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Balancing Act

Rocks perched at odd angles may hold clues about future earthquakes

By Jennifer Barone

From the May/June 2021 Issue

Key Focus Area: Plate Tectonics and Landforms,

Other Focus Areas: Earth's Materials & Systems

As you read, think about how earthquakes can change natural landscapes.

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).

How Balanced Rocks Can Form

The balancing boulders in the American Southwest often form over millions of years through a process of weathering. Here’s one way that can happen.

A diagram of cracks forming between underground rock

1. Cracks form in bedrock, a layer of solid rock that sits at ground level or beneath a layer of soil or sand.

A diagram of weathered rock rising to the surface of the ground

2. Water seeps into the cracks and breaks down bits of rock around them. The bedrock breaks into large chunks.

A diagram of weathered rock balancing on other rocks pushed to the surface of the ground

3. Wind and water wear away the ground. The rocks settle on one another. Some are perched at odd angles.

Think: What happens to the bits of rock worn away by wind and water?

^{_{Images: Alex Gontar/Shutterstock.com}}

How Balanced Rocks Can Form

The balancing boulders in the American Southwest often form over millions of years through a process of weathering. Here’s one way that can happen.

1. Cracks form in bedrock, a layer of solid rock that sits at ground level or beneath a layer of soil or sand.

2. Water seeps into the cracks and breaks down bits of rock around them. The bedrock breaks into large chunks.

3. Wind and water wear away the ground. The rocks settle on one another. Some are perched at odd angles.

Think: What happens to the bits of rock worn away by wind and water?

^{_{Images: Alex Gontar/Shutterstock.com}}

A person examining a large boulder with another boulder perched on top

Anna Rood & Dylan Rood

SAN LUIS OBISPO, CALIFORNIA:
Geologist Anna Rood examines one of the area’s many balanced rocks.

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.

How Strong Is an Earthquake?

Scientists measure earthquake strength using the moment magnitude scale. Here is how earthquakes of different strengths can affect areas near the place where the crust shifts.

A cartoon of a glass of water with tiny ripples

LESS THAN 2.5
Quakes not often felt.

A cartoon of a lightbulb swinging side-to-side

2.5-5.4
Houses shake, and items fall off shelves

A cartoon of a house shaking and its windows shattering

5.5-6.9
Windows and walls may crack. Buildings have some damage.

A cartoon of a house whose roof and walls have cracked

7.0-7.9
Windows and walls may crack. Buildings have some damage.

8.0 or greater
Large buildings and bridges collapse.

^{_{Illustrations: Shutterstock.com}}

How Strong Is an Earthquake?

Scientists measure earthquake strength using the moment magnitude scale. Here is how earthquakes of different strengths can affect areas near the place where the crust shifts.

LESS THAN 2.5
Quakes not often felt.

2.5-5.4
Houses shake, and items fall off shelves

5.5-6.9
Windows and walls may crack. Buildings have some damage.

7.0-7.9
Windows and walls may crack. Buildings have some damage.

8.0 or greater
Large buildings and bridges collapse.

^{_{Illustrations: Shutterstock.com}}

Shaky Science

A huge red boulder perched on a tall rock

iStockPhoto/Getty Images

MOAB, UTAH:
This 3,600-ton boulder in Arches National Park sits on a thin rock that has been wearing away for millions of years.

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.”

Ancient Rocks

Rood’s team focused on seven balanced rocks at the San Luis Obispo site. For each rock, they asked two questions: How long has the rock balanced in place? How much force would it take to topple it?

To learn the age of the balanced boulders, Rood’s team took samples of each one. They tested the samples for a rare chemical element that builds up on rock surfaces over time. The boulders were much older than scientists thought. Some had stood in place for 20,000 years!

Rood also took detailed photos of the rock formations. She used the images to build a digital model of each balanced rock. Other researchers had figured out how to calculate how much the ground would need to shake in order to tip over a rock. Their tests involved using machines to shake small rock structures until they tumbled.

Using these calculations with her models, Rood was able to determine the size of the earthquake needed to topple each rock. Since the actual boulders are still standing, Rood knows that no quake that large has struck since the balanced rocks formed.

Balanced Boulder Locations

The fact that balanced boulders are still standing means that no earthquake strong enough to topple them has shaken their area. This map shows the locations of the balanced rocks described in the article and the risk of experiencing a damaging earthquake across the U.S.

Balanced Boulder Locations

Rocking Out

Scientists assume that an area’s future earthquakes will be similar to what has happened in the past. Now that Rood understood the types of earthquakes that might have already struck the area, she was able to estimate what the future might look like.

Earlier groups of scientists had used other methods to calculate the earthquake risk near San Luis Obispo. Rood’s research suggested that the strongest earthquake expected in the next 10,000 years was about 27 percent weaker than what those researchers had found. That’s great news for nearby communities!

Rood is currently working on another study of balanced rocks across Southern California. Meanwhile, the impressive rock formations continue to draw tourists, amazed that they’re still standing. If you ever spot a balancing boulder in nature, Rood says, don’t knock it over: You might erase thousands of years’ worth of data!

Continue Your Learning

Have you ever experienced an earthquake? Write a short story about the experience. If you don’t live in an earthquake-prone area, try to imagine what one of those events would be like. What would you see, hear, and feel? How would you react?

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