Plate Tectonics. The Plates

When the theory of continental drift was joined with the idea of seafloor spreading, it led to the formation of the integrated concept of crustal movement known as plate tectonics. Plate tectonics views the lithosphere as a series of lithospheric slabs, or plates, each made up of continental or oceanic crust, or some of each, with its attached rigid mantle material. The plates are outlined by the major earthquake belts of the world (refer to fig. 2.9); this relation was first pointed out in 1965 by J. T. Wilson, a geophysicist at the University of Toronto.

Seven major plates are recognized— the Pacific, Eurasian, African, Australian, North American, South American, and Antarctic—as well as smaller plates off South and Central America, in the Mediterranean area, and along the northwestern United States, for a total of about twelve plates (fig. 2.19).

Fig. 2.19. Major lithospheric plates of the world. Arrows indicate direction of plate motion. The new plate boundary separates India from the Australian Plate

In 1970 a zone of compressed sea floor running in an east-west direction was detected in the Indian Ocean. In 1990, after further study, it was proposed that this zone represented a new plate boundary. More recent acoustical measurements of the sea floor in this area have confirmed the plate boundary and the presence of a thirteenth plate.

The Plates. The plate boundaries are trenches, ridges, or faults. The direction of plate motion at each boundary depends on whether lithospheric plates move together, move apart, or slide past each other. Plate boundaries diverge or move apart at the mid-ocean ridges where new lithosphere is formed; these are known as divergent plate boundaries. At the trenches, plates converge, or move toward each other, destroying old lithosphere at subduction zones and forming convergent plate boundaries. Convergent boundaries also form where landmasses collide.

Plates move past each other along the faults, where there is a break in the rocky crust with displacement of one side relative to the other. There are many types of faults; in some the movement is vertical, in some horizontal, and in some oblique. Close inspection of the mid-ocean ridge and rise system shows sections of the ridge that are offset or displaced laterally from each other along a special kind of fault called a transform fault.

The opposite sides of a transform fault are two different plates that are moving in opposite directions (see area A in fig. 2.20). This movement creates a fault zone that is active and may be the seat of frequent and severe but shallow earthquakes. Where this same fault line extends outside both ridge axes, the crustal plates are moving in the same direction, and differential motion along the fault is much reduced (see area В in fig. 2.20). These regions of the faults are quiet, but the faults can still be detected.

Fig. 2.20. Relative plate motion and displacement of the crust along a fault. Plates at the transform fault, A, move in opposite directions. Plate motion at В is in the same direction

The lateral motion of the ridge along transform faults produces sharp vertical displacements called escarpments across the width of the ridge. These are regions where there are sudden changes in the depth of the ocean. The escarpments and transform faults are boundaries where one piece of crust moves relative to another. Crust is neither created nor destroyed along these faults.

The San Andreas Fault, an excellent example of one of the larger transform faults, extends from the northern end of the East Pacific Rise up through the Gulf of California, to the San Francisco Bay region, and out to sea to the north (fig. 2.21), terminating at the Gorda and Juan de Fuca ridges.

Fig. 2.21. (a) The sunken portion of the fault south of San Francisco and west of Palo Alto is used to store water for both cities use. (b) The San Andreas Fault runs north-south from the Gulf of California through the San Francisco peninsula

The San Andreas Fault system allows crustal sections on either side to move horizontally relative to each other. The land on the Pacific side, including the coastal area from San Francisco to the tip of Baja California, is actively moving northward past the land on the east side of the fault. The motion is not uniform; when stress accumulates, there is a sudden movement and displacement along the fault, causing severe earthquakes. Movement along this fault system caused the famous 1906 and October 1989 (Loma Prieta) earthquakes in the San Francisco Bay area and the 1994 (Northridge) Los Angeles earthquake.

The reason for the many transform faults that are found associated with the ridge system is related to changes in the speed and direction of the plates as they move apart on a spherical surface. Variations in the strength or location of convection cells and collisions between sections of crust may also result in transform faults.