Gravitational (density) controls on volcanism, magma chambers and intrusions

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Abstract
Gravity controls the movement of magmas through the density‐zoned lithosphere, and plays a pre‐eminent role in determining the course of magmatism. It is inferred from the widespread nature of volcanism that hydrostatic pressure (mainly gravitational in origin) is capable of bringing most mafic magmas and all silicic magmas to the surface. The abundance of intrusions and the presence of long‐lived magma chambers in many volcanoes suggests that ascending magma may alternatively stop at the level of neutral buoyancy (LNB). This is the level at which the density/depth profile for a magma intersects the density/depth profile for the lithosphere; the level above which the rocks are less dense than the magma. It is the level where a magma is in gravitational equilibrium. If a magma chamber develops and swells there, gravity may then cause lateral magma excursions along the LNB. Several cases illustrate possible consequences of magmatic intrusion at the LNB. Where a large volume of mafic magma accumulates at a deep crustal LNB, changes (fractionation, assimilation) may cause its density to decrease and enable it to rise to the surface when it acquires positive buoyancy. This storage can explain how single flood basalt lava flows can have a great volume (≥ 10 km3), though monogenetic volcanoes demonstrate that a small volume (< 1 km3) of mafic magma is all that is required to penetrate the lithosphere. Deep‐crustal storage also readily explains the absence of mantle xenoliths from flood‐basalt flows. In another case the ponding of mafic magma at the LNB beneath an intracrustal silicic magma body assists the ascent and eruption of the silicic magma. An example from Hawaii illustrates that intrusions have changed the crustal‐density zonation and shifted the LNB, influencing the localization of subsequent intrusions as non‐vertical bladed dykes. Finally, two situations are considered in which great intracrustal prisms of mafic and ultra‐mafic intrusions and cumulates, such as give rise to very large positive Bouguer gravity anomalies, can develop as the traces of upwardly migrating magma chambers at a rising LNB.