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RedDeerGuy1's avatar

What is the science of water erosion on rocks (details inside)?

Asked by RedDeerGuy1 (17698points) March 20th, 2017

Is it stripped one atom at a time per second or is it random? When water runs over rocks? How does erosion work? How does one calculate the speed of erosion?

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5 Answers

jwalt's avatar

Some erosion is due to direct friction of wind, water or sand on the rock’s surface. Large rocks are more quickly broken into smaller pieces when water sleeps in small cracks and freezes, the expansion speeds up the growth of the crack until the rock fractures.

The rate that you are asking for depends on the type of rock (soft sandstone or hard granite, for example), rain rates, winds, and other factors. The following article quotes rates on a particular type of rock in Australia that range from 0.15 mm to 2.3 mm per 1000 years. In this case they are concerned about the preservation of petroglyphs from both natural and artificial effects.

Patty_Melt's avatar

Wellllll, I guess ^^^ that pretty much covers it.

CWOTUS's avatar

I’m not a geologist, but having worked in the construction industry for nearly all of my adult life I have learned a little about abrasives.

Water, by itself, is not a great eroding medium. Pure water, that is. Wind – as moving air – even less so.

It’s the things which those media carry that cause erosion: sand, silt, acids and bases, and even the vegetation carried away, which loosens more soil and exposes more rock. Those hard things cut rock. (And some “rock” is cuttable by water alone, when the rock is a soft sedimentary rock to begin with, such as sandstone and some shales.) Wind, by itself, also doesn’t do a lot to rock. But when high winds carry sand, that’s nature’s sandblaster.

However, to get back to “pure water”: over changes of season and during periods of glaciation, ice is good at breaking and grinding rock. And rocks falling on other rocks – including the scree that forms at the base of many cliffs as they decay with seasonal change and freezing water breaking into fissures of the rock face – that creates the smaller rocks, and sand that flows in rivers to erode rock faces.

And erosion obviously feeds itself, too.

You might say, “But floods are great masses of water, surely they have a cutting action.” And you’d be partially right, but if you look at videos of floods (flash floods, especially), you never see very clean water. It’s the things carried by the water that cause the erosion.

dappled_leaves's avatar

Yeah, I would imagine the rates are variable depending on what is being carried by the fluid. Keep in mind also that with erosion by water, you have not only physical weathering, but also chemical weathering. The type of rock matters in both cases, but especially in chemical weathering. If you have slightly acidic water (rainwater), that’s going to erode limestone much more quickly than quartz, as a random example.

LostInParadise's avatar

Erosion rate varies with what is coming in contact with the rock. Because water expands when it is frozen, ice can cause erosion by expanding cracks. Earthquakes and volcanoes also cause erosion. Even the slowest erosion rate will have a large impact over hundreds of millions of years. The Grand Canyon wasn’t built in a day.

Erosion is also be caused by plants. Lichen and moss can wear down the surface of a rock, creating a rudimentary soil that can be used by other plants, leading to further erosion. Link I also just found out that lichen can protect against erosion according to the abstract of this article
This and the following page is a short summary of what causes erosion.

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