1.2 Earth's internal structure and composition

Earth's structure is like a layered onion, with distinct zones from crust to core. Each layer has unique properties that shape our planet's behavior. Understanding these layers helps explain earthquakes, plate tectonics, and Earth's magnetic field.

Seismology reveals Earth's hidden depths. By studying how seismic waves travel through the planet, scientists map out its internal structure. This knowledge is crucial for grasping Earth's dynamics and evolution over time.

Earth's Major Layers

Crust and Mantle Characteristics

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• Crust forms Earth's outermost layer ranging from 5-70 km thick
  • Oceanic crust: thinner (5-10 km), denser, composed mainly of basaltic rocks
  • Continental crust: thicker (30-70 km), less dense, composed primarily of granitic rocks
• Mantle extends from base of crust to 2900 km depth
  • Divided into upper mantle and lower mantle
  • Composed mainly of silicate rocks rich in iron and magnesium
  • Convection currents in mantle drive plate tectonics

Core Structure and Properties

• Outer core spans from 2900 km to 5150 km depth
  • Liquid iron-nickel alloy with some lighter elements
  • Generates Earth's magnetic field through convection and rotation
• Inner core extends from 5150 km to center of Earth (6371 km)
  • Solid iron-nickel alloy due to extreme pressure
  • Rotates slightly faster than rest of planet (superrotation)

Structural Boundaries

Major Discontinuities and Transitions

• Mohorovičić discontinuity (Moho) marks boundary between crust and mantle
  • Identified by sudden increase in seismic wave velocities
  • Depth varies: ~5-10 km beneath oceans, ~30-50 km beneath continents
• Lithosphere includes crust and uppermost solid mantle
  • Behaves as a rigid layer, broken into tectonic plates
  • Thickness varies from ~50-280 km
• Asthenosphere lies beneath lithosphere in upper mantle
  • Partially molten, ductile layer allowing plate movement
  • Extends to ~700 km depth

Other Significant Boundaries

• Conrad discontinuity separates upper and lower continental crust
  • Not present globally, more prominent in some regions
• 410 km and 660 km discontinuities mark phase transitions in mantle minerals
  • Affect seismic wave velocities and mantle dynamics
• Core-mantle boundary (CMB) separates mantle from outer core
  • Dramatic change in composition, density, and physical properties

Composition

Density Stratification

• Earth's layers organized by increasing density with depth
  • Crust: ~2.7-3.0 g/cm³ (continental), ~3.0-3.3 g/cm³ (oceanic)
  • Mantle: ~3.3-5.7 g/cm³
  • Outer core: ~9.9-12.2 g/cm³
  • Inner core: ~12.8-13.1 g/cm³
• Density increases result from:
  • Gravitational compression
  • Changes in chemical composition
  • Phase transitions of minerals

Chemical Composition and Variations

• Bulk Earth composition differs from crustal rocks
  • Crust enriched in lighter elements (silicon, aluminum)
  • Mantle and core contain higher proportions of iron and magnesium
• Crust primarily composed of:
  • Continental: felsic rocks (granite, rich in silica and aluminum)
  • Oceanic: mafic rocks (basalt, rich in magnesium and iron)
• Mantle composed mainly of ultramafic rocks
  • Peridotite dominant rock type
  • Key minerals include olivine, pyroxene, and garnet
• Core primarily iron-nickel alloy
  • Outer core contains ~10% lighter elements (sulfur, oxygen, silicon)
  • Inner core nearly pure iron-nickel under extreme pressure