Phenomenon: Glaciers

Glaciers are all about power. They sculpt mountains, dredge valleys – and turn hikes into mettle-testing climbs. The secret to all that power? Ice – lots of ice. These masses of frozen H2O are named after the different places where they form, including cirques, valleys, and piedmonts (bases of mountains). They can be long and narrow like Alaska’s 100-mile Bering Glacier, or broad, like the Seward-Malaspina glacier system in Wrangell-St. Elias National Park , which covers nearly 2,000 square miles – an area larger than Rhode Island.

SNOW TO ICE

Glaciers form in areas where summer temperatures never rise enough to melt all of winter’s snowfall. The powder settles, and subsequent storms pack it into dense layers. This consolidated snow, called firn, continues to compress each season. Gradually, it recrystallizes into glacial ice – a process that can take anywhere from 3 to 20 years, depending on climate and annual snowfall.

ICE TO GLACIER

If new snow and cold temperatures persist, the ice mass expands. When it is larger than a football field and exceeds 50 feet in depth, the weight of the ice and snow in the accumulation zone (the upper elevations) pushes the ice below it downhill. This movement, or glacial flow, transforms the ice mass into a glacier.

GLACIAL SPEED

Although a glacier appears static, it’s actually a dynamic, flowing river of ice. Its progress hinges on two things: the grade of the slope and the amount of new glacial ice in the accumulation zone. The rate of movement – a foot a day for many glaciers – is called flow rate or ice velocity.

EXPANDING OR SHRINKING

The difference between flow rate and the amount of melting in the ablation zone (the lower section) determines if a glacier advances or retreats. If flow is a foot a day and melt is 6 inches, the glacier will advance 6 inches each day.