Volcanic eruptions and earthquakes: WACE Year 12 Earth and Environmental Science
“Explain how plate boundary processes cause volcanic eruptions and earthquakes”
Volcanic eruptions and earthquakes are driven by plate tectonics. Most occur at plate boundaries, where plates converge, diverge or slide past one another. Convergent boundaries with subduction produce explosive volcanoes and large earthquakes, while transform boundaries mainly produce earthquakes. The Ring of Fire around the Pacific concentrates both hazards.
What this dot point is asking
SCSA wants you to link plate boundary type to the hazards produced. A strong answer explains the process at each boundary, not just where hazards happen, and distinguishes magnitude from intensity.
Plate tectonics as the driver
Earth's lithosphere is broken into plates that move slowly over the weaker asthenosphere, driven by heat from Earth's interior through processes such as convection and slab pull. Where plates interact, stress builds and is released as earthquakes, and magma can rise to form volcanoes. Plate boundaries are therefore where most geological hazards concentrate.
Subduction: one plate sinking beneath another into the mantle. Magnitude: the energy released by an earthquake, measured on the moment magnitude scale. Intensity: the shaking and damage experienced at a place. Focus and epicentre: the underground point where rupture begins and the point on the surface above it.
Convergent boundaries
At convergent boundaries plates move toward each other.
- Oceanic-continental and oceanic-oceanic convergence cause subduction. The descending plate heats and releases water, which lowers the melting point of the overlying mantle and generates magma. This magma is often viscous and gas-rich, producing explosive volcanoes such as those along the Andes and in Indonesia.
- Subduction zones also produce the largest earthquakes, because huge areas of plate lock together and then rupture suddenly. The 2004 Sumatra-Andaman earthquake occurred at such a zone.
Divergent boundaries
At divergent boundaries plates move apart, usually at mid-ocean ridges. The reduced pressure allows mantle rock to melt and rise, forming new crust. Volcanism here is generally less explosive because the magma is more fluid, and earthquakes are typically smaller and shallow. Iceland sits on a divergent boundary above a hotspot.
Transform boundaries
At transform boundaries plates slide past one another. Friction locks the plates until accumulated stress is released as an earthquake. There is little volcanism. The San Andreas Fault is the classic example, capable of large and damaging earthquakes.
Hotspots
Some volcanoes form away from plate boundaries above hotspots, where a plume of hot mantle melts the crust above it. As the plate moves over the stationary plume, a chain of volcanoes forms, as in Hawaii.
The Ring of Fire is a belt of subduction zones around the Pacific Ocean where most of the world's active volcanoes and large earthquakes occur. Australia sits in the stable interior of its plate, which is why it experiences far fewer of these hazards than its neighbours.
Measuring earthquakes: magnitude versus intensity
Magnitude measures the energy released and is a single number for the whole event, recorded on the moment magnitude scale. Intensity measures the shaking and damage at a particular place and varies with distance from the epicentre, depth of focus, and local ground conditions. Soft sediments amplify shaking, so two places at the same distance can suffer very different damage.
Australia is intraplate but not aseismic: the 1968 Meckering earthquake in Western Australia, with surface rupture, shows that significant earthquakes can occur within a plate along old fault lines.
Two earthquakes have the same magnitude, but one has a shallow focus and one a deep focus. The shallow event releases its energy close to the surface, so intensity and damage are greater. The deep event spreads its energy over a longer path to the surface, reducing intensity even though the magnitude is identical.
- Confusing magnitude with intensity
- Magnitude is the energy of the event; intensity is the shaking felt at a place and varies with location.
- Saying all volcanoes are explosive
- Subduction volcanoes are explosive; divergent and hotspot volcanoes are often more effusive.
- Forgetting subduction releases water
- Water from the sinking plate lowers the mantle melting point, which is why subduction generates magma.
- Assuming Australia has no earthquakes
- Intraplate earthquakes such as Meckering do occur, just less often than at boundaries.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2022-style7 marksExplain how the process of subduction at a convergent plate boundary generates both explosive volcanic eruptions and great earthquakes.Show worked answer →
A 7 mark answer must link the single subduction process to both hazards with mechanism.
- Subduction setting
- Where a denser oceanic plate converges with another plate, it sinks (subducts) into the mantle.
- Explosive volcanism
- As the descending slab heats, it releases water into the overlying mantle wedge. The water lowers the mantle's melting point, generating magma. Rising through the crust, this magma becomes viscous and gas-rich (high silica), so when it erupts it does so explosively, building composite volcanoes (as in Indonesia and the Andes).
- Great earthquakes
- The two plates do not slide smoothly; friction locks large areas of the interface. Stress accumulates until the locked zone suddenly ruptures, releasing enormous energy as a great earthquake (for example the 2004 Sumatra-Andaman event). Because the locked area can be huge, subduction zones produce the largest earthquakes.
Markers reward the water-release-and-melting mechanism for volcanism and the lock-and-rupture mechanism for earthquakes, both tied to the same subduction process.
WACE 2020-style6 marksCompare the volcanic and earthquake hazards produced at convergent, divergent and transform plate boundaries.Show worked answer →
A 6 mark comparison rewards a clear hazard profile for each boundary type.
- Convergent (subduction)
- Explosive, viscous volcanism from water-fluxed melting, and the largest earthquakes from sudden rupture of locked plate interfaces. The most hazardous boundary.
- Divergent (mid-ocean ridge)
- Reduced pressure melts rising mantle, producing relatively fluid, less explosive (effusive) volcanism, with smaller, shallow earthquakes.
- Transform
- Plates slide horizontally past each other; friction locks then releases as earthquakes, which can be large and shallow, but there is little volcanism (no melting mechanism).
- Summary
- Convergent boundaries combine both severe hazards; divergent boundaries give gentle volcanism and minor quakes; transform boundaries give earthquakes but essentially no volcanism.
Markers reward distinguishing the volcanism style and earthquake character at each boundary, especially the absence of volcanism at transform boundaries.