Fatal Rockslide in Major National Park—What You Need to Know

On rare occasions, Yosemite's unpredictable cliffs can create dangerous conditions for park visitors.

Witnessed the Yosemite Valley El Capitan rockslide from Half Dome this afternoon. Sorry to hear at least one fatality.

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At least one person is dead and another injured after a deadly rockslide fell from El Capitan in Yosemite National Park in northern California.

The rockslide began near the park's "Waterfall Route" on the East Buttress of El Capitan at around 1:55 p.m. Wednesday afternoon. Shortly after the rockslide began, the area was surveyed by park helicopters and rescue workers transported the affected park visitors to receive emergency medical care outside the park.

At this time it's unclear if the injured people were climbing the rock face or walking below it. However, in a statement, the National Park Service noted that it's currently climbing season in Yosemite, meaning a number of climbers have been in the area.

See First Video of Most Dangerous Rope-Free Climb Ever (Alex Honnold)

While the size of the rockslide is still being determined, photos show plumes of dust rising from the fall's impact. El Capitan is one of the nation's most recognizable national park landmarks. The steep, sheer cliff face is frequented heavily by climbers, and at 3,000 feet, it's one of the most challenging climbs in the park.

Fatal climbing accidents in the park are quite rare, but not unheard of. An analysis by the National Park Service found that about 100 accidents occur in Yosemite every year, but they're rarely fatal. From 1970 to 1990, 51 climbing deaths were reported. Many of the accidents, the report noted, were preventable, but around 10 percent came from rockfalls.

Park officials are still accessing what caused the incident. Rockslides, a type of landslide, are defined as a rocky avalanche falling from a cliff or mountainside. Many disturbances, from earthquakes to erosion, can trigger them.

A detailed study of El Capitan was conducted in 2013 by researchers hoping to better understand the rare but fatal rockfalls. Using rock samples and laser scans of the cliff's topography, they mapped the mountain's granite, finding some areas were weaker than others. (Learn more about the effort to map El Capitan.)

Predicting how and when a rockslide can occur, however, is difficult, leaving climbers exposed to danger.

In June of this year, climber Alex Honnold made history as the first person to free solo climb up the face of El Capitan, without any safety ropes, an endeavor that took him almost four hours. His climb was preceded by weeks of surveying the cliff to mark key hand holds to assist him up the mountain. (Honnold explained what it was like to climb El Capitan to National Geographic.)

The NPS recommends experienced climbers ascend El Capitan.