Our earth is composed of three main types of rocks, each having been formed in its own special way.
The first type, igneus rock, was formed when hot (2,000°F.), melted rock material, magma, deep inside the earth rose to the surface during earthquakes, volcanic eruptions, or other movements of the earth’s crust. This magma cooled and hardened. Granite is an example of igneus rock.
The second type, sedimentary rock, was formed millions of years ago mostly on the bottoms of lakes and oceans, when parts of plants, animals, and other, older rocks piled loosely upon each other in layers. Over millions of years, these layers squeezed together to form solid rock. Sandstone and limestone are examples of sedimentary rock.
The third type of rock, metamorphic rock, was formed when great heat or pressure, or chemical actions of liquids and gases changed either the igneus or sedimentary rock in appearance and composition. Marble is an example of metamorphic rock, since it was originally limestone, a sedimentary rock formed from hardened shells and coral upon which chemicals acted.
Smaller rocks of all three types are formed when water seeps into rocks, freezes, then expands. This expansion causes huge rocks to break apart. The movement of glaciers has also ground larger rocks into smaller, more rounded forms.
There is actually a rare kind of thin sandstone, itacolumite, found in North Carolina, which can be bent out of shape by hand!