Magmatic underplating occurs when

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 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 s ...

s are trapped during their rise to the surface at the

Mohorovičić discontinuity The Mohorovičić discontinuity ( , ), usually referred to as the Moho discontinuity or the Moho, is the boundary between the Earth's crust and the mantle. It is defined by the distinct change in velocity of seismic waves as they pass through ch ...

or within the crust. Entrapment (or 'stalling out') of magmas within the crust occurs due to the difference in relative densities between the rising magma and the surrounding rock. Magmatic underplating can be responsible for thickening of the crust when the magma cools.

Geophysical Geophysics () is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. The term ''geophysics'' som ...

seismic Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other ...

studies (as well as

igneous petrology Igneous petrology is the study of igneous rocks—those that are formed from magma. As a branch of geology, igneous petrology is closely related to volcanology, tectonophysics, and petrology in general. The modern study of igneous rocks utilizes a ...

and

geochemistry Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing the e ...

) utilize the differences in densities to identify underplating that occurs at depth.

Evidence

Magmatic underplating has been identified using multiple techniques that are non-specific to the area in which they are used.

Geochemistry Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing the e ...

allows geologists to determine levels of association between igneous units: in the

Karoo Province The Karoo Supergroup is the most widespread stratigraphic unit in Africa south of the Kalahari Desert. The supergroup consists of a sequence of units, mostly of nonmarine origin, deposited between the Late Carboniferous and Early Jurassic, a perio ...

of southern Africa, large volumes of

rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals ( phenocrysts) in an otherwise fine-grained groundmass. The miner ...

along the continental margin were produced from melts with initially basaltic compositions. Xenoliths of mantle material can carry information about the ultimate source of a magma, as well as reveal heterogeneities within the magma mixing and assimilation of host magmas at depth.

Gabbro Gabbro () is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ch ...

fractionation Fractionation is a separation process in which a certain quantity of a mixture (of gases, solids, liquids, enzymes, or isotopes, or a suspension) is divided during a phase transition, into a number of smaller quantities (fractions) in which the ...

allows geologists to determine the smallest possible mass of concealed material. Studies of

geomorphology Geomorphology (from Ancient Greek: , ', "earth"; , ', "form"; and , ', "study") is the scientific study of the origin and evolution of topographic and bathymetric features created by physical, chemical or biological processes operating at or ...

in the Karoo Province have identified regional uplift, associated with the underplating and consequent thickening of the crust. Seismic studies of the crust at depth have done a great deal to identify magmatic underplating, but without direct samples to look at, it can be problematic for geologists to agree on the source of an anomaly. Seismic studies of the

Laccadive Islands The Laccadive or Cannanore Islands are one of the three island subgroups in the Union Territory of Lakshadweep, India. It is the central subgroup of the Lakshadweep, separated from the Amindivi Islands subgroup roughly by the 11th parallel ...

in the

Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by t ...

revealed a high-velocity layer of thickened crust between 16 and 24 km below the surface; these were corroborated with tomographic work in the nearby Kutch District, which identified a large mafic body at depth, close to the mantle. Gravity modelling also found mafic intrusive body in the lower crust in the Kachchh rift However, tomographic studies in Norway undertaken to identify the source of locally thickened crust found uneven thicknesses restricted by several

lineaments ''See also Line (geometry)'' A lineament is a linear feature in a landscape which is an expression of an underlying geological structure such as a fault. Typically a lineament will appear as a fault-aligned valley, a series of fault or fold-align ...

. The morphology of the lower crust was not conclusively identified as magmatic underplating, and may in fact be the remnants of the Caledonian root. Proximity to

large igneous provinces A large igneous province (LIP) is an extremely large accumulation of igneous rocks, including intrusive (sills, dikes) and extrusive (lava flows, tephra deposits), arising when magma travels through the crust towards the surface. The formation ...

may also be helpful in identifying magmatic underplating. Unsolidified areas of magmatic underplating (a

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 upw ...

