Types of Intrusive Igneous Bodies

Types of Intrusive Igneous Bodies. Plutons can have a variety of shapes, and be positioned in a variety of ways relative to the surrounding rocks. They are named according to these characteristics. [Karla Panchuk CC-BY-ND 4.0]

In most cases, a body of hot magma is less dense than the rock surrounding it, so it has a tendency to creep upward toward the surface. It does so in a few different ways:

Filling and widening existing cracks
Melting the surrounding rock (called country rock)
Pushing the rock aside (where the rock is hot enough and under enough pressure to deform without breaking)
Breaking the rock.

When magma forces itself into cracks, breaks off pieces of rock, and then envelops them, this is called stoping. The resulting fragments are called xenoliths.

Plutons can have different shapes and different relationships with the surrounding country rock. These characteristics determine what name the pluton is given.

Large, irregularly shaped plutons are called stocks or batholiths, depending on size. Tabular plutons are called dikes if they cut across existing structures, and sills if they do not. Laccoliths are like sills, except they have caused the overlying rocks to bulge upward. Pipes are cylindrical conduits.

Types of Plutons

Intrusions can be classified according to the shape and size of the intrusive body and its relation to the other formations into which it intrudes:

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

Hosta Beach rock formations - North Uist, Outer Hebrides, Scotland

Characteristics

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 depending on the area exposed at the surface.

If the body has an exposed surface area greater than 100 km2, then it’s a batholith, otherwise it’s a stock. Batholiths are typically formed when a number of stocks coalesce beneath the surface to create one large body.

Intrusive rocks are characterized by large crystal sizes, and as the individual crystals are visible, the rock is called phaneritic. Tabular (sheet-like) plutons are classified according to whether or not they are concordant with (parallel to) existing layering (e.g., sedimentary bedding or metamorphic foliation) in the country rock.

This is 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. A sill is concordant with existing layering, and a dike is discordant. If the country rock has no bedding or foliation, then any tabular body within it is a dike. Note that the sill-versus-dike designation is not determined simply by the orientation of the feature. A dike could be horizontal and a sill could be vertical- it all depends on the orientation of features in the surrounding rocks.

If an intrusion makes rocks above rise to form a dome, it is called a laccolith. A laccolith is a sill-like body which has expanded upward by deforming the overlying rock.

A pipe, as the name suggests, is a cylindrical body with a circular, ellipitical, or even irregular cross-section, which serves as a conduit (or pipeline) for the movement of magma from one location to another. Pipes may feed volcanoes, but pipes can also connect plutons. It is also possible for a dike to feed a volcano.

How deep-seated intrusions burst through the overlying strata causes intrusive rock to be recognized: 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 form cavities, which can often be observed. Because their crystals are of the rough equal size, these rocks are said to be equigranular.

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.

The most obvious effect that country rock can have on magma is a chilled margin along the edges of the pluton. The country rock is much cooler than the magma, so magma that comes into contact with the country rock cools much faster than magma toward the interior of the pluton. Rapid cooling leads to smaller crystals, so the texture along the edges of the pluton is different from that of the interior of the pluton, and the colour may be different.

See also:
What Gems Are Found in Igneous Rock?
General Classification of Igneous Rocks
The Texture of Igneous Rocks