Petrology or lithology is a branch of geology that is dedicated to studying rocks, especially their physical, chemical and mineralogical properties, but also studies rock associations and the respective formation processes. This fascinating science encompasses the study of both igneous rocks and sedimentary rocks and metamorphic rocks. In relation to what has just been described, volcanic rocks are the subject of petrology study. This type of rock, also called effusive rocks or extrusive rocks, is broadly one of the products that arise from volcanic eruptions.
What are volcanic rocks
To better understand this article, we first need to review the difference between magma and lava. Magma is the mass of rock molten from the center of the Earth, while lava is the magma that, during a volcanic eruption, reaches the earth’s surface.
Now, as to what volcanic rocks are, we can detail that they are defined as a type of igneous rocks , which originate thanks to the cooling and solidification of lava on the earth’s surface or magma that is close to the surface. Therefore, volcanic rocks are particularized by having formed from a rapid and, sometimes, abrupt cooling . This phenomenon results in volcanic rocks being composed of small crystals called microcrystals.
Types of volcanic rocks
Volcanic rocks can be classified in multiple ways, by their abundance, size and even by the type of crystal they are formed. According to the texture of the rocks and the size of the crystals that compose them, the classification of volcanic rocks is as follows:
- Glassy Rocks: As the name refers, rocks of this type have a texture similar to glass. This is because the cooling of the lava is so fast that it causes a disorder in the ions, without being able to constitute an ordered crystalline structure. A typical example of rock with this structure is obsidian .
- Rocks of aphanitic texture: this type of rock originates from a considerably rapid cooling of lava. As a result, the crystals are so small that they are impossible to see with the naked eye. Among the examples of volcanic rocks with this texture, we find rhyolite .
- Rocks of porphyritic texture: they originate as a result of the different crystallization temperatures of minerals. Meanwhile, this type of rock presents large crystals immersed in a matrix of smaller crystals. As an example, basalt presents this type of texture, although sometimes it can present parts with other textures.
- Rocks of pyroclastic texture: during volcanic eruptions the so-called pyroclastic flow is emitted , made up of gases and different rock fragments. Said fragments are classified, according to their diameter, into: ash (diameter less than 2 millimeters), lapilli (diameter between 2 to 64 millimeters) and bombs or blocks (diameter greater than 2 millimeters). These rock fragments can consolidate and give rise to rocks with a pyroclastic texture. The most common example of rocks with this type of texture is volcanic tuff .
Now that we have talked about what volcanic rocks are and their types and characteristics, it will be easy for us to understand the difference between these and plutonic rocks. Before starting with the differences, we must know that both types of rocks are igneous rocks , that is, rocks that originate from the cooling and solidification of magma near the surface or lava already outside. The main differences between rocks are around the place where they originate and the time in which they cool. Others of the differences are related to their respective location. Let’s see here:
- Place of origin: volcanic rocks originate, as we have already learned, from the cooling of lava on the earth’s surface, while plutonic rocks originate from the solidification of magma below the earth’s crust.
- Cooling time: when it originates from the earth’s surface, the lava that forms volcanic rocks is exposed to room temperature, causing rapid and abrupt cooling, on the other hand, the magma that forms plutonic rocks cools very slowly thanks to the rocks that it has around it.
- Location: volcanic rocks are formed on the earth’s surface and remain there accumulating and forming geological structures such as volcanic buildings. For their part, plutonic rocks are only visible when they emerge, that is, when the earth’s crust rises and the rocks that cover them are removed. Interestingly, the center of the mountain ranges is made up of the latter type of rocks.