Glaciers & ice sheets
Understand how accumulated snow becomes dense glacier ice that moves under its own weight. Explain how glaciers wear away valleys and leave moraines. Greenland and Antarctica have large ice sheets on land. Their complete melting would raise global sea level by about 65 metres. Ice cores preserve past air and clues about temperature; some Antarctic records span 800,000 years.
What a learner can do afterwards
- Describe how glaciers form: snow accumulates, compresses, and becomes dense ice that flows slowly under its own weight
- Explain that the Greenland and Antarctic ice sheets hold enough water to raise sea levels dramatically if melted
- Describe how ice cores reveal climate history through trapped air bubbles from hundreds of thousands of years ago
1 · Read
You have seen how snow becomes ice. A glacier is a large mass of ice on land that moves under its own weight. It forms where more snow builds up than melts away. New snow presses older snow into dense ice. Gravity pulls thick ice downhill. Ice slowly changes shape inside the glacier. Some glaciers also slide over the ground beneath them.
Moving ice and the rocks it carries wear away valley floors and sides. This often makes a broad U-shaped valley. Glaciers also carry rock and soil. Piles of this material deposited by a glacier are called moraines. A glacier can change land over many years even when its movement is slow.
An ice sheet is a glacier covering a huge land area. Earth's two ice sheets are in Greenland and Antarctica. Complete melting would raise global sea level by about 65 metres. Rounded estimates are 7 metres from Greenland and 58 from Antarctica. Combine estimates by adding their values. This is not a forecast for a particular year.
Scientists drill cylinders called ice cores from glaciers and ice sheets. Air bubbles sealed as snow becomes ice preserve samples of past air. Scientists measure gases such as carbon dioxide in them. The ice itself holds clues about past temperatures. Deeper layers are usually older, though ice movement can disturb them. Some Antarctic ice-core records span 800,000 years. Different cores cover different times.
To find half an age, divide by 2. Half of 600,000 years is 300,000 years. Check by adding two equal halves: 300,000 + 300,000 = 600,000. A sample's age alone does not tell us its exact depth.
Glaciers form from packed snow and move under their own weight. They shape valleys and leave moraines. Ice sheets store water on land. Ice cores preserve clues about past air and climate.
This lesson, practice and flashcards were written for this dataset.
The readings were written for this dataset. The linked lesson, When will the next ice age happen? - Lorraine Lisiecki, is credited to TED-Ed in the source record. Its licensing terms have not been verified.
More to read
Moving ice and the land
A glacier forms where snow builds up faster than it melts away. New layers bury older snow. Their weight presses it into dense ice. A glacier is more than a pile of snow: its ice moves under its own weight.
Ice can slowly change shape inside a glacier. Some glaciers also slide over the ground below. These are different ways that glacier ice moves.
Moving ice carries pieces of rock. These rocks can scrape the valley floor and sides. Over time, a glacier can leave a broad U-shaped valley.
Rock and soil carried by the glacier may also be deposited. Piles of this material are called moraines. Wearing away a valley removes material. Depositing a moraine leaves material behind.
Air inside an ice core
Scientists drill a cylinder of ice called an ice core. Its layers hold different kinds of evidence about the past.
Buried snow has spaces containing air. As it becomes dense ice, some spaces close into sealed bubbles. These preserve samples of past air. Scientists measure gases such as carbon dioxide in them. The bubbles close after the snow first falls, so the air can be younger than the surrounding ice.
The ice itself also holds clues about past temperatures. Scientists compare these clues with the gases in the bubbles and with other climate records.
Deeper layers are usually older, but ice movement can disturb them. Some Antarctic ice-core records span 800,000 years. Different cores cover different periods; that number is not an age limit for all ice cores.
Water stored on land
Greenland and Antarctica have Earth's two ice sheets. These enormous glaciers store frozen water on land.
When land ice melts and its water reaches the ocean, it adds water to the sea. Ice that flows off land into the ocean also contributes to sea-level rise.
Rounded complete-melt estimates are about 7 metres of global sea-level rise from Greenland and 58 metres from Antarctica. Together that is about 65 metres. This describes complete melting, not what is expected by a particular year.
Floating sea ice is different. It already pushes aside seawater. Its melting has little direct effect on sea level. Naming the kind of ice matters when explaining why seas rise.
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