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Igneous and metamorphic rocks make up 90–95% of the top 16 kilometres (9.9 mi) of the Earth's crust by volume. [1] Igneous rocks form about 15% of the Earth's current land surface. [note 1] Most of the Earth's oceanic crust is made of igneous rock. Igneous rocks are also geologically important because:
When extrusive rocks make contact with the atmosphere they cool quickly, so the minerals do not have time to form large crystals. The individual crystals in an aphanitic igneous rock are not distinguishable to the naked eye. Examples of aphanitic igneous rock include basalt, andesite, and rhyolite.
Nephelinite – Igneous rock made up almost entirely of nepheline and clinopyroxene – A silica-undersaturated plutonic rock with >90% nepheline; Norite – Mafic intrusive igneous rock – A hypersthene-bearing gabbro; Obsidian – Naturally occurring volcanic glass; Pegmatite – Igneous rock with very large interlocked crystals
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
Cumulate rock – Igneous rocks formed by the accumulation of crystals from a magma either by settling or floating. Flow banding – Bands or layers that can sometimes be seen in rock that formed from magma; Fractional crystallization (chemistry) – Method for refining substances based on differences in their solubility
Metamorphic rocks are formed by subjecting any rock type—sedimentary rock, igneous rock or another older metamorphic rock—to different temperature and pressure conditions than those in which the original rock was formed. This process is called metamorphism, meaning to "change in form". The result is a profound change in physical properties ...
In geology, texture or rock microstructure [1] refers to the relationship between the materials of which a rock is composed. [2] The broadest textural classes are crystalline (in which the components are intergrown and interlocking crystals), fragmental (in which there is an accumulation of fragments by some physical process), aphanitic (in which crystals are not visible to the unaided eye ...
Conglomerate (/ k ən ˈ ɡ l ɒ m ər ɪ t /) is a sedimentary rock made up of rounded gravel-sized pieces of rock surrounded by finer-grained sediments (such as sand, silt, or clay). The larger fragments within conglomerate are called clasts, while the finer sediment surrounding the clasts is called the matrix.