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rope up this 8=D


gapertimmy

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Most basalt is columnar, meaning it has vertical cracks. As the basaltic lava cools it must contract. It's quite easy for it to contract in the vertical direction, the flow merely gets "shorter" or thinnner as it cools. However, in the horizontal dimensions it cannot "shrink" because the flow is too large to overcome the shear stresses at it's base, therefore it cracks into collumns to accomodate the contraction due to cooling. Why these things crack into such perfectly polygonal shapes is not understood, and rather counter-intuitive when thought about from the perspective of fracture propogation (i.e. once a fracture starts propogating it should continue indefinitely in the direction that it initiated until intersecting a barrier).

 

The second link I gave outlines the SJ Gould article rather well, but it's not nearly as well written. I'm leaving it up to Fern to find a copy of the original. I'm tired and going home.

Gonna stop in my office to lock up and say good-night then I'm outta here.

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I believe columnar basalt cracks in hexagonals mirroring the underlying crystal lattice structure formed from Si MG and O2. Microscopic structure reconstructed at the macro level. Just got back from climbing on some slightly andesitic basalt (beacon baby beacon!) [Razz]

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quote:

Originally posted by iain:

I believe columnar basalt cracks in hexagonals mirroring the underlying crystal lattice structure formed from Si MG and O2. Microscopic structure reconstructed at the macro level. Just got back from climbing on some slightly andesitic basalt (beacon baby beacon!)
[Razz]

Nice try, Iain but, it's not. The mineralogy of Basalt is totally random (and primarily Ca-Feldspar anyway, which isn't hexagonal). Columnar jointing has nothing to do with the macroscopic expression of the internal micro-structure. But I can see how people might think so.

 

[ 10-20-2002, 12:09 AM: Message edited by: E-rock ]

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Good night everyone, it's been fun (a better substitute for climbing than I had hoped).

 

And Muffy, the uniformitarian debate I think is one of the more interesting ones in scientific philosophy. One of the big questions that my old Sed-Stat professor used to like to ask was:

 

Is the sedimentary record representative of normal sedimentary events, or are only the really BIG events preserved?

 

Food for thought... The sahara desert sands are on the order of tens of meters thick. The Navajo sandstone is on the order of 100 to 1000m thick.

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Originally posted by E-rock:

Food for thought... The sahara desert sands are on the order of tens of meters thick. The Navajo sandstone is on the order of 100 to 1000m thick.

So what are you suggesting, that climates were extremely different in the Mesozoic? Of course! [smile] The Navajo Erg explanation was good enough for me. (I think the Navajo tops out at 700-something meters but I'm rusty on that strata).

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