EARTHQUAKES AND VOLCANOES
The distribution of volcanic hazards in relation to plate boundaries:
Volcanic activity is located mainly along plate margins; either at destructive plate margins (approx. 80%) or constructive plate margins. As a result, they are commonly seen:
1. Along oceanic ridges where plates move apart (constructive), e.g. the mid-Atlantic ridge.
2. Associated with rift valleys e.g. Mt Kenya and Mt Kilimanjaro along the African rift valley.
3. On or near subduction zones e.g. the Pacific ‘ring of fire’ which is essentially a line of violent volcanoes running from Alaska to Chile where the Pacific plate and the smaller Juan de Fuca, Nazca and Cocos Plates meet the North and South American Plates.
4. Over hot spots where there is concentration of radioactive elements inside the mantle. From this, a plume of magma can rise towards and eat into the over lying plate. Where lava breaks through to the surface, active volcanoes occur above the hot spot. For example, a hot spot in the middle of the Pacific Ocean has given rise to the volcanic chain of Hawaiian islands (not connected to any plate boundary).
1. Along oceanic ridges where plates move apart (constructive), e.g. the mid-Atlantic ridge.
2. Associated with rift valleys e.g. Mt Kenya and Mt Kilimanjaro along the African rift valley.
3. On or near subduction zones e.g. the Pacific ‘ring of fire’ which is essentially a line of violent volcanoes running from Alaska to Chile where the Pacific plate and the smaller Juan de Fuca, Nazca and Cocos Plates meet the North and South American Plates.
4. Over hot spots where there is concentration of radioactive elements inside the mantle. From this, a plume of magma can rise towards and eat into the over lying plate. Where lava breaks through to the surface, active volcanoes occur above the hot spot. For example, a hot spot in the middle of the Pacific Ocean has given rise to the volcanic chain of Hawaiian islands (not connected to any plate boundary).
The distribution of earthquakes in relation to plate boundaries:
Most earthquakes occur along plate boundaries (approx. 90% of all EQ activity). Seismologists have identified 4 zones believed to be most at risk from earthquakes:
1. Constructive boundaries along the oceanic ridges. Earthquakes in this zone are mainly shallow and tend to be submarine and therefore pose little hazard to people. However there are areas of crustal tensions on land, e.g. the East African Rift Valley.
2. Destructive boundaries where major earthquakes result and represent areas of major hazard. Tsunamis are most commonly generated by these earthquakes.
3. Destructive collision margins where shallow earthquakes occur in a relatively broad zone resulting in a high hazard risk.
4. Areas of lateral crust movement (Conservative) in continental regions producing mainly shallow depth earthquakes, such as the San Andreas fault system in western USA.
It is also important to note that some earthquakes occur away from plate boundaries and are associated with the reactivation of old fault lines.
1. Constructive boundaries along the oceanic ridges. Earthquakes in this zone are mainly shallow and tend to be submarine and therefore pose little hazard to people. However there are areas of crustal tensions on land, e.g. the East African Rift Valley.
2. Destructive boundaries where major earthquakes result and represent areas of major hazard. Tsunamis are most commonly generated by these earthquakes.
3. Destructive collision margins where shallow earthquakes occur in a relatively broad zone resulting in a high hazard risk.
4. Areas of lateral crust movement (Conservative) in continental regions producing mainly shallow depth earthquakes, such as the San Andreas fault system in western USA.
It is also important to note that some earthquakes occur away from plate boundaries and are associated with the reactivation of old fault lines.