Lithosphere

Lithosphere (Greek lithos, “stone”), the near-rigid outer shell of the Earth, comprising the crust and the uppermost layer of the mantle, and made up of numerous blocks that are in motion relative to each other—the plates of plate tectonics theory. Above the lithosphere are the atmosphere and the hydrosphere, or ocean waters. Below it is the asthenosphere (Greek, aesthenes; “weak” or “devoid of force”), a relatively narrow, dense, mobile layer within the upper mantle. The asthenosphere is believed to be composed of hot, semi-molten material that can soften and flow after being subjected to high temperatures and pressures over geological time. The plates of the lithosphere, which contain the world’s continents and oceans, thus “float” on the slowly flowing asthenosphere, drifting and jostling against one another.

The lithosphere tends to be thinnest under the oceans and in volcanically active areas, and thickest under the continents. Under the oceans lithosphere varies in thickness from about 1 to 2 km (0.6 to 1.2 mi) at mid-oceanic ridges, the places where according to plate tectonics theory lithosphere wells up from the Earth’s interior to spread out and form new oceanic crust, generally increasing in depth away from the ridges; beneath the oldest oceanic crust, that is the crust furthest away from the mid-oceanic ridges, it is believed to be between 40 and 140 km (29 to 87 mi) thick. The actual thickness of lithosphere under the continents is unknown, partly because the asthenosphere here is very difficult to detect, but it is estimated to be between 250 and 500 km (155 to 311 mi) thick under cratons, the ancient, tectonically stable cores of the continents. The differences in lithospheric depth, as also in the depth of crust, are thought to reflect adjustments to the differences in the nature of oceanic and continental crust. Oceanic crust is composed of volcanic lava rocks, predominantly basalt, which are much heavier and denser than the granitic rocks of which most continental crust is composed.

Earth scientists have distinguished about 12 major lithospheric plates and a large number of smaller ones. Movement between the plates occurs along a relatively narrow zone where plate tectonic forces are most active. It is these zones along which is found the vast majority of volcanic and seismic activity on the Earth (see Volcanism). Lithospheric plates move relative to each other in three main ways. At divergent, or constructive, boundaries—the mid-oceanic ridges—they are moving apart and new oceanic crust is being formed as lithosphere wells to the surface. At convergent, or destructive, boundaries they are colliding with each other. Depending on the nature of the plates involved, whether they are composed mainly of continental or oceanic lithosphere, different things happen at convergent boundaries. If two predominantly oceanic plates collide then the leading edge of one plate is forced down under that of the other; the lithospheric material of the descending plate eventually being reincorporated into the mantle in a process called subduction. A similar process happens when a predominantly continental plate collides with a predominantly oceanic one, only in this case the lighter continental lithosphere rides up over the heavier oceanic lithosphere, which begins to descend and is subducted. These boundaries are marked by world’s deep ocean trenches, and it is along them that most of the world’s volcanoes occur. If two continental plates collide, then neither is subducted because both are relatively light. Instead they buckle and are pushed up vertically or sideways, leading to the creation of mountain chains. The most spectacular example of this process is the on-going creation of the Himalaya ranges by the collision between the Indian and Eurasian plates that began more than 20 million years ago. The third kind of movement occurs along transverse or transform boundaries when two plates slide past each other. The most famous example of a transverse boundary is the San Andreas Fault in California in the western United States.