GEOLOGY

The Other Half of Half Dome

Most geologists believe there never was a fully-developed "other half" to this dome. Present in the granite of Half Dome and in most of the rock formations in Yosemite are numerous parallel, vertical fractures called joints (zones of breakage formed in response to regional stresses) that cause structural weakness. Water seeped into these cracks and widened them through freeze and thaw cycles. Glaciers moving through Yosemite Valley undercut and carved away rock beneath the dome. Without supporting rocks below, the dome was destabilized and a portion (perhaps 20%) of the dome's mass fell away along a joint surface, leaving the sheer cliff which today so markedly identifies "Half Dome."

Geologic Story of Yosemite

Yosemite is a part of the of the Sierra Nevada batholith -- a conglomeration of plutonic rocks that solidified underground and were later uplifted and exposed. About 50 million years ago the landscape was very different than what we see today: rolling hills, broad valleys and meandering streams. About 10 million years ago, the Sierra buckled, tilted and uplifted in response to regional tectonic stresses. The westward-tilting landscape steepened drainages, accelerating river-caused erosion. A series of glacial advances began about two million years ago, gouging out steep-sided U-shaped valleys. Yosemite's last, fairly small glacier left a natural series of rock-debris dams (moraines) near El Capitan, which impounded "ancient Lake Yosemite." Eventually this lake filled in with sediment, leaving today's level Valley floor.

The 1996 Rockfall

Rockfalls occur frequently in Yosemite due to temperature fluctuations, water seeping into crevices and freezing, the effects of growing tree roots and ongoing tectonic stresses. On July 10, 1996, a 80,000 ton slab of granite detached from the southeast slope of the Glacier Point Apron in the Happy Isles area and slid down a 500-foot ramp of granite before hitting a vertical section and free falling 400 feet. The impact generated 160-mph winds that blew down 10 acres of trees. Numerous people were injured and one person was killed by a falling tree. The size of the rock and its lengthy free fall were what made this rockfall so remarkable. The dust created by the impact billowed hundreds of feet into the air and covered everything nearby with up to 2 inches of dust.

There was a large rockfall (about 75,000 tons) between February 22 and March 15, 2000, off of the Middle Brother. These fall areas are clearly visible from the south side of the valley, particularly from Swinging Bridge and the Four-Mile Trail. Approximately 10 acres of trees were leveled.

On June 13, 1999, a 525-ton rockfall occurred from Glacier Point above Curry Village. A rock climber was killed by falling debris. This rockfall resulted in the closing of the Terrace area of Curry Village. There is a large color poster of the rockfall behind the Visitor Center Desk. This rockfall is best viewed from Stoneman Meadow and looks like an American Bison (Buffalo) running to the west.