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,
Obsidian manufacture cutting and piercing tools and it has been used experimentally as surgical scalpel blades. The translation into English of "Natural History" written by Pliny the Elder of Rome shows a few sentences on the subject of a volcanic glass called obsidian ("lapis obsidianus"), discovered in Ethiopia by Obsidius, a Roman explorer. Obsidian is the rock formed as a result of quickly cooled lava, which is the parent material. Extrusive formation of obsidian may occur when felsic lava cools rapidly at the edges of a felsic lava flow or volcanic dome or when lava cools during sudden contact with water