Balancing Act

In Colorado Springs, Colorado, a boulder named Balanced Rock stands delicately upright on a small point, looking as though it could tip over at any moment. The rock seems to defy the laws of physics, yet it may have perched in this unlikely position for thousands of years—making it beloved by locals and attracting millions of visitors annually.

Scientists call such landforms precariously balanced rocks. They can form in many ways. In some cases, ancient glaciers—slow-moving masses of ice—deposited the boulders. Balanced boulders can also form through weathering, as wind or water wear away rock.

A boulder named Balanced Rock stands in Colorado Springs, Colorado. It rests upright on a small point. The rock looks like it could tip over at any moment. It seems to defy the laws of physics. But it may have perched in this unlikely position for thousands of years. Locals adore the rock, and it attracts millions of visitors each year.

Scientists call such landforms precariously balanced rocks. They can form in many ways. Some were left behind by ancient glaciers. Glaciers are slow-moving masses of ice. Balanced boulders can also form through weathering. That’s when wind or water wears away rock.

The formations are more than just curiosities. They can reveal a lot about an area’s geologic history—particularly when it comes to earthquakes. The fact that a balanced rock remains standing tells scientists that a quake strong enough to topple the boulder hasn’t occurred in its lifetime.

That idea prompted a team of researchers to investigate the unusual rocks. Teetering boulders can provide important information about the strength of an area’s past earthquakes—which could help scientists estimate future risks.

The formations aren’t just interesting objects. They can reveal a lot about an area’s geologic history—especially earthquakes. A balanced rock is still standing. That tells scientists that a quake strong enough to topple it hasn’t happened in its lifetime.

A team of researchers decided to study the unusual rocks. Balanced boulders can tell them about the strength of an area’s past earthquakes. That could help scientists estimate future risks.

Scientists create detailed computer models to understand a region’s earthquake risks. Their calculations rely partly on historical data from seismometers—instruments that record ground movement. But reliable seismometers have been operating for only the past century or so. Information about older quakes comes from physical evidence they left behind, such as breaks in rock layers or marks where shifting rocks scraped against one another. There’s often a lot of uncertainty in this data.

Precariously balanced rocks represent an additional source of seismic information. Geologist Anna Rood of Imperial College London says the rocks are natural scientific instruments: They act like “inverse seismometers,” she says. “They record earthquakes that haven’t happened, just by the fact that they’re still standing.”

To find out what the rocks could reveal about earthquake risks, Rood and a team of researchers headed to a spot that’s home to many balanced boulders in San Luis Obispo, California. The area also happens to be an earthquake zone.

Scientists create computer models to help understand a region’s earthquake risks. They use past data from seismometers. These instruments record ground movement. But good seismometers have been around for only about a hundred years. Scientists learn about even older quakes from physical evidence. For example, past quakes left behind breaks in rock layers. And moving rocks scraped against each other, leaving marks. But there can be a lot of uncertainty in this kind of evidence. 

Precariously balanced rocks provide another source of earthquake information. Anna Rood is a geologist at Imperial College London. She says the rocks are natural scientific instruments. They act like “inverse seismometers,” she says. “They record earthquakes that haven’t happened, just by the fact that they’re still standing.”

What could the rocks reveal about earthquake risks? Rood and a team of researchers wanted to find out. So they headed to a spot in San Luis Obispo, California. It has many balanced boulders. The area also happens to be an earthquake zone.

Rood’s team chose seven precarious rocks at the site. For each one, they determined two key pieces of information: how long it has balanced in place, and how much force would be required to topple it.

To learn the rocks’ ages, Rood’s team tested the boulders for a rare isotope, or one of multiple forms of the same element. An isotope called beryllium-10 forms only on rock surfaces that have been struck by cosmic rays—high-energy particles from outer space. The longer a rock surface has been exposed, the more beryllium-10 it contains. Previously, scientists believed that most balanced rocks in California were about 10,000 years old. Rood discovered that some at her research site had stood in place for twice that long.

Rood’s team chose seven precarious rocks at the site. For each one, they answered two key questions: How long has it balanced in place? And how much force would be needed to topple it?

