Plate Tectonics

Structure of the Earth

  • Earth formed around 4.6 billion years ago.
  • Surface initially molten, cooling over time.
  • Heavier metals sank to the center; lighter materials remained on the surface.

Inner Core

  • Central part of Earth, depth: 5150 km to 6371 km.
  • Composed mainly of iron, minor nickel and lighter elements.
  • Solid due to high pressure (~6000°C).
  • Conducts heat to the outer core.

Outer Core

  • Liquid layer, depth: 2900 km to 5150 km.
  • Mainly liquid iron and nickel.
  • Motion generates Earth's magnetic field, protecting from solar radiation.

Mantle

  • Thickest layer (about 75% of Earth's volume).
  • Divided into:
    • Lower Mantle: Hottest, fully molten, convection currents pattern.
    • Asthenosphere: Semi-molten, about 100-200 km deep, acts as lubricant for lithosphere.
    • Lithosphere: Rigid outer layer, 50-100 km thick, brittle material.
  • Moho: boundary separating lithosphere and asthenosphere.

The Crust

  • Oceanic Crust:
    • Thinner (average 8 km, as thin as 3 km).
    • Heavier materials (sima: silica, magnesium).
  • Continental Crust:
    • Typically 45 km thick.
    • Lighter materials (sial: silica, aluminum).

Theory of Plate Tectonics

  • Earth divided into moving plates (5-10 cm per year).
  • Helps understand earthquakes, tsunamis, volcanoes.
  • Plate boundaries are sites of interaction.

Continental Drift

  • Proposed by Alfred Wegener, connected continents fit together (Pangea).
  • Evidence supports drift:
    • Continental fit: Map alignment, jigsaw pattern.
    • Matching fossils: Identical fossils on distant continents.
    • Identical Mountain Ranges: E.g. Blue Stack Mountains vs Appalachian Mountains.
    • Glacial deposits: Found in currently warm places, indicating past positions.

Convection Currents

  • Discovered by Arthur Holmes (1928).
  • Heated magma rises, cools at crust, then sinks, driving plate movements.

Mid Ocean Ridges and Seafloor Spreading

  • Mid-Atlantic Ridge: location of new oceanic crust formation.
  • Proposed by Harry Hess using age analysis of basalt.
  • Sediment accumulation shows older crust away from the ridge.

Subduction Zones

  • Occurs at destructive convergent boundaries.
  • Oceanic plate subducts under continental plate due to weight.
  • Creates trenches (Marianas Trench) and volcanic arcs.

Types of Plate Boundaries

  1. Divergent (Constructive):

    • Plates move apart, magma rises to create new land (e.g., Mid Atlantic Ridge).
    • Features: Mid-ocean ridges, volcanic islands, rift valleys.

  2. Convergent:

    • Plates collide; subduction leads to volcanic activity.
    • Oceanic vs Oceanic: volcanic island arcs.
    • Oceanic vs Continental: e.g., Andes mountains.
    • Continental vs Continental: Mountain ranges form (e.g., Himalayas).

  3. Transformative:

    • Plates slide past one another (e.g., San Andreas Fault).
    • Friction can cause significant earthquakes.

Ireland's Geological History

  • Caledonian Orogeny: Collision of Laurentia and Avalonia around 400 million years ago formed the Blue Stack Mountains.

  • Early climate was desert-like; sediments from erosion led to red sandstone.

  • Armorican Orogeny: Collision of Eurasian and African plates around 250 million years ago; formed the Munster Ridge and Valley system.

  • Divergence/separation of NA Plate and Eurasian plate leads to modern landscape.

    • Formation of features: Giant’s Causeway, Antrim Derry Plateau.

  • Current Movement: Ireland moving north at 5 cm/year, potential for future geological evolution.