}}: CS1 maint: multiple names: authors list (link) (Sverdrup, Johnson & Fleming 1942)
• Thompson, Graham R. and Turk, Jonathan (1991). Modern Physical Geology. Saunders College Publishing.{{cite book}}: CS1 maint: multiple names: authors list (link) (Thompson & Turk 1991)
• Stanley, Steven M. (1999). Earth System History. W.H. Freeman. pp. 211–228. (Stanley 1999)
• Schubert, Gerald; Turcotte, Donald L.; Olson, Peter (2001). Mantle Convection in the Earth and Planets. Cambridge: Cambridge University Press. (Schubert, Turcotte & Olson 2001)
• Oreskes, Naomi, ed. (2003). Plate Tectonics: An Insider's History of the Modern Theory of the Earth. Westview. (Oreskes 2003)
• Winchester, Simon (2003). Krakatoa: The Day the World Exploded: August 27, 1883. HarperCollins. (Winchester 2003)
• McKnight, Tom (2004). Geographica: The complete illustrated Atlas of the world. New York: Barnes and Noble Books. (McKnight 2004)

References

Notes

References to Books, Articles and Web sites, in order of appearance in the text

Cited Books

Listed in alphabetical order

• Butler, Robert F. (1992). "Applications to paleogeography". Paleomagnetism: Magnetic domains to geologic terranes (PDF). Blackwell. ISBN 086542070X. Retrieved 18 June 2010.{{cite book}}: CS1 maint: ref duplicates default (link)
• Carey, S. W. (1958). "The tectonic approach to continental drift". In Carey, S.W. (ed.). Continental Drift—A symposium, held in March 1956. Hobart: Univ. of Tasmania. pp. 177–363. Expanding Earth from p. 311 to p. 349.
• Condie, K.C. (1997). Plate tectonics and crustal evolution (4 ed.). Butterworth-Heinemann. p. 282. ISBN 9780750633864. Retrieved 2010-06-18.{{cite book}}: CS1 maint: ref duplicates default (link)
• Frankel, H. (1987). "The Continental Drift Debate". In H.T. Engelhardt Jr and A.L. Caplan (ed.). Scientific Controversies: Case Solutions in the resolution and closure of disputes in science and technology. Cambridge University Press. ISBN 9780521275606.{{cite book}}: CS1 maint: ref duplicates default (link)
• Hancock, Paul L.; Skinner, Brian J.; Dineley, David L. (2000). The Oxford Companion to The Earth. Oxford University Press. ISBN 0198540396.{{cite book}}: CS1 maint: ref duplicates default (link)
• Hess, H. H. (1962). "History of Ocean Basins". In A. E. J. Engel, Harold L. James, and B. F. Leonard (ed.). Petrologic studies: a volume to honor of A. F. Buddington. Boulder, CO: Geological Society of America. pp. 599–620. {{cite book}}: External link in |chapterurl= (help); Unknown parameter |chapterurl= ignored (|chapter-url= suggested) (help); Unknown parameter |month= ignored (help)CS1 maint: multiple names: editors list (link) CS1 maint: ref duplicates default (link)
• Holmes, Arthur (1978). Principles of Physical Geology (3 ed.). Wiley. pp. 640–641. ISBN 0471072516.{{cite book}}: CS1 maint: ref duplicates default (link)
• Joly, John (1909). Radioactivity and Geology: An Account of the Influence of Radioactive Energy on Terrestrial History. London: Archibald Constable. p. 36. ISBN 1402135777.{{cite book}}: CS1 maint: ref duplicates default (link)
• Kious, W. Jacquelyne; Tilling, Robert I. (2001) [1996]. "Historical perspective". This Dynamic Earth: the Story of Plate Tectonics (Online ed.). U.S. Geological Survey. ISBN 0160482208. Retrieved 2008-01-29. Abraham Ortelius in his work Thesaurus Geographicus... suggested that the Americas were 'torn away from Europe and Africa... by earthquakes and floods... The vestiges of the rupture reveal themselves, if someone brings forward a map of the world and considers carefully the coasts of the three [continents].' {{cite book}}: External link in |chapterurl= (help); Unknown parameter |chapterurl= ignored (|chapter-url= suggested) (help); Unknown parameter |month= ignored (help)
• Lippsett, Laurence (2006). "Maurice Ewing and the Lamont-Doherty Earth Observatory". In William Theodore De Bary, Jerry Kisslinger and Tom Mathewson (ed.). Living Legacies at Columbia. Columbia University Press. pp. 277–297. ISBN 0-231-13884-9. Retrieved 2010-06-22.{{cite book}}: CS1 maint: ref duplicates default (link)
• Little, W.; Fowler, H.W.; Coulson, J. (1990). Onions C.T. (ed.). The Shorter Oxford English Dictionary: on historical principles. Vol. II (3 ed.). Clarendon Press. ISBN 9780198611264.{{cite book}}: CS1 maint: ref duplicates default (link)
• Lliboutry, L. (2000). Quantitative geophysics and geology. Springer. p. 480. ISBN 9781852331153. Retrieved 2010-06-18.{{cite book}}: CS1 maint: ref duplicates default (link)
• McKnight, Tom (2004). Geographica: The complete illustrated Atlas of the world. New York: Barnes and Noble Books. ISBN 076075974X.

