What Are Mafic Rocks, their Composition and Examples

Mafic rocks, minerals, and magmas are common terms in geology, geosciences, or Earth sciences. You often come across them in igneous rocks and when looking at the composition of the Earth.  

Mafic is a portmanteau created from two words: magnesium and ferric + ic. Ferric means iron and comes from the Latin word ferrum.

On the other hand, mafic rocks or minerals refer to igneous rocks or silicate minerals rich in iron and magnesium.

Silicate minerals are rock-forming minerals. They are the largest and most important group, forming over 90% of the Earth’s crust.

What are they? Silicate minerals refer to any mineral with a silicate group. A silicate group is a polyatomic anion with silicon, oxygen, and a basic tetrahedron unit, i.e., SiO₄^4-.

Polyatomic anions describe a group of covalently bonded atoms with an overall negative charge. Covalent bonds share electrons.

On the other hand, the tetrahedron unit has a central atom bonded to four other bonds. In our case, silicon is connected to four oxygen atoms.

Examples of silicate minerals are quartz, feldspar, pyroxene, mica, olivine, amphibole, etc. However, not all silicate minerals are iron-rich or magnesium. Some are not.

Charles Whitman Cross, Louis V. Pirsson, Joseph P. Iddings, and Henry Stephens Washington coined mafic in 1912. They preferred the term over femag, coined a year earlier by A. Johannsen, which they felt sounded awkward.

Examples of mafic minerals

Mafic minerals are mostly dark-colored iron and magnesium-rich silicates compared to felsic counterparts. Felsic minerals have lighter-colored elements like silicon, potassium, oxygen, aluminum, and sodium.

Examples of mafic minerals are pyroxene, olivine, hornblende, and biotite. Common pyroxenes include enstatite, pigeonite, augite, and hedenbergite.

Less common mafic minerals examples include humite, chloritoid, and garnets like pyrope or almandine.

Besides being dark-colored, these minerals are denser than their felsic.  

What are mafic rocks?

Mafic rocks are dark-colored igneous rocks rich in magnesium and calcium. These rocks are rich in minerals like pyroxene, olivine, hornblende, and biotite and are associated with calcium-rich plagioclase.

Chemically, mafic rocks are relatively low in silica. They usually have 45-52 wt. % silica. Rocks with less than 45 wt. % silica are ultramafic. Those with 52-63 wt. % are intermediate, while felsic rocks have > 63 wt.% silica.

Common examples are basalt, gabbro, and diabase (dolerite). Other less common ones are norite, basanite, sideromelane, palagonite, Pele’s tears, and Pele’s hair.

These rocks can be intrusive, like peridotite, or extrusive, like basalt. Intrusive ones are coarse-grained as they cool slowly inside magma chambers deep in the crust.

On the other hand, extrusive or volcanic forms near or on the Earth’s surface. These types cool faster and will have a fine-grained to glassy texture.

Some are subvolcanic, i.e., form close to the Earth’s surface. Such have a medium grain texture like diabase.

While popular in the field, mafic isn’t an official IUGS classification scheme term. Also, it may indicate rocks with rocks whose color index is ‘M’ is 50 and 90.

Lastly, mafic rocks often correspond to basic rocks. Basic rocks are low in silica, contrasted with acidic which are high in silica.

Where are mafic rocks found?

Mafic rocks cover the oceanic crust, which is about 6-12 km (4-7 mi) thick. These rocks form at mid-ocean ridges. Mid-ocean ridges are divergent boundaries, i.e., where two plates move away from each other. They are where a new ocean lithosphere (the outer, rigid solid part of the Earth) forms.

These rocks also occur in volcanic hot spots. These are magma plumes that rise from deep inside the Earth. They occur on oceanic islands like Hawaii and form large igneous provinces. These provinces have huge deposits of mostly mafic rocks, including flood basalts.

Lastly, basalt may also occur in volcanic arcs and continental rifts but in smaller amounts.

Six characteristics or facts about mafic rocks

Mafic rocks usually form from less viscous, low-volatile magmas that erupt effusively. Also, they are more abundant, crystallize at higher temperatures, and are denser than felsic rocks.  

Here is more on these facts:

1. Crystallize at higher temperatures

Minerals that make mafic rocks crystallize in the Bowen reaction series at a relatively higher temperature range.

2. Form from low-viscosity magma

The low silica content makes mafic magma have a lower viscosity than felsic. Such magma flows easily when on the surface of the Earth. Usually, they flow as pahoehoe.

3. mafic volcanoes erupt effusively

Since they tend to be low viscosity, mafic magma tends to erupt effusively. This happens because the low viscosity allows gases to escape. However, they can also erupt explosively, especially those rich in volatiles.

4. Less stable on the surface

Most minerals in mafic rocks form at higher temperatures in drier magmas. Therefore, they are less stable in colder, wetter places near or on the Earth’s surface. Most will undergo alteration to form chlorite, uralite, iddingsite, smectite, etc.

5. Are denser

Their high amount of iron and magnesium makes them denser than felsic rocks. Therefore, they will occur deeper in the Earth’s crust. However, some, like basalt, erupt on the surface.

6. Are the most abundant rocks

Mafic rocks, especially basalt and gabbros, are the most abundant rocks in the Earth’s crust.

They cover the ocean’s crust, which makes up 70% of the Earth’s crust. Also, they occur in large igneous provinces on continental crust.

Lastly, over 50% of the world’s volcanoes are mostly or entirely basaltic.

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