El Capitan

El Capitan's rock was once hot, melted rock deep under the ground.

It cooled very slowly and became hard granite. Much later, glaciers carved the valley next to it.

Granite forms from magma that cools very slowly miles underground, growing large crystals that lock together, the National Park Service explains. The NPS also says El Capitan's 3,000-foot vertical face shows how strong its granite is and how few cracks it has.

Most of the granite in the Sierra Nevada formed between 120 and 85 million years ago. Starting about 3 million years ago, glaciers flowed down Yosemite's valleys, deepening and widening them.

NPS granite: an intrusive igneous rock crystallized from magma miles underground; slow cooling grows large interlocking crystals. Magma from subduction rose to chambers 3-10 miles down, building plutons; about 35 plutons in the Yosemite area, named for landmarks (the El Capitan Granite). Yosemite lies in the 70-mile-wide, 300-mile-long Sierra Nevada Batholith, mostly emplaced 120-85 million years ago.

NPS geology: from about 3 million years ago glaciers repeatedly advanced from the range crest and deepened and widened stream-cut valleys; the Tioga Glaciation began about 26,000 years ago and peaked about 18,000 years ago. UNESCO: Yosemite Valley is a 1 km deep, glacier-carved cleft with massive sheer granite walls.

El Capitan is made of more than one kind of rock.

Some parts are light, and some parts are dark.

El Capitan is not just one rock color. On its southeast face, a dark rock called the Diorite of North America pushed into the older, lighter El Capitan and Taft granites.

Even this famous face was not mapped by geologists until 2011 to 2013. The team used climbing, remote sensing and chemistry to make the first geologic map of the wall.

NPS: on the southeast face, the dark, hornblende-rich Diorite of North America intruded the older, lighter, quartz- and feldspar-rich El Capitan and Taft granites. The 3,000-foot southeast face remained unmapped until a team made the first geologic map between 2011 and 2013 using remote sensing, climbing and geochemistry; the map reveals the vertical structure of plutons and interactions between intrusive suites. NPS: few places display the internal complexity of a batholith better than El Capitan.

Trees grow at the bottom of the rock wall.

Meadows and giant trees grow in other parts of the park.

UNESCO says Yosemite's cliffs, domes and waterfalls are a backdrop for mountain meadows and groves of giant sequoias, and that the park ranges from 600 to 4,000 meters high, with a great variety of plants and animals.

In the photo, a band of dark green trees grows along the foot of El Capitan.

UNESCO (Yosemite National Park, 308): repeated glaciations created soaring cliffs, domes and free-falling waterfalls; these features form a scenic backdrop for mountain meadows and giant sequoia groves; elevations 600-4,000 m support a great variety of flora and fauna.

The photo shows a band of trees at the base of the wall.

Sometimes rocks fall from Yosemite's cliffs.

Out in the sun, wear a hat and sunscreen.

The National Park Service says rockfalls are the most active geological process in Yosemite today. Rain, earthquakes and even heating by the sun can set them off, and they pose a real risk to people.

Out in the sun, the CDC advises a hat with a brim all the way around, sunglasses and sunscreen of SPF 15 or higher, even on cloudy days.

NPS geology: rockfalls are the most active geological process in the park, triggered by rain, earthquakes and even solar heating; they continue to shape the valley walls and pose considerable risk to people.

CDC sun safety: UV reaches you on cloudy and cool days; use shade, protective clothing, a wide-brimmed hat, UVA/UVB sunglasses and broad-spectrum sunscreen of SPF 15 or higher.