Plate tectonic theory and the global distribution of hazards: the structure of the Earth, convection and plate movement, the types of plate boundary (constructive, destructive, conservative, collision), and the global distribution of earthquakes and volcanoes.

What this dot point is asking

This is the opening idea of Eduqas GCSE Geography A (C111) Theme 3, Tectonic Landscapes and Hazards, the optional theme in Component 1. Eduqas expects you to know the structure of the Earth, how convection currents move the plates, the four types of plate boundary (constructive, destructive, conservative and collision), and the global distribution of earthquakes and volcanoes and why it follows the plate boundaries.

The structure of the Earth

The Earth is layered, like an onion.

Convection and plate movement

The plates move because of heat from the core.

• Heat from the core warms the lower mantle, making it less dense so it rises; near the surface it cools, becomes denser and sinks again.
• This circular flow is a convection current, and the currents drag the plates above them, moving them a few centimetres a year (about as fast as fingernails grow).
• Additional forces include ridge push (new crust at a ridge pushes plates apart) and slab pull (a sinking subducted slab drags the rest of the plate with it).

The four types of plate boundary

Where plates meet, four things can happen.

• Constructive (divergent): plates pull apart; magma rises to fill the gap, forming new crust, gentle volcanoes and shallow earthquakes (the Mid-Atlantic Ridge).
• Destructive (convergent): oceanic and continental plates move together; the denser oceanic plate is subducted beneath the continental plate, melting to feed explosive volcanoes and triggering powerful earthquakes (the Andes, Japan).
• Conservative (transform): plates slide past each other; they stick, pressure builds and they suddenly slip, causing earthquakes but no volcanoes (the San Andreas Fault).
• Collision: two continental plates of similar density push together; neither subducts, so the crust crumples upwards into fold mountains, with earthquakes (the Himalayas).

The global distribution of hazards

Earthquakes and volcanoes are not spread evenly.

• They cluster in narrow belts that follow the plate boundaries.
• The clearest is the Pacific "Ring of Fire" around the edge of the Pacific Ocean (the Americas, Japan, the Philippines, New Zealand), where most of the world's earthquakes and active volcanoes occur.
• Other belts run through the Mediterranean and Asia to the Himalayas, and along the Mid-Atlantic Ridge.
• A few volcanoes sit far from boundaries over hotspots, where a plume of magma burns through the middle of a plate (Hawaii).

A note on magnitude

Earthquake size is measured on a magnitude scale (the moment magnitude scale, $M_w$, which has replaced the older Richter scale). The scale is logarithmic: each whole number up represents about $31.6$ times more energy released. So a magnitude $7$ earthquake releases roughly $31.6 \times 31.6 \approx 1000$ times more energy than a magnitude $5$. This is why a small rise in magnitude means a very large rise in destructive power.

Try this

Q1. Name and describe what happens at a conservative plate boundary. [4 marks]

• Cue. Two plates slide past each other; they stick, pressure builds, and a sudden slip causes an earthquake (the San Andreas Fault). No volcanoes form.

Q2. Explain why volcanoes form at a destructive plate boundary. [4 marks]

• Cue. The subducted oceanic plate melts as it sinks; the molten rock (magma) rises through the crust and erupts as a volcano.