How is a cave formed?

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How is a cave formed?

Postby Vader » May 3, 2006 11:03 pm

I was preparing to speak to my neighbors Earth Science class about caves and caving and I was reading the Text book that they are working from.

This is probably a very oversimplified version of how the process works, but this is what I allways thought.

Slightly acidic rain water percolates into the ground. It becomes more acidic as the rain water reacts with dying rotting plant life at ground level. As this acidic water reacts with limestone it creates and washes away Calcium Carbonate. Where this Calcium carbonate is washed away a void will be created and this results in a cave. some of this Calcium Carbonate is then redeposited in different areas of the cave as formations.

OK...I know it's simple and basic, but while I was reading this text book it mentions that this is not possible for acidic water to react this way with limestone. It states that the Acidity of the surface water dissapears in the first 4-10 meters of travel downward into rock.

I looked on the NSS site, but a brief search did not find any mention of this.

Any Ideas or good links you can think of.

Thanks :-)
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Postby Squirrel Girl » May 4, 2006 6:04 am

That is basically right. Crudely described, but not wrong. The deal is that the acidic groundwater much find a crack to work it's way down or along. And that is how it can move much longer distances. If the groundwater percolates uniformly through the rock, you don't get significant caves. But if it finds a crack (a "bedding plane" is a crack between layers of rock and a "joint" is a crack at an angle), it can flow just a little better. And once it does, the crack begins to enlarge because it is *dissolved* by the acidic ground water. That is a positive feedback that makes the crack larger, and this more water can use that crack. A few thousand (or hundred thousand) years, and whammo, you have a cave.
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Whammo

Postby CaverScott » May 4, 2006 7:23 am

Squirrel Girl wrote: A few thousand (or hundred thousand) years, and whammo, you have a cave.


Barbara - I can't explain it, but I just found your use "whammo" highly entertaining! Thanks for making my morning smile. :grin:
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Postby Vader » May 4, 2006 7:40 am

Squirrel Girl wrote:The deal is that the acidic groundwater much find a crack to work it's way down or along. And that is how it can move much longer distances. If the groundwater percolates uniformly through the rock, you don't get significant caves.


That makes perfect sense to me now..

Thanks :kewl:
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Re: How is a cave formed?

Postby Dwight Livingston » May 4, 2006 7:50 am

Flash wrote:As this acidic water reacts with limestone it creates and washes away Calcium Carbonate.


Limestone is already calcium carbonate for the most part. Acidic water dissolves it and carries it away.

Your description looks good.

There's a good poster about living with karst (areas with caves and related features) at Speleobooks:

Image
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Postby erebus » May 4, 2006 9:18 am

There's also another mechanism, where acids percolate up from underneath. I'm not qualified to write much about that, but I understand that it's thought to be how Carlsbad and Lechuguilla formed.
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Something to add

Postby CaverScott » May 4, 2006 9:34 am

You might like this:

Create a Cave - KARTCHNER CAVERNS FORMED

http://www.friendsofkartchner.org/lessons/guide1b.html

Or this which is about Jewel Cave-

http://www.nps.gov/jeca/geology.htm
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Postby Cheryl Jones » May 4, 2006 10:50 am

Here are some Web sites with info on speleogenesis that might be helpful to you.
BLM's Welcome to the Underground
The Virtual Cave
NPS Cave and Karst Knowledge Center

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Postby bigalpha » May 4, 2006 11:13 am

Cave and doline (sinkhole) formation is dominated by lithologic and structural features.

In order to have significant doline occurrence, you must have:
1. Massive to thick bedded limestone (or other soluble rock)
2. at least 60% calcite
3. A minimum of interbeds, allowing vertical movement of ground water
4. A well defined joint set in the limestone
5. Highly reactive ground water (aka carbonic acid)

In our area, there are subsidence and collapse dolines. Usually subsidence dolines are more common, but not around here. Subsidence dolines form from the surface downward, while collapse dolines occur from the ground to the surface. Subsidence dolines are pan shaped and collapse dolines are bowl shaped, etc.

As for caves, some factors controlling their formation and length can be:
1. If the soluble rocks are capped or unlain by nonsoluble rocks...i.e. sandstone. A good example is Mammoth cave. The Big Clifty (sandstone) overlays the Girkin (LS). The cap allows water to move laterally for long distances
2. Whether the soluble rocks extend to the surface
3. Structure controls, like joint sets in the LS

Also, the longer that groundwater stays in contact with Limestone, the less chance you'll have with dissolving the calcite. The more turbulently ground water flows, the better dissolving. There are some more factors, but my fingers are tired.
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Postby Vader » May 4, 2006 9:13 pm

Thanks for all the information :-)

The links were very useful :kewl:
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Postby paul » May 5, 2006 6:40 am

erebus wrote:There's also another mechanism, where acids percolate up from underneath. I'm not qualified to write much about that, but I understand that it's thought to be how Carlsbad and Lechuguilla formed.


