Three main features of plate tectonics help geologists explain the mechanics of plate tectonics:1) The layers of the earth
The earth's surface is covered by a series of brittle plates; below these plates are other layers of the earth including
- the inner and outer coresthe innermost zone of the earth;
- the mantlethe intermediate zone of the earth;
- and the crustthe outermost shell of the earth.
Each of these is shown in the graphic below.
The crust and the uppermost part of the mantle lie within an environment of low temperature and pressure within which rocks are brittle when exposed to stress. This layer is called the lithosphere, the outermost brittle layer of the earth. Starting at the bottom of the lithosphere and extending to depths of approximately 300 km (180 miles) or so is a region of high temperature and intermediate pressure, the asthenosphere. The rocks of the asthenosphere are partly (1 to 10 percent) molten and otherwise hot enough to behave plastically when stressed.
According to plate tectonic theorists, the continents and ocean basins occupy lithospheric plates that actually float on top of the asthenosphere. The continental lithosphere is, on average, about 100 kilometers (60 miles) thick, and the oceanic lithosphere is, on average, about 50 kilometers (30 miles) thick.2) Lithospheric plate boundaries
Along their edges, the lithospheric plates interact forcefully, by either moving apart, colliding, or grinding by each other. These plate boundaries are sites of intense geologic activity, such as
- and mountain building.
The plates under the ocean floors are continually moving. They spread from the center, sink at the edges, and are regenerated from lava that comes up through cracks in the earth's crust. This continual recycling of the oceanic lithosphere explains why, although the continents contain rocks up to 4 billion years old, the rocks of the ocean basins are everywhere less than about 200 million years old.3) Continental drift and sea-floor spreading
Plate tectonics is a combination of two earlier ideas, continental drift and sea-floor spreading. Continental drift is the movement of continents over the earth's surface. Continental drift also describes the change in position of the continents in relation to each other. Sea-floor spreading is the creation of new oceanic crust at mid-ocean ridges and the movement of the crust away from the mid-ocean ridges. The sea floor spreads, and new ocean floor is created by magma that seeps from the mantle into the crust at mid-ocean ridges.