intrusion

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Devils Tower, an igneous intrusion exposed when the surrounding softer rock eroded away.

In geology, an intrusion is a body of igneous rock that has crystallized from molten magma below the surface of the Earth. Bodies of magma that solidify underground before they reach the surface of the earth are called plutons, named for Pluto, the Roman god of the underworld. Correspondingly, rocks of this kind are also referred to as igneous plutonic rocks or igneous intrusive rocks. This is to be contrasted with extrusive rocks. The rock surrounding a pluton is called country rock.

Varieties

In composition, intrusive rocks include the entire sequence of igneous rock types from the dense and dark ultramafic peridotites to the very light-colored and low-density alkali granites and syenites. A well-known example of an igneous intrusion is Devil's Tower in Wyoming, USA.

Structural types

A dike intrudes into the country rock, Baranof Cross-Island Trail, Alaska, USA.

Intrusive rocks also exist in a wide range of forms from mountain range sized batholiths to thin vein-like fracture fillings of aplite. Intrusive structures are often classified according to whether or not they are parallel to the bedding planes or foliation of the country rock: if the intrusion is parallel, the body is concordant, while if it cuts across the country rock, it is discordant. Structural types include:

• batholith: large irregular discordant intrusions.
• stock: smaller irregular discordant intrusions.
• dike: a relatively narrow tabular discordant body, often with near-vertical attitude.
• sill: a relatively thin tabular concordant body intruded along bedding planes, often near-horizontal when emplaced, but also may be intruded into tilted beds or the entire package may be tilted by later deformation.
• pipe or volcanic neck: circular or tube shaped nearly vertical body which may have been a feeder vent for a volcano.
• laccolith: concordant body with essentially flat base and dome shaped upper surface, usually has a feeder pipe below.
• lopolith: concordant body with a relatively flat to sagging top and a shallow convex base (spoon-shaped), may have a feeder dike or pipe below.
• phacolith: a concordant lens-shaped pluton that typically occupies the crest of an anticline or the trough of a syncline.

Characteristics

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Deep-seated intrusive rock formations are shown by the way in which they have burst through the superincumbent strata. The ramifying veins resulted from filled cracks. That the rock was at a very high temperature is equally clear from the changes that have been induced in the rocks that were in contact with it. But since heat can only dissipate slowly, and since the rock is under pressure, crystals form and no vitreous rapidly chilled matter is present. As they have had time to rest before crystallizing, they are not fluidal. Their contained gases have not been able to escape through the thick layer of strata, beneath which they were injected. Such gases form cavities, which can often be observed in these minerals. Such gases have also resulted in many important modifications in the crystallization of the rock. Because their crystals are of approximately equal size these rocks are said to be granular.

There is typically no distinction between a first generation of large well-shaped crystals and a fine-grained ground-mass. Their minerals have formed, however, in a definite order, and each has had a period of crystallization which may be very distinct or may have coincided with or overlapped the period of formation of some of the other ingredients. The earlier crystals originated at a time when most of the rock was still liquid and are more or less perfect. The later crystals are less regular in shape because they were compelled to occupy the interspaces left between the already formed crystals. The former is said to be idiomorphic (or automorphic), the latter is anidiomorphic (allotriomorphic, xenomorphic). There are also many other characteristics which serve to distinguish the members of these two groups. Orthoclase, for example, is typically feldspar from granite, while its modifications occur in lavas of similar composition. The same distinction holds between elaeolite and nepheline. Leucite is common in lavas, but very rare in plutonic rocks. Muscovite is confined to the intrusions. These differences show the influence of the physical conditions under which consolidation takes place.

There is a certain class of intrusive rocks which have risen towards the surface, but have failed to reach it, and have solidified in fissures as dikes and intrusive sills at no great depth. These types are given the name "intrusive" (or "hypabyssal") or "plutonic" (or "abyssal") which formed at greater depths. As might be expected, they show structures intermediate between those of the effusive and the 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.^[1]

See also

• Diapir
• Pluton
• Layered intrusion

References

1. ^ This article incorporates text from the article "Petrology" in the Encyclopædia Britannica, Eleventh Edition, a publication now in the public domain.