Mafic or Basaltic Rock

The class of rock which crystallizes from silicate minerals at relatively high temperatures is sometimes referred to as "mafic" rock. It is also sometimes called basaltic since the class includes basalt and gabbro. It is in a high temperature range of the Bowen reaction series.

The term "mafic" is a short form indicator of the presence of a relatively large concentration of iron and magnesium. The term "ferromagnesian" is also used.

Basalt and Gabbro

Basalt is a dark gray to black, fine-grained, volcanic rock. The fine-grained (aphanitic) texture indicates that it is an extrusive rock, formed at or near the surface from volcanic eruptions. It is a major constituent of lava flows on the surface and of the ocean bottoms. It is in fact the most common extrusive igneous rock. It is the major component of volcanic islands like the Hawaiian Islands and Iceland. Large portions of the states of Oregon and Washington are basalt from the volcanic activity in the region.

Basalt is typically composed of pyroxene and calcium-rich plagioclase with lesser amounts of olivine and amphibole. This fits the position in the Bowen reaction series associated with mafic rocks. Basalt with a porphyritic texture is also found, associated with partial crystallization at depth and then eruption from the surface. In these cases, the larger crystals (phenocrysts) tend to be olivine or light-colored calcium feldspar.

Gabbro can be considered to be roughly the same as basalt in compostion, but different in texture. With larger crystals, gabbro is typical of an intrusive rock, formed deep within the Earth. Its texture is called phaneritic. Pyroxene and calcium-rich plagioclase are its major constituents.