The rocks that comprise the Earth’s crust consist of a variety of minerals with different chemical compositions and physical properties. Minerals originate in igneous rocks, which have solidified from magma, and the types of mineral present depend not only on the chemical composition of the original magma, but on its temperature, pressure and the rate at which it cooled. In the early 20th century, the geologist Norman L. Bowen of the Geophysical Laboratory at the Carnegie Institution of Washington, D.C. carried out a series of experiments aimed at determining the sequence of crystallization of different minerals from magma. He melted powdered samples of igneous rock, then allowed them to cool to predetermined temperatures so that he could observe the formation of mineral crystals and the sequence in which they appeared. From these results, he compiled what became known as Bowen’s reaction series, a sequence of mineral formation that is widely used in geology, petrology and volcanology.
When molten rock is cooled very rapidly, there is insufficient time for minerals to form crystals; the result is instead an amorphous glass. The experimental procedure used by Bowen was designed to exploit this phenomenon to “freeze” the crystallization process at different stages. The rock samples were placed in an extremely robust container known as a “bomb” and heated to about 2,912 °F (1,600 °C), ensuring that all the material would melt. The sample was allowed to cool to a certain temperature and maintained at that temperature long enough to allow crystallization of some minerals to take place, then cooled suddenly with water in order to provide a “snapshot” of the process at that particular stage. The minerals that had already crystallized were preserved, while the rest of the material, which had still been molten, solidified into glass.
By repeating this procedure for different temperatures, Bowen’s reaction series was expanded, giving a picture of the crystalline minerals produced at temperatures ranging from 2,552 °F (1,400 °C) down to 1472 °F (800 °C). Bowen identified two distinct branches of the series, distinguished by the chemistry of the minerals, which united at lower temperatures. One, which he called the continuous series, described the sequence of crystallization of minerals rich in sodium, calcium, aluminum and silica, collectively known as plagioclases. The other, called the discontinuous series, described the sequence for minerals rich in iron and magnesium, known as mafic minerals.
The continuous series is so-called because it shows a smooth transition in the composition of the minerals formed as the temperature decreases. This is best illustrated by the relative proportions of calcium and sodium. When crystallization takes place at very high temperatures, the crystalline material is very rich in calcium and very low in sodium. With declining temperature, the ratio of sodium to calcium steadily increases, until these proportions are reversed. The proportion of silica in the minerals also increases with decreasing temperature.
In the discontinuous branch of Bowen’s reaction series, the processes are more complex. As with the continuous series, the proportion of silica increases as the temperature drops; however, instead of a steady increase in the silica content there is a sequence of quite distinct minerals: olivine, pyroxene, amphibole and biotite. Olivine is the first to crystallize — at about 2,552 °F (1,400 °C), but as the temperature drops it reacts with the still molten material, forming the next mineral in the series, pyroxene. Similar processes convert pyroxene to amphibole, and amphibole to biotite; however, each change from one mineral to the next will only occur if there is sufficient silica still present in the magma. The sequence can also stop at any point if the magma is cooled very quickly by reaching the surface, leaving minerals such as olivine, pyroxene and amphibole still present in the solidified rock, just as in Bowen’s experiments.
Where the two branches merge, the sequence continues. The remaining minerals, in increasing order of silica content, are orthoclase — also known as potassium feldspar — muscovite and quartz. Overall, Bowen’s reaction series goes from rocks that are high in calcium, magnesium and iron, and low in sodium and silica — such as basalt — to rocks that are low in calcium, magnesium and iron, and high in sodium and silica — such as granite. In a large, underground, magma chamber that cools very slowly, the olivine and high-calcium plagioclase will crystallize first and sink through the liquid magma to the bottom of the chamber, followed by other minerals in the sequence, leaving granite and similar rocks at the top by the time the whole mass has solidified. Good examples of this sequence, going from granite at the top to gabbro — a coarse crystalline rock with the same composition as basalt — at the bottom can be found at a number of locations throughout the world.