Thursday, February 12, 2015

Geo 1095: February 10, Day 771: Cracked Ribs

I suppose I'd call this a solution fissure. It seems evident that the diagonal fracture in the top left middle was the weak zone allowing solution along its plane, though whether that's a joint or a fault is unclear from the photo. The features I find interesting, though, are the "ribs" along the sides of the fissure. They look close to horizontal, but that could be misleading. They could just happen to align with whatever direction my camera was pointing, which might not have been horizontally. If we assume that I was shooting horizontally, the most likely explanation for the ribs is that they represent intervals where groundwater levels were dropping rapidly compared to periods where water levels were stable for longer times. Putting it another way, the deeper furrows along the sides may represent periods when groundwater levels were stable, allowing more time for solution to occur.

On the other hand, if I wasn't looking along a horizontal plane, but up or down at some angle, those ribs may represent impurities in the marble, such as silt and/or mud, which would have gone to various silicates such as wollastonite during metamorphosis, rendering those layers less soluble than the purer calcite bands.

Photo unmodified. May 9, 2013. FlashEarth Location. (Since we're underground, I have only a vague idea where this is with respect to the surface.)

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