Extrusion refers to the process by which magma reaches the Earth’s surface and solidifies to form volcanic rocks. The study of extrusion processes is important in understanding volcanic activity, the formation of volcanic landforms, and the petrology of extrusive rocks.
Reference: Williams, H., Turner, F. J., & Gilbert, C. M. (1982). “Petrography: An Introduction to the Study of Rocks in Thin Sections.” W. H. Freeman.