Magmatic underplating

Magmatic underplating occurs when basaltic magmas are trapped during their rise to the surface at the Mohorovičić discontinuity 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 seismic studies (as well as igneous petrology and geochemistry) utilize the differences in densities to identify underplating that occurs at depth.

Magmatic underplating has been identified using multiple techniques that are non-specific to the area in which they are used. Geochemistry allows geologists to determine levels of association between igneous units: in the Karoo Province of southern Africa, large volumes of rhyolite 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 fractionation allows geologists to determine the smallest possible mass of concealed material. Studies of geomorphology 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 in the Indian Ocean 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.

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