Onion skin weathering is a form of physical weathering caused by temperature changes. This process helps to strip off the outer layers of rocks without the need for chemical or erosive processes.
Onion skin weathering often occurs in areas of great daily temperature changes. Rocks heat during the day, causing them to expand. When temperatures fall during the night the rocks cool and contract. This expansion and contraction can apply stress to the outer layers of the rock, which, over time, causes the outer layers to peel away or flake off.
Onion skin weathering can also be caused by forest fires, which rapidly heat the surface layers of a rock causing expansion and, in turn, stress. Both chemical and erosive processes, while not key contributors to onion skin weathering, can also help to speed up the process.
Onion skin weathering can often be seen in hot desert environments where the day-to-night temperature difference is great.
Desert areas in both Arizona, in the U.S., and Alice Springs, in Australia, provide numerous examples of onion skin weathering. Half Dome in the Yosemite National Park and Sugar Loaf Mountain in Rio de Janeiro are two further examples of onion skin weathering on a grand scale.
Onion skin weathering is also known as “thermal expansion,” or “exfoliation."