Natural Agents of Change

• Earth’s surface changes constantly. Natural processes that occur on the Earth’s surface change the shape of the land. These processes usually occur over a very long time.

• Features on the surface of the Earth are called landforms.

• A mountain is an example of a landform.

• Landforms are made of rocks. When rocks are exposed to air and water at Earth’s surface, they change. As rocks change, so do the landforms.

• Rocks change all the time as forces act to break them up. The two most important natural forces are weathering and erosion.

Weathering• When a rock is broken down into smaller

pieces it is called weathering. A rock weathers in response to changes in its environment.

• Weathering can occur by three different methods:

1. Mechanical weathering 2. Chemical weathering 3. Biological weathering

Mechanical Weathering

• Mechanical weathering is a process where rocks are physically broken into smaller pieces by wind, water, ice or heat. The common product of mechanical weathering is silt, a form of very finely-ground rock.

• Examples of mechanical weathering is everywhere. One famous example is the Grand Canyon.

• Freezing and thawing cycles also weather rock mechanically. When water freezes, it expands. The ice crystals push against the solid rock, weakening its structure.

• When it warms up again, the ice crystals melt and the pressure is released—but the rock structure is weakened even more. Each time the rock freezes and thaws, it cracks a little more.

Chemical Weathering

• Chemical weathering is a process in which the rocks are broken down by removing or altering elements that make up the minerals. The most common form of chemical weathering is from carbon dioxide (CO2) in the air. When carbon dioxide combines with rain, a chemical change occurs.

• The end result of chemically weathered rock is clay. Clay is made up of finely ground rock pieces that are a little larger than silt.

Biological Weathering

• Biological weathering is caused by living organisms. For example, burrowing animals or plant roots can break up soil and rocks.

Erosion

• Erosion is the transport of soil or rock by water, ice or wind. Erosion is the movement of weathered rock, and not the actual weathering itself. So, streams and rivers not only weather rocks by carrying water past the rock. They also transport the fragments downstream (erosion). Small fragments of rock and soil are called sediment.

Erosion by Water

• Rivers move sediment by carrying them in the water or rolling them along the bottom. This is a form of water erosion.

Erosion by Wind

• Wind erosion occurs all over the Earth. It is most noticeable in desert climates, where water is scarce. The wind picks up small pieces of rock fragments and carries them along until they reach an obstacle, such as a hill. The fragments, pushed by the wind, act as a sandblaster eroding the hill slowly over time. The wind also causes erosion by scooping up large areas of loose soil and transporting them to another location.

Erosion by Ice

• Ice on Earth’s surface also causes erosion. Ice erosion is usually the result of glaciers. A glacier is a large body of moving ice. It flows downhill slowly, acting like a frozen river. A glacier moves downhill due to the force of gravity. As it moves, it picks up and carries any loose material in its path.

Erosion by Gravity

• Gravity pulls everything towards the center of the Earth. It is constantly pulling at rocks, soil, water and anything on Earth’s surface.

• When a pile of rocks falls down the side of the mountain as one single block it is called a landslide. Landslides erode mountains and change their shape very quickly.

Vegetation Affects Erosion

• The amount of vegetation in the area has a big affect on how fast or slow erosion takes place. Vegetation means plants, including trees, shrubs and flowers. The roots of the vegetation reach into the soil and hold it in place. The leaves and limbs of the vegetation shield the ground from wind. When vegetation is growing well, soil does not erode quickly. When vegetation is removed by fire, animals or humans, soil can erode quickly.

The End!!!