How is a cave formed?

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Postby hydrology_joe » May 19, 2006 9:31 am

Amemeba wrote:
hydrology_joe wrote:In this case, the simple answer described previously in this thread is actually more applicable to the formation of epikarst, not cave passage formation. That is exactly why I provided the KISS description of mixing water corrosion.


Wrongo, Hydro, "epikarst" is but an aspect of poor hydrologic circulation.

The dissolution effects of renewed acidity by a recharging liberation of CO2 through mixed waters is dwarfed by the effects of a time enhanced hydrologic flow. A directional flow through time is paramount to linear cave developement.

Ain't it the truth?



I don't know how I was wrong as I never claimed that epikarst wasn't an aspect of poor hydrologic flow nor did I claim that time wasn't a factor... but for further clarification:

Epikarst is formed as a result of poor hydrologic flow and results is the formation of perched epikarstic watertables. Because of this, the water becomes saturated with calcite at its pCO2 near the surface and loses its corrosivity long before it can form caves.

from Ford and Williams, Karst geomorphology and hydrology, 1994:
Recharge from rainfall will be relatively diffuse, depending on soil cover, and percolating water will accomplish 50-80% of its solutional work within about 10 m of the surface. Hence fissures that are considerably widened by corrosion beneath the soil close rapidly with depth. As a result, infiltration into the top of this highly corroded subcutaneous zone is much easier than drainage out of it.The bottleneck effect results in much storage of water in this zone after heavy rain, constituting a perched epikarstic aquifer with a base that is essentially a leaky capillary barrier. Because of initial spatial variability in fissure frequency and permeability arising from tectonic and lithological influences, preferred (low resistance) vertical leakage paths develop down connected pipes at the base of this aquifer. These paths are enlarged by solution with the result that a depression develops in the overlying subcutaneous water table similar to the cone of depression around a pumped well. Flow paths then adjust in the epikarst aquifer to converge on the dominant leakage route. The extra flow encourages more solution and with it the enhancement of vertical permeability.

The mixing water corrosion that occurs below the water table as I described previously is enhanced by time. As with the old racing addage "there's no replacement for displacement"; there is no replacement for time. The longer the time-span the process is allowed to take place over, the more dissolution will occur assuming continued input & discharge of water. Flow alone cannot replace the chemistry required to dissolve the limestone. Increased flow can began physical erosion, but the initial flow paths need to be developed through dissolution first (unless we're getting into tectonic caves).
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Postby Teresa » May 19, 2006 12:20 pm

hydrology_joe wrote:


from Ford and Williams, Karst geomorphology and hydrology, 1994:
Recharge from rainfall will be relatively diffuse, depending on soil cover, and percolating water will accomplish 50-80% of its solutional work within about 10 m of the surface. Hence fissures that are considerably widened by corrosion beneath the soil close rapidly with depth. As a result, infiltration into the top of this highly corroded subcutaneous zone is much easier than drainage out of it.The bottleneck effect results in much storage of water in this zone after heavy rain, constituting a perched epikarstic aquifer with a base that is essentially a leaky capillary barrier. Because of initial spatial variability in fissure frequency and permeability arising from tectonic and lithological influences, preferred (low resistance) vertical leakage paths develop down connected pipes at the base of this aquifer. These paths are enlarged by solution with the result that a depression develops in the overlying subcutaneous water table similar to the cone of depression around a pumped well. Flow paths then adjust in the epikarst aquifer to converge on the dominant leakage route. The extra flow encourages more solution and with it the enhancement of vertical permeability.

The mixing water corrosion that occurs below the water table as I described previously is enhanced by time. As with the old racing addage "there's no replacement for displacement"; there is no replacement for time. The longer the time-span the process is allowed to take place over, the more dissolution will occur assuming continued input & discharge of water. Flow alone cannot replace the chemistry required to dissolve the limestone. Increased flow can began physical erosion, but the initial flow paths need to be developed through dissolution first (unless we're getting into tectonic caves).


I disagree,and I don't think that is what Ford and Williams are saying.

1) Mixing water corrosion should decrease with time as the waters reach equilbrium with each other. In the case of recurrent recharge, there are a whole bundle of factors which makes chemical dissolution a pulse phenomenon-- just as a river goes up, into flood, down, dries unde drought, etc. Chemical equilibrium is a moving target.

