Plutons: Types of Intrusive Igneous Bodies

Pluton is a large underground body of igneous rock that forms when magma cools and crystallizes beneath the Earth's surface. Magma is molten rock that originates deep within the Earth's mantle. When magma rises into the crust, it can either erupt onto the surface as lava, or it can cool and solidify underground to form a pluton.

They are typically composed of plutonic rocks such as granite, diorite, and gabbro. Plutons vary in size, shape, and orientation, ranging from small, irregularly shaped bodies to massive, tabular structures.

Intrusive igneous bodies are formed when magma cools and solidifies beneath the Earth's surface. Magma is less dense than the surrounding rock, so it tends to rise towards the surface. If it encounters a crack or weakness in the rock, it can intrude into the surrounding rock and form an intrusive igneous body.

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.

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]

The intrusive rocks and bodies are further subdivided into hypabyssal and plutonic (derived from Pluto, the Greek god of the underworld), depending on whether they crystallize near the surface or at great depth, respectively. The distinction, which does not imply a specific depth, is based mainly on grain size. Plutonic magma cools and crystallizes so slowly that few crystal nuclei form, which results in coarse-grained rock. Hypabyssal rocks crystallize relatively rapidly and have large numbers of nuclei and are, therefore, finer grained. Indeed, most hypabyssal rocks resemble volcanic rocks more than they do plutonic rocks.

Shallow intrusive igneous bodies: dikes, sills, laccoliths, cone sheets, ring dikes, and diatremes

Plutonic igneous bodies: lopoliths, batholiths, and stocks

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

Batholiths are the largest type of intrusive igneous body, with a surface area of greater than 100 square kilometers. They are typically formed from the crystallization of large volumes of magma. Think of them as giant underground mountains of granite, diorite, or other felsic (silica-rich) rocks. Batholiths often form during mountain building events due to the melting of continental crust.

Chonolith

A chonolith is a large, cylindrical intrusive igneous body that is more than 100 square kilometers in area. Chonoliths are thought to form when magma rises from the mantle and intrudes into surrounding rocks. The magma can solidify in the cylinder, forming a chonolith.

Cupola

A cupola is a small, mushroom-shaped intrusive igneous body that is typically less than 1 square kilometer in area. Cupolas are often found in the central parts of larger intrusive igneous bodies, such as batholiths. Cupolas are typically composed of more felsic (silica-rich) igneous rocks than the surrounding rocks.

Laccolith

Laccoliths are dome-shaped intrusive igneous bodies that form when magma intrudes into a layer of rock, pushing the overlying rock upwards. They are typically composed of felsic or intermediate igneous rocks, such as granite or syenite. Laccoliths can be important sources of mineral resources, such as copper, molybdenum, and uranium.

Lopolith

Lopoliths are intrusive igneous bodies that have a saucer-shaped shape. They form when magma intrudes into the Earth's crust and spreads out laterally, pushing the overlying rock down. Lopoliths are often composed of mafic igneous rocks, such as basalt or gabbro. They can be important sources of platinum group elements (PGEs) and other valuable minerals.

Phacolith

A concordant lens-shaped pluton that typically occupies the crest of an anticline or trough of a syncline. They are typically composed of felsic or intermediate igneous rocks, such as granite or diorite. Phacoliths can be important markers of past tectonic activity and can provide insights into the history of deformation in an area.

Volcanic pipe or volcanic neck

These are cylindrical bodies that were once the conduits for feeding magma to a volcano. After the volcanic activity ceases, erosion wears away the surrounding softer rock, leaving the harder volcanic neck exposed. Volcanic necks are typically composed of solidified lava, breccia (fragments of rock cemented together), or volcanic ash.

Dike

Dikes are vertical or steeply dipping intrusive igneous bodies that form when magma intrudes into cracks in the rock. They can be very thin, just a few centimeters wide, or very thick, up to hundreds of meters. Dikes are often composed of mafic (silica-poor) igneous rocks like basalt.

Ring dikes

Ring dikes are circular or elliptical dike intrusions that form around a volcanic vent or conduit. They are formed by magma intruding along ring-shaped fractures in the surrounding rock.

Sill

Sills are horizontal or gently dipping intrusive igneous bodies that form when magma intrudes between layers of existing rock.

Stock

Stocks are similar to batholiths, but they are smaller, with a surface area of less than 100 square kilometers. Examples of stocks include the Henry Mountains of Utah and the Shiprock in New Mexico.

Cone sheets

Cone sheets are funnel-shaped or cone-shaped igneous intrusions that are typically inclined and have a sheet-like form. They are formed by magma intruding along cone-shaped fractures around a volcanic vent.

Diatremes

Diatremes are cylindrical or pipe-shaped volcanic conduits that are breccia-filled. Breccia is a rock composed of fragments of other rocks cemented together. Diatremes form when violent gas explosions fragment rock, allowing magma to intrude and solidify.

Boss

A small stock. A boss is a small, dome-shaped intrusive igneous body that is less than 100 square kilometers in area. Bosses are often found in groups, and they can be formed when magma intrudes into overlying rocks and pushes them up into a dome shape. Bosses are often found in areas of volcanic activity, and they can be composed of a variety of igneous rocks, including granite, gabbro, and diorite.

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

Plutonic Igneous Bodies 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.

Intrusive igneous bodies are also important sources of mineral resources. Many metallic and non-metallic minerals are concentrated in intrusive igneous rocks. For example, copper, gold, and silver are often found in porphyritic intrusions. And tin, lithium, and tantalum are often found in pegmatitic intrusions.