Introduction
As described in some of the previous topics, igneous rocks are produced by the crystallization and solidification of molten magma. Magma forms when rock is heated to high temperatures (between 625 and 1200° Celsius) beneath the Earth's surface. The exact temperature needed to melt rock is controlled by several factors. Chemistry of the rock material, pressure, presence of gases (like water vapor) all influence when melting occurs. Most of the heat required to melt rock into magma comes from the Earth's central internal region known as the core. Scientists estimate that the temperature of the Earth's core is about 5000° Celsius. Heat moves from the Earth's core towards the solid outer crust by convection and conduction. Convection moves hot plumes of magma vertically from the lower mantle to the upper mantle. Some of these plumes melt through the Earth's solid lithosphere and can produce intrusive igneous features and extrusive igneous features on the surface. Heat can also be generated in the lower lithosphere through friction. The tectonic movement of subducted crustal plates can generate enough heat (and pressure) to melt rock. This fact explains the presence of volcanoes along the margin of some continental plates.
Types of Igneous Rocks
The type of igneous rocks that form from magma is a function of three factors: the chemical composition of the magma; temperature of solidification; and the rate of cooling which influences the crystallization process. Magma can vary chemically in its composition. For example, the amount of silica (SiO 2 ) found in magma can vary from 75% to less than 45%. The temperature of cooling determines which types of minerals are found dominating the rock's composition. Rocks that begin their cooling at low temperatures tend to be rich in minerals composed of silicon, potassium, and aluminum. High temperature igneous rocks are dominated by minerals with higher quantities of calcium, sodium, iron, and magnesium. The rate of cooling is important in crystal development. Igneous rocks that form through a gradual cooling process tend to have large crystals. Relatively fast cooling of magma produces small crystals. Volcanic magma that cools very quickly on the Earth's surface can produce obsidian (see image link) glass which contains no crystalline structures.
Geologists have classified the chemistry of igneous rocks into four basic types: felsic, intermediate, mafic, and ultramafic. Igneous rocks derived from felsic magma contain relatively high quantities of sodium, aluminum, and potassium and are composed of more than 65% silica. Rocks formed from felsic magma include granite (see image link), granodiorite (see image link), dacite, and rhyolite (see image link). All of these rock types are light in color because of the dominance of quartz, potassium and sodium feldspars, and plagioclase feldspar minerals (Figure 10e-1). Dacite and granodiorite contain slightly more biotite and amphibole minerals than granite and rhyolite. Rhyolite and dacite are produced from continental lava flows that solidify quickly. The quick solidification causes the mineral crystals in these rocks to be fine grained. Granite and granodiorite are common intrusive igneous rocks that are restricted to the Earth's continents. Large expanses these rocks were formed during episodes of mountain building on the Earth. Because granite and granodiorite form beneath the Earth's surface their solidification is a relatively slow process. This slow solidification produces a rock with a coarse mineral grain.
Mafic magma produces igneous rocks rich in calcium, iron, and magnesium and are relatively poor in silica (silica amounts from 45 to 52%). Some common mafic igneous rocks include fine grained basalt (see image link) and coarse grained gabbro (see image link). Mafic igneous rocks tend to be dark in color because they contain a large proportion of minerals rich in iron and magnesium (pyroxene, amphiboles, and olivine). Basalt is much more common than gabbro. It is found in the upper portion of the oceanic crust and also in vast continental lava flows that cover parts of Washington, Oregon, Idaho, and California. Gabbro is normally found in the lower parts of oceanic crust and sometimes in relatively small intrusive features in continental crust.
Andesite (see image link) and diorite are intermediate igneous rocks that have a chemistry between mafic and felsic (silica amounts between 53 to 65%). These rocks are composed predominantly of the minerals plagioclase feldspar, amphibole, and pyroxene. Andesite is a common fine grained extrusive igneous rock that forms from lavas erupted by volcanoes located along continental margins. Coarse grained diorite is found in intrusive igneous bodies associated with continental crust.
Ultramafic igneous rocks contain relative low amounts of silica (< 45%) and are dominated by the minerals olivine, calcium-rich plagioclase feldspars, and pyroxene. Peridotite is the most common ultramafic rock found in the Earth's crust. These rocks are extremely rare at the Earth's surface.
| Figure 10e-1: The classification of igneous rocks. This graphic model describes the difference between nine common igneous rocks based on texture of mineral grains, temperature of crystallization, relative amounts of typical rock forming elements, and relative proportions of silica and some common minerals. |
Igneous Rocks and the Bowen Reaction Series
In the 1920s, N.L. Bowen created the following model to explain the origin of the various types of igneous rocks (Figure 10e-2). This model, known as the Bowen reaction series, suggests that the type of igneous rocks that form from magma solidification depends on the temperature of crystallization and the chemical composition of the originating magma. Bowen theorized that the formation of minerals, which make up igneous rocks, begins with two different chemical sequences at high temperatures that eventually merge into a single series at cooler temperatures. One sequence, the discontinuous series, involves the formation of chemically unique minerals at discrete temperature intervals from iron and magnesium rich mafic magma. In the other sequence, known as the continuous series, temperature reduction causes a gradual change in the chemistry of the minerals that form calcium and sodium rich felsic magma. The discontinuous series starts with the formation of rocks that are primarily composed of the mineral olivine. Continued temperature decreases change the minerals dominating the composition of the rock from pyroxene, to amphibole, and then biotite. The continuous series produces light colored rocks rich in plagioclase feldspar minerals. At high temperatures, the plagioclase feldspar minerals are dominated with the element calcium. With continued cooling, the calcium in these minerals is gradually replaced with sodium. The convergence of both series occurs with a continued drop in magma temperature. In the merged series, the minerals within the crystallizing rock become richer in potassium and silica and we get the formation of first potassium feldspars and then the mineral muscovite. The last mineral to crystallize in the Bowen reaction series is quartz. Quartz is a silicate mineral composed of just silicon and oxygen (SiO2).
Figure 10e-2: Bowen reaction series.