Author Topic: Dusty question  (Read 3972 times)

Offline Pitlamp

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Dusty question
« on: December 23, 2013, 03:22:38 pm »
Here's one for people familiar with lava tube caves. I understand that many of the lava caves in Lanzarote have thick accumulations of fine powdery white material. The question as to what this stuff is made of cropped up in a discussion recently. Suggestions varied from gypsum to dust associated with pyroclastic clouds during subsequent eruptions. I know there are some forum users who know a lot about caves in volcanic rock - any ideas?

Thanks.

Online Aubrey

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Re: Dusty question
« Reply #1 on: December 23, 2013, 04:48:06 pm »
We brought some samples back from Lanzarote for testing last year. It was found to be calcium sulphate (Gypsum).

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Offline Pitlamp

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Re: Dusty question
« Reply #2 on: December 23, 2013, 05:06:09 pm »
Thanks Aubrey - out of interest, what's the mechanism for its emplacement? I've never been lucky enough to visit one of these places but, having been shown some photographs, it looks like it's settled from airborne dust.

Offline menacer

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Re: Dusty question
« Reply #3 on: December 23, 2013, 06:37:12 pm »
From an observational perspective only, quite often, in narrower passages, the gypsum is prolific in one direction of rock faces ( similar to lichen on rock)
Javier tried explaining its formation to me once...in Spanglish. my interpretation was, it leeches out the rock from rainwater percolation...so my guess would be - then falls to the floor and blown in draught suspension or doesnt even make it to the floor in some instances and settles on the rock.
However, which draught direction I wouldnt know.
Is there more facing the predomint wind direction because it effectively splats against the rock which catches it, or is it less because its blown off...
As you may have realised this is a WAG answer ( wild arsed guess) hopefully Ed maybe along with some more sound scientific reasoning...


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Offline Pitlamp

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Re: Dusty question
« Reply #4 on: December 24, 2013, 10:22:13 am »
Cheers menacer - I guess that all makes sense.

It occurs to me that, at times, there will be very fast draughts in caves in volcanic areas. As we all know, the temperature outside a cave, which has two separate entrances at different altitudes, often results in a strong draught. In summer for example, when it's (say) 30 degrees C on the surface, the air howls down into Bar Pot. But in a volcanic area I'd have thought that subsequent eruptions would result in massive temperature differences between the cave (about 14 degrees in Lanzarote?) and upstairs (hundreds of degrees). So, assuming there's an upper & lower entrance to a lave tube, there must be times when it's occupied by a raging (downwards) gale. This might account for leached minerals being blown off the walls? Just a thought - but I know nowt about lava caves and it'd be interesting to read the ideas of more experienced folk (particularly Ed if he spots this topic).

Offline Ed W

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Re: Dusty question
« Reply #5 on: December 24, 2013, 05:10:57 pm »
The Gypsum is what is known as a "secondary" formation, i.e. it is formed some time after the formation of the cave, unlike lava stalagmites and stalactites that form as the cave is forming.  I think Menacer is right, in that the Gypsum is deposited from water leaching through the rock that the cave is formed in and dissolving it.  In principal this is little different to calcite formations in a limestone cave.  Such secondary formation are relatively common in lava tubes, and though Gypsum seems to be the most common mineral they can be formed from many others (I have seen calcite, silicates and opal to name a few).

The deep drifts of dust, almost like snow, I have only seen on Lanzarote.  These can be really deep, as shown by the "snow" crater in the picture below.




I have also caved in lava caves in Iceland, Sicily, Mexico and Korea (as well as Tenerife and El Hierro which also lie in the Canaries archipelago) and seen many gypsum formations but not the fine dust.  The formation of the dust is not something I know about.  However, there are some clues in some of the caves.  In some caves on the island there are rather beautiful and delicate gypsum flowers;



And in at least one other there are some very fine crystalline filaments coating the walls (the cave also has masses of the white dust).  I suspect (though do not know) that the dust is somehow formed by breakdown of these formations.  It is also possible that there is at least some biological factors involved in its formation.

As to the draughts.  Strong draughts are present in many of the Lanzarote caves, but apart from a few constricted passages I have not seen them strong enough to blow the dust about (in some of the constrictions the wind speed is high enough to kick dust into your eyes).  As to eruptions causing the equivalent of speleological gales, I am not convinced of this.  I would suspect that any temperature effects would be very limited in area, and if the cave entrance(s) were that close to an eruption subsequent to that in which the cave was formed I think that it would be very likely that the entrance would be blocked by ash, tephra or even newer lava flows (I have seen evidence of this from inside at least one cave in Iceland).  In practice it seems rare that a lava tube is subjected to harsh volcanic conditions post its formation without it being destroyed (as some caves on Hekla were some years back).

I have to add that I am supposing as much as anyone else in this debate, all my knowledge is second-hand from someone who knew far more than me about this subject, but is alas not able to educate me any further.  I think that this white powder on Lanzarote is fascinating, and I would suggest that Paolo Forti (co-author of cave minerals of the world) or Diana Northup (http://www.caveslime.org/) may well be best placed to give definitive advice - or sponsor some research!
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Online Fulk

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Re: Dusty question
« Reply #6 on: December 24, 2013, 05:31:09 pm »
Pitlamp:
Quote
In summer for example, when it's (say) 30 degrees C on the surface, the air howls down into Bar Pot.

