Earth’s Structure and Plate Tectonics: A Comprehensive Study

Posted on Mar 14, 2025 in Geology

Earth’s Interior: Methods of Study

  • Surveys and Mines: Limited to a depth of 13 km compared to Earth’s radius of 6371 km.
  • Study of Rocks: Natural processes like erosion and volcanic eruptions bring interior rocks to the surface.
  • Meteorites: Provide insights into solar system materials, similar to Earth’s composition.
  • Seismic Methods: The most effective, analyzing seismic waves from earthquakes or blasts.

Seismic Waves: P and S Waves

  • P Waves (Primary): Faster, travel through all mediums, faster in solids than liquids. Cause compression and decompression as particles vibrate in the wave’s direction.
  • S Waves (Secondary): Slower, only travel through solids. Particles vibrate perpendicularly to the wave’s direction.

Seismic Discontinuities: Changes in Velocity

Sudden changes in seismic wave velocity indicate changes in Earth’s composition.

  • Mohorovičić Discontinuity: Boundary between the crust and mantle, at 30-40 km under continents and 10 km under the ocean floor.
  • Gutenberg Discontinuity: At 2900 km, S waves stop, and P waves slow significantly, indicating a molten outer core.

Earth’s Layers: Compositional and Dynamic

  • Compositional Layers:
    • Crust and Mantle: Composed of rocks.
    • Core: Metallic, mainly iron.
  • Dynamic Layers:
    • Lithosphere: Includes the crust and a portion of the mantle.
    • Mesosphere: The rest of the mantle, plastic and malleable but solid.

Continental Drift Hypothesis: Wegener’s Theory

Alfred Wegener proposed that continents were once united. Evidence included:

  • Better fit using continental shelves, not coastlines.
  • Matching mountain ranges and rocks of the same age on different continents.
  • Similar fossils across continents.

In 1912, he presented the theory of continental drift: 200 million years ago, continents were united in Pangaea (all land). Continents slid on a continuous layer beneath them. His hypothesis was initially rejected.

Ocean Floor Relief and Composition

Reliefs:

  • Mid-Ocean Ridges: A 60,000 km range crossing ocean centers, with a central rift and fractures.
  • Trenches: Narrow, deep trenches along continental margins or volcanic island arcs.

Composition:

Volcanic rocks with accumulated marine sediments. Young lavas near the rift become older further away.

Seismic and Volcanic Belts: Active Zones

Earthquakes and volcanoes are concentrated in seismic belts. Internal activity is focused on rifts, mid-ocean ridges, and young ridges. Volcanic and seismic activity is related to the formation of these reliefs.

Lithospheric Plates: Fragments of the Lithosphere

The lithosphere is divided into plates separated by seismic belts.

Types:

  • Size: Large plates or microplates.
  • Lithosphere Type: Oceanic, continental, or mixed.

Subduction: Destruction of Oceanic Crust

Occurs in ocean trenches where the seafloor bends and sinks into the mantle. Material partially melts, feeding volcanoes on the overriding plate.

Plate Tectonics: Key Principles

  1. The lithosphere is divided into plates that fit together.
  2. Most geological activity is concentrated at plate boundaries.
  3. The ocean floor is continuously generated at ridges and destroyed by subduction.
  4. Plate movements cause new oceans to form where plates move apart, and mountain ranges to rise where they collide.

Types of Plate Boundaries

  • Constructive or Divergent: Separation creates new oceanic lithosphere at ridges and continental rifts.
  • Destructive or Convergent: Approaching and collision destroy oceanic lithosphere at trenches, island arcs, and mountain ranges.
  • Transform: Side-slide neither creates nor destroys lithosphere at transform faults.