, an igneous ''intrusion'' exposed when the surrounding softer rock eroded away
Intrusive rock is formed when magma
penetrates existing rock, crystallizes, and solidifies underground to form ''intrusions
'', such as batholith
s, and volcanic neck
accessdate: March 27, 2017.
[Igneous intrusive rocks]
accessdate: March 27, 2017.
intrusive rock | geology | Britannica.com
accessdate: March 27, 2017.
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).
Because the solid country rock
into which magma intrudes is an excellent insulator, cooling of the magma is extremely slow, and intrusive igneous rock is coarse-grained (phaneritic
). However, the rate of cooling is greatest for intrusions at relatively shallow depth, and the rock in such intrusions is often much less coarse-grained than intrusive rock formed at greater depth. Coarse-grained intrusive igneous rocks that form at depth within the earth are called ''abyssal'' or ''plutonic'' while those that form near the surface are called subvolcanic
Plutonic rocks are classified separately from extrusive igneous rocks, generally on the basis of their mineral
content. The relative amounts of quartz
, alkali feldspar
, and feldspathoid
are particularly important in classifying intrusive igneous rocks, and most plutonic rocks are classified by where they fall in the QAPF diagram
ic rocks are further classified by their relative contents of plagioclase and various iron
- or magnesium
-rich minerals (mafic
minerals) such as olivine
, and orthopyroxene
, which are the most common mafic minerals in intrusive rock. Rare ultramafic rocks, which contain more than 90% mafic minerals, and carbonatite
rocks, containing over 50% carbonate minerals, have their own special classifications.
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
ic composition often show grain sizes intermediate between plutonic and volcanic rock, and are classified as diabase
s or dolerite
s. Rare ultramafic hypabyssal rocks called ''lamprophyre
s'' have their own classification scheme.
An intrusion (pink _[[monzonite">Notch_Peak
_[[monzonite)_inter-fingers_(partly_as_a_[[dike_(geology).html" style="text-decoration: none;"class="mw-redirect" title="monzonite.html" style="text-decoration: none;"class="mw-redirect" title="Notch Peak Notch_Peak
_[[monzonite)_inter-fingers_(partly_as_a_[[dike_(geology)">dike)_with_highly_metamorphosed_black-and-white-striped_host_rock_([[Cambrian.html" style="text-decoration: none;"class="mw-redirect" title="monzonite">Notch Peak [[monzonite) inter-fingers (partly as a [[dike (geology)">dike) with highly metamorphosed black-and-white-striped host rock ([[Cambrian">monzonite">Notch Peak [[monzonite) inter-fingers (partly as a [[dike (geology)">dike) with highly metamorphosed black-and-white-striped host rock ([[Cambrian [[carbonate rocks) near Notch Peak, [[House Range, [[Utah
Intrusive rocks are characterized by large [[crystal sizes, and as the individual crystals are visible, the rock is called ''[[phaneritic''.
There are few indications of flow in intrusive rocks, since their texture and structure mostly develops in the final stages of crystallization, when flow has ended. Contained gases cannot escape through the overlying strata, and these gases sometimes form cavities
, often lined with large, well-shaped crystals. These are particularly common in granites and their presence is described as ''miarolitic texture''. Because their crystals are of roughly equal size, intrusive rocks are said to be ''equigranular
[rocks and minerals]
Geology - rocks and minerals
accessdate: March 28, 2017.
Plutonic rocks are less likely than volcanic rocks to show a pronounced porphyritic
texture, in which a first generation of large well-shaped crystals are embedded in 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 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
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.
Hypabyssal rocks show structures intermediate between those of extrusive
and plutonic rocks. They are very commonly porphyritic
, and sometimes even vesicular
. In fact, many of them are petrologically
indistinguishable from lavas of similar composition.
Plutonic rocks form 7% of the Earth's current land surface.
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 pipe or 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
* Ellicott City Granodiorite
* Guilford Quartz Monzonite
* Methods of pluton emplacement
* Norbeck Intrusive Suite
* Subvolcanic rock
* Tuolumne Intrusive Suite
* Volcanic rock
* Woodstock Quartz Monzonite