Rocks are all around you; they are set into the pavement that you drive on, they are tracked into your house by the family pet in the form of dirt, and you empty them out of your shoes as sand in the summertime. All rocks, including the fantastic rock formations in Monument Valley National Park on the border of Arizona and Utah, are made of one of three basic types of rock.
Igneous rocks start off as liquid magma from an erupting volcano or from beneath the surface of the earth. As the magma cools from contact with the air or water, the liquid undergoes a phase change and the crystals in the magma form solids. Many famous rock formations in the United States are igneous in nature, including Shiprock in New Mexico and The Devil's Tower in Wyoming. Igneous rock formations such as these have a characteristic gray hue.
Sedimentary rocks are formed over millions of years as various layers are deposited and compressed into a striped pattern. Examples of these layers include rock made from fossilized plant and vegetation, known as organic layers; rock made from minerals that collect when standing water evaporates, known as chemical layers; and rock that forms as broken rock collects and becomes compressed, known as clastic rock.
Sedimentary rock formations are common in the desert Southwest in the United States. The most well known of the sedimentary rock formations is the Grand Canyon, where the layers of rock are visible after millenia of erosion by wind and water.
Metamorphic rocks are rocks that undergo a change. Diamonds are metamorphic rocks that are formed when carbon is highly compressed. However, to view metamorphic rock formations in the United States, usually you need to travel to places where intense pressure or heat have changed the sedimentary or igneous rocks.
Many metamorphic rock formations are formed by plate tectonics. All of the planet's surface rests on giant plates known as tectonic plates, which float on magma under the surface of the earth.The Rocky Mountains are examples of rocks that underwent intense pressure and temperature as two plates moved in contrary directions: the North American Plate moved westward and its neighbor, the Pacific Plate, moved north. As the North American Plate moved over the Pacific Plate, rock was upthrust under intense pressure until high mountains were formed.
Caves often contain more than one type of rock formation. Caves sometimes feature vast marble formations, which are formations made from metamorphic rock. Because marble is water soluble, dripping water deposits minute amounts of marble until a formation is created. Caves also are often created from sedimentary rock as water flowing from underground rivers erodes the rock under the earth's surface. Some caves were formed from volcanic eruptions when magma ate away the bedrock under the soil, leaving igneous formations.
