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November 14, 2010


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Rather fascinating as ever. A Quick question. Unsure how short the answer. I assume that I understand (to some degree) what happens when organic matter rots. I associate rotting with the work of living agents such as bacteria, molds, fungi, etc. All sorts of buggy things.

What happens when rocks rot? I would guess that the answer would vary with type of minerals involved, the climate and other environmental factors. Dissolution? Chemical reactions causing change in structure and consequential mechanical integrity (and solubility, etc.)?

Or are biological processes sometimes (or always) involved?

I've heard of rocks rotting before but just realized that I have no idea what it means.



Hi Walter - you make a good point, and here's a reasonably short answer. Yes, "rotting" is generally associated with organic materials, but I'm exercising a little poetic license here (not that I pretend to be a poet)and using the term rather broadly. Because, in many ways, rocks do rot - it's primarily a chemical process of breakdown, caused by the atmosphere, water - and organic processes. Any mineral that formed below the surface of the earth under higher temperatures and pressures will, once it's exposed at the surface, be out of equilibrium, unstable. The greater the difference between surface temperature and pressure and the conditions of original formation, the more unstable it will be.

In, for example, an igneous rock, say a granite, the feldspars and micas are extremely unstable and will succumb first to chemical attack, dissolving and disintegrating. The relatively stable quartz grains will then be left without support, and, like old teeth, drop out to form quartz sand grains. We know that microbes can penetrate deep into rock via cracks and fissures and their biochemical activity accelerates the whole process.

You can walk up to an outcrop of weathered granite, once a hard and solid rock, and simply scrape it off into your hand - it has lost its integrity and original character, and, I think it's fair to say, rotted....

In the short term (for geology), this loss of stability has serious consequences for buildings. Somewhere I saw an architectural conservator for Venice quoted as lamenting the absence of treatments for "diseases of stone."
"Competent rock" is a commonplace, a lovely Disneyish image of stone dutifully and skillfully holding itself up. Deep time reveals process analogies among animal, vegetable, mineral, as speeding up earthquake vibrations makes them sound like whale song.

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