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Messages 7901 - 7920 of total 8661 in this topic << First  |  < Previous  |  Show All  |  Next >  |  Last >>
Oplopanax

Mountain climber
The Deep Woods
Apr 20, 2015 - 11:28am PT
Better annotated photo, thanks to Mike for the before and after!

Note lowest scar: green outline, brand new dihedral!
Then probably the orange stuff failed in sequence, and this removed enough material for the red block to go as a single mass.
Credit: Oplopanax
Big Mike

Trad climber
BC
Topic Author's Reply - Apr 20, 2015 - 12:05pm PT
Drew you stole my thunder! ;)

Here's the original before and after.




After, little further out


Before from the bluffs


Original after


Original before

TheSoloClimber

Trad climber
Vancouver
Apr 20, 2015 - 12:20pm PT
It looks like that massive block underneath it is just waiting for a reason to go as well.
Big Mike

Trad climber
BC
Topic Author's Reply - Apr 20, 2015 - 12:31pm PT
From way higher up, massive block realized and rotated out from the face. I could see debris at the toe of the block ravelling off, sensing something big was about to happen. The sound was incredible. Anybody who has climbed Angel's Crest knows that the sound echos off across the North Gully and the North Walls. At first I saw the base of the block moving and dust clouds forming. I've watched more than my share of rock fall and landslide videos through school and work and for fun, but this was a first experience for me.

It was hard to judge how close it was to me. The view of the mountains was momentarily blocked by the free-falling granite. There was no rotation, just one monster chunk of granite aimed straight down. I looked up and Mark, jaw-dropped and speechless. Once the block hit the forest below, a huge plume of rock dust engulfed the area below. The volume of rock is estimated at 1000-5000 cubic metres. I couldn't see the parties below at this point but hoped that they were ok.
Richard So

http://richso.blogspot.ca/2015/04/angels-crest-and-zodiac-wall-rockfall.html

edit
ya Nathan. could be.. the boot looks sketchy too though and it's still there...
Impaler

Social climber
Oakland
Apr 20, 2015 - 01:02pm PT
I'm really glad this pitch was far enough to the left. Probably the best dihedral pitch on the planet.

Pitch 2 of Calling
Pitch 2 of Calling
Credit: Impaler
Big Mike

Trad climber
BC
Topic Author's Reply - Apr 20, 2015 - 01:07pm PT
The base of your pic is the impact zone.
Tami

Social climber
Canada
Apr 20, 2015 - 01:13pm PT
What Nathan said. I was thinking that exact same thing when I read his post. Looks like detachment all around. Maybe it's also held on by the brown poo that held the upper part.

Wow all around. Great shots Mike & thanks for the link to Richard So. Nice blog.
Big Mike

Trad climber
BC
Topic Author's Reply - Apr 20, 2015 - 01:18pm PT
I think all that brown stuff is evidence that water and dirt was getting behind there for awhile. If you look at Dru's orange and green outlines, the rock fractures there are clean. So I think that thing was probably just sitting there being held by the flakes and pillar below it.

The upper right corner of the block fracture looks fairly clean too so, that was probably holding it as well.

Gstock? :)
Big Mike

Trad climber
BC
Topic Author's Reply - Apr 20, 2015 - 01:36pm PT
Supercrop of the scar
Big Mike

Trad climber
BC
Topic Author's Reply - Apr 20, 2015 - 01:55pm PT
Before supercrop


Notice all the dirt on the slope below? and in the groove above it. This thing's been lurking for awhile I bet.



It would appear that the block slid over/impacted that slab. If that lower chunk was about to go, one would think that it would have gone then??
Oplopanax

Mountain climber
The Deep Woods
Apr 20, 2015 - 02:23pm PT
Hard to blame this one on rainfall and pore pressure along the fault since we're in the middle of sunny and dry conditions.

Given the dirt behind the main scar, I think root wedging from the trees on the ledge might be to blame. We're in the middle of spring and the trees are in the middle of their annual growth spurt, putting on mass in their roots too.

