Monday, October 10, 2011

Quaking Aspen, Sherman Granite

Today’s post follows the overall theme of my blog -- plants and rocks -- which I chose because I can’t decide which I am at heart, a botanist or a geologist.

Last week I headed to the crest of the southern Laramie Range to enjoy another Indian Summer day, hoping to capture the glowing yellow aspen leaves with my point-and-shoot camera.  It’s fall and already the sun is low enough to make things sparkle -- leaves on trees, crystals in rocks.  But capturing glowing yellow leaves back-lit by the sun is not easy.  I think my camera’s auto exposure function doesn’t deal well with back-lighting.
Sherman granite on left; yellow (and some green) quaking aspen in distance.
The Sherman batholith was emplaced ca 1.4 billion years ago, one of the many granite plutons of that age in southeast Wyoming, Colorado, New Mexico and Arizona ... but not further north. This must have something to do with the Cheyenne belt, our local suture zone where arcs or microterranes were added to the south margin of Laurentia 1.9 to 1.6 billion years ago.  The Wyoming craton itself had merged with Laurentia just a few hundred million years earlier (wow! how do they know all this stuff?!! ... I have so much to learn).

This granite is infamously coarse, just ask Vedauwoo rock climbers

Grus, to ca 0.5 cm diameter ... with lichen.

The granite weathers to grus -- “accumulation of angular, coarse-grained fragments (sand and gravel) resulting from the granular disintegration of crystalline rocks (most notably granitoids) generally in arid or semiarid regions” (Bates and Jackson, Dictionary of Geological Terms).

Spectacular washboard topography develops on the unpaved roads each season, I suspect because grus doesn’t compact, is easily moved.  Locals disagree on to how to drive -- creep along to maximize control vs. drive at 30 mph to smooth out effects of the bumps.

Not all the granite in this area is coarse; occasionally there is a fine-grained reddish type for which I’ve yet to find an explanation or reference ... sorry, you’ll have to wait for the next exciting episode.
This looks like yet another photo of a rock, but turn your attention please to the bright yellow leaves behind -- backlit quaking aspen.  Why are they no longer green?  Steady lengthening of nights triggers the shut-down of (green) chlorophyll production, allowing the yellow carotenoid and reddish anthocyanin pigments their days in the sun.  Carotenoids are always present, so the yellows are predictable.  Anthocyanins are produced in the fall, dependent on amount of sugar in the leaves.  Warm sunny days with cool but not freezing nights maximize sugar production and red to crimson colors.  We don’t have much in the way of red leaves here, but occasionally our aspen turn orange.

I hiked down into the Enchanted Forest with its large quaking aspen.  There was the soft sound of leaves fluttering in the wind, but somehow this place always seems very still.  If I’m ever going to see the Buddha, I think it will be here.

1 comment:

  1. I have a small planting of Quaking Aspen (Populus tremuloides) on my property. It is a clone. Trees-Plants Nursery