Post by diane9247 on Jul 17, 2015 3:04:32 GMT
...and no one noticed! One of the country's most iconic geological formations is Yosemite's Half Dome in California. It's also one of the most challenging locations in the world for rock climbers and Yosemite National Park is filled with tourists all summer long, many camping overnight. So, you would expect that someone would notice when a five million pound slab of rock crashes to the valley floor.
(Source for above information and photo is SmithsonianMag.)
Nick Stockton reported in Wired that the 2,500 ton rock fell sometime before July 5th, when it was discovered by two climbers that a massive section was missing that would have allowed them to reach the next place to anchor their rope. The slab, measuring around 200 feet tall by 100 feet wide and 3 to 10 feet thick, probably fell off late at night between between July 2 and 3 during heavy rainfall in the park. The natural expansion and shrinkage of the stone due to heat and cold causes cracks, which leads to sloughing off of layers of rock. This natural exfoliation happens about 60 times per year in the park. The heavy rain probably contributed to the rock finally giving way. Presumably, 2,500 tons of falling rock hitting the ground make a very loud noise, which could be heard by rangers and campers day or night. It's a puzzle how this event passed without notice.
Roger Putnam, a geologist and climber quoted in Climbing.com, wants climbers to stay off of Half Dome. "[He] warned that more rockfall is likely in this area. 'I see that as a possible progressing failure up an exfoliation slab,' he said. 'Often, when you see rockfalls coming from underneath a roof at the base of an exfoliation slab, more will follow.'
Illustration from Climbing.com, linked above.
Slab falls off Half Dome.kmz (913 B)
In fact, writes The Guardian’s Caty Enders, the new topography of Half Dome is now so different that “the race is on to be the first to remap some of the most spectacular and influential routes across the new rock face in Yosemite.” Hopefully whoever maps the park’s new look will be paying attention if other rocks fall (and stay out of harm’s way if they do).
Nick Stockton reported in Wired that the 2,500 ton rock fell sometime before July 5th, when it was discovered by two climbers that a massive section was missing that would have allowed them to reach the next place to anchor their rope. The slab, measuring around 200 feet tall by 100 feet wide and 3 to 10 feet thick, probably fell off late at night between between July 2 and 3 during heavy rainfall in the park. The natural expansion and shrinkage of the stone due to heat and cold causes cracks, which leads to sloughing off of layers of rock. This natural exfoliation happens about 60 times per year in the park. The heavy rain probably contributed to the rock finally giving way. Presumably, 2,500 tons of falling rock hitting the ground make a very loud noise, which could be heard by rangers and campers day or night. It's a puzzle how this event passed without notice.
So what happens next? “Climbers are always at their own risk in Yosemite,” says [Greg] Stock, [Yosemite Park's chief geologist], though he adds that the [climbing] community will probably take a break from Half Dome until they can fully assess the stability of the remaining rock by measuring the widths of visible cracks, or using acoustic sensors to listen for subsurface fissures. But the cliff is over 1,000 feet tall, so that’s easier said than done. “We’ve never been able to document anything that’s a clear precursor to imminent rock fall,” says Stock. Occasionally, a gunshot-like cracking sound precedes a collapse, but that’s not usually the case. [Wired]
Illustration from Climbing.com, linked above.
Slab falls off Half Dome.kmz (913 B)
