Icebergs: top to bottom: plunge under Antarctic ice to learn about the ocean's frozen wonders ... (Earth science: icebergs/glaciers).

It's October--high spring in the Southern Hemisphere--when nature photographer Norbert Wu arrives at McMurdo Station, a U.S. research base on the eastern tip of Antarctica's Ross Ice Shelf. The thick slab of floating ice is as large as France (see map, p. 16). "Antarctic waters are exceptionally clear at this time," Wu says. He'll have only a brief window to dive beneath Antarctic icebergs and capture images of a twilight zone few humans have seen.

Every summer--December through February--Antarctica's glacial icecap launches about 5,000 giant icebergs into the sea. Finding one to shoot should be a breeze, but cutting through sea ice to swim in frigid waters will test Wu's mettle.

Undaunted, Wu plunges feet first down a 3 meter (10 foot)-long dive hole in the ice. The sea beneath the ice shelf is-1.8[degrees]C (28.7[degrees]F), chilly enough to freeze blood. "The cold wreaked havoc on my cameras," says Wu. But it was worth the trouble. "There's something special about peering beneath the bottom of the world."

Most experts agree. Read on to find out what they have to say about icebergs.

Q: What's an iceberg?

A: An iceberg is a floating piece of glacial ice. Unlike most continents, Antarctica doesn't have rivers that pump freshwater into the ocean. Instead, it has a two-mile-thick mass of flowing ice called a continental glacier. "Ice is the overwhelming physical presence in Antarctica," Wu says.

The glacier is made of 500,000 years' worth of snow, compacted under its own weight into a rock-solid icecap over 14 million square kilometers (5.4 million sq mi) of land. "The glacier flows very slowly down the continent into the ocean," says engineer David Long at Brigham Young University in Utah. Ice near the coasts can advance 200 m (660 ft) a year. "When ice reaches the edge of the continent, it flows right out onto the sea. So you get enormous floating ice shelves 900 feet thick."

Huge cracks form in these walls of frozen water, and chunks of ice break off into the sea, a process called calving. The planet's glaciers calve more than 100,000 icebergs each year, 90 percent of them from Antarctica, which dumps 46 times the annual flow of the Mississippi river into the sea. "That's how freshwater recycles off the continent, back into the ocean," says Long, who uses satellite images and supercomputers to track Antarctic icebergs. Freshwater helps keep ocean salinity (saltiness) in check.

Q: How long does it take for an iceberg to form?

A: It takes only a few seconds for a block of ice to calve off a glacier and crash into the ocean. Yet it may take thousands of years for that ice to accumulate and flow from the South Pole to the ocean. The average age of ice in an iceberg is 5,000 years.

Q: What's the largest iceberg ever identified?

A: "Last year, the largest iceberg ever observed calved off the Ross Ice Shelf," Long says. It was 340 km (211 mi) in length and 111 km (69 mi) wide. That's larger than the entire state of Maryland.

Q: How is something that big able to float?

A: An iceberg is just a giant ice cube, Long explains. Toss an ice cube into water and it floats. Icebergs float--just barely--because floating has nothing to do with size, but rather density, or mass per unit of volume. Ice is slightly less dense than water, so in equal volumes of water and ice the water will be heavier. But because the densities of ice and water are so close, only the tip of the iceberg is visible above the surface. Nearly 90 percent remains submerged, where the sea gradually melts the ice.

Q: How long do icebergs last?

A: Some bergs survive 12 years, but most don't last that long. "The deciding factor is how fast ocean currents move a berg north toward the equator," says Long. "After a big berg cracks off the ice shelf, it breaks into smaller pieces as it melts." Piano-size pieces are called growlers; chunks the size of SUVs, bergy bits.

The smaller the berg, the faster it melts. A 100 m (328 ft)-long berg lasts about 179 days in--1[degrees]C (30.2[degrees]F) water, and just 5 days in 15[degrees]C (59[degrees]F) water.

Q: Are icebergs dangerous?

A: The iceberg that sank the world's first "indestructible cruise liner," Titanic, on April 14, 1912, certainly was. When the ship rammed the berg, it ripped open the hull 6 m (20 ft) below the waterline. But it's not just icebergs' underwater girth that's hazardous. The ice itself is unstable because the melting process makes bergs prone to sudden cracking and tumbling in the sea.

Q: Is there life on an iceberg?

A: Not only do creatures live on and around icebergs, the ice itself teems with life. As glaciers creep down the continent, they gather microscopic animal and plant life as well as dust and minerals. When a berg calves and begins melting it releases these nutrients into the sea, setting in motion a unique food chain. Add summer sun to the mix, and algae cells in the ice photosynthesize (convert light into food and energy) and rapidly reproduce, turning the water to a cloudy soup. Inconvenient for underwater photography, but excellent for jumpstarting the food chain.

