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Bowen's Reaction Series

© Geoff Habiger

We have already discussed the basic types of igneous rocks (see my early article from the rock cycle on igneous rocks), and have discussed the basic types of igneous intrusions and the interior of the Earth (see my articles on igneous intrusions and the earths interior). We have discussed how igneous rocks can be identified based on whether they erupted from a volcano (extrusive rocks) or were intruded by a large pluton (intrusive rocks). But, how do the different rock types (granites, basalts, syenites) get to their final composition?

Many petrologists (a geologist who specializes in studying rocks) have spent their careers studying how different rocks can form from the magma in the Earth. What they have learned is that magmas evolve by a variety of different methods. One of these methods is called differentiation, a process where a single homogeneous (uniform) magma can produce a variety of chemically different igneous rocks.

The main process of differentiation is called fractional crystallization. In this process the minerals that precipitate out of the magma are prevented from reacting with the magma, either by removal from the magma or separation from the still liquid part of the magma. This causes the magma to become depleted in some minerals and saturated in others, producing different rock compositions as the magma cools. Another method for magmas to evolve, but not by differentiation, is called equilibrium crystallization. In this method, as the magma cools different minerals precipitate from it. But instead of being separated from the magma the minerals continue to react chemically and form different minerals. Since the magma and the minerals that precipitate are of different compositions, the magma will change composition as minerals crystallize and react chemically with the magma. The difference between these two methods is that during fractional crystallization it is possible for magma of one composition (basalt) to produce rocks of a different composition (granite). In equilibrium crystallization a magma that was basaltic in composition results in basalt, and a magma that was granitic in composition results in granite.

In the early 1900’s a Canadian geologist, Norman L. Bowen (1887 - 1956) combined field studies with laboratory work to come up with a theory for fractional crystallization. Bowen assumed that there was a single parent magma that was basaltic in composition from which all other magma types were evolved. Bowen’s work was summarized by a process called Bowen’s Reaction Series. Today we know that all igneous rocks are not created from the differentiation of a parent melt of basaltic composition. (Melt is a term used to describe the liquid state of magma.) In fact, there are probably several types of reaction series for the formation of igneous rocks from magmas. Despite this, Bowen’s work is still important as it helps a geologist understand the sequence and relationship of a variety of igneous rock types.