Thursday, October 12, 2017
You Just Can't Take Sonora Pass for Granite: Travels Around the Sierra Nevada High Country
"granitics" are truly beautiful rocks. At Sonora Pass, the common granitic rock is called the granodiorite of Topaz Lake, and it is one of the younger intrusions in the Sierra Nevada, having cooled about 89-93 million years ago. It has massively large crystals of potassium feldspar, 2-3 inches long, embedded in a groundmass (matrix) of smaller crystals of quartz and plagioclase. Such rocks with two distinct grain sizes are described as porphyritic. Glaciers have smoothed off the rock in many places ('glacial polish'), almost providing the effect of a polished countertop.
If you long to spend time in the Sierra Nevada, and just can't fathom the thought of fighting the crowds at Yosemite Valley and other parts of the national park, give Sonora Pass a look. I'm pretty sure you won't be disappointed. If you need a guide, my colleague Noah Hughes and I edited a field trip guide for the region for a meeting of the National Association of Geoscience Teachers in 2012, and the book we published is for sale (details are here; all proceeds support the scholarship program of the NAGT-Far Western Section).