As simple as black and white

I am absolutely convinced that we are all born geologists. I mean, really, what kid doesn’t have a rock collection? Sadly, most of us lose the love by middle school when the burdens and amusements of adulthood begin to cloud our realities. The lucky ones get it back.

Look at any pre-school kid’s collection, and nearly all of our budding Earth Scientists will have started to organize their treasures. Most— at least at the beginning — will have constructed two separate piles: one with dark-colored rocks and the other light.

And from such humble roots their exploration into one of the fundamental, and most important, differences between our planet’s crustal composition begins.

Which brings us to the acronyms “mafic” and “felsic” — made-up words used to indicate the chemical composition of silicate minerals, magmas, and igneous rocks, as well as a glimpse into the plate tectonic settings where they form.

Mafic is used for silicate magmas, minerals, and rocks that are enriched in the heavier elements. The term is derived from the MA in magnesium and the FIC from the Latin word for iron. (Please note that some geologists with questionable motives switch the order of the magnesium and iron and come up with the acronym “femag.” This term is not to be confused with Femag, the dull-witted henchman of the Diabolical Dr. Saprolite.)

Mafic minerals and rocks are usually dark in color and have higher specific gravities (greater than 3.0). Common rock-forming mafic minerals include olivine, pyroxene, amphibole, biotite mica, and the plagioclase feldspars. Mafic magmas represent material that is newly differentiated from the upper mantle. Common mafic rocks include basalt and gabbro. Most (but not all) mafic materials are associated with zones of crustal divergence (spreading centers), and oceanic plates.

Felsic, on the other hand, is used for silicate magmas, minerals, and rocks which have a lower percentage of the heavier elements, and are correspondingly enriched in the lighter elements, such as silicon and oxygen (together called “silica”), aluminum, and potassium. The term comes from FEL for feldspar (in this case the potassium-rich variety) and SIC, which indicates the higher percentage of silica. Most (nearly all) felsic materials are associated with the continental plates and most (but not all) are created at zones of crustal convergence (subduction zones).

Felsic minerals are usually lighter in color and have specific gravities less than 3.0. Common felsic minerals include quartz, muscovite mica, and the orthoclase feldspars. The most common felsic rock is granite, which represents the purified end product of the earth’s internal differentiation process.

It may be important to note that there are many intermediate steps in the purification process, and that many discrete varieties of intermediate magma can be produced during the conversion from mafic to felsic. We call the magmas associated with these intermediate stages “intermediate” (just one more clever name from the fine folks at GeoSpeak).