Chapter 4 Summary
The topics covered in this chapter can be summarized as follows:
| 4.1 | Plate Tectonics and Volcanism | Volcanism is closely related to plate tectonics. Most volcanoes are associated with convergent plate boundaries (at subduction zones), and there is also a great deal of volcanic activity at divergent boundaries and areas of continental rifting. At convergent boundaries magma is formed where water from a subducting plate acts as a flux to lower the melting temperature of the adjacent mantle rock. At divergent boundaries magma forms because of decompression melting. Decompression melting also takes place within a mantle plume. |
| 4.2 | Magma Composition and Eruption Style | The initial magmas in most volcanic regions are mafic in composition, but they can evolve into more felsic types through interaction with crustal rock, and as a result of crystal settling within a magma chamber. Felsic magmas tend to have higher gas contents than mafic magmas, and they are also more viscous. The higher viscosity prevents gases from escaping from the magma, and so felsic magmas are more pressurized and more likely to erupt explosively. |
| 4.3 | Types of Volcanoes | Cinder cones, which can form in various volcanic settings, are relatively small volcanoes that are composed mostly of mafic rock fragments that were formed during a single eruptive event. Composite volcanoes are normally associated with subduction, and while their magma tends to be intermediate on average, it can range all the way from felsic to mafic. The corresponding differences in magma viscosity lead to significant differences in eruptions style. Most shield volcanoes are associated with mantle plumes, and have consistently mafic magma which generally erupts as lava flows. |
| 4.4 | Volcanic Hazards | Most direct volcanic hazards are related to volcanoes that erupt explosively, especially composite volcanoes. Pyroclastic density currents, some as hot as 1000˚C can move at hundreds of km/h and will kill anything in the way. Lava flows will destroy anything in their paths, but tend to move slowly enough so that people can get to safety. |
| 4.5 | Monitoring Volcanoes and Predicting Eruptions | We have the understanding and technology to predict volcanic eruptions with some success, and to ensure that people are not harmed. The prediction techniques include monitoring seismicity in volcanic regions, detecting volcanic gases, and measuring deformation of the flanks of a volcano. |
| 4.6 | Volcanoes in British Columbia | There are examples of all of the important types of volcanoes in British Columbia, including subduction volcanism north of Vancouver, mantle-plume volcanism along the Nazco trend, and rift-related volcanism in the Wells Gray and Stikine regions. |
Questions for Review
- What are the three main tectonic settings for volcanism on Earth?
- What is the primary mechanism for partial melting at a convergent plate boundary?
- Why are the viscosity and gas content of a