The Earth’s structure is often described by its chemical layering, which includes the crust, mantle, and core. A different way to understand the planet’s outer structure is through its mechanical properties, which define the lithosphere. This rigid outer shell is the planet’s outermost layer, encompassing the surface we inhabit and extending far beneath it. The lithosphere is the hard, strong layer that forms the tectonic plates responsible for shaping the Earth’s surface over geologic time.
Defining the Lithosphere: The Rigid Outer Shell
The lithosphere is defined by its physical state—rigidity and strength—rather than its chemical composition. This layer acts as a cold, brittle, and solid shell that can fracture when subjected to stress. It is composed of rocks that are strong enough to behave elastically over long periods, meaning they can bend or break rather than flow. This mechanical strength allows the lithosphere to break up into the large, moving segments known as tectonic plates.
The boundary separating the lithosphere from the layer beneath it is the lithosphere-asthenosphere boundary (LAB). Below this boundary lies the asthenosphere, a layer of the upper mantle that is weaker and more ductile. Although the asthenosphere is solid rock, high temperature and pressure bring its material close to the melting point, allowing it to deform and flow very slowly over geologic time.
The rigid lithosphere floats and moves across the underlying, plastic asthenosphere. The asthenosphere’s ductile nature allows for slow convective currents, which drive the movement of the rigid lithospheric plates above it. The depth of the LAB is approximated by the isotherm where mantle rock transitions from brittle to viscous behavior, typically around 1,300 degrees Celsius.
The Structural Components: Crust and Uppermost Mantle
The lithosphere is composed of two distinct chemical layers: the entire crust and the uppermost portion of the mantle. The crust is the chemically distinct, outermost layer and forms the very top of the lithosphere. Beneath the crust lies the lithospheric mantle, which is the solid, rigid part of the upper mantle mechanically bound to the crust above it.
The division between the crust and the mantle is a chemical boundary known as the Mohorovičić discontinuity, or Moho. This discontinuity marks a sharp change in chemical composition and rock density. Since the Moho is contained within the mechanically defined lithosphere, the lithosphere incorporates both the crust and the cold, rigid portion of the mantle.
The thickness of the lithosphere is highly variable, depending on whether it is beneath an ocean basin or a continent. Under the oceans, the lithosphere can be thin, sometimes only 5 to 10 kilometers thick near mid-ocean ridges where new crust is forming. The oceanic lithosphere thickens as it moves away from the ridge, reaching thicknesses of about 50 to 100 kilometers in older regions.
The continental lithosphere is much thicker, typically ranging from 150 kilometers to as much as 280 kilometers beneath the oldest continental interiors. This greater thickness is primarily due to the deep root of rigid mantle material that extends beneath the continental crust. The continental crust itself accounts for about 30 to 70 kilometers of the total thickness.
Compositional Differences: Continental Versus Oceanic
The lithosphere is divided into two types that differ significantly in chemical makeup, density, and thickness. The oceanic lithosphere is associated with the ocean basins and is created at mid-ocean ridges. Its crustal component is composed mainly of dark, dense rocks like basalt and gabbro, which are rich in iron and magnesium, classifying them as mafic.
This mafic composition results in a higher average density for oceanic crust (2.9 to 3.0 grams per cubic centimeter). The oceanic lithosphere is relatively thin and young, with the oldest sections being less than 200 million years old. Because it is denser than the continental lithosphere, the oceanic plate tends to subduct back into the mantle when the two types collide.
In contrast, the continental lithosphere is associated with the land masses and has a complex, older history. Its crust is primarily made of lighter-colored, silica-rich rocks, such as granite, which are classified as felsic. This chemical difference gives continental crust a lower average density, typically around 2.7 grams per cubic centimeter.
The continental lithosphere is thicker than its oceanic counterpart and contains some of the oldest rocks on Earth, with ages reaching billions of years. The lower density and greater thickness allow it to float higher on the asthenosphere, explaining why continents stand above sea level. This buoyancy prevents continental lithosphere from being easily recycled back into the mantle.