shield formations

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shield formations

Postby shottheradio » Jun 22, 2007 10:08 pm

what are the major thoughts that they have for the growth of these formations???
Thank ya.
Andrew
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Postby wendy » Jun 23, 2007 2:43 am

I've only once, maybe two, one was at Black Casam Cavern in Cali and the other was probably not a shield, but was in a lava tube and looked like one. What was the question again? :-)
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Postby shottheradio » Jun 23, 2007 2:33 pm

haha,.. i was wondering people's thoughts, on how these formations grow.
So you have only seen these things,like once? I work in Endless Caverns,In Virginia. We have them all over the place. And Grand Caverns which is down the road from me probably has more Shields than any other cave in the US.
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Postby Teresa » Jun 24, 2007 11:01 am

Calcite shields are usually hollow, somewhat like a slice of pita pocket bread.

They grow out of the rock from a very thin crack, at an oblique angle from the wall, as two rims which almost seal along the edges. The most common explanation I've ever heard is that the water is under hydrostatic pressure-- squeezing out through the crack as a very slow rate, so that the calcite forms before the water dribbles down the wall. Mechanism: surface tension in local excess of gravity persisting long enough for the CO2 to off gas and the calcite to deposit.

This is a similar mechanism to that of helictites, which form from pore space sized exudations, forming a tube.
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Postby shottheradio » Jun 24, 2007 7:34 pm

ok yeah,.. thats what I have been saying for the most part as well.
thanks for the info.
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Postby fuzzy-hair-man » Jun 24, 2007 10:11 pm

Dave Bunnel's Virtual Cave has a bit about shields:

http://www.goodearthgraphics.com/virtcave/shield/shield.html

And some pictures of some for anyone wondering what they are or what they look like.

Myself, I may have seen one, but I suspect it really wasn't.... :doh:
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Postby baa43003 » Jun 25, 2007 12:24 pm

And Grand Caverns which is down the road from me probably has more Shields than any other cave in the US.


I've never been to Grand Caverns but Lehman Caves in Great Basin National Park has quite a few also. Their website claims over 300 of them. Teresa'a explanation is the one I've heard most often.

An interesting side note: On a recent visit to Australia, I saw quite a few shields in Tasmanian caves. And every time I'd comment on them, our Tasmanian host would ask, "What is it with Americans and their fascination with shields?"

Are they so common in other parts of the world that our fascination with them should seem commical?
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Postby Ralph E. Powers » Jun 25, 2007 2:23 pm

Lehmans does have some fine examples/specimen's of sheilds. Here are a few pics and one shield "table" in Cedar Ridge Cave in TAG. How they're formed... wish I knew... they're unique to be sure.
Image

Image

Image
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Postby shottheradio » Jun 25, 2007 10:10 pm

baa43003 that is a bit funny. I'm not sure. I get were she's coming from though. I see them here in Virginia all the time. I just think people like us find them so interesting because we don't know for sure how they form. and in the states they are quite rare.
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Postby ArCaver » Jul 13, 2007 9:56 pm

Don't know if they're rare in the states. they aren't rare in the Ozarks.
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Postby Bruce Rogers » Jul 27, 2007 5:47 pm

Listeros,

As a suppliment to the above comments, I offer this "Short Life History of a Shield."

1. Start with a crack in a cave wall.

2. Cover said crack with a thin flim of calcite to block any seepage, thud building up a hydrostatic head of ground water pressure behind the film.

3. Crack this film via earth tides (the solid part of the Earth also moves in diurnal waves just as the oceans do, but on a vastly smaller scale).

4. Allow mineral-laden water to seep out, boil off carbon dioxide, and deposit another thin film of calcite over said crack, blocking it.

5. Crack open the newly deposited film via next round of Earth tides in concert with ground water hydrostatic pressure built up behind the calcite film.

6. Repeat as necessary to form a circular, disk-shaped, hollow speleothem.

7a. If ground water conditions change, then increased water flow may deposit draperies along the crack to make the shield look like a pararchute - as in Lehman Caves famous Parachute.

7b. As an alternative, if the upper plate of the shield cracks open, then either another shield may form along this crack or possibly a fringe of helictites may form.

If one is lucky enough to view the inside of a broken shield one can see the concentric growth lines as well. If the shield grew tilted up at some angle from the horizon, lines of calcite may form across the inside of the lower disk representing "shore lines," so to speak, of where the mineral-laden water deposited horizontal welts of calcite.

Cheers,
Bruce rogers, Earth scientist on a good day
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Postby ArCaver » Jul 27, 2007 7:40 pm

A few from an Arkansas cave.
Image

Image

Image

Image
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Shield Formations

Postby Larry E. Matthews » Aug 8, 2007 9:03 am

There are shield formations here is Tennessee that grow out of columns. Sometimes, there are 2 or 3 on the same column, all at different angles.

This pretty much discounts the "crack in the wall" theory, don't you think?

Some of the active ones that I have observed have calcite crystals pointing directly outward from the edge. I don't know if this is similar to "twining", with all the crystals in the same plane. There may be another name for it.

At any rate, where they occur, there are frequently lots of them, as a previous poster noted. Personally, I vote for something going on the the chemistry of the water.

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Postby ArCaver » Aug 8, 2007 9:40 pm

The shields I see are almost always near or often covered with helectites. Any thoughts on what causes this? I've read that shields and helectites may both be caused by hydrostatic pressure. I personally think helectite gnomes hide in the cave, probably in the water, and occasionally emerge to twist each one a small amount...
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Postby Teresa » Aug 8, 2007 11:28 pm

ArCaver wrote:A few from an Arkansas cave.


I wouldn't call any but the first image a shield. Those round ceiling patches of speleothem are common as dirt. They are also attached. Usually, they are a 'healed spot' where there was a previous stal fall, which took some of the ceiling with it; creating a sideways seep which eventually became filled with calcite again. There is usually too much water to make a good bond with the limestone or dolostone, and such stal falls are habitual over time.

A shield has to be at least [partially freestanding, IMHO, for it to be a shield, not just circular. And it has to be somewhat saucer shaped.

Shields growing from columns as Larry M. suggests don't bother me-- a stal could have water flowing out from a crack under pressure as well as bedrock can.

"What is an expert? A drip under pressure!" Maybe that's the answer-- shields are all experts.
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