) may feed magma to volcanoes. In the

Rajmahal Traps The Rajmahal Traps is a volcanic igneous province in Eastern India, covering the parts of Jharkhand, West Bengal and Meghalaya. The Rajmahal Hills of Jharkhand is the type area of this province. Multiple layers of solidified lava made the Rajma ...

, there is a 10–15 km thick igneous layer at the base of the crust beneath this area. The thickness of the layer is different in various parts of the area; it is in the center, where the thickness is the greatest, where it is possible that the magma is being fed to the Rajmahal Traps up above. Presence of underplating is also found in the Cambay rift at the depth range of 25 and 31 km through gravity modeling.

Denudation

In the

British Isles The British Isles are a group of islands in the North Atlantic Ocean off the north-western coast of continental Europe, consisting of the islands of Great Britain, Ireland, the Isle of Man, the Inner and Outer Hebrides, the Northern Isles, ...

, (

Paleogene The Paleogene ( ; also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period million years ago ( Mya) to the beginning o ...

)

denudation Denudation is the geological processes in which moving water, ice, wind, and waves erode the Earth's surface, leading to a reduction in elevation and in relief of landforms and landscapes. Although the terms erosion and denudation are used interch ...

is linked with magmatic underplating. It has been shown that the

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...

and

amplitude The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of ampl ...

of denudation can be determined by the density and distribution of the underplating in a given area. Modeling of data brought on by studies of the British Isles shows that a large amount of high velocity material occurs around the

under the

Irish Sea The Irish Sea or , gv, Y Keayn Yernagh, sco, Erse Sie, gd, Muir Èireann , Ulster-Scots: ''Airish Sea'', cy, Môr Iwerddon . is an extensive body of water that separates the islands of Ireland and Great Britain. It is linked to the C ...

. Epeirogenic uplift is a long-wavelength form of uplift and be split up into two separate categories, which is transient and permanent. Permanent epeirogenic uplift is possibly mainly produced by magmatic underplating, while transient uplift is more involved with

mantle convection Mantle convection is the very slow creeping motion of Earth's solid silicate mantle as convection currents carrying heat from the interior to the planet's surface. The Earth's surface lithosphere rides atop the asthenosphere and the two form ...

. Magmatic underplating is important for causing quick epeirogenic uplift in certain areas. It has been argued that the greatest denudation happened in the Paleogene due to records of clastic deposition in

sedimentary basin Sedimentary basins are region-scale depressions of the Earth's crust where subsidence has occurred and a thick sequence of sediments have accumulated to form a large three-dimensional body of sedimentary rock. They form when long-term subsidence ...

s. Some of these sedimentary basins include the

North Sea The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegia ...

Basin and the Porcupine Basin off the southwest coast of Ireland. It has also been argued that Paleogene denudation was mainly caused by magmatic underplating.

Effects of magmatic underplating: a case study

Studies have been done on the phenomenon of magmatic underplating in various areas around the world. In northern Italy, the effects of magmatic underplating were studied along a traverse through the Strona-Ceneri Zone and the

Ivrea Verbano Zone Ivrea (; pms, Ivrèja ; ; lat, Eporedia) is a town and ''comune'' of the Metropolitan City of Turin in the Piedmont region of northwestern Italy. Situated on the road leading to the Aosta Valley (part of the medieval Via Francigena), it strad ...

. The studies included a thermal modeling method which split the cross section up into three different sections: the upper crust, the lower crust, and the

upper mantle The upper mantle of Earth is a very thick layer of rock inside the planet, which begins just beneath the crust (at about under the oceans and about under the continents) and ends at the top of the lower mantle at . Temperatures range from appro ...

. The model displayed multiple magmatic intrusions spreading over time, which resulted in the heating up of the lower crust causing

metamorphism Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of chem ...

and

anatexis Anatexis (via Latin from Greek roots meaning "to melt down") is the partial melting of rocks. Traditionally, anatexis is used specifically to discuss the partial melting of crustal rocks, while the generic term "partial melting" refers to the part ...

