Classification of Igneous Rocks Most basically, igneous rocks are classified by their texture and mineral composition. Some textures of rocks are described below: Plutonic Texture - having a coarse, crystaline texture, which cools slowly underground. Volcanic Texture - having a fine crystaline texture, cools rapidly on the surface. Porphyritic Texture - contains both coarse and fine crystaline structures. Glassy Texture - extremely smooth, like glass, cools extremely fast on the surface. Fragmental Texture - fragments, which results from an exploding volcano. As Magma becomes Lava, temperature decreases and the pressure decreases. The first minerals to crystalize are Olivine, Pyroxene, Amphibole and Biotite. Next to crystalize are Orthoclays, Muscovite and Quartz. Then the feldspars crystalize (Plagioclaise) Ca feldspar, CaNa feldspar and Na feldspars.

To Mineral Composition Return To Geology Page