Quick questions on Volcanic eruptions and earthquakes: WACE Year 12 Earth and Environmental Science

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.

At convergent boundaries plates move toward each other.

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.

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.

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.

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.