A glacier is a large, dense ice mass made from compacted snow that slowly moves under its own weight over a long period of time. Unlike seasonal snowpack, glacial ice does not fully melt in summer. Although glaciers move slowly, often just centimeters per day, they may cover great distances over many years. There are two types of glaciated landscapes on earth. Alpine glaciation is found in tall mountains where the high elevation creates cold enough conditions for glaciers to form. Continental glaciation is when glacial ice fully covers a very large area of land – sometimes an entire continent!
How does glacial ice form?
Glacial ice begins forming when snowflakes fall and accumulate. Compared to ice, freshly fallen snow has a very low overall water content and is mostly made of air. As snowfall continues, snow compacts under its own weight. The delicate snowflake forms start breaking under the weight of overlying snow. Snowflakes may also recrystallize into more grain-like shapes if they partially thaw and refreeze. These forces reshape the snowflakes into granular snow.
Next, over several winters, additional snowfall buries and compacts the granular snow. Grains become weakly cemented together into firn. There is still air between the individual grains, but less than in granular snow. Over many years, the firn is buried deeper and deeper in snow. The mass of the overlying snow applies great pressure on the firn. This pressure forces more air out and causes the firn to recrystallize into glacial ice.
Glaciers form when the amount of snow that accumulates on the ground in an area is more than the amount of snow that ablates – or is lost. Glacial ice ablates by melting and calving (breaking off into a body of water and floating away). Year after year, as snow builds up, it compacts and recrystallizes into dense glacial ice. An advancing (growing) glacier accumulates more ice than it ablates. A receding (shrinking) glacier accumulates less ice than it ablates. Glaciers also move downhill due to gravity via basal sliding over bedrock and sediments.
What are the different types of glaciers?
A valley glacier is a large area of ice confined to a mountain valley. As valley glaciers move down valleys, they carve out many unique landforms. As the ice bends to go around corners or over surfaces with changing slopes crevasses, or cracks, form in the surface of the glacier. Today, you will find valley glaciers in many tall mountain ranges worldwide and even in some high-elevation areas close to the equator! However, many valley glaciers are receding due to our warming climate.
Ice sheets are huge areas of ice that cover 50,000 square kilometers or more! Unlike valley glaciers, which move in just one direction, ice sheets spread outward in all directions. Ice sheets cover large parts of continents, or sometimes, entire continents. Today, ice sheets are only found in Antarctica and Greenland. However, during the last glacial maximum, ice sheets also covered large parts of North America and Eurasia. Unlike valley glaciers, ice sheets are not contained within valleys. They may span across and cover entire mountain ranges in a thick layer of ice.
Ice caps are like miniature ice sheets. They are areas of ice that cover less than 50,000 square kilometers but are not confined to mountain valleys. Ice caps are more common than ice sheets today, and are found in many places, including Iceland, the Andes, and the Himalayas.
Other masses of ice
Ice shelves are parts of glaciers that extend outward over the open ocean or sea. They are still attached to land-based glaciers along an edge. Ice shelves and pieces of ice shelves break off (or “calve”), forming icebergs. Icebergs as well as frozen ocean water form sea ice. Some sea ice melts and reforms each year while some lasts year-round. However, anthropogenic climate change reduces the amount of sea ice that makes it through summers.
Why are glaciers important?
Glacial ice stores almost 70% of the world’s freshwater! Additionally, rapid anthropogenic (human-caused) climate change causes glaciers to recede at unusually fast rates. This results in sea level rise and harms coastal communities and ecosystems. Glaciers are also important because they shape the landscape. Furthermore, wildlife live on and around glaciers and sea ice (e.g., glacier finches, penguins) and in landscapes shaped by glaciation (e.g., pikas, Dall sheep).
Scientists who study glaciers are called glaciologists. They measure glacial movement and monitor the recession of many glaciers. Glaciologists study and predict the ways our changing climate affects glaciers. Scientists also learn about past climates by drilling deep into glacial ice and studying ice that formed long ago. The ice deep within some very old glaciers comes from snow that fell hundreds of thousands of years ago! Glacial ice acts as a time capsule, preserving frozen water, gas bubbles, and dust/ash from the time it formed. When scientists study ancient ice, they can learn about past temperatures, atmospheric conditions, volcanic eruptions, and more.
Where can I find glaciers?
Today, you can find alpine glaciation on most continents. However, most glacial ice is found near the poles. In the past, glaciers covered larger areas of land. During the last glacial maximum (the time when the glaciers were largest during the last ice age), ice sheets covered large parts of northern Eurasia and North America.
Teaching a glaciation unit?
I’ve put together a list of free resources for teaching kids about glaciation! Click the link below to view the list:
Also, check out the Glaciers Unit that I’ve put together! It features tons of my watercolor illustrations of glaciers and the post-glacial landscape! Click below to learn more!
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References and Further Reading
- Davies, B. (2022). Ice shelves, icebergs and sea ice. AntarcticGlaciers.org. Available:
- NASA (2017). Core questions: An introduction to ice cores. Available: https://www.antarcticglaciers.org/glacier-processes/glacier-types/ice-shelves-sea-ice-icebergs/https://climate.nasa.gov/news/2616/core-questions-an-introduction-to-ice-cores/
- National Park Service (2018).Glaciers and Climate Change. Available: https://www.nps.gov/articles/glaciersandclimatechange.htm
- National Snow and Ice Data Center (n.d.) Glaciers. University of Colorado Boulder. Available: https://nsidc.org/learn/parts-cryosphere/glaciers
- Plummer, C. C., Carlson, D. H., & Hammersley L. (2019). Physical Geology. New York, NY: McGraw-Hill Education. Pg 163-170, 283 – 287