Mafic, or basaltic, igneous rocks contain a greater portion of iron and magnesium than silica, and they are often much darker in color, typically black, dark brown or dark gray, compared to the lighter-colored igneous rocks which contain a high silica content. Mafic igneous rocks include basalt, gabbro, scoria, dolerite and tachylite. Like all igneous rocks, mafic rocks are formed from magma.
The descriptive term "mafic" is derived from the words "magnesium" and "ferric," which refers to the two elements that are found in higher concentrations within mafic rocks. These rocks are on the opposite side of the chemical-composition spectrum from the feslic rocks, which contain a higher concentration of silica.
The magma that produces mafic igneous rocks originates in the Earth's mantle, where higher temperatures, which range from about 1850 to 2150 degrees Fahrenheit, are able to melt rock that has a high content of magnesium and iron. Felsic magma is produced in the Earth's crust, which is closer to the surface, and where the temperature ranges from about 1200 to 1450 F.
Igneous rocks are also classified by how they are formed. Plutonic, or intrusive, rocks are formed by molten rock solidifying deep with the Earth. Volcanic, or extrusive, rocks are formed when magma erupts through the Earth's surface and then cools to form igneous rock.