Lithosphere

The lithosphere is the rigid outer layer of Earth's structure. It consists of the crust and the uppermost part of the mantle, which move together as a single rigid layer that does not deform easily. The term comes from the Greek 'lithos', meaning rock.

The lithosphere is not a single unified whole. It is divided into several large and small sections called tectonic plates (saxanada litosfeerka). These plates move systematically, supported by the layer beneath them.

Division of layers

Beneath the lithosphere lies the asthenosphere, a layer of rock with higher temperatures, which allows it to move slowly over long periods. The boundary between the lithosphere and asthenosphere is determined by how they behave—rigidity versus deformation—rather than only by differences in composition. This is why the division differs from the separation of crust and mantle, which is based on chemical change.

Oceanic and continental types

There are two types of lithosphere. Oceanic lithosphere underlies the ocean basins and forms at mid-ocean ridges, where new rock emerges. Continental lithosphere is thicker but lighter in density, which allows it to persist for longer periods. Oceanic lithosphere, when it cools and becomes denser, can enter subduction, meaning it descends beneath another plate.

Plate motion

Tectonic plate theory explains how lithosphere movement produces earthquakes, volcanoes, mountains, and ocean trenches. Plate boundaries are classified into three main types: divergent, convergent, and transform. Much of the earthquake activity around the world occurs at these locations.