|Tis-sa-ack (Half Dome) from Glacier Point. For the origin of the name Tis-sa-ack, check my post at http://geotripper.blogspot.com/2014/11/if-these-cliffs-could-talk-tis-sa-ack.html|
|A view from Washburn Point, a bit more to the south than Glacier Point. Both views are looking roughly east.|
Many of the granitic rocks of Yosemite Valley are highly jointed, meaning that the rocks fractured as they were exposed at the surface by erosion. Jointing is also influenced by regional stresses, such as nearby fault zones. Jointed rocks are easily quarried by glacial action, and in Yosemite Valley, such rocks form the recessed parts of the canyon walls. Rocks like El Capitan and Half Dome have very few joints, and thus stand out as a monolithic cliffs and peaks.
The rounded shape of the dome is the result of exfoliation jointing. Exfoliation is the tendency to form slabs of rock parallel to the surface of the rock. Corners and edges are preferentially broken off, leading to the dome shape. The cause of exfoliation is still being researched, with a traditional explanation of pressure relief as the rock is exposed by erosion. More nuanced causes are related to thermal stresses and compressional forces within the rock. A rather stunning example of exfoliation in action was caught on film last year. Check out my post on the subject here: http://geotripper.blogspot.com/2014/09/ongoing-exfoliation-event-at-twain.html.
Whatever the cause of the exfoliation and doming, glaciers did play a role in the unusual shape of Half Dome. A planar joint forms the face of Half Dome, and it was glaciers in the valley below that quarried away the rocks on the other side of the joint. Boulders and blocks of Half Dome Granodiorite can be found miles downstream in Yosemite Valley.
It takes just a bit of walking to Mirror Lake and the valley above to see Half Dome from the base. Somehow the dome looks huge from below.
As noted in the last post, a huge rockfall came off the rock on the flank of Half Dome in March of 2009. The Ahwiyah Point rock fall forever changed the appearance of Tis-sa-ack. But that's the nature of geology. Nothing on Earth remains the same, and some of those changes happen in a human time frame. Just ask the people of New Hampshire about their state emblem (story here).
here and here.