Lithosphere

The lithosphere is Earth's outermost solid shell, comprising the crust and the uppermost portion of the mantle. It is mechanically rigid and relatively cool compared with underlying layers. Its thickness varies: continental lithosphere averages about 100 to 200 kilometers, oceanic lithosphere about 60 to 100 kilometers, and it thickens with age away from mid-ocean ridges.

The lithosphere is divided into tectonic plates that float on the underlying asthenosphere, a partially molten layer of the mantle. The boundary between crust and the mantle is defined by the Mohorovičić discontinuity (the Moho); the boundary between the lithosphere and the asthenosphere is called the lithosphere-asthenosphere boundary (LAB). The continental lithosphere is less dense and often granitic in composition, while the oceanic lithosphere is denser and basaltic and enriched in magnesium and iron.

Plate tectonics explains the movement and interaction of lithospheric plates, producing divergent, convergent, and transform boundaries. Along divergent margins, seafloor spreading creates new lithosphere; subduction at convergent margins recycles lithosphere into the mantle; transform faults accommodate lateral motion. These processes drive earthquakes, volcanism, mountain building, and formation of ocean basins and continental margins.

Geophysical methods, including seismic profiling and tomography, constrain lithospheric structure, thickness, and variations in rigidity. The lithosphere plays a central role in Earth's geodynamics and surface geology, governing the distribution of continents, oceans, and tectonic activity.