A mineral is made up of one or more elements, such as iron or gold, that are fused together. A rock is classified as igneous, sedimentary or metamorphic based on how it was formed.
Igneous rocks are formed by volcanic action, such as molten lava that solidifies, or granite that is created by magma that solidifies underground. The most common igneous rock is basalt, which covers the sea floor. It is a dark lava rock, just like the rocks formed on the Big Island of Hawaii when Kilauea's lava cools. Granite is much older, sometimes taking four billion years to break the surface.
Sedimentary rocks form when bits of other rocks, as well as plant and animal remains, accumulate in low-lying areas and are compressed. These are most commonly found near lakes and oceans. Limestone is an example, formed from seashells and the skeletons of microscopic creatures. Sedimentary rocks are fossil-rich and form in layers, called strata, making it easier for scientists to date specimens.
Metamorphic rocks are igneous or sedimentary rocks that have been altered by seeping fluids, heat or pressure. This could happen deep underground when magma compresses the minerals. Examples include marble, which comes from limestone, and quartzite, which was created from sandstone.