Igneous intrusion

Intrusions have a wide variety of forms and compositions, illustrated by examples like the Palisades Sill of New York and New Jersey;[3] the Henry Mountains of Utah;[4] the Bushveld Igneous Complex of South Africa;[5] Shiprock in New Mexico;[6] the Ardnamurchan intrusion in Scotland;[7] and the Sierra Nevada Batholith of California.

The relative amounts of quartz, alkali feldspar, plagioclase, and feldspathoid is particularly important in classifying intrusive igneous rocks.

The question of how this takes place is called the room problem, and it remains a subject of active investigation for many kinds of intrusions.

[19][20][21] Dikes are tabular discordant intrusions, taking the form of sheets that cut across existing rock beds.

They vary in thickness from millimeter-thick films to over 300 meters (980 ft) and an individual sheet can have an area of 12,000 square kilometers (4,600 sq mi).

Dikes form by hydraulic fracturing of the country rock by magma under pressure,[23] and are more common in regions of crustal tension.

A stock is a non-tabular discordant intrusion whose exposure covers less than 100 square kilometers (39 sq mi).

[28] Batholiths are discordant intrusions with an exposed area greater than 100 square kilometers (39 sq mi).

[29] A sill is a tabular concordant intrusion, typically taking the form of a sheet parallel to sedimentary beds.

Laccoliths typically form at shallow depth, less than 3 kilometers (1.9 mi),[30] and in regions of crustal compression.

[24] Lopoliths are concordant intrusions with a saucer shape, somewhat resembling an inverted laccolith, but they can be much larger and form by different processes.

Their immense size promotes very slow cooling, and this produces an unusually complete mineral segregation called a layered intrusion.

[31] The ultimate source of magma is partial melting of rock in the upper mantle and lower crust.

For example, a granitic magma, which is high in silica, has a density of 2.4 Mg/m3, much less than the 2.8 Mg/m3 of high-grade metamorphic rock.

[24] Ring dikes and cone sheets form only at shallow depth, where a plug of overlying country rock can be raised or lowered.

[38] Zircon zoning provides important evidence for determining if a single magmatic event or a series of injections were the methods of emplacement.

Such limited mixing as takes place results in the small inclusions of mafic rock commonly found in granites and granodiorites.

Such intrusions are interpreted as occurring at shallow depth, and are commonly associated with volcanic rocks and collapse structures.

Crystals formed early in cooling are generally denser than the remaining magma and can settle to the bottom of a large intrusive body.