9 Hazardous environments 9.1 Hazardous environments resulting from crustal (tectonic) movement Global distribution of tectonic hazards
Revised
Tectonic hazards include seismic activity (earthquakes), volcanoes and tsunamis. Most of the world’s earthquakes occur in clearly defined linear patterns (Figure 9.1). These linear chains generally follow plate boundaries.
Earthquakes
Typical mistake
Broad belts of earthquakes are associated with subduction zones (where a dense ocean plate plunges beneath a less dense continental plate) whereas narrower belts of earthquakes are associated with constructive plate margins, where new material is formed, and plates are moving apart.
Although most earthquakes are associated with plate boundaries and tectonic activity, many earthquakes occur at great distances from plate boundaries and are not readily explained by tectonic activity.
Collision boundaries, such as in the Himalayas, are also associated with broad belts of earthquakes, whereas conservative plate boundaries, such as California’s San Andreas fault line, give a relatively narrow belt of earthquakes (this can still be over 100 km wide). In addition, there appear to be occurrences of earthquakes related to isolated plumes of tectonic activity, known as hotspots.
Eurasian plate
North American plate
Aleuthian Trench ‘Ring of Fire’ African plate
Philippine plate Arabian plate
San Andreas fault Hawaiian ‘Hot Spot’
Java Trench
Cocos plate
East Pacific Rise
Indo-Australian plate
Pacific plate
Antarctic plate
Caribbean plate Nazca plate
South American plate
Antarctic plate
Volcanoes
Subduction zone
Spreading ridge offset by transform faults
Earthquake zones
Motion of plate
Collision zone
Figure 9.1 Distribution of plates, plate boundaries, volcanoes and earthquakes
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MidAtlantic ridge
Cambridge International AS and A Level Geography Revision Guide
