Volcanic and igneous plumbing systems (VIPS) consist of interconnected

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...

channels and

chambers Chambers may refer to: Places Canada: *Chambers Township, Ontario United States: *Chambers County, Alabama * Chambers, Arizona, an unincorporated community in Apache County * Chambers, Nebraska * Chambers, West Virginia * Chambers Township, Hol ...

which are responsible for the production, storage and transportation of magma in Earth's crust. Volcanic plumbing systems can be found in all active

tectonic Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents ...

settings, such as mid-oceanic ridges, subduction zones, and

mantle plumes A mantle plume is a proposed mechanism of convection within the Earth's mantle, hypothesized to explain anomalous volcanism. Because the plume head partially melts on reaching shallow depths, a plume is often invoked as the cause of volcanic hot ...

, when magmas generated in

continental lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the portion of the upper mantle that behaves elastically on time scales of up to thousands of years or ...

oceanic lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the portion of the upper mantle that behaves elastically on time scales of up to thousands of years or ...

, and in the sub-lithospheric mantle are transported. Magma is first generated by

partial melting Partial melting occurs when only a portion of a solid is melted. For mixed substances, such as a rock containing several different minerals or a mineral that displays solid solution, this melt can be different from the bulk composition of the soli ...

, followed by segregation and extraction from the source rock to separate the melt from the solid. As magma propagates upwards, a self-organised network of magma channels develops, transporting the melt from lower crust to upper regions. Channelled ascent mechanisms include the formation of dykes and ductile fractures that transport the melt in conduits. For bulk transportation, diapirs carry a large volume of melt and ascent through the crust. When magma stops ascending, or when magma supply stops, magma emplacement occurs. Different mechanisms of emplacement result in different structures, including

plutons In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

, sills,

laccolith A laccolith is a body of intrusive rock with a dome-shaped upper surface and a level base, fed by a conduit from below. A laccolith forms when magma (molten rock) rising through the Earth's crust begins to spread out horizontally, prying apar ...

s and

lopolith A lopolith is a large igneous intrusion which is lenticular in shape with a depressed central region. Lopoliths are generally concordant with the intruded strata with dike or funnel-shaped feeder bodies below the body. The term was first defin ...

Magma production

Partial melting

Partial melting Partial melting occurs when only a portion of a solid is melted. For mixed substances, such as a rock containing several different minerals or a mineral that displays solid solution, this melt can be different from the bulk composition of the soli ...

is the first step for generating magma and magma is the basis of VIPS. After magma is generated, it will travel across the crust and lead to the formation of magma conduits and chambers. In

continental crust Continental crust is the layer of igneous, sedimentary, and metamorphic rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called '' sial'' be ...

, partial melting occurs when a portion of the solid rock melts into felsic magma. Rocks in the lower crust and the upper mantle are subject to partial melting. The rate of partial melting and the resultant silicate melt composition depend on temperature, pressure, flux addition (water,

volatiles Volatiles are the group of chemical elements and chemical compounds that can be readily vaporized. In contrast with volatiles, elements and compounds that are not readily vaporized are known as refractory substances. On planet Earth, the term ...

) and the source rock composition. In

oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic ...

decompression melting Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...

of mantle materials forms

basaltic Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...

magma. When the mantle materials rise, the pressure greatly decreases which significantly lowers the

melting point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depen ...

of the rock.

Melt segregation and extraction

After magma is generated, magma will migrate out of its source region by the process of magma segregation and extraction. These processes define the resulting composition of the magma. Depending on the efficiency of the segregation and extraction, there will be different structures of the volcanic and igneous plumbing systems.

Segregation

Melt segregation is the process of melt separating from its source rock. After the

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...

-rich melt is generated by partial melting, melt segregation is achieved by the

gravitational In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stron ...

compaction of the source rock. It causes the squeezing of the melt through the pores and the melts are produced at

grain boundaries In materials science, a grain boundary is the interface between two grains, or crystallites, in a polycrystalline material. Grain boundaries are two-dimensional defects in the crystal structure, and tend to decrease the electrical and thermal ...

