A sheet intrusion is where a mass of molten magma takes advantage of a pre-existing linear feature in a host rock, such as a long rupture or fault, and forces its way into these spaces. Thus the magma, intruded between existing rocks, solidifies into large thin sheets of igneous rock.
They are among the most extensive igneous features on Earth, in the form as dikes, dike swarms, laccoliths, and sills.