Geochemistry, Mineralogy, Volcanology Feature

The Delicacy of Arches

Living in Geologic Time: How long will Utah’s arches grace the red rock desert?

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Most people know Utah’s most famous rock feature, Delicate Arch, from the state’s license plate and the America the Beautiful commemorative quarter, but if you’ve ever hiked to its bow-legged base in Arches National Park, you know it’s really not all that delicate: The soaring 16-meter-high arc of Entrada sandstone is the largest freestanding arch in the park.

On geologic timescales, Delicate Arch will exist for only the blink of an eye. But don’t despair—although all the famous arches will crumble and collapse within a few thousand years, replacements will continue to be sculpted out of Utah’s bedrock for a very long time to come.

How Old Are Arches?

Arches National Park, near the town of Moab, boasts around 100 arches with openings that span at least 1 meter across. If the catalog is expanded to include smaller arch openings, over 2,000 can be found within the 310-square-kilometer park.

Arches can form in many types of rock, but around Moab, Entrada sandstone is the star. Originally laid down in the Jurassic period between 180 and 140 million years ago as sand dunes and beaches, this coppery orange sandstone is relatively soft and readily sculpted by flowing water, as well as freeze and thaw cycles.

Around Arches National Park, the Entrada layers tend to erode into fins along parallel fractures, setting the stage for arches to form over thousands of years. As water flows over and around these fins, weaker areas of rock will erode away faster, sometimes creating an alcove or shallow cave that eventually wears into a hole through the fin. Once a hole forms, chunks of rock fall from the ceiling, enlarging the opening, in some places creating arches more than 90 meters across.

Two holes eroded through a sandstone fin, creating Double O Arch in Arches National Park.
Double O Arch stands in front of a lineup of sandstone fins that may erode into arches in a few thousand years. Credit: Mary Caperton Morton

Arches come in many shapes and sizes, but they all follow the same engineering principles that govern bridges and cathedrals: Certain shapes, especially circles and arcs, are stronger than others. Arches naturally erode into self-supporting shapes with the weight of the overlying rock compressing the base of the structure into a strong abutment, much like how the thick base of a stone wall supports the arched roof of a cathedral.

Entrada sandstone dates back to the time of the dinosaurs, but the arches themselves are much younger, with most existing arches likely forming within the past few thousand years, said Paul Geimer, a geophysicist at the University of Utah in Salt Lake City. Erosive features are notoriously hard to date, he said, because most of the diagnostic evidence is transported away by wind and water. “Our best guess is that the smaller arches are on the order of hundreds to thousands of years old, while the largest arches may be in the range of 100,000 years old.”

Ever Changing Arches

In 1968, Arches park ranger Edward Abbey wrote Desert Solitaire: A Season in the Wilderness, his ode to Arches and the one book I would choose to keep me company if I were marooned on a desert island. At that time, the park was a little-known and seldom-seen national monument.

Arches has changed dramatically in the years since it became a national park in 1971: The roads are paved, the campground has flush toilets, and at least 43 arches have collapsed. The risk of rockfall hitting park visitors is low, said Terry Fisk, chief of resource stewardship and science for the southeast Utah group of national parks. “Rockfall is pretty rare, and it’s even rarer that it’s actually witnessed by people.”

A thin band of rock stands against a bright blue sky at Ring Arch.
Chunks of rock fell from Ring Arch in 2014, leaving only a thin band of rock standing in the Courthouse Wash area of Arches National Park. Credit: Jeff Moore

The park service does not prevent people from walking up to, under, and around most arches in the park. “It’s difficult to justify closing the area around an arch based on the potential for rockfall,” Fisk said. “It could happen today or 500 years from now. We just hope people pay attention when they’re out there.”

A few decades ago, people could walk or even drive cars on top of some features. “Now we prohibit people from walking on, scaling, or swinging on the arches,” Fisk said. “They’re not jungle gyms.”

