1: Earth's formation and structure

Earth's Formation and Structure

Earth originated approximately 4.54 billion years ago within the solar nebula—a rotating disk of gas and dust surrounding the young Sun. Through accretion, dust particles collided and adhered, forming planetesimals (1–10 km bodies). These grew via gravitational attraction, culminating in planetary embryos (Moon-to-Mars-sized). Earth’s final assembly involved a cataclysmic impact with a Mars-sized body named Theia, ejecting debris that formed the Moon. This collision also liquefied early Earth, enabling global chemical reorganization.

Intense heat from three sources drove planetary differentiation:

1. Gravitational compression
2. Radioactive decay of isotopes (e.g., uranium-238, potassium-40)
3. Kinetic energy from impacts

Molten iron and nickel sank toward the center, forming the core, while lighter silicate minerals migrated upward, creating the primitive mantle and crust. This process established Earth’s layered structure:

1. Core:

   • Inner core (1,220 km radius): Solid iron-nickel alloy. Temperatures reach 5,700°C, but immense pressure prevents melting.
   • Outer core (2,260 km thick): Liquid iron-nickel. Convective motion generates Earth’s magnetic field via the geodynamo effect.

2. Mantle (2,900 km thick):

   • Composed of ultramafic silicates (olivine, pyroxene).
   • Divided into upper mantle (rigid) and lower mantle (high-pressure phases like bridgmanite).
   • The asthenosphere (upper mantle, 100–660 km depth) is partially molten (1–5%), enabling ductile flow that drives plate tectonics.

3. Crust:

   • Oceanic: Thin (5–10 km), dense basalt/gabbro.
   • Continental: Thick (20–70 km), less dense granite.

Earth’s mechanical layers further refine this:

• Lithosphere: Brittle crust and uppermost mantle (0–100 km), fragmented into tectonic plates.
• Mesosphere: Rigid lower mantle (660–2,900 km).

Seismic wave studies (P-waves and S-waves) reveal these layers. S-waves cannot traverse the liquid outer core, while P-waves refract at core-mantle boundaries, creating shadow zones. Gravity and magnetic field data corroborate core dynamics.

This structure regulates geologic activity: core convection powers the magnetic field shielding Earth from solar radiation, mantle convection drives plate tectonics, and crustal interactions recycle surface materials. Understanding Earth’s formation and stratification is foundational for interpreting volcanism, seismicity, and resource distribution.