Geochemistry: Earth's Layers, Plate Tectonics, and Wilson Cycle

Geochemistry: Understanding the Earth's Composition and Dynamics

Earth's Layers Based on Chemical Composition

1. Crust: Extends to the Mohorovicic seismic discontinuity. It is a solid layer composed of silicified rock. There are two types:
   • Continental crust
   • Oceanic crust (thinner and denser)
2. Mantle: Located between the Moho and Gutenberg discontinuities. It consists of silicate rocks that are slightly denser than the crust. It is divided into the upper and lower mantle.
3. Core: The innermost layer of the Earth, beneath the Gutenberg seismic discontinuity. It is composed of metal. The Lehmann seismic discontinuity separates the outer core from the inner core.

Earth's Layers Based on Dynamic Properties

1. Lithosphere: Composed of rigid and fragile rock. It includes the crust and part of the upper mantle.
2. Upper Mantle: An area where mantle rocks exhibit partial melting and behave plastically.
3. Mesosphere: Characterized by a change in the mineral phase, where minerals become denser.
4. Endosphere: Has an outer layer that is melted, where materials occur in streams or flows, and an inner solid, very dense layer.

Tectonic Plate Boundaries

• Divergent Boundaries: Characterized by the separation of two plates.
• Convergent Boundaries: Where two tectonic plates collide:
  • Ocean-Ocean: Leads to subduction, forming island arcs (e.g., Japan).
  • Ocean-Continent: Results in subduction, creating marginal mountain ranges (e.g., Himalayas).
  • Continent-Continent: Causes obduction, forming intracontinental mountain ranges (e.g., Andes).
• Neutral or Passive Boundaries: Where two plates slide horizontally along a plane, known as transform faults.

The Wilson Cycle

1. Hot mantle material ascends, pushing up the continental lithosphere and causing it to swell until it fractures due to its rigidity.
2. The expansion of the ocean floor creates a narrow sea (e.g., Red Sea).
3. Continued growth of the ocean floor leads to the development of a mature oceanic basin (e.g., Atlantic Ocean).
4. Stable continental margins become active when the oceanic lithosphere begins to subduct (e.g., Pacific Ocean).
5. The reduction of the ocean becomes evident.
6. The oceanic lithosphere subducts completely, the ocean closes, and continents collide through obduction, forming an intracontinental mountain range.