Summary
Highlights
Earth's crust is divided into continental and oceanic varieties. Continental crust is felsic, silica-rich, and 20-70 km thick, while oceanic crust is mafic, rich in iron and magnesium, and 5-10 km thick. Oceanic crust is denser than continental crust, leading to subduction when they collide, a process that destroys oceanic crust over time.
Below the crust is the mantle, a solid but malleable layer reaching 3500 degrees Celsius. It convects over millions of years, driven by Earth's internal heat. Subducting oceanic crust creates a slab-pull force, which is the primary driver of plate tectonics. The mantle extends from 70 to 2900 km deep and is mainly composed of peridotite.
The D'' layer is a mysterious, geologically important layer at the bottom of the mantle, characterized by a thermal gradient and heterogeneity. Hypotheses suggest it's a "slab graveyard" where subducted slabs settle or the origin point for mantle plumes, which cause significant volcanic activity. This layer also has implications for magnetic field reversals.
The D'' layer transitions into the liquid outer core at 2900 km, composed mainly of iron and nickel, with temperatures reaching 5500 degrees Celsius. This liquid layer convects, and its rotation creates the geodynamo, which generates Earth's magnetic field through the movement of electrically conductive iron.
At 5100 km, immense pressure forces iron into a solid phase, forming the inner core. This solid inner core, about the size of the moon and as hot as the sun's surface, helps stabilize the magnetic field from the outer core and rotates faster than the rest of Earth due to magnetic torque, a phenomenon called super-rotation.
Earth's magnetic field undergoes periodic reversals where its strength drops, and the poles switch. The last reversal was 780,000 years ago. One hypothesis for these reversals involves the sudden collapse of subducted slabs into the D'' layer, reorganizing outer core convection and altering the magnetic field.
Earth initially formed as a hot, homogeneous planet. Early melting due to radioactive decay and kinetic energy led to differentiation, with heavier materials sinking to form the core and lighter materials rising to form the crust and mantle. The inner core began crystallizing between 2 billion and 570 million years ago and continues to grow at about a millimeter per year as the outer core solidifies.