Rocks Rocks seem commonplace enough to most people, but to others they are fascinating, and rightly so. Rocks are divided into three main types, according to how they were made. Igneous rock is fire-born. It was once magma, or liquid rock, hidden deep inside the earth. When the hot magma hit the cool air, it hardened, forming igneous rock.
Sedimentary rock is formed from earth materials gradually worn down by water, wind, and snow.
Tiny particles of dirt, sand, and clay are washed to the bottoms of lakes, rivers, and oceans. Rocks are formed as layers of this sediment build up, each pressing down and hardening the layer underneath. Again, shifts in the earth's crust cause the layers to come to the surface. Either igneous or sedimentary rock is metamorphosed, or changed, usually through extreme heat or pressure under the earth's crust. This ever-continuing process has formed the various landscapes of the earth since the beginning of time.