Chapter 12: Earth’s Interior

Early in Earth's history, gravitational heating, planetary collisions, and radioactive decay melted the planet's interior, allowing chemical differentiation to occur as dense iron and nickel materials sank toward the center while lighter silicate minerals rose upward, eventually creating three major compositional divisions: the crust, mantle, and core. The crust consists of two distinct types—thin oceanic crust composed of basalt and gabbro beneath the ocean floor, and thicker continental crust made predominantly of granodiorite—separated by the Mohorovičić discontinuity. Beneath the crust lies the mantle, a vast region of ultramafic peridotite that extends nearly 2,900 kilometers deep and contains the rigid lithosphere in its uppermost portion, the weak asthenosphere that permits plate motion, a transition zone marked by significant mineral phase changes, and the lower mantle where extreme pressures create denser perovskite-structured silicates. The innermost region comprises two cores: a liquid iron-nickel outer core responsible for generating Earth's magnetic field through convection, and a solid iron inner core that continues crystallizing today. Seismic waves generated by earthquakes serve as primary tools for mapping Earth's interior, with P waves and S waves traveling at different velocities through various materials and revealing layer boundaries through reflection and refraction patterns. The absence of S waves in certain regions confirms the outer core's liquid state, while three-dimensional seismic tomography reveals internal temperature variations and convection patterns analogous to medical imaging. Heat escapes Earth's interior through conduction in the crust and core, convection in the mantle and outer core, and radiation to space, with regional variations reflecting structural heterogeneity. Mantle convection driven by sinking cold slabs and rising hot plumes directly powers plate tectonics, volcanism, and earthquakes, while outer-core convection sustains the geodynamo. Studies of gravity anomalies, seismic patterns, and magnetic field behavior reveal lateral variations including subducting oceanic plates and massive superplumes beneath Africa and the Pacific Ocean, demonstrating that Earth's interior remains dynamic and complex rather than static.