All rocks found on the Earth are classified into one of three groups: igneous, sedimentary, or metamorphic. This rock classification is based on the origin of each of these rock types, or if you prefer, based on the rock-forming process that formed the rock. The focus of this chapter will be on igneous rocks, which are the only rocks that form from what was once a molten or liquid state. Therefore, based on their mode of origin, igneous rocks are defined as those rock types that form by the cooling of magma or lava. You would be right in thinking that there is more to the classification of igneous rocks than stated in the previous sentence, as there are dozens of different igneous rocks that are considered commonplace, and dozens of more types that are less common, and also quite a few igneous rock types that are quite scarce, yet each igneous rock has a name that distinguishes it from all the rest of the igneous group of rocks. So, if they all start out as molten material (magma or lava), which must harden to form a rock, then it is logical to assume that these igneous rocks differ from one another primarily due to
- the original composition of the molten material from which the rock is derived, and
- the cooling process of the molten material that ended up forming the rock.
These two parameters define the classification of igneous rocks, which are simplified into two terms: composition and texture. Igneous rock composition refers to what is in the rock (the chemical composition of the minerals that are present), and the word texture refers to the features that we see in the rock such as the mineral sizes or the presence of glass, fragmented material, or vesicles (holes) in the igneous rock.
- Felsic (Silicic)