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Glaciation and geological timescales - Not an ice cube: how glaciers work

This lesson focuses on the reasons why glaciers flow, how this happens and why glaciers change in size.

Key questions

  • Why and how do glaciers flow?
  • What causes glaciers to change size during a year, or over many years?
  • At what speeds do glaciers flow, and how can this be measured?

Subject content areas

  • Physical Geography: glaciation (focusing on the glacier as a ‘system’ with inputs and outputs), the basic differences between the workings of ice sheets versus alpine (mountain) glaciers
  • Geographical skills: ‘field sketching’ glaciers from photographs, measuring glacier movement (simple fieldwork on a glacier)

Glaciers are sometimes called ‘rivers of ice’ – but ice is solid, so how can it flow? If you have ever seen a glacier, or even walked or skied on one, you will know that it feels pretty solid. However, the amazing thing is that these large masses of ice actually do flow and move: it just happens slowly enough that it’s only noticeable over relatively long periods of time.

It is better to think of glaciers as more like rivers than giant ice cubes. As discussed in Lesson one, glacier ice forms in a completely different way to ice cubes in your freezer. As snow piles up in a cold, sheltered area it compresses the snow beneath, turning it into glacier ice, and as the snow and glacier ice continue to pile up, the lower layers get squeezed and pushed downslope.

In other words, glacier ice formed where snow piles up (zone of accumulation/zone of ablation) is forced downwards to the area where there is less snowfall and more melting (the ‘ablation’ area). In this way, a glacier ‘discharges’ ice away from where it is forming; similar to a river discharging water away from where the water fell as rain.

Not only do these ‘rivers of ice’ move huge quantities of frozen H2O that fell as snow, but they also carry along large amounts of broken up rock (called ‘moraine’), and they erode the landscape as they go.

No matter how big or small, all glaciers work in the same basic way – moving ice away from where it is forming. This means that the ice making up the glacier is always moving forwards even when the glacier itself is getting smaller and its lower end (or ‘snout’) is retreating back.



Because of how slowly it occurs, it can be hard to imagine how glaciers flow. Time lapse photography (where images are taken over a long period of time and then shown speeded up), makes it much easier to understand.

Click on the links below to watch time lapse video clips of different glaciers, and then discuss the questions which follow with a partner in your class.

1) Describe how the glaciers look when viewed in time lapse.

2) In the video clips what evidence is there that broken up rock is being transported by the glaciers?

3) How does the Aletsch Glacier (and Crater Glacier) look different between summer and winter? Can you think of a reason why?

4) Are there any other features that you notice on the surface of the glaciers?

You can also listen to the sounds made by glaciers as they flow by visiting Antarctica 2000 website:  (Try the Taylor Glacier, ‘listening inside with a hydrophone’.)

Finish the starter by watching this video clip from BBC Class Clips website (about four minutes) about the Franz Josef Glacier, New Zealand.


To understand how glaciers flow, the speeds at which this happens, and why glaciers can either expand in size or shrink, we need to think of a glacier as a ‘system’ that has ‘inputs’ and ‘outputs’.

The input is of course snow. If the lower end of the glacier is on land, then the output is through melting. If the end of the glacier extends into the sea (e.g. as an ‘ice shelf’ along the coast of Antarctica) then the output is through pieces of the glacier breaking off and floating away as icebergs. This process is called ‘calving’. Whether the ice is lost by melting or calving, this output from the glacier is called ‘ablation’.

All glaciers have an upper area where more snow piles up during the year than can be lost (the ‘accumulation zone’) and a lower area where there is more ablation than snow accumulation. So, the glacier ice moves from the accumulation zone to replace the ice lost in the ‘ablation zone’. The dividing line between these two zones is called the ‘equilibrium line’.

Over a year a glacier (in the Alps for example) will tend to expand a little in the winter (with more snowfall and less melting) and then shrink back a little in the summer as the area of the glacier experiencing melting increases. If on average the glacier remains the same size over many years, then the glacier is neither ‘advancing’ nor ‘retreating’ – it is in balance. However, if the climate changes (e.g. temperature rises and snowfall decreases), then over many years the glacier will shrink and its snout will retreat back.

View these two interactive diagrams to see how a glacier responds to changes in climate (the interactive diagram shows a glacier ending in the sea so it could represent a glacier along the coastline of Antarctica or Greenland).

  • What do you think happens to the position of the equilibrium line as a glacier advances? Retreats?
  • Why is the glacier ice still moving forward, even once you’ve changed the climate to make the glacier retreat?

The balance between a glacier’s accumulation and ablation is called the ‘mass balance’ or ‘glacier budget’. The speed at which the ice flows depends on the amount of accumulation and ablation that happens in a year. If there is a lot of snowfall near the top of the glacier, and a lot of melting near the bottom, then there will also be relatively fast movement of glacier ice (e.g. alpine glaciers). With low accumulation and low ablation (such as occurs in polar areas) the flow of glacier ice is slower.

Download and complete the Glacier image annotation task. Then learn and review the different terms and definitions relating to how glaciers work by downloading the Glacier system definitions matching task.


Different glaciers flow at different speeds, ranging between about three and 300 metres per year (this works out to between one cm and 80 cm a day). However, where snowfall is very high, glaciers can flow at speeds over a few metres or more per day, and some types of glaciers occasionally collapse (or ‘surge’) causing the ice to move at much higher speeds for a brief period.

The specific ways in which glaciers flow are complicated, but there are three main processes.

  • The first is called ‘internal deformation’ and this involves the weight of ice above squeezing, fracturing, and pushing on the ice beneath.
  • The second is called ‘basal sliding’, and this involves the base of the glacier sliding along the rock beneath when there is meltwater under the glacier.
  • Lastly, sometimes the ground beneath the glacier can be weak and loose, so giving way under the glacier and helping to carry it along – a little like the way you might slip on a banana skin!

Measuring the movement of glaciers is important both for understanding how glaciers work and also for the information it can give geographers about climate change (because glaciers are sensitive to changes in temperature and precipitation).

There are many different ways to measure a glacier’s flow, and this can involve doing fieldwork on a glacier or looking at images of a glacier taken at different times. The simplest approach is to drive several stakes into the surface of a glacier, record their exact position using a GPS (Global Positioning System), and then return at different times to record changes in their position.

Download the Flow data task, follow the instructions on the spreadsheet for analysing the data, and then answer the questions that are shown on the bottom of the spreadsheet. To do a similar task in more detail, you can go to this page of the Exploring Earth website.

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