Earth’s Interior: Composition, Seismic Waves, and Plate Tectonics

Posted on Nov 29, 2024 in Geology

Earth’s Interior

Composition of Earth’s Interior

Direct Methods: Surveys (max. 13km), analysis of meteorites, analysis of lava from volcanoes (crust and mantle materials).

Indirect Methods: Studying Earth’s density (compared to the theoretical density of crustal material), studying seismic waves (behavior of primary and secondary waves).

Why is the outer core liquid and the inner core solid?

Iron Core = 90% + 10% nickel. Core temperature is 5000°C. At these temperatures, these materials are molten; thus, the outer core is fluid. However, at Earth’s center, the immense pressure forces the materials into a solid state, despite the high temperatures (all particles are compacted).

The Seismic Method

P-waves: Easily detected, transmitted through all mediums, and faster.

S-waves: Only travel through solids.

All waves: Higher speed indicates a denser, harder material; waves curve as they travel.

Surface waves: (description needed)

Representation of seismic wave propagation: What information does it give us?

Mohorovičić discontinuity: The first sudden change in P and S waves; marks the boundary between the crust and mantle (6-12km under the ocean floor and 25-70km under continents).

At around 670km, wave speed reduction occurs. A more fluid area exists between the upper and lower mantle.

With increasing depth, wave speed increases as mantle materials become more compacted.

Wiechert-Gutenberg discontinuity: No S-waves, and P-waves sharply reduce speed. Marks the boundary between the mantle and the molten outer core (around 2900km).

At 5120km, P-wave velocity increases. The change in velocity between the outer and inner core is called the Lehmann discontinuity.

Earth’s center is at 6378km.

Earth’s Internal Energy Source

  • Planetesimal impacts during Earth’s formation. This energy is retained as heat.
  • Decay of radioactive elements in Earth’s materials (Uranium and Thorium). This decay releases energy as heat.

Is Earth Cooling? Consequences?

The remaining heat from Earth’s formation is slowly radiating into space. Radioactive decay continues, but the energy generated does not offset the heat loss through radiation. The consequence is that the outer core may solidify, eliminating the convective movements that generate Earth’s magnetic field.

Continental Drift

Wegener’s Theory of Continental Drift

Alfred Wegener, a German scientist, proposed the theory of continental drift. His evidence included:

  • Geographical: Matching shapes of continental coastlines.
  • Paleontological: Identical fossils found in geographically distant locations, suggesting past connections.
  • Geological and Tectonic: Matching rock types, ages, and mountain ranges.
  • Paleoclimatic: Evidence of past climates in areas with different current climates.

Global Tectonics

Background to the Theory of Global Tectonics

Wegener’s theory, geographical coincidence of volcanoes and earthquakes, study of ocean floor relief (ridges, volcanoes), ocean floor formed by volcanic material, younger rocks closer to ridges, symmetrical age of rocks, symmetrical magnetic stripes on either side of ridges, and the theory of mantle convection.

Theory of Global Tectonics

The lithosphere is fragmented into plates that move, split, or merge. Global tectonics explains that continents move because they are carried on tectonic plates. Mantle convection drives plate movement: heat from the core causes convection currents, with hot material rising and cool material sinking. This movement agitates the lithosphere, causing plates to break.

Plates are created at mid-ocean ridges, where ocean expansion occurs as plates separate. When two plates collide:

  • If one is continental and one is oceanic, subduction occurs (the oceanic plate sinks beneath the continental plate).
  • If both are continental, obduction occurs (the continents collide and uplift, forming mountain ranges like the Himalayas).

The Wilson Cycle: Tectonic Plates Through Time

Milestones:

  1. Rift: Upwelling mantle currents thin the lithosphere, creating rifts that fill with rivers and lakes (e.g., the Red Sea).
  2. Young Ocean: Continued rifting leads to seafloor spreading and the formation of a new ocean basin.
  3. Mature Ocean: (e.g., the Atlantic Ocean). The ridge may become inactive.
  4. Fragmentation: Extensive oceans adapt to Earth’s curvature, fragmenting the lithosphere, especially at continental edges.
  5. Subduction and Collision: Oceanic crust subducts under continental crust, closing the ocean. Continental collision leads to mountain building and plate merging.