The Motion of the Plates. Mechanisms of Motion. Rates of Motion

Mechanisms of Motion. The mechanism that drives the plates apart is still not fully understood. The plates could be forced apart at the ridges by the formation of new crust, moved sideways, and then thrust downward at the trenches. Another possibility is that the thick, dense, oceanic crust and its sediment load sink into the mantle and the resulting tension drags the remainder of the plate with it.

This latter mechanism could result in tension cracks through which the mantle material escapes upward to form the ridges and new crust. In this theory the weight of the dense, cold, descending slab of oceanic lithosphere pulls the remainder of the plate with it, and the friction between the moving plate and the asthenosphere helps to drive the convection cell rather than the reverse.

New computer models of this “slab pull” theory lead some researchers to suggest that the fracturing and sinking of old ocean plates can drive plate tectonics without the forces of convection cells driving seafloor spreading. The fact that the speed of a plate seems to increase as the amount of subducting lithosphere along the edge of the plate increases further supports theories that slab pull is an important driving force.

The situation is probably more complex, combining both mechanisms. Other factors that may be involved include the shape of the Earth, changes in the weight of the plates resulting from land erosion or the accumulation of sediments, the rate at which magma wells up into the ridges, and the thickening of the older lithosphere as it cools.

Rates of Motion. Acting like a conveyor belt, the sea floor moves away from the ridge system at rates typically between 2 and 10 cm (0.8-4 in)/year. Spreading rates vary between 1 and 24 cm (0.4-9.4 in)/year and average about 6 cm (2.4 in)/year. Old crust disappears into the trenches at a comparable rate. Although the rates are small by everyday standards, they produce large changes over geologic time.

For example, at the rate of just 1.6 cm (0.6 in) per year it takes 100,000 years to move 1.6 km, or 1 mi; therefore, in the 200 million years since the breakup of Pangaea the crust could have moved more than 3200 km, or 2000 mi, which is more than half the distance between Africa and South America. Spreading rates vary in magnitude and with time. A ridge system with a steep profile (the Mid-Atlantic Ridge) has a slower spreading rate than a ridge system with less-steep sides (the East Pacific Rise). Spreading rates are estimated at 3 cm (1 in) per year for the Mid-Atlantic Ridge and at 6-17 cm (2-6 in) per year for the East Pacific Rise.

The largest spreading velocity known is 24 cm (9.4 in) per year along a portion of the boundary between the Pacific and Australian plates near Samoa. Keep in mind that the process of spreading does not occur smoothly and continuously but goes on in fits and starts, with varying time periods between occurrences. Compare the width of the age band centered on the East Pacific Rise with the width of the band centered on the Mid-Atlantic Ridge in figure 2.18; the wider the age band, the faster the spreading rate.

Although seafloor spreading can be observed directly only with great expense and difficulty at sea, there is one place where many of the processes can be seen on land—in Iceland, the only large island lying across a mid-ocean ridge and rift zone. Spreading in Iceland occurs at rates similar to those found at the crest of the Mid-Atlantic Ridge. Northeastern Iceland had been quiet for 100 years, until volcanic activity began in 1975; in six years this activity widened by 5 m (17 ft) an 80-km (50-mi)-long stretch of the ridge’s rift zone.

Over 100 years this spreading rate is 5 cm (2 in) per year, which is within the typical range of spreading rates. The spreading motion between the Earth’s surface plates can now be monitored by satellite. The satellite system known as GPS for Global Positioning System (see chapter 1 box and section 1.6) detects the positions of land control points with great accuracy. Recent GPS measurements in the eastern Mediterranean Sea indicate that the African plate is moving northward at 10 mm per year (0.4 in/yr).

Recent investigations of ancient granitic rocks, formed during the Archean Eon and found exposed in West Greenland, eastern Labrador, Wyoming, western Australia, and southern Africa, indicate that 3.5 billion years ago crustal plates existed and moved granitic continental blocks at an average rate of about 1.7 cm (0.67 in) per year, again within today’s average rates of plate motion.