Half Dome, Yosemite. Image scanned from a photo taken in 1965!
"Half Dome is a granite dome at the eastern end of Yosemite Valley in Yosemite National Park, California. It is a well-known rock formation in the park, named for its distinct shape. One side is a sheer face while the other three sides are smooth and round, making it appear like a dome cut in half. The granite crest rises more than 4,737 ft (1,444 m) above the valley floor."
About 25 million years ago, the North American plate started overriding the Pacific plate. As the leading edge of the Pacific plate was pushed into the hot mantle below, it melted, sending bubbles of magma up through layers of continental crust. Half Dome, El Capitan, and many of Yosemite's other peaks began as these bubbles, called intrusions. The bubbles didn't actually reach the surface. But over time, they were exposed as ice age after ice age sent glacier after glacier carving through the nascent Sierra Nevadas. Softer rock cut away, leaving the tough, granite intrusions sticking out—Half Dome among the largest and most striking.
The last glaciers retreated about 15,000 years ago. Since then, the dominant process shaping the granite extrusions of Yosemite Valley has been exfoliation. This is exactly what it sounds like: thin flakes of rock sloughing away. "Rock will expand or contract as it heats and cools," says Stock, Yosemite's chief geologist. That means it swells out when the sun is up, and shrinks in the shade or nighttime. But the heating is never even—because the sun hits different parts of the dome at different times. Or not at all."
Half Dome, Yosemite. Image scanned from a photo taken in 1965!
"Half Dome is a granite dome at the eastern end of Yosemite Valley in Yosemite National Park, California. It is a well-known rock formation in the park, named for its distinct shape. One side is a sheer face while the other three sides are smooth and round, making it appear like a dome cut in half. The granite crest rises more than 4,737 ft (1,444 m) above the valley floor."
About 25 million years ago, the North American plate started overriding the Pacific plate. As the leading edge of the Pacific plate was pushed into the hot mantle below, it melted, sending bubbles of magma up through layers of continental crust. Half Dome, El Capitan, and many of Yosemite's other peaks began as these bubbles, called intrusions. The bubbles didn't actually reach the surface. But over time, they were exposed as ice age after ice age sent glacier after glacier carving through the nascent Sierra Nevadas. Softer rock cut away, leaving the tough, granite intrusions sticking out—Half Dome among the largest and most striking.
The last glaciers retreated about 15,000 years ago. Since then, the dominant process shaping the granite extrusions of Yosemite Valley has been exfoliation. This is exactly what it sounds like: thin flakes of rock sloughing away. "Rock will expand or contract as it heats and cools," says Stock, Yosemite's chief geologist. That means it swells out when the sun is up, and shrinks in the shade or nighttime. But the heating is never even—because the sun hits different parts of the dome at different times. Or not at all."