Glaciers are a powerful example of a type of physical weathering, called abrasion.
Weathering is a slow, constant process whereby large rocks are broken down into either smaller rocks or new, more stable minerals. There are two major types of weathering: physical and chemical. Physical weathering results in the disintegration of rocks into small pieces. There are five primary methods of physical weathering: frost wedging, thermal expansion and contraction, wetting and drying, exfoliation, and abrasion.
Frost Wedging
Frost wedging occurs when water seeps into the cracks and crevices of a rock. When this water freezes, it expands in volume by around 9 percent. This expansion generates tremendous force, expanding the size of the fractures. If the fracture expands enough, the rock will break apart. Frost wedging is most common in high-altitude terrains, where the frequency of freeze-thaw cycles are higher.
Thermal Expansion and Contraction
The heating of a rock causes it to expand, while cooling results in contractions. This thermal cycle can produce fractures and breakdown rocks. Furthermore, the different minerals present in a rock expand and contract at different rates, amplifying this weathering process and producing fractures along mineral boundaries. Normal temperature ranges tend to have a minimal affect; however, extreme temperature ranges, such as those found in deserts or fires, can produce more rapid and intense weathering.
Wetting and Drying
All rocks, to at least some extent, are porous. As they absorb water and dry out, they can expand and contract. This cycle can lead to the creation and expansion of fractures, culminating in rock disintegration. This process also can lead to crystallization, produced when ion rich water works its way into cracks and crevices. As the water evaporates, the ions left behind can form into crystals. As these crystals grow, they can exert forces capable of fracturing and crumbling rocks.
Exfoliation
Exfoliation occurs when overlying material is removed, such as by erosion, from over a rock layer. This relieves the pressure on the rock, allowing it to expand. As the rock expands, joints form. Eventually, pieces of rock will begin to break off in sheets. This weathering process can be further amplified if the rock layer experiences any geological uplift. Exfoliation produces dome-shaped rock formations, such as those found in Yosemite National Park.
Abrasion
Abrasion is the form of weathering people are most likely to think of first whenever erosion is mentioned. Often invisible to the naked eye, wind and water contain suspended particles that act like a natural sandpaper, scouring the surfaces of rocks as they flow over them. A good example of this weathering process is the abrasive power of glaciers.
Biological Weathering
In addition to these five primary types of physical weathering, there is another type of weathering, called biological weathering, which is sometimes considered a part of physical weathering. Biological weathering includes root wedging and animal activity. Root wedging occurs when plant roots extend into cracks and crevices, called joints. As they continue to grow, they can cause the fractures to expand, leading to the breakdown of the rock. Animal burrowing and persistent movement also can cause rock to fracture and crumble.
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