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Exfoliation
Exfoliation, or unloading, is created by uplift and erosion. Rock forms under a certain pressure. As this rock is uplifted to the Earth's surface, this pressure decreases, causing the rock to expand. This expansion creates joints. The erosion of overlying material also results in reduced pressure on the underlying rock, producing expansion. This process is typical in layers of granite rock, and results in rounded or dome-shaped rock formations. These can be observed in Yosemite National Park, the Quiet City of Rocks near Oakley, Idaho, and the Idaho Batholith.
Thermal Expansion
Thermal expansion occurs when there is a wide temperature gradient. The rock heats up during the day and cools off at night. These fluctuations in temperatures cause the rock to expand and contract, leading to the formation of joints. Furthermore, the expansion and contraction is not uniform throughout the rock, as different minerals within it expand and contract at different rates. This uneven expansion and contraction further contributes to joint formation. The effects of this weathering process are very small and require long periods of time to produce results. However, the process can be accelerated by rapid heating, such as what occurs during forest and grass fires.
Frost Wedging
Frost wedging is one of the primary physical weathering processes that leads to expansion of rock joints. This process occurs when water seeps into joints and cracks in the rock during the warm hours of the day. As temperatures drop at night, the water freezes. Water expands by roughly nine percent when it freezes, producing up to 4.3 million pounds of pressure per square foot. This extreme pressure forces the fracture to expand further. A common sign of frost wedging is a slope of gravel, called talus, at the base of cliffs or mountains.
Crystallization
Crystallization is another form of physical weathering that leads to joint expansion. It occurs when water filters down into rock fractures and then evaporates. Though the water is gone, it leaves behind minerals that precipitate out during the evaporation process. These minerals form crystals, which then grow over time. Like frost wedging, the growing crystals exert pressure on the joints, causing further fracturing and joint expansion. This type of weathering is common in arid climates.
Organic activity
Organic activity, also known as biological weathering, also leads to joint expansion. There are two primary types of organic activity: root wedging and animal activity. Root wedging occurs when roots grow into rock fractures. As they continue to grow, they exert pressure on the joint, causing it to expand. Organic activity also includes animal burrowing, which can lead to the expansion of rock joints.