"Not being a geologist, I can't get very technical but it just went kaboom," Chief Ranger Denny Ziemann said. "The middle of the arch just collapsed under its own weight. It just happens."
Wall Arch, located along the popular Devils Garden Trail, was 71 feet tall and 33 1/2 feet wide, ranking it 12th in size among the known arches inside the park. Lewis T. McKinney first reported and named Wall Arch in 1948.
No one reported observing the arch collapse and there were no visitor injuries, the National Park Service said.
Geologists from the Park Service's Geologic Resources Division and the Utah Geologic Survey visited the site Thursday and noticed obvious stress fractures in the remaining Wall Arch formation, spokesman Paul Henderson said in a statement. Rock debris completely blocked a section of the trail about a mile from the Devils Garden parking area.
As a result, the trail has been closed just beyond Landscape Arch. Landscape is the longest arch in the park, which, with 2,000 natural arches, contains the largest number of these rock formations in the world. Chunks of Landscape Arch have fallen in recent years as well but the arch itself remains intact.
The trail will remain closed until visitor safety issues can be resolved.
"Geologists are concerned it is not done falling," said Ziemann, who could not recall the last time a major arch at the park collapsed. "It could be 24 hours or 2,400 years. It's kind of a mystery. We'll watch it for a while and will know soon. With the middle missing, the weight shifts and the remaining pieces will ultimately fall. It's a matter of how long."
Ziemann said the trail closure extends from Double 0 Arch to Wall Arch. If the rest of Wall Arch falls soon, the Park Service will clear off the trail to make it passable. If it continues to teeter over the trail, it may be a while before the trail reopens.
Robert Iberia of the Moab Area Travel Council called the collapse "geology in action. You hate to see those go down. But it happens. It's a natural phenomenon."
The Park Service's Henderson noted that all arches are temporary features and will eventually succumb to the forces of gravity and erosion. While the geologic forces that create the arches continue, in human terms it's rare to see such dramatic changes as a major collapse of an arch.