Asthenosphere
The asthenosphere is the hot, weak, slowly flowing layer of the upper mantle beneath the lithosphere on which the tectonic plates move.
Definition
The asthenosphere is a soft, hot layer of the mantle right under the stiff plates; it is solid rock, but so hot and squishy that it slowly flows, letting the plates on top of it move. It is the ductile, partially molten zone of the upper mantle from the base of the lithosphere (about $100\ \text{km}$) to roughly $350$ to $700\ \text{km}$ depth, where rock is near its melting point and contains a small percentage of melt, and its melt is the source of magma at mid-ocean ridges. Rheologically it is the layer where mantle viscosity drops to around $10^{19}$ to $10^{20}\ \text{Pa}\cdot\text{s}$, one to three orders of magnitude below the lithosphere and lower mantle. Its weakness arises because temperature approaches the peridotite solidus, aided by small amounts of water and perhaps $1$ to $2\%$ partial melt. Decompression melting occurs when asthenosphere rises adiabatically beneath spreading ridges, producing basaltic melt at roughly $8$ to $10\%$ melt fraction. The asthenosphere accommodates isostatic adjustment, so glacial rebound rates give a direct measurement of its viscosity.
Example
Imagine a cracker floating on thick honey: the cracker is a plate, and the honey is the asthenosphere. Beneath the Mid-Atlantic Ridge, asthenosphere rises, decompresses, and partially melts to create new oceanic crust, and beneath Yellowstone, unusually hot asthenosphere feeds the magma system that powers the geysers. Post-glacial uplift of Hudson Bay and Fennoscandia, still proceeding at about $1\ \text{cm/yr}$, matches models with an asthenospheric viscosity near $10^{20}\ \text{Pa}\cdot\text{s}$. Seismic anisotropy in the asthenosphere records the alignment of olivine crystals by flow in the direction of plate motion.
Key Insight
"Astheno" means weak in Greek, and the asthenosphere is the weak layer that makes plate movement possible. Earthquake waves slow down when they pass through it, which is how it was discovered; geologists call it the low-velocity zone. Whether plates move because the asthenosphere drags them or whether plates move and drag the asthenosphere is a real debate. Current evidence favors plates driving the flow, with slab pull as the dominant force.