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Devil’s Postpile 15 June 2021


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It being Tuesday (take out the trash day) I’m on breakfast patrol, and it’s bacon and scribbled eggs with onions and sheep cheese.

Then, before the day gets hot, its away to the Postpile trailhead. Once again, no idea how far or how up-and-down, but this is what we came for. Turns out, quite a lot of up-and-down, but not as far as yesterday, and coming around the corner and catching the first glimpse of the postpile brought an “Oh my goodness!” out of Rochelle. Worth the work. 

Being early, the sun was right in our faces, not good for photos. This is from the north side of the pile:


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What is going on here?

Stunning by itself – wait ‘til you see the bent piles! – but there’s so much science here. How in the world could this formation (and others like it) have come about? How long did it take?

As usual with geology, a very long time, even though it likely began very quickly, when a hot flow of lava from the very active (still) Mammoth Mountain pooled here, and then s-l-o-w-l-y cooled, giving the liquid magma time to organize itself into convection cells, like the ones you can sometimes see in tea with cream.

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Convection cells are common, but seldom seen this well delineated – for that to happen, circumstances need to be perfect, and as Mary Hill, author of the excellent Geology of the Sierra Nevada, notes, in nature, that is exceedingly rare. In addition to teacups, they happen wherever a fluid is being heated or cooled. In a room with a fireplace, warm air rises near the fire and cool air flows down the exterior walls to replace it. Mud puddles tessellate because the sun warms the tops of the cells and the moist earth below cools it; under favorable conditions, these tiles are hexagonal.

The size of the cells is determined by the temperature differential, the viscosity of the fluid, and the ambient temperature. The cells we are most familiar with, the crackles on top of dried mud puddles, start out cool and runny, and usually make big, irregular tiles. At the Postpile, the fluid was hot, viscous molten rock, and apparently the cooling took a long time. The cells organized themselves and their walls solidified before their cores, initiating a virtuous cycle that reinforced their formation. As the whole mass cooled, the cells shrank and their boundaries – cell walls – hardened and constrained the fluid currents. The space between allowed rain and snow (if there was rain and snow in this era; according to Hill, the pool probably happened quickly, probably “less than 100,000 years ago” and then cooled for years or even decades. Their hexagonal shape is “as the honeybee knows,” Hill points out, the most efficient tessellation, optimizing the ratio of area to circumference while enabling tight packing. (Squares and triangles work too, but the ratios aren’t as low.) 

I cannot resist another foray back in time: in my high school days, I first heard of convection cells, but they were called Bénard Cells then. A few years later I remember trying to explain Bénard convection to a friend, who fact-checked me and found nothing: the cells had been renamed “convection cells” (because that's what they are!) and somehow their former name hadn't yet appeared on the internet. I am happy, this morning, to find that I was not hallucinating. 

 


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The configuration of the cells can be seen clearly at the top – somewhat random, as Nature prefers, but generally very regular. 

Were they this smooth when they were cool? Probably not, but this science lesson has been simplified for us by the Ice Age: a glacier, bearing stones and rubbish from up canyon ground and polished the cells as recently as 35,000 years ago.

The cells are basalt, but the patches that have peeled off were likely baked (and thus metamorphosed) by heat generated by the glacier’s passage.

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The glacier’s skid marks, where its “teeth” – rocks embedded in the underside of the glacier – were dragged across the tops of the postpiles. The shiny surface is called “glacier polish,” caused by the tiny teeth of the rocks being milled down by the glacier’s slow but inexorable downhill flow. 

 

Most likely, that wasn’t the only heat these posts experienced. Earthquakes, continental subduction, plate tilting, and volcanism all played their part. My favorite part of the face of the Postpile is the twisted posts in one section: what caused that?


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Along the trail down from the top, another pile appears to have been tilted whole by the disorderly process of mountain building that is responsible for this range, the Sierras: according to Hill, the longest range of mountains on the North American Continent.

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