Plate Tectonics and Tectonic Hazards
Plate Tectonics Theory
The Earth's structure consists of four main layers: the inner core (solid iron and nickel, ~5,500°C), outer core (liquid iron and nickel), mantle (semi-molten rock called magma), and crust (thin outer shell). There are two types of crust:
- Continental crust — thicker (30–70 km), less dense, older (up to 3.8 billion years)
- Oceanic crust — thinner (5–10 km), denser, younger (less than 200 million years)
The crust is broken into large pieces called tectonic plates that float on the mantle. These plates move due to convection currents — heated rock in the mantle rises, spreads, cools, and sinks, dragging plates along. Additional forces include ridge push (new crust pushes plates apart at mid-ocean ridges) and slab pull (denser oceanic crust sinks at subduction zones, pulling the rest of the plate with it).
Plate Boundaries
| Boundary Type | Movement | Features | Example |
|---|---|---|---|
| Constructive (divergent) | Plates move apart | Mid-ocean ridges, shield volcanoes, shallow earthquakes | Mid-Atlantic Ridge |
| Destructive (convergent) | Plates move together | Subduction zones, fold mountains, deep ocean trenches, composite volcanoes, strong earthquakes | Andes (Nazca under South American) |
| Conservative (transform) | Plates slide past each other | Earthquakes only (no volcanoes) | San Andreas Fault, California |
| Collision | Two continental plates collide | Fold mountains, earthquakes, no volcanoes | Himalayas (Indo-Australian and Eurasian) |
Distribution of Earthquakes and Volcanoes
Earthquakes and volcanoes are concentrated along plate boundaries. The Pacific Ring of Fire contains approximately 75% of the world's active volcanoes and accounts for about 90% of earthquakes. Some volcanoes occur at hotspots away from boundaries (e.g. Hawaii) where a plume of magma rises through the crust.
Measuring Tectonic Hazards
- Magnitude — measured on the Moment Magnitude Scale (replaced the Richter Scale). Each whole number increase represents roughly 32 times more energy released.
- Intensity — the Mercalli Scale (I–XII) measures the effects felt at the surface based on observations and damage.
Why People Live Near Tectonic Hazards
- Fertile volcanic soils support agriculture (e.g. slopes of Mount Etna, Sicily)
- Geothermal energy (e.g. Iceland generates ~25% of electricity from geothermal)
- Tourism and employment opportunities
- Mineral deposits near volcanic zones (sulphur, gold, diamonds)
- Poverty and lack of choice — people cannot afford to relocate
Reducing the Risk
- Monitoring — seismometers, tiltmeters on volcanoes, gas sampling, satellite imagery
- Prediction — still impossible for earthquakes; volcanic eruptions can sometimes be forecast days/weeks ahead
- Protection — earthquake-resistant buildings (cross-bracing, deep foundations, rubber shock absorbers), land-use planning, exclusion zones
- Planning — emergency drills, evacuation routes, stockpiling supplies, public education