Although rocks appear to be hard and not easily broken, they are not immune to nature's forces, which erode and change them over time. This process of rock transformation is known as weathering. Rocks weather through a number of different agents. However, some of the most strange and beautiful rocks owe their shapes to the forces of weathering.
Nature (and people) can wear away at rocks through any number of ways, and each style has its own definition. Rock weathering falls into three main categories. Biological weathering occurs when living things alter rocks. Chemical weathering happens when chemical agents break down the rock, on a molecular or atomic level. Physical weathering (sometimes called mechanical weathering) wears down the rocks without changing the elements inside them or their overall identity.
The process of biological weathering occurs when living things--from people to plants--change rocks. The roots of even tiny weeds can split rocks, while lichen growing on rocks can also change them. Additionally, animals are known to alter rocks (some advanced species may even use them for tools). Finally, one only has to look at companies that extract materials like coal from the ground to see how radically humans can alter rocks with explosives and other machinery.
Chemical weathering occurs when the actual composition of a rock changes. This process happens most frequently below the ground surface, as water and rocks are constantly in contact in the soil. Two common minerals, halite and calcite, are often found in water and are able to dissolve rocks. When oxygen combines with iron-bearing silicate, the rock can begin to "rust." However, gases and other solutions can also chemically alter rocks (pollution in cities demonstrates this well).
Many agents, including water, carry out physical weathering (when a rock changes form but not its elemental make up). Water can expand as much as 9 percent when it freezes. Thus, freezing and thawing cycles expand the cracks in rocks, and can ultimately wedge them apart.
Clays, known as smectites, can absorb water, which causes them to expand and push rocks apart. The high temperatures of fire may make a rock's exterior expand and crack parallel to the surface in a process called "spalling" off.
Famous Weathering Patterns
Different kinds of weathering processes have different definitions, and the results also vary. Some weathering is purely destructive, as in the case of eroding monuments and headstones in cemeteries. Yet, other weathering can create spectacular scenery, such as the Grand Canyon (caused by water and sediment in the Colorado River eroding the walls). Utah is also famous for its rock formations that were created through weathering processes.