A glacier is a river of ice that flows downhill under its own weight. In Landforms Made by Water, Wind and Ice you saw what glaciers carve. Here we look at the glacier's budget, its income and its spending, and how that decides whether its end advances or retreats.
Why it works
High on a glacier, more snow falls each winter than melts each summer: this is the accumulation zone. Low on it, melting wins: the ablation zone. The year's mass balance is gains minus losses. Meanwhile, gravity makes the ice flow downhill all the time, carrying ice from the accumulation zone to the ablation zone. The end of the glacier (the terminus) stays where ice arrives as fast as it melts. If melting wins, the terminus retreats, even though the ice keeps flowing forward, like walking forward on a moving walkway that runs backward faster than you walk.
Key idea
A glacier gains snow up high and loses ice down low; its net mass balance is gains minus losses. The ice always flows downhill; the terminus advances or retreats depending on whether ice arrives faster than it melts.
Worked examples
Worked example: A year's balance
A glacier gains the equivalent of 1.5 m of water over its surface and loses 2.3 m. What is its net balance?
- $1.5 - 2.3 = -0.8\,\text{m}$ of water.
- Negative: the glacier lost mass. Most mountain glaciers worldwide have had negative balances for decades.
Answer: $-0.8\,\text{m}$.
Worked example: Moving the terminus
At its end, a glacier's ice flows forward 40 m a year, while melting removes 55 m of length a year. What happens to the terminus?
- $40 - 55 = -15\,\text{m}$ per year.
- It retreats 15 m a year, while the ice itself still moves forward 40 m a year.
Answer: It retreats 15 m a year.
Common mistake
It is natural to think a retreating glacier is flowing backward, uphill. It isn't: the ice always flows downhill. Its end retreats because it melts faster than new ice arrives. Check with stakes drilled into the ice: they keep moving down-glacier even as the terminus pulls back.