. When the melt

droplets A drop or droplet is a small column of liquid, bounded completely or almost completely by free surfaces. A drop may form when liquid accumulates at the lower end of a tube or other surface boundary, producing a hanging drop called a pendant d ...

continue to build up and the proportion of melt continues to increase, they tend to gather together as melt pools. The interconnectivity of the melt determines whether and when melt may be extracted. When the melt percentage in the source rock approaches the first percolation threshold at 7%, the melt starts to migrate. At this point, 80% of the grain boundaries are melted and the rock becomes very weak. As melting advances and the melt continues to accumulate, it reaches the second percolation threshold at a melt percentage of 26% to 30%. The matrix of the source rock will start to break down and the melt will start to be extracted.

Extraction

After the melt segregates from the solid, melt extraction takes place. The rate of magma extraction depends on the spatial distribution and interconnectivity of the magma channel network developed out of its source rock. There are two end members of melt extraction: melt can be extracted in pulses if the development of magma channels are rapid and the network is highly interconnected, or melt can be constantly drained from the source if the magma channels are developed in a continuous and steady manner. Also, magma extraction controls the chemical composition of the melt, the amount of

transported by dykes, and consequently, the volume flux of magma into

. These will eventually control the overall structure of the VIPS such as the formation of dykes and plutons. For instance, if the magma channels are not well connected, the source may not be drained successfully, and dykes may freeze before propagating far enough to feed plutons. If the source rock could not initiate dyke ascent with sufficient melt, the source rock may remain undrained, favouring diapiric ascent of the source rock.

Magma ascent and transportation

When there is sufficient melt accumulation, the magma in the source will migrate from the source to the shallower level of the crust through magma conduits to feed and form different magma reservoirs and structures in VIPS. The

buoyancy Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the ...

of magma is the main driving force of all types of transportation mechanism.

Diapirism

A diapir forms when a blob of buoyant, hot, and

ductile Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stres ...

magma ascends to a higher lithospheric layer. Diapirism is considered as the main mechanism of magma transport in lower to middle crust and it is one of the viable transportation mechanisms for both felsic and

mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks incl ...

magmas.

The process of diapirism only begins when there is sufficient volume of melt accumulated in the source region. When a blob of melt is generated in the source region and it is about to ascend, the distortion causes periodic Rayleigh-Taylor instabilities at the interface of the melt and the surrounding country rock as a result of

density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...

difference. As the melt is less dense than the surrounding rock, Rayleigh-Taylor instabilities will grow and amplify, and eventually become diapirs. Numerical models and laboratory experiments demonstrate that if the upwelling melt is less

viscous The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...

than the surrounding country rock, a spherical shaped diapir connected to a stalk will be formed, which is called Stokes diapir. Stoke diapirism is a viable mechanism preferably for the ascent of massive magma bodies in a weak and ductile crust. Small diapirs are likely to freeze in the middle of the ascent due to heat loss and

solidification Freezing is a phase transition where a liquid turns into a solid when its temperature is lowered below its freezing point. In accordance with the internationally established definition, freezing means the solidification phase change of a liquid ...

. Recent studies demonstrated that a dyke-diapir hybrid model may be a more realistic mechanism of diapir formation. The

numerical simulation Computer simulation is the process of mathematical modelling, performed on a computer, which is designed to predict the behaviour of, or the outcome of, a real-world or physical system. The reliability of some mathematical models can be dete ...

of dyke-diapir pair shows that a

pseudo The prefix pseudo- (from Greek ψευδής, ''pseudes'', "false") is used to mark something that superficially appears to be (or behaves like) one thing, but is something else. Subject to context, ''pseudo'' may connote coincidence, imitation, ...

-dyke zone may develop at the top of the diapir as it propagates, which is essential for softening the roof rocks and allowing the diapir to ascend. It also demonstrates that episodic injection of magma is crucial in maintaining the temperature of the diapir system and preventing it from freezing. Diapirs can also be categorised into crustal and mantle diapirs. Crustal diapirs accents from the lower crust due to partial melting. On the other hand, mantle diapir forms in the mantle, and eventually ascends across the MOHO or underplate the lower crust to provide heat for partial melting.

Channeled ascent

Dykes

Dykes are vertical to sub-vertical fractures filled with magma that cut through layers, and they connect the source rock to

magma chamber A magma chamber is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma, in such a chamber is less dense than the surrounding country rock, which produces buoyant forces on the magma that tend to drive it up ...

