The word “igneous” is a Latin word for fire. This means igneous originate from hot, fluid material called “lava” that erupts on the earth’s surface during a volcanic activity or un-erupted lava at shallow depths within bodies. These types of rock are either formed at the surface of the earth or deep within the volcanic bodies. Those that are formed at the surface (extrusive) are as a result of the cooling of the lava, while intrusive are those that form as a result of the solidification of the Magma before reaching the earth’s surface. According to Alden, “Magma crystallizes into a collection of minerals, and some crystallize sooner than others. Not just that, but when they crystallize, they leave the remaining liquid with a changed chemical composition.” The two types of igneous rocks are intrusive and extrusive igneous rocks. This classification is basically dependent on the place where the rocks are formed within volcanic bodies.
What are the distinguishing characteristics between the two categories of rocks?
These types of igneous rocks are distinguished from one another by their textures. For instance, intrusive rocks have small to medium-sized grains, unlike extrusive which have invisible or very small grains. This is because extrusive solidifies faster on the surface of the earth. The different textures come along due to the amount of time the rocks take to solidify. Alden illustrates that “Some extrusive rocks have distinctive texture such as obsidian, formed when lava hardens quickly and has a glassy texture; Pumice and scoria are volcanic froth, puffed up by millions of gas bubbles giving them a vesicular texture”. This is not so intrusive as granite, gabbro, diorite, and peridotite are the rocks formed as a result of their slow solidification.