Overview of Igneous Rocks
How Igneous Rocks Form
Magma Composition
Before any igneous rock can form, molten material—known as magma—must be produced. That means you must have a rock to melt in the first place to make the magma that will eventually cool to become an igneous rock. The composition of the original rock (or rocks) that melted is one of the factors that controls the composition of the igneous rock that forms once the melt cools. Other factors are how much of the original rock actually melts, and the cooling process of the magma.
Rocks are often made up of a mixture of minerals. For each mineral, there is a unique set of conditions (such as pressure and temperature) under which that mineral can melt. For a rock with a mixture of minerals, this means that under certain conditions, some of the minerals in the rock may melt, while other minerals remain solid. Because some minerals melt at lower temperatures than others, temperature conditions determine which minerals will add their chemical components to the magma that forms. If temperatures are low enough, some of the minerals might not melt at all. Therefore, even if the same types of rocks are melting, different magma compositions can be generated simply by melting at different temperatures!
Cooling & Mineral Formation
Eventually, magma will start to rise through Earth’s lithosphere, because it’s more buoyant than its source rock. When the magma moves away from its source region, it encounters new thermal conditions, and begins to cool. As the magma cools, the temperature begins to drop beneath the melting points of different minerals. The sequence in which minerals crystallize is the opposite of the melting sequence, such that minerals with high melting points form first as the magma cools. The order is summarized in Bowen’s reaction series (Figure 3.2) named after Normal L. Bowen, who performed early experiments on cooling melts.
“Reaction series” refers to the sequence of chemical reactions between elements within magma that result in the formation of minerals as the temperature falls. On the diagram, the sequence proceeds from top to bottom. The length of the arrow indicates the range of temperatures at which a particular mineral can form.
The first mineral to crystallize in a cooling magma of ultramafic composition is olivine. Once the temperature falls below this range, olivine crystals will no longer form; instead, other minerals such as pyroxene will start to crystallize. Note that more than one mineral might be forming at a given temperature; for example, within a certain range of temperatures, chemical reactions are forming both olivine and pyroxene.
As mineral crystals form in cooling magma, they take some chemical elements from the magma into their crystal structure, and exclude others. In the case of olivine, magnesium (Mg) and iron (Fe) are taken in, leaving the remaining magma with less Mg and Fe than before crystallization started. This means that the composition of the