#1007 How quickly does a river cut through rock?

How quickly does a river cut through rock? It depends on the type of rock, the material the water is carrying, the amount of water, the speed of the water, and the acidity of the water.

This is not a simple question. When we think about the erosive power of water we often think of the Grand Canyon. It is roughly 1,200 meters deep and is 2,000 meters deep at its deepest point. It was formed by the Colorado River, and it has taken the river about five or six million years to erode as far as it has. However, the Colorado River has only been able to erode as much as it has because of a few specific conditions that we can look at.

The most important thing when we talk about the speed at which a river can erode rock is the type of rock. The rock of the Grand Canyon is made up of different sedimentary rocks. The area that is now the Grand Canyon used to be under a shallow sea, and sedimentary rocks form at the bottom of a sea. They are made when pieces of other rocks, sand, dirt, and dead animals and plants sink to the bottom of the sea and become compressed. The rock in the upper layers of the Grand Canyon is limestone and sandstone, which are not very strong rocks. They crumble and break down fairly easily, which is one reason why the Colorado River could cut through them so easily. If the plain that became the Grand Canyon had been made out of granite, we probably wouldn’t have a Grand Canyon. The river would have formed a lake until it found a different route to take. Softer rocks, like sandstone, are easily eroded. Harder rocks like granite and basalt are not eroded so easily. Also, rocks that are made of large grains, rather than smaller grains like sandstone, will hold together more effectively in the face of water. Rocks that are set down in layers, like sedimentary rocks, erode more easily. As do rocks that fracture easily. So, where a river has cut down through a plain, we can usually assume that it was a softer rock.

The next thing to consider is the type of sediment that the river carries. Water on its own cannot erode rock very quickly. It can erode rock by getting in cracks, but it is mainly the sediment that the river carries which is the real power of erosion. If a river is carrying lots of sand, or small pieces of rock, those act like sandpaper and gradually scrape off the rock on the edges and the bottom of the river. Once this process has started, there is more rough sediment in the river and the more the process continues. The Colorado River had lots of sand from the sandstone, which sped up the process.

The volume of the water is obviously a big factor as well. The more water that a river has, the more sediment it can carry, and the more it can erode the rocks around it. The larger the volume of water, as well, the larger the amount of energy that the river has. The Colorado River has a flow rate of 109,557 liters of water per second. This is far lower than it was when the river eroded the Grand Canyon because the river has been diverted for irrigation purposes. The river with the highest flow rate in the world is the Amazon River, which has a flow rate of 220,000,000 liters of water per second. This speed is generated because of the amount of water that feeds into the river, but also because of the descent of the river. The Amazon River starts high in the mountains and ends up at sea level. This heigh difference creates a lot of speed as gravity pulls the water down. The same was true for the Colorado River, although the river has reduced most of that height difference through erosion. IN the beginning, the tectonic plates shifted and pushed the river and the plains high up. This height gave the river a lot of speed, and it was able to cut through the soft rock fairly quickly. A large and steep river will have more power and be able to carry more sediment than a small river that only has a small difference in height between source and end.

The last thing is the acidity of the water. If water is more acidic, it can dissolve the minerals in rocks, rather than eroding them by carrying rough sediment. Some sedimentary rocks can react to acids. When limestone is exposed to acid, the calcium carbonate that makes up the rock becomes carbon dioxide and water, disappearing into the river. This acidity is increasing because the excess carbon dioxide we put into the atmosphere is absorbed by rivers, lowering their pH and making them more acidic. However, acidic rivers cannot erode the harder rocks.

So, the speed at which a river erodes through rock depends on the type of rock, the amount of sediment the river can carry, its speed, its volume, and how acidic it is. Even then, there are other variables, so it is a pretty impossible question. And this is what I learned today.

Photo by Mohammed Abubakr: https://www.pexels.com/photo/scenic-view-of-grand-canyon-national-park-13572581/

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