That's correct. The usual mechanism of cave formatin involves dissolved CO2 in water which creates the weak acid Carbonic Acid.

In the case of Lechuguilla, Carlsbad and some other caves, depending on the geology, it is thought that the mechanism involves the much stronger Sulphuric (or Sulfuric) Acid formed by microbial action. A by-product of this mechanism is Gypsum whcih is why some caves have an abundance of this material.
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Postby ian mckenzie » May 5, 2006 9:32 am

"It has long been thought that caves form by dissolution of the limestone by carbonic acid... however, the presence of sulphate in karst waters has been largely ignored... sulphate is usually present in large quantities in the waters that initiate conduit flow so these may play a pivotal role in the formation of these conduits." (from Steve Worthington's 1991 thesis Karst Hydrogeology of the Canadian Rocky Mountains)

Similar theorizing by Tim Atkinson in Arctic and Alpine Research Vol 15 No 4.
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Postby Teresa » May 5, 2006 10:34 am

Here's another link on the karst question:

http://members.socket.net/~joschaper/sprkarst.html
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Postby hewhocaves » May 8, 2006 12:23 am

Hey Flash...

the only thing you might want to add to your description is that most of the dissolution occurs at or just below the water table. This Winds up being a very important, but often omitted detail as it can be partly responsible for the gridlike patterns in some caves (along with many caves developing in levels which make no sense otherwise)

The 'average joe' explanation I usually gives goes as follows:

"Big cave systems need a few basic things to form. 1) Rainwater. 2) Limestone. 3) a water table. Now rain, which becomes slightly acidic from rotting plants passes downward to the water table, dissolving little bits of rock along the way. When it hits the water table, all those little reactions which started on their own can come together and collectively eat away far more limestone than they could individually. As they do, they release carbon dioxide into the air, actually changing the atmosphere! Given enough time, large rooms and long passages can be formed this way, confined only by the folding of the layers of rock or the edges of the limestone deposits."
(at this point I usually pause for questions befor continuing...)

"Now, the reason I explained it this way is that a giant cave with no entrance is a pretty useless thing. And even if you could get into you, you'd find a very dull and formationless cave. This is because, for the most part, the environment that makes a cave bigger isn't conducive to making it pretty. But the natural world is really cool in that with just a slight change the exact same preocesses will fix all that up for us. going back to our little cave, we know that little bits of rock are being dissolved as the water trickels downward to the water table. And though this isn't the quickest way to make a cave, over a long long time, it will still make a hole big enough to go through and eventually that hole will come close enough to the surface that a hole will open up and we'll be able to get in! Or maybe a horizontal passage will come close enough to the edge of a hill that a small crack will open to the surface! But the point is, entrances are accidents. Happy accidents, though. Because when they do open up, all that changed atmosphere leaves the cave and the air is back to normal (this happens so well that it's really, really rare to find bad air in a cave).

But the story isn't done yet, because after all that is done, the slightly acidic rain won't stop... it will still keep coming. But this time when it does, something totally different will happen. As it passes through solid rock, it picks up little bits of it as before. But now, when it reaches a room, it immediately drops it load and becomes regular water once again. These little molecules of calcium carbonate (the dissolving part of limestone) arrange themselves (like a natural game of Tetris) into long colums we call stalacties and soda straws. If there's an awful lot of CC, some small amount will splash onto the bottom of the floor, creating a stalagmite. You can see this happen over the course of years rather than centuries by looking under concrete bridges and buildings.

So the same process does double duty - first, to dissolve away the cave and later to make it pretty."

And from that point, I can split the converastion into formations, hydrology, etc... anything.

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Postby George Dasher » May 8, 2006 10:34 am

My understanding is that the dissolution or calcium precipitation can occur anywhere in the vadose or phreatic zone, not just at or below the water table. Both can occur at the same time, and on different sides of the passage (depending on the CO2 concentrations). In addition, the dissolution and calcium precipitation can move upstream and downstream in the passage depending on the amount of groundwater flowing into the cave.

Some caves are vadose, some caves are phreatic, and some caves are both.

And some have nothing to do with water, but that is a different story.
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