The fact that the plates include both crustal material and lithospheric mantle material makes it possible for a single plate to be made up of both oceanic and continental crust. Tectonic plates consist of lithosphere, which includes the crust and the lithospheric (rigid) part of the mantle.Īt spreading centres, the lithospheric mantle may be very thin because the upward convective motion of hot mantle material generates temperatures that are too high for the existence of a significant thickness of rigid lithosphere (Figure 10.3.8). Figure 10.4.2 The crust and upper mantle. It is assumed that the relative lack of strength of the partial melting zone facilitates the sliding of the lithospheric plates. Plates are thought to move along the lithosphere-asthenosphere boundary, as the asthenosphere is the zone of partial melting. The plates are made up of crust and the lithospheric part of the mantle (Figure 10.4.2), and even though they are moving all the time, and in different directions, there is never a significant amount of space between them. Before we talk about processes at plate boundaries, it’s important to point out that there are never gaps between plates. The North American Plate, for example, rotates counter-clockwise the Eurasian Plate rotates clockwise.īoundaries between the plates are of three types: divergent (i.e., moving apart), convergent (i.e., moving together), and transform (moving side by side). The explanation is that plates move in a rotational manner. Plates move as rigid bodies, so it may seem surprising that the North American Plate can be moving at different rates in different places. The North American Plate is one of the slowest, averaging around 1 cm/y in the south up to almost 4 cm/y in the north. The Pacific Plate is the fastest, followed by the Australian and Nazca Plates. Rates of motions of the major plates range from less than 1 cm/y to over 10 cm/y. Figure 10.4.1 A map showing 15 of the Earth’s tectonic plates and the approximate rates and directions of plate motions. For example the Juan de Fuca Plate is actually three separate plates (Gorda, Juan de Fuca, and Explorer) that all move in the same general direction but at slightly different rates. There are also numerous small plates (e.g., Juan de Fuca, Cocos, Nazca, Scotia, Philippine, Caribbean), and many very small plates or sub-plates. The major plates are Eurasia, Pacific, India, Australia, North America, South America, Africa, and Antarctic. By the end of 1967 the Earth’s surface had been mapped into a series of plates (Figure 10.4.1). 10.4 Plate, Plate Motions, and Plate Boundary ProcessesĬontinental drift and sea-floor spreading became widely accepted around 1965 as more and more geologists started thinking in these terms.

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