what are the four layers of which the earth is composed?

Earth is composed of four main layers, defined by their chemical and physical properties. From the outermost to the innermost, they are:

1. Crust – The thin, solid outermost layer, made mostly of silicate rocks. It's divided into oceanic crust (thinner, denser, basaltic) and continental crust (thicker, less dense, granitic). It ranges from about 5 km thick under oceans to 70 km thick under mountain ranges.

2. Mantle – The thickest layer, extending to a depth of about 2,900 km. It's made of hot, semi-solid silicate rock rich in iron and magnesium. Although mostly solid, it flows very slowly over geological timescales (convection), which drives plate tectonics. The upper mantle, together with the crust, forms the rigid lithosphere, while the layer below it (asthenosphere) is more ductile.

3. Outer Core – A liquid layer composed mainly of iron and nickel, located roughly 2,900–5,150 km deep. Its swirling molten metal motion generates Earth's magnetic field through the geodynamo effect.

4. Inner Core – The solid, innermost layer, also made of iron and nickel, extending from about 5,150 km to the planet's center at roughly 6,371 km. Despite extremely high temperatures (comparable to the sun's surface), immense pressure keeps it solid.

Some geologists further subdivide these into additional mechanical layers (lithosphere, asthenosphere, mesosphere, outer core, inner core) based on physical behavior rather than composition, but the classic four-layer model — crust, mantle, outer core, and inner core — is the most commonly taught framework in Earth science.