2) Flow cannot replace the chemistry, but if solution occurs under no or low flow conditions, it greatly expands the time needed for dissolution to occur, and increases the chances that you will end up with a clay filled filled subsidence, not a cave.

3) Initial flow paths MUST be mechanically derived before they can enlarged by chemical solution. Even if it is a teeny crack, you need that crack there to make karst, unless you have an enormously porous limestone (not the usual case, since most limestone has a fair to high admixture of clay, and silica, and therefore, lower porosity, permeability and solubility than pure calcite.) And in the case of a porous limestone, cracks form quite quickly.
You don't want your limestone to be too porous or permeable, because then there is nothing to hold up the cave, and you get subsidence, not cave.

4) There are replacements for time. They're velocity, volume, hydraulic pressure in a conduit, relative acidity, and higher vs lower differentials in the original chemistry of the mixing waters.


5) Despite hydrology_joe's fine and largely successful effort at reducing carbonate dissolution kinetics from equations to English, this entire discussion is way way WAY above the level of a non-geologist. They quit reading the first time pCO2 showed up and they said, "Say what?"I even know a caver with a master's degree who read the mixing waters thing, and said, Huh? If hydrogeos want to be listened to, they have to refrain from speaking in hydrogeology in public. It just scares people.



:hairpull:
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Postby hydrology_joe » May 19, 2006 2:27 pm

Teresa wrote:
I disagree,and I don't think that is what Ford and Williams are saying.


The Ford & Williams citation was to demonstrate the loss of corrosivity of waters percholating through the subcutaneous zone and the applicability of the cave formation discussion as it relates to epikarst... not mixing water corrosion.

Teresa wrote:1) Mixing water corrosion should decrease with time as the waters reach equilbrium with each other. In the case of recurrent recharge, there are a whole bundle of factors which makes chemical dissolution a pulse phenomenon-- just as a river goes up, into flood, down, dries under drought, etc. Chemical equilibrium is a moving target.


Mixing water corrosion does decrease with time as the waters reach equilibrium. That is why I explicitly stated that there is continual recharge and discharge to the system. There are pulse recharge events and gradual recharge events. The epikarstic perched aquifers are key in providing the gradual recharge and "fresh" water to continue the mixing water corrosion over the long-term.

Teresa wrote:2) Flow cannot replace the chemistry, but if solution occurs under no or low flow conditions, it greatly expands the time needed for dissolution to occur, and increases the chances that you will end up with a clay filled subsidence, not a cave.


Mixing water corrosion cannot occur under no-flow conditions as the water would reach saturation under stagnation. Flow must be there to provide the "fresh" water to drive the kinetics of the reaction. However, once you reach too high of a flow, dependent upon the system, the residence time is shorter than the time required for the reaction to occur and results in no solutional enlargement but scouring.

Teresa wrote:3) Initial flow paths MUST be mechanically derived before they can enlarged by chemical solution. Even if it is a teeny crack, you need that crack there to make karst, unless you have an enormously porous limestone (not the usual case, since most limestone has a fair to high admixture of clay, and silica, and therefore, lower porosity, permeability and solubility than pure calcite.) And in the case of a porous limestone, cracks form quite quickly. You don't want your limestone to be too porous or permeable, because then there is nothing to hold up the cave, and you get subsidence, not cave.


*Mechanical Seperations* as Preferential flowpaths DO NOT NEED TO EXIST before enlargement by chemical dissolution. This is exactly what my graduate research was investigating. (Trying to back-quantify & identify preferential pathways based upon existing cave passage morphologies) The preferential flowpaths need not be mechanical separations (joints, fractures, bedding planes) but can be higher permeability portions in an anisotropic massive limestones. If mechanical seperations are the controlling factor for cave formation, what is the explanation for the caves that do not show any control to orientation or level; or caves in massive (relatively fractureless) limestones?

Teresa wrote:4) There are replacements for time. They're velocity, volume, hydraulic pressure in a conduit, relative acidity, and higher vs lower differentials in the original chemistry of the mixing waters.


The factors may reduce the amount of time required, but nothing can replace time.

