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You might expect that heavy snowfall is the main requirement for the formation of a glacier. But if you stop and think about it, you’ll realize there are lots of areas of the world that receive huge amounts of snow but have no glaciers. Snowfall alone isn’t enough. For a glacier to form, the snow can’t melt. It has to be conserved. In the extremely cold Arctic and Antarctic, there are large areas that receive minimal precipitation and actually qualify as desert. Although there’s little snow, there are massive ice sheets because the snow that does fall is conserved and transformed into ice.
Of course, lots of places do get large amounts of snow, and they do have glaciers ... high mountains, for example. In high mountains where the climate is humid and cold, snowfall is frequent. Above the snowline, the annual snowfall exceeds the annual melting, so snow builds up. The accumulation is thickest in the hollows at the heads of valleys, because these locations are relatively high and cool, and are protected from sun and wind.
As snow accumulates in a hollow, it’s gradually converted to ice. First, the fragile snow crystals break as they’re compressed by the weight of more snow settling on top of them. There’s some melting and refreezing because of compaction, earth heat, and seasonal temperature fluctuations. So, the snow crystals are broken, then they’re wetted by meltwater, and refrozen, over and over again.
Gradually, over time, the snowflakes change into grains. They become rounded and granular, like the grains of coarse sugar. There are pockets of air between the grains, connecting the grains to one another. This old snow, called "firn," is generally created after one complete winter-summer cycle.
Firn is actually bits of ice. The firn begins to re-crystallize, and eventually, it combines and crystallizes into solid ice—a glacier. What happens is, the small grains coalesce to form large interlocking crystals of ice, with air trapped as bubbles inside the crystals. In the end, it’s pure ice, with all the air squeezed out. The flow of the glacier down the mountain contributes to crystal growth, as the movement helps to compress the air out.
As the hollow in the valley head fills with snow turning to ice, the hollow enlarges, and the rock walls are carved out by shifting ice. As new snow is added, the lower part of the snow-and-ice mass bulges out, kind of like a mud pie. As the mass continues to bulge, part of the ice moves over the edge of the hollow and starts moving down the valley. Large glaciers usually move faster than small ones. Also, the movement is faster in the summer, when more meltwater is present beneath and around the ice mass to lubricate it and buoy it up.
Most valley glaciers move at a rate of... oh ... between a few inches and a few feet a day. But some glaciers—called surging glaciers—can travel as much as 300 feet a day. There are at least 200 of these surging glaciers in Alaska and the Northwest Territories. The surging is caused by a number of conditions, like ... oh ... sudden adjustment to an increase in the snow load.on top, or, more likely, an increase in the production of meltwater due to a rise in temperature. Glaciers that have more meltwater are better lubricated and tend to move faster than drier ones. In very cold climates, glaciers are quite dry because of the lack of melting. The amount of water is slight, so the glacier does not slide as quickly.
In warmer climates, glaciers are better lubricated with meltwater. They also cause more erosion, more carving out of the valley floor. This is because during the melt-freeze cycle, parts of the glacier freeze to the bottom and sides of the valley, and then, as the ice moves on, large chunks of glacier pluck out rock. So you can see why glaciers in warmer climates have a greater impact on the landscape than those in very cold climates.