The weak, ductile layer of the upper mantle on which tectonic plates move is the asthenosphere.
The asthenosphere lies beneath the rigid lithosphere, which includes Earth’s crust and the uppermost solid mantle. Although the asthenosphere is mostly solid rock, its high temperature and long-term mechanical behavior allow it to deform and flow slowly. This contrast lets lithospheric plates move over it rather than behaving as one unbroken shell.
The layer is associated with the upper mantle and is composed largely of peridotite, rich in minerals such as olivine and pyroxene. Small amounts of partial melt, temperature effects, volatile elements, and deformation processes may all contribute to its weakness. Rising asthenospheric material can also undergo decompression melting, producing much of the magma at mid-ocean ridges.
A frequent mistake is calling the asthenosphere liquid. It is not an underground ocean of molten rock; it is predominantly solid but ductile over geological timescales. The lithosphere is the strong plate, while the asthenosphere is the weaker underlying zone that helps accommodate plate motion.