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Weathering and erosion

Water, a powerful force, shapes our world through weathering and erosion. Rain dislodges soil particles, leading to mechanical weathering. As water accumulates, it moves soil and rock particles, causing erosion. Fast-moving water erodes more than slow water, creating landscapes like meandering rivers and Oxbow lakes. Water also causes chemical weathering, dissolving rocks and minerals, forming caves and cliffs. Created by Khan Academy.

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Video transcript

- [Instructor] Growing up in Southern Idaho, one of my favorite places to visit was Swan Falls, a gorgeous canyon formed by the snake river. All throughout my childhood, I remember scrambling over the boulders down in this canyon and playing in the cool water at the edge of the river. Over thousands and thousands of years, water slowly eroded away rock and sediment. Water made this canyon and water forms many landscapes, including valleys, ravines, cliffs, coastlines, all of these were largely shaped and affected by water. And given that water covers most of the earth, or over 70% of it, it's no surprise that water also shapes the earth. Water is a powerful force. It can literally wash away soil and carve away rock bit by bit, which scientists describe through two separate processes. When water breaks rocks apart, it's called weathering. And when water carries away surface soil, sediments, and that broken down rock it's called erosion. Erosion by water starts with rain. During a rain storm the impact of raindrops is actually powerful enough to dislodge soil particles and damage soil structure. This is called mechanical weathering. As rainwater starts to accumulate, it can erode a landscape by moving soil and rock particles, called sediment, away from their original location. And as water flows down the slope of hills and mountains, it transports sediment along with it. Eventually water gathers into small streams and then rivers, carrying more and more sediment. In fact, through erosion, water carries away billions of tons of sediment to the oceans every year. Plant roots and vegetation can hold soil in place and prevent or slow soil erosion. But if soil has nothing to hold it down, water can erode away soil very quickly. As the volume of water increases, so does its power of erosion. In turn, a large river can erode more soil and rock quicker than a small river can. The speed of the water flow also affects its erosional power. Simply put, faster moving water can carry more sediment. This relationship is one reason why fast moving water causes more erosion than the same volume of slow moving water. A great example of water erosion in relation to speed is that of meandering rivers and Oxbow lakes. You know, the weird horseshoe looking lake in this image. Let's take a look at how these Oxbow lakes form. The current of the water is fastest on the outside of the bend. In turn, these are the areas where water tends to erode away the river bank. Conversely, the current of the water is slowest on the inside of a bend. Here, the water slows down so much that some of the sediment that is being carried is dropped, or deposited. This is called sedimentation. Over time, through erosion and deposition, an Oxbow lake will form, which is eventually cut off from the river. Chemical weathering as another way that water can break rocks, such as when acids in water dissolve certain types of rocks and minerals. Over time, flowing water can dissolve the minerals and rocks on or under the surface. This chemical weathering can cause sinkholes, caves, and cliffs to form. Certain rocks like limestone are especially vulnerable to erosion. In particular, limestone easily erodes due to carbonic acid and rainwater, which forms when carbon dioxide, or CO2, from the atmosphere dissolves in water. In turn limestone erosion can produce intricate cave systems. Along beaches and the ocean front, coastal erosion breaks up and wears away rock. As waves sweep in over and over, they cause rock to weather away and break off. Going back to the Snake River and Swan Falls, I can see examples of weathering and erosion all around me. I look to the cliffs which have been slowly carved away by the Snake River over time to reveal this amazing valley, which was formed by weathering and water erosion. Water erosion is incredibly powerful and although it can be destructive by removing soil and rock over thousands and thousands of years, water erosion is also responsible for creating the beautiful and spectacular landscapes that we enjoy today.