Plate tectonics

The theory of plate tectonics is the foundation for understanding earthquakes, volcanoes, and major landform features. CCEA examiners expect you to explain the theory, describe the structure of the Earth, and explain what happens at each type of plate boundary.

Structure of the Earth

The Earth has four main layers:

• Inner core: solid iron and nickel; extremely hot (~5,000°C). About 1,200 km radius.
• Outer core: liquid iron and nickel; generates Earth's magnetic field. ~2,200 km thick.
• Mantle: semi-molten rock (magma); 2,900 km thick. Convection currents in the mantle drive plate movement.
• Crust: thin, rigid outer layer. Two types:
  • Continental crust: thicker (30-50 km), less dense, older; mostly granite.
  • Oceanic crust: thinner (5-10 km), denser, younger; mostly basalt. Denser oceanic crust can subduct under continental crust.

The theory of plate tectonics

The tectonic plates are rigid sections of the Earth's crust and upper mantle (together called the lithosphere) that float on the semi-molten asthenosphere (upper mantle).

Convection currents in the mantle drive plate movement: hot magma rises, moves laterally, cools, and sinks — dragging the plates above it. Plates move approximately 2-10 cm per year.

Evidence for the theory:

• Continental fit: South America and Africa fit together like jigsaw pieces.
• Fossil evidence: identical fossils found on continents now far apart (e.g. Mesosaurus in both South America and Africa).
• Paleomagnetism: magnetic stripes on the ocean floor show seafloor spreading.
• Earthquake and volcano distribution: concentrated at plate boundaries.

Types of plate boundaries

1. Constructive (divergent) plate boundary

Plates move apart. Magma rises to fill the gap, creating new oceanic crust.

Features: mid-ocean ridges (e.g. Mid-Atlantic Ridge); rift valleys on land (e.g. East African Rift Valley). Shallow-focus, relatively small earthquakes; volcanic activity creating shield volcanoes (basaltic, fluid lava, gentle slopes).

NI/British context: Iceland sits on the Mid-Atlantic Ridge — a constructive boundary. Iceland has numerous active volcanoes and geysers (Eyjafjallajökull 2010; Fagradalsfjall ongoing eruptions).

2. Destructive (convergent) plate boundary

Plates move together. Dense oceanic crust subducts beneath less dense continental (or oceanic) crust into the mantle, where it melts.

Features: ocean trenches (deepest points on Earth — Mariana Trench, 11 km deep); fold mountains (e.g. Andes, Rockies); violent volcanic eruptions; deep-focus earthquakes.

Volcanic type: composite volcanoes (stratovolcanoes) — steep sides, alternating layers of ash and lava, highly explosive (silica-rich, viscous magma). Example: Mount St Helens, Washington (1980 eruption).

Oceanic-oceanic subduction: the denser plate subducts; volcanic island arcs form (e.g. Japan, Philippines, Caribbean).

3. Conservative (transform) plate boundary

Plates move past each other horizontally. Crust is neither created nor destroyed.

Features: major fault lines (e.g. San Andreas Fault, California). No volcanic activity. Severe, shallow-focus earthquakes (e.g. San Francisco 1906; Los Angeles area regularly affected).

4. Collision plate boundary

Two continental plates converge. Neither can subduct (both less dense). The crust crumples and is pushed upward.

Features: the highest fold mountain ranges — Himalayas (India-Eurasia collision; still rising ~5 mm/year), Alps (Africa-Europe collision). Earthquakes are common; very limited volcanic activity.