Igneous rocks can be formed under two main different conditions: intrusively, or extrusively. This means the rock can be formed insie the earth, or upon the earth, most usually in open air. The conditions under which an igneous rock can form extrusively usually occur after a volcano erupts. The magma violently escapes from the earth, becomes lava, and when the lava comes in contact with the cool air, it cools down. The hardened lava is one type of extrusive igneous rock. One example of this type of rock is Pumice. Intrusive rocks form inside the earth. Usually, these intrusive rocks take much more time to form than extrusive rocks, due to the subtlety of the temperature changes within the earth as opposed to the drastic changes in temperature outside the earth. Many factors affect the development of a rock within the earth, including changes in temperature, pressure, and the presence of water. When temperature and water levels are increased, the rock will become hotter, and more likely to melt. When pressure is increased, the rock is more likely to solidify because it does not have much room to expand. For both intrusive and extrusive rocks, the slower the rocks cool, the larger the crystals will be. This is because when lava or magma is cooled very rapidly, the crystals have less time to form. In rocks such as obsidian, a glassy texture is a result of the extremely rapid cooling, and there are no visible crystalline shapes. In rocks such as granite, crystals are clearly visible.
This brings us to the first way igneous rocks can be classified by: texture. There are five main ways to classify the texture of an igneous rock: Phaneritic, Aphanitic, Porphyritic, Glassy, an Vescular. First on the list, Phaneritic, can be classified by having large crystals that the human eye can easily discern. This texture is a very obvious one that any beginning rock classifier can figure out. An example of this texture is Granite. The second, Aphaneritic, is distinguished by its small, sand-like crystals. These rocks may feel smooth. Their crystals are so small that the human eye cannot see them, and a microscope is necessary. An example of this type of rock would be basalt. The third clasification, Porphryitic, is unique in that it features both small and large crystals. Because one may impulsively assume that its large crystals make it Phaneritic, its looks can be decieving. One example of this rock is Andesite. Fourthly, there is the glassy texture. This type of rock is very obvious. it may feel smooth or greasy, and it always looks shiny. This type of rock is formed when molten rock cools very quickly. An example of this rock is Obsidian. The final texture is Vescular. Vescular means that the rock has many holes. These types of rocks are usually extrusive an used for cosmetic purposes. One of the most well-known of these is pumice.
Also, igneous rocks can be classified by their silica content. Igneous rocks with more silica are referred to as felsic and igneous rock that are comprised of iron and magnesium are called mafic. Felsic rocks are usually lighter in color and mafic rocks are dark in color.