When wind, water or other forces carry particles that wear away rocks, abrasion has occurred. In the photo at left, a natural bridge developed as water-carried particles ate away the rock. Sulfuric acid is a much stronger acid than carbonic acid, and may wear away rocks, metals and other substances very quickly. When carbon dioxide in the air dissolves in rain, a weak carbonic acid is formed. This weak acid, while harmless to plants and animals, is able to dissolve some kinds of rocks, like feldspar and limestone, in a process called carbonation.
Many caves begin forming during the process of carbonation.
Caused by changes in temperature, exfoliation is the breaking off of rock in curved sheets or slabs parallel to the rock's surface. During the day, the outside of the rocks heats up and expands. The same rock cools at night, and the process repeats itself day after day, causing the rock to break off in slabs. Exfoliation, shown at left, is a common sight at Enchanted Rock State Park in Texas. Because water expands when it freezes, it can cause rocks to break apart.