L19 - Building Continents

Building Continents

  • Key Questions Addressed:
    • Why are continents elevated?
    • How are mountains supported?
    • What is isostasy?
    • How are continents built?
    • Processes at convergent boundaries

Continental Structure

  • Continental Areas:
    • Comprises continental land areas, margin, shelf, and slope.
    • Areas underlain by continental crust (less dense).

Elevation of Continents

  • Gravity and Density:
    • Denser materials sink lower (less buoyant).
    • Oceanic Plates: More dense (sinks)
    • Continental Plates: Less dense (floats)
  • Analogy: Wood vs. Styrofoam floating in water.
    • Representation of how different densities affect elevation relative to water level.

Understanding Isostasy

  • Definition:

    • Isostasy = "Iso" (equal) + "Stasy" (standing).
    • Plates float on the mantle at a height that maintains gravitational equilibrium.

  • Adjustments:

    • Loading of mass (increases load) vs. removal of mass (lightens load).
    • Isostatic Adjustment: Results in subsidence or uplift.
    • Related to the asthenosphere properties.

Processes Involved in Building Continents

  1. Addition of Igneous Material:

    • Occurs at volcanic arcs (e.g., Cascades, Japan).

  2. Accretion:

    • Accumulating material at the edges of continents (e.g., scraping sediments, volcanic seamounts).

  3. Collision and Orogenesis (Mountain Building):

    • Example: The collision of the Indian and Eurasian plates resulting in the Himalayas.
    • Features folding and thrusting of the crust, remnants of subducted slabs.

Volcanic Arcs and Subduction Zones

  • Mechanics of Magma Production:

    • Subduction involves oceanic crust recycling into the mantle, creating magma via flux melting.
    • Importance of hydrous minerals and water released during subduction.

  • Hydrothermal Activity:

    • Circulation of ocean water through crust at mid-ocean ridges contributes to geological processes.

Example of Accretion in Western Canada

  • Comprised of various accreted terranes, each with distinct geological histories.
  • Origin elements include island arcs and materials too buoyant to subduct (ophiolites).

Summary of the Wilson Cycle

  1. Opening of Ocean Basins:
    • Divergent margins resulting in continental rifts.
  2. Closure of Ocean Basins:
    • Convergent margins yielding subduction zones and continental collisions.
  3. Current Status:
    • Ongoing tectonic processes involving deformation and crustal thickening as seen in the Himalayas.