The term “mafic” is a fundamental classification used by geologists to describe a specific group of igneous rocks. This categorization centers on the rock’s chemical and mineral composition, which dictates its physical appearance and the conditions under which it forms. Mafic rocks are defined by a high concentration of relatively heavy elements, distinguishing them from other rock types formed from molten material.
The Chemical Definition of Mafic
The word “mafic” is a portmanteau created from the first letters of the two metallic elements that dominate the chemical makeup of these rocks. The “Ma” stands for Magnesium, and the “Fic” is derived from ferric, the geological term for Iron. This high concentration of iron and magnesium ions defines the rock’s chemistry and its subsequent physical properties.
These high-density metallic ions mean that mafic rocks have a relatively low silica content compared to other igneous rock types. Geologists generally classify mafic rocks as those containing between 45% and 55% silicon dioxide (\(\text{SiO}_2\)). This lower silica percentage contrasts sharply with the composition of rocks like granite, which are much richer in the lighter element silicon.
Physical Characteristics and Identification
The abundance of iron and magnesium directly causes mafic rocks to exhibit a distinctly dark coloration, typically appearing black, dark gray, or sometimes greenish-black. This dark color comes from the specific silicate minerals that crystallize from mafic magma, such as pyroxene, amphibole, and olivine, which are naturally dark due to their high iron content.
The presence of these heavier elements also gives mafic rocks a high specific gravity. Consequently, mafic rocks feel noticeably denser than lighter-colored rocks of the same size. This dark color and high density serve as the two most reliable visual and tactile features for identifying mafic rock samples. When molten, this low-silica chemistry also results in a fluid, low-viscosity magma that flows easily.
Common Mafic Rock Types and Formation
Mafic magma originates primarily from the partial melting of the Earth’s mantle, making it the main constituent of oceanic crust. This magma rises to the surface in areas where tectonic plates are pulling apart, such as mid-ocean ridges, or over mantle plumes, like the one beneath the Hawaiian Islands. The location where the magma cools determines the final texture and rock name, even though the chemical composition remains mafic.
Basalt is the most common example, forming when mafic lava erupts onto the Earth’s surface and cools rapidly. This fast cooling rate does not allow large crystals to grow, resulting in a fine-grained, aphanitic texture where individual mineral grains are too small to be seen without magnification. Basalt forms the vast majority of the ocean floor and is the rock type that makes up shield volcanoes, which have broad, gentle slopes due to the runny nature of the lava.
Gabbro is the intrusive equivalent of basalt, sharing the same chemical composition but forming deep beneath the surface. Here, the magma cools much more slowly, allowing mineral crystals to grow larger over time. This slow process produces a coarse-grained, phaneritic texture, where individual crystals of dark minerals like pyroxene are clearly visible to the naked eye.