The main difference between intrusive and extrusive igneous rocks is a. intrusive is due to magma cooling deep within earth's interior and extrusive is due to it cooling at earth's surface. b. extrusive rocks form quickly and intrusive rocks form over millions of years. c. extrusive is due to magma cooling deep within earth's interior and intrusive is due to it cooling at earth's surface. d. intrusive rocks are made of magma and extrusive rocks are made of volcanic ash.