Mega melt: get set for Ice Age 2: The Meltdown with our science-guide to glaciers.

A squirrel-like creature claws its way up a giant wall of ice. This animal--named Scrat--is after an acorn lodged high up in the ice. Scrat pries the acorn loose--POP. Suddenly, a stream of water gushes through a hole in the ice. As Scrat plugs the hole with his claw, another spout forms, and another. Finally, the hapless critter tries to drink up the water, causing his belly to balloon.

The gusher is just one sign that the chilly environment that set the backdrop in the original Ice Age film is ending. In the sequel, Ice Age 2--which hits theaters March 31--Scrat, Manny the woolly mammoth, and the entire cast of characters are enjoying a warm spell. But soon, they realize that the toasty weather means melting ice, which could release torrents of water and flood their home.

Thawing ice ages and massive floods ... could this really happen? To find out, Science World asks scientists, including Ross MacPhee, a paleontologist at the American Museum of Natural History in New York, who was a consultant for the film.

JUST CHILLIN'

When the new movie opens, the animals are still living in an icy landscape. MacPhee says that this ice-covered world is not just Hollywood fantasy. Ice ages have occurred many times in Earth's past.

The most recent ice age (see Nuts & Bolts, p. 17) began approximately 4 million years ago. By roughly 100,000 years ago, Earth had entered a glacial period, or a time during an ice age when temperatures plunge. During that time, 30 percent of Earth's land was covered with massive sheets of ice, such as glaciers. These moving slabs of ice form when the amount of snowfall exceeds the melting of snow.

With each new snowstorm, the fresh snow layer presses down on the layers beneath it. Gradually, the lower sections get compacted like a well-packed snowball. Over hundreds of years--and many snowfalls--the underlying snow turns into a thick slab of ice. Some sheets of ice can tower at a whopping 3,000 meters (9,840 feet).

Often called "rivers of ice," glaciers slowly creep over land. How? A valley glacier forms high up on a mountainside that overlooks a valley. Once the depth of the glacier reaches 30 to 40 m (100 to 130 ft), Earth's gravity, or the force that pulls two objects together, tugs the glacier downhill.

CRACKING THE ICE

But Earth's frozen coating during an ice age is just temporary. When the climate (average weather in an area) warms up--called an interglacial period--glaciers melt. "If more ice is melting than accumulating, glaciers begin to shrink," says Tom Hooyer, a geologist at the Wisconsin Geological and Natural History Survey.

That's what happens in Ice Age 2. As the glacier in the movie thaws, water begins to puddle and slick, wet surfaces of ice form. The characters splash and slide in the new environment. "They see this melting world as a tropical paradise," says Lori Forte, the movie's producer.

But the fun doesn't last. This "water park" is located in a bowl-shaped valley surrounded by glaciers. Also in the valley is a towering wall of ice. Curious, Manny climbs up this ice dam and peeks over the wall. What's behind it? A giant lake of water--and the water level seems to be rising!

UNDER PRESSURE

What caused this lake-blocking ice dam? One way: A glacier can move slowly downhill into a valley. As this slab of ice creeps down and across the valley, it can form a solid wall.

When the climate begins to warm, melt-water from glaciers builds up on one side of the wall. "During these melting phases, you can get hundreds and even thousands of feet of water building up [behind an ice dam]," says MacPhee.

The rising water exerts pressure on the dam. At some point, this water pressure can cause the dam to fall. "When this happens, you can have a mountain of water rushing down, all at once, taking out everything in front of it," says MacPhee.

To survive, the entire cast of characters must find a way to escape the valley. They must leave before the gushing water turns their "water park" into a giant aquarium.

WIPEOUT

Besides fleeing the flood, the animals are dealing with their own issues. Take Manny: He wants to start a family. "But he fears that he might be the last woolly mammoth on Earth," says Forte. If Manny were the only woolly mammoth alive, that would mean extinction, or a dying out of his species.

According to scientists, Manny's fears are well-founded. When tile last glacial period ended--roughly 10,000 years ago--more than 70 animal species, including the woolly mammoth, became extinct.

Scientists have different theories to explain the wipeout. One idea: Since the animals had become so adapted (adjusted) to a chilly climate, their bodies were no longer able to cope with the heat of an interglacial period.

Luckily, Manny and his friends think up ingenious ways to survive the challenges of The Meltdown.

Nuts & Bolts

During the past billion years, Earth has experienced several ice ages. These are long periods of time during which the climate varies between colder glacial periods--when sheets of ice cover large parts of Earth's surface--and warmer interglacial periods.

DURING

During the last glacial period, ice covered approximately 30 percent of Earth's land surface, including parts of Europe and North America.

AFTER

The current interglacial period began roughly 10,000 years ago. That's when the climate warmed, causing glaciers to melt and recede.

WEB EXTRA

For more on Ice Age 2, visit: www.iceage2.com

DID YOU KNOW?

* Today, approximately 10 percent of Earth's land surface is covered in ice sheets, such as glaciers. Glacial ice stores 75 percent of Earth's freshwater.

* Glaciers often appear blue. Why? Glacial ice is sometimes so compressed that it has few air bubbles inside. Trapped air bubbles reflect all colors of visible light. But when a glacier contains few air bubbles, the dense ice absorbs all colors of visible light, except blue light. This makes the glacier appear blue.

CRITICAL THINKING:

* The characters in the movie Ice Age 2 are dealing with the effects of a warming climate, including an impending flood. Discuss how a warming climate could affect animals today.

CROSS-CURRICULAR CONNECTIONS:

LANGUAGE ARTS: In Ice Age 2, the animal characters must flee across a valley to escape an impending flood. Have students write an essay, describing their own ending to this story.

RESOURCES

* For more on glaciers see: http://nsidc.org/glaciers/

* Discover the life cycle of a glacier. Follow its journey from a snowflake to a giant ice sheet at: www.pbs.org/wgbh/nova/mtblanc/glacier.html

* At this site, students can click on an interactive geological time line to learn about time periods, such as the Cenozoic era--when the last glacial period occurred: www.cotf.edu/ete/modules/msese/earthsysflr/geotime.html

DIRECTIONS: Fill in the blanks to complete the following sentences.

1. The most recent ice age began approximately -- years ago. By roughly -- years ago, Earth had entered a(n) -- period, or a time during an ice age when temperatures plunge. During that time, roughly -- percent of Earth's land surface was covered with massive sheets of ice.

2. -- are slowly moving slabs of ice that form when the amount of snowfall exceeds the -- of snow.

3. A -- is a type of slowly moving slab of ice that forms high up on a mountainside that overlooks a valley. Once the icy slab's depth reaches 30 to 40 meters, Earth's --, or the force that pulls two objects together, tugs it downhill.

4. When the --, or the average weather in an area, warms up, it's called a(n) period. The current period began approximately -- years ago.

5. Scientists have different theories to explain why the woolly mammoth became -- (dying out of a species). One idea: The animal became so --, or adjusted, to a chilly climate, its body couldn't cope with the warming climate.

1. 4 million; 100,000, glacial; 30

2. Glaciers, melting

3. valley glacier; gravity

4. climate, interglacial; 10,000

5. extinct; adapted