Half Dome and its Shape…

half-dome-in-snow-by-jeff-kreiderFor most of my visits to the park, I was more wrapped up in the magnificence of the view rather than how it got that way. When I learned (or discovered) that glaciation is what formed the valley, I assumed that a glacier had sheered off Half Dome as it went through. It turns out that is not the case. The highest level of any of the glaciers never got closer than 900 feet from the top.

The question of where the missing half of Half Dome has gone, presupposes there actually was another half. A better question is, “How did it form?” Unfortunately, it’s not a simple answer. Books have been written about the process. We will need to get, at least, ankle deep into geology and the formation of the whole valley (and maybe should get into the formation of the Earth, itself, for that matter, but I’ll spare us that). Nevertheless, it is kind of interesting and somewhat a-intuitive.

Actually, it wasn’t always called “Half Dome” even though the name was first applied the very day the valley was “discovered” back on March 25, 1851. During most of the rest of that century, it was more commonly called “South Dome.” It was noted as South Dome in the hotel register of La Casa Nevada commemorating George Anderson[i] and his party that journeyed to the top on October 16, 1875, noting, “…The ascent of South Dome was accomplished this morning by a party of…” John Muir made reference to “South Dome” in a letter to his sister, Sarah[ii].

Even for a short period of time, it was referred to as Sentinel Rock because it stood at the end of the half-dome-close-up-by-jeff-kreidervalley as if watching over it. The name was soon dropped and applied to the existing “Sentinel Dome” and rock for much of the same reason. As can be seen by the view of Half Dome from Glacier Point, maybe a better name for Half Dome might be “Three-Quarter Dome” or, maybe, “Seven-Eighths Dome.” The original formation that is now called Half Dome was only a little bit fuller than it is now. Granite is formed in the earth’s crust when magma is ejected into the earth’s crust from the core, but doesn’t break the surface. It cools underground slowly and under a great deal of pressure, hence its high density. It is left for further upheavals (such as the plate tectonic action that formed all of the Sierra Nevada) or water and wind erosion to expose the granite. In the case of the Yosemite Valley, it was probably a little bit of all three. Though it is true that no less than three glaciers cut through and helped form most of the valley, it is doubtful that they had anything to do with the shaping of Half Dome

The kind of granite that comprises Half Dome has layers or fissures known as “joints”. Being underground, the pressure helped to hold the dome together. Once exposed, the rock was subjected to natural expansion and weathering, such as the freeze-thaw cycle of water seepage into the joints (this causes the ice’s natural expansion to separate the rock in layers and fall away). But there are other causes, but what those are is not clear. Whatever the cause, the process is called exfoliation and it strips off shells of rock in clean straight sheets. This process could have shaped Half Dome before the Sherwin glacier of about a million years ago. That glacier left only about 900 feet of Half Dome exposed above its surface. Neither of the two major glaciers since (Tahoe and Tioga) came even close to that height. They never even made it to the foot of Half Dome. Glaciation played a much more significant role in the formation of the valley itself, but, apparently, very little with Half Dome. It may be that Half Dome did not complete its metamorphosis until, during, or even after the Sherwin glacier passed through. I’m not positive anyone really knows for sure. The point is that glaciation had nothing to do with its shaping, it was exfoliation.[iii]

Exfoliation is not unusual. You can see it all over the park, what is somewhat unusual is that these joints run perpendicular to the dome rather than along its perimeter. You can see the normal exfoliation of domes readily in the back country. It looks kind of like an onion unpeeling. This is quite clear at Olmsted Point near Tenaya Lake along Tioga Road. Nor is geological exfoliation unique to domes. It happens to all the granite as well, except that we usually call them rock slides or rockfall.

There is a Yosemite Notes video of about 8 minutes long discussing what is known (and not known) about rockfall.

Though the Freeze-Thaw cycle explains some rockfall, it doesn’t explain all of them or even most of them. In fact, according to the USGS[iv] it is unclear what is the cause of most rockfalls[v]. There are a number of known causes, but to pinpoint THIS cause with THAT rockfall isn’t easy. Other causes are vegetation dislodging some sections, earthquakes (of course), and just the normal day/night temperature changes could cause one.

It appears that the rockfall from the face of Half Dome reported a few days ago (July 7, 2015) is an example of the exfoliation and is one of an average of 45 rockfalls a year[vi]. Though a rockfall is not predictable, with the help of current technologies and computer modeling, the USGS and National Park Service can do a “risk assessment” and will relocate or re-purpose buildings to avoid high risk areas. They believe they have reduced rockfall related risk by 95%. In fact, a small slide on February 11, 2014 in Camp Curry was of little concern because the cabins it would have crushed were relocated because of this risk assessment. You would think that rockfall, especially large ones like the one reported earlier this month, would be easy to notice. But this one wasn’t’ reported when it happened, but when it was discovered by climbers. Apparently, it compromised a popular passage to the top of Half Dome via the North Face by climbers using the route. It appears the fall, about 100×200 feet in size, occurred a week earlier.

So what does this mean? If I go to Yosemite, am I going to die? Well, not to put too fine a point on it; Yep, you’re going to die even if you DON’T go to Yosemite. Life is fatal. But the park is taking all kinds of precautions to make sure visitors are safe (witness the risk assessment that had them move some of the Camp Curry cabins). They also outline some things you can do to avoid issues with rockfall. So check their web site for details. As it stands, Rockfall fatalities, are comparatively rare. More common are water related accidents and most common are traffic accidents. Stats are hard to come by, but, all in all, there are less than 20 deaths per year out of about 4 million visitors[vii]. That’s about 1 chance in 200,000 of dying from any cause while in the park. But, to put that into perspective, the chance of dying at all is 1 in 1. You will die. The chance of dying in a car accident is 1 in 272. Death by Suicide is 1 in 115. Dying in an Earthquake is 1 in 153,597[viii]. So if I were you, I’d run over to Yosemite, pick any cliff on the North or South rim and stand under it and wait for a rockfall…At least there, you’ll have a fighting chance.


[i] George Anderson was the first to climb Half Dome, which he accomplished a few days earlier than this climb.

[ii] I wonder if she had a mule…

[iii] The Geologic Story of Yosemite Valley by N. King Huber (1987) Yosemite Association (reprinted 1989)

[iv] See the referenced video.

[v] I would argue that number one cause of rockfall is Gravity…but what do I know?

[vi] However, in 2014 there were 77 counted rockfalls in the park.

[vii] The highest number I’ve seen is less than 20, but as of this writing I have not seen a source the boasts comprehensiveness.

[viii] www.MedHelp.org the 25 most common causes of death