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如果大峡谷足够深，它会被海洋淹没吗?

科罗拉多河已经雕刻了大峡谷几百万年了，因此峡谷底部的海拔比周围地区低6000英尺左右，海拔约2400英尺。如果事情继续这样发展下去，科罗拉多河将继续使峡谷变得更深。如果河流侵蚀掉足够多的岩石，使河底达到海平面，这个过程继续向上游延伸，从河流流入太平洋，最终使峡谷底部侵蚀到海平面以下，峡谷会逐渐被海洋填满吗?这可能吗?如果不是，为什么我们没有看到科罗拉多河口(或任何河流)逐渐向上移动，海洋填满了河流曾经所在的地方?

回答

这不太可能仅仅是因为侵蚀。这有几个原因，但由于这一过程发生的时间尺度不同，其他现象可能会完全改变地质图景。峡谷最初形成的原因是地壳构造使岩石隆起。科罗拉多高原的隆起是大峡谷最终形成的关键一步。板块构造运动把岩石抬升得又高又平，形成了科罗拉多河可以通过的高原。坡度越陡，侵蚀的速度就越快，所以河流流速越平缓，峡谷的雕刻速度就越慢。所以峡谷雕刻得越深，失去的势能就越多。从前面引用的同一来源，地层剖面图:[!侵蚀速度显著减缓的另一个原因是，水流越深，所流经的岩石类型就越不同。这条河现在已经到了变质层，比上层的石灰石和砂岩坚硬得多。 The Vishnu Group is practically bedrock. [This page][3] has a nice explanation: > The steeper the slope, the faster the runoff flow and the more force it will have to move material. The less vegetative (plant) cover and leaf material on the ground, the more open the soil will be to the flowing runoff water. Four things cause slope erosion: the amount and rate of rainfall, the steepness or gradient of the slope, the amount and nature of plant cover, and the type of soil and bedrock underneath. Regarding this question: > why don't we see the mouth of the Colorado (or any river for that matter) gradually move upstream, with ocean filling in where the river once was? Erosion takes material away but this material has to go somewhere. Sediments are carried downstream and deposited at the mouth of those rivers, becoming what we call river deltas. The deltas usually advance **into** the sea as the mass of sediments is incredibly huge. Take a look at this picture: [![enter image description here][4]][4] This is the eastern end of Lake Mead where the Colorado river would flow when the reservoir level was higher. The entire flat brownish area is deposited silt from around 50 years of flow. I rafted through that now exposed silt and the walls on either side of the new channel were over 30 feet high. The same thing occurs in the oceans, but over much longer timescales. So, to summarize: - There is much less water flowing through now than in the past. If Colorado winters become too mild and dry, then the river's power will be even further diminished. - The slope gradient of the river can only get gentler, so erosion will slow down. - The river has reached much harder rock that resists erosion much better. [1]: https://www.nps.gov/grca/learn/nature/grca-geology.htm [2]: https://i.stack.imgur.com/GOEIu.png [3]: http://www.watersheds.org/earth/field.htm [4]: https://i.stack.imgur.com/nbktZ.jpg

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