plate tectonics, principles

plate tectonics, principles The theory of plate tectonics describes tectonic events in terms of the motions of a small number of rigid plates which move relative to one another on the surface of the Earth, interacting and deforming only along their common boundaries. Because it provides an overarching, self-consistent, and global view of tectonic activity, its development in the 1960s ushered in a true revolution in the Earth sciences. Previous ad hoc explanations for local and regional events could be seen to be part of a wider, ultimately global, pattern. Moreover, by providing an understanding of the types of tectonic and volcanic activity that are associated with different types of plate boundary, it brought an important predictive power to geology. It was also important in finally providing a physically realistic mechanism to explain the observations of continental drift.

Approximately 12 major plates are recognized, together with a number of smaller ones (Fig. 1). They are outlined by narrow bands of seismic activity, and have almost aseismic interiors (see seismology and plate tectonics). It was this observation, which resulted from the setting up of the World Wide Standard Seismograph Network in the 1960s, that led to the idea that the lithosphere is divided into a number of large, rigid plates that are tectonically active only at their boundaries.

Plate tectonics thus relies on the concept of a rigid lithosphere (see lithosphere). This layer, which is rheologically defined as the strong outer layer of the Earth, generally includes both the crust and the uppermost part of the upper mantle. It is approximately 100 km thick, whereas the major plates are many thousands of kilometres wide; the plates thus behave as thin shells sliding on the surface of the Earth. The asthenosphere (see asthenosphere) is the ductile layer immediately below the lithosphere, and is believed to decouple the plates from the deeper mantle.

An important difference between plate tectonics and earlier ideas of continental drift is that plates may comprise both continental and oceanic lithosphere. One of the earliest objections to continental drift was the absence of a mechanism that would enable the continents to move through what was perceived as a rigid mantle without producing major disruption at the leading and trailing continental edges and in the ocean basins. In plate tectonics, the oceans themselves are part of the plates, moving rigidly with the continents. The motion is facilitated by the presence of the asthenosphere, whose high ductility results from thermally activated creep, a process unknown to the physicists of the early twentieth century.

In its broadest sense, plate tectonics can be thought of as covering two distinct areas: plate kinematics, which restricts itself to describing the movements of plates and the changing geometry of their boundaries, and plate tectonics sensu stricto, which deals with the geological consequences of the creation, destruction, and interactions of plates.

Plate kinematics

As stated above, plate boundaries are primarily defined by the seismicity along their boundaries. However, plate boundaries are also marked by distinctive morphological features, narrow bands of tectonism, and sometimes by individual major faults. The active plate boundary is never more than a few tens of kilometres wide, and may be as narrow as a few kilometres or even less.

Three types of plate boundary are recognized, corresponding to the three types of relative motion (Fig. 2); divergent (also ridge or accretionary) plate boundaries, transform (or conservative) plate boundaries, and convergent (subduction or destructive) plate boundaries. In general, any type of plate boundary can join any other type. Divergent boundaries occur only at mid-ocean ridges, and are the sites of creation of new lithosphere by sea-floor spreading. Transform boundaries are those where plates slide past one another with no change of area; they join one plate boundary to another, and can thus transform one type of tectonic boundary (e.g. extensional) to another (e.g. compressional), hence the name. They are found in both continental and oceanic lithosphere. Convergent plate boundaries in the strict plate-tectonic sense occur only at the deep-sea trenches and related subduction zones.

There are also zones of compressional deformation between converging plates in continents (such as the Alpine–Himalayan zone), but these are much broader than normal plate boundaries. This is partly because continental lithosphere is weaker and more easily deformed than oceanic lithosphere, and partly because it is less dense and is therefore not readily removed by subduction. Plate tectonics does not provide a very useful description of such broad continental convergence zones.

Apart from transform faults, plate tectonic theory does not prescribe the precise directions of relative motion at plate boundaries. However, although small degrees of oblique spreading and subduction are relatively common, strongly oblique motions are rare. In the same way, plate tectonics allows asymmetrical spreading (one plate accreting faster than the other). Although temporary asymmetrical spreading is common, the net effect averaged over millions of years is usually approximately symmetrical. The precise reasons for this are not fully understood.

Outlines of the geological consequences of the different types of plate boundaries are given under their individual entries.

Rotation poles

The great strength of plate kinematics is that it can describe plate motions in terms of simple geometry, and hence make precise predictions of relative motions anywhere on the globe. At the heart of this geometry is the concept embodied in Euler's theorem, which states that any displacement of a rigid body on the surface of a sphere can be described in terms of a single rotation about a specified axis. Such axes cut the Earth's surface at pairs of points called ‘rotation (or Euler) poles’. Once the poles and the angular rotations are specified, the whole motion is completely determined. Thus the motion of a given plate is specified in terms of its Euler pole and a corresponding angular rotation rate.

