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The texture of extrusive rocks is characterized by fine-grained crystals indistinguishable to the human eye, described as aphantic. Crystals in aphantic rocks are small in size due to their rapid formation during eruption. [3] Any larger crystals visible to the human eye, called phenocrysts, form earlier while slowly cooling in the magma ...
Diorite results from the partial melting of a mafic rock above a subduction zone. It is found in volcanic arcs , and in cordilleran mountain building , such as in the Andes Mountains . However, while its extrusive volcanic equivalent, andesite, is common in these settings, diorite is a minor component of the plutonic rocks, which are mostly ...
[5] [6] [8] Felsic volcanic rocks are rare in the early Earth and only contribute to less 20% of rocks in the Archean greenstone belts worldwide. [6] In contrast, mafic volcanic rocks (such as basalt and komatiite, silicate content <52% [3]) occupy about 50% in the greenstone belts. [6] Thus, felsic volcanic rocks are rare members in the ...
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.
The series is divided into two branches, the continuous (felsic minerals: feldspars) and the discontinuous (mafic minerals).The minerals at the top of the illustration (given aside) are first to crystallize and so the temperature gradient can be read to be from high to low with the high-temperature minerals being on the top and the low-temperature ones on the bottom.
Felsic refers to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium. Molten felsic magma and lava is more viscous than molten mafic magma and lava. Felsic magmas and lavas have lower temperatures of melting and solidification than mafic magmas and lavas.
Total alkali vs. silica classification scheme (TAS), as proposed in Le Maitre's 2002 Igneous Rocks – A classification and glossary of terms [3]: 237 Phonolite is a fine-grained equivalent of nepheline syenite. They are products of partial melting, are silica-undersaturated, and have feldspathoids in their normative mineralogy.
Some syenites contain larger proportions of mafic components and smaller amounts of felsic material than most granites; those are classed as being of intermediate composition. The extrusive equivalent of syenite is trachyte. [1]