The crust, mantle and core describe what Earth is made of. Geologists also sort the outer layers by how they behave: are they stiff and brittle, or soft and able to flow?
- Lithosphere ("stone sphere"): the crust plus the cold, stiff top of the mantle, about 100 km thick on average (thinner under oceans, thicker under old continents). It is broken into the tectonic plates.
- Asthenosphere ("weak sphere"): hot, soft mantle rock below the lithosphere, down to a few hundred kilometers. It is solid, but it can slowly flow.
Key idea
Tectonic plates are pieces of lithosphere: crust plus the uppermost mantle. They slide over the hot, weak but solid asthenosphere.
Why it works
Whether rock bends, breaks or flows depends on how hot it is. Near the surface the rock is cool and brittle, so it cracks. Deeper down it is close to its melting point. It stays solid, but over thousands of years it can creep, like a candle that is warm but not melted. The boundary between stiff and soft rock lies inside the mantle, so the lithosphere includes the crust and the top of the mantle.
Worked example: Under the crust
Under a continent, the crust is 35 km thick and the lithosphere is 120 km thick. How much of the lithosphere is mantle rock?
- The lithosphere is the crust plus the top of the mantle.
- $120 - 35 = 85$ km of it is mantle.
Answer: 85 km
Worked example: Two ways to name a layer
Rock 200 km below the surface is in which compositional layer, and which behavior layer?
- The crust is at most about 70 km thick, so 200 km down is the mantle.
- The lithosphere is about 100 km thick, so 200 km down is below it, in the soft asthenosphere.
Answer: The mantle, in the asthenosphere
Common mistake
Thinking the plates are only crust, or that they float on liquid. Plates are lithosphere: crust plus the top of the mantle, about 100 km thick. They ride on asthenosphere that is hot and weak but still solid. Check: earthquake S waves travel through the asthenosphere, so it can't be a liquid layer.