extrusion The emission of magma from a vent or fissure on to the Earth's surface where it forms a lava flow. Gas is released quietly, rather than explosively, from the magma at the source vent and consequently there is little associated pyroclastic activity. Gas may also be vented from a different source, or extrusion may be of lava (magma) which has been degassed previously.

extrusion In geology, the breaking-out of igneous rock from below the Earth's surface. Any volcanic product reaching the surface becomes extrusive material whether it is ejected through a volcano's cone or through pipe-like channels or fissures in its crust. In industry, extrusion is the forcing of metals or plastics at optimum temperature through a die to make rods, tubes and various hollow or solid sections.