Igneous Rocks: Their Formation and Uses Over the past week (and since the start of this class), Professor Pillar has repeatedly mentioned igneous rocks, and has alluded to their formation. Although I had a very basic understanding of how they are formed, I felt that it would be beneficial to me to investigate deeper into the process of their creation. When asked how an igneous rock is formed, I would like to be able to respond with an answer more in-depth than, “It has something to do with volcanoes or molten lava.” I found a few articles on the formation and uses of igneous rocks that aided in my investigation. According to the introductory page of the first website I visited, “Igneous rocks are called fire rocks and are formed either underground or above ground.” This is something I didn’t know. I always assumed that igneous rocks would be formed underground, where most molten rock is. But this would make sense, considering the hot liquid would have to cool before solidifying into hard rock. Igneous rocks can be separated into five categories: ultra-mafic, mafic, intermediate, intermediate-felsic, and felsic. These classifications are used to differentiate between rocks by their amounts of SiO2. In fact, the process of underground formation of igneous rocks takes much longer than the above ground process. They are called intrusive. This process takes longer because the lava takes longer to cool when it is submerged underground. When underground, the lava, which is comprised of a vast array of different minerals, solidifies at different rates, depending on the temperature at which the specific minerals become solid. Extrusive igneous rocks are formed on the earth’s crust. These are usually formed when volcanoes erupt and the magma cools outside the volcano. Because of the way igneous rocks are formed, they are extremely resistant to the processes of weathering and erosion. Their durability makes them very useful. Some common examples of igneous rocks are granite, scoria, and pumice. Pumice and scoria are actually glassy. You may recognize some of these and already know what they can be used for. Granite usually forms below ground, and commonly is used for building. This rock is very common, and most people will recognize it. Scoria, on the other hand, is formed above ground. The minerals in scoria never combine with each other. Pumice, another extremely popular rock, is very porous, can actually float, and is created above ground when lava cools very quickly. It is used for cosmetic purposes, especially in exfoliating products. Gabbro is used in the transportation system. It is implemented in railroads and highways. Basalt is an extremely weather-resistant igneous rock that is used in aggregate.