Teresa wrote:5) Despite hydrology_joe's fine and largely successful effort at reducing carbonate dissolution kinetics from equations to English, this entire discussion is way way WAY above the level of a non-geologist. They quit reading the first time pCO2 showed up and they said, "Say what?"I even know a caver with a master's degree who read the mixing waters thing, and said, Huh? If hydrogeos want to be listened to, they have to refrain from speaking in hydrogeology in public. It just scares people.
:hairpull:


I purposely dumbed the topic down and refrained from the kinetic reaction equations, etc to avoid the :? stares from the gallery. Unfortunately I know of no way to further dumb-down the topic so it is not scary to people.
Last edited by hydrology_joe on May 20, 2006 8:08 pm, edited 1 time in total.
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Postby Amemeba » May 19, 2006 10:43 pm

Preferential flowpaths DO NOT NEED TO EXIST before enlargement by chemical dissolution. This is exactly what my graduate research was investigating. (Trying to back-quantify & identify preferential pathways based upon existing cave passage morphologies) The preferential flowpaths need not be mechanical separations (joints, fractures, bedding planes) but can be higher permeability portions in an anisotropic massive limestones. If mechanical seperations are the controlling factor for cave formation, what is the explanation for the caves that do not show any control to orientation or level; or caves in massive (relatively fractureless) limestones?


Tell us more, hydrology_joe.

Maybe the evidence for cave enlargement in massive bedded limestones by way of linear mechanial openings was dissolved away within that enlargement and so left your study ripe for exotic speculation.

Would you mind listing the depositional conditions that might have led to an increase in the permeability of the subsequent rock?
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Postby Teresa » May 20, 2006 9:38 am

I would encourage hydrology_joe to give us the citation for his graduate research (published in JCKS) so we can look at the published summary, and find out more about these caves formed without preferential flowpaths.

I read the paper some time back; one thing which sticks in my mind is that many of the caves of his study area aren't in limestone at all, but in secondary dolomite, which has different dissolution kinetics--being as it is a different mineral. I am unaware of any 'massive limestones' (cf: Bedford, or Bloomington, Indiana) in his study area.
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Postby hydrology_joe » May 20, 2006 8:08 pm

Teresa wrote:I would encourage hydrology_joe to give us the citation for his graduate research (published in JCKS) so we can look at the published summary, and find out more about these caves formed without preferential flowpaths.


Easy enough...
http://www.caves.org/pub/journal/PDF/V65/v65n3-Dom.pdf

However, you must note that the emphasis of my work was to back out preferential flowpaths based on resulting cave passage morphologies. I was not specifically looking at cave passage formation in massive host rocks. I am just trying to point out that mechanical seperations are not needed as preferential flowpaths for cave formation. Rocks are not homogeneous... preferential flowpaths can exist in massive rocks without joints or fractures or beddingplanes or faults but through higher permeability sections of the anisotropic rock. If you don't want to believe me, please go look at Dr Palmer's 1991 work Origin and Morphology of Limestone Caves... I have included Fig 25:

Check out the 3rd row which identifies the dominant type of porosity as intergranular (in addition to the fractures and bedding planes)

Image


Teresa wrote:I read the paper some time back; one thing which sticks in my mind is that many of the caves of his study area aren't in limestone at all, but in secondary dolomite, which has different dissolution kinetics--being as it is a different mineral. I am unaware of any 'massive limestones' (cf: Bedford, or Bloomington, Indiana) in his study area.


The caves were formed in Mississippian Limestones and Ordovician Dolomites. (There was no statistical differences between the host rock.) As for the massive limestones, no there weren't any in the study area. (BTW, the study area was Missouri, not Indiana)


Amemeba wrote:Maybe the evidence for cave enlargement in massive bedded limestones by way of linear mechanial openings was dissolved away within that enlargement and so left your study ripe for exotic speculation.


That was a major assumption in the work. In order to try and minimize the loss of original preferential flowpath, we used a large data set and utilized straight line orientations of the center of cave passages (rather than sidewalls). Some caves showed control related to jointing in the area, other caves showed control related to bedding planes. However, some of the caves showed absolutely no control to the orientation of the cave passages even though the bedrock was highly jointed.


Amemeba wrote:Would you mind listing the depositional conditions that might have led to an increase in the permeability of the subsequent rock?


um.. how about coarser grained clasts? partial dolomitization?


Oh boy, now we are way off the KISS topic... :panic:
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Postby tropicalbats » May 21, 2006 12:18 am

I suspect the original poster's question has been answered. This is the speleology section, and his question was well-placed. All is good. So why not now break this off into a new thread and fire up the equations and jargon? I doubt anyone would mind a good debate over cave science. This is interesting stuff.

