So

Figure 1.4 Distribution of world earthquakes 1961-69 (after National Earthquake Information Center Map NEIC-3005).

plates: constructive margins (oceanic ridges), destructive margins (subduction zones or trenches) and conservative margins (e.g. transform faults). The motion of the plates can be described in terms of a pole of rotation via Euler's 'fixedpoint' theorem, which states that 'the most general displacement of a rigid body over the surface of a sphere can be regarded as a rotation about a suitable axis which passes through the centre of that sphere'. The motion of plates can be determined in a number of ways. These include palaeomagnetic reconstructions, as described by Smith et al. (1981) and available in software packages such as Atlas 3.3. Contemporary motions can also be determined by satellite laser ranging and very-long-baseline interferometry (VLBR) which uses the signals from quasars and terrestrial radio telescopes as receivers. Moreover, by assuming that certain volcanic features were associated with stationary hotspots (generally viewed as being generated by plumes upwelling from the mantle), the absolute plate motions were determined.

It can be seen from Figure 1.7 that plates range widely in size from the small Cocos and Nazca Plates to the huge Pacific Plate. These plates are viewed as being parts of the Earth's rigid lithosphere (discussed in more detail below) which moves over a more plastic (or viscous) deformable asthenosphere. The lithosphere contains the crust (both oceanic and continental) and part of the upper mantle. The areas of the twelve plates and the proportions of lithosphere containing continental crust are given, in alphabetical order, in Table 1.1.

The plate tectonic model is now generally accepted, and is routinely used to describe the evolution of oceanic regions. In addition, with this model, the general distribution of the continents over the past 500 Ma or so can now be inferred with some accuracy. Full and more comprehensive reviews of the foundations of the plate tectonic model can be found in many excellent texts by, for example, Kearey and Vine (1990) and Cox (1988). From these and other works, it is apparent that the plate tectonic model is good at explaining what happens to the Earth's lithosphere. However, it must be noted that, as Molnar and Stock

Figure 1.5 Map of the Kamchatka-Kurile volcanic arc with topographic profiles and a composite seismic profile (after Benioff, 1954).
Figure 1.6 Schematic three-dimensional section diagram showing the major features of sea-floor spreading (after Isaaks et al, 1968).
Figure 1.7 Plate movement vectors relative to fixed hotspot frame of reference (after Forsyth and Uyeda, 1978 and Morgan, 1972).
Table 1.1 Areas of the plates (in millions of km2).

Plate

Total area

Continental area

Africa

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