Igneous Rocks

All igneous rocks form from magma, but there are strong differences based on where they crystallize. Rocks formed on the earths surface (extrusive igneous rocks), and rocks formed inside the earth (intrusive igneous rocks) have unique textures and mineral compositions.

The size of mineral grains is a function of how fast it cools. The slower a rock mass cools the larger the individual crystal will become. Intrusive rocks are well insulated and cool very slowly. A coarse grained rock is called a phanerite, which means that the grains must be at least 2 mm in diameter and viewable by the naked eye.

Intrusive rock that contain really large (larger than ordinary) grains is called a pegmatite. Pegmatites are often locations where large fine mineral specimens are found. The average grain diameters must be at least 2 cm for the body to be called a pegmatite.

Extrusive igneous rocks crystallize much faster as they have immediate contact with the atmosphere and/or water. They form finer grain structures and are called affinities. If an igneous rock is quench cooled, it may form a glass not having sufficient time to crystallize at all. This is called obsidian (volcanic glass). Aphanites have grain sizes less than 2 mm and require some kind of magnification to see. One exception is a rock type called a pophyry, it is made up of large course grains cemented by the fine grain material. The large grains then are called phenocryst.

What does this indicate?
The large grained material must have been formed slowly at depth, the fine grained material crystallized quickly at the surface. It probably means that the rising magma never fully melted all of its mass before it arrived at the surface. It is a fine grained material cooled rapidly encapsulating the phenocryst.