Do magma that cools slowly form extrusive igneous rock?
Your scenario of cooling slowly and extrusive is rare. Extrusive igneous rocks are essentially volcanic rocks, meaning magma that has been erupted onto the earth’s surface. Once magma has been erupted onto the surface it is called lava, and will cool very quickly – at least in geological terms. In general, the quicker that an igneous melt (magma or lava) cools, the smaller the crystals. Now back to your scenario of “cools slowly” and “extrusive” – if you have an igneous melt in a magma chamber (say below a caldera) that begins to cool slowly, certain ealry crystals will start to form (such as feldspar). Then if the partly crystallized magma is erupted onto the surface to form a lava, the remaining melt would cool quickly, forming an irregular crystal size texture called a porphyry. The rock would be extrusive, such as an andesite porphyry, definitely not a granite.
