Magmas can vary widely in composition, but in general they are made up of only eight elements. In order of importance these are oxygen, silicon, aluminum, iron, calcium, sodium, magnesium, and potassium (Figure 7.1). Of those elements, oxygen comprises a little less than half, and silicon is just over one-quarter.
The composition of magma depends on the rock it was formed from (by melting), and the conditions of that melting. Magmas derived from the mantle have higher levels of iron, magnesium, and calcium, but they are still likely to be dominated by oxygen and silicon. All magmas have varying proportions of elements such as hydrogen, carbon, and sulphur, which are converted into gases like water vapour, carbon dioxide, and hydrogen sulphide as the magma cools.
Virtually all of the igneous rocks that we see on Earth are derived from magmas that formed from partial melting of existing rock, either in the upper mantle or the crust. Partial melting is what happens when only some of the minerals within a rock melt. It takes place because different minerals have different melting temperatures.
To understand how this works, consider the mix of materials in Figure 7.2a. It contains blocks of candle wax (white), black plastic pipe, green beach glass, and pieces of aluminum wire. If you put the mixture into a warm oven at 50 °C, the wax would begin to melt into a clear liquid (Figure 7.2b), but the other materials in the mix would stay solid. That’s partial melting. Next, imagine that the oven is heated up to 120 °C. The plastic would melt too and mix with the liquid wax, but the aluminum and glass would remain solid (Figure 7.2c). This is also partial melting. If at the end of the experiment the plastic and wax “magma” mixture were poured into a separate container and let cool, the result would be a solid with a very different composition from the original mixture (Figure 7.2d).
Of course partial melting in the real world isn’t as simple as in the example in Figure 7.2. One difference is that rocks are much more complex than the four-component system used here. Another difference is that the mineral components of most rocks have similar melting temperatures, so two or more minerals are likely to melt at the same time to varying degrees. Yet another important difference is that when rocks melt, the process takes thousands to millions of years, not the 90 minutes it took in the example.
Adding Heat Does Not Melt Rocks, But Dropping Pressure and Adding Water Can
Contrary to what one might expect, most partial melting does not involve adding heat to rock. The two main mechanisms through which rocks melt are decompression melting and flux melting.
Decompression melting takes place when a body of rock within the Earth is held at approximately the same temperature but the pressure is reduced. This can happen if the rock is being moved toward the surface, either at a mantle plume (a hot spot), or in the upwelling part of a mantle convection cell. The mechanism of decompression melting is shown in Figure 7.3a. If a rock that is hot enough to be close to its melting point is moved toward the surface, the pressure is reduced, and the rock can pass to the liquid side of its melting curve. At this point, partial melting begins.
Flux melting occurs when a substance such as water is added to the system, causing the melting point of a rock to decrease. If a rock is already close to its melting point, the effect of adding water can be enough to trigger partial melting. In Figure 7.3b this is illustrated by the melting curve for dry rock (dotted line) shifting to the left.
Partial melting of rock happens in a wide range of situations, most of which are related to plate tectonics (Figure 7.4). Decompression melting happens at both mantle plumes and in the upward parts of convection systems. Rock is being moved toward the surface in these locations, so the pressure is dropping. At some point the rock crosses to the liquid side of its melting curve. At subduction zones, pressure and heat cause minerals within subducting ocean crust to release water into the overlying hot mantle. This provides the flux needed to lower the melting temperature.
In both of these cases, only partial melting takes place, typically melting only about 10% of the rock. It is always the most silica-rich components of the rock that melt, creating a magma that is more silica-rich than the rock from which it is derived. By analogy, the melt from the experiment in Figure 7.2 is richer in wax and plastic than the mixture from which it was derived. The magma that is produced is less dense than the surrounding rock. It moves up through the mantle and eventually into the crust.
Cooling Magma Becomes More Viscous
As the magma moves toward the surface it interacts with the surrounding rock, especially when it moves from the mantle into the lower crust. This typically leads to partial melting of the surrounding rock because the magma is often hotter than the melting temperature of rocks in the crust. (This is one of the less common instances where melting is caused by an increase in temperature.) Again, the more silica-rich parts of the surrounding rock are preferentially melted, and this contributes to an increase in the silica content of the magma.
At very high temperatures (over 1300°C), most magma is entirely liquid because there is too much energy for the atoms to bond together. As the magma loses heat to the surrounding rocks and the temperature drops, things start to change. Silicon and oxygen combine to form silica tetrahedra. As cooling continues, the tetrahedra start to link together to make chains (they polymerize). These silica chains have the important effect of making the magma more viscous (less runny), and as we’ll see in Chapter 11, magma viscosity has significant implications for volcanic eruptions. As the magma continues to cool, crystals start to form.
Exercise 7.1 Making Magma Viscous
This is a quick and easy experiment that you can do at home to help you understand the properties of magma. It will only take about 15 minutes, and all you need is half a cup of water and a few tablespoons of flour.
If you’ve ever made gravy, white sauce, or roux, you’ll know how this works.
Place about 1/2 cup (125 mL) of water in a saucepan over medium heat. Add 2 teaspoons (10 mL) of white flour (this represents silica) and stir while the mixture comes close to boiling. It should thicken like gravy because the gluten in the flour becomes polymerized into chains during this process.
Now you’re going to add more “silica” to see how this changes the viscosity of your magma. Take another 4 teaspoons (20 mL) of flour and mix it thoroughly with about 4 teaspoons (20 mL) of water in a cup and then add all of that mixture to the rest of the water and flour in the saucepan. Stir while bringing it back up to nearly boiling temperature, and then allow it to cool. This mixture should slowly become much thicker because there is more gluten and more chains have been formed (see the photo).
This is analogous to magma, of course. As we’ll see below, magmas have variable contents of silica and therefore have widely varying viscosities (thicknesses) during cooling.
- Mantle plumes and mantle convection are described in Chapters 3 and 4. ↵