Mafic: composition, properties, and geological significance
Mafic describes silicate minerals and igneous rocks rich in magnesium and iron; typically dark, dense, and low in silica. This article explains composition, common minerals and rocks, volcanic behavior, formation, and uses.
Overview
Mafic is an adjective used in geology to describe silicate minerals and igneous rocks that are relatively rich in magnesium and iron. The name is a shorthand formed from those two elements. Mafic materials contrast with silica-rich types and form an important part of Earth's crust, especially the oceanic crust and many volcanic rocks.
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Key characteristics
Mafic rocks and minerals are commonly dark in color and tend to have higher density. Their bulk density and mass are often reflected by a specific gravity that is noticeably greater than that of lighter, silica-rich rocks. Chemically they contain lower silica and higher iron-magnesium components than felsic rocks, which affects their melting behavior and physical properties.
- Common mafic minerals: olivine, pyroxene, amphibole, various micas such as biotite, augite, and calcium-rich plagioclase feldspars.
- Typical mafic rocks: basalt (fine-grained, volcanic) and gabbro (coarse-grained, plutonic).
Formation and chemistry
Mafic magmas derive largely from partial melting of the upper mantle or from melting of pre-existing mafic crustal rocks. Because they are lower in silica than felsic magmas, mafic melts are less polymerized and therefore typically less viscous. Variations within mafic compositions lead to subclassifications such as ultramafic (very low silica) and intermediate compositions that bridge toward felsic types.
Volcanic behavior and tectonic setting
Mafic lava flows more readily than silica-rich lavas because of its lower viscosity. Volcanic eruptions of mafic composition commonly produce extensive lava flows rather than highly explosive eruptions, since gases escape more easily from low-viscosity magma. Many oceanic volcanoes and hotspot islands—Hawaiian eruptions being a familiar example—are dominated by mafic volcanism; see Hawaii as a classical case.
Uses, importance, and distinctions
Mafic rocks like basalt are widely used in construction as crushed stone and aggregate and form significant portions of seafloor and island-building volcanic material. In petrology, recognizing mafic versus felsic composition helps interpret tectonic environment, magma source, and eruption style. Notable distinctions include "mafic" versus "felsic" and further extremes such as "ultramafic," each implying different mineral assemblages and physical behavior.
History and terminology
The term "mafic" evolved as a concise descriptor in modern petrology and roughly corresponds to the older classification term "basic" for silica-poor rocks. Its formation from the words magnesium and ferric underscores the chemical signature that defines this category in igneous geology.
Microscopic image of the thin section of a diorite. The grains of non-opaque mafic minerals typically appear greenish (here predominantly hornblende) or brownish, the felsic minerals typically appear colourless to "dirty-cloudy" (width of the image section: approx. 2 mm).
Microscopic image of a thin section of the same rock with crossed polarizers. Many mafic minerals are characterized by variegated interference colours and can thus be easily distinguished from the often greyish and whitish appearing felsic minerals (here, clearly recognizable on the basis of the twin lamellae, predominantly plagioclase) (width of the image section: approx. 1.5 mm).
Questions and answers
Q: What does the term "mafic" refer to?
A: Mafic is an adjective describing a silicate mineral or igneous rock which is rich in magnesium and iron.
Q: How is the term "mafic" derived?
A: The term "mafic" is a portmanteau made from "magnesium" and "ferric".
Q: What are some features of mafic minerals?
A: Mafic minerals are usually dark in color and have a specific gravity greater than 3.
Q: What are some common rock-forming mafic minerals?
A: Common rock-forming mafic minerals include olivine, pyroxene, amphibole, biotite and other micas, augite and the calcium-rich plagioclase feldspars.
Q: What are some common mafic rocks?
A: Common mafic rocks include basalt and gabbro.
Q: What distinguishes mafic rocks from felsic rocks?
A: In terms of chemistry, mafic rocks are on the other side of the rock spectrum from the so-called felsic rocks.
Q: What is the difference between mafic and felsic lava in terms of viscosity and explosiveness?
A: Mafic lava, before cooling, has a lower viscosity than felsic lava due to its lower silica content. Water and other volatiles can more easily and gradually escape from mafic lava, so eruptions of volcanoes made of mafic lavas are less explosively violent than felsic lava eruptions.
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AlegsaOnline.com Mafic: composition, properties, and geological significance Leandro Alegsa
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