Tuesday, April 2, 2013

Stone lace?


It is Easter, and I am spending the long weekend back home near the coast. Later I expect we will take a walk by the beach, maybe explore some rock pools. And there, doubtless, we shall see strange patterns on the surface of the rocks, like lace latticework across the surface.

Patterns of honeycomb weathering, typical on sandstones at the coast, also known as “tafoni”, have attracted the attention of rock-hoppers like me over the decades. Charles Darwin mentioned them in 1839, in his account of his voyage on HMS Beagle. They point to Nature’s inherent capacity to self-organise – the weathering may begin as isolated pock-marks on an otherwise pristine surface, but then seems to progress to a point where these depressions are arranged on a characteristic, narrowly defined, length scale. Eventually this type of weathering matures to a point where the ridges between the depressions are the features that attract our attention, standing proud like the web of some demented stone-weaving spider.

Less fantastical explanations are usually invoked to describe the onset of honeycomb weathering. It is often seen in rocks at the inter-tidal zone of the coast, but also in desert environments, and from Antarctica to the equator. Most commonly, it is believed that these patterns are the result of salt weathering. This was first suggested back in 1925, when it was noticed that honeycomb weathered limestones near the Nile, in Egypt, showed clumps of fibrous salt crystals on their surface. It seems that as salty water percolates through the pore spaces of rocks and evaporates at the surface, it acts to disaggregate the grains and break up the clay and cement that holds them together. Very concentrated salt solutions accumulate in any depression, and as they evaporate and concentrate further become more aggressive in their attack on the rock fabric.

As such cavities enlarge you would expect them to coalesce. One argument is that they will only develop to a certain depth, beyond which the water within them will not evaporate sufficiently to induce further erosion. Another is that the walls between cavities are stronger, perhaps due to microscopic algae growing within the porous rock and protecting them from chemical attack. Variations in mineral content within the rock will also, surely, play a role, as can be seen from the ridges running parallel to the bedding in the photo here,

Whatever the precise origins, I see that the sun is rising. Maybe spring is, at last, appearing … time to go an explore the coast. I must go down to the sea again.


Photo: Honeycomb weather in sandstone at Elgol, west Scotland, looking towards the Cuillin of Skye (credit: Martin Sharman, creative commons).


Links:
http://gsabulletin.gsapubs.org/content/93/2/108.short
http://gsabulletin.gsapubs.org/content/111/8/1250.short

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