How Old Is Yosemite Valley?

[maverick]

Interesting article.
https://news.berkeley.edu/2022/10/20/ho ... epOwMkDjA0

[balzaccom]

All that granite is young--one of the reasons that it is in such great shape!

[Gogd]

The granite rock itself is actually pretty old, >40M years. Then it was lifted by the effects of tectonic plate shift; then glaciation of the last 4-10M years cut the valley from the rock. The relative youth of the valley only partially explains why the more notable features of the valley are less subject to cracking and wear. I can't recall the precise explanation for the rock's exceptional hardness, if it is the chemistry or the affect processes and subterranean conditions had on the rock when it was formed, or a combination of factors.

Hey @ giantbrookie , please elucidate us with your expertise! Why is Yosemite granite so tough?

Ed

[giantbrookie]

The "young" nature of the valley refers to the carving of the canyon by stream and subsequent glacial erosion instead of the granitic bedrock which is ca. 100 million years old or so. The granitic bedrock crystallized several km beneath the Earth's surface and was then exhumed in the solid state, primarily by erosion. The massive and solid nature of those walls is primarily a result of the very sparse fracturing of this granitic rock. You will note that a lot of the rock adjacent to the more massive cliffy stuff is more rubbly and has a lower angle of repose. You see in this in other Sierra canyons such as the S Fork Kings (ie near Road's End/Cedar Grove), the Tuolumne (Hetch Hetchy), etc. The joints/fractures formed during the exhumation of the granitic rocks as a result of various differential stresses imparted during this process. How much of the fracture spacing is controlled by small spatial variations in said differential stresses, or small spatial differences in mineralogy that imparted different physical properties to the rock (ie stronger and less likely to develop the brittle fractures) is unclear.

[balzaccom]

Cool! Brookie strikes again!