Students in the fifth and sixth grades study weathering and erosion as part of their earth science curriculum. These two geological processes combine to cause natural wonders, such as canyons, rock formations, stone bridges and caves. They also cause less impressive sights, such as ditches and dust-devils. Students can see examples of weathering and erosion almost anywhere.
Weathering is the name for the processes that break rock into pieces. Rock starts out solid, but over time small bits of it break off, changing the shape of the original rock. One kind of weathering is called physical or mechanical weathering. When a rock undergoes mechanical weathering, something physically breaks it into pieces. For example, a landslide from above crashes over it, a tree's roots grow into it, or water inside its cracks freezes and expands. Any of these things will break chunks or fragments off the rock.
Another kind of weathering is called chemical weathering. In mechanical weathering, the rock only changes size and shape. Its actual material does not change. In chemical weathering, a chemical reaction happens to the rock and changes it into a different material. Some kinds of rock react with oxygen or carbon dioxide, so simply coming into contact with the air can be enough to cause chemical weathering. This is how rocks turn into sand, soil or rust.
Erosion is the name for any process that moves rock particles from one place to another. If ocean waves or a river washes over a rock, the water will carry pebbles or rock chunks to new places. On high mountains or flat plains, strong winds can pick up dirt and sand and blow them away. In icy areas, the slow, powerful pressure from moving glaciers can push stone fragments along. Eventually, these eroding forces deposit the fragments in new places, where they become beaches, mounds and other formations.
Weathering and erosion work together to change the appearance of land. After weathering processes have broken small pieces from rocks or chemically caused rocks to dissolve, eroding forces can move them to new places. Students can see the effects of weathering and erosion in their own neighbourhoods, whether they live in a big city or a rural area. They can look at stone buildings and see the places where wind and rain have roughened the edges and corners. They can watch dust-devils pick up soil in the fields. They can see where tree roots have pushed up through the sidewalk. All of these are examples of weathering or erosion.