From Academic Kids
In geology, mafic minerals and rocks are silicate minerals, magmas, and volcanic and intrusive igneous rocks that have relatively high concentrations of the heavier elements. The term is a combination of "magnesium" and ferrum, the Latin word for iron [ma(gnesium) + f(errum) + ic]  (http://dictionary.reference.com/search?q=mafic). In spite of the name, mafic magmas also are rich in calcium and sodium.
Mafic minerals are usually dark in color and have a specific gravity greater than 3. Common rock-forming mafic minerals include olivine, pyroxene, amphibole, biotite and other micas, augite and the calcium-rich plagioclase feldspars. Common mafic rocks include basalt and gabbro.
In terms of chemistry, mafic rocks are on the other side of the rock spectrum from the so-called felsic rocks. The term roughly corresponds to the older basic rock class.
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. Most mafic lava volcanoes are oceanic volcanoes, like Hawaii.
|Rock Texture||Name of Mafic Rock|
|Coarse grained (phaneritic)||Gabbro|
|Coarse grained and porphyritic||Porphyritic gabbro|
|Fine grained (aphanitic)||Basalt|
|Fine grained and porphyritic||Porphyritic basalt|
|Pyroclastic||Basalt tuff or breccia|
|Many small vesicles||Scoria|
|Glassy||Tachylyte, sideromelane, palagonite|