Breaking Down the World: Weathering and Erosion

Pick up an imaginary handful of sand from a beach. Now think: where did those tiny grains come from? Every grain was once part of something much bigger — a cliff, a boulder, a mountain. Slowly, over thousands of years, that rock was broken apart and carried away. Two processes did that work: weathering and erosion. By the time you finish this lesson, you will understand exactly how it happened — and why the Grand Canyon in the USA is getting a tiny bit wider every single day.

Weathering and erosion are partners in reshaping the Earth's surface, but they do different jobs. Weathering is the breaking down of rock in place. The rock does not move — it is simply broken into smaller pieces or chemically changed. There are three main types. Mechanical (physical) weathering breaks rock without changing its chemical make-up. The most important example is freeze-thaw action. Water seeps into a crack in a rock. When temperatures drop below 0 °C, the water freezes and expands by about 9%. This expansion pushes the crack wider. When the ice melts, more water enters. Repeated over many winters, this splits the rock apart. You can see this process at work in mountain ranges such as the Alps in Europe. Chemical weathering changes the minerals inside the rock. Rainwater is naturally slightly acidic because it absorbs carbon dioxide from the air, forming weak carbonic acid. This acid dissolves limestone and chalk — a process called carbonation. The White Cliffs of Dover on the south coast of England are made of chalk, and chemical weathering slowly dissolves their surface year after year. Biological weathering is caused by living things. Plant roots grow into cracks and widen them; lichens (simple organisms that grow on rock) release weak acids that slowly eat into the rock surface. Erosion is the picking up and carrying away of weathered material by a moving agent — water, wind, ice (glaciers), or waves. The Colorado River in the USA has been eroding the rock of the Grand Canyon for around 5 to 6 million years, cutting the canyon more than 1,800 metres deep. Without weathering first loosening the rock, erosion would have far less material to transport. The two processes work together: weathering prepares the material, and erosion removes it. Once eroded material is transported and dropped somewhere else, that final step is called deposition — for example, sand deposited at the mouth of a river to form a delta.