Chapter 3. Igneous Rocks
Overview of Igneous Rocks
3.2 IGNEOUS ROCK ORIGIN
3.2.1 Magma Composition
Before any igneous rock can form, there must be molten material, known as magma, produced; therefore, you must have a rock to melt to make the magma, which then cools to become an igneous rock. There are many factors which may affect the composition of the igneous rock, including the initial rock or rocks that melt to form the magma, the degree of melt, the cooling process of the magma, etc. These are things one should consider when studying the origin of igneous rocks.
Most rocks contain minerals that are crystalline solids composed of the chemical elements. All minerals have a certain set of conditions, such as temperature, at which they can melt; since rocks contain a mixture of minerals, some of the minerals in a rock may melt, while others remain solid. Temperature conditions are important, as only minerals that can melt at “lower” temperatures may experience melting, whereas the temperature would have to increase in order for other minerals to also melt and add their chemical components to the magma that is being generated. Therefore, if the same types of rocks are melting, different magma compositions are generated simply by melting at different temperatures!
Eventually, magma will start to rise upward through the Earth’s lithosphere, as it is more buoyant than its source rock. This separation of magma from source region will result in new thermal conditions, which results in the magma cooling. As with melting, minerals also have a certain set of conditions at which they form, or crystallize, from within a cooling magma body. The sequence of mineral crystallization is the opposite sequence of crystal melting, which had been experimentally determined by Norman L. Bowen in the early 1900’s, and known as Bowen’s reaction series (e.g. Figure 3.1)
This “reaction series” refers to the chemical reactions that are the formation of minerals, through chemical bonding of elements within the magma, in a sequence that is based on falling magma temperatures. Close examination of Figure 3.1 shows that the first mineral to crystallize in a cooling magma of ultramafic composition is olivine; the length of the arrow indicates the range of temperatures at which olivine can form. Once temperatures fall below this range, olivine crystals will no longer form; instead, other minerals such as pyroxene will start to crystallize (a small interval of temperatures exists where both olivine and pyroxene can crystallize). Minerals that form in cooling magma are called crystals, or phenocrysts; as these phenocrysts are forming, they are removing chemical elements from the magma. For example, olivine phenocrysts take magnesium (Mg) and iron (Fe) from the magma and incorporate them into their crystal structure. This behavior of mineral phenocrysts to take certain chemical elements into their structure, while excluding other elements, means that the composition of the magma m