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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.
These structures are lighter colored than the host rock and typically range in diameter from 0.05mm to over 5 cm. [1] [2] [3] In 1648, Aldrovandi created the term variolite for aphanitic or fine-grained igneous rocks containing varioles. [4] [5] The weathering of varioles often cause variolites to have a pock-marked appearance.
Flaked stone artefacts from Australia, made of tachylite. A second mode of occurrence of tachylite is in the form of lava flows.Basaltic rocks often contain a small amount of glassy ground-mass, and in the limburgites this becomes more important and conspicuous, but vitreous types are far less common in these than in the acid lavas.
Leucocratic mafic rocks such as leucotroctolite and leuconorite; Though co-eval, these rocks likely represent chemically-independent magmas, likely produced by melting of country rock into which the anorthosites intruded. [2] Importantly, large volumes of ultramafic rocks are not found in association with Proterozoic anorthosites. [7]
Serpentinization is a form of low-temperature (0 to ~600 °C) [5] metamorphism of ferromagnesian minerals in mafic and ultramafic rocks, such as dunite, harzburgite, or lherzolite. These are rocks low in silica and composed mostly of olivine ( (Mg 2+ , Fe 2+ ) 2 SiO 4 ), pyroxene ( XY(Si,Al) 2 O 6 ), and chromite (approximately FeCr 2 O 4 ).
A rock with over 90% mafic mineral content will be classified instead as an ultramafic rock. A gabbroic rock with less than 10% mafic mineral content will be classified as an anorthosite. [8] [13] A more detailed classification is based on the relative percentages of plagioclase, pyroxene, hornblende, and olivine. The end members are: [8] [13]
Serpentinite is a metamorphic rock composed predominantly of serpentine group minerals formed by serpentinization of mafic or ultramafic rocks. The ancient origin of the name is uncertain; it may be from the similarity of its texture or color to snake skin. [1] Greek pharmacologist Dioscorides (AD 50) recommended eating this rock to prevent ...
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 ...