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At the head of the valley in Yosemite National Park, as if on a pedestal, stands Half Dome, the most colossal and recognizable rock monument in the Sierra Nevada, smoothly rounded on three sides and a sheer vertical face on the fourth. From its summit, over 4,800 feet above the valley, you look southeast into Little Yosemite Valley, which is broad floored and has granite walls more gently sloping than in its larger namesake. Half Dome, which stands nearly 8,800 feet above sea level, is composed of quartz monzonite, an igneous rock that solidified several thousand feet within the Earth. Half Dome, is the remains of a magma chamber that cooled slowly and crystallized beneath the Earth's surface. The solidified magma chamber was then exposed and cut in half by erosion, therefore leading to the geographic name Half Dome. Similar, still molten magma chambers are thought to underlie many active volcanoes.
The geologic story of Yosemite National Park
Geologic Unit: Half Dome Web site.
You can order hard copies of topo maps for a fee or download a free pdf file from our USGS Store Web site. (The product ID numbers for ordering maps of Yosemite National Park & vicinity are: Topographic Folded (45928), Topographic Flat (45929), and Geologic (28027).)
Images of Half Dome can be found at this USGS Web site.
Yosemite National Park Web site.