, sills and may eventually reach the surface. The transportation of magma in dyke is caused by the buoyancy of magma, and also the

reservoir A reservoir (; from French ''réservoir'' ) is an enlarged lake behind a dam. Such a dam may be either artificial, built to store fresh water or it may be a natural formation. Reservoirs can be created in a number of ways, including contro ...

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...

if it is connected to the source rock. Dykes transport magma at a higher

velocity Velocity is the directional speed of an object in motion as an indication of its rate of change in position as observed from a particular frame of reference and as measured by a particular standard of time (e.g. northbound). Velocity i ...

than diapirs because dykes are usually in an extended network of narrow channels which have a large surface area. However, the large surface area implies that magma crystallization is easier to occur. Therefore, some dykes may rise to the surface, but the majority of them terminates at depth because of solidification of a blockage of rigid layer. There are two types of dyke, including regional

dyke swarm A dike swarm (American spelling) or dyke swarm (British spelling) is a large geological structure consisting of a major group of parallel, linear, or radially oriented magmatic dikes intruded within continental crust or central volcanoes ...

s which originate from a deep magma source, and local sheet swarms which originate from a shallow

magma reservoir A magma chamber is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma, in such a chamber is less dense than the surrounding country rock, which produces buoyant forces on the magma that tend to drive it u ...

. Regional dyke swarms are usually elongated where local sheet swarms are inclined and circular, also known as ring dykes. Marquenas Formation

The

geometry Geometry (; ) is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space such as the distance, shape, size, and relative position of figures. A mathematician who works in the field of geometry is ...

of the dyke is related to the

stress field A stress field is the distribution of internal forces in a body that balance a given set of external forces. Stress fields are widely used in fluid dynamics and materials science. Consider that one can picture the stress fields as the stress cre ...

and the distribution of pre-existing faults and

joints A joint or articulation (or articular surface) is the connection made between bones, ossicles, or other hard structures in the body which link an animal's skeletal system into a functional whole.Saladin, Ken. Anatomy & Physiology. 7th ed. McGraw- ...

in the country rock. Therefore an extensional tectonic setting favours the formation of dykes.

Ductile fractures

Ductile fractures are formed by rock creep in which the

recrystallisation produces tiny

void Void may refer to: Science, engineering, and technology * Void (astronomy), the spaces between galaxy filaments that contain no galaxies * Void (composites), a pore that remains unoccupied in a composite material * Void, synonym for vacuum, a ...

s that connect and eventually fracture the rock. Ductile fractures can be found in the deeper crust, as the mode of deformation transforms from brittle to ductile. Ductile fractures are associated with magma conduits in the deeper region of the crust.

Faults and shears

Fault and shear zones act as lines of weakness for magma to flow in and transport to upper levels. Regional deformation may result in the three main types of faults including

normal fault In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tecton ...

reverse fault In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectoni ...

s, and strike-slip faults. Particularly, a transpressional fault that cuts through layers is related to the transportation and ascent of magma by creating space for emplacement. Emplacementstructures

Magma emplacement

When magma stops ascending, the freezing of magma bodies or the arrest of magma supply lead to the formation of magma reservoirs. Magma emplacement can take place at any depth above the source rock. Magma emplacement is primarily controlled by the internal forces of magma including

and magma

. Magma pressure changes with depth as vertical stress is a function of the depth. Another parameter of magma emplacement is the rate of magma supply. From field evidence, the formation of plutons involves multiple stages of magma injection instead of a single pulse. Small batches of magma will accumulate incrementally for several million years until the magma supply ceases. According to the depth of formation and geometry, magma emplacement can be classified into

, sills,

s and

Middle to lower crust

Plutons

Magma bodies emplaced in lower crust can be classified as

. They are tabular bodies with a larger thickness than its length. It implies that at the level of emplacement, magma mainly flows horizontally. The thicknesses of pluton ranges from one kilometres to about tens of kilometres. And it takes about 0.1 Ma to 6 Ma for plutons to be constructed in multiple magma pulses. The growth of plutons in different environments can be a function of the country rock characteristics and the depth of emplacement. From field evidence, when plutons are formed in a

environment, it will displace the surrounding rocks both laterally and vertically. However, for

brittle A material is brittle if, when subjected to stress, it fractures with little elastic deformation and without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Br ...

environments, as there is no evidence for

strain Strain may refer to: Science and technology * Strain (biology), variants of plants, viruses or bacteria; or an inbred animal used for experimental purposes * Strain (chemistry), a chemical stress of a molecule * Strain (injury), an injury to a mu ...