• Meissner, Rolf (2002). The Little Book of Planet Earth. New York: Copernicus Books. p. 202. ISBN 978-0-387-95258-1.{{cite book}}: CS1 maint: ref duplicates default (link)
• Moss, S.J.; Wilson, M.E.J. (1998). "Biogeographic implications from the Tertiary palaeogeographic evolution of Sulawesi and Borneo". In Hall R, Holloway JD (eds) (ed.). Biogeography and Geological Evolution of SE Asia. Leiden, The Netherlands: Backhuys. pp. 133–163. ISBN 9073348978. {{cite book}}: |access-date= requires |url= (help); |editor= has generic name (help); |format= requires |url= (help); External link in |chapterurl= (help); Unknown parameter |chapterurl= ignored (|chapter-url= suggested) (help)CS1 maint: ref duplicates default (link)
• Read, Herbert Harold; Watson, Janet (1975). Introduction to Geology. New York: Halsted. pp. 13–15. ISBN 9780470711651. OCLC 317775677.{{cite book}}: CS1 maint: ref duplicates default (link)
• Schmidt, Victor A.; Harbert, William (1998). "The Living Machine: Plate Tectonics". Planet Earth and the New Geosciences (3 ed.). p. 442. ISBN 0787242969. Retrieved 2008-01-28. {{cite book}}: External link in |chapterurl= (help); Unknown parameter |chapterurl= ignored (|chapter-url= suggested) (help)CS1 maint: ref duplicates default (link)
• Smoot, N. Christian; Choi, Dong R.; Meyerhoff, Arthur Augustus; Bhat, Mohammad I.; Morris, A. E. L.; Agocs, W. B.; Kamen-Kaye, M.; Taner, I. (1996). Surge tectonics: a new hypothesis of global geodynamics. Solid-State Science and Technology Library. Springer Netherlands. p. 348. ISBN 978-0792341567. {{cite book}}: |first9= missing |last9= (help)CS1 maint: ref duplicates default (link)
• Oreskes, Naomi, ed. (2003). Plate Tectonics: An Insider's History of the Modern Theory of the Earth. Westview. ISBN 0813341329.
• Stanley, Steven M. (1999). Earth System History. W.H. Freeman. pp. 211–228. ISBN 0716728826.
• Sverdrup, H. U., Johnson, M. W. and Fleming, R. H. (1942). The Oceans: Their physics, chemistry and general biology. Englewood Cliffs: Prentice-Hall. p. 1087.{{cite book}}: CS1 maint: multiple names: authors list (link)
• Torsvik, Trond Helge; Steinberger, Bernhard (2006). "Fra kontinentaldrift til manteldynamikk". Geo (in Norwegian). 8: 20–30. Retrieved 22 June 2010. {{cite journal}}: Unknown parameter |month= ignored (help); Unknown parameter |trans_title= ignored (|trans-title= suggested) (help), translation: Torsvik, Trond Helge; Steinberger, Bernhard (2008). "From Continental Drift to Mantle Dynamics". In Trond Slagstad and Rolv Dahl Gråsteinen (ed.). Geology for Society for 150 years - The Legacy after Kjerulf. Vol. 12. Trondheim: Norges Geologiske Undersokelse. pp. 24–38 [Norwegian Geological Survey, Popular Science]. {{cite book}}: |access-date= requires |url= (help); External link in |chapterurl= (help); Unknown parameter |chapterurl= ignored (|chapter-url= suggested) (help)CS1 maint: postscript (link)
• Schubert, Gerald; Turcotte, Donald L.; Olson, Peter (2001). Mantle Convection in the Earth and Planets. Cambridge: Cambridge University Press. ISBN 052135367X.{{cite book}}: CS1 maint: ref duplicates default (link)
• Thompson, Graham R. and Turk, Jonathan (1991). Modern Physical Geology. Saunders College Publishing. ISBN 0030253985.{{cite book}}: CS1 maint: multiple names: authors list (link)
• Turcotte, D.L.; Schubert, G. (2002). "Plate Tectonics". Geodynamics (2 ed.). Cambridge University Press. pp. 1–21. ISBN 0-521-66186-2.{{cite book}}: CS1 maint: ref duplicates default (link)
• Wegener, Alfred (1929). Die Entstehung der Kontinente und Ozeane (4 ed.). Braunschweig: Friedrich Vieweg & Sohn Akt. Ges. ISBN 3443010563.{{cite book}}: CS1 maint: ref duplicates default (link)
• Wegener, Alfred (1966). The origin of continents and oceans. Biram John (translator). Courier Dover. p. 246. ISBN 0486617084.{{cite book}}: CS1 maint: ref duplicates default (link)
• Winchester, Simon (2003). Krakatoa: The Day the World Exploded: August 27, 1883. HarperCollins. ISBN 0066212855.