, and even managed to moderately heat up the top of the lower crust. The results also showed that final heating began at the same time as extension in shallower crustal levels, while in deeper parts, extension occurred later than the thermal peak of metamorphism. It was also shown that magmatic underplating during a time period of about thirty million years was strong enough to erase all tectono–

metamorphic Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causin ...

history in the Ivera Verbano Zone. This information was preserved in the Strona-Ceneri Zone because areas in the upper crust were not affected nearly as much. Other research has been conducted in the Kutch District of Northwest India. It was concluded that the uplift that occurred in the area was due to intrusions of magma in the lower part of the crust. This uplift occurred because of two separate processes. One of these processes is due to magmatic underplating, while the other involves only

isostasy Isostasy (Greek ''ísos'' "equal", ''stásis'' "standstill") or isostatic equilibrium is the state of gravitational equilibrium between Earth's crust (or lithosphere) and mantle such that the crust "floats" at an elevation that depends on its ...

. Research has shown that during the

Oxfordian Stage The Oxfordian is, in the ICS' geologic timescale, the earliest age of the Late Jurassic Epoch, or the lowest stage of the Upper Jurassic Series. It spans the time between 163.5 ± 1.0 Ma and 157.3 ± 1.0 Ma (million years ago). The Oxfordian ...

a peak transgressive event occurred which was followed by the deposition of

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especially ...

and

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...

. It is possible that the lower units may represent a decrease in sea level; the sea began to withdraw because of the uplift related to the magmatic underplating.

Underplating

Underplating is the accumulation of partial melts at the base of the crust where an ocean

plate Plate may refer to: Cooking * Plate (dishware), a broad, mainly flat vessel commonly used to serve food * Plates, tableware, dishes or dishware used for setting a table, serving food and dining * Plate, the content of such a plate (for example: ...

subducting Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...

under continental crust. Underplating is the result of partial melts being produced in the

mantle wedge A mantle wedge is a triangular shaped piece of mantle that lies above a subducting tectonic plate and below the overriding plate. This piece of mantle can be identified using seismic velocity imaging as well as earthquake maps. Subducting oceanic ...

above a subducting

. The partial melting is induced by a lowering of the melting temperature, the

solidus Solidus (Latin for "solid") may refer to: * Solidus (coin) The ''solidus'' (Latin 'solid'; ''solidi'') or nomisma ( grc-gre, νόμισμα, ''nómisma'', 'coin') was a highly pure gold coin issued in the Late Roman Empire and By ...

, by the input of water and other volatiles supplied by

phase transition In chemistry, thermodynamics, and other related fields, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic states o ...

s in the subducting slab. When the buoyant partial melt rises upwards through the mantle, it will usually stall at the base of the crust and pond there. This is because the crust is usually less dense than the underplating

, and this is the point at which the ascending magma reaches a level of neutral buoyancy. The evolving melt will remain there until it fractionates enough (through melting-assimilation-storage-homogenization (MASH) processes) that the remaining melt is less dense than the surrounding rock; the melt will then continue up into the crust, leaving behind the heavier

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 include ...

minerals which were crystallized during

fractional crystallization Fractional crystallization may refer to: * Fractional crystallization (chemistry), a process to separate different solutes from a solution * Fractional crystallization (geology) Fractional crystallization, or crystal fractionation, is one of th ...

. The assemblage of minerals remaining behind are typically mafic or

ultramafic Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are comp ...

, and are responsible for the observed seismic anomaly which indicates underplated material.

References

{{reflist Chouhan, A.K., Choudhury, P. & Pal, S.K. New evidence for a thin crust and magmatic underplating beneath the Cambay rift basin, Western India through modelling of EIGEN-6C4 gravity data. J Earth Syst Sci 129, 64 (2020). https://doi.org/10.1007/s12040-019-1335-y Plate tectonics Igneous petrology