In practice, determining the motions of individual plates relative to some common reference frame is not easy; however, determining relative motions between pairs of plates is quite straightforward. Such relative motions can also be described in terms of rotation poles (although the pole position must be specified relative to one or other of the plate pair), and such relative rotation poles are the basis of most plate-kinematic descriptions.

Since Euler poles define the directions of the rotation axes, and the angles or rates of rotation define their magnitudes, poles can also be described as vectors. Vector and matrix algebra can then be used to calculate plate motions, greatly simplifying and speeding up such calculations, which can be performed routinely by digital computers.

It is, however, important to distinguish between so-called ‘instantaneous poles’, which describe motion at an instant only, and ‘finite rotation poles’, which may describe the net result of motion over long periods of time. In descriptions of current plate motions, ‘instantaneous’ is usually taken to be about the past 1–3 million years. Motions over longer periods can be approximated by successions of so-called ‘stage poles’, each of which may describe the motion over a period of a few million years.

We can imagine the Euler poles, like the geographic poles, as centres of coordinate grids (Fig. 3). Equivalent to lines of latitude are ‘small circles’ centred on the poles. Plate relative motions are everywhere parallel to these small circles. Moreover, angular separation rates are constant along a given small circle, and are proportional to the sine of the radius of the circle (measured as a geocentric angle from circle to pole). ‘Great circles’ (for which the angular radius is 90°) through the pole are equivalent to lines of longitude, and cut the small circles at right angles.

Measuring plate motions

Plates move at average rates of a few tens of millimetres per year; consequently, relatively indirect methods have generally been used to determine plate motions, although more recently sufficiently precise direct geodetic measurements have been developed. The most common way of determining plate divergent rates is via the linear magnetic anomalies (Vine—Matthews anomalies) produced during sea-floor spreading (see sea-floor spreading). Such anomalies mark isochrons (lines of equal age) of crustal creation which have been dated by a variety of methods. Thus, measuring the distance between a recent magnetic isochron on one plate and its conjugate on the other allows the ‘instantaneous’ divergence rate to be determined. (The commonly quoted ‘spreading rate’ is the rate at which a single plate accretes—for symmetrical spreading it is half the divergence rate.) Since the spreading or divergence rate varies with distance from the Euler pole, in principle one can determine the distance to the pole by measuring the spreading rate at several places along the plate boundary. This distance, combined with the linear divergence rate, gives the angular rate of opening about the pole.

The best measure of the direction of relative plate motion is the azimuth (bearing) of transform faults. At these, the relative motion is purely strike-slip (i.e. along the fault direction). Transform faults are readily recognized by their morphology (for example, a narrow linear valley for ridge—ridge transforms along mid-ocean ridges). Transform faults thus follow small circles about Euler poles. A great circle at right angles to a small circle passes through the pole. Thus, if the azimuths of several transforms along a plate boundary are determined, great circles can be constructed normal (at right angles) to them, and should intersect at the Euler pole.

In practice, both spreading rates and transform azimuths are used together to solve for pole positions, sometimes supplemented by other data such as earthquake focal mechanisms (see seismology and plate tectonics) to estimate relative motion directions. Euler poles may be calculated for individual plate pairs, for groups of plates, or for the global plate system. The determination of global plate motions performed by C. DeMets and others from Northwestern University, Illinois, provided a remarkably precise and self-consistent description of these motions.

As stated above, an important attribute of plate kinematics is its ability to predict plate motions. An interesting example of this is the possibility of determining convergent rates across subduction zones. Even at ocean—ocean subduction zones, one plate is destroyed, together with the record of magnetic lineations carried on it. Thus there was no direct way of measuring such motion until the development of sufficiently precise geodetic methods. However, the relative motions of the plate pair can be determined by global fits as described above, and then the motion at any point on a plate boundary, including subduction zones, can be calculated from the Euler pole data.

Geodetic methods have now been developed to the level where they can be used to measure plate motions directly. Where plate boundaries exist on land (such as the Mid-Atlantic Ridge in Iceland or the San Andreas Fault in California), standard geodetic methods such as electronic distance measurement can be used at a local scale (over ranges of a few kilometres). On a slightly larger scale of tens to hundreds of kilometres, precise relative determinations of position (to precisions of a few millimetres) can be made by careful use of the Global Positioning System satellite network. Relative positions between widely separated continents can be determined by very long baseline interferometry (VLBI), in which the variation in phases of radio signals from distant quasars is used. Repeat measurements by these methods over times of a few years can now resolve plate motions and give results that, in general, agree well with the more traditional determinations. Palaeomagnetism can on occasion also be used to determine rotation rates for rapidly rotating microplates.

The rotation rates of the major plates about their Euler poles range from about 2.1° per million years for the Cocos—Pacific pair to about 0.1° per million years between Africa and Europe or Africa and Antarctica, and only 0.03° per million years for India—Arabia. Many of the minor (or ‘micro’) plates rotate much faster than this, at tens of degrees per million years. In terms of linear rates, the fastest plate-divergence rate at present is on the East Pacific Rise between the Pacific and Nazca plates, at about 160 km per million years (or 160 mm per year). At the slow end, the North America—Eurasia plate boundary passes very close to its Euler pole in northern Siberia, where the relative motion becomes essentially zero.

