Laying the Groundwork: Earth's Layers and Magnetism

1. Seismic Waves and the Earth's Core

  • Seismic Waves: The primary tools used to probe Earth's interior.

    • P waves (Primary): Longitudinal waves that can travel through both solids and liquids.

    • S waves (Secondary): Transverse waves that can only travel through solids.

  • Shadow Zone: A region where no seismic waves are detected due to the refraction caused by changes in material density.

  • Core Characteristics:

    • Radius: Approximately 3,400\text{ km}.

    • Composition: Primarily iron and nickel.

    • Structure: Divided into a liquid outer core and a solid inner core.

    • Magnetic Generation: Convection within the outer core creates Earth's magnetic field.

2. The Crust and the Mantle

  • Mantle Structure:

    • Composed mostly of peridotite.

    • Solid but behaviorally plastic, undergoing slow convection.

    • Divided into the upper mantle and lower mantle.

  • Moho Barrier: The seismic boundary between the crust and the mantle, averaging a depth of 40\text{ km}.

  • Crustal Comparison:

    • Oceanic Crust: Thinner, younger, and composed of basalt.

    • Continental Crust: Thicker, older, and composed of granite.

3. Earth's Magnetism

  • Types of Magnets: Includes permanent magnets and electromagnets (induced by electricity).

  • Magnetic Field: Includes a magnetosphere that shields the planet from solar particles. Note that magnetic poles do not coincide with geographic poles.

  • Dynamo Theory: Attribute the magnetic field to the circulation of liquid iron in the outer core.

  • Pole Reversals: Happen roughly every 250,000 years. This is evidenced by magnetic stripes in rocks on the ocean floor.

4. Plate Tectonics

  • Continental Drift: Theory proposed by Alfred Wegener. Evidence includes:

    • Jigsaw fit of continents.

    • Matching fossils and mountain ranges across oceans.

    • Glacial evidence in currently warm regions.

  • Pangaea: A super-continent that existed 300\text{ million} years ago.

  • Mechanical Classification:

    1. Lithosphere: Rigid outer layer.

    2. Asthenosphere: Soft, flowing layer.

    3. Mesosphere: Lower mantle.

  • Plate Drive: Movements are powered by slab-pull and slab-push.

    • Subduction: Convergent boundaries.

    • Ridges: Divergent boundaries.

    • Faults: Transform boundaries.

5. Faults and Movements

  • Fault Classifications:

    • Normal Fault: Tension-based (plates move apart); the footwall rises.

    • Reverse Fault: Compression-based (plates move together); the footwall falls.

    • Strike-Slip Fault: Shearing-based (plates slide past each other).

  • Surface Expressions:

    • Fault Scarps: Result from sudden displacements.

    • Fault Creep: Result from gradual, slow movements.