How do the three types of plate boundary control where Earth hazards occur?
Describe the three plate boundary types and explain how they control hazard distribution
A focused answer to the WACE Year 12 Earth and Environmental Science dot point on plate tectonics. Covers the driving mechanism, divergent, convergent and transform boundaries, the processes at each, and how they explain the distribution of volcanoes and earthquakes, including why most of Australia is tectonically quiet.
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What this dot point is asking
SCSA wants you to describe the plate boundary types and explain how they govern the distribution of Earth hazards. This is the foundation for the whole hazards section, because volcanoes, earthquakes and tsunamis all cluster at plate boundaries.
What drives plate motion
The lithosphere is broken into rigid plates that move over the weaker, slowly flowing asthenosphere. The motion is driven by:
- Mantle convection, the slow churning of the mantle as hot material rises and cool material sinks, and
- Slab pull, the dominant force, where a dense, cold subducting slab sinks into the mantle and drags the rest of its plate along.
The three boundary types
- Divergent boundaries are where plates move apart. Mantle rises to fill the gap, partly melts, and forms new oceanic crust, as at mid-ocean ridges. They produce gentle volcanism and shallow earthquakes.
- Convergent boundaries are where plates move together. Where ocean meets continent or another ocean plate, the denser plate subducts, melting at depth to feed explosive volcanoes and generating powerful earthquakes. Where two continents collide, neither subducts easily, building mountains and causing large earthquakes.
- Transform boundaries are where plates slide horizontally past each other. They produce major earthquakes but little volcanism.
Explaining hazard distribution
Plate tectonics explains why hazards are not spread evenly. Volcanoes and earthquakes concentrate in narrow belts along plate boundaries, above all around the Pacific Ring of Fire, where subduction is widespread. Mid-ocean ridges add a belt of gentle volcanism and shallow quakes, and transform faults add belts of strong earthquakes.
Australia sits in the stable interior of its plate, far from any active boundary, which is why most of the continent has few volcanoes and only modest earthquakes. The risk to Australia comes mainly from hazards generated at distant boundaries, such as tsunamis from the subduction zones to the north, which threaten the WA coast.