1.3 Earth's internal structure and composition

Earth's internal structure is like a cosmic onion, with distinct layers from crust to core. Each layer has unique properties that shape our planet's behavior. Understanding this structure is crucial for grasping plate tectonics and Earth's dynamic processes.

Seismic waves act as Earth's X-rays, revealing its hidden layers. By studying how these waves travel through the planet, scientists have mapped out Earth's interior, uncovering the secrets of its composition and physical properties.

Earth's Interior Structure

Layered Structure of Earth's Interior

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• Earth's interior divides into distinct layers based on composition and physical properties
  • Crust: Outermost layer
    • Thickness varies from 5-70 km
    • Composed primarily of silicate rocks
  • Mantle: Largest layer
    • Extends from base of crust to ~2900 km depth
    • Predominantly composed of iron and magnesium-rich silicate minerals
  • Outer core: Liquid layer
    • Extends from ~2900 km to 5150 km depth
    • Mostly iron and nickel composition
  • Inner core: Solid sphere
    • Radius of ~1220 km
    • Primarily iron and nickel composition
• Layer boundaries marked by seismic wave velocity discontinuities
  • Mohorovičić discontinuity separates crust and mantle
• Temperature and pressure increase with depth
  • Influences physical state and behavior of materials in each layer
  • Affects rock properties (density, melting point)

Seismic Discontinuities and Layer Properties

• Crust-mantle boundary (Mohorovičić discontinuity)
  • Marks transition from less dense crustal rocks to denser mantle material
  • Depth varies (5-70 km) depending on location (oceanic vs continental crust)
• Mantle transition zone (410-660 km depth)
  • Marks changes in mineral crystal structures due to increasing pressure
  • Olivine transforms to wadsleyite at 410 km, then to ringwoodite at 520 km
• Core-mantle boundary (Gutenberg discontinuity)
  • Separates silicate mantle from iron-rich core at ~2900 km depth
  • Significant change in composition, density, and seismic wave behavior
• Inner core boundary
  • Transition from liquid outer core to solid inner core at ~5150 km depth
  • Marked by changes in seismic wave velocities and attenuation

Lithosphere vs Asthenosphere

Characteristics and Behavior

• Lithosphere: Rigid outer layer of Earth
  • Comprises crust and uppermost solid mantle
  • Average thickness ~100 km
  • Behaves elastically on geological timescales
  • Broken into tectonic plates
• Asthenosphere: Partially molten, ductile layer beneath lithosphere
  • Extends from ~100 km to 400 km depth
  • Allows for plate tectonic movements
  • Behaves plastically, enabling mantle convection
• Lithosphere-asthenosphere boundary (LAB)
  • Critical zone for plate tectonics
  • Marks transition from rigid to more ductile behavior
  • Depth varies with tectonic setting (shallower under oceans, deeper under continents)

Mantle and Core Divisions

• Mantle divisions
  • Upper mantle: Includes asthenosphere and part of lithosphere
  • Transition zone: 410-660 km depth
  • Lower mantle: Extends to core-mantle boundary
• Core divisions
  • Outer core: Liquid layer (2900-5150 km depth)
  • Inner core: Solid sphere (5150 km to center)
• Rheological properties
  • Lithosphere: Behaves rigidly, transmits tectonic stresses
  • Asthenosphere: More plastic, allows for mantle convection and plate movement
  • Lower mantle: Higher viscosity than asthenosphere
  • Outer core: Liquid, convects and generates Earth's magnetic field
  • Inner core: Solid, rotates slightly faster than the rest of the Earth (superrotation)

Seismic Waves and Earth's Interior

Types of Seismic Waves and Their Behavior

• Primary (P) waves
  • Compressional waves
  • Travel through solids and liquids
  • Fastest seismic waves
• Secondary (S) waves
  • Shear waves
  • Only propagate through solids
  • Slower than P-waves
• Surface waves (Rayleigh and Love waves)
  • Travel along Earth's surface
  • Cause most earthquake damage
• Wave behavior at boundaries
  • Refraction: Bending of waves at layer interfaces
  • Reflection: Bouncing of waves off layer boundaries
  • Used to map internal structures and discontinuities

Seismic Methods for Probing Earth's Interior

• Travel-time curves
  • Plot arrival times of seismic waves against distance from source
  • Reveal changes in wave velocity with depth
  • Used to determine layer depths and properties
• Shadow zones
  • Areas where certain seismic waves do not arrive directly
  • P-wave shadow zone: 103°-140° from earthquake source
  • S-wave shadow zone: Beyond 103° from source
  • Provide evidence for Earth's core and its properties
• Seismic tomography
  • 3D imaging of Earth's internal structure
  • Uses multiple seismic wave paths
  • Reveals mantle heterogeneities and subducting slabs
• Receiver functions
  • Analyze converted seismic waves at boundaries
  • Used to study crust-mantle boundary and lithosphere-asthenosphere boundary

Composition of Earth's Layers

Crustal and Mantle Composition

• Crust composition
  • Continental crust: Rich in silicon and aluminum (sial)
    • Average composition similar to granite
    • Examples: quartz, feldspar, mica
  • Oceanic crust: Rich in silicon and magnesium (sima)
    • Composition similar to basalt
    • Examples: pyroxene, plagioclase feldspar
• Mantle composition
  • Predominantly silicate minerals rich in magnesium and iron
  • Major mineral phases
    • Olivine ($\text{(Mg,Fe)}_2\text{SiO}_4$)
    • Pyroxene ($\text{(Mg,Fe)SiO}_3$)
    • Garnet ($\text{(Mg,Fe,Ca)}_3\text{Al}_2\text{Si}_3\text{O}_{12}$)
  • Composition changes with depth due to phase transitions

Core Composition and Physical Properties

• Outer core composition
  • Iron-nickel alloy with some lighter elements (oxygen, sulfur, silicon)
  • Liquid state due to high temperatures and pressures
  • Responsible for Earth's magnetic field through geodynamo process
• Inner core composition
  • Primarily solid iron with some nickel
  • Maintained in solid state despite extreme temperatures (~5700°C) due to immense pressure
• Density variation
  • Increases significantly with depth
  • Crust: ~2.7 g/cm³
  • Lower mantle: ~5.5 g/cm³
  • Outer core: ~10-12 g/cm³
  • Inner core: >13 g/cm³
• Temperature gradients
  • Crust-mantle boundary: ~1000°C
  • Core-mantle boundary: ~4000°C
  • Center of Earth: >5000°C
• Viscosity variations
  • Lithosphere: High viscosity ($10^{21}-10^{23}$ Pa·s)
  • Asthenosphere: Lower viscosity ($10^{19}-10^{21}$ Pa·s)
  • Lower mantle: Increases with depth ($10^{21}-10^{23}$ Pa·s)
  • Outer core: Very low viscosity (liquid)