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a. (context geology English) Describing rocks, such as silicate minerals, magmas, and volcanic and intrusive igneous rocks, which contain relatively high concentrations of magnesium and iron. n. A rock with such properties.


Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron, and is thus a portmanteau of magnesium and ferric. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include basalt, dolerite and gabbro.

Chemically, mafic rocks are on the other side of the rock spectrum from the felsic rocks. The term roughly corresponds to the older basic rock class.

Mafic lava, before cooling, has a low viscosity, in comparison to felsic lava, due to the lower silica content in mafic magma. 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. Most mafic-lava volcanoes are shield volcanoes, like those in Hawaii.

| Rock texture

| Name of mafic rock

| Pegmatitic

| Gabbro pegmatite

| Coarse grained ( phaneritic)

| Gabbro

| Coarse grained and porphyritic

| Porphyritic gabbro

| Medium grained

| Diabase or Dolerite, Microgabbro

| Fine grained ( aphanitic)

| Basalt

| Fine grained and porphyritic

| Porphyritic basalt

| Pyroclastic

| Basalt tuff or breccia

| Vesicular

| Vesicular basalt

| Amygdaloidal

| Amygdaloidal basalt

| Many small vesicles

| Scoria

| Glassy texture

| Tachylyte, sideromelane, palagonite

Usage examples of "mafic".

Some of them had cracked over a platinoid-rich mafic igneous intrusion that the miners had named the Merensky Reeflets.

Once they passed a new Amex mining station manned mostly by Americans, perched on one of the rare big veins of mafic rock rich in platinoids, in Tantalus Fossae near Alba Patera.

They had been made over an area of about a million square miles, mainly in the mafic and ultra-mafic rocks of the greenstone belt.

Progressively deeper levels within the magma chamber were tapped, until after about seven hours the more mafic grey pumice was reached.

As the crust heaved and shifted, many of those mafic barges collided and merged until newborn continents towered over the waters.