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In a widely accepted silica-content classification scheme, rocks with more than 65 percent silica are called felsic; those with between 55 and 65 percent silica are intermediate; those with between 45 and 55 percent silica are mafic; and those with less than 45 percent are ultramafic.
Mafic rocks have between 45 and 55% of silica, whereas felsic rocks have over 65% of silica, the highest of all types. Due to the color of the minerals forming them, the rocks are either light or dark.
For igneous rock, the composition is divided into four groups: felsic, intermediate, mafic, and ultramafic. These groups refer to differing amounts of silica, iron, and magnesium found in the minerals that make up the rocks.
Mafic and felsic rocks represent the extremes of igneous rock compositions, each playing a crucial role in the Earth’s geology. Mafic rocks, with their dark color, higher density, and formation in oceanic settings, contrast with the lighter, less dense felsic rocks that form much of the continental crust.
Felsic rocks crystallize at lower temperatures of 600-750°C (1112°F-1382°F), while mafic rocks crystallize at higher temperatures of 1000°C (1832°F) to 1200 °C (2192°F). Therefore, in magmas, mafic minerals tend to crystallize earlier and mafic later.
Felsic rocks are high in silica (SiO2), while mafic rocks are low in silica. This difference in silica content gives the two types of rocks different physical and chemical properties. Mafic is a term used to describe silicate minerals, magma, and igneous rocks which are rich in magnesium and iron.
Igneous rocks can be classified based on their chemical and mineral composition into four primary categories: ultramafic, mafic, intermediate, and felsic. Here’s a detailed overview of each category including their composition, characteristics, examples, and formation processes.