Difference between Mafic and Felsic

Updated on June 1, 2017

Geologists use the terms “mafic” and “felsic” to classify rocks into specific groups. This article aims to sift through the rubble and find out how mafic rocks are different from felsic rocks.

Definitions

Mafic
Gabbro is an example of a felsic rock

In geology, mafic is a term which describes an igneous rock or a silicate mineral rich in iron and magnesium. The term mafic is a combination of the words “magnesium” and “ferric.” The majority of mafic minerals are dark green or greenish black and are medium to coarse-grained. Minerals that usually form mafic rocks are clinopyroxene and/or orthopyroxene, olivine, biotite, and amphibole. Mafic rocks usually found in abundance include dolerite, gabbro, and basalt.

Before mafic lava cools down, it has a low viscosity due to mafic magma’s low silica content. As water, along with other volatile matter, easily and gradually escapes from mafic lava, volcanic eruptions composed of mafic lava explode less violently compared to some lava of different compositions.

felsic
Granite is a felsic rock

The term felsic describes igneous rocks that are relatively rich in the elements that form quartz and feldspar. Felsic rocks are composed of high levels of magma, silicate materials, and rocks which are also rich in potassium, oxygen, silicon, and sodium. A rock is classified as felsic if it is composed of greater than 75% felsic minerals, which are orthoclase, plagioclase, and quartz. The word “felsic” is a combination of the words “feldspar” and “silica.”

Felsic rocks are mostly light-colored and the most common type is granite. Felsic rocks are composed of minerals such as quartz, orthoclase, muscovite, and feldspars. When it comes to chemistry, felsic minerals and rocks are at the opposite end of the elemental spectrum as mafic minerals and rocks are.

Sometimes, felsic volcanic rocks contain small amounts (i.e. phenocrysts) of mafic minerals such as pyroxene, hornblende, or a feldspar mineral. In such cases, it’s customary for the rock to be named after the phenocryst mineral e.g. “hornblende-bearing felsite.”

Felsic rocks are given chemical names as determined by the TAS (Total Alkali Silica) classification, a system of assigning types of volcanic rocks relative to their alkali and silica content. In addition, if a felsic rock is found to be metamorphic with no traces of volcanic protolith (i.e. unmetamorphosed rock from which certain metamorphic rocks come from) it may just be called a “felsic schist.”

Phaneritic felsic rocks are named using the QAPF diagram. A phaneritic felsic rock is composed of crystals large enough to be seen with the naked eye. It is also common for the species of mafic minerals to be used in the name, such as pyroxene tonalite or hornblende-bearing granite – the term “granite” indicates feldspar and quartz content. Thus, the basic name of a felsic rock is determined by the rock’s texture.

Mafic vs Felsic

So what’s the difference between mafic and felsic? In geology, these terms are descriptive of the chemicals that make up igneous rocks, magmas, and silicate minerals.

“Mafic” is a term that describes igneous rocks or silicate material characterized by a rich content of iron and magnesium while felsic rocks are igneous rocks that are rich in light elements such as oxygen, aluminum, silicon, potassium, and sodium. In terms of chemistry, this means that mafic and felsic minerals and rocks are at opposite ends of the elemental spectrum.

The term “mafic” combines the words “magnesium” and “ferric” and are usually dark-colored rocks. In contrast, felsic rocks are made up of a combination of feldspar and silica and are typically light colored rocks.

Comparison Chart

MaficFelsic
Igneous rocks rich in iron and magnesiumIgneous rocks rich in oxygen, aluminum, silicon, potassium, and sodium
Usually dark-coloredUsually light-colored
Gabbro, dolerite, basaltGranite, rhyolite

Video

Here’s a lesson in geology discussing mafic and felsic rocks.