# river polish stoneAugust 29, 2007 5:33 AM   Subscribe

How long does it take a river to polish a jagged stone smooth?
posted by wobh to Science & Nature (12 answers total) 1 user marked this as a favorite

How long is a piece of string?

Which river? Which part of that river? A small, slow moving river won't do it very fast. A big river will do it very fast at the bottom of a big waterfall (e.g. Niagara).

The answer to your question is, "Anywhere from 50 years to 1 million years."
posted by Steven C. Den Beste at 5:51 AM on August 29, 2007

is it rivers, or glaciers? i would guess jagged stones tend to come from frost shattering, which might be more likely to fall on glaciers (since it will occur in very cold areas, on sun-facing slopes). thinking back to when i have been at the foot (tail?) of a glacier, i don't remember seeing jagged stones, so i guess they come out smooth (and i suspect trapped under a glacier is a more effective grinding proccess than rolling along a stream bed).

anyway, to answer the question (slightly) the reason i mention this is that it seems to me that the process requires movement of the stone (i'm assuming it's stone on stone grinding that is where the real action is :o). in which case you could get the answer from the average speed of rocks carried by water/ice and the distance from source of jagged rocks to location of smooth rocks.

so if it's glaciers, the time a glacier takes to travel from source (what's the right word for that? col?) to tail (foot?) is an upper limit.
posted by andrew cooke at 5:55 AM on August 29, 2007

Also strongly depends on (a) the stone being abraded, and (b) the surfaces doing the abrading...
posted by Chunder at 6:00 AM on August 29, 2007

There are far too many random factors to answer that question. For instance:

What is the Mohs hardness rating of said stone?

What is the Mohs hardness rating of the other stones in the environment which help polish it?

What is the Mohs hardness rating of the grit suspended in the water?

What is the ratio of grit to water in the immediate environment?

Consider that even given an optimal environment such as this tumbler, with a far greater ratio of grit to water and stones selected for relative pliability, it still can take more than a month to fully polish a stone.

In a natural environment you can add years, decades, centuries, or even longer to that figure. Take your pick.
posted by The Confessor at 6:04 AM on August 29, 2007

Another difficulty, of course, is that the surrounding environment tends to change somewhat in the course of time.
posted by The Confessor at 6:09 AM on August 29, 2007

How big is your rock? Is it rolling, saltating, or in suspension in the current? Does it travel downstream where current energy is less but sedimentation in the water may be more? What is the shape of the stone? What minerals and elements make up the rock? Something largely composed of feldspars will break down and "round" faster than something largely made of quartz.

Rivers are not the only thing that shape rocks. Stream and glacier abrasion are two mechanical processes; a process called weathering also affects stones. Mechanical weathering such as wind and water shape stones and polish them; some of the most beautiful stones in the world have been polished by the sand and grit in wind, are called ventifacts. Wind/water can turn giant granite monoliths into smooth, beautiful rounded boulders in a process called exfoliation or sphearoidal weathering. There is also chemical weathering, but that doesn't polish so much as it erodes.

And lets not forget the greatest polisher of rocks in the world: a beach! (Tidal currents/wave action, bouncing against other rocks, etc.)

To sum up: it takes a long time and depends on the rock and its conditions. Or what everybody else said.

Rock abrasion party!
posted by barchan at 6:30 AM on August 29, 2007

longer than in a rock tumbler.
posted by pmbuko at 7:14 AM on August 29, 2007

the answer is blowin' in the wind.

Anyway, there are too many variables in your question.
So I'll just have to say "a while".
posted by Green Eyed Monster at 7:48 AM on August 29, 2007

Oh, and saltating is when a grain or rock "bounces" along the river bottom or wind dune.
posted by barchan at 9:12 AM on August 29, 2007

*holds up hands, indicating a space between them*

That long.
posted by Aquaman at 10:22 AM on August 29, 2007

Two things:

First, an apology for my poorly considered question. I'm sorry. It seemed like a good question when I got up this morning.

Second, my gratitude for everyone who was irritated enough to try and answer it. Thank you. In spite of myself your answers have helped me understand what I want to know better. Even the snarky ones.

I'm going to search the net for rock formation and geology resources, but if anyone has any of those they happen to know and want to share in this space, please do.

Once again: I'm sorry and thank you.
posted by wobh at 6:23 PM on August 29, 2007

Wobh, I teach geology, and yours is a pretty common question. Don't beat yourself up over it. I actually love it when students ask questions like this because it helps them gain an understanding of "geologic time." Yay for asking rock questions!
posted by barchan at 3:41 PM on September 3, 2007

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