1.2 Earth's structure and composition

Earth's interior is a complex system of layers, each with unique properties. From the thin crust we live on to the scorching inner core, these layers interact to shape our planet's geology and habitability.

The structure and composition of Earth's layers drive plate tectonics and other geological processes. Understanding these internal dynamics helps explain everything from earthquakes to the formation of continents and oceans.

Earth's Internal Structure

Structure of Earth's interior

• Crust forms outermost layer ranging 5-70 km thick composed mainly of silicate rocks (granite, basalt)
• Mantle extends from base of crust to core-mantle boundary divided into upper and lower sections primarily composed of iron and magnesium-rich silicates (olivine, pyroxene)
• Outer Core consists of liquid iron and nickel generating Earth's magnetic field with temperatures 4,400-6,100℃
• Inner Core forms solid sphere at Earth's center composed of iron and nickel under extreme pressure and heat up to 5,400℃

Lithosphere vs asthenosphere

• Lithosphere forms rigid outer layer 50-280 km thick including crust and uppermost mantle broken into tectonic plates (Pacific, North American)
• Asthenosphere comprises weak, ductile upper mantle layer beneath lithosphere allowing plate movement through convective flow
• Plate tectonics driven by lithospheric plate interactions at boundaries (convergent, divergent) and asthenospheric convection currents

Composition of crust and mantle

• Crust major elements: oxygen 46.6%, silicon 27.7%, aluminum 8.1%, iron 5.0%, calcium 3.6%
• Mantle major elements: oxygen 44.8%, magnesium 22.8%, silicon 21.5%, iron 5.8%
• Common crust minerals: quartz, feldspars, micas, amphiboles
• Common mantle minerals: olivine, pyroxenes, garnets

Oceanic vs continental crust

• Oceanic crust:
  1. Basaltic composition
  2. 5-10 km thick
  3. Density ~3.0 g/cm³
  4. Younger than 200 million years
• Continental crust:
  1. Granitic composition
  2. 30-50 km average thickness
  3. Density ~2.7 g/cm³
  4. Can be billions of years old
• Oceanic crust thinner but denser subducts beneath continental crust at convergent boundaries (Mariana Trench)
• Continental crust more buoyant resists subduction forms stable landmasses (Eurasia, North America)