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updated: 11/3/2010 4:23 PM

Kids ask: How much does a glacier weigh?

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  • Portland State University associate professor and glaciologist Christina Hulbe gets ready to research a glacier.

    Portland State University associate professor and glaciologist Christina Hulbe gets ready to research a glacier.
    Courtesy of Christina Hulbe


Polly Kluver's seventh-grade science class at West Oak Middle School in Mundelein asked: "How much does a glacier weigh?"

Glaciers are very large chunks of ice and snow. Think of a frozen river, one that's been frozen for thousands of years.

Glaciers exist in the very coldest regions of the Earth. They are features on mountains across the continents, except in Australia, where there are no glaciers now.

As glaciers melt, they provide fresh water for plants and animals. Too much glacial melt can cause environmental concerns, though. The study of glaciers is called glaciology and involves math, physics, chemistry and computer programming.

"Measuring the volume of glaciers and ice sheets is a very important topic in glaciology today," said Christina Hulbe, associate professor in the department of geology at Portland State University.

"Most glaciers are getting smaller right now due to global warming. As glacier volume decreases, sea levels rise."

You need a map of a glacier in order to measure it, Hulbe said. The parts of the glacier that are visible can be charted using standard measurement tools. Hulbe said NASA and the National Science Foundation sponsor scientists to study changing ice sheet and glacier volume.

"Scientists use data collected on the surface from airplanes and satellites. They write computer programs to analyze all the data," she said.

It's the underside of the glacier that's tough to measure.

"Sometimes we can use radar to look right through the glacier and make a map of the land surface right underneath the glacier," Hulbe said.

"There are a few other methods as well. Sometimes one method works in a particular place while another method does not."

Once a map is created, scientists can determine the volume, or "the space the glacier occupies in all three dimensions," Hulbe said.

Multiply volume and density that's the mass contained in a specific volume of ice and the result is the weight.

Density is a little tricky because it changes with the depth of the glacier.

"New snow is a mixture of air and ice, mostly air," Hulbe said. "As new snow falls on top of old snow, the old snow begins to compact and the air is eventually either forced out or trapped in bubbles in the ice."

A series of processes can cause the ice to become denser, such as the way the snow settles and changes that occur in the ice crystals.

The changes in ice density from the top of the glacier to the bottom would need to be taken into account when calculating a glacier's weight.

Air bubbles inside the glacial ice tell a story.

"When air that started out mixed in with the snow becomes trapped in bubbles, the bubbles become samples of what the air was like when the snow fell," Hulbe said.

"The oldest air samples yet that have been retrieved from glacier ice in this way are about 700,000 years old and come from ice near the bottom of the East Antarctic Ice Sheet at a place called Concordia Station."

Knowing the weight of a glacier helps scientists to understand how glaciers change over time.

Hulbe said scientists use that information in computer programs that simulate how glaciers flow and respond to changing climate, analyzing what's happening today, as well as events from the distant past.

Check these out

The Wauconda Area Library suggests these titles on glaciers:

• "Why Do Glaciers Grind?" by Helen Bethume

• "Bodies From the Ice: Melting Glaciers and the Rediscovery of the Past" by James Deem

• "An Inconvenient Truth: The Crisis of Global Warming" by Albert Gore

• "The Creation of Glaciers" by Carol Hand

• "Discovering Antarctica" by June Loves

• "Frozen Secrets: Antarctica Revealed" by Sally M. Walker