Balancing Act

A 700-ton boulder tilts at an angle on a small point in Colorado Springs, Colorado. The enormous rock looks like it could tip over at any moment. Yet scientists believe that it may have stood in this position for thousands of years! 

This boulder isn’t the planet’s only rock perched in a precarious position. There are many balanced boulders around the world that settled into place because of natural causes. They can form in several ways. In some cases, the rocks were moved into their positions by slow-moving masses of ice called glaciers. Others formed after wind or water wore away parts of rock during a process called weathering (see How Balanced Rocks Can Form).

There’s a 700-ton boulder. It’s in Colorado Springs, Colorado. The rock sits at an angle. Its whole mass rests on a small point. It looks like it could tip over at any second. Yet scientists think it has stood like this for thousands of years! 

This isn’t the planet’s only oddly placed rock. There are many balanced boulders found all over the world. They settled into place because of natural causes. Glaciers, slow-moving masses of ice, can push rocks into tilted positions. Other balanced rocks form through weathering. This process happens when wind or water wears away parts of the rock (see How Balanced Rocks Can Form).

Recently, researchers set out to study some of these odd formations. The rocks contain clues about an area’s past earthquakes. During an earthquake, the giant slabs of rock that make up Earth’s crust suddenly slip past each other, shaking the ground. Strong earthquakes can do major damage to communities.

If a balanced rock is standing today, it means that no earthquake strong enough to knock it down has struck  since it formed (See How Strong Is an Earthquake?). By learning about the history of earthquakes in an area, scientists hope to better understand quakes that are likely to strike in the future.

Recently, scientists studied some of these rocks. The boulders may hold clues about an area’s past earthquakes. Earthquakes occur when the giant slabs of rock that make up Earth’s crust move. The crust is the planet’s outer layer. When the rock slabs suddenly slip past each other, it causes the ground to shake. Strong earthquakes can damage communities.

Balanced rocks remain standing today. That means there hasn’t been an earthquake strong enough to knock the rocks over (see How Strong Is an Earthquake?). This tells scientists something about the area’s earthquake history. And it could reveal what quakes are likely to strike in the future. 

Scientists have long measured earthquakes as they happen using tools called seismometers. These instruments contain a magnet and electronic devices. When an earthquake rattles the ground, the devices record how much the magnet moves. 

To study earthquakes that happened in the past, researchers look at the physical evidence that’s been left behind in rock layers. Scientists observe breaks in rock or marks where rocks scraped against one another. Scientists use this information to build computer models. Studying the models can show how the crust likely moved during the quakes. But there are many questions that these models can’t answer.

Anna Rood is a geologist at Imperial College London. A few years ago, she and a research team set out to study earthquakes in San Luis Obispo, California. That area is an “earthquake zone,” a site where earthquakes often occur. In her study, Rood decided to use a nontraditional tool: the area’s many balanced rocks! She calls these formations “natural scientific instruments.” 

Scientists use tools to measure earthquakes as they happen These devices are called seismometers. They contain a magnet and electronic parts. The magnet moves as the ground shakes. That records the quake’s strength.

Scientists also study past earthquakes. To do this, they look at breaks in rocks. The breaks are caused by the ground shifting. Scientists also look at marks on rocks where they scraped against one another. Scientists use this data to create computer models. The models can show how the crust likely moved during the past quakes. But those models can’t answer some questions.

Anna Rood is a geologist. She works at Imperial College London. A few years ago, she and a team set out to study earthquakes. They visited San Luis Obispo, California. That area is an “earthquake zone.” It’s a site where earthquakes often occur. Rood aimed to use a new tool to study the area—the local balanced rocks! Rood calls the rocks “natural scientific instruments.”