The structure of the Earth and the theory of plate tectonics; the types of plate boundary and hotspots; and how these explain the global distribution of tectonic hazards.

What this dot point is asking

Eduqas wants you to explain the structure of the Earth and the theory of plate tectonics, identify the types of plate boundary and hotspots, and use them to explain the global distribution of tectonic hazards.

The answer

The structure of the Earth and plate tectonics

The Earth is layered: a dense iron inner (solid) and outer (liquid) core, a thick mantle, and a thin crust (oceanic, dense and thin; continental, less dense and thick). The rigid lithosphere (crust plus uppermost mantle) sits on the weaker, partly molten asthenosphere and is broken into tectonic plates. These plates move a few centimetres a year, driven by mantle convection (heat from the core circulating the mantle), ridge push (gravity sliding new lithosphere away from mid-ocean ridges) and, most importantly, slab pull (the weight of a subducting plate dragging the rest down). Because the motion is concentrated at plate edges, so are the hazards.

Types of plate boundary and hotspots

Each boundary produces characteristic hazards. At constructive boundaries (the Mid-Atlantic Ridge, Iceland) plates diverge and basaltic magma rises, giving relatively gentle effusive eruptions and shallow, low-magnitude earthquakes. At destructive boundaries (the Pacific Ring of Fire) denser oceanic plate subducts, melting to feed explosive andesitic volcanoes and generating the deepest, most powerful earthquakes and tsunamis. At conservative boundaries (the San Andreas Fault) plates slide past, building and releasing strain as powerful earthquakes, but no volcanoes. At collision zones (the Himalayas) continental plates collide to build fold mountains, with strong earthquakes but no volcanoes. Hotspots, fixed plumes of rising mantle, create volcanic chains such as Hawaii far from any boundary.

Explaining the global distribution

Plate tectonics explains why tectonic hazards are not random but cluster in narrow belts along plate boundaries. The Pacific Ring of Fire (a ring of subduction zones around the Pacific) concentrates most of the world's earthquakes and volcanoes; the mid-ocean ridges mark constructive boundaries; and the Alpine-Himalayan belt marks collision and convergence from the Mediterranean to the Himalayas. Plate interiors are relatively stable, with few hazards, and the main exceptions, hotspot volcanoes like Hawaii, prove the rule because they form over plumes rather than boundaries. This is why a distribution map of earthquakes and volcanoes traces the plate boundaries.

Examples in context

Example 1. The Pacific Ring of Fire. The Pacific Ring of Fire, a horseshoe of subduction zones around the rim of the Pacific Ocean, is the clearest demonstration of plate tectonics controlling hazard distribution. Around $75\%$ of the world's volcanoes and roughly $90\%$ of its earthquakes occur here, including the most powerful (megathrust) quakes and most damaging tsunamis, because dense oceanic plate is subducting all around the basin. The Ring of Fire is the standard Eduqas case for linking destructive boundaries to explosive volcanism, deep earthquakes and tsunamis, and for explaining why hazards concentrate in belts.

Example 2. Iceland on the Mid-Atlantic Ridge. Iceland sits astride the Mid-Atlantic Ridge, a constructive (divergent) boundary where the North American and Eurasian plates pull apart, and over a hotspot, which together make it intensely volcanic but with relatively gentle, effusive basaltic eruptions (such as fissure eruptions) and mostly shallow, low-magnitude earthquakes. Iceland contrasts sharply with the explosive subduction volcanism of the Ring of Fire, and is the textbook Eduqas example of how a constructive boundary (plus a hotspot) produces a very different hazard style from a destructive one.

Try this

Q1. Name the four main types of plate boundary. [2 marks]

• Cue. Constructive (divergent), destructive (convergent), conservative (transform) and collision boundaries.

Q2. Explain why explosive volcanoes occur at destructive plate boundaries. [3 marks]

• Cue. Oceanic plate subducts and melts, and water released lowers the melting point, generating silica-rich, viscous (andesitic) magma that traps gas, so the build-up of pressure produces explosive eruptions.