Among the different types of rocks are igneous rocks . The surface of our planet is covered in rocks and a wide variety of minerals. However, igneous rocks are of particular importance since they make up 95% of the Earth's crust. Some are very well-known, such as granite and obsidian, although there are many other igneous rocks that you are likely familiar with. For a deeper understanding, it is helpful to know how rocks are formed and classified, which can be found in this article.
Therefore, we are going to dedicate this article to tell you about all the characteristics and origin of igneous rocks.
Key features
These are also called igneous rocks and are formed when molten rock in the form of magma begins to cool. It is as the magma begins to cool that the minerals begin to crystallize and bond together. Magma can cool in two ways. One is cooling at the Earth's surface, which occurs through volcanic eruptions. The other is cooling within the lithosphere. The lithosphere is the solid layer of the Earth's surface. The vast majority of these rocks form beneath the Earth's crust and are called plutonic igneous rocks, which can be explored further at this link . Rocks that cool at the surface are known as volcanic igneous rocks.
Although these types of rocks make up a large percentage of the upper part of the Earth's crust, they are usually found beneath layers of metamorphic and sedimentary rocks. They are of great importance in geology, and their characteristics and composition help us understand the Earth's mantle. The composition of the Earth's mantle and all past tectonic events help us understand the formation and characteristics of our planet.
Classification of igneous rocks
Let's look at the classifications that exist for igneous rocks. As we saw earlier, they are usually classified directly based on their formation. If they cooled in the upper part of the Earth's crust, they are called volcanic igneous rocks; on the other hand, if they cooled within the lithosphere, they are known as plutonic igneous rocks. Plutonic rocks are also called intrusive rocks since they formed within the lithosphere. Here, the magma cools in a much slower process, resulting in rocks with larger crystals. These crystals can be observed more easily.
Plutonic igneous rocks are transported to the Earth's surface by erosion or tectonic deformation. We must not forget that the Earth's surface is composed of shifting tectonic plates. This shift is almost imperceptible to humans, but we are talking about a geological time scale. Plutonic plates are called plutons because they are large magma intrusions from which they form. It is worth noting that the core of the largest mountain ranges is made up of intrusive rocks such as granite.
On the other hand, extrusive igneous rocks, or volcanic rocks, are formed when magma is expelled from the Earth's surface and cools much more rapidly . The vast majority of these rocks are generated by volcanic eruptions and the rapid cooling of magma. The crystals that form inside these rocks are smaller and less visible to the naked eye. It is very common to see cavities or holes left by gas bubbles during the solidification process in this type of rock.
Besides these two main classifications, there are others. These are called dike rocks . These rocks are intermediate between the two main classifications. When a large volume of magma rises to the surface and solidifies along the way, dike rocks are formed, intermediate between volcanic and plutonic igneous rocks.
Types of igneous rocks
We are going to see what are the different classifications of igneous rocks according to their composition and texture.
Texture
Igneous rocks have the following textures:
- Vitreous: This is a very common texture in volcanic rocks. This texture forms when rocks are violently ejected into the atmosphere and subjected to rapid cooling.
- Aphanitic: They are volcanic rocks that have crystals of a microscopic size.
- Phaneritics: They are formed by large quantities of magma that have solidified more slowly and at great depth.
- Porphyritic: These are rocks that have large crystals in the center and smaller ones on the outside. This is due to uneven cooling. The area with the larger crystals has cooled more slowly, while the outer part with the smaller crystals has cooled much more quickly.
- Pyroclastic: pyroclasts are generated in explosive-type volcanic eruptions. They do not usually have crystals and are made up of rock fragments.
- Pegmatitics: They are those that have a very coarse grain and are made up of crystals of more than one centimeter in diameter. They are formed when magma has a large amount of water and other volatile elements.
Chemical composition
We are going to see what are the different types of igneous rocks depending on the chemical composition that each of them has:
- Felsic: They are those rocks that are made up mostly of low-density silica and light colors. We see that the continental crust is formed mainly by this type of rocks and they contain approximately 10% pure silicates.
- Andesitic: they contain at least 25% dark silicates.
- Mafic: this type of rock is usually quite rich in dark silicates. They have a higher density and darker colors and usually make up the oceanic crust.
- Ultramafic: they have 90% of their composition dark silicates. They are usually rare rocks to find on the surface of the planet.
Among the best known examples of igneous rocks we have granite, which is the most common plutonic rock. The assault is also one of the widely known volcanic rocks due. As you can see, there are different types of igneous rocks depending on their formation.
I hope that with this information you can learn more about igneous rocks and their characteristics.