Of course, the response time to glacial unloading can be tens of thousands of years, so in our time scale, things can fall off for no immediate cause. They are responding to an event (deglaciation) thousands of years ago.

Hydraulic conductivity varies along joints too. But it's hard to state with a straight face that the pore pressure varies enough that this thing popped off now in response to record rainfall months ago.
Big Mike

Trad climber
BC
Topic Author's Reply - Apr 20, 2015 - 02:28pm PT
Maybe a combination of factors Drew? Perhaps the roots wedged it out enough to fracture the support structure below?

Tami

Social climber
Canada
Apr 20, 2015 - 03:37pm PT
I think it was group suicide of the trees on that ledge. Too bad they didn't ask for help....

Ha.

But I agree with the note about it being that time of year when roots are growing fast; that might have had some influence on this release.

gstock

climber
Yosemite Valley
Apr 20, 2015 - 03:45pm PT
You guys are up against the problem we often deal with in Yosemite in terms of rockfall triggering, namely that several possible environmental triggers are present (water, root wedging, etc.) but there is no "smoking gun". Plenty of Yosemite rockfalls occur on bluebird days when you just wouldn't expect it.

The fresh scar in Mike's photos clearly shows weathering and staining indicative of probably decades of seepage, plus some residual soil. This suggests that the block was partially detached for quite awhile.

However, I'm most intrigued by the reports of precusor rockfalls that occurred in the cleaner rock beneath the big block; if that was indeed the case, the partial detachment and weathering of the big block may have been mostly irrelevant, as the block was undermined by smaller rockfalls that fell for some other reason.

Speculation from afar...

In case you get really interested in this stuff, the Yosemite rockfall database and a paper we wrote on the progressive Rhombus Wall rockfalls in 2009-2010 are available here: http://www.nps.gov/yose/learn/nature/rockfall.htm

Big Mike

Trad climber
BC
Topic Author's Reply - Apr 20, 2015 - 03:52pm PT
Greg there are reports of new routers hearing small gravel sized rocks falling last week. They thought someone was throwing rocks from the top!

Perhaps the roots made enough space for the medium behind the block to escape?
Oplopanax

Mountain climber
The Deep Woods
Apr 20, 2015 - 04:00pm PT
Causes of rockfall are of professional interest to me.

In this case there definitely were two separate, smaller, audible rockfalls in the half hour before the big one.

It's also true that Paul M reported small rocks falling while new routing in the area over the last week.

This does point to progressive loss of support for the big block that ultimately fell, and it also suggests that frequent, small rockfalls can serve as a useful indicator for a future larger rockfall, which is an easy idea to convey to non-geoscientists.
Big Mike

Trad climber
BC
Topic Author's Reply - Apr 20, 2015 - 04:39pm PT
I've never really heard rockfall while climbing on the Chief, so i would say it is definitely abnormal to hear it. If i heard stuff falling down like Richard did that morning i'd think twice about climbing there that day..
Jim Brennan

Trad climber
Canada
Apr 20, 2015 - 05:15pm PT
I think there's some route wedging at the base of the Split Pillar that needs remediation.
Jim Brennan

Trad climber
Canada
Apr 20, 2015 - 07:31pm PT
Rock fall is a wild card for climbers...

Dave V. Craig T and I were ready to do Exasperator for some mid day kicks. A rock slipped by what held it, crashed into Bellygood and continued to the ground, peppering us puny humans.

We stepped left and ran right with only hope and every man for himself on offer...

There were people on the Grand and people on Cruel Shoes. It didn't matter, the stones stopped whizzing and in the space of 1 minute, everything was pleasant and the same again.

Big Mike

Trad climber
BC
Topic Author's Reply - Apr 21, 2015 - 08:58pm PT
Nice story Jim. I seem to remember having some shrapnel fall on us when we were at Exasperator too.. Maybe i'm just blocking out all the close calls? :)
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