Glaciers are rivers of ice. They actually flow, and they really are made of ice. Wherever glaciers are found, the average temperature for the whole year is below freezing. This means that ice can last, year after year, even if some of it melts in summer. But where does the ice come from in the first place?
It comes from snow which can change into ice in two different ways. When the sun melts the surface of snow, water seeps down and melts more snowflakes underneath. But the temperature deep in the snow is still below freezing. When the seeping water reaches this super-cold snow, it changes to ice.
But this is not the main way in which snow becomes glacial ice. Snow piles up so deep that the individual snowflakes are pressed close together. When this happens, the tiny snow crystals begin to act according to a special habit they have. The smaller snowflakes join larger ones. The result is new and larger snowflakes or crystals. These crystals then join still larger ones, and so on until solid crystals the size of marbles have been formed. Sometimes in very big glaciers the crystals grow to the size of baseballs.
Glaciers would be mountains of ice getting higher every year if gravity didn't pull on them and make them move downward from the places where they form. Glaciers move slowly, compared to rivers of water, but they do move.
