Subvolcanic Intrusions

Volcanic plumbing systems are made by complex networks of dykes, sills, laccoliths, cryptodomes, and other intrusions, which may interconnect to form more or less temporary magma reservoirs through which magma is stored and migrates before reaching the Earth’s surface. We first summarise the physical factors that determine whether or not magmas rise and erupt, or form subvolcanic intrusions. The competing forces of buoyancy and viscosity determine the ability of magma to move upwards, as well as the structural fabric of country rocks and their mechanical competence. Different types of intrusions are then described. In recent years, the study of petroleum industry 3D seismic reflection data sets from offshore sedimentary basins has revealed that mafic sills are often emplaced as a series of variably interconnected saucer-shaped bodies, fed by inclined sheets at their margins, and composed of magma lobes, similar to lava lobes. This has greatly advanced knowledge of mafic sheet-like complexes. We then consider the architecture and processes within magma plumbing systems in different volcanic systems. Complex magma plumbing systems have been mapped and imaged in stratovolcanoes (e.g. Kora stratovolcano, offshore New Zealand). There are differences in the architecture and processes that occur in thick regionally extensive mafic sills associated with large igneous provinces, and more evolved, and more localised magma reservoirs and chambers associated with caldera volcanoes. Magma plumbing systems not only contribute to the growth of volcanic edifices, but they can also contribute to their destruction. In both young and ancient volcanic terrains, it can be difficult to distinguish lavas from intrusions, especially sills, an ambiguity that can have a significant impact on paleoenvironmental and tectonic reconstructions, and we therefore provide guidelines to differentiate them. The most important criterion that distinguishes intrusions from lavas is the nature of the lower and upper contact relationships with the country rock or host sediments. Finally, we highlight the study of shallow intrusions for paleostress analysis in older and modern volcanic terrains.