3-2 Volcanic Eruptions

The geological settings of volcanic eruptions are summarized on Figures 7.1.1 and 7.1.2 of the textbook. It’s important to know where, how, and why volcanoes occur in order to be able to understand the risks associated with volcanic eruptions. Almost all volcanic eruptions start with processes that take place in the Earth’s mantle—such as mantle convection, mantle plumes, or subduction of oceanic crust into the mantle—and most involve the partial melting of mantle rock, either because of a reduction in pressure (e.g., decompression melting at mantle plumes or areas of upward-moving convection) or because of the addition of water (e.g., flux melting above subduction zones). In most cases, the magma generated in the mantle then moves up into the crust prior to an eruption. Volcanoes and volcanic eruptions are covered in Chapter 7 of the Environmental Geology textbook. Please read that chapter before or while working through this part of the course.

The importance of magma characteristics for volcanic eruptions is summarized in section 7.2, and also here in Figure 3-4. Felsic magmas have more silica, less iron and magnesium, more volatiles (water vapour and other gases) than mafic magmas, and they tend to be more viscous and lighter in colour. Eruptions involving felsic magma are also typically more violent (explosive) than eruptions of mafic magma.

The magma that erupts at subduction-related volcanoes is typically stored in a chamber in the middle to upper part of the crust, and as a result of the processes that take place there, the magma becomes more felsic than that at volcanoes above mantle plumes or at divergent boundaries. These processes are summarized on Figure 7.2.1 in the textbook, which also explains how a magma chamber can become zoned, from relatively mafic at the bottom to more felsic at the top. That zonation also contributes to the variable nature of eruption styles at composite volcanoes—from effusive to explosive, depending on the composition of the magma that is erupting.

The various types of volcanoes and volcanic eruptions are summarized and described in section 7.3 in the textbook. For the purposes of this course, we don’t need to concern ourselves with kimberlites or large igneous provinces (as they are both extremely rare), nor do we need to worry much about sea-floor volcanism which continues to tick along, largely unseen to us, at about the same steady rate that it has for hundreds of millions of years. Cinder cone eruptions tend to be relatively small, and since most are just one-time events, they don’t have that much impact on people or the environment, unless one starts erupting in your neighbourhood.

That leaves us with shield volcanoes and composite volcanoes as the really significant types of volcanic eruptions—ones that can kill or injure people (in several different ways), damage infrastructure over a wide area, and change the climate—at least on a short time scale.

Volcanic hazards are described in section 7.4 of the textbook, and summarized on Table 7.4.1. Make sure you are familiar with the seven types of hazards listed there, and how they can affect people and infrastructure near to volcanoes.

Composite volcanoes can have significant human and environmental effects for a number of reasons. One reason is that eruptions of composite volcanoes are commonly explosive, and so can produce many of the risks listed in Table 7.4.1. Another is that composite volcanoes are typically quite steep (slopes to 10 to 30°) and can be high (up to 5000 m above the surrounding terrain). Steep slopes are subject to failures such as sector collapse, and also increase the speed and severity of pyroclastic density currents. High elevation peaks are commonly snow- and ice-covered, and that contributes to the risk of lahars.

In the case of shield volcanoes, the most obvious risk comes from lava flows (which tend to be relatively slow), but gas emissions from any type of eruption can have serious implications, both globally and locally. Emissions of SO₂ and CO₂ from volcanic eruptions have both short- and long-term implications for climate, and those can have deadly consequences, as was the case following the 1783-84 eruption in Iceland. Another example of the danger of volcanic gas is the eruption of the Tseax Volcano in northern BC, as described in Box 3-3.

We are getting better at measuring the signs of pre-eruption volcanic activity, predicting eruptions and warning people to get out of harm’s way. The types of signals that are now routinely monitored around potentially active volcanoes are summarized in the list at the top of section 7.5. Some of these detection systems are only installed around volcanoes that we worry about, but, because minor seismic tremors and small earthquakes are typically the first sign that something is starting to happen, almost any volcano, except those in very remote locations, can be monitored using existing seismic networks. Exercise 7.6 Volcano Alert! gives you a chance to think about what additional types of monitoring and observation would be useful in an area that has a sufficiently good seismic network to detect the first signs of life from a dormant volcano.

Although there have been some devastating tragedies associated with volcanic eruptions, humans have gained much more from volcanic eruptions than we have lost. Some of those benefits are described in the first part of section 7.6, and illustrated on Figure 7.6.3.

Volcanic eruptions have important implications for Earth systems.  They transport solids (silicates) and gases (especially water) from the Earth’s interior to the surface, and they affect our climate, both by emitting gases like SO₂ and CO₂, and by creating mountains that influence climate patterns and consuming CO₂ when they are eroded.