Origins of Mountains - Mountain Geography: Physical and Human Dimensions - page 31

This conveyor-belt system involves creation of new crust at the divergent mid-ocean
ridges and reincorporation of the plate's leading edge back into the mantle at the con-
vergent subduction zones. The material in between is slowly transported from one point
to another. Continents drift, but they do not, as formerly envisioned, plow across a
viscous sea bottom; rather, they are passive passengers on lithospheric plates moving
outward from oceanic spreading centers. Mechanisms that drive plate tectonics are
thought to be largely due to convective flow in the mantle—in which warm, less dense
rock rises and cooler, more dense material sinks. In this context, ridge-push results from
the elevated position of the oceanic ridge system, so that oceanic lithosphere slides
gravitationally down the flanks of the ridge, and slab-pull occurs where old oceanic
crust, which is relatively cool and dense, sinks into the mantle and “pulls” the trailing
lithosphere along. Slab-suction occurs at a plate margin where the descending plate
pulls at the upper plate, and as a result the subduction zone can migrate in the direction
opposite that of the lower plate's movement. The suction force pulling on the overriding
lithosphere can move it toward the trench, or even rift it apart.
The lithospheric plate system consists of twelve major plates. Six of the twelve are
“great plates.” The remaining six are intermediate to comparatively small. The great
Pacific Plate occupies most of the Pacific Ocean basin and consists almost entirely of
oceanic lithosphere. Its relative motion is northwesterly, so that it has convergent sub-
duction zones along much of its northern and western edges, and huge island arcs of
volcanoes have developed on the peripheral parts of the plates above the subduction
zones. A spreading boundary occurs along much of its eastern and southern edges. The
North American Plate consists of the continent and the western half of the Atlantic
Ocean; the entire unit is moving to the west, where it is colliding with the Pacific Plate.
The eastern side of the Atlantic Ocean basin is part of the vast Eurasian Plate, which
is generally moving east and colliding with the western edge of the Pacific Plate. Thus
the Atlantic Ocean basin is opening and the Pacific Ocean basin is closing. Because con-
tinental crust is composed of low-density rock and is more buoyant than oceanic crust,
it cannot be subducted. Consequently, the Pacific Plate, which is composed of oceanic
crust, is descending beneath the North American and Eurasian Plates, which carry con-
tinental crust on them, and it is undergoing subduction in the deep-sea trenches that
occur on its margin—the Aleutian, Kuril, Japan, and Marianas trenches (Fig. 2.10).