Asthenosphere
The weak, partly ductile layer of the upper mantle beneath the lithosphere, roughly 80–200 km deep, that allows tectonic plates to move.
Mantle convection: Slow solid-state circulation of Earth's mantle driven by heat loss, moving material over centimeters per year. The convecting asthenosphere carries the plates riding on top of it.
Lithosphere: Earth's rigid outer shell, comprising crust plus stiffest upper mantle, broken into tectonic plates. Rigid lid above, weak layer below: the contrast defines both.
Joseph Barrell: American geologist who coined 'asthenosphere' in 1914 for a weak layer beneath the lithosphere. He named the layer to explain isostatic compensation of crustal loads.
Lithosphere–asthenosphere boundary: The transition from rigid plate to ductile mantle, whose depth varies with age, temperature and water content. Whether the boundary is thermal, compositional or melt-controlled is unsettled.
Partial melting: Melting of only a fraction of a rock's grains, lowering bulk strength; small melt fractions may exist in the asthenosphere. Trace melts and volatile-rich zones contribute to the asthenosphere's low viscosity.
Mantle: The silicate rock shell between crust and core, about 2,900 km thick and 84% of Earth's volume. The asthenosphere is the upper mantle's weak middle portion.
Beno Gutenberg: Seismologist who, with Charles Richter, documented a low-velocity zone in the upper mantle. His seismic evidence gave the named layer physical reality.
Melt fraction in the asthenosphere: The unresolved amount of silicate melt present, with seismic anisotropy suggesting distributed pockets. Tiny melt fractions could explain seismic anomalies without whole-layer melting.
Geothermal gradient: The increase of temperature with depth inside Earth, roughly 25–30 °C per kilometer in the crust. Depth-dependent temperature is why rock turns weak below the rigid lithosphere.
Mohorovičić discontinuity: The seismic boundary between crust and mantle, found by Andrija Mohorovičić in 1909. Crust–mantle boundary sits above the asthenosphere but does not coincide with its top.