Magma composition is partly related to the chemical composition of the original rock that melted, certain igneous rock types are associated with certain plate tectonic settings. In this lab we will learn how to describe igneous rocks, give them names, and a little about the plate tectonic settings in which the different igneous rock types typically form. You need to learn how to describe these two important characteristics. The most important aspect of igneous rock texture is the size of individual mineral crystals. Crystal size is related to the cooling history of the magma.
Phaneritic textures form if magma cools slowly so that crystals can grow to larger sizes.
Slow cooling occurs underground, where surrounding rock insulates the magma so that it loses heat more slowly. Aphanitic textures result from cooling magma quickly, so that crystals don't have much time to grow. Magma erupted onto earth's surface, or magma that cools close to earth's surface, will solidify into a fine-grained, or aphanitic igneous rock. Determining whether the rock has phaneritic or aphanitic texture is the first step in determining the name for an igneous rock. The large crystals in a porphyritic rock are called phenocrysts , and the fine-grained rock surrounding them is called groundmass .