Intrusive rock
Intrusive rock is formed when
Intrusion is one of the two ways igneous rock can form. The other is extrusion, such as a volcanic eruption or similar event. An intrusion is any body of intrusive igneous rock, formed from magma that cools and solidifies within the crust of the planet. In contrast, an extrusion consists of extrusive rock, formed above the surface of the crust.
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 (typically formed deeper in the Earth's crust in batholiths or stocks) and medium-grained subvolcanic or hypabyssal rock (typically formed higher in the crust in dikes and sills).[4]
Classification
Because the solid
Plutonic rocks are classified separately from extrusive igneous rocks, generally on the basis of their
Hypabyssal rocks resemble volcanic rocks more than they resemble plutonic rocks, being nearly as fine-grained, and are usually assigned volcanic rock names. However, dikes of basaltic composition often show grain sizes intermediate between plutonic and volcanic rock, and are classified as diabases or dolerites. Rare ultramafic hypabyssal rocks called lamprophyres have their own classification scheme.[7]
Characteristics
Intrusive rocks are characterized by large
Plutonic rocks are less likely than volcanic rocks to show a pronounced
There are also many other characteristics that serve to distinguish plutonic from volcanic rock. For example, the alkali feldspar in plutonic rocks is typically orthoclase, while the higher-temperature polymorph, sanidine, is more common in volcanic rock. 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 crystallization takes place.[12]
Hypabyssal rocks 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.[12][7]
Occurrences
Plutonic rocks form 7% of the Earth's current land surface.[13] Intrusions vary widely, from mountain-range-sized batholiths to thin veinlike fracture fillings of aplite or pegmatite.
- Batholith: a large irregular discordant intrusion
- Chonolith: an irregularly-shaped intrusion with a demonstrable base
- Cupola: a dome-shaped projection from the top of a large subterranean intrusion
- Dike: a relatively narrow tabular discordant body, often nearly vertical
- Laccolith: concordant body with roughly flat base and convex top, usually with a feeder pipe below
- Lopolith: concordant body with roughly flat top and a shallow convex base, may have a feeder dike or pipe below
- Phacolith: a concordant lens-shaped pluton that typically occupies the crest of an anticline or trough of a syncline
- volcanic neck: tubular, roughly vertical body that may have been a feeder vent for a volcano
- Sill: a relatively thin tabular concordant body intruded along bedding planes
- Stock: a smaller irregular discordant intrusive
- Boss: a small stock
See also
- Ellicott City Granodiorite
- Guilford Quartz Monzonite
- Pluton emplacement
- Norbeck Intrusive Suite
- Subvolcanic rock
- Tuolumne Intrusive Suite
- Volcanic rock
- Woodstock Quartz Monzonite
References
- ^ Intrusive Rocks: Intrusive rocks, accessdate: March 27, 2017.
- ^ Igneous intrusive rocks: Igneous intrusive rocks Archived 2018-05-12 at the Wayback Machine, accessdate: March 27, 2017.
- ^ Britannica.com: intrusive rock | geology | Britannica.com, accessdate: March 27, 2017.
- ^ ISBN 9780521880060.
- S2CID 28548230.
- ^ "Rock Classification Scheme - Vol 1 - Igneous" (PDF). British Geological Survey: Rock Classification Scheme. 1: 1–52. 1999.
- ^ a b Philpotts & Ague 2009, p. 139.
- ISBN 0716724383.
- ^ Philpotts & Ague 2009, p. 48.
- ^ Blatt & Tracy 1996, p. 44.
- ^ rocks and minerals: Geology - rocks and minerals, accessdate: March 28, 2017.
- ^ a b public domain: Flett, John Smith (1911). "Petrology". In Chisholm, Hugh (ed.). Encyclopædia Britannica. Vol. 21 (11th ed.). Cambridge University Press. p. 327. One or more of the preceding sentences incorporates text from a publication now in the
- doi:10.1130/B26457.1.