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.

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: batholiths, stocks, dikes, and sills.

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.

Stock

Stocks are similar to batholiths but are smaller, typically less than 100 square kilometers in size. While stocks often resemble the exposed tops of larger batholiths, they can form independently as smaller, isolated magma bodies. Stocks may become visible through erosion and are frequently found in association with larger intrusive bodies. Examples of stocks include the Henry Mountains of Utah and the Shiprock in New Mexico.

Laccolith

A laccolith is a lens-shaped or mushroom-shaped igneous intrusion that forms when magma is injected between layers of sedimentary rock, forcing the overlying strata to dome upward. Unlike a lopolith, which sags downwards, a laccolith's magma doesn't usually cause the overlying rock to break but instead creates a dome. Laccoliths are concordant, aligning with the layering of the surrounding rock. They typically have a flat floor and a domed roof and are indeed smaller than batholiths. Compositionally, laccoliths are often made of felsic to intermediate rocks like granite or diorite, though syenite can also occur. While they can host mineral deposits, their economic significance might not be as directly emphasized as with other types of intrusions, but they can still be associated with mineralization like copper or uranium, depending on the geological context

Cupola

A cupola is a small, dome-shaped protrusion extending from a larger igneous body like a batholith, often covering an area less than 1 square kilometer. These features occur when magma pushes upward but does not erupt, forming a bulb-like structure. Cupolas can be associated with mineral deposits due to the concentration of minerals as the magma cools. They usually consist of igneous rocks that might be more felsic than the main body, reflecting a later stage in the magma differentiation process where silica-rich magma fractions rise to the top.

Lopolith

A lopolith is a large, basin-shaped or lens-shaped intrusive body that is typically concave, where the center dips lower than its edges. It forms when magma intrudes into the crust and spreads laterally, but unlike the upward pushing dome of a laccolith, the weight of the injected magma itself, or the overlying strata, causes it to sag or be pushed downward, forming a bowl-like structure. Lopoliths are concordant with the rock layers into which they intrude, lying parallel to the strata. They are commonly associated with mafic or ultramafic magmas, leading to compositions like gabbro or norite, rather than basalt for large intrusions. Lopoliths can indeed be significant for their mineral wealth, including deposits of nickel, copper, and platinum group elements (PGEs), often linked to extensive magmatic systems.

Phacolith

Phacoliths are lens-shaped plutons that form along the crests or troughs of folded rock layers. These bodies are also concordant, meaning they follow the curvature of the folded layers. 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 Neck

A volcanic neck, also known as a volcanic plug, forms when magma solidifies within the vent or conduit of a volcano. This occurs either during an active eruption or after the volcano has become extinct. The magma, hardening into a solid igneous rock structure, typically consists of basalt, andesite, or rhyolite, depending on its composition. Over geological timescales, the softer surrounding volcanic material erodes away due to natural processes like weathering and erosion. This erosion leaves the more resistant plug standing alone, often appearing as a steep-sided, cylindrical or conical protrusion on the landscape, with a flat or rounded top shaped by its origin in a vertical conduit.

Volcanic pipe

Volcanic pipes 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. Dikes are discordant plutons, meaning they cut across pre-existing rock layers. 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. Sills are concordant bodies, meaning they run parallel to the existing rock layers.

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.

Types of Plutons: like batholiths, stocks, dikes, sills, Cone sheets, Ring dikes, Volcanic pipe or volcanic neck.

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 boss is a small, rounded, dome-shaped mass of intrusive igneous rock. While bosses are often composed of resistant rocks like granite, they can also be made of other intrusive rock types. They form from magma that cools and solidifies below the surface, and subsequent erosion of the less resistant surrounding rock can lead to the boss being exposed at the surface, sometimes appearing as a prominent hill or mound due to its more durable nature.

Chonolith

A chonolith is a less common type of igneous intrusion that is defined by its irregular shape, which does not conform to any typical geometric pattern. Unlike other intrusions that have predictable forms (such as the sheet-like form of sills or dikes), chonoliths can have complex shapes due to the variable paths magma follows during its emplacement. The term "chonolith" is somewhat of a catch-all for intrusions that don’t neatly fit into other categories. These intrusions can take on any number of forms, depending on local geological conditions.

Discordant and Concordant bodies

Discordant and concordant bodies refer to the relationship between igneous intrusions and the pre-existing layers of rock they penetrate.

Discordant bodies are igneous intrusions that cut across the existing layering or structure of the surrounding rock, examples include dikes, volcanic necks, and batholiths, which do not conform to the bedding planes or foliation of the host rock.

Concordant bodies are igneous intrusions that are injected parallel to the existing rock layers or foliation, examples include sills, which spread between rock beds, and laccoliths, which create a dome by pushing overlying strata upward.

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.

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

 

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.

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