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The answer to how are igneous rocks formed is pretty straight forward: Igneous rocks are called fire rocks and are formed either underground or above ground. Underground, they are formed when magma deep within the Earth becomes trapped in small pockets. As these pockets of magma cool slowly they become igneous rocks. Igneous rocks are also formed when volcanoes erupt. Igneous rocks are formed as the lava cools above ground. The upper 16 km of the Earth’s crust is composed of 95% igneous rock.
Intrusive igneous rocks are formed from magma that cools and solidifies underground. These rocks are coarse grained. The mineral grains in such rocks can generally be identified with the unaided eye. They can be classified according to the shape and size of the intrusive body and its relation to the other formations into which it intrudes. Intrusive formations are batholiths, stocks,laccoliths, sills, and dikes.
The central core(batholiths) of major mountain ranges consist of intrusive igneous rocks, usually granite and occupy huge areas of the Earth’s surface. Coarse grained intrusive igneous rocks which form deep in the crust are termed as abyssal and those that form near the surface are called hypabyssal.
Extrusive igneous rocks are formed at the crust’s surface as a result of the partial melting of rocks within the mantle and crust. Extrusive igneous rocks cool and solidify quicker than intrusive igneous rocks and are fine grained.
Igneous rocks are classified according to mode of occurrence, texture, mineralogy, chemical composition, and the geometry of the igneous body. Two important variables used for the classification of igneous rocks are particle size and the mineral composition of the rock. Feldspar, quartz, olivines, micas, etc are all important minerals in the formation of igneous rocks, and are important to their classification. Types of igneous rocks with other essential minerals are very rare. In simplified classification, igneous rock types are separated by the type of feldspar present, the presence or absence of quartz, and in rocks with no feldspar or quartz, the type of iron or magnesium minerals present. Rocks containing quartz are silica-oversaturated. Rocks with feldspathoids are silica-undersaturated.
Igneous rocks which have crystals large enough to be seen with the unaided eye are called phaneritic. Those with crystals too small to be seen are called aphanitic. Usually phaneritic are intrusive rock and aphanitic are extrusive. An igneous rock with larger, clearly discernible crystals embedded in a finer-grained matrix is called porphyry. Porphyritic texture develops when some of the crystals grow before the main mass of the magma crystallizes as finer-grained material.
While this gives you a basic understanding of ‘how are igneous rocks formed’ and a brief brush over of their classification, there is still plenty of additional research that you can do here on Universe Today.
We have written many articles about igneous rocks for Universe Today. Here’s an article about the types of rock, and here’s an article about how rocks are formed.
We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.