A glacier is a large mass of ice that is formed primarily from atmospheric precipitation and then moves slowly by its own gravity. In other words, these are giant natural bulldozers, whose movement constantly changes the Earth’s topography. In this lesson of the Windy.app Meteorological Textbook (WMT) and newsletter for better weather forecasting you will learn what are glaciers and how exactly do they work.
In the mountains and at high latitudes, where there is year-round frost and lots of solid precipitation, snow doesn’t have time to melt and accumulates in layers. The more layers there are, the more pressure they put on the layers below.
Under high pressure, the crystals that make up snow press tighter together and change shape, losing their porosity — recrystallization occurs. In other words, once soft snow becomes solid ice.
When there is too much ice, it starts moving under its own weight down the mountains. The entire mass of moving ice is called a glacier. Ice often accumulates in natural bowl-shaped depressions surrounded by mountains on three sides, cirques. The glacier begins to descend as soon as it passes the edge of the depression.
Argentine Perito Moreno glacier. Those sharp peaks seracs are formed by intersecting crevasses in the ice. Jason Bright
The movement happens because of both gravity and the structure of the glacier itself. Ice is structured as horizontal layers of water molecules, relatively loosely bound together. Therefore, if the pressure from the outside — gravity or obstacles under the glacier — becomes stronger than the bonds between the different ice layers at the point of tension, the glacier ceases to be a single structure and splits into several horizontal layers.
Moreover, the layers of the glacier move at different speeds. The lowest ones rub against the ground, and thus move slower, while the upper ones move faster, as they slide on the ice, or even melt due to the friction of the liquid layer.
The zone where the glacier accumulates mass is called the accumulation zone. Approximately 60% of a glacier’s surface area is usually here, among the mountain peaks. And the area below, where the warm air causes the glacier to melt, break off in pieces and icebergs, and eventually end, is called the ablation zone.
If the mass at the top accumulates faster than it has time to melt at the bottom, the glacier is advancinп — its front, that is, its frontal and lowest boundary, is advancing. In other words, the glacier continues descending or sliding down the plain (if it has already descended into the plain).
In the reverse situation, the glacier retreats. It can also happen that the feeding and melting of the glacier counterbalance each other — in which case, the glacier seems to be standing still.
Glacial ice can be white, gray, or even black, depending on the presence of dirt and rocks. But often, especially in ablation areas, the glacier has a rich blue hue, like seawater.
The blue color comes from the compression of the ice — not only do the snowflakes change shape under pressure, but they also squeeze out the air bubbles that are used to make the snow white and opaque. As a result, the most visible ice begins to reflect light with a “blue” wavelength — just like the molecules of liquid water.
A glacial crevasse filled with water (photo taken underwater). Jason Bright
On average, glaciers move at a rate of 1 meter (3.2 feet) per day, with a maximum speed of 30 meters per day (98.4 feet). On rare occasions, inhomogeneities in the ground and the accumulation of liquid water beneath the glacier can cause short-term but dramatic jumps in velocity — up to 90 meters (almost 300 feet) per day!
Add to this the enormous mass (for example, the weight of an average glacier in the Alps is 90 thousand tons (almost 200 million pounds!), and it is clear that the movement of such a giant simply can not pass without leaving its mark on the landscape.
As the glacier descends from the mountains, it grinds the ground beneath it and the mountains around it. It also traps and absorbs rocks of all sizes, which then travel inside the ice to the melting zone, where they come to the surface again.
The landscape that a long-melted glacier can leave behind. Jason Bright
When the ice recedes or melts completely, not only are new depressions and U-shaped plains and fjords pressed by its weight found underneath, but also all sorts of furrows, polished rocks, hills, streams, and lakes that were not there before. The lakes form long before the glacier finally melts — primarily at its front, where century after century of melting ice is replaced by new ice.
For example, the Great Lakes on the U.S.-Canadian border are depressions where massive glaciers melted thousands of years ago. Argentina’s largest lake, Lago Argentino, was also formed by melting glaciers.
Glaciers are divided according to various parameters, but most often, they are divided into two main types: ice sheets, or ice floes, and mountain glaciers, or simply glaciers in the narrow sense. Ice sheets are masses of solid ice the size of a continent and kilometers deep, simultaneously descending in all directions from their center. There are only two of these on Earth, over Greenland and Antarctica.
The most common and visually impressive — mountain, or alpine, glaciers — can be found on all continents except Australia. More can be found in the Himalayas, Argentina, Chile, Pakistan, and Scandinavia, as well as in Iceland, Iran, the Caucasus and Alaska, eastern Africa, and even New Zealand — almost anywhere there are high mountains.
The cracks are formed by the unevenness of the ground beneath the glacier, among other things, and then eroded further by streams of liquid water on the glacier’s surface. Jason Bright
Therefore, getting to the glacier is not that difficult. If you don’t do ice climbing, climbing crampons — supports with spikes on your shoes — are enough to navigate the glacier.
But if you are inexperienced, you should climb the glacier with a trained guide (and more than once), who will show you what the dangerous places look like and how to avoid slipping into the ice cracks up to 40 meters deep, and, in rare cases, up to 300 meters deep (over 130 feet and almost a thousand feet, respectively). After all, a glacier is a constantly changing structure, so safety rules are especially important there.
Text: Jason Bright, a journalist, and a traveler
Cover photo: S B Vonlanthen / Unsplash
Safe ice thickness for various sports