1.2 Plates, Plate Motions, and Plate Boundaries
By the end of 1967 the Earth’s surface had been mapped into a series of plates. The major plates are Eurasia, Pacific, India, Australia, North America, South America, Africa, and Antarctic. There are also numerous small plates (e.g., Juan de Fuca, Cocos, Nazca, Scotia, Philippine, Caribbean), and many very small plates or sub-plates. 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.
Rates of motions of the major plates range from less than 1 cm/y to over 10 cm/y. The Pacific Plate is the fastest, followed by the Australian and Nazca Plates. 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. 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 explanation is that plates move in a rotational manner. The North American Plate, for example, rotates counter-clockwise; the Eurasian Plate rotates clockwise.
Boundaries between the plates are of three types:
- divergent (i.e., moving apart),
- convergent (i.e., moving together), and
- transform (moving side by side).
Before we talk about processes at plate boundaries, it’s important to point out that there are never gaps between plates. The plates are made up of the lithosphere. The lithosphere consists of the crust, which can be thin, relatively dense oceanic crust or thick, relatively buoyant continental crust, and the rigid uppermost part of the mantle (Figure 1.2.1). Even though tectonic plates are moving all the time, and in different directions, there is never a significant amount of space between them. Plates are thought to move along the lithosphere-asthenosphere boundary, as the asthenosphere is the zone of partial melting. It is assumed that the relative lack of strength of the partial melting zone facilitates the sliding of the lithospheric plates. 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. For example, the North American Plate includes most of North America (continental crust), plus half of the northern Atlantic Ocean (oceanic crust). The Pacific Plate is almost entirely oceanic, but it does include the part of California west of the San Andreas Fault.
Divergent Boundaries
Divergent boundaries are spreading boundaries, where new oceanic crust is created from magma derived from partial melting of the mantle caused by decompression as hot mantle rock from depth is moved toward the surface (Figure 1.2.2). The triangular zone of partial melting near the ridge crest produces oceanic crust that is about 6 km thick. Most divergent boundaries are located at the oceanic ridges, although some are on land.
Convergent Boundaries
Convergent bo