in the lateral margins, plutons must be displaced in a vertical manner. Therefore, the chances of lateral displacement decrease with decreasing

ductility Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile str ...

of country rocks. Plutons can be categorised into two types depending on the geometry of the pluton floor. They are called wedged-shape plutons and tablet-shaped plutons. Wedge-shaped plutons typically have irregular shapes. They may have roots that tapers downwards which eventually become cylindrical-shaped feeder structures which cause the floors to dip inward at different angles. Tablet-shaped plutons have parallel pluton floors and roofs, and steeper sides compared to wedge-shaped plutons. Some plutons may exhibit features of the two types.

Middle to upper crust

Sills

An Igneous Sill Intrusion in Yellowstone National Park Wyoming USA

Sills are generally defined as sheet intrusions which are tabular in shape and dominantly concordant to the surrounding rock layers. They are commonly emplaced within three kilometres below the Earth surface. Most sills are sub-horizontal in shape as they are usually found in sedimentary layers. However, in some cases, sills may deform

sedimentary Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

layers and exhibit other geometries such as inclined or sub-vertical shapes. The length of sill can extend up to tens of kilometres. Depending to its shape and concordance to the country rock, sills can be classified into five different types based on field evidence. They are strata-concordant sills, transgressive sills, step-wise transgressive sills, saucer-shaped sills, V-shaped sills, and hybrid sills. Strata-concordant sills are the classic representation of a sill. They develop continuously and concordantly with the host rock and are often found in deeper part of the upper crust. Transgressive sills cut through and propagate to higher layers with an oblique angle to the host rock, displaying discordant properties. It is straighter in shape. Step-wise transgressive sills are similar to transgressive sills, but there are alternating concordant and discordant segments, producing step-like features. Saucer-shaped sills have a lower central concordant sill, and two higher outer transgressive sills that flatten out at the tips. They usually have a thicker inner sill and thinning outwards. V-shaped sills are somewhat similar to saucer-shaped sills, but it has a shorter inner part. Hybrid sills shows mixed features of the above-mentioned sills. Sills

Laccoliths

Laccoliths A laccolith is a body of intrusive rock with a dome-shaped upper surface and a level base, fed by a conduit from below. A laccolith forms when magma (molten rock) rising through the Earth's crust begins to spread out horizontally, prying ap ...

forms from the stacking of sills. They typically display dome-shaped structures with slightly elevated roofs and flat floors that are concordant to rock layers. They are formed at depths that do not exceed three kilometres. It typically takes 100 to 100,000 years for enough magma to emplace as sills, and the grouping of sills form laccoliths. The formation of laccolith is governed by the jointing and

faulting In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

of the country rocks when emplacement begins. These lines of weakness provide pathways for the formation of initial sill-like structures that are horizontal in shape. At this stage, sheet intrusion is a more favourable mechanism of emplacement because the margins of the sheet cool faster, which creates

shear zones Boudinaged quartz vein (with strain fringe) showing ''Fault (geology)">sinistral shear sense'', Starlight Pit, Fortnum Gold Mine, Western Australia In geology, shear is the response of a rock to deformation usually by compressive stress and f ...

that allow further horizontal displacement. After some time, when the cooling rate decreases, and when the sills continue to stack onto one another, sheet intrusion is no longer a favourable mechanism because the zones of weakness diminish. The cohesion between the

layers is also reducing because of displacement and deformation of the rock. Here, inflation is a possible mechanism to continue the growth of intrusion. If, at this point, the surface area of the magma is large enough to generate a magma force that can overcome the lithostatic load of the overlying layer, vertical inflation can take place. The vertical inflation of magma chambers creates laccoliths.

Lopoliths

Lopoliths are lenticular concordant intrusive masses that display a convex-down shape. It typically involves floor depression. Two models were proposed for the formation of lopoliths. They are the cantilever model and the piston model. The cantilever model describes the formation of the lopoliths as a result of the tilting of floor about a point at the pluton margin. It deforms the underlying crust by simple shear and leads to the sinkage of partial melt. In the piston model, the formation of lopolith begins when the central block floor sinks. The floor continues to thicken and creates tabular-shaped lopoliths.

References

{{Reflist Volcanism Lithosphere