Earth's Interior

Earth's Interior

Probing Earth’s Interior

  • Most knowledge of Earth's interior comes from seismology.
    • Travel times of seismic waves vary depending on the rock type.
    • Travel paths are complex, enabling the identification of resources like oil.

Seismic Wave Properties

  • Important characteristics of seismic waves:
    • S waves only travel through solids.
    • P waves travel faster than S waves.
    • Seismic waves travel faster through stiffer rock.
    • Waves can reflect and refract at boundaries between different materials.

Earth's Layer Structure

  • By analyzing changes in wave behavior, scientists locate boundaries within the Earth.
  • Types of layers by composition:
    • Crust
    • Mantle
    • Core
  • Types of layers by physical properties:
    • Lithosphere
    • Asthenosphere
    • Transition Zone
    • Mesosphere
    • D” Layer
    • Outer Core
    • Inner Core

Layers by Composition

  • Crust:

    • Thin outer layer of the Earth.
    • Oceanic Crust:
    • Averages 7 km thick; typically younger and denser.
    • Continental Crust:
    • Averages 40 km thick; can reach up to 70 km.

  • The Moho (Mohorovicic discontinuity):

    • The boundary between the crust and the mantle, where P waves are refracted.

  • Mantle:

    • A solid rocky shell made primarily of peridotite, extending to a depth of approximately 2900 km.

Layers by Physical Properties

  • Upper Mantle:

    • Extends to 660 km depth.
    • Lithosphere:
      • Comprises the crust and upper mantle, ranging from a few to about 200 km deep.

  • Asthenosphere:

    • Extends up to 410 km; noted for its weaker properties.

  • Transition Zone:

    • Ranges from 410 km to 660 km with a rapid increase in density and phase changes in olivine.

  • Lower Mantle (Mesosphere):

    • Extends from 660 km to ~2800 km.
    • D” Layer:
    • Boundary between core and mantle, with variable composition and temperature.

  • Core-mantle boundary:

    • P waves die out around 100°, reappearing around 140° creating a P-wave shadow zone.

Core Composition

  • Core:

    • Primarily made of iron, some nickel, with trace amounts of other elements.
    • Radius is approximately 3500 km.

  • Outer Core:

    • Liquid layer about 2300 km thick, responsible for Earth's magnetic field.

  • Inner Core:

    • Solid, with a diameter of roughly 1200 km. It will grow as Earth cools.

  • Inner-core / outer-core boundary:

    • P waves passing through the inner core are faster, demonstrating refraction.

Earth's Temperature

  • Earth remains hot due to:
    • Collisions during formation.
    • Impact events that led to the formation of the Moon.
    • Radiogenic Heat: Caused by short-lived radioactive atoms and long-lived isotopes effecting decay.

Heat Transfer Mechanisms

  • Convection:

    • Movement of material in fluid-like manner.

  • Conduction:

    • Collisions of atoms; transmission of energy through electron flow.

  • Radiation:

    • Transfer of energy via waves, predominantly occurring in the atmosphere.

Geothermal Gradient

  • The geothermal gradient fluctuates significantly across the surface and varies with depth.

Earth's Structure Characteristics

  • Earth is not perfectly spherical; it is larger at the equator, leading to gravity anomalies.

Earth's Magnetic Field

  • The magnetic field has a similar structure to that of a bar magnet and is thought to form like an electromagnet via a process known as geodynamo.

Requirements for Magnetic Field Production

  • An electrically conducting, fluid interior is essential.
  • Convection within the fluid layer is necessary.
  • Some minimum rotation of the Earth is needed.

Polar Wandering and Field Reversal

  • The inner core rotates faster than the Earth’s surface, contributing to polar wandering.
  • The magnetic field not only wanders but can also reverse, a process still under investigation.

Seismic Tomography

  • Utilizes signals from numerous earthquakes and seismograph stations to create a comprehensive image of the Earth’s interior, akin to a CT scan.

Summary Notes

  • Understanding the internal structure of Earth.
  • Methods of studying Earth's interior.
  • Specifics of different geological layers.
  • Internal temperature dynamics.
  • Gravitational variations and implications of seismic tomography.
  • Characteristics of Earth's magnetic field.