Igneous rocks

The igneous, or magmatic rocks, are those that result from the solidification of molten rocky material: the magma. The latter is formed by the fusion of rocks at the base of the earth’s crust or in the mantle and tends to rise towards the most superficial areas of the crust, because of having a lower density than the rock where it is imbedded.

The genetic classification of the igneous rocks is based on the interpretation of the conditions in which the process of cooling and crystallization of the magma was developed: those that crystallize in the inside of the crust are called intrusive; if they make it in magmatic chambers the resulting rocks are called plutonic and form the batholiths, while if they are located in tabular conduits, forming dikes, they are called hypabyssals. When the magmas reach the surface of the Earth and they cool and crystallize on land or under water conditions, the resulting rocks are called extrusive or volcanic.

The igneous rocks consist of silicate minerals. The plutonic rocks are formed by regular-sized crystals, generally identifiable with the naked eye, that is to say, they have a granular texture; this is an indication of the slow cooling of the magmas, which gives time to mineral crystals to grow. The volcanic rocks and the hypabyssals usually have relatively large and isolated crystals, which stand out in a mass of very small-sized crystals, the matrix; this texture, called porphyritic or microgranular, is an indication of a more rapid cooling.

The classification of the igneous rocks is based on the mineralogical composition, the geochemical composition, the occurrence and the texture. One of the simplest classification criteria is the “degree of acidity”, based on the amount of quartz (SiO2): going from greater or lesser proportion they are classified as acid or intermediate, and basic or ultrabasic when they do not contain quartz. The most used mineralogical classification is based on the proportions of quartz, feldspars minerals (plagioclase and potassium feldspar) and feldspathoids, and also on the type of igneous rock. This classification is very useful for the plutonic rocks, since the minerals can be identified with the naked eye, but for the volcanic rocks, due to their small-sized grain, geochemical analyses are necessary.