Rock­fall by the num­bers: the stats and sci­ence be­hind Yosemite’s con­stantly ex­fo­li­at­ing gran­ite.


YOSEMITE IS FAMED for its per­fect gran­ite, but the ge­o­logic re­al­ity is that the Val­ley is a dy­namic land­scape in which the cliffs are con­stantly slough­ing. Take the Reg­u­lar North­west Face of Half Dome: In July 2015, it shed a mas­sive amount of stone, chang­ing the char­ac­ter of the clas­sic for­ever. Here, Park Ge­ol­o­gist Greg Stock and oth­ers help us un­der­stand the forces at play— forces, given the dan­ger of rock­fall, that are of great im­por­tance to climbers. ( For more, visit climb­­fall.)


Num­ber of rock­falls of all sizes recorded each week in Yosemite, on av­er­age. Many small rock­falls go un­de­tected, how­ever.


Years ago that Yosemite’s glaciers re­treated, re­veal­ing kilo­me­ter- tall un­sta­ble cliffs. Since then, the gran­ite domes have con­tin­u­ously shed their top lay­ers of rock. The fall­ing rock be­comes talus and the boul­ders that climbers now fre­quent, in­clud­ing the blocks in Camp 4 and Half Dome Vil­lage ( for­merly Curry Vil­lage).


The first year a Yosemite rock­fall was doc­u­mented. James Ma­son Hutch­ings, busi­ness­man and early pro­moter of Yosemite, wrote in his Hutch­ings’ Cal­i­for­nia Magazine that the de­bris was “said to cover over thirty acres.” Stock in 2011 up­dated the his­toric en­try, stat­ing that the fall may have oc­curred on the Lost Brother for­ma­tion of Pro­file Cliff at Taft Point, and was 50,000– 500,000 cu­bic me­ters in vol­ume. The writ­ings of famed nat­u­ral­ist John Muir also contributed to nu­mer­ous early records of rock­falls. Since then, ge­ol­o­gists have doc­u­mented over 1,000 rock­falls. Now, the Yosemite rock­fall data­base is used to look for pat­terns and trends.


Es­ti­mated per­cent­age of rock­falls doc­u­mented in the data­base (the rest ei­ther oc­cur at night and/ or in low- traf­fic ar­eas). Of these, ge­ol­o­gists at­tribute 29 per­cent to wa­ter leak­ing into cracks and build­ing up pres­sure be­hind the outer flakes. Other causes—in­clud­ing earthquakes, freeze-thaw cy­cles, and snowmelt— make up 18 per­cent. Causes for the rest re­main elu­sive: Ei­ther there’s too lit­tle info or sci­en­tists can’t de­ter­mine a cause.


Num­ber of wash­ing ma­chines (vol­ume: 1 cu­bic me­ter) needed to equal the Val­ley’s ma­jor rock­fall events, of which there is roughly one an­nu­ally.


Num­ber of peo­ple killed in a Novem­ber 1980 rock­fall that crashed down upon the up­per Yosemite Falls Trail. At least 19 were in­jured. Rock­falls in Yosemite cause rel­a­tively few fa­tal­i­ties com­pared to the Merced River or traf­fic ac­ci­dents, but a big rock­fall in a fre­quented area can be cat­a­strophic.


Num­ber of peo­ple killed by nat­u­ral rock­fall in Yosemite since record­keep­ing be­gan. Of these, only a small por­tion were climbers. Most in­fa­mously, in June 1999, rock shards show­er­ing down from 1,000 feet up Glacier Point Apron killed Peter Ter­bush as he be­layed his part­ner on Apron Jam ( 5.9; see climb­ing. com/ ter­bush). How­ever, hu­man- caused rock­falls kill far more peo­ple.


Num­ber of build­ings de­stroyed or dam­aged by rocks break­ing off below Glacier Point and tum­bling into Half Dome Vil­lage on Oc­to­ber 8, 2008. For­tu­nately, there were only mi­nor in­juries.


Num­ber of climb­ing rangers a nd vol­un­teer climb­ing ste­wards at the Yosemite Climb­ing Man­age­ment of­fice. The rangers team up with ge­ol­o­gists like Stock to help build a bet­ter un­der­stand­ing of rock­falls on Yosemite’s com­pli­cated cliffs. ( Read the full story of their ef­forts here: climb­ing. com/ rock­fall.)


Tons of rock that peeled off the fifth pitch of Kali Yuga on Half Dome dur­ing three rock­falls in June 2003. One climber at the base of the cliff tripped and broke an an­kle while run­ning away; two oth­ers re­turned to Camp 4 “pale as ghosts”— per­haps both from shock and the dust that cov­ered them as a re­sult of the rock­fall.


The year Stock and fel­low ge­ol­o­gist Brian Collins dis­cover that heat can trig­ger rock­fall. Ev­ery day—but most dra­mat­i­cally in sum­mer—flakes of rock ex­pand out­ward dur­ing warm hours and con­tract at night. This causes the flakes to grad­u­ally crack and split away from the rock, un­til they break off. The re­search may ex­plain why a sur­pris­ing amount of rock­falls oc­cur, seem­ingly spon­ta­neously, on sunny days.


The num­ber of pitches lost from the Reg­u­lar North­west Face of Half Dome dur­ing a July 2015 rock­fall. The sheet of rock was about 200 feet tall and 100 feet wide, and ripped off from pitches 11 and 12, above the Rob­bins Tra­verse. Now, climbers cir­cum­nav­i­gate the blank scar by us­ing a bolt lad­der to move over onto Arc­turus, from where they ei­ther toss a knot to pull them­selves into the chim­ney sys­tems of pitch 13 or climb higher on Arc­turus and pen­du­lum back onto the RNF. The up­per Death Slabs ap­proach and bivy ar­eas at the base of the climb are di­rectly below the scar, which is framed by flakes of loose rock— Climb­ing Ranger Eric Bis­sell warns that these ar­eas may be hit by fall­ing rock.


The year the United States Ge­o­logic Sur­vey pub­lished their “Quan­ti­ta­tive rock- fall haz­ard and risk as­sess­ment for Yosemite Val­ley, Yosemite Na­tional Park, Cal­i­for­nia” ( Greg M. Stock, et al.). The study mapped out a “haz­ard zone” most prone to rock­fall, al­low­ing the Na­tional Park Ser­vice to move struc­tures out of harm’s way. A row of camp­sites at Camp 4 was re­lo­cated far­ther from the cliff, and sev­eral cab­ins in Half Dome Vil­lage were also closed, free­ing up climb­ing po­ten­tial on pre­vi­ously in­ac­ces­si­ble boul­ders. Two times since these and other pre­ven­ta­tive ac­tions, boul­ders have crashed into the old foot­prints of cab­ins.


Newspapers in English

Newspapers from USA

© PressReader. All rights reserved.