字幕列表 影片播放 列印英文字幕 You know, nature can create some pretty amazing stuff. We've got waterfalls, mountains, and beaches. And Canyons! A canyon is a deep gorge or gully in the earth. It usually has steep walls and, most of the time, there is a river flowing at the bottom of it. And one of the biggest canyons in the world is in the southwestern United States, The Grand Canyon, in Arizona. How grand is the Grand Canyon? Well, say you took the tallest building in the world and stacked it on top of itself twice. The Grand Canyon is deeper than that. And it's also more than 450 kilometers long That's so long that it would take you about 4 days to walk its length from end to end. So how did this giant groove in Earth get there? You can see the secret in this picture. Do you notice anything besides the Grand Canyon in this photo? How about at the bottom? That's right, water! That's a river called the Colorado River. And it's what carved the Grand Canyon that we know today. How did it do that? Very slowly. A long time ago, the land where the Grand Canyon is was just a big, flat landscape. But,then, water started to flow. The water flowed over the plain, picking up little pieces of dirt and rock as it went along. The water carried the stuff from one place to another, moving it downstream. It happened just one tiny piece at a time but over a really, really long time, it made a big difference. The natural process of dirt and rock getting picked up from one place and moved to another is called erosion. And you've probably seen it happen. Have you ever built a sandcastle at the beach? And then watched as a wave crashed over it and moved the sand? The water eroded your castle, but rocks are a lot harder to break down than a sandcastle, so it takes a lot more time. A river can create something as big and breath-taking as the Grand Canyon because a river is always moving so it's always eroding the rocks. And sometimes, special things happen to really speed things up, like a flood. When a river floods, way more water flows into the river than it usually holds and the river starts to overflow. And when more water rushes through a river, that means it can pick up more stuff. A big flood can even move big, heavy rocks and carry them down along the river and all of these big moving rocks can act like chisels, carving away at the land. And a series of big floods, just like that, made the Grand Canyon deeper and deeper. But the Grand Canyon isn't just deep, it's wide, too. What makes it so wide? Erosion, again. And water, again. But this time, it's rainwater. When it rains, water flows down from the sides of the Canyon, acting like a bunch of teeny-tiny rivers, picking up more pieces of rock, and making the Canyon wider. Today, the Grand Canyon is almost 30 kilometer across at its widest point. That's like 30 football fields. But even with the occasional big flood or heavy rain, it still takes a really, really long time to make a Canyon as long and wide and deep as the Grand Canyon. Scientists are pretty sure that the Grand Canyon started forming 5 to 6 million years ago. So erosion happens very slowly. Sometimes it happens so slowly, we can't even see it working. But sometimes, we can! When you see a rock moving with water in a stream, that's erosion. When you see a river full of muddy water, that's erosion, too. All those pieces of dirt in the river are flowing with the water, moving from one place to another. And since the Colorado River is still flowing, erosion is still happening which means the Grand Canyon is still growing, kinda like you. Thank you for joining us on SciShow Kids! Do you have a question about space, animals, or anything else? Get help from a grown-up and let us know in the comments below or send us an e-mail to firstname.lastname@example.org. And we'll see next time!