Cited Articles

Listed in alphabetical order

• Andrews-Hanna, Jeffrey C.; Zuber, Maria T.; Banerdt, W. Bruce (2008). "The Borealis basin and the origin of the martian crustal dichotomy". Nature. 453 (7199): 1212. doi:10.1038/nature07011. PMID 18580944.{{cite journal}}: CS1 maint: ref duplicates default (link)
• Blacket, P.M.S.; Bullard, E.; Runcorn, S.K., eds. (1965). A Symposium on Continental Drift, held in 28 October 1965. Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences. Vol. 258. The Royal Society of London. p. 323.{{cite book}}: CS1 maint: ref duplicates default (link)
• Bostrom, R.C. (31 December 1971). "Westward displacement of the lithosphere". Nature. 234: 536–538. doi:10.1038/234536a0.{{cite journal}}: CS1 maint: ref duplicates default (link)
• Connerney, J.E.P.; Acuña, M.H.; Wasilewski, P.J.; Ness, N.F. (1999). "Magnetic Lineations in the Ancient Crust of Mars". Science. 284 (5415): 794–798. doi:10.1126/science.284.5415.794. PMID 10221909. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)CS1 maint: ref duplicates default (link)
• Connerney, J.E.P.; Acuña, M.H.; Ness, N.F.; Kletetschka, G. (2005). "Tectonic implications of Mars crustal magnetism". Proceedings of the National Academy of Sciences. 102 (42): 14970–14975. doi:10.1073/pnas.0507469102. PMC 1250232. PMID 16217034. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)CS1 maint: ref duplicates default (link)
• Conrad, Clinton P.; Lithgow-Bertelloni, Carolina (2002). "How Mantle Slabs Drive Plate Tectonics". Science. 298 (5591): L45. doi:10.1126/science.1074161. PMID 12364804.{{cite journal}}: CS1 maint: ref duplicates default (link)
• Dietz, Robert S. (1961). "Continent and Ocean Basin Evolution by Spreading of the Sea Floor". Nature. 190: 854–857. doi:10.1038/190854a0. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: ref duplicates default (link)
• van Dijk, Janpieter; Okkes, F.W. Mark (1990). "The analysis of shear zones in Calabria; implications for the geodynamics of the Central Mediterranean". Rivista Italiana di Paleontologia e Stratigrafia. 96 (2–3): 241–270. ISSN 0035-6883.{{cite journal}}: CS1 maint: ref duplicates default (link)
• van Dijk, J.P.; Okkes, F.W.M. (1991). "Neogene tectonostratigraphy and kinematics of Calabrian Basins: implications for the geodynamics of the Central Mediterranean". Tectonophysics. 196: 23–60. doi:10.1016/0040-1951(91)90288-4.{{cite journal}}: CS1 maint: ref duplicates default (link)
• van Dijk, Janpieter (1992). "Late Neogene fore-arc basin evolution in the Calabrian Arc (Central Mediterranean). Tectonic sequence stratigraphy and dynamic geohistory. With special reference to the geology of Central Calabria". Geologica Ultrajectina. 92: 288.{{cite journal}}: CS1 maint: ref duplicates default (link)
• Frankel, Henry (1978). "Arthur Holmes and continental drift". The British Journal for the History of Science. 11 (2): 130–150. doi:10.1017/S0007087400016551. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: ref duplicates default (link)
• Harrison, C.G.A. (2000). "Questions About Magnetic Lineations in the Ancient Crust of Mars". Science. 287: 547a. doi:10.1126/science.287.5453.547a.{{cite journal}}: CS1 maint: ref duplicates default (link)
• Heezen, B. (1960). "The rift in the ocean floor". Scientific American. 203: 98–110. doi:10.1038/scientificamerican1060-98.{{cite journal}}: CS1 maint: ref duplicates default (link)
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External links

• Web site dedicated to plate tectonics.
• This Dynamic Earth: The Story of Plate Tectonics. USGS.
• The PLATES Project. Jackson School of Geosciences.
• Paleomap Project, Christopher Scotese.
• 750 million years of global tectonic activity. Movie.
• An explanation of tectonic forces. Example of calculations to show that Earth Rotation could be a driving force.
• Bird, P. (2003); An updated digital model of plate boundaries.
• Map of tectonic plates.
• Understanding Plate Tectonics. USGS.
• The geodynamics of the North-American-Eurasian-African plate boundaries.
• Huang Zhen Shao (1997). "Speed of the Continental Plates". The Physics Factbook.
• GPlates, desktop software for the interactive visualisation of plate-tectonics.
• MORVEL plate velocity estimates and information. C. DeMets, D. Argus, & R. Gordon.