When was it ever 30 deg C on the surface at Bar Pot?  ;D. Last time I was there it was hardly 30 F (and it felt like 30 K).

Anyway, how does the gypsum get into lava tubes of Lanzarote? I don't know, but it strikes me that there's a helluva lot of it down there, in a place where the rainfall is pretty meagre.

Could all the gypsum in these caves be accounted for by the sparse rainfall on Lanzarote?

Offline Cap'n Chris

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Re: Dusty question
« Reply #7 on: December 24, 2013, 05:57:10 pm »
And in at least one other there are some very fine crystalline filaments coating the walls (the cave also has masses of the white dust).

In many, Ed!.... I imagine there's hectares of these super-fine crystals in Lanzarote tubes:


Offline menacer

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Re: Dusty question
« Reply #8 on: December 24, 2013, 06:06:15 pm »
There seems to be a connection with calcium rich magmas and Olivines which are also prolific on Lanzarote. http://en.wikipedia.org/wiki/Basalt
Scroll down to "petrology" and look at the bit about calcium rich plagioclases and the inclusion of phenocryst minerals, of which one can be Olivine.
It may be simply down to the type of lava spewed up all those years ago that accounts for the prolific gypsum in the islands caves

Although rainfall on the island is low, the rocks are quite porous so when it does rain, what little water there is will leech its way through quite easily and possibly condensation and humidity play a part.

Miguel showed us a new high level gallery in one of the popular caves this year, it was stunning, untouched white powder covering the entire passage width, feet thick. ( Nigella would have had a field day)
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Offline menacer

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Re: Dusty question
« Reply #9 on: December 24, 2013, 06:49:32 pm »
High level gallery with Gypsum
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Online Fulk

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Re: Dusty question
« Reply #10 on: December 24, 2013, 07:04:27 pm »
Which cave is that?

Offline Cap'n Chris

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Re: Dusty question
« Reply #11 on: December 24, 2013, 08:16:31 pm »
We agreed with the discoverer not to divulge its whereabouts, for conservation reasons.

Offline Cap'n Chris

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Re: Dusty question
« Reply #12 on: December 24, 2013, 09:11:05 pm »
Referral to Hill & Forti suggests that other common gypsum-like deposits abound in lava tubes - namely Epsomite and Mirabilite; the latter looks to be very very similar to formations in Naturalistas and Cueva Aubrito, viz:



?Mirabilite in Cueva Aubrito

Online Fulk

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Re: Dusty question
« Reply #13 on: December 25, 2013, 01:12:55 pm »
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We agreed with the discoverer not to divulge its whereabouts, for conservation reasons.

Fair enough.

Offline mrodoc

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Re: Dusty question
« Reply #14 on: December 25, 2013, 04:50:26 pm »
Gypsum soon comes off cave walls. We were exploring/surveying virgin cave in Meghalaya and the walls were covered in cotton wool deposits that just fell off as you passed so I imagine this is what has happened over thousands of years in Lanzarote. The amount of dust in the caves there is quite extraordinary I agree but the tubes are quite old as well.

Offline Cap'n Chris

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Re: Dusty question
« Reply #15 on: December 25, 2013, 07:38:23 pm »
Bear in mind that some of the lava tubes (more properly called pyroducts, apparently) with thick deposits of gypsum (or derivatives) are those in the National Park of Timanfaya where the caves are less than a few hundred years old. The mechanism for deposition seems to be that mentioned earlier by Menacer, namely evaporation deposition of percolated sulphate-rich moisture and subsequent settling of the deposits.

Offline Ed W

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Re: Dusty question
« Reply #16 on: December 25, 2013, 09:48:22 pm »
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more properly called pyroducts, apparently

Personally I'm not keen on the "P" word.  There are a few academics trying to retro-name lava tubes as Pyroducts based on the name being used by an early observer.  However, I think lava tubes is a perfectly good name that everyone understands, so why change.
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Offline menacer

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Re: Dusty question
« Reply #17 on: January 31, 2014, 08:05:22 pm »
Ive just noticed on the NSS website that Dave Bunnell has released an update to his Caves of Fire book(Caves of Fire 2)
http://nssbookstore.org/index.php?mode=store&submode=showitem&itemnumber=07-1131-2819
Quote
Available exclusively through the NSS Bookstore!  Author Dave Bunnell has made an extensive revision to his full color First Edition, Caves of Fire: Inside America's Lava Tubes, published by the NSS in 2008.  Twenty new pages have been added and many revisions have been made to the existing pages including many new photos taken by the author and his friends.  The 144 page Second Edition has expanded coverage of lava tube genesis including spectacular new images which peek into active tubes to reveal lava formations in the making.  The book has two purposes: first, to highlight the many bizarre forms that can be found in lava tubes and explain how they came to be using the most photogenic examples the author knows of; and second, to present descriptions and images from dozens of lava tubes open to the general public on state, federal, or private lands for those who want to enjoy exploring lava tubes for themselves.  Because of lower printing costs we are able to offer this larger edition at less than half the price of the First Edition.
Definately worth a read to anyone out there questioning how lava formations occur.
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