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Some examples of felsic rocks include granite and rhyolite, while examples of mafic rocks include gabbro and basalt. [1] According to the Encyclopædia Britannica, color indices, 0–50 are felsic, 50–90 are mafic, and 90–100 are ultramafic. [6] An online geology textbook provides an example of the use of another classification scheme, in ...
Phonolite is an uncommon shallow intrusive or extrusive rock, of intermediate chemical composition between felsic and mafic, with texture ranging from aphanitic (fine-grained) to porphyritic (mixed fine- and coarse-grained).
In igneous petrology, an intermediate composition refers to the chemical composition of a rock that has 51.5–63 wt% SiO 2 being an intermediate between felsic and mafic compositions. Typical intermediate rocks include andesite and trachyandesite among volcanic rocks and diorite and granodiorite among plutonic rocks
Peridotite, a type of ultramafic rock. Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are usually composed of greater than 90% mafic minerals (dark colored, high magnesium and iron content).
Felsic magmas and lavas have lower temperatures of melting and solidification than mafic magmas and lavas. Felsic rocks are usually light in color and have specific gravities less than 3. The most common felsic rock is granite. Common felsic minerals include quartz, muscovite, orthoclase, and the sodium-rich plagioclase feldspars (albite-rich).
The rocks are therefore ultramafic, mafic and intermediate in their geochemical composition. Felsic end members can reach up to 72.1 weight % SiO 2. The SiO 2 contents correspond with the rock types cortlandtite (a melagabbro), hornblendite, hornblende diorite, meladiorite and diorite, the felsic end members with granodiorite till granite.
Assimilation is a popular mechanism to partly explain the felsification of ultramafic and mafic magmas as they rise through the crust: a hot primitive melt intruding into a cooler, felsic crust will melt the crust and mix with the resulting melt. [2] This then alters the composition of the primitive magma.
The mafic rocks also typically have a higher density than felsic rocks. The term roughly corresponds to the older basic rock class. [9] Mafic lava, before cooling, has a low viscosity, in comparison with felsic lava, due to the lower silica content in mafic magma. Water and other volatiles can more easily and gradually escape from mafic lava.