Overview
Over time, the earth can change in three basic ways.
1. The first of these is called weathering. Weathering breaks rocks apart and creates pebbles and soil. There are two kinds of weathering: physical weathering and chemical weathering. Both kinds happen over a period of many years.
Physical WeatheringPhysical weathering involves the forces of nature. During this process, rock is broken apart little by little. You can see physical weathering at work in a number of ways. For example, when plants begin to grow inside cracks in rocks, their roots push outward to find water and to establish a stronger anchor. The pressure of the roots can break even a large rock completely in two. Physical weathering can also happen when rain enters the cracks in rock and freezes. Because water expands when it becomes ice, this drives the rock apart. |
Chemical WeatheringChemical weathering takes place when chemicals break down rocks. One example is when oxygen combines with the materials that make up rocks. This can create a chemical change. In a chemical change, the materials react with each other and form a new substance. When this substance is softer than the original rock, it is more likely to break off. Water and rock can combine to form a chemical change as well. Rust is a good example of chemical weathering. |
2. The second process is erosion. Erosion is similar to physical weathering because it causes changes to rocks and other materials. Instead of breaking rocks apart, though, erosion moves them.
Erosion is sometimes caused by wind, which blows small rocks and soil away. Erosion can also be caused when gravity pulls material down hillsides. Most often, though, erosion is the result of moving water, such as oceans, rain, or rivers. The steady movement of the water picks up rocks and other materials from the land and carries them along.
3. The third process is deposition. When particles of rocks are swept away by erosion, they eventually come to rest in a new place. That might be a beach, the bottom of a creek, or a forest floor. Deposition takes place when the particles come to rest, or are deposited somewhere new. Over time, many particles may come to rest in the same spot. That creates new soil and changes the look of the landscape. It may also affect the plants and animals in an area.
