Magmatic Differentiation

The defining characteristic of igneous rocks is that at one time they were molten and part of magmas or lavas. A magma is a body of molten rock that occurs below the surface of the earth. When magma rises along a deep fault and pours out on the earth's surface, it is termed lava. This material then cooled to form a variety of intrusive and extrusive igneous rocks. Extrusive rocks crystallized from liquid magmas that reached the surface and were generally vented as volcanic lavas. Intrusive rocks crystallized from magmas that did not reach the surface but moved upward into cracks and voids deep in the crust.

When a magma cools, chemical reactions occur that create a series of different minerals. This process of differentiation occurs along two branches: discontinuous and continuous.

The discontinuous branch. The minerals that form in the discontinuous branch are all ferromagnesian—that is, they contain high percentages of iron and magnesium, which impart a dark green to black color. The branch is called discontinuous because the minerals form at discrete temperatures and not continuously during cooling. The first mineral to crystallize is olivine, followed by pyroxene, amphibole, and biotite.

The continuous branch. The continuous branch is made up of the plagioclase feldspars. The calcium/sodium ratio in this mineral type changes continuously as the magma cools. The first feldspars to form contain the highest amounts of calcium; subsequent feldspars have progressively less calcium and more sodium. These minerals tend to be pink, tan, brown, or whitish.

Any magma left over after all these reactions have been completed crystallizes at the lowest temperature as quartz.

These theories were first proven in the laboratory by N. L. Bowen in the early 1900s and are also known as Bowen's reaction series. The progression in the series explains why the first lavas from a volcanic vent are rich in iron, magnesium, and calcium, are low in quartz, and are dark green to black and why the later lavas are lighter colored and contain more quartz.