Exfoliation weathering

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Last week I stopped by the office of WSC Board President Todd Foutz for a meeting. There were several decorative granite boulders in the landscaping outside his office with interesting features that caught my attention.

The boulders are more-or-less ovoid, but with a slightly depressed area on the top surface that’s a few millimeters below the boulder edges. The lower area is visible in the photo at the top because of its jagged edges and somewhat different color; it’s more white, and less brown, than the higher surrounding areas. It almost appears as if the rock is composed of concentric layers that are peeling away like an onion skin, but this rock is a granite with little or no internal layering.

This is caused by a type of weathering called exfoliation (or sometimes exfoliation jointing). The onion analogy is somewhat apt, because the outer parts of the rock are in fact flaking away in thin sheets. That’s why the depressed area is whiter in color; it has only recently been exposed to the air, and as a result hasn’t experienced as much oxidation as the surrounding rock.

But, if the granite doesn’t have internal layering, what causes it to flake off in sheets? There are actually several different ways that exfoliation joints can form. A rock that has been buried can start to exfoliate if the overlying rock has been removed, releasing pressure on the deeper rocks; these exfoliation events can be rather dramatic. But that’s not the case in our rock, which has been quarried and moved to this location.

Another possible cause of exfoliation is freeze-thaw cycles. If the outer part of a rock soaks up water and then freezes, the resulting ice can expand and split off the outer part of the rock. That’s not likely the case here for several reasons. This granite is not very porous and so has a hard time absorbing much water (although some would get in eventually). More significantly, this rock is sitting in Hemet, which only averages about 28 cm of rain a year and where the temperature almost never drops below freezing.

I think the most likely cause of exfoliation in these rocks is thermal expansion. It may almost never drop below freezing here in Hemet, but it does get hot! Temperatures above 38C are not uncommon for much of the year, and that heats up the outer surface of the rock. As with most materials, rocks expand when they heat up. But rocks are terrible conductors of heat. That means that even though the outside of a rock may get painfully hot sitting in the sun, just a few millimeters below the surface the rock’s temperature barely changes at all. So the outer part of the rock expands and contracts as the temperature cycles throughout the day and eventually weakens to the point that pops off of the rest of the rock. (Incidentally, the low thermal conductivity of rocks is also the reason most caves stay at a fairly constant temperature inside, no matter what the weather is like outside.)

Of course, this type of weathering can potentially occur anywhere there is variation in temperatures, which is pretty much everywhere on the Earth’s surface. Yet this is not something I frequently observed in Virginia, not because it wasn’t happening but because other processes have such a big influence. In Virginia higher rainfall amounts, sub-freezing winter temperatures, and abundant vegetation with rock-splitting roots often make it almost impossible to determine the cause of exfoliation weathering. Here in Southern California those other variables are minimized, making the effects of thermal expansion more obvious.

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