The topics covered in this chapter can be summarized as follows:
11.1 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.
11.2 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.
11.3 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.
11.4 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. Lahars, volcano-related mudflows, can be large enough to destroy entire towns. Lava flows will destroy anything in their paths, but tend to move slowly enough so that people can get to safety.
11.5 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.
11.6 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 magma important in determining the type of volcanic rocks that will be formed when that magma is extruded?
- Why do the gases in magma not form gas bubbles when the magma is deep within the crust?
- Where do pillow lavas form? Why do they form and from what type of magma?
- What two kinds of rock textures are typically found in a composite volcano?
- What is a lahar, and why are lahars commonly associated with eruptions of composite volcanoes?
- Under what other circumstances might a lahar form?
- Explain why shield volcanoes have such gentle slopes.
- In very general terms, what is the lifespan difference between a composite volcano and a shield volcano?
- Why is weak seismic activity (small earthquakes) typically associated with the early stages of a volcanic eruption?
- How can GPS technology be used to help monitor a volcano in the lead-up to an eruption?
- What type of eruption at Mt. St. Helens might have produced columnar basalts?
- What is the likely geological origin of the Nazko Cone?
- What might be the explanation for southwestern B.C. having much less subduction-related volcanism than adjacent Washington and Oregon?
- What was the likely cause of most of the deaths from the most recent eruption at the Tseax River Cone?