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.

On the left side of Bowen’s reaction series, the minerals olivine, pyroxene, amphibole, and biotite all remove iron (Fe), magnesium (Mg), and manganese (Mn) from magma during crystallization, but do so over different temperature ranges. These iron- and magnesium-rich minerals are referred to as ferromagnesian minerals (ferro = iron) and are usually green, dark gray, or black in colour due to the absorption of visible light by iron and magnesium atoms.

On the right side of Bowen’s reaction series is a long arrow labelled plagioclase feldspar. Plagioclase treated separately because it crystallizes continuously over a large temperature range. As the magma temperature drops and plagioclase first begins to crystallize, it will take calcium atoms into its crystal structure, but as the temperature drops, plagioclase takes in sodium atoms in increasing abundance, and less and less calcium. The difference in calcium and sodium content make a difference in the appearance of plagioclase: the higher temperature calcium-rich plagioclase is dark gray in colour, while the lower temperature sodium-rich plagioclase is white.

At the bottom of Figure 3.2 are potassium feldspar, muscovite, and quartz, the low-temperature minerals that are the last to form during cooling (and therefore the first to melt as a rock is heated). These minerals form from magma that has been depleted of iron and magnesium, and so are referred to as non-ferromagnesian minerals. Non-ferromagnesian minerals are much lighter in colour. For example, the potassium-rich feldspar (also known as orthoclase) can be a pale pink or white in colour. The colour of an igneous rock will be affected by its mineral content, so a general knowledge of mineral colour is helpful for identifying and classifying igneous rocks.

Igneous Rock Composition

Effect of Mineral Content on Rock Colour

Igneous Rock Texture

The classification of igneous rocks is based not just on composition, but also on texture, the features and surface characteristics that we see in a rock. These can include the sizes of minerals, the presence of glass or rock fragments, and holes related to gas bubbles.

Mineral Crystal Size (Grain Size)

For mineral crystals to form, the chemical elements that become part of the mineral must migrate from the liquid magma to where the crystal is growing, then bond with other elements in a particular way to form the unique crystal structure for that mineral. When magma cools slowly, the chemical elements needed to form a certain mineral have more time to migrate and accumulate; that mineral can become large enough to see without the aid of a microscope.

Rocks having crystals that are visible to the unaided eye have a phaneritic (or coarse-grained) texture (from the Greek word phanerós, meaning “visible”). The rocks in Figure 3.3 all have a phaneritic texture. For most of them this is easy to spot because the different minerals show up as different colours. For the gabbro in the upper left, however, this is more difficult to see because of the dark colour. The rock in Figure 3.5B is also phaneritic. If a phaneritic rock has especially large crystals (with grains being larger than 1 cm on average), it’s called pegmatite.

When the opposite scenario happens, and magma cools quickly, there is less time for chemical elements to migrate and form large mineral crystals. In this case, many microscopic mineral crystals of a particular mineral will form; these igneous rocks have an aphanitic (or fine-grained) texture. (“Aphanitic” means “not visible.”) Figure 3.4A and 3.4C are good examples of aphanitic rocks.

Sometimes an igneous rock will have some crystals that are distinctly larger than others in the same rock. This occurs when cooling happens at different rates: slower cooling permits the larger crystals to form, and then a switch to rapid cooling caused the remaining melt to crystallize as much smaller crystals. The distinctly larger crystals are called phenocrysts. An igneous rock with phenocrysts is said to have a porphyritic texture.

Figure 3.5 shows two examples of porphyritic igneous rocks, but notice that the over-all crystal sizes of each rock are different. Figure 3.5A is both aphanitic and porphyritic, while Figure 3.5B is both phaneritic and porphyritic. In Figure 3.4B, the white spots are phenocrysts, making it aphanitic and porphyritic also.

If magma cools so quickly that there isn’t time for the chemical elements in the magma to migrate into a crystal structure, glass forms. Glass is a chaotic arrangement of chemical elements, so by definition it is not considered a mineral. Igneous rocks composed primarily of glass are said to have a glassy texture.

Obsidian (Figure 3.6A) is a dense type of glass that looks as you might expect glass to look. The identification of a glassy rock such as obsidian is easy once you recall the properties of glass; any thick glass pane or a glass bottle that is broken will have a smooth, curve shaped pattern on the broken edge called conchoidal fracture. If you look closely at the obsidian in Figure 3.6A, you will see curved surfaces on the rock. Obsidian appears dark in colour regardless of its composition because it is a dense glass, and light cannot pass through this thick glass.

Pumice (Figure 3.6B) is another igneous rock also composed primarily of glass due to a very fast rate of cooling. In pumice, the glass is stretched into very fine fibers of glass which formed during a volcanic eruption from lava that was frothy with gas. Pockets of air within the pumice are why it can float on water. The glass fibers in pumice are very thin, so they break easily, but any conchoidal fracture in these fibers is too small to see without the aid of a microscope. Pumice is distinctive because it’s remarkably light for a rock its size.

Light-coloured pumice is felsic or intermediate in composition. The mafic counterpart, called scoria, comes in dark grey, red, or black.