Bowen's Reaction Series - How rocks are born

Where do rocks come from? It may sound like a simple question, but the answer is full of fascinating nuances!

To understand the relationship between various minerals, and to appreciate the incredible diversity of mineral types, it's helpful to first understand how rocks actually form, deep within the earth, and what kind of processes are at work in the creation of minerals. The Bowen Reaction Series (BRS) is a great starting point for hobbyists to gain a deeper understanding of mineral formation.

You might remember from Elementary school science classes that there are three main rock types: Igneous, Sedimentary, and Metamorphic. There are many common rocks types you might see around every day - from great, granite mountains carved by glaciers, to oozing, dark basaltic lava flows that erupted into new islands, to the neatly layered grainy sandstone of rocky cliffs and beaches, to weird, folding, mashed-up metamorphic Schist and Gneiss. All of these different rock types are, or were once, an igneous rock! This is why igneous rocks are considered the "parent" of all other rocks- erosion, heat, weathering, and intense pressures can transform these seminal rocks into other types like the sedimentary, metamorphic, and geothermal rocks we also love to collect; but, to understand all other rock types, you need to first understand igneous rocks. And all igneous rocks are born based on the rules laid out in the BRS.

Bowen's Reaction Series was the invention of the Canadian Petrologist Norman L. Bowen (1887-1956), a pioneer in the field of geology. Bowen conducted the forst controlled experiments in magma crystallization, repeatedly heating and force cooling magma of different characteristics and observing the resulting rock forming minerals varying states of crystallization based on the temperature at the time that the natural cooling was stopped. During these experiments Bowen observed that chemical reactions were occuring at specific temperature ranges and causing the minerals in the mixture to change.

The BRS describes the temperatures and characteristics of igneous rock types as they crystallize out of magma. The series is divided into two groups that start separate at the top, and converge at the bottom into one group. At the top are minerals that crystallize at higher temperatures, and at the bottom, lower temperatures. On the left are the discontinuous series starting with Olivine, then Pyroxene, Amphibole, and Biotite. On the right are the continuous series that Starts with Calcium-rich Anorthite, progressing continually into Sodium-rich Oligoclase. At this point, the Oligoclase from the continuous series, and and Biotite from the discontinuous series converge into K-Spar (Potassium Feldspar), and then begin the residual phase of the series, which further goes on to Muscovite, and finally Quartz. 

The theory here, is that as magma cools, Certain types of crystals will form, and fall out (precipitate) from the mixture, and in that process, the magma will become depleted of the constituent elements and chemicals that are required to form that type of mineral. The magma continues to cool, and only the remaining leftover components are then available to form other types of mineral. This process keeps repeating until the only mineral that can form, is quartz, which is primarily silica, after the magma has been depleted of all the other "nutrients" (elements) that fed the creation of other mineral types. Think of a volume of magma flowing underground like a great vat of chicken soup, and as it slowly cools, hungry diners pick out the chicken, leaving only noodles, peas and carrots, then as it further cools, the noodles start disappearing, then the veggies, and finally all that is left is the broth. Well this happens naturally in magma, as the crystals form and precipitate out of the magma, leaving behind a soup with fewer and fewer components. 

At the top of the series, the crystals which formed at the hottest temperatures, are rocks that are considered Mafic, and at the bottom Felsic. Mafic is a portmanteau for the element symbols for Ma (Magnesium) and Fe (Iron). A magma chamber starts higher in Magnesium, progressing from Ultramafic (a relatively rare rock type) to Mafic, then Intermediate, then Felsic rocks, like Quartz that are relatively common. Felsic, as you might have inferred, is a portmanteau for Fe (Iron) and Si (Silicon). 

As a general rule, rocks that form at the top of the series are darker in color, and those forming lower are also lighter in color; the darker minerals tend to precipitate out of the mixture, leaving the lightest colored white to clear silica at the end of this long process. So dark colored, mafic rocks like komatite and olivine-rich peridotite form first, followed by medium gray basalt and black & white speckled gabbro, then lighter diorite and andesite, and finally the felsic rhyolite and granite which is mostly white with some dark specks left scattered throughout. These rocks are off similar composition within each stage of this process but are named differently depending on whether they formed as intrusive (underground) or extrusive (surface) rock.