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Mighty Hiker
climber
Vancouver, B.C.
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Mar 28, 2012 - 07:25pm PT
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The rockfalls from El Cap Tree area, starting spring 1977? Or did someone mention them already?
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Melissa
Gym climber
berkeley, ca
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Mar 28, 2012 - 09:02pm PT
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No worries. Did we win a prize?! :-)
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Minerals
Social climber
The Deli
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Mar 28, 2012 - 10:07pm PT
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Wasn’t the Space rockfall in January? I don’t remember exactly, but have a vague recollection that it was during winter. ’97 sounds about right; it was in the late ‘90s. The fresh talus and mangled oak trees were covered with a ton of rock dust for at least a few months after the event and there were a bunch of impact marks on the slabs below The Alcove where the blocks first impacted.
Roger and I heard the rockfalls on 6/13/99 and 6/15/99 on Glacier Point while we were on the Falls Wall. 10:13 PM sounds right for the second rockfall (as listed in the database that Clint linked to); I remember hearing it around 10 PM.
The first A5 Adventures catalog has a hand-drawn topo on the inside of the back cover – “El Capitan, S.E. Face Topos” – and the Footstool rockfall is listed as ’78. I’m not sure if this is correct or not. There have been several releases out of the Footstool scar, some as recent as the last 5 or 10 years because the talus keeps changing. The sharp, mega tombstone that was so cool is no longer standing at the base; it was right up next to the wall, on the far right side of the scar, at the edge of the trees. Greg, it’s the tombstone that Kent is leaning against in Figure 6. (C) in his 2001 GSAB paper (to be specific…).
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gstock
climber
Yosemite Valley
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Topic Author's Reply - Mar 29, 2012 - 12:56pm PT
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Just for persepctive, January 1997 was during the big flood, after which the park was closed for many weeks for repairs. Does this timing sound right for the Space rockfall?
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Melissa
Gym climber
berkeley, ca
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Mar 29, 2012 - 01:35pm PT
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I sent a note to Chris. Maybe he'll remember more.
Have you guys sifted through old rec.climbing posts? Might be something there.
This one wasn't as big as the others, but it was kind of a wild realization...In August of 2002, I think, MInerals led us on geology tour. We went to the base of Mecca to see "his favorite rock", but it was missing. I was taking a load off on a big boulder, and when I looked next to where my butt was parked, there was a 3/8" bolt and hanger affixed. My seat had apparently replaced the favorite rock. He could probably tell you when his last visit to the not-yet-smooshed rock would have been.
The moderate (compared to the one on the right side from Aug. 2006) sized fall I saw from by Trundling Juan was likely March 2004.
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Minerals
Social climber
The Deli
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Mar 31, 2012 - 05:22pm PT
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Greg, that sounds about right, but again, I am not 100% sure. I suppose it could have also been in February. If the rockfall occurred while the park was closed for repairs after the flood, then there obviously wouldn’t be any climbers around to witness the event. Is this why none of us can really remember exactly when it was? Where’s Werner and his internal database of Valley history?
I seem to remember talking to climbers about the rockfall in the spring, not too long after it happened, and no one knew exactly when it cut loose but figured that it was in January because when they showed up in late winter/early spring, it had already occurred. The flood peaked on January 2nd, so it would make sense if the extraordinary amount of rainfall and snowmelt took a few days or a couple of weeks to seep through the fracture systems on El Cap and eventually contributed to greater freeze-thaw action. But I really don’t know for sure…
Questions: With respect to diurnal fluctuations of the surface temperature of the rock on El Cap, would this be greater during the winter than in the summer? During the summer, nighttime air temperatures are relatively warm and even though daytime air temperatures can be really hot, the angle of the sun is fairly oblique to the vertical surface of El Cap. During the winter, nights are much longer and air temperatures dip well below freezing, yet the sun is much lower in the sky and so the angle of the sun is much more direct on the face of El Cap. On clear winter days, sunlight would quickly warm the surface of the rock on El Cap while the south side of the Valley would remain shaded and frigid. But, how about the effects of colder daytime air temperatures and convection? The base of El Cap can feel like a toaster in the winter while El Cap Meadow feels like an icebox (although this is also due to a temperature inversion). Does this make any sense?
How do heating rates of granitic rock compare to cooling rates of granitic rock when in direct sunlight versus when in the shade/dark, and how much of a factor is rock albedo? We would expect dark diorite to reach a higher temperature than light granite when in direct sunlight, but I am curious about heating and cooling rates.
With a quick Google search, I found some answers here:
http://web.unbc.ca/~lindgren/pdf/hall_et_al_2005.pdf
(Edit: POS box of electrons doesn’t recognize the entire link… Copy and paste…)
Yeah, Melissa, that was fun! Here’s a photo of part of the boulder that was still intact in ’98 or ’99. It was pretty cool looking – coarse-grained gabbro (related to the Diorite of the Rockslides) that was brecciated by the intrusion of numerous aplite/leucogranite dikes. Too bad it was decimated. It seems that there have been several releases from Lower Cathedral Rock over the last couple of decades.
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