Absolute plate motions

The discussion so far has dealt only with relative motions, which are fairly easy to determine. There is also interest in determining so-called ‘absolute’ plate motions, in which the motions of all plates are related to some common reference frame. The possibility of doing this arises from the recognition of mantle plumes, which rise as narrow columns of relatively hot rock from deep in the mantle, possibly from the core–mantle boundary. They reach the Earth's surface in so-called ‘hot-spots’ where they are manifest by clusters of intense volcanic and seismic activity. Well-known examples are Iceland and Hawaii, but there are thought to be many tens of such hot-spots and associated plumes.

Hot-spots leave clear trails on the Earth's surface, which comprise lines of volcanoes or volcanic sea mounts and zones of thickened, volcanically produced crust. The Hawaii– Emperor sea-mount chain, trending north-west from Hawaii, is an excellent example, but there are many other sub-parallel sea-mount trails in the south-western Pacific that have resulted from plumes. If points along these trails are dated (e.g. by radiometric dating of volcanic products), the relative motion between the plumes and the plate and between given plumes can be determined. We can also calculate the motions of individual plates relative to the average plume motion.

It turns out that the relative motion between plumes is quite small, and significantly less than average relative motions between plates. From this, and the fact that plumes are thought to rise through the mantle, it seems reasonable to assume that the average plume motion relative to the mantle is quite small. If it is assumed that it is zero, plate motions can be given relative to the mantle, and these are referred to as absolute plate motions. They can also be described in terms of Euler poles, and are shown in Fig. 4.

Mechanisms and driving forces of plate tectonics

Plate motions are ultimately driven by Earth's heat energy, and they are intimately related to the mantle convection that is driven by this heat. Indeed, one view of plates is that they simply represent the surficial parts of mantle convection cells: as hot, ductile mantle rises to the surface it cools and becomes brittle—a plate—then moves as a rigid block over the surface before being subducted, gaining temperature, and becoming ductile again. Results from seismic tomography suggest that around the Pacific rim, sheets of cold material descend below subduction zones deep into the lower mantle, implying a strong coupling of mantle motion and subducted plates.

However, the coupling is not perfect. For example, there are some parts of mid-ocean ridges (divergent plate boundaries) where it seems that the deeper mantle (below the asthenosphere) may be descending rather than rising. One such is the so-called Australo-Antarctic Discordance south of Australia. Also, some plates, such as Africa, are almost entirely surrounded by ridges and have very few subduction zones on their boundaries. In such instances, a rigid coupling of plates to convection cells would imply the unusual scenario of upwelling along an expanding ring, with a downwelling column inside it. In fact one of the advantages of plate tectonics is that it allows partial decoupling of plate motions from deeper mantle flow via the ductile asthenoshphere.

Another way of looking at the problem of the driving mechanism is to consider the forces acting directly on the plates. There are many possible forces, but among the most important are ridge push, slab pull, trench suction, and mantle drag. Ridge push arises from the tendency of the plates on a mid-ocean ridge flank to slide down the slopes of the wedge of thermally expanded asthenosphere that lies beneath the ridge. Slab pull arises from the negative buoyancy of the subducted plate which tends to drag the rest of the plate down with it. Trench suction is an additional force tending to pull plates together at subduction zones, perhaps as a result of local convection driven by the subduction. Mantle drag is the frictional force between the base of the plate and the underlying asthenosphere.

It is possible to estimate some of these forces, at least approximately, and their relative importance can also be estimated by considering the observed stresses in plates and inferred absolute plate velocities. The plate velocities are particularly instructive. It is found that absolute velocities are largely independent of the total area of the plate, and so it is unlikely that mantle drag is an overall driving force, as was once thought: in other words, plates do not ride as passive passengers on top of mantle convection cells. However, plate velocities are inversely correlated with the area of continental lithosphere, which suggests that large areas of continent act as a brake (perhaps because such lithosphere is very thick or because sub-continental asthenosphere is rather viscous). The fastest plates are those (mainly in the Pacific) that have large lengths of subduction zone along their boundaries. This implies that slab pull or trench suction, or both, are important driving forces, as has been suggested by calculation. There is a modest correlation with the effective length of ridge on the plate, indicating (again, as suggested by calculations) that ridge push is a driving force, but less strong than trench pull. Observations of intraplate stresses agree with these conclusions.

Tests of plate tectonics

There have been many tests of plate tectonics theory. Its self-consistency, direct measurements of predicted plate motions, earthquake focal mechanisms, and distribution of earthquakes have all played their part in confirming the theory.

Roger Searle

Bibliography

DeMets, C.,, Gordon, R. G.,, and Argus, D. F. , et al. (1990) Current plate motions. Geophysical Journal International 101, 425–78.
Kearey, P. and and Vine, F. J. (1996) Global tectonics. Blackwell Science, Oxford.