Rocks: Kinds of Igneous Rock
Igneous rocks are made from lava or magma. Lava and magma are made of hot, melted minerals. Lava is found on or near the Earth’s surface. Magma is found far beneath the Earth’s surface.
Andesite is a kind of extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture.
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey.
Dike is a planar wall rock structures, such as bedding or foliation. massive rock formations, like igneous/magmatic intrusions and salt diapirs.Dikes can therefore be either intrusive or sedimentary in origin.
Diorite is a grey to dark grey intermediate intrusive igneous rock composed principally of plagioclase feldspar biotite, hornblende, and/or pyroxene. It may contain small amounts of quartz, microcline and olivine. Zircon, apatite, sphene, magnetite, ilmenite and sulfides occur as accessory minerals
Dolerite is a mafic, holocrystalline, intrusive igneous rock equivalent to volcanic basalt or plutonic gabbro.
Felsite is a very fine grained volcanic rock that may or may not contain larger crystals. Felsite is a field term for a light colored rock that typically requires petrographic examination or chemical analysis for more precise definition
Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth’s surface and cools into a crystalline mass.
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite has a medium to coarse texture, occasionally with some individual crystals larger than the groundmass forming a rock known as porphyry. Granites can be pink to dark gray or even black, depending on their chemistry and mineralogy. Outcrops of granite tend to form tors, and rounded massifs. Granites sometimes occur in circular depressions surrounded by a range of hills, formed by the metamorphic aureole or hornfels.
Microdiorite is a medium-grained intrusive igneous rock. It contains crystals that are smaller than grains of rice, which are interlocking and randomly oriented. It is dark grey or greenish brown.
Microgranite is a medium-grained intrusive igneous rock. It contains crystals, smaller than grains of rice, which are interlocking and randomly oriented. It is pale grey and can sometimes be pinkish in colour. It contains a number of minerals, mostly feldspars, which are pale grey or pinkish, and quartz, which is grey or white. It also contains small specs of mafic minerals.
Obsidian is a kind of igneous rock that form when lava cools quickly above ground. Obsidian is actually glass and not a mixture of minerals. The edges of this rock are very sharp.
Pegmatite is a plutonic rock with exceptionally large crystals. It forms at a late stage in the solidification of granite bodies.
Peridotite is a dark igneous rocks consisting mainly of olivine and other ferromagnesian minerals
Pitchstone is a dull black glassy volcanic rock formed when lava cools swiftly.
Pumice is a kind of igneous rock which were formed when lava cooled quickly above ground. You can see where little pockets of air had been. This rock is so light, that many pumice rocks will actually float in water. Pumice is actually a kind of glass and not a mixture of minerals. Because this rock is so light, it is used quite often as a decorative landscape stone. Ground to a powder, it is used as an abrasive in polish compounds and in Lava soap.
Rhyolite has a composition similar to granite but has a much smaller grain size. It is composed of the light-colored silicates and is usually buff to pink in color. Occasionally it is light gray. In texture it is classified as aphanitic. It frequently contains voids and glassy fragments, evidence of having formed in a surface environment with rapid cooling.
Serpentinite is a kind of rock composed of one or more serpentine minerals. Minerals in this group are formed by serpentinization, a hydration and metamorphic transformation of ultramafic rock from the Earth’s mantle. The alteration is particularly important at the sea floor at tectonic plate boundaries.
Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or present in relatively small amounts.
Trachyte a fine-grained, light-colored, extrusive igneous rock, consisting chiefly of alkali feldspars. It is the extrusive equivalent to syenite.
Xenolith is a rock fragment which becomes enveloped in a larger rock during the latter’s development and hardening.