The lithosphere is a rigid mechanical layer (crust + uppermost mantle) that floats on a weaker, more ductile asthenosphere. This thermal boundary layer (~100-200 km deep) is defined by rheology (strength), not composition. The asthenosphere's low viscosity allows the rigid lithosphere to move horizontally.
Compare seismic velocities and temperature profiles to understand the thermal structure. Examine how strain rates differ between rigid and ductile layers.
From your study of Earth's interior, you know that the planet is layered by composition — a metallic core, a silicate mantle, and a thin crust. But composition is not the only way to slice the Earth. The lithosphere and asthenosphere are defined not by what they are made of, but by how they behave mechanically — how they respond to stress over geological timescales. This distinction is what makes plate tectonics possible.
The lithosphere includes the crust and the uppermost part of the mantle, bonded together into a single rigid shell roughly 100–200 km thick. It is cool enough (below about 1,300°C) that rock behaves as a brittle or elastic solid — it breaks rather than flows when stressed. Think of it like a frozen crust on a pond: stiff, capable of fracturing, and floating on something softer beneath. Oceanic lithosphere is thinner (as little as 7 km at mid-ocean ridges, thickening to ~100 km as it ages and cools) and denser than continental lithosphere, which can be 150–250 km thick but is more buoyant because of the low-density continental crust.
Directly beneath the lithosphere lies the asthenosphere, extending from roughly 100 km to about 660 km depth. Here, temperatures are high enough — close to the melting point of mantle rock — that the material deforms by slow, viscous creep rather than brittle fracture. The asthenosphere is not liquid; it is solid rock that flows extremely slowly, like warm wax or glacier ice, under sustained pressure over millions of years. Seismologists detect the asthenosphere as a low-velocity zone where seismic waves slow down slightly, indicating reduced rigidity. A small fraction of partial melt (perhaps 1–2%) may exist in some regions, further reducing viscosity.
The practical consequence of this layering is straightforward: the rigid lithospheric plates can slide horizontally over the weak asthenosphere. Without the asthenosphere's low viscosity, the plates would be locked to the deeper mantle and could not move independently. This is why the lithosphere-asthenosphere boundary matters more for plate tectonics than the crust-mantle boundary — the Moho (the compositional boundary) tells you what the rock is made of, but the lithosphere-asthenosphere boundary tells you whether the rock can move. Convection currents in the asthenosphere and deeper mantle help drive plate motion, while the rigid lithosphere transmits stress across thousands of kilometers, linking mid-ocean ridges to subduction zones in a single coherent mechanical system.