page_title: Intrusive rock
section_title: Characteristics
hierarchical_section_title: Intrusive rock / Characteristics
caption_attribution_description: English: An intrusion (Notch Peak monzonite) inter-fingers (partly as a dike) with highly-metamorphosed host rock (Cambrian carbonate rocks). From near Notch Peak, House Range, Utah. According to Utah Geological Survey, this is the Notch Peak Sill, a sill of quartz monzonite which, about 17 million years ago, intruded into the layers of white marble and grey argillite of the Marjum Formation. Reference: http://geology.utah.gov/surveynotes/geosights/notch_peak.pdf .
context_page_description: Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form intrusions, for example plutons, batholiths, dikes, sills, laccoliths, and volcanic necks. Some geologists use the term plutonic rock synonymously with intrusive rock but other geologists subdivide intrusive rock, by crystal size, into coarse-grained plutonic rock and medium-grained subvolcanic or hypabyssal rock.
context_section_description: A body of intrusive igneous rock which crystallizes from magma cooling underneath the surface of the Earth is called a pluton. If the pluton is large, it may be called a batholith or a stock. 
  Intrusive rocks are characterized by large crystal sizes, and as the individual crystals are visible, the rock is called phaneritic. This is formed as the magma cools underground, and while cooling may be fast or slow, cooling is slower than on the surface, so larger crystals grow. If it runs parallel to rock layers, it is called a sill. If an intrusion makes rocks above rise to form a dome, it is called a laccolith.
  How deep-seated intrusions burst through the overlying strata cause intrusive rock to be characterised: Veins spread out into branches, or branchlike parts result from filled cracks, and the high temperature is evident in how they alter country rock. As heat dissipation is slow, and as the rock is under pressure, crystals form, and no vitreous rapidly chilled matter is present.
  The intrusions did not flow while solidifying, hence do not show lines. Contained gases could not escape through the thick strata, thus they form cavities, which can often be observed. Because their crystals are of roughly equal size, these rocks are said to be equigranular.
  There is typically no distinction between a first generation of large well-shaped crystals and a fine-grained ground-mass. The minerals of each have formed in a definite order, and each has had a period of crystallization that may be very distinct or may have coincided with or overlapped the period of formation of some of the other ingredients. Earlier crystals originated at a time when most of the rock was still liquid and are more or less perfect. Later crystals are less regular in shape because they were compelled to occupy the spaces left between the already-formed crystals. The former case is said to be idiomorphic (or automorphic); the latter is xenomorphic. There are also many other characteristics that serve to distinguish the members of these two groups. For example, orthoclase is typically feldspar from granite, while its modifications occur in lavas of similar composition. The same distinction holds for nepheline varieties. Leucite is common in lavas but very rare in plutonic rocks. Muscovite is confined to intrusions. These differences show the influence of the physical conditions under which consolidation takes place.
  Intrusive rocks formed at greater depths are called plutonic or abyssal. Some intrusive rocks solidified in fissures as dikes and intrusive sills at shallow depth and are called subvolcanic or hypabyssal. They show structures intermediate between those of extrusive and plutonic rocks. They are very commonly porphyritic, vitreous, and sometimes even vesicular. In fact, many of them are petrologically indistinguishable from lavas of similar composition.
caption: An intrusion (pink Notch Peak monzonite) inter-fingers (partly as a dike) with highly metamorphosed black-and-white-striped host rock (Cambrian carbonate rocks) near Notch Peak, House Range, Utah
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