Rocks may seem incredibly hard, but, like nearly everything else in nature, eventually wear away. Scientists call this process, where the forces of nature consume rocks and them back into sediment, weathering. There are many different materials that erode rocks over time, including water. Given its ubiquity, water is one of the most common agents of rock weathering, especially when it freezes and melts over time. Still, there are many other weathering agents that eat away at rock.
There are three kinds of rock weathering, but the freeze-thaw cycle falls under category of mechanical (also called physical) weathering. According to Georgia Perimeter College, mechanical weathering is a process in which a weathering agent wears away at a rock without altering its mineral makeup or its molecular structure (as happens with rust or oxidation). A rock weathered through mechanical weathering is chemically identical before and after the process, only its size and shape are different.
As the Water Encyclopedia reports, water expands 9 percent when it freezes. This makes the freeze-thaw cycle a powerful weathering agent. If, for example, water seeps into a crack in a rock, freezes overnight and then melts again in the morning, the expansion of the ice during the night will make the crack bigger. In the morning, that water will melt, but because the crack is bigger, it can now take on more water. That night, this even greater volume of water will expand, making the crack even bigger. Over time, this freeze-thaw process easily causes pieces of the rock to break off into smaller fragments.
The freeze-thaw cycle is what gives water the ability to break rocks apart, but the process is also sometimes called frost wedging. Either term is acceptable.
The Power of Water
However, the freeze-thaw cycle is not the only way that water can eat away at rock. Rivers and streams can erode rock because their waters carry debris and other sediment that constantly flows over a rock's surface, wearing it down. One of the most famous examples of rock weathering in the world, Arizona's Grand Canyon, is a result of this form of mechanical water weathering. However, water alone did not sculpt the canyon, as wind, as well as other chemical processes, contributed to the contours and colors as well, according to Arizona State University.
Other weathering processes
The Grand Canyon is the result of multiple forms of weathering creating its current form. Its colors are due to chemical weathering, in which the actual mineral composition of the rock breaks down.
Another form of weathering, biological weathering, occurs when living things alter rocks. Tree and plant roots, much like the freeze-thaw cycle, exploit the cracks in rocks, and as they grow, push the rock apart.
- The Rock image by dr.cornelius from Fotolia.com