Igneous rocks: types, characteristics, classification and examples

The magmatic rocks are formed when molten rock as magma is cooled and minerals that form and crystallize the particles intertwine. This magma can cool on the surface, due to the effect of volcanic eruptions , or inside the lithosphere, which is the solid layer of the earth’s surface. The vast majority of these rocks are formed under the earth’s crust.

Although igneous rocks form a very high percentage of the upper part of the earth’s crust, they are usually found under a layer of metamorphic rocks and sedimentary rocks . They are of great importance in geology, since their characteristics and composition help to understand the earth’s mantle and its composition, as well as to read past tectonic events.

Magmatic rocks can be classified in various ways, although their most general classification divides them according to their origin.

The igneous plutonic rocks , also called intrusive, have formed inside the lithosphere. There, the magma cools in a much slower process, giving rise to rocks with large crystals, which can be easily observed. These rocks are transported to the surface by erosion processes or by tectonic deformations. Plutonic rocks get their name from plutons, which are the large magmatic intrusions from which they form. It must be said that the heart of the largest mountain ranges is formed by intrusive rocks.

The extrusive or volcanic igneous rocks are formed when magma is ejected to the surface and rapidly cooled. The vast majority of these rocks are created by the effect of volcanic eruptions , and as magma cooling occurs at high speed, the crystals that are created inside the rocks are much smaller, invisible to the human eye. In this type of rocks, the formation of holes or holes left by gas bubbles that are formed in the solidification process is common.

In addition to these two major classifications, we also have the Philonian rocks , which are halfway between the plutonic and the volcanic, when a magma seam heading towards the surface solidifies midway. The magmatic rocks can also be divided according to their texture and composition.

Types of igneous rocks according to their texture

• Vitreous: they are volcanic rocks that are formed when they are violently thrown into the atmosphere, where they cool at high speed.
• Aphanitic: volcanic with microscopic crystals.
• Phaneritics: formed when large amounts of magma solidify very slowly and at great depth.
• Porphyritic: these rocks have large crystals in the center and small crystals near the outside, due to uneven cooling.
• Pyroclastic – Pyroclasts form in explosive volcanic eruptions. They do not have crystals and are made up of rock fragments.
• Pegmatitic: very coarse-grained, these rocks are made up of crystals of more than one centimeter in diameter. They are formed when magma has large amounts of water and volatile elements.

Types of igneous rocks according to their chemical composition

Igneous rocks can have different levels of light or dark silicates.

• Felsic: they are made up of 70% silica, low density and light colors. The continental crust is made up mainly of them, and they contain approximately 10% dark silicates.
• Andesitic: they contain at least 25% dark silicates.
• Mafic: they are very rich in dark silicates. They are of greater density and of darker colors, and they constitute the oceanic crust.
• Ultramafic: with at least 90% dark silicates, they are rare rocks to find on the planet’s surface.

To finish, these are some simple examples of igneous rocks :

• Granite is the most common plutonic rock.
• Basalt is a widely known volcanic rock.
• Obsidian, widely used by our first ancestors in prehistory, is an igneous rock with a glassy texture.
• Peridotite is one of the rare ultramafic rocks.
• Rhyolite, an extrusive igneous rock, is considered the volcanic equivalent of granite.