Rock
A rock is a naturally formed solid made of one or more minerals or mineral-like materials, classified as igneous, sedimentary, or metamorphic.
Definition
A rock is a solid chunk of Earth made of one or more minerals stuck together, and it can be as big as a mountain or as small as a pebble. Formally it is a naturally occurring aggregate of minerals, mineral fragments, glass, or organic material such as coal. Rocks are classified first by how they form: igneous rocks cool from melt, sedimentary rocks form from compacted sediment or precipitated material, and metamorphic rocks are changed by heat and pressure. Within each group, texture and composition refine the name. Igneous rocks are described by grain size and silica content from felsic to ultramafic, sedimentary rocks by clast size or chemical origin, and metamorphic rocks by foliation and grade. Texture preserves a physical history: cooling rate, transport distance, or peak metamorphic temperature and pressure.
Example
Granite, the speckled gray and pink rock used for countertops, is made of the minerals quartz, feldspar, and mica. Charlotte sits on the Carolina terrane, where the bedrock includes metamorphosed volcanic rock and granite, and Stone Mountain in the North Carolina foothills is a dome of igneous granite. Across the state, the Blue Ridge exposes Precambrian gneiss over a billion years old, the Coastal Plain is built of unconsolidated to weakly cemented Cretaceous and younger sediments, and the Piedmont between them is largely metamorphic and plutonic rock of the Carolina and Inner Piedmont terranes.
Key Insight
Minerals are the ingredients and rocks are the finished dish; most rocks are a mix of several minerals. The same minerals can build different rocks: quartz and feldspar make granite when they crystallize from magma, sandstone when their grains are cemented, and gneiss when they are squeezed and heated. A rock is therefore a record, not just a material. Reading its texture, mineral assemblage, and structure lets a geologist reconstruct pressures, temperatures, and environments that no longer exist.