The interior of the Earth can be classified into three major compositional layers: the core, the mantle, and the crust which differ by their chemical properties (Fig. 1, left side). When looking at their physical properties or mechanical behavior, these can be further divided into distinct layers (Fig. 1, right side).

Figure 1: The layers of the Earth, determined by chemical properties (core, mantle, and crust) and physical properties (inner and outer core, lower mantle, asthenosphere, and lithosphere).

The core is Earth’s innermost layer and is composed of metals such as iron (Fe) and nickel (Ni). It can be split into the inner core and outer core due to its mechanical behavior. The inner core is solid despite experiencing the highest temperature on Earth. The immense pressure from the overlying layers of Earth prevents it from melting. The outer core is a liquid layer. Convection currents in the outer core create Earth’s magnetic field.

The mantle is Earth’s largest layer by volume and is sandwiched between the crust and the core. The mantle is largely composed of silicates (minerals containing silicon and oxygen). The mantle material behaves differently depending on depth. The lower mantle is less plastic than the asthenosphere above it due to higher pressure. While the lower mantle is solid, it still deforms and flows in convection currents over long timescales. The asthenosphere is also solid, but large parts behave like a viscous liquid. This allows for the convection currents that drive the movement of tectonic plates.

The crust is the outermost and thinnest compositional layer. There is oceanic crust and continental crust. Oceanic crust is the denser of the two and underlies the oceans. It largely consists of basaltic rocks that are high in magnesium (Mg) and iron (Fe). Continental crust is comprised of less dense rocks, in composition similar to granite, and forms nearly all of Earth’s land surface. The crust and the upper mantle together make up the lithosphere. This layer is cold, brittle, and broken up into many pieces, which we refer to as tectonic plates.