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.