Earth's Layers
Crust — outermost solid layer, 5–70 km thick. Continental crust is thicker; oceanic crust is thinner and denser.
Mantle — thick rock layer, ~2,900 km. The upper mantle is partly molten (asthenosphere), allowing plates to move.
Outer Core — liquid iron and nickel, ~2,200 km thick. Its motion generates Earth's magnetic field.
Inner Core — solid iron and nickel, radius ~1,200 km. Extremely hot (~5,400°C) but kept solid by pressure.
Plate Tectonics
Earth's crust is broken into ~15 major tectonic plates that move ~2–10 cm/year on the partly molten asthenosphere. Three boundary types: convergent (plates collide → mountains/trenches), divergent (plates separate → rift valleys/new ocean floor), transform (plates slide past → earthquakes).