Weathering
Erosion starts with weathering. Weathering is the process by which rocks are chemically altered or physically broken down into fragments at or near Earth's surface. 
There are two forms of weathering: mechanical and chemical. They cause rocks to disintegrate or decompose.
Figure 8 When water expands to form ice, it can pry open cracks in rock. This is called frost wedging. Classifying Is frost wedging a form of mechanical weathering or chemical weathering?
Mechanical Weathering
Under the right conditions, even the hardest rock can be broken into smaller pieces. Mechanical weathering is the process of physically breaking rock into smaller fragments. Mechanical weathering occurs through frost wedging, abrasion, growth of plant roots, and other processes.
The most common form of mechanical weathering is frost wedging, shown in Figure 8. When water freezes to form ice, the water expands. This expansion pries open any cracks that the water has seeped into. Frost wedging is an important cause of weathering in mountains, where water often freezes at night and melts during the day. Frost wedging pries rocks apart a little bit every night. If you live in a region that has cold winters, you may see how frost wedging damages roads. Many of the potholes that form in roads during winter are the result in part of frost wedging.
Another form of mechanical weathering, called abrasion, occurs when rocks scrape or grind against one another. For example, abrasion occurs when sand carried by water or wind causes bits of rock to break off as if they were being sandblasted.
Mechanical weathering also occurs in other ways. Plant roots can grow into cracks in a rock. Roots exert a powerful force that can slowly pry the rock apart. In another form of weathering, erosion removes material from the surface of a mass of rock, reducing pressure on the rock that remains. This release of pressure allows the rock to expand, causing the outside of the rock to crack and flake off like layers of pastry.
Chemical Weathering
Chemical weathering is a process in which rock is broken down by chemical reactions. Chemical weathering involves chemical reactions that dissolve the minerals making up rock or change them into new minerals. Over time, the effects of chemical weathering can be dramatic. Pits or holes may form in the rock surface. Eventually, the rock crumbles and disintegrates.
Water is the main agent of chemical weathering. Water is an effective solvent. All minerals dissolve in water, though most do so very slowly.
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