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.