Rock ClassificationRocks are divided into three main classes based on how they form:1. Igneous2. Sedimentary3. MetamorphicEach type of rock can be further classified based on: Composition and texture
Composition Thechemical make up of the rock or what it is made of. This can be minerals or other materials: shells, clay, plants, sand, pebbles
Texture/Grains Determined by the grains size, shape and position. Igneous can be fine or coarse grain Sedimentary can be characterized by observing lithified (cemented) inorganic grains, minerals, organic material, or fossil fragments. The wide range of textures common in sedimentary rocks is separated into clastic , chemical, and bioclastic (biochemical) groups.
Texture/Grains Metamorphic textures consist of two main types: foliated and non-foliated textures. Texture can reveal the process that formed the rock.
Texture/Grain Coarse Grained = large grains, cooled slowly Think about it…. If it cools over a long time – the grains have time to get big! Most abundant intrusive rock is GRANITE Fine Grained = small grains, cooled fast Think about it… If it cools over a short time – the grains don’t have time to get big! Basalt
Igneous Rock Formation-Origin Extrusive – formed ABOVE GROUND. It cools fast so it has small grains. Intrusive – formed UNDERGROUND. It cools slowly so it will have large grains. Laccolith
Examples of each type ofROCKFine GrainedBasalt = Extrusive Igneous rockCourse GrainedGranite = Intrusive Igneous rock
Mineral Composition How much silica is in the magma/lava? – Low silica content = dark colored = Mafic – High silica content = light colored = Felsic High Silica Content Low Silica Content Felsic Igneous Rock Mafic Igneous Rock
Big Crystals Small or No Crystals Lighter Darker