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Postby Amemeba » May 21, 2006 8:40 pm

Fair enough, Keith Christenson, I'll post the great cave type chart supplied by Hydrology_Joe in a new thread and ask him some questions about it if I can figure out how to enlarge the chart and transfer it.
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Postby George Dasher » May 22, 2006 10:47 am

I don't understand how the mixing of two waters saturated with pCO2 can equal a water than isn't saturated...
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Postby Squirrel Girl » May 22, 2006 10:50 am

George Dasher wrote:I don't understand how the mixing of two waters saturated with pCO2 can equal a water than isn't saturated...
Because the saturation level isn't linear. It would be easier to show with a picture, but I don't have time to hunt the beast down.
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Postby Phil Winkler » May 22, 2006 11:52 am

This is a fun read and isn't the mixed water corrosion topic one originally postulated by Fred Bogli from Switzerland concerning how passages had formed in Holloch back in 40s or 50s? Mischungs wasserkorrosion or something like that?

Fascinating stuff, I think.
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Postby Dwight Livingston » May 22, 2006 12:21 pm

George Dasher wrote:I don't understand how the mixing of two waters saturated with pCO2 can equal a water than isn't saturated...


George

I think we're talking saturation of calcium carbonate, not of carbon dioxide. Water containing different amounts of C02 can contain different amounts of calcium carbonate. The more CO2, the more rock dissolves before the water becomes saturated with calcium carbonate. And it's that relationship that is nonlinear.

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Postby Teresa » May 22, 2006 12:53 pm

George Dasher wrote:I don't understand how the mixing of two waters saturated with pCO2 can equal a water than isn't saturated...


Disclaimer: Ok, this is an analogy--not science. Men = positive ions (H+, Ca+ , and other minor positive cations) Women = negative ions--CO3 2-, HCO3-, CO2 (dissolved gas)other negatively charged free radicals and polyatomic ion groups.)

You have a church group of married couples come to a square dance at the VFW hall. (saturated solution with X pCO2). Unknown to them, a second set of swinging singles, (temporarily paired off, but with some loose singles of both genders) is also holding a square dance that night at the VFW (saturated solution with Y pCO2).

Once inside the VFW (with an open free for the evening bar!), the two groups intermingle, both because of the mixer nature of the square dance, and in random patterns due to the alcohol. By the end of the evening you have one big group, some of whom have switched partners, some partners who have become singles, and some singles who have found soulmates. When the clock strikes twelve, everyone is so drunk, no one knows who brought them anymore, and the two groups cannot sort themselves out.
As a result of this some fistfights, screaming, and other interpersonal friction (=mixing corrosion) ensues, and they end up tearing the VFW down. When the cops get there, you have a single mob of strangely paired up people, and no walls left.

I'm sure some hydrologists are snarfing into their keyboards at this explanation, but this is about what happens within the equations of mixing water kinetics to take 'apart' two saturated solutions, and end up with a different solution approaching equilibrium, while corroding cave walls in the process.
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Postby George Dasher » May 22, 2006 1:16 pm

Well, the good news is: I did understand the part about getting drunk with a woman. :banana:

And yes I did mean CaCO3 saturation, not pCO2 saturation. Sorry about that. And if the pCO2-CaCO3 saturation is non-linear, then I guess the resulting combined stream could be non-saturated. The key words in that sentence are "I guess." Of course, that combined water is immediately going to work toward being saturated. "More cave is us!" :grin:

I don't think it is correct to say that epikarst has poor hydraulic circulation. It has, rather, slower groundwater flows and greater groundwater storage, but not poor circulation. Heck, you could say conduit karst has the poorest hydraulic circulation of all. After all, that water moves fast and usually is just plain gone somewhere else.

And I think the Ford and Williams description of dissolution may be antiquated. My understanding is that, while dissolution takes place "near the surface" or in the epikarst during low or normal flows, this dissolution area will move downstream in the cave during high or flood flows. If I recall correctly, there is now a monitoring well installed in one of the water passages in Mammoth Cave. Calcium precipitates near this well during normal flow conditions, but dissolves away during high flow conditions. So I think it is important to realize that the calcium dissolution and precipitation, as well as the groundwater conditions, are always changing in an active water cave. :hairpull:

That's all.
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Postby Squirrel Girl » May 22, 2006 1:45 pm

George Dasher wrote:Well, the good news is: I did understand the part about getting drunk with a woman. :banana:
Harumph! I don't seem to recall ME every being able to get you drunk! :whiskey:
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