In the previous article in this series, we found that the chemistry of rocks affects their density, which is important to life on earth, because the differences in various rocks’ density give us the continents on which we live. But that led us to the question of how the different rock chemistries arise, which we’ll examine in this article by looking at how plate tectonics operate.

It turns out that these different rock chemistries also have another important effect: they determine whether a volcano is likely to be dangerously explosive or relatively safe (for a volcano) in its eruptions. We will use various visuals to collect evidence to complete a data table that we — or students — can use to explain conclusions about our questions: Why do some volcanoes erupt explosively while others usually do not, and how does earth chemistry lead to this result?

Figure 1. Mineral composition of common igneous rocks. Original Source: https://commons.wikimedia.org/wiki/File:Mineralogy...

Recall that the minerals toward the right in Figure 1, like olivine and pyroxene, are mafic minerals containing iron and magnesium, while the ones on the left, such as orthoclase and quartz, are felsic minerals. Plagioclase feldspar is a continuous series of substitution of sodium or calcium, considered more mafic when high in calcium, and more felsic when high in sodium.

To review major igneous rock types and compositions, and to provide a source for new facts we’ll use for the student activity that accompanies this article, Table 1 (at the end of this article) consolidates facts. Column 1 of that table lists the rocks we will use for this study, which are also shown at the top of Figure 1. The resulting general composition of each of the rock types included in Table 1 is given in column 2 of the table.

The mechanics of different refining processes

When humans want to refine a material from a mixture, we often use physical properties of the chemicals to do so. For example, to separate all the fractions in petroleum, we vaporize the mixture and then use the varying condensation temperatures of the fractions to re-condense them in different parts of a distillation tower. To refine iron, we put ore in a blast furnace with other chemicals and eventually get the iron out at the bottom, because it is denser than the other materials. So we are using physical properties to separate and concentrate the fractions — and it turns out the same thing happens in the earth as part of plate tectonics processes.

For igneous rocks, variations in the melting temperature of minerals, which is the same as their temperature of crystallization, result in separating minerals so they aggregate as particular rock types. The range of melting temperatures for different common minerals, and the resulting rock aggregations of minerals that crystallize at similar temperatures, are shown in Figure 2. Given this data, we can see that the listed mafic minerals melt at higher temperatures than the felsic minerals; this information is what appears qualitatively as the third column of Table 1. You can also note the agreement between the general compositions listed in column 2 of Table 1 and the rock types on the right side of Figure 2.

Figure 2. Bowen's Reaction Series, showing melting point temperatures for common minerals and igneous rock types that form from minerals crystallizing at similar temperatures. Source: https://opentextbc.ca/physicalgeology2ed/chapter/3..., © Steven Earle.

This also means that if a solid rock is heated, it will be the felsic minerals that melt first. Importantly in that situation, pressures in the earth may then squeeze off the more-felsic liquid, thus physically separating the mafic and felsic fractions of the rock. This is the mechanism by which rocks that start as dense ultramafic composition in the mantle can be refined to the less-dense felsic rocks we find in the continental crust. This process is shown in Figure 3, going from upper left to lower right, showing how the mafic and felsic fractions separate in several steps, resulting in different rocks that might be plutonic or volcanic.

Figure 3. Igneous rock evolution: Sequence of steps to refine igneous rocks from ultramafic to felsic, and letter designations to match plate tectonics locations in Figure 4. Note this is a diagram of the processes only, and does not imply they occur at increasing depths in the earth. See sources at the end of this article.

Figure 3 also shows, next to the red arrows, the locations of plate tectonics processes that accomplish these steps of separating and concentrating the elements in minerals to make these rocks. Plate tectonics is the movement of the pieces of the crust of the earth, driven by heat in the earth and gravity. Plate tectonics not only moves continents around over time; it also acts as a chemical refinery, thus making this a relevant subject for chemistry.

How plate tectonics affect rock chemistry

It may help at this point to review some of the mechanisms of plate tectonics, as shown in the cross-section in Figure 4. These mechanics also align with the processes affecting the rock chemistry labeled A–D in Figure 3.

Location A is a place from which the mantle material is rising to the surface at B; this mantle material will partially melt due to decompression and make new basaltic crust. This is where the oceanic crust is diverging or spreading out and pushing plates apart, such as the rift in the middle of the Atlantic Ocean. This oceanic crust moves outward until it is pushed back into the mantle at a convergent boundary at C, also called a subduction zone. Here the basalt is partially melted. Addition of water from oceanic sediments here has lowered the melting point temperatures, and intermediate-composition igneous rocks form. This magma moves upward because it is less dense than its surroundings, and further fractionation and/or additions can occur, making it more felsic. These movements may place diorite or granite as intrusions below area D, or reach the surface as andesite or rhyolite as volcanoes above the subduction zone at D. The letters and plate tectonic processes are also shown in columns 4 and 5 of Table 1.

Figure 4. Cross-section of outer layers of earth showing plate tectonics mechanisms. By Jose F. Vigil. USGS ([1]) [Public domain], via Wikimedia Commons, with long black arrows and letters added. Note the convergent boundary at C and D is also called a “subduction zone.”

