Formation of stalactites

[Fulk]

It?s often said that all stalactites start life as straw stals ? and, indeed, it is a matter of observation of broken stals that they generally (always?) have a tiny hole running roughly down their centre. Then, so the argument goes, water laden with calcium bicarbonate starts to run down the outside of some stals, where calcite is deposited, to give the typically pointed stalactites that we see. But how does this putative process work in practice?

If water somehow seeps out of a hole at the top of a straw (because a blockage develops lower down?) one would expect it to do so on one side only, hence one might expect bigger stalactites to be asymmetrical, rather than the quasi-symmetrical ones we see in practice. I guess that for a small-diameter straw the influence of something such as capillary forces may tend to cause it to run all the way round the top of the straw and hence slowly run down the outside in a symmetrical fashion ? but I wouldn?t expect this to happen once this process of building up CaCO3 on the outside has proceeded to any great extent.

So how do tapered stalactites form in a quasi-symmetrical fashion?

 

[Bob Mehew]

Er, try these:

http://www.damtp.cam.ac.uk/user/gold/pdfs/stalactite_prl.pdf

http://www.damtp.cam.ac.uk/user/gold/pdfs/stalactite_long.pdf

Sorry, I don't have the time (or at this moment the power of concentration thanks to a vomiting bug) to read, let alone summarise.

 

[TheBitterEnd]

you can borrow my copy of Speleothem Science if you like,  https://www.wiley.com/en-gb/Speleothem+Science%3A+From+Process+to+Past+Environments-p-9781405196208

I'm not sure it answers your question directly because as is so often the case there probably isn't  simple answer.

 

[TheBitterEnd]

Having mentioned that I couldn't help but have a look, even though I'm about to go out for the evening and probably going to get into trouble 

It does say that Maltsev (1998) has disputed that water flows entirely internally in straws...  it's online here
https://books.google.co.uk/books?id=aNlhAvqnTDYC&pg=PT250&lpg=PT250&dq=Maltsev+(1998)+soda+straw&source=bl&ots=VVGQKLkthq&sig=JB1zvGO6Y81rdFjrhNGLYukRKVU&hl=en&sa=X&ved=2ahUKEwj90J_r2srfAhUFRBoKHXYAAY8Q6AEwDnoECAkQAQ#v=onepage&q=Maltsev%20(1998)%20soda%20straw&f=false

 

[Bob Mehew]

I have recevied a prompt that the Maltsev paper is available at http://www.ubss.org.uk/resources/proceedings/vol21/UBSS_Proc_21_2_149-158.pdf .

There is a copy of Speleothem Science in the British Caving Library but it is not available for loan.  So either you make the journey to BCL or else seek a Inter Library Lending loan via your local library (cost circa ?5).  (Or possibly a local caving club library may have a copy.)

 

[mikem]

Water takes the path of least resistance, so once a small amount of calcite forms on one side the water will divert around it - a fresh calcite flow down a wall tends to run in various directions rather than all follow the same route, so maybe same on stalactites (just not so obvious).

Mike

 

[Fulk]

Thanks for your replies.

However, I have to say that I found the papers you cite, Bob, somewhat beyond my mathematical capabilities; in any case, unless I've missed something, neither answers my question as to how the water flows uniformly over the outside of stals; they both seem to just assume this:

(Paper 1): 'The fluid layer flowing down the surface of a growing stalactite controls precipitative growth, so we first establish its typical thickness and velocity.'

(Paper 2): 'Next, we establish the properties of the aqueous fluid layer on the stalactite surface by considering a cylindrical stalactite of radius R, length , and coated by a film of thickness h.'

I was, however, quite taken by:

'After straightforwardly solving Eq. 39,'

Eq 39 being:

 

[droid]

Gotta love calculus.....

 

[Fulk]

Yeah, especially second-order differential equations.

 

[Bob Mehew]

Yeah, especially second-order differential equations.

Strangely I do like them.  But I will admit it has been a few years since I last did something like that one so no, it is not straight forward to me either.   

But a possible answer to your question re "how the water flows uniformly over the outside of stals" is perhaps where it does not, it forms a curtain.  That is assuming all the other conditions are right for the formation of stal.

 

[Fulk]

Actually, I enjoyed calculus when I was at school.

Anyway, I really don't see that the formation of curtains (flowstone, flowing down the wall of a cave) is relevant to the formation of stals (dripstone, dripping out of the roof of a cave).

 

[Kenilworth]

Anyway, I really don't see that the formation of curtains (flowstone, flowing down the wall of a cave) is relevant to the formation of stals (dripstone, dripping out of the roof of a cave).

Draperies sometimes hang directly from the ceiling, but I mostly agree that stalactite formation is difficult to explain and poorly understood. How water travels to, over, and through stal is puzzling to me at least.

 

[Bob Mehew]

My take on the formation of stal of any type is that it requires water to transport the key species of Ca(HCO3)2 and dissolved CO2 through micro fissures in the  limestone rock.  (Micro because to a first approximation, you can't see them.)  Once that water reaches the surface of the limestone with air, then the equilibrium of dissolved CO2 and Ca(HCO3)2 (as well as other related species) is disturbed and relatively slowly (relative compared to the movement of water cross the surface) allows some CO2 to be released to the air which in turn upsets the Ca(HCO3)2 / CaCO3 equilibrium which allows some CaCO3 to deposit out.  The contours of the limestone surface around the point where the water exits the micro fissures then dictate how the water flows across the surface and hence deposits CaCO3. And I suspect the cross sectional area of the micro fissures controls the flow rate of water also has a bearing on the nature of the deposition.  (I admit this is not based on any scientific papers I have read.)

One interesting allied point, is that the associated CO2 release with the deposition of CaCO3 does not account for the 'bad atmospheres' due to CO2 occasionally observed in caves.  (In simple terms there is not enough deposited stal.)  Those atmospheres are thought to be due to a sightly simpler process of dissolved CO2 in water 'degassing' as the water reaches the surface of the limestone with air.  The only problem with that statement is I am troubled by the absence of sufficient water flows to transport sufficient CO2 to generate such bad atmospheres.  But I will concede that there are a number of variables for which I / we don't have a decent value which could make this a plausible mechanism.

Oh and to emphasises a particular bee in my bonnet, I would prefer to see the emphasis on CO2 in the atmosphere is always dispersing.  There is no atmospheric transport system which concentrates CO2.  Where the overall rate is sometimes slow enough to make it appear like an accumulation is because the feed rate by the non atmospheric transport system (water) overwhelms the dispersion via the fairly slow specific atmospheric transport system governed by diffusion.  Obviously drafts have a much greater capacity as a dispersing atmospheric transport system and winds even more so. 
 

 

[Ad W]

This is an interesting topic. Does calcite have any appreciable permeability once deposited?



Sent from my iPhone using Tapatalk

 

[Bob Mehew]

Does calcite have any appreciable permeability once deposited?

I assume you are ignoring diffusion.  And in any case since researchers are using C14 measurements across a stal to age the stal, there is presumably evidence to show that CO3 ions do not migrate. 

I have not read up on this aspect (my focus of interest is CO2).  But in single crystal form I would guess no.  Even in multiple crystal form I would expect the crystal boundaries to be effectively impermeable.  I can foresee foreign bodies causing some degree of permeability by incomplete bonding between stal and body, though I think not enough to be appreciably permeable.

So if you were thinking of a transport of CaCO3, CO2 or H2O though stal, then it seems unlikely.