“Park staff, and there were people in the campground, who reported hearing some popping sounds,” says Nathan Hatfield, the supervisory park ranger of interpretation at Aztec Ruins National Monument & Chaco Culture National Historical Park. “Shortly thereafter, the rockfall occurred.”
“There is a structure very close to where this rockfall occurred – it’s a cliff dwelling and we think it was built approximately around the 1130s,” Hatfield says.
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The National Park Service posted photos of the rockfall on July 27. Courtesy NPS.
The rocks fell from a cliff near the park’s Gallo Campground. Park staff temporarily closed five campsites with no timeline on when they might re-open. Officials say there might be a connection between the intense heat New Mexico saw in July and the recent rockfall.
“We’re hearing that this most recent rockfall event, one of the factors that lead up to this could have been the excessive heat we’ve been having,” Hatfield says. “They’re saying it’s the hottest month in the recorded history of our planet.”
National Park geomorphologist Eric Bilderback took a look at pictures of the rockfall and says cracks in the rock are expected, given how the rocks formed under pressure and underground. But heat can speed up the spread of those cracks and ultimately lead to a rockfall.
“The rock temperature in the area is hitting its annual high,” Bilderback wrote to the park. “There have been high rock temperatures at 10 cm depth of over 40 degrees C (104° F) this month starting on the 13th of July. Rock temperature has been daily cycling between about 25° and 40° C (77-104° F), this can certainly drive rock fracture for these kinds of exfoliation slabs.”
Bilderback says temperatures usually decrease in August. But there’s no guarantee.
This isn’t the first rockfall at Chaco Canyon. The most infamous event was in 1941 when aptly named “Threatening Rock” came crashing down. The rock was a 30,000-ton sandstone slab that had separated from the canyon wall above Pueblo Bonito, one of Chaco Canyon’s most famous historical structures dating back around 1,000 years.
Threatening Rock was – as its name implies – a looming threat long before it fell. Ancestral Puebloans even built support structures for the massive rock that towered above their buildings.
The National Park Service says snow and rain may have been the straw that broke Threatening Rocks’ balance that fateful day in 1941. Now, the park actively monitors key sections of rock above Chaco Canyon sites.
“We actually have devices that are up on the canyon wall that are giving us information about what’s happening with temperature and potential movement,” Hatfield says. “We’re monitoring. And if the data tells us that something could happen . . . we’ll have to make a decision on what to do.”
“In some cases, if the rockfall is going to be far enough away from any kind of facilities or archaeological sites, I’m guessing we would just let nature take its course,” Hatfield says. “If we determine that there’s going to be rockfall in very close proximity to facilities or archaeological sites, then we’re going to have to figure out: ‘What can we save?'”
To some extent, rockfall is just an inevitable part of the site’s wonder. “You can drive along the canyon, and you can see slabs that you can tell: That’s going to fall one day,” Hatfield says. “It might not be for a hundred years or a thousand years from now but, it could fall.”