9.6: Magma Composition and Viscosity - Geosciences
In the chapter on igneous rocks, you learned that the igneous rock classification is in part based on the mineral content of the rock. For example, ultramafic rocks are igneous rocks composed primarily of olivine and a lesser amount of calcium-rich plagioclase and pyroxene, whereas quartz, muscovite and potassium feldspar are the typical minerals found in felsic rocks (Figure 9.7). We need to review the mineral content of these rocks, because igneous rocks are the crystallized result of cooled magma, and the minerals that form during cooling depend on the chemical composition of the magma; for example, a mafic magma will form a mafic rock containing a large amount of the ferromagnesian minerals pyroxene and amphibole, but will not contain quartz (the mineral that is always present in felsic rocks)
Recall that most of the minerals in igneous rocks are silicate minerals, and all of the minerals shown in Bowen’s Reaction Series belong to the silicate mineral group. All silicate minerals have crystal structures containing silica tetrahedron (a silicon atom linked to four oxygen atoms), and these silica tetrahedra can be linked in a variety of ways to form sheets, linked chains, or a 3-dimensional framework. The lower temperature minerals (quartz, muscovite and orthoclase) have more linked tetrahedra than the high-temperature minerals (olivine and pyroxene). Figure 9.8A shows how the silica tetrahedra are linked to form the mineral quartz. Note that this is a 2-dimensional diagram of a 3-dimensional structure, and there are a lot more tetrahedra connected in the area above and behind the typed page. What is important about this figure is what happens to the silica tetrahedra when quartz melts; Figure 9.8B shows that even though the crystal structure is lost (the regularly repeated structure is gone), the tetrahedral links are maintained, albeit distorted. The bonds that link these tetrahedra are strong, and magma temperatures are not high enough to break these bonds. This means that magmas that can crystallize quartz will have a lot of these tetrahedra linked in the magma, whereas mafic magmas which do not contain enough silicon to crystallize quartz, will instead crystallize minerals that have fewer linked tetrahedra.
Why does the silica content (the amount of linked tetrahedra) of magma matter so much? A large amount of linked silica tetrahedra will result in magma or lava that is very viscous, meaning that it cannot flow easily (viscosity means resistance to flow). The temperature of lava also affects the viscosity; think of how ketchup from your refrigerator flows and how ketchup stored in your pantry flows; of these two fluids, the colder ketchup has the higher viscosity. In the case of magmas or lavas, the hotter the lava, the easier it flows, and the less silica that is present, the lower the viscosity (see right side of the diagram in Figure 9.7). This means that mafic lavas can flow faster than intermediate or felsic lavas. The silica content of magma affects not only the shape of the volcano but the style of eruption, whether an eruption will be lava that flows, or a magma that blows (up).
Magma (Characteristics, Types, Sources, and Evolution)
Melting of solid rocks to form magma is controlled by three physical parameters: its temperature, pressure, and composition. Mechanisms are discussed in the entry for igneous rock.
When rocks melt they do so incrementally and gradually most rocks are made of several minerals, all of which have different melting points, and the physical/chemical relationships controlling melting are complex. As a rock melts, its volume changes. When enough rock is melted, the small globules of melt (generally occurring in between mineral grains) link up and soften the rock. Under pressure within the earth, as little as a fraction of a percent partial melting may be sufficient to cause melt to be squeezed from its source. Melts can stay in place long enough to melt to 20% or even 35%, but rocks are rarely melted in excess of 50%, because eventually the melted rock mass becomes a crystal and melt mush that can then ascend en masse as a diapir, which may then cause further decompression melting.
- Basalticmagma -- SiO2 45-55 wt%, high in Fe, Mg, Ca, low in K, Na
- Andesiticmagma -- SiO2 55-65 wt%, intermediate. in Fe, Mg, Ca, Na, K
- Rhyoliticmagma -- SiO2 65-75%, low in Fe, Mg, Ca, high in K, Na
At depth in the Earth nearly all magmas contain gas dissolved in the liquid, but the gas forms a separate vapor phase when pressure is decreased as magma rises toward the surface. This is similar to carbonated beverages which are bottled at high pressure. The high pressure keeps the gas in solution in the liquid, but when pressure is decreased, like when you open the can or bottle, the gas comes out of solution and forms a separate gas phase that you see as bubbles. Gas gives magmas their explosive character, because volume of gas expands as pressure is reduced. The composition of the gases in magma are:
- Mostly H2O (water vapor) with some CO2 (carbon dioxide)
- Minor amounts of Sulfur, Chlorine, and Fluorine gases
Temperature of Magmas
- Basaltic magma - 1000 to 1200 o C
- Andesitic magma - 800 to 1000 o C
- Rhyolitic magma - 650 to 800 o C.
Viscosity of Magmas
Viscosityis the resistance to flow (opposite of fluidity). Viscosity depends on primarily on the composition of the magma, and temperature.