Understanding Plate Tectonics: Subduction, Boundaries, and Earth’s Interior

Posted on Apr 3, 2025 in Geology

Understanding Plate Tectonics

Subduction is the process occurring in ocean trenches where the ocean floor bends and sinks into the Earth’s mantle.

Plate Tectonics Theory

  1. The lithosphere is divided into blocks (tectonic plates) that cover the Earth’s surface and fit together.
  2. Most geological activity occurs at the boundaries between these plates.
  3. The seafloor is generated at mid-ocean ridges and destroyed at subduction zones (trenches).
  4. Plates move and, in doing so, continents interact.

Plate Boundaries

  • Constructive (Divergent): Zones where two plates move apart, creating new oceanic lithosphere.
  • Destructive (Convergent): Areas where two continents collide or where subduction zones cause the ocean floor to descend into the mantle.
  • Transform: Fractures (transform faults) where two plates slide past each other horizontally, often causing earthquakes.

Subduction Types

  • Subduction under continental lithosphere: Occurs when a trench is located near a continental edge. Pressure causes a volcanic mountain range to form along the continental margin.
  • Subduction under oceanic lithosphere: Results in an arc of volcanic islands on the overriding plate, pointing towards the subducting plate.

Methods of Studying Earth’s Interior

  • Surveys and Mines
  • Study of Rocks
  • Meteorites
  • Seismic Methods

Earth’s Layers

Compositional Layers

  • Crust: Composed of rocks rich in silicon and aluminum.
  • Mantle: Composed of rocks rich in iron and magnesium.
  • Core: Primarily composed of iron; it is metallic.

Dynamic Layers

  • Lithosphere: A rigid surface layer that includes the entire crust and the uppermost part of the mantle.
  • Mesosphere: A more plastic and ductile layer, but still solid.
  • Asthenosphere: A layer beneath the lithosphere where the mantle is partially molten.
  • Outer Core (Endosphere): A molten outer layer.

The Wilson Cycle

  1. Formation of a Thermal Dome: Heat accumulates beneath the continent, causing expansion and bulging.
  2. Continental Rift Stage: Large fractures appear, thinning the lithosphere and leading to the formation of a continental rift.
  3. Narrow Sea Stage: Separation is completed, and new oceanic lithosphere begins to form with a small mid-ocean ridge.
  4. Atlantic Ocean Stage: Separation continues, and the ocean floor expands.
  5. Pacific Ocean Type Stage: The ocean begins to close due to the appearance of subduction zones along its edges.
  6. Approach Stage: Closure is nearly complete.
  7. Continental Collision: The edges of both continents and sediments in between are deformed.
  8. Final Stage: Suturing of the continental masses forms a continental collision mountain chain.

Plate Patterns

The lithosphere is divided into fragments separated by seismic belts.

Types of Plates

  • Based on size: Large plates or microplates.
  • Based on lithosphere type: Oceanic, continental, or mixed plates.

Seafloor Relief

  • Mid-Ocean Ridges: A mountain range extending over 60,000 km with a central rift valley, giving it a backbone-like appearance.
  • Trenches: Narrow and deep depressions located at continental margins.