The lithosphere is made up of two distinct layers: Earth’s crust and the rigid uppermost portion of the mantle beneath it. Together, these layers form a solid, brittle shell that ranges from about 85 kilometers thick under the oceans to more than 150 kilometers thick beneath continents. This is the layer you live on, and it’s the layer that breaks into tectonic plates.
Two Layers, One Rigid Shell
The lithosphere isn’t defined by what it’s made of chemically. It’s defined by how it behaves mechanically. Both the crust and the upper mantle portion of the lithosphere are cool and rigid enough to act as a single solid unit. Below the lithosphere sits the asthenosphere, where rock is hot enough to flow slowly like thick putty over geological time. The boundary between these two layers sits at roughly the depth where temperatures reach about 1,300°C.
At shallower depths within the lithosphere, rock behaves in a brittle way. It cracks and fractures under stress, which is why earthquakes originate here. Deeper in the lithosphere, rock reaches a transitional zone where it’s at its strongest before eventually giving way to the softer, more deformable material of the asthenosphere below.
The Crust: Earth’s Thin Outer Skin
The crust is the outermost and thinnest part of the lithosphere. It comes in two varieties. Continental crust is thicker (typically 30 to 70 kilometers), less dense, and composed largely of granite-like rocks rich in silicon, aluminum, and oxygen. Oceanic crust is thinner (around 7 kilometers), denser, and made primarily of basalt, a darker volcanic rock.
By weight, the crust is dominated by just a handful of elements. Oxygen accounts for about 46.6% of the crust’s mass, silicon for 27.7%, and aluminum for 8.1%. Iron, calcium, sodium, potassium, and magnesium make up most of the rest, with all other elements combined totaling roughly 1.5%. These elements don’t exist as pure metals. They’re locked into mineral structures, primarily silicate minerals, which are compounds built around frameworks of silicon and oxygen atoms.
The Lithospheric Mantle: The Thicker Lower Layer
Below the crust, the lithospheric mantle makes up the bulk of the lithosphere’s thickness. This layer is chemically different from the crust. It’s composed predominantly of peridotite, a dense rock rich in iron- and magnesium-bearing minerals like olivine and pyroxene. Peridotite is darker, heavier, and far less familiar than crustal rocks because it rarely reaches the surface. When it does get exposed, typically on the ocean floor near mid-ocean ridges, it reacts with seawater and transforms into a greenish rock called serpentinite.
A thin but important boundary called the Moho (short for Mohorovičić discontinuity) separates the crust from the mantle portion of the lithosphere. This isn’t a visible gap. It’s a zone where the chemical composition shifts from the lighter, silicon-rich rocks of the crust to the denser, magnesium- and iron-rich rocks of the mantle. Seismic waves speed up sharply as they cross this boundary, which is how geologists first detected it.
Continental vs. Oceanic Lithosphere
The lithosphere under continents and the lithosphere under oceans differ in both thickness and composition. Continental lithosphere extends to depths of 150 kilometers or more beneath stable continental interiors, sometimes reaching over 200 kilometers under ancient cratons (the oldest, most stable cores of continents). It includes thick continental crust on top and a deep root of lithospheric mantle below.
Oceanic lithosphere is thinner overall, with an asymptotic thickness of about 85 to 95 kilometers. It also gets thicker as it ages. When new oceanic lithosphere forms at mid-ocean ridges, it’s hot and thin. As it moves away from the ridge and cools over millions of years, the rigid layer deepens. The elastic thickness of oceanic lithosphere grows roughly in proportion to the square root of its age, meaning a 100-million-year-old piece of ocean floor has a substantially thicker lithospheric mantle than a 10-million-year-old piece.
This age-related thickening matters because older, cooler oceanic lithosphere becomes denser than the asthenosphere below it, which is what allows it to sink back into the mantle at subduction zones.
How the Lithosphere Becomes Tectonic Plates
The lithosphere’s rigidity is what makes plate tectonics possible. Because it behaves as a brittle solid, the lithosphere doesn’t deform smoothly. Instead, it fractures into discrete pieces. Earth’s surface is currently divided into seven large tectonic plates of roughly similar size, plus a population of smaller plates that follow a fractal size distribution, meaning there are many more small plates than large ones.
This fragmentation isn’t random. Three-dimensional models of mantle convection show that the pattern of plates is driven by a feedback loop between the flowing mantle below and the strength of the lithosphere above. The spacing between subducting slabs (places where one plate dives beneath another) controls where large plates form, while the bending stresses at curved trench boundaries crack the lithosphere into smaller fragments. In other words, the lithosphere’s composition and mechanical properties don’t just define what it is. They determine how Earth’s surface moves.