What it is: Outermost, thinnest layer.
Composition: Silicate minerals rich in oxygen and silicon.
Types:
- Oceanic Crust: Thinner ($\sim 5-10 \text{ km}$), denser (basaltic).
- Continental Crust: Thicker ($\sim 30-50 \text{ km}$), less dense (granitic).
Physical State: Rigid and brittle; causes earthquakes.

What it is: Thick, solid layer, $\sim 84\%$ of Earth's volume.
Composition: Ultramafic silicate rocks rich in iron and magnesium (e.g., peridotite).
Physical State & Behavior: Solid, but upper part forms rigid lithosphere with crust. Below is asthenosphere (solid but plastic, flows slowly) driving plate tectonics.

What it is: Innermost layer, primarily iron and nickel.
Layers:
- Outer Core: Liquid. Movement generates Earth's magnetic field (geodynamo).
- Inner Core: Solid. Immense pressure keeps it solid despite higher temperature.

Identified by analyzing seismic wave speed/path changes.
Seismic Waves:
- P-waves (Primary/Compressional): Fastest, travel through solids, liquids, gases.
- S-waves (Secondary/Shear): Slower, travel only through solids.
Key Seismic Discontinuities:
- Mohorovičić Discontinuity ("Moho"): Boundary between crust and mantle. Discovered by Andrija Mohorovičić (1909).
- Core-Mantle Boundary: Boundary between mantle and outer core. S-waves disappear (outer core is liquid). P-waves refract sharply.
- Inner Core Boundary: Boundary between liquid outer core and solid inner core. Discovered by Inge Lehmann (1936).

Lithosphere: Rigid, brittle outer layer (crust + uppermost mantle). Breaks into tectonic plates.
Asthenosphere: Beneath lithosphere, rock is solid but flows plastically. Low-velocity zone for seismic waves.
Mesosphere (Lower Mantle): Stronger and more rigid than asthenosphere due to pressure.

What it is: Primary driver of geological activity. Solid mantle flows like viscous fluid over millions of years due to temperature differences.
How it works: Hot material rises, cools, sinks, creating slow-moving conveyor belts.
Surface Impact: Drives plate tectonics.

Subduction Slabs: Cold, dense oceanic plates sinking into mantle; show faster seismic wave speeds.
Mantle Plumes: Narrow, upwelling columns of hot, buoyant rock; show slower seismic wave speeds. Can create hotspot volcanism.
Large Low-Shear-Velocity Provinces (LLSVPs): Massive, mysterious structures on core-mantle boundary; very hot, possibly partially molten.
Inner Core's Anisotropy: Seismic waves travel at different speeds depending on direction, indicating preferred crystalline orientation.