Corona Arch photographed from underneath, showing a curve of red rock against a blue sky
Corona Arch is one of hundreds of arches found outside Arches National Park. This arch was used to erect a giant rope swing until the extreme sport was banned at Corona and other arches in 2017. Credit: Mary Caperton Morton

I’ve visited dozens of arches both inside and outside the park and always hold my breath when I walk under them to admire the improbable curve of red rock against a bright blue sky. In those moments, I sometimes flash back to a dramatic rockfall I witnessed in New Mexico. That morning, I was sitting outside my camper, enjoying the damp desert after a rare overnight rainstorm. The moment the first rays of light hit the top of one white sandstone fin, it exploded. With a startling shotgun crack and a rumble, the whole top of the formation collapsed, spraying bright white rock all over the ground—a startling reminder that geologic time really does include now!

Over the past 15 years, the Geohazards Research Group led by Jeff Moore at the University of Utah has been using seismometers and other noninvasive methods to monitor the stability of arches. “These arches are iconic, even sacred, and draw millions of people from all over the world,” he said. “So of course, there is a lot of pressure to balance visitation with conservation.”

Moore and colleagues have collected data on 25 arches in the national park, as well as other arches and rock formations outside the park. Their research is being used to better understand which features are most vulnerable to erosion, as well as visitor impacts. Their findings were summarized in two recent reports in Geophysical Research Letters and Geomorphology.

Landscape Arch arcs across a fence and sign warning “Do not go beyond fenceline!”
Long, flat arches like Landscape Arch are under more stress than more curvilinear structures. Credit: Mary Caperton Morton

By installing seismometers on the arches and assimilating the data into computer models, the team studied how individual arches move and vibrate in response to wind, earthquakes, and helicopters.

“Arches are just like guitar strings that are plucked by the wind,” Geimer said. “The thinner they are, the more flexible they are.” An arch’s shape also plays a significant role in its overall stability. U-shaped cantilever arches like Delicate Arch are actually quite strong and stable, whereas long, flat arches like Landscape Arch are under more stress.

Small earthquakes regularly strike Utah and the Colorado Plateau, and the ground vibrations generated by the quakes are greatly amplified in the arches. “Earthquakes shake these features very strongly,” Geimer said. “The vibrations come through loud and clear.” The team has also recommended that helicopters steer clear of arches and natural bridges like Rainbow Bridge to protect the rock features from human-made vibrations.

Kachina Bridge is a large arch spanning a creek in Natural Bridges National Monument, Utah.
Kachina Bridge in Natural Bridges National Monument, Utah, is a natural bridge, a type of arch that forms over a creek or river. In 1992, approximately 4 tons of rock fell from the underside of the bridge. Credit: Mary Caperton Morton

The team’s studies have also shown that arches respond to daily and seasonal temperature fluctuations. Five years of continuous monitoring of North Window Arch by an instrument nicknamed the “crackmeter” revealed that a crack running through the center of the formation moves regularly as the rock expands in the desert heat and contracts at night.

“Rocks seem so stable, but these studies have shown that arches are more sensitive than you might think,” Fisk said. “It makes me wonder how much modern vibrations are playing a role in degrading the life span of arches.”

A Delicate Future

The continued life spans of individual arches are as mysterious as their age, but longevity is likely on the order of tens of thousands of years, Geimer said. “I’d like to think Delicate Arch will stand for thousands more years, but we really can’t say for sure,” Fisk said.

In the meantime, new arches will keep eroding into existence out of the 100-meter-thick wedge of Entrada sandstone that underlies southern Utah. “If you look around the park, you see so many huge fins and alcoves that are on their way to becoming the next North Window Arch,” Geimer said.

A few thousand years may sound like a long life span on human timescales, but in the grand scheme of geology it’s vanishingly fast. “I feel very fortunate to exist in the same eyelash blink of geologic time as Delicate Arch,” Fisk said.

—Mary Caperton Morton (@theblondecoyote), Science Writer

Living in Geologic Time is a series of personal accounts that highlight the past, present, and future of famous landmarks on geologic timescales.

Citation: Morton, M. C. (2020), The delicacy of arches, Eos, 101, https://doi.org/10.1029/2020EO146523. Published on 13 July 2020.
Text © 2020. The authors. CC BY-NC-ND 3.0
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