Basic Earth Structure

The earth is composed of four layers: the inner core, the outer core, the mantle and the crust.

Each layer can be characterized by its distinctive composition and physical properties.

Most of the heaviest elements are found closer to the center of the earth while the lighter elements are concentrated near the Earth's surface.

Crust

• thin rigid layer of igneous, metamorphic and sedimentary rocks.
• made mainly of silicate minerals (light elements).
• thickness of oceanic crust 5 - 10 km, continental crust 30 - 40 km.

Mantle

• plastic-like layer capable of flow.
• made of Fe-Mg silicate minerals.
• thickness about 2900 km.

Outer Core

• thick liquid.
• made of Fe and other elements.
• thickness about 2300 km.

Inner Core

• solid sphere.
• made of metals such as Fe and Ni.
• diameter 2400 km.

Earth Structure Inferred From Seismic Waves

• From below-ground nuclear weapons tests, geophysicists inferred the Earth's structure.
• Seismic waves curve as they travel into the Earth, due to:
  • increased temperature with depth.
  • pressure with depth.
  • varying composition.
• Boundaries of the Earth's structure mapped by reflection and refraction of the seismic waves.
• Secondary waves (S-waves) were blocked by the outer core, suggesting it is molten.
• P-wave shadow zone:
  • zone where P-waves of earthquake not detected.
  • due to refraction.
• S-wave shadow zone:
  • zone where S-waves of earthquake not detected.
  • due to combination of blocking by the outer core and refraction.