Friction and Tectonic Forces
Some of the first forces that may begin the process of breaking down rock are underground tectonic forces. As the plates of the Earth's crust move against one another, they create friction and pressure, and rocks caught between these plates may fracture and grind themselves into smaller fragments. If any of the broken pieces make their way to the surface, they may experience weathering, the next step in the process of breaking down.
Chemical Weathering
Chemical weathering occurs when a rock encounters a liquid or gas that damages it. For instance, any rock exposed to air undergoes oxidation, in which the oxygen in the air reacts with metallic elements to cause rust. This process gives soil that is rich in iron oxides a reddish color. Similarly, exposure to water can alter certain types of minerals, as with hydrolysis that changes feldspar into clay. Feldspar is the most common mineral found in rock. Dissolved carbon dioxide in rainwater can form carbonic acid, which will break down minerals like calcite -- a calcium-containing mineral found in limestone. These chemical processes can further weaken rocks, making them more susceptible to other forces.
Physical Weathering
Physical forces can also weather rocks. Water that freezes inside the cracks of rock expands, pushing apart the mineral deposits and causing it to fracture. Similarly, the roots of plants can work their way into rocks as they grow, and the pressure caused by their expansion can break apart the rock into smaller pieces. Temperature extremes can cause rocks to expand and contract, increasing the stress on fracture lines and causing them to break apart.
Wind and Water Erosion
Once weathering has damaged rocks and broken them down, the forces of erosion can take over to redistribute the material. Wind and water passing over rocks can pick up small particles, carrying them downstream away from the original deposit. Over time, erosion can turn mountains into hills, carry topsoil into the oceans, and carve channels into solid stone. For instance, scientists believe that one of the primary forces that shaped the Grand Canyon was erosion -- due to the waters of the Colorado River carrying away lightweight soil and limestone from the surface, and the winds blowing dust and smaller particles through the resulting channels.
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