Weather, water and people led to the collapse of the Double Arch at Lake Powell
The falling water level of Lake Powell
With Lake Powell’s water levels declining, many people are concerned about how to maintain access to the lake and how to preserve it for the future.
Mark Henle, The Republic
For decades, visitors to Rock Creek Bay in Utah’s Glen Canyon National Recreation Area have marveled at Double Arch, a rock formation carved out of sandstone by wind, water and other natural resources. But the forces that created it may have contributed to its demise.
On August 8, Double Arch collapsed into Lake Powell. While scientists believe all arches have a finite lifespan, Double Arch’s collapse was likely accelerated by weather, environmental factors, and human stressors.
“What happened is part of a natural process in the life cycle of an arch,” said Jeffrey Moore, a geologist at the University of Utah. “We expect arches to collapse at some point, and the high number of events in the last year shows that these are ephemeral landforms that are constantly changing and sometimes collapsing.”
The arch probably formed about 190 million years ago in the Navajo Sandstone and dates from the Triassic to early Jurassic period.
Arches develop cracks over time and become larger when exposed to water, wind, seismic vibrations and other elements. Moore says that if an arch is naturally geometrically stable, it is more likely to withstand disturbances for longer.
While it’s impossible to say what ultimately caused the arch to collapse, Moore said it was likely a mix of natural and man-made disturbances that accumulated over time.
“This event reminds us of our responsibility and need to protect the natural resources surrounding Lake Powell,” said Michelle Kerns, recreation area manager. “These features have a lifespan that can be impacted or damaged by human activity.”
What role the weather plays
Moore says weather has a major impact on the formation and ultimate collapse of natural monuments like arches. Wind, rain and temperature changes affect rock formations and enlarge cracks in them.
Wind causes vibrations that can enlarge cracks. Temperature fluctuations can trigger thermal elastic expansion and contraction, or cause a stone to expand and contract as it heats and cools.
The process can occur over a 24-hour period as temperatures rise and fall with the sun, or seasonally as temperatures tend to get warmer or colder.
Precipitation also plays a role. If an arch is wet after heavy rain, the stone will become heavier until the water evaporates, compromising stability.
Rain can also cause a salt weathering effect: rainwater can carry dissolved salts into cracks in the sandstone. When the water evaporates, the salts crystallize and expand, exerting pressure on the stone.
Over time, this process causes cracks and weakens the sandstone, which may also have contributed to the gradual decay of the Double Arch.
What role does water play in
Double Arch was located above Lake Powell, and although the reservoir was only recently added to the landscape, changes in water levels and waves crashing against the rock may have contributed to its long life.
The arch was formed thousands of years before the Glen Canyon Dam formed Lake Powell in the 1960s. Water levels rose as the reservoir filled, but have dropped sharply over the past two decades due to a relentless drought, and rise and fall with the amount of runoff.
“Over time, when the sandstone is flooded, some of the cement that holds the sand grains together in the sandstone can dissolve,” Moore said. “That weakens the rock.”
Although the arch has been exposed to the lake’s waters since the dam was built, Moore says fluctuations are “more problematic.” Cracks tend to expand more quickly in humid or wet conditions, especially when the stone is prone to getting wet and drying cyclically.
Falling water levels or waves hitting the stone can cause rapid wet and dry cycles. Repeated contact with water has likely increased the cracks and weakened the strength of the sandstone.
Human activities contributed to
Although rocks seem sturdy enough to withstand anything, natural landmarks are more fragile than you might think.
“When arches collapse, it reminds us that they are fragile,” Moore said. “They are in this critical state of stability throughout their development. Cracks get bigger, pieces fall down.”
“There are more recent factors. Bows evolved over tens of thousands of years, and within that time span, the rise in human energy has only occurred in the last 100 years,” he added.
Vibrations from human activity can affect the stability of arches. Moore has observed people walking on arches, pushing off of them and even driving over them with vehicles. He says arches that are within easy reach of people are at greater risk.
The National Park Service has banned potentially harmful activities around arches throughout the Southwest, but some activities are unavoidable. If an arch is located near a highway, vibrations from vehicles can reach the arch and cause additional stress.
“It’s a reminder to be respectful,” Moore said. “Think of them as fragile, delicate and naturally beautiful.”
Hayleigh Evans writes about extreme weather and related topics for The Arizona Republic and azcentral.com. Email her story tips to [email protected].
