Oceanic Lithosphere, Plate Boundaries, and Mountain Formation

Posted on Feb 8, 2025 in Geology

Mid-Ocean Ridges and Oceanic Lithosphere

Mid-ocean ridges are seamounts rising above the abyssal plain, broken by transform faults. The Atlantic Ridge features a central rift valley.

Oceanic lithosphere forms in a column. Subduction zones, often located near deep ocean trenches, are areas where the lithosphere is recycled back into the Earth’s interior.

Lithospheric Plate Boundaries

The boundary between two lithospheric plates is called a plate boundary. There are three main types:

  • Oceanic ridges: Boundaries where new oceanic lithosphere is generated from material within the Earth.
  • Subduction zones: Boundaries where the lithosphere is destroyed. The displacement of one plate beneath another, at irregular rock mass and thickness, occurs in leaps. These jumps generate major earthquakes.
  • Transform faults: Boundaries where lithosphere is neither created nor destroyed. One plate moves laterally with respect to another.

A Puzzle in Motion

The edges of these moving plates define seven major plates: Eurasian, African, Indo-Australian, Pacific, North American, South American, and Antarctic. Smaller plates include the Nazca, Caribbean, and Arabian plates.

Process of Ocean Formation

  1. Lithospheric Uplift and Bowing: The lithosphere bulges and thins under pressure.
  2. Rift Formation: Tension creates fractures, causing a central sink, forming a crest with a central valley.
  3. Formation of Oceanic Lithosphere: Magma solidifies in the rift through fractures, forming a narrow sea.
  4. Ocean Formation: If the process continues, it eventually results in a fully formed ocean.

Fractures: Joints and Faults

Fractures occur when the stress on a rock exceeds its breaking point.

Joints are fractures where the blocks do not move relative to each other. Joints usually appear grouped, forming joint systems.

Faults are fractures where there has been displacement of one block relative to the other. A fault has four elements:

  • Fault plane: The fracture surface.
  • Fault lips: Two (raised lip and sunken lip).
  • Fault scarp: The measure of displacement.
  • Fault orientation: The direction and dip.

Types of Faults:

  • Normal fault: The fault plane dips toward the downthrown block.
  • Reverse fault: The fault plane dips toward the upthrown block.
  • Strike-slip fault (Tear fault): The relative displacement of the blocks is horizontal.

Mountain Formation

In subduction zones, the thinner, denser oceanic lithosphere is subducted beneath the thicker, less dense continental lithosphere. This initiates the long and complex process that results in a mountain range.

  • Formation of an accretionary prism.
  • Magmatism and metamorphism.
  • Uplift of the orogen: Crustal thickening in the continental lithosphere leads to isostatic elevation.
Elements of a Fold
  • Axial plane: Divides the fold into two halves.
  • Hinge: The area of maximum curvature.
  • Fold axis: The intersection of the axial plane with the hinge.
  • Flanks: The sides of the fold on either side of the hinge.
  • Core: The inner zone of the fold.