ZingPath: Plate Tectonics and Earth's Dynamics

The Earth's interior is frequently divided into crust, mantle, and core, based on compositional differences between these layers. Another method of describing the Earth's outer layers depends on the structural properties of the layers. With this approach, the outermost part of the Earth is rigid and brittle. This layer is called the lithosphere, and it consists of the crust and the uppermost part of the mantle. Below the lithosphere is a relatively soft layer called the asthenosphere. Although the asthenosphere is still mostly solid, it is capable of flowing slowly through internal deformation. The theory of plate tectonics states that the Earth's lithosphere is broken into large, rigid pieces called plates, which float and move on the underlying asthenosphere. Most major geologic features on the Earth, such as fold mountain belts, ocean trenches, volcanic arcs, and rift valleys, are the result of movements of the plates that cover the Earth's surface.

The lithosphere, which underlies the Earth's ocean basins, contains a crust that is relatively thin (approximately 7-10 km) and fairly dense, allowing oceanic lithosphere to sink into the asthenosphere without much difficulty. Thus, as long as at least one of the plates at a convergent plate boundary comprises oceanic lithosphere, subduction will occur.

Continental crust, on the other hand, is much thicker (averaging approximately 40 km, and as much as 70 km in places) and much less dense. As a result, it is too buoyant to sink into the asthenosphere. At convergent boundaries where both plates include continental crust, neither plate is able to move out of the way, and a collision occurs. As a result of such collisions, the lithosphere breaks, folds, and buckles, lifting up large mountain ranges.

Other than the Himalayas, this processes is also responsible for forming the Alps and the Zagros Mountains of Iran. Much earlier continental collisions formed the Appalachian and Atlas Mountains (due to a collision between North America and Africa) and the Ural Mountains (due to a collision between Europe and Asia).