Extrusive rock Extrusive rock Extrusive rock refers to the mode of igneous volcanic rock formation in which hot magma from inside the Earth flows out (extrudes) onto the surface as lava or explodes violently into the atmosphere to fall back as pyroclastics or tuff.This is as opposed to intrusive rock formation, in which magma does not reach the surface. The main effect of extrusion is that the magma can cool much more quickly in the open air or under seawater, and there is little time for the growth of crystals. Sometimes, a residual portion of the matrix fails to crystallize at all,
Hell's Half Acre Lava Field was created when one or more magmatic dikes (sheets or tubes of magma cutting across the existing geologic features) found their way to the surface. At the northwestern edge of the lava field is a basaltic volcano, with the fissure vent extending toward the southeast and the Hell's Half Acre site. The basaltic volcano is a shield type. Pit craters and spatter cones follow this active fissure line. Two non-erupting fissures extend northwest from the shield volcano for about . One scientific team hypothesized that the seven lava fields in the vicinity of Hell's Half Acre may belong to as