Nearly all rocks found on our planet can be divided into three categories, depending on the natural forces that created them. Igneous rocks are those formed by magma or lava that has cooled and solidified. Sedimentary rocks are formed by the sedimentation of materials on the earth's surface, or in bodies of water. Metamorphic rocks are the result of other rock types that undergo changes due to intense heat or pressure. To better demonstrate these rock types to an elementary school science class, it can be highly effective to bring in rock samples from each category.
Geologists have identified over 700 types of igneous rocks in the earth's crust and on its surface. Those that form within the earth's crust are known as intrusive, or plutonic, rocks. Those that form on the surface of the earth are known as extrusive, or volcanic, rocks. Both categories are the result of cooling lava or magma that originates below the earth's crust. Varieties of igneous rock that are readily available and identifiable for the classroom include basalt, granite, pumice, and obsidian.
Sedimentary rocks are caused by the accumulation of sediment that forms into rock over the course of many years. This often occurs when materials are dislodged due to weathering or erosion, and settle in bodies of water. These deposits occur in layers, and can often be seen within the rocks themselves. For this reason, sedimentary rocks are important in understanding the history of the earth, as the different layers, or strata, can be used to determine the order and age of geologic events. Varieties of sedimentary rock that work well in a classroom include quartz, sandstone, shale, coal, limestone, and halite (rock salt).
Metamorphic rocks are formed when a rock of an existing type -- known as a protolith -- is subject to amounts of heat and pressure that cause a physical or chemical change. The protolith can be an igneous, sedimentary, or existing metamorphic rock, before undergoing its transformation. Metamorphic rocks are important because they allow geologists to understand activity that is occurring deep beneath the earth's surface. Slate, marble, and schist are all varieties of metamorphic rock that can be readily found and used as samples in a science class.