The shallow plumbing system of volcanoes usually comprises countless magmatic sheet intrusions that can transport magma from a magma reservoir towards the surface. These sheet intrusions have characteristic shapes with straight, steeply dipping sheets generally referred to as dykes and conical, shallow- to moderately dipping sheets called cone sheets. Magmatic sheets arrange in systematic patterns, such as elongate volcanic rift zones, concentric cone-sheet swarms and radial dyke swarms. All three types of sheets may form in the same volcano and be fed from the same magma chamber. It is the geometrical relationship between the depth and the size of the feeder, as well as viscous stresses that result from the interplay between the properties of the magma and its host rock that control whether cone sheets or dykes form. Moreover, the gravitational force exerted by the volcanic edifice and regional tectonic stresses that produce a radial or elongate rift-one arrangement of dykes. At the Earth’s surface, sheet intrusions feed fissure eruptions that with time can develop into pipe-like conduits.