Glaciers occur on all continents, as far as New Zealand and New Guinea are included in the continent of Oceania. The formation and growth of a glacier takes (quite literally) ages and depends on many factors.
Glacier formation depends on specific conditions in the local climate: the amount of precipitation, temperature and the presence of an accumulation zone. Alpine glaciers consist of this accumulation zone, ablation zone and firn zone.
Accumulation zone of a glacier
The accumulation zone is the area where snow falls and lands on the glacier. Compare this to a river: it’s not just the snow that falls on the glacier itself. It is also the snow that ends up on the glacier in other ways: this can be caused by, for example, the wind, but also by avalanches.
Not infrequently, the highest peaks around a glacier are snowless, simply because they are too steep for snow to accumulate: it falls down into the accumulation zone through avalanches.
The Firn Zone
The firn zone is where this snow ends up and is subsequently converted into “firn” (compacted snow) and later into ice. Firn is still open and porous, but more compact than snow. It is the hard snow layer that is created by compression from above: the weight of the snow compresses the snow. This also forms the “eternal snow”: the snow from the previous year remains until winter, where a fresh layer of snow is then added. However, the pressure and volume is insufficient to convert the firn into ice – and it doesn’t flow downhill.
The image below shows the black line drawn by me on the Glacier de Saleina in Switzerland, on the border with Italy and France. This line more or less indicates the accumulation zone: for example, snow falls on the Aiguille d’Argentiere and falls as an avalanche on this glacier. The blue line indicates the firn zone: in principle the snow remains here all year round and thus changes into firn. When snow falls on it for several years, the increased pressure makes it ice and behaves like a liquid and flows downwards.
As it flows down, the ice enters the ablation zone. It starts at the yellow arrow and is the line where the ice is no longer covered by firn.
A rule-of-thumb is that 1 centimeter of snow flows into 1 mm of ice. One meter of ice means at least 10 meters of snow. This also explains the large glaciers in the south of Norway: the western mountain areas receive extreme amounts of precipitation: up to 5,000 mm per year. If this all fell like snow (which it doesn’t), it would be about 50 meters of snow.
Equilibrium line of a glacier
The separation between the ablation zone and the firn zone is the “equilibrium line”. So this is the line where the annual amount of snow is just as great as the loss through melting or evaporation. A glacier grows when more snow falls than melts in the ablation zone. The size of the firn basin determines to a large extent how long a glacier can get: there is a large supply of ice that can form and flow further down.
The amount of precipitation also strongly determines the amount of ice that is formed, as does the temperature in summer (and at altitude). In years with a lot of snow falling in winter and with a cool summer, a glacier can grow (both in length and in mass).
Likewise, glaciers can melt due to increased temperatures, or shrink due to droughts. Droughts are detrimental to glaciers in normally-wet areas: there is no fresh snow that keeps the pressure on the firn, and firn is not transitioned into ice.
A field with perennial snow is not yet a glacier: only when the mass of ice starts to move under the influence of gravity, we call it a glacier.