Magma Composition Depends on Tectonic Setting
The types of magma produced in different volcanic settings can vary significantly. At divergent boundaries and oceanic mantle plumes, magma is consistently mafic. This is because little interaction with crustal materials occurs, and magma fractionation to create felsic melts does not take place.
At subduction zones, where the magma ascends through a significant thickness of crust, a variety of processes can occur that make magma stored within a magma chamber more felsic (Figure 11.7):
- As the magma begins to cool, higher-temperature minerals such as olivine and pyroxene begin to crystallize. The minerals are denser than the surrounding magma, so they sink to the bottom of the magma chamber. The magma at the top of the chamber is more felsic because of the loss of these minerals.
- Heat from the magma can trigger partial melting of the country rock around the magma chamber. Lower-temperature felsic minerals will be the first to melt, causing the magma to become more felsic.
- If fragments of felsic country rock have broken from the walls of the magma chamber, and begin melting within the chamber, this will cause the magma to become more felsic.
- If mafic minerals have settled to the bottom of the magma chamber, and are heated again, it is possible that they could melt and mix back in with the magma. This would make the magma at the base of the chamber more mafic.
From the perspective of what happens during a volcanic eruption, there are two important differences between felsic and mafic magmas: viscosity (how easily the magma flows), and volatile (or gas) content.
Felsic magmas are more viscous than mafic magmas because they have more silica, and hence more polymerization. Felsic magmas also contain more volatile compounds. Volatile compounds behave as gases when volcanoes erupt. The most common volatile compound in magma is water, followed by carbon dioxide (CO2), then sulphur dioxide (SO2). A general relationship between the SiO2 content of magma and the amount of volatiles is shown in Figure 11.8. Although there are exceptions to the trend show in in Figure 11.8, mafic magmas typically have 1% to 3% volatiles, intermediate magmas have 3% to 4% volatiles, and felsic magmas have 4% to 7% volatiles.
Differences in viscosity and volatile level have significant implications for the nature of volcanic eruptions. When magma is deep beneath the surface and under high pressure from the surrounding rocks, the volatile compounds within it remain dissolved. As magma approaches the surface, the pressure exerted on it decreases. Volatile compounds come out of solution, and accumulate as gas bubbles. The more volatile compounds in the magma, the more bubbles form.
If the gas content is low or the magma is runny enough for gases to rise up through it and escape to surface, the pressure will not become excessive. Assuming that it can break through to the surface, the magma will flow out relatively gently. An eruption that involves a steady non-violent flow of magma is called effusive.
If the magma is felsic, and therefore too viscous for gases to escape easily, or if it has a particularly high gas content, it is likely to be under high pressure. Viscous magma doesn’t flow easily, so even if there is a way for it to move out, it may not flow. Under these circumstances pressure will continue to build as more magma moves up from beneath and more gas bubbles form. Eventually some part of the volcano will break, releasing the pent-up pressure in an explosive eruption.
Because the composition of magma depends on tectonic setting, the behaviour of volcanoes does too. Mantle plume and spreading-ridge magmas tend to be consistently mafic, so effusive eruptions are the norm. At subduction zones, the average magma composition is likely to be close to intermediate, but as we’ve seen, magma chambers can become zoned and so compositions ranging from felsic to mafic are possible. Eruption styles can be correspondingly variable
Exercise 11.2 Gas Under Pressure
A good analogy for a magma chamber in the upper crust is a plastic bottle of pop on the supermarket shelf. Go to a supermarket and pick one up off the shelf. You’ll find that the bottle is hard because it was bottled under pressure. You should be able to see that there are few gas bubbles inside.
Buy a small bottle of pop and open it. The bottle will become soft because the pressure is released, and small bubbles will start forming. If you put the lid back on and shake the bottle (best to do this outside!), you’ll enhance the processes of bubble formation. When you open the lid, the pop will come gushing out, just like an explosive volcanic eruption.
A pop bottle is a better analogue for a volcano than the old baking soda and vinegar experiment that you did in elementary school, because pop bottles, like volcanoes, come pre-charged with gas pressure. All we need to do is release the confining pressure (remove the cap) and the gases come bubbling out.
[Image: Niels Noordhoek CC-BY-SA 3.0 http://upload.wikimedia.org /wikipedia/commons/6/64/Champagne_uncorking_photographed_with_a_high_speed_air-gap_flash.jpg]