Igneous Intrusions: Sills, Dykes and Plutons

Igneous intrusions record the movement and storage of magma within the Earth's crust and understanding how intrusions are emplaced is key to our understanding of how magma is transported from its source of origin, to in many cases, it's eventual eruption. Igneous intrusions come in many different shapes, sizes and compositions, with the terminology used to describe them being equally diverse. However, intrusions can be fundamentally split into two main categories; tabular intrusions, in the form of sills and dykes; and plutons, which are often volumetrically large, and are generally less tabular in morphology when compared to sills and dykes. Whilst classification of intrusions as sills, dykes or plutons (or other types of intrusion) is effective on an outcrop scale, the applicability of such classifications on regional-to-crustal scales can be more problematic. In the case of plutons, it is now generally understood that large, long-lived magma chambers do not necessarily form major components of the magmatic system. In many cases, the sills, dykes and plutons form complex interconnected networks within the crust, with sills intrusions for example inflating and coalescing with other intrusions leading to the formation of large plutonic bodies. Tabular intrusions are emplaced within the upper crust via two end-member modes. The first is the emplacement of magma in a brittle fashion, whereby fracture(s) extends ahead of the tip of the magma in a sill or dyke. In the second mode, the host rock does not behave in a brittle manner during magma intrusion, and the magma and host rock interact in a fashion akin to the interaction of two viscous fluids. Both modes of emplacement create distinctive morphologies and structures within the resulting intrusions, which can be observed in field outcrops to assist the identification and understanding of intrusion emplacement mechanisms.