SC.4.E.6.4 Weathering and Erosion
SC.4.E.6.4: Describe the basic differences between physical weathering (breaking down of rock by wind, water, ice, temperature change, and plants) and erosion (movement of rock by gravity, wind, water, and ice).
How do rocks break apart and move?
The processes of weathering, erosion, and deposition work together to change the surface of the Earth. Some changes can be fast, such as the side of a mountain breaking of in a landslide. Some changes can be slow, such as a river cutting a canyon into a landscape.
Weathering is the breaking of rock by wind, water, ice, temperature change, and plants. Erosion is the movement of broken rock by gravity, wind, water, and ice. Deposition is when broken rock which has been carried by erosion has settled in one place. Most of the time, these three processes work together at the same time.
Gravity and temperature change can work together to break apart a mountain. When the Sun rises, heat energy from the Sun causes the rock on top of a mountain to heat up. The temperature change caused by the heat of the Sun causes the rock to expand or get larger. Sometimes this causes the rock to break apart (weather). When a rock breaks apart at the top of a mountain, gravity can cause the rock to tumble down the side of the mountain (erosion.) When the rock comes to rest at the bottom of the mountain, it has been deposited (placed) in a new location.
A desert contains many rocks and sand. Winds blow across the desert. The wind can pick up sand. The sand is now being moved by wind: this is a form of erosion. Sometimes the wind blows the sand into a rock. Just as sandpaper can abrade, or rub, a piece of wood and make the wood smooth, blowing sand abrades rock. Tiny pieces of the larger rock are broken away, making new sand. The wind continues to blow the sand across the desert. At some point the sand will begin to pile up in a new location in the desert. The sand has been deposited by the wind. Wind being blown across the desert is an example of erosion. Broken pieces of rock (sand) are being moved (eroded) by wind. As the sand blows across the desert, it abrades (rubs) another rock, and weathers that rock, breaking pieces off. Finally, the sand is deposited in a new location.
This same combination of processes can happen with water. A fast-moving river can pick up rocks and other sediments, pushing them down the river. This movement of rock and sediments is called erosion. At the same time, the moving rocks are tumbling against each other. This causes the rocks to break apart: another example of weathering. Whenever rocks are broken apart, weathering has happened. As the river weather the rocks into smaller and smaller pieces, the sediments carried by the river will settle when the water slows down. Then the sediments (broken pieces of rock) will be deposited by the moving water.
Glaciers also carry out the processes of weathering, erosion, and deposition. Glaciers move very slowly, sometimes only inches a year. This form of weathering, erosion, and deposition can take thousands or even tens of thousands of years to happen. Glaciers, large rivers of ice, are pulled downhill by gravity. As the ice moves across the surface of the land, rocks are picked up and moved (eroded) by the ice. As the rocks scrape across the ground as they are moved, they are slowly abrading other rocks, causing them to break apart (weather). If the glaciers melt, as they did at the end of the last ice age around ten thousand years ago, piles of rock and sediment can be left behind (deposition).
Plants can cause weathering. The roots of plants will grow down into the cracks in a rock. As the plant grows, the roots get larger. Sometimes this can cause the rock to break apart (weather). This is easy to observe along sidewalks, roads, and driveways. Roots often crack these surfaces apart.
The processes of weathering, erosion, and deposition work together to change the surface of the Earth. The landscape around us is always in change. Sometimes these changes happen fast enough for us to observe, such as a new pass being opened on a barrier island when a hurricane pushes enough sand out of the way to make a new opening. Sometimes these changes happen too slowly to notice, such as glacial weathering, erosion, and deposition.