This is chemistry at work! This mechanism explains the separation of dense oceanic crust in some areas of the earth and less-dense continental crust that we examined in Part 2 of this series. Since this separation gives us the continents above the oceans on which we live, we can see that this refining of earth chemicals by plate tectonics is vital to our life on the planet. This is an excellent example of how crossing interdisciplinary boundaries to connect chemistry and earth science can lead to a broader understanding in science.

And this leads to the question: Why are some volcanoes explosive, while others seldom have explosions? To understand this, we need to look at another property that changes as the mineral composition in the igneous rocks evolves. We already recognize they change from mafic to felsic, and become less dense in the process; now we can also use the values along the bottom of Figure 1 to identify their silica percentage. These values are shown in column 6 of Table 1. We will consider the boundary between granite and diorite to be about halfway between them, and so can estimate the range of percent silica for each rock. We can see a clear trend of increasing percent of silica as the compositions evolve from the mantle at A to continental rocks at D.

Making chemistry connections

Why does this insight matter? Recall from Part 1 of this series that we noticed silicon is in the same column of the periodic table as carbon, so it makes sense that they can behave in similar ways. As previously noted, carbon makes many shapes for molecules in organic compounds, and silicon (with oxygen) makes many shapes for inorganic mineral structures; both of these indicate they develop significant inter-molecular forces in making their structures. Looking at igneous rocks, this will mean that a liquid magma that is high in silica will be held together by its inter-molecular forces, and so will be more viscous than a magma that is low in silica, and we see this in volcanoes. Since we’re dealing with rock material that is much more dense than water, we might think of low-viscosity “runny” magma as being somewhat like wet concrete coming down the trough of a concrete truck, while viscous magma would seem more like a deformable solid like wet clay or peanut butter.

To see the effect of this process, consider a general description of an explosion: a sudden release of pressure from expansion of a gas. In order to have an explosion, we have to confine (or rapidly create) a gas until it builds up pressure, then release that pressure. It is normal for a volcano to produce large quantities of gas, so the question becomes whether that gas can build up pressure. Of course, it builds up some pressure simply due to the weight of magma, but then viscosity of the magma becomes an important additional factor. Using an analogy, blowing bubbles through a straw in water does not produce explosions, but rather only sprays of water; but if you managed to blow a bubble in peanut butter, its high viscosity would result in an explosion.

And so it is with volcanoes: Using the silica percentage data in Table 1, we would predict in column 7 of Table 1 that basaltic magma would be relatively non-viscous and so would not hold gases to the point of explosion. This is correct: basaltic volcanoes such as those in Hawaii can produce high fountains of lava when there is gas pressure driving it, but then it can flow off, much like a thick river to the ocean. This also controls the shape of the volcano: the lava runs off and creates a broad mountain with a low gradient called a “shield volcano,” like the gentle curve of a warrior’s shield lying on the ground.

However, the higher silica content of andesite and rhyolite lavas will cause stronger intermolecular forces and make the lava more viscous. So this lava is thicker and will hold together as pressure builds up and finally explodes. These are dangerous volcanoes to be around, because the moment of explosion is difficult to predict, and it is the chemistry of the magma that is a dominant factor is this behavior. Also because this lava is not runny, it builds up right around the volcano and so creates a high, steep mountain: the classic cone shape. This volcano type is called a “stratovolcano,” having both layers of ash (created by explosions turning rock into powder) and layers of lava.

All of this means that the silica chemistry of a magma can be of life-and-death importance! This happens because of the actions of plate tectonics that affect rock chemistry. This connection allows us to predict where we will have explosive and non-explosive volcanoes. In Figure 4, we can see examples: Just to the left of point B is a shield volcano, which has had only one refinement step from the mantle and, like point B, is low-viscosity basalt. Again, our best example is Hawaii, which can have eruptions that are dangerous but rarely explosive. But the rock at the D points in Figure 4 has been refined so it contains more silica, and there we find the more explosive stratovolcanoes. Examples are the Cascade Range in the U.S., the Andes in South America, and indeed the entire “Ring of Fire” volcanoes around the edge of the Pacific Ocean, because subduction is occurring all around the Pacific. These effects resulting from chemistry are all around our planet.

Seeing the big picture

In this series of articles, we have examined the chemistry of the earth and some ways it affects life, particularly how the earth has built continents and volcanoes. There will be one more article to look at the opposite process: how the surface of the earth breaks down. This is generally the process of weathering; reactions to aspects of the weather and other conditions at or near the surface of the earth. There are a variety of physical actions and chemical reactions going on that, again, are vital to life on earth, and contribute to biogeochemical cycles, and we’ll look at those in the next article.

Table 1. Data summarized from this article.

Resources

Figure 3 modified from charts at

• https://slideplayer.com/slide/3909162/13/images/22/Igneous+Evolution+All+rocks+on+Earth+have+evolved+from+the+first+igneous+rocks..jpg
• http://csmgeo.csm.jmu.edu/geollab/Fichter/IgnRx/IgEvweb.html
• https://csmgeo.csm.jmu.edu/geollab/Fichter/IgnRx/Introigrx.html

Photo credit (article cover): scott/Bigstockphotos.com