Rood’s team tested the boulders to learn their ages. They looked for a rare isotope, or one of multiple forms of the same element. An isotope called beryllium-10 forms on rock surfaces. But it forms only if the surfaces are struck by cosmic rays. These high-energy particles come from outer space. The longer a rock surface is exposed, the more beryllium-10 it contains. Scientists thought most balanced rocks in California were about 10,000 years old. Rood found that some rocks at her site had balanced for twice that long.

To find out how much shaking it would take to knock the boulders over, Rood took photos of them and the points where they made contact with the surface beneath. She used these images to make a digital 3-D model of each boulder. “Then we thought about the forces that act on a rock during an earthquake,” says Rood. “A medium-sized earthquake might make a boulder wobble back and forth. A really big earthquake could push it over.” That tipping point happens when the boulder’s center of mass—the point around which an object’s mass is distributed—moves so that it’s no longer above the area supporting the rock’s base.

Previous experiments by other researchers had revealed how to calculate the force needed to tip over a rock. The tests involved shaking rock samples on machines until they tumbled. By applying these calculations to her computer models, Rood was able to estimate the strength of the earthquake needed to topple each of her rocks.

Rood also figured out how much shaking would be needed to knock the boulders over. She took photos of them and the place where they touched the surface beneath. She used these images to make a digital 3-D model of each boulder. “Then we thought about the forces that act on a rock during an earthquake,” says Rood. “A medium-sized earthquake might make a boulder wobble back and forth. A really big earthquake could push it over.” The boulder’s mass is arranged around a point called its center of mass. The tipping point happens when the center of mass moves too far. When it’s no longer above the area that supports the rock’s base, the rock falls over.

Other researchers had already done experiments on the force needed to tip a rock. They shook rock samples on machines until they tumbled. Rood used this research on her computer models. That way she could estimate the strength of the earthquake needed to topple each of her rocks.

Scientists had already made a detailed assessment of earthquake risks at Rood’s research site. That’s because along with balanced rocks, the location is home to the Diablo Canyon nuclear power plant, which for safety reasons had to be built to withstand quakes. Rood’s team compared this existing assessment with their new results. Their findings suggested that the biggest expected earthquake over the next 10,000 years would be about 27 percent weaker than previously projected. That’s great news for the nuclear plant and surrounding communities.

Their results also suggest that data from balanced rocks could reduce the uncertainty in local earthquake estimates by half. But scientists shouldn’t overhaul current risk assessments just yet. “These are models based on very detailed studies and data,” says Rood. No individual part seems wrong. Before making changes, says Rood, it’s important to find out why previous assessments varied so dramatically from her team’s findings.

Rood is planning a larger study of rocks from several sites across Southern California. Bringing together data from multiple locations may help unravel the key factors needed to make more accurate earthquake risk estimates. Rood looks forward to getting out into the field with balanced boulders again. And if you ever spot a cool balanced rock in nature, she says, you should resist the urge to give it a push: You might unwittingly erase tens of thousands of years’ worth of important data!

Scientists had studied Rood’s research site before. They’d made a detailed report on earthquake risks. That’s because the spot doesn’t just contain balanced rocks. It’s also home to the Diablo Canyon nuclear power plant. For safety reasons, the plant had to be built to withstand quakes. Rood’s team compared this previous report with their new results. They found the strength of the biggest earthquake expected over the next 10,000 years. Their results said it would be about 27 percent weaker than the old report said. That’s great news for the nuclear plant and local communities.

Rood’s results suggest that data from balanced rocks could help in another way. It could remove half of the uncertainty in local earthquake estimates. But scientists shouldn’t redo the risk reports just yet. “These are models based on very detailed studies and data,” says Rood. None of the parts seem wrong. It’s too soon to make changes, says Rood. First, they must find out why older reports were so different from her team’s findings.

Rood is planning a larger study. It will include rocks from several sites across Southern California. She will bring data together from multiple locations. That may help reveal the factors needed to better estimate earthquake risks. Rood looks forward to working in the field with balanced boulders again. Someday, you might spot a cool balanced rock in nature. But don’t give it a push, says Rood. You might accidentally erase tens of thousands of years’ worth of important data!