Most tectonic hazards cluster near plate boundaries. Hotspots and hazards that travel far from their source create important exceptions.
Earthquakes occur at divergent, convergent and transform boundaries because moving plates deform rock and cause faults to slip. Their largest concentration lies around the Pacific Ring of Fire. This is a broad belt around the Pacific margin, not one circular fault. Some earthquakes also occur within plates.
Many volcanoes form at divergent boundaries, where rising mantle melts as pressure falls. Others form at subduction zones, where water from a sinking slab promotes melting in the mantle above it. Continental collisions and transform boundaries can produce earthquakes without the usual volcanic arc.
Hotspots can produce volcanoes away from boundaries. Hawaii is an oceanic volcanic chain formed as the Pacific Plate moves over a hotspot. In Iceland, a hotspot overlaps a plate boundary, so both help explain the volcanism.
A hazard can affect places far from its source. Shaking is usually strongest nearer the rupture, modified by local conditions. Tsunamis can cross oceans, and winds can carry fine ash far from a volcano. Distant ports and flight routes can therefore be disrupted.
To describe a distribution, state the main pattern, name supporting locations and identify exceptions. To explain it, link those locations to the physical processes. For example, connect volcanoes around parts of the Pacific margin to subduction.
Step by step
Plate boundaries
Movement produces earthquakes; spreading and subduction also create many volcanoes.
Hotspots and other exceptions
Some volcanoes and earthquakes occur within plates.
Travelling hazards
Tsunamis and wind-blown ash can affect places far from their source.
Worked example: Far from the volcano, still affected
An ash-producing eruption can send particles into winds that carry them across a distant air route. The aircraft does not have to fly directly above the vent to be affected. The relevant evidence includes eruption height and winds as well as distance.
Watch out for this
Every place outside the Ring of Fire has no tectonic risk.
Other plate boundaries, hotspots and intraplate faults exist, and tsunamis or ash can travel beyond their source regions.
Check your understanding
A mapped belt has earthquakes along a transform boundary but no volcanic arc. Is this surprising?
- Yes. Every earthquake must be accompanied by an eruption.
- No. Sliding and locking can produce earthquakes without the usual magma-generation process.
- Yes. Earthquakes occur only where plates converge.