The mantle is the widest layer (~2900 km thick), situated between the crust and the core. It has distinct upper and lower regions.
Lithosphere: The rigid outer layer comprising the crust and the rigid upper mantle. It is broken into tectonic plates that move over the asthenosphere. The lithosphere "floats" because it is less dense than the underlying mantle.
Asthenosphere: A plastic-type layer directly beneath the lithosphere. It moves very slowly under high pressure and temperature, behaving like a viscous fluid over geological time. The lithospheric plates are carried on top of this convective layer.
Lower mantle: Hotter and denser than the upper mantle. Intense pressure at depth keeps it solid despite high temperatures. Heat transferred from the core drives convection currents that rise through the mantle.
The core is divided into two parts: the inner core (solid) and the outer core (semi-molten, mostly liquid iron and nickel).
The core is the hottest part of the Earth, and this heat is the main driver of tectonic activity. Without heat from the core, convection currents would cease and plate movement would stop.
Two key causes of extreme heat:
Key insight: The inner core is solid because of immense pressure, while the outer core remains liquid. Heat loss from the core drives mantle convection and, ultimately, plate tectonics.
| Layer | State | Key property |
|---|---|---|
| Crust | Solid | Two types: continental vs oceanic |
| Upper mantle (rigid) | Solid | Part of lithosphere |
| Asthenosphere | Plastic/ductile | Carries lithosphere |
| Lower mantle | Solid | Hot, dense |
| Outer core | Liquid | Iron, nickel |
| Inner core | Solid | Mostly iron |
Lithosphere vs asthenosphere: The lithosphere is rigid and brittle; the asthenosphere is ductile and allows slow flow. This contrast enables plate movement.
Continental vs oceanic subduction: Only oceanic crust subducts because it is denser. Continental crust is too buoyant to sink into the mantle.
Always identify which type of crust is involved when discussing plate boundaries. Oceanic–continental and oceanic–oceanic boundaries involve subduction; continental–continental does not.
Remember the lithosphere includes BOTH the crust and the rigid upper mantle — it is not just the crust.
Common mistake: Confusing the asthenosphere with the mantle. The asthenosphere is a specific zone within the upper mantle.
Verification: If asked about heat sources, include BOTH radioactive decay and primordial heat — marks are often given for each.
