Describe the three plate boundary types and explain how they control hazard distribution

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.

Hotspots: hazards away from boundaries

Not every volcano sits on a plate boundary, and SCSA expects you to recognise the exception. A hotspot is a plume of unusually hot mantle that melts through the overlying plate, producing volcanism in a plate interior, far from any boundary. As the plate moves over the relatively fixed plume, a chain of volcanoes forms, progressively older away from the active centre; the Hawaiian island chain is the classic example. Australia carries its own record of this: the long line of extinct volcanoes down the eastern highlands, youngest in the south, formed as the Australian plate drifted northward over a hotspot. Hotspots explain why a tectonically stable plate interior can still show past volcanic activity, and they reinforce the central idea that volcanism marks where the mantle can melt and reach the surface, whether at a boundary or over a plume.