Glaciers are more than fancy words for snowfields. In the mountains, they’re the main obstacles to reaching peaks. In Alaska, they calve icebergs into the sea and have origins in icefields the size of Rhode Island. And they’re melting, which will have some big ripple effects that Al Gore has warned us about. And what are terminal and lateral moraines anyway? Here’s what happens to a snowfield when it comes alive and starts to move.
A Cold, Cold River
The magic happens when enough snow accumulates on a downward facing slope, which gravity then starts to move it all downhill. A glacier is simply a frozen river. When ice accumulates to a certain thickness, the weight of all that ice, combined with gravity, causes the bottom section of the glacier to flow and slip along the surface. The result is a river that gradually flows downhill, at rates that may be measured inches, or in the case of Glacier Bay’s moderately hyperactive Johns Hopkins Glacier, as much as 8 feet a day. As it flows downhill, the glacier hits warmer temperatures, and the terminus starts to melt. The rates of flow and the rate of melt determine whether a glacier advances or retreats. And like any river, glaciers have steep sections and calm ones.
Crevasses and Icefalls and Seracs, Oh My!
When the glacier moves downhill, the brittle upper section that can’t bend ends up cracking. The results are crevasses, which climbers struggle to cross and dread falling in. Seracs are ice towers that form below steep sections, which form icefalls: the glacial equivalent of a waterfall. These are particularly treacherous for climbers, because they’re both difficult terrain and the most prone to shifting, avalanches, and tumbling chunks of ice and rock. The most notorious is the Khumbu Icefall on the approach to Everest.
Beware the Bergschrund
At the top of every glacier is the bergschrund (German for “mountain cleft”) a large crevasse that marks where the glacier breaks away from the headwall or snowfield to begin flowing downhill. If it were a river, this would be the headwaters. Bergschrunds are big, deep, and hard to cross. On many climbs, especially ice cascade stratovolcanoes, they’re one of the biggest obstacles. The later the season, the wider the bergschrund yawns and the harder it is to cross.
Wet Winters, Cool Summers
If you look at a map of the most glaciated mountains, one thing is clear: they’re near the sea, even though many coastal mountain ranges are lower than the Rockies. Glaciers form when snow accumulates rapidly in wet winters and doesn’t melt much in cool summers. The moist marine air that hits the coast of Alaska, British Columbia, and the Cascades of Washington and Oregon, as well as Patagonia and the monsoon sweeping over the Himalayas are cases in point: we get the massive glacier-covered Cascade volcanoes, and the tidewater glaciers of Alaska and Patagonia and the Himalayan ice fields. Drier places may be colder, but unless they also get enough precipitation, the glaciers will be smaller, unless we’re talking Antarctica.
3 Types: Valley, Piedmont, Tidewater
Valley glaciers are the ones we see in most of the mountains in the lower 48: they form near the top of peaks, often where snow accumulates below a mountain headwall, where it’s protected from the wind. Over time it accumulates enough weight to start flowing, and flows downhill, carving a valley. Most end when they reach lower altitudes, releasing silt-laden torrents down steep valleys.
If a valley glacier flows out of the mountains onto a flat plain, it does just what a river would: spread out into a wide, flat sheet called a piedmont glacier. The most famous of these is the Malaspina Glacier in Alaska’s Wrangell St. Elias National Park.
But the most dramatic are tidewater glaciers, when glaciers flow all the way to the sea, calving icebergs into the ocean. The most famous of these are in Alaska: Glacier Bay, Prince William Sound, and Kenai Fjords, but they exist elsewhere in the world: Greenland, southern South America and elsewhere. Tidewater glaciers act like “glaciers on steroids, advancing and retreating faster than most other glaciers.
Blue, Blue Baby I Love You
Glacial ice is an enchanting shade of deep blue-turquoise. You’ll notice it most on cloudy days, or when an iceberg breaks off. Because glacial ice is super-compressed from millennia under the weight of all that other ice, the gases have been squeezed out of it, unlike the clear ice in your freezer. This dense ice absorbs the rest of the colors of the spectrum, so blue is what’s left for you to see.
The Moraine Drag
Glaciers are enormous bulldozers pushing massive piles of rock and soil in slow motion. On the sides and end of the glaciers, you’ll find ridges of rocks and debris from previous advances and retreats. The ones on the sides are called lateral moraines; at the end of the glacier, they’re called terminal moraines. Where two glaciers meet, they form a stripe of debris in the middle of the new glacier, called medial moraines. The landscape around glaciers is full of past moraines, some exposed rock and gravel some overgrown with vegetation.
Is Al Gore Right?
Yes, he is. Most glaciers in the world are melting faster than they are accumulating new ice. As a case in point, North Sister’s Collier Glacier has been steadily shrinking. Climate change is very real. So are the effects of melting glaciers. Smaller glaciers mean an increasing risk of summer low flows, meaning less water for crops, people, and fish down the road. And the more glaciers melt, the more the earth warms, because the white surface of ice reflect heat, while dark rocks absorb it. And as the vast glaciers of Greenland melt and release cold water into the Atlantic Gulf Stream, a warm current that keeps Europe fertile for its latitude. Climate change could hit the UK particularly hard.
Dangerous but Beautiful
Glaciers are one of the most stunningly beautiful things on earth. Their color and shape are amazing, especially when seen at sunrise on a high peak. The blue of crevasses, the strange forms of seracs and the indescribable colors of meltwater ponds are one thing. But a few hours spent on an Alaskan glacier will reveal that they have their own mountains, lakes, canyons, and rivers. Needless to say, glaciers should be approached with skill, care, and awareness of the conditions. But there are few more irresistible things on earth.