The lithosphere and asthenosphere form the upper two layers of the earth. The lithosphere, Greek for "stone," is composed of brittle rock. Below the lithosphere, the asthenosphere, Greek for "weak," is composed of ductile and semi-fluid rock. The lithosphere rides atop the slowly flowing asthensophere. The differences between these two layers include locations, physical properties, chemical properties and roles in plate tectonics.
The lithosphere represents the earth’s top layer, consisting of the crust and the upper reaches of the mantle. The depth of this layer is between 49 and 62 miles. The upper lithosphere is composed of both oceanic crust, about 4.5 miles thick, and continental crust, about 22 miles thick. The lithosphere plunges into and through the asthenosphere in tectonic subduction zones. The lithospheric layer is also deeper under mountain ranges. The asthenosphere is below the lithosphere, composed of the upper mantle. The thickness of this layer ranges between 62 and 217 miles. The asthenosphere rises to the surface at mid-oceanic ridges.
The lithosphere is composed of relatively cool, rigid rocks. These rocks behave elastically, although they are brittle and can break, fracture or fault. At its lower limit, the lithosphere contains mantle rock. This rock is similar in composition to the asthenosphere, but it is cooler and less fluid. The asthenosphere is a semi-fluid layer of partly molten rock. A balance between temperature and pressure maintains a consistency similar to that of warm tar. This ductile material consists of solid particles, with liquid filling the space between them. This state causes asthenospheric rock to behave like a plastic, capable of gradual flow.
The slush-like rock material of the asthenosphere is composed of iron-magnesium silicates. This chemical composition is nearly identical to the lower mesospheric layer. In contrast, lithospheric rocks contain more silica, but less aluminum, sodium and potassium. Within the lithosphere, composition varies between oceanic crust and continental crust. Oceanic crust contains less silica than continental crust, producing a darker color. Oceanic crust also contains more magnesium and iron than continental crust, making it much denser.
Roles in Plate Tectonics
The lithosphere, being rigid, is broken into pieces called tectonic plates. These plates ride on top of the semi-fluid asthenosphere. The asthenospheric flow is driven by convection, caused by heat deep within the earth. As this layer slowly flows, the lithosphere’s tectonic plates are moved laterally, as if on a conveyor belt. The asthenosphere is also responsible for the creation of new crust. This occurs at mid-ocean ridges where convection forces the asthenosphere to the surface. As the partly molten material extrudes, it cools and forms new crust. The convection also forces the lithospheric plates to move apart at these ridges, called divergent boundaries, or zones.