Bright red amphipods (small crustaceans) come to graze on the algae, and icefish feed on the amphipods. Enormous jellyfish billow through the water catching fish with their long tentacles. "Antarctic biodiversity is richer than scientists ever imagined," says Wu. "But you'd never guess it from the surface."

Q: Will global warming affect Antarctica's icebergs?

A: "Science doesn't clearly understand how global warming affects icebergs, but this is something we're actively studying," Long says. Some scientific models predict Antarctic waters could rise 3 degrees in the next 100 years--which could wipe out thousands of the world's most exotic sea animals. "At a temperature rise of two to three degrees, many sea animals asphyxiate [suffocate]," says Lloyd Peck of the British Antarctic Survey.

But that's just part of the complex picture. Antarctica's icecap holds over 7 million cubic miles of ice--90 percent of the world's ice and 68 percent of the world's freshwater. "One thing is pretty clear," says Long. "If it got warm enough that glaciers flowed faster off the continent and formed more bergs, it could raise sea level. And that would have serious consequences for the world."

How Antarctic Iceberg Form

1. Snow dusts the South Pole.

2. The snow builds up inch by inch over millions of years into a compact freshwater-ice dome, or icecap.

3. This icecap flows down the continent to the ocean, where it becomes an ice shelf.

4. The ice shelf floats on seawater up to 1,000 feet deep.

5. Chunks of ice calve off the ice shelf, crashing into the ocean as icebergs.

6. Ocean currents carry icebergs away from Antarctica. Eventually, they melt into the sea.

NAME THAT BERG!

Some icebergs are as large as small U.S. states, others as small as washing machines. The world's largest are flat-topped tabular icebergs that calve from Antarctic ice shelves. As icebergs break up, they're given names to describe their shape. Read the clues on the left. Then match each clue with the photo it describes.

1. A PINNACLE iceberg has floated in the sea for a long time, sometimes several years. As seawater melts the berg's underside, it grows top heavy and eventually rolls over, revealing sculpted peaks and spires.

2. BLUE icebergs cane off the bottom of a glacier, where the ice is under extreme pressure for thousands of years.

3. A BLOCKY iceberg has a flat top and steep vertical sides.

4. Wind and water erode U-shape slots in DRY-DOCK bergs.

5. A WEDGE iceberg's flat side slopes into the water like a chunk of cheese.

Lsson Plans

Did You Know?

* In recent years, two gigantic Antarctic icebergs collided at the shoreline at Cape Crozier, which caused an Emperor penguin breeding colony to fragment into at least five groups. As a result, the colony's population has dramatically dwindled.

* Blue icebergs aren't really blue. They're perfectly clear, which allows light to pass through. Since ice filters out all colors except blue, the iceberg emits blue light.

* Antarctica's icecap weighs so much it actually pushes the continent's landmass below sea level, a process called isostasy. If the icecap melted, Antarctica would rise another 450 m (1,500 ft) above sea level.

Cross-Curricular Connection

History: Research the history of Antarctic exploration. Then draw a map tracing the course of Robert Falcon Scott's ill-fated South Pole expedition.

Critical Thinking:

Pure water normally freezes at 32[degrees]F. The freezing point of seawater decreases about 0.5[degrees]F for each 5 parts per thousand (ppt) of increase in salinity, or salt level. If the salinity of sea water is about 30 ppt near Antarctica, what is its freezing point?

CHECK FOR UNDERSTANDING

Directions: For 1 to 6, match the word in the left column with the correct phrase in the right column. For 7 and 8, circle the correct

word(s) to complete the sentences.

1. salinity     a. suffocate
2. amphipods    b. floating ice chunk
3. asphyxiate   c. salt level
4. icecap       d. vast, flowing ice mass
5. iceberg      r. compact freshwater ice dome
6. glacier      f. small crustaceans

7. An iceberg floats because its (density, volume, mass) is (slightly less than, slightly more than, the same as) that of water.

8. The average age of ice in an iceberg is (50; 500; 5,000) years.

ANSWERS

1. c 2. f 3. a 4. e 5. b 6. d 7. density, slightly less than 8. 5,000

Thrills, Chills and Spills

1. gravity 2. outside, kinetic 3. center of gravity, femur 4. friction

Name That Berg!

1. a 2. d 3. e 4. b 5. c

Resources

"Terminal Ice" by Ian Frazier, Outside Magazine, Oct. 2002

Antarctica and the Arctic: The Complete Encyclopedia by David McGonigal and Dr. Lynn Woodworth, Firefly Books, 2001

Learn how scientists work in Antarctica at this Web site, Secrets of the Ice: An Antarctic Expedition www.secretsoftheice.org