Introducing Professor Incognito!

For my teaching colleagues, I introduce Professor Incognito, who says all the things you want to say, but is not constrained by any sense of politeness or decorum. Incognito is brought to you courtesy of Zeo at the Rant Hour, who in my opinion is one of the sharpest online comic artists and social critics out there (but who is rated kind of a PG to R for language and adult situations). I can't imagine who Zeo used as an inspiration; I almost never say things like this myself......

The Ahwiyah Point Rockfall: the View I Wish I Had

Photos courtesy of the National Park Service. Photographer: Greg Stock, Park Geologist

The National Park Service now estimates that the March 28 Ahwiyah Point Rockfall had a volume of 43,000 cubic meters, or 115,000 tons. This means that the event was almost 50 percent larger than the 1996 Happy Isles Fall (30,000 cubic meters), and is the largest event since the 1987 Middle Brothers slide (1.5 million tons). The airblast was reminiscent of the Happy Isles event.

These pictures, taken by Greg Stock, the park geologist, show the views I wish I had when I was there last weekend. The top photo is from the switchbacks to Snow Creek, and the other is obviously from the edge of the destruction zone.

For some strange reason, rock climbers have been inordinately interested in this event, and the effect it may have had on some of the Half Dome climbing routes. Via Geology.com, here is a comment string replete with photos from a number of climbers.

Tragic Quake in Italy: a peripheral note

Tragedy this week in L'Aquila, Italy, when a 6.3 magnitude quake collapsed buildings and killed 235 people (at last report). Many in the geoblogosphere have commented about the tectonics of the quake (check here for the latest posts), and the seemingly odd juxtaposition of extensional quakes in an ostensibly compressional landscape associated with convergent boundaries.

I don't have much to add to the discussion, but I did want to share a few photos of another town in the Appennines in central Italy where the quake occurred. Gubbio is small medieval village tucked along the mountain flanks of the fault systems that slice northwest across the region. The valley is a prominent fault graben and provides a vivid example of the appearance of the many fault zones in the region.

In Roman times, the inhabitants of the region felt secure enough to live out on the valley plain, under the protection of the Roman Empire. When the empire crumbled, the cities became isolated and vulnerable to attack, so they moved the towns to a more defensible position at the base of the steep mountains. Walls and buildings were robustly constructed out of thick walls of mortar and rock. Unfortunately, little thought was given to the possibility of earthquakes, and many of the buildings (as we saw this week) are seismic death traps.

When a valley sinks along a fault system, a mountain block rises in a relative sense, forming a prominent ridge. Erosion produces deep gorges, and the original fault surface (the scarp) is shaped into a series of triangular facets , separated by wineglass canyons: alluvial fan (base of glass), deep mouth of gorge (the stem of the glass), and the open canyon above (the cup). These features are visible in the photos above. Unfortunately, Gubbio is threatened by the same kind of earthquake activity.

The top photo shows the triangular facet that forms the backdrop of Gubbio, while the second reveals part of the wineglass canyon formed by the erosion of the Bottaccioni Gorge. The pictures are a record of one of the great geologic days of my life: a visit to the Gubbio exposure of the clay layer where the Alvarez team made the discovery of the iridium anomaly that led to a new interpretation of the extinction of the dinosaurs and much of the other life on the planet at the K/T boundary. Unfortunately, I was trapped on a bus that was not under my control, and we didn't have time to stop for better composed pictures.

A Final Look (for now): The Ahwiyah Point Slide in Yosemite

When the Ahwiyah Point Rockfall impacted after a tumble of 1,800 feet, the slabs and boulders of granitic rock exploded, producing an airblast that is reported to have flattened hundreds of trees (and also producing a magnitude 2.4 earthquake). It also pulverized the rock. Completely. As I walked on the north side of the Mirror Lake Loop Trail, I didn't see anything right away, but then I nearly slipped and fell on a granite boulder that would normally provide fine traction. I looked and realized the boulders around me were covered with a fine powder up to a quarter inch thick. It looked like volcanic ash, and it occurred to me and my students that the dust would actually be chemically indistinguishable from rhyolite tuff (granite and rhyolite are coarse- and fine-grained varieties of silica-rich igneous rock). It really didn't even feel gritty, and it got all over our shoes and pants.

We were on the far outer periphery of the impact zone, so only a few weakened tree trunks were knocked over. Several are shown in the second photo.

That's all I have on the slide for now. I hope to be able to get closer to the impact zone on my next trip in the area.

Another Look: The Ahwiyah Slide in Yosemite

The Ahwiyah Point rockfall in Yosemite Valley was probably the largest rockfall since 1996, when the Happy Isles fall killed one person and severely damaged the Nature Center (photo here). Luckily the slide a week ago occurred early in the morning so no one was on the trails covered by the event. Access is a bit tricky, but good views of the entire pathway of the slide can be had from the trail north and east of Mirror Lake (the south side of the loop is covered with rockfall debris). Unfortunately (for us geologists) views of the impact area are not so easy; I didn't have time to bushwack across the river, and it was probably still very dangerous in the impact zone anyway.

The first photo shows the entire rockfall zone, from near the top of Ahwiwah Point to the base of the canyon slope on Tenaya Creek. The high rock on the right side is Half Dome, from an angle not often seen by casual tourists (it requires a one and a half mile hike, after all!). The total length of the fall was 1,800 feet.

The second is a zoom on the release point (sorry for the intervening branches; I took about 30 pictures and zoomed only the one time! There were clearer views elsewhere). The nearby trees provide scale; the rock mass was dozens of yards across.

The third and fourth photos provide the only view I could get of the impact zone. Lots of new talus slopes up there, lots of mangled logs, and if you look very carefully at the top of the talus pile, you can see a single tree that survived the onslaught of falling rock.

One more set of photos in the next post!

First Look: Ahwiyah Rock Fall in Yosemite

For a first look at the latest rock fall in Yosemite National Park, I offer two nearly identical perspectives of Ahwiyah Point: I didn't predict the rock fall! I was able to crop a shot of Half Dome from a trip in 2004 that included Ahwiyah Point and almost match it from a shot taken today. The release point is on the left side, near the top of the cliff. The scar of the rock fall is visible from many, many points in the valley, including perhaps the most famous at the Wawona Tunnel. The appearance of Yosemite Valley is changed forever, even if in a relatively small way (small from a distance, anyway; it is greatly changed from up close)!

Details are starting to emerge about the event, including seismic information. The rock mass fell some 1,800 feet, and more or less exploded on impact, causing an airblast that spread pulverized granite dust throughout the area. The rocks and blast knocked down hundreds of trees, according to a preliminary note from the park geologist, Greg Stock (from the park's daily report at http://www.nps.gov/yose/parknews/upload/daily.PDF). Thankfully, no one was injured.

More pictures soon!

How it was: Yosemite Valley on a Saturday in April

I had the privilege today of taking 25 students to Yosemite National Park on a geology field studies trip; despite living in Modesto, only 2 hours away, some of them had never seen Yosemite Valley before in their lives. The valley did not disappoint (it rarely does): sunny, middle fifties, scattered snow patches, wonderful spring wildflowers in the foothills (very little in the valley), and a bear.

I'm tired tonight, so I leave you with a view from the newly renovated Tunnel View Parking Lot below Inspiration Point. The snow covered peak on the left side is Cloud's Rest, the highest point visible from the valley floor at just over 9,900 feet, and of course the iconic Half Dome on the right. The unobtrusive outcrop in the lower center part of the photo was the site of a significant rock fall a week ago, and we made a reconnaissance trip to the vicinity of the impact. Pictures to follow in the next post!