Yosemite’s Iconic El Capitan Mapped in High-Res 3-D

New geologic map helps scientists understand ancient volcano’s roots and contemporary rock falls.

While many graduate students spend days in a lab or in front of a computer, Roger Putnam, a master's candidate at the University of North Carolina, spent up to three days at a time on the sheer face of a cliff, suspended thousands of feet above the ground.

At night, he slept on a tiny platform. During the day, he climbed up the face of El Capitan, taking rock samples and detailed field notes along the way. (See "Climbing El Capitan.")

Putnam, a student of Allen Glazner, a professor of geology at the University of North Carolina, was gathering data for the first-ever high-resolution geologic map of El Capitan, one of America's most iconic natural landmarks and a jewel in the crown of Yosemite National Park in central California. The work was supported by the National Geographic Society.

"This map of El Capitan is the only map with this kind of detail on a 3,000-foot (900-meter) tall cliff anywhere," Glazner said about the project's outcome. (Related: "El Capitan: Scaling the Wall.")

The new geologic map of El Capitan's southeast face will be published soon in a scientific journal. Putnam said researchers in Switzerland are already using it to better understand rockfalls, which have killed more than a dozen people in Yosemite.

"My biggest hope for the map is getting people thinking about the complexity of the granite that makes up Yosemite," said Putnam. "I hope it will reveal complex processes happening tens of kilometers below the Earth's surface." (See: "Climbing Yosemite.")

Glazner added that previous geologic surveys of El Capitan had relied on rough mapping done about a hundred years ago, largely by geologists on the ground looking through scopes. "We had a very poor understanding of the geology of that face," said Glazner. This despite the fact that the dramatic granite cliffs of El Capitan are a key landmark in the northern Yosemite Valley.

In the past few years, Glazner and his students had begun to map the complex pattern of joints and different igneous rock types that make up El Capitan, but they had done so only on the bottom and the top of the cliff face, where they could access the rocks without the difficulty of technical climbing.

Painstaking Process

To get a more complete and detailed picture, Putnam spent six weeks doing fieldwork on El Capitan, in May and June of last year. During that time he climbed the entire face three times and rappelled all the way down twice. He also climbed numerous shorter routes, always working with an experienced climber as partner.

"I suppose I was the perfect person for the job because I had already climbed El Capitan 12 times, and I was very comfortable up there," said Putnam. After getting his bachelor's degree in geology from Vassar College in 2006, Putnam spent six seasons in Yosemite, doing high-altitude ecological restoration work with the National Park Service.

Because he was doing fieldwork, Putnam had more to worry about than just climbing knots. "Climbing El Capitan is a very difficult task, so doing that while getting work done was challenging. But I felt blessed to apply my love of that rock to my master's project," he said.

Putnam chose his routes carefully to get a close look at rock that had never been studied up close. He examined the layers and chipped off small samples of rock for geochemical analysis back at the lab.

"I took every effort to sample in a way that could not be noticed by any climbers, because as a climber myself I would be loath to hear of someone chipping away at a route," said Putnam. He was also careful not to drop anything on the way up. "Even a piece of rock the size of a sugar cube could really hurt someone down below, so I couldn't just hammer away," he said.

Time on the wall was precious. The climbers had to haul a gallon of water per person per day, including an extra gallon as a backup, which added up to about 56 pounds (25 kilograms) of water for a three-day climb. With sleeping platforms, food, and climbing gear included, the total weight they had to hoist up the face was about 100 pounds (45 kilograms).

To bolster his efforts, Putnam enlisted local climbers to take photos along their routes on El Capitan. Twenty-nine climbers contributed information to widen the database. Putnam estimates there are at least a hundred different climbing routes on the massive mountain.

"Crowdsourcing was a really cool way of having the climbing community work with the Park Service and academia," said Putnam. "It is sometimes a contentious relationship, and 'El Cap' is a central rock in the climbing ethos."

On the ground, the team made a series of high-resolution photographs of every nook and cranny in the cliff rocks. They used a digital camera mounted on a GigaPan, a small robotic mount that allows the camera to focus on one small area at a time; all the frames are then stitched together into a detailed record.

Yosemite's park geologist, Greg Stock, supported the effort by taking LiDAR (laser topographic) scans of the face of El Capitan, also from the ground. (Stock co-wrote the 2010 book Geology Underfoot in Yosemite National Park with Glazner.)

When he returned to North Carolina, Putnam overlaid the photos and the LiDAR data in a Geographical Information Systems (GIS) program to make the geologic map of the face. He ended up with more than 850,000 individual points representing different features in the rock, so it was painstaking work.

As Putnam built a 3-D model of the physical structure of El Capitan, he also populated it with information about the specific makeup of the individual rock layers, based on information learned from his climbs. "That made for a well-rounded study of this rock," he said.

Investigating a Rockfall

Before he started mapping El Capitan, Putnam had assisted Stock in assessing rockfalls in the park. The National Park Service hopes that important work will help keep climbers and visitors safer from hazards. Over the years, more than a dozen people have died in the area in rockfalls.

One prehistoric rockfall in particular had long intrigued Stock. Its remnants lie on the floor of the valley below El Capitan. It was a huge rockfall, said Stock, consisting of millions of tons of material. Stock dated the fall to about 3,600 years ago and said if, as recent evidence suggests, Native Americans had witnessed it, they would have found it "absolutely devastating."

"There would have been a thunderous sound; it certainly would have shaken the ground, and if you were within a half mile of it you probably would have perished," said Stock. "It filled the entire valley with dust for hours, diverted a river, and changed the face of Yosemite Valley."

Geologists had long been puzzled about exactly where on El Capitan the rockfall originated. Thanks to Glazner and Putnam's mapping work, Stock was able to match the specific granite in the fall's debris with a zone of the same granite high up on the face of El Capitan.

"It came from the top few hundred feet of the cliff, and then the rock was free-falling for about 2,500 feet (760 meters). That explains why it had so much energy when it hit the ground," said Stock.

Understanding a Mountain

El Capitan is actually the remnant of an ancient volcanic system. "A hundred million years ago," Glazner explains, "there would have been a big volcano on top, [and] the stuff we see as El Capitan today would have been about 10 kilometers (6.2 miles) deep; it's the remnants of the volcano's plumbing system." That's because the weaker rock that surrounded the core of the ancient volcano eroded away, leaving the central plug, which is harder granite, exposed.

"Most of the time you are looking at a complexly deformed system," he said, "often one that had been turned on its side over time, but here it has stayed vertical, so we can get a good look at it."

Thanks to details revealed by the new map, Glazner said he has learned there were different magma types inside the volcano over a shorter time span than he previously thought. "It's given me a much greater appreciation for how complex things are," he said.

Glazner added that the map may also give climbers a better sense of the complexity of the rocks they struggle with. "It's very important to understand the geology," he said.

Stock said he hopes the Park Service can develop some interpretive displays and other materials based on the map, "to teach people about the complex history of granite, and show how it continues to be sculpted by rockfalls," he said. An estimated four million people visit Yosemite National Park every year.

"It's challenging to be mapping a vertical cliff like that, to think deeply about the rocks when you're also trying to remember that your knots are tied correctly," said Stock. "It's very all-consuming, but it's great."

Putnam added that the view from El Capitan remains breathtaking. "It's different depending on where you are on El Cap," he said. “It never disappoints.”

Photograph courtesy xRez Studio; overlay and legend courtesy Roger Putnam

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