Classification of Igneous rocks.

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Josey Bellamy

Classification of Igneous rocks

Igneous Rocks are classified is several ways, and methods of classification have evolved a lot over the past 100 years. Each classification is useful for a certain purpose and reflects a particular way of looking at igneous rocks. Early in the days of geology there were few rocks described and classified.  In those days each new rock described by a geologist could have shown characteristics different than the rocks that had already been described, so there was a tendency to give the new and different rock a new name. Because such factors as cooling conditions, chemical composition of the original magma, and weathering effects, there is a potential to see an endless variety of igneous rocks, and thus a classification scheme based solely on the description of the rock would eventually lead to a lot of rock names.  

There are various ways that could be used to classify igneous rocks…

Crystal size: - Igneous rocks are formed by the crystallisation of a rock melt or magma. The crystallisation occurs during cooling, as the atoms become organised into crystals. Eventually all the crystals will grow until they meet each other, forming an interlocking three dimensional structure when crystallisation is complete.

                Magmas that reach the surface of the Earth in volcanoes cool quickly, forming fine-grained extrusive volcanic rocks. If the rock is cooled extremely rapidly a volcanic glass results, where no crystals have had time to form.

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                Rocks forming mid- point within the Earth form medium- grained hypabyssal rocks. These are intrusive, that is they are forced into existing rocks.

                Crystallisation also takes place very slowly deep within the earth’s crusts, to form coarse-grained plutonic rocks. These are also intrusive.


  • > 5mm, and easily visible with the naked eye.
  • They form at plutonic depth- with slow cooling rate.
  • Batholiths
  • Limited nuclei-, which grow to bigger crystals- ionic diffusion.
  • Example- Granite


  • 1-5mm- crystals are visible with the naked eye, but you may need ...

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