Rivers of ice
A glacier is a huge mass of ice that moves slowly over land under its own weight. Glaciers form where more snow falls in winter than melts in summer. Over hundreds of years, the snow piles up, gets squeezed and turns into ice.
Today, glaciers and ice sheets cover about 10% of Earth’s land and hold about 69% of the world’s fresh water.
How a glacier forms
| Step | What happens |
|---|---|
| 1. Snow falls | Fluffy snowflakes pile up year after year |
| 2. Snow compacts | The weight of new snow squeezes out air |
| 3. Firn forms | After about a year, snow becomes grainy, like wet sugar |
| 4. Glacier ice forms | After many years, firn becomes dense, blue-tinted ice |
| 5. Ice moves | Once thick enough (about 50 m), the ice begins to flow downhill |
Glacier ice often looks blue because thick ice absorbs red light and lets blue light through.
How glaciers move
Glaciers move in two ways:
- Sliding: meltwater at the bottom lets the glacier slide over the rock below.
- Flowing: under great pressure, the ice deforms and creeps like very thick toothpaste.
Most glaciers move a few centimetres to a few metres a day. Some fast glaciers in Greenland move more than 30 metres a day.
The brittle surface ice can’t bend as it moves, so it cracks, forming crevasses that can be tens of metres deep. Snow bridges can hide them, which makes glaciers dangerous to cross.
Types of glaciers
| Type | Where | Example |
|---|---|---|
| Valley (alpine) glacier | Flows down mountain valleys | Athabasca Glacier, Alberta |
| Ice sheet | Covers huge areas of land | Greenland and Antarctic ice sheets |
| Ice cap | A smaller dome of ice | Ice caps on Canada’s Arctic islands |
| Tidewater glacier | Flows into the sea | Glaciers in Alaska and Greenland |
The Antarctic ice sheet is up to about 4.8 km thick. If it all melted, sea level would rise by about 58 m.
How glaciers shape the land
Glaciers are powerful sculptors. As they move, they pick up rocks that scrape the land like sandpaper.
| Landform | How it forms |
|---|---|
| U-shaped valley | A glacier widens and deepens a river valley |
| Fjord | A glacial valley flooded by the sea (Newfoundland’s Western Brook Pond was once a fjord) |
| Cirque | A bowl carved at the head of a glacier |
| Horn | A sharp peak carved by glaciers on several sides |
| Moraine | A ridge of rock and soil left at the glacier’s edges or end |
| Erratic | A large boulder carried far from where it came from |
| Drumlin | A smooth, egg-shaped hill |
| Esker | A long, winding ridge of sand and gravel left by meltwater rivers |
| Kettle lake | A lake formed where a block of ice melted in the ground |
Canada and the ice age
During the last ice age, which peaked about 20 000 years ago, the Laurentide Ice Sheet covered almost all of Canada, up to 3 km thick. As it melted, it carved the Great Lakes, left thousands of lakes across the Canadian Shield, and dropped erratics across the Prairies. The Okotoks Erratic in Alberta is a 16 500-tonne boulder carried hundreds of kilometres from the Rockies.
Icebergs and calving
When a glacier reaches the sea, chunks break off in a process called calving, forming icebergs. About 90% of an iceberg is underwater. Icebergs from Greenland drift down "Iceberg Alley" past Newfoundland each spring. It was one of these that sank the Titanic in 1912.
Glaciers and climate change
Glaciers are shrinking around the world as temperatures rise. The Athabasca Glacier in Jasper National Park has lost about half its volume and retreated more than 1.5 km in about 125 years. Melting glaciers raise sea levels and reduce the fresh water that many rivers depend on in summer.
Common mistakes
- Thinking glaciers are frozen in place. They are always moving.
- Thinking icebergs are made of frozen seawater. They are freshwater ice from glaciers.
- Thinking glaciers move fast. Most move very slowly.
Try it yourself
- Freeze sand and pebbles in an ice block. Push it across modelling clay. What marks does it leave?
- Put an ice cube in a glass of water. Estimate how much is below the surface.
- Find a landform near you shaped by glaciers.
Quick recap
- Glaciers form from snow compacting into ice over many years.
- They move by sliding and flowing.
- Valley glaciers, ice caps and ice sheets are types of glaciers.
- Glaciers carve U-shaped valleys and leave moraines and erratics.
- Glaciers are shrinking as the climate warms.