The Mystery of Tuolumne Meadows

In Environment, Geology

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If you’ve never been to Tuolumne Meadows then you’re missing out on one of the best parts of Yosemite, in my opinion. It’s a broad, grassy valley surrounded by sharp, knife-edged peaks and domes that makes for some very beautiful scenery. But it’s always been a bit of a mystery how the meadow and the unique mix of geological features surrounding it formed. Now researchers think they may have uncovered clues to how the meadow formed and likely it has to do with the way the Sierra Nevada itself formed.

LiveScience.com. When ice age glaciers carved Yosemite’s distinctive features, flowing ice easily scooped out the shattered granite, leaving only intact rock behind, the researchers think.

The Sierra Nevada mountains are glued together by large masses of cooled magma called plutons. Each pluton was once a blob of rising molten rock that stalled underground and slowly crystallized miles from the surface.

Several plutons form the stunning domes and steep walls of Yosemite National Park, each one overlapping the next, like a pile of sleeping puppies. One of the youngest, called the Cathedral Peak granodiorite, crosses through the Tuolumne Meadows region.

The Cathedral Peak granodiorite was shattered and cracked about 85 million years ago, when new magma pushed upward into the cooling pluton, according to earlier research by Becker’s UW colleagues. Gas or fluids from the younger magma blasted open escape routes in the older granite.

“These volatiles explosively fractured the rock,” said Becker.

The peculiar cracks, called tabular fracture clusters (TFCs), are tightly spaced within Tuolumne Meadows and few and far between elsewhere. The clusters are zones of intensely fractured rock about 3 feet to 320 feet long (3 to 100 meters), with at least four cracks within a 4-inch (10 centimeters) span. The researchers say the valley’s vanished rock was likely pulverized, too. Clumps of clusters plunge toward the meadows on rocky slopes bordering the valley. “When we go to higher elevations, we see TFCs heading straight for the valley air, so we infer that the ghost rocks were filled with TFCs as well,” Becker said.

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