Glaciers are large masses of ice that form on land and move slowly under their own weight. This cheat sheet helps students connect glacier formation, movement, erosion, and deposition to the landforms seen on Earth today. It also explains how ice ages happen and how scientists study past climates.
These ideas are important for understanding landscapes, sea level, and long-term climate change.
The core idea is that glaciers form where more snow accumulates in winter than melts in summer over many years. Gravity causes glacial ice to flow downhill or outward, carrying rock material that can scrape, carve, and reshape land. Glaciers erode by plucking and abrasion, then deposit sediment as till, moraines, and outwash.
Ice ages are linked to changes in Earth's orbit, atmospheric gases, ocean circulation, and feedbacks such as ice-albedo feedback.
Key Facts
- A glacier forms when annual snow accumulation is greater than annual melting for many years, so snow compacts into firn and then glacial ice.
- The glacier mass balance is accumulation minus ablation, where positive mass balance means the glacier gains ice and negative mass balance means it loses ice.
- Glaciers move by internal flow of ice and by basal sliding when meltwater reduces friction at the glacier bed.
- Glacial erosion happens mainly by plucking, which pulls rocks from the ground, and abrasion, which scrapes bedrock like sandpaper.
- A U-shaped valley forms when a glacier widens and deepens a river valley that was originally V-shaped.
- Glacial deposits include till, which is unsorted sediment, and outwash, which is sorted sediment carried by meltwater streams.
- Moraines are ridges of till, and a terminal moraine marks the farthest advance of a glacier.
- Ice ages are long cold periods when continental ice sheets expand, often influenced by Milankovitch cycles, greenhouse gas levels, ocean circulation, and ice-albedo feedback.
Vocabulary
- Glacier
- A large, long-lasting mass of ice on land that moves under its own weight.
- Accumulation
- The gain of snow and ice on a glacier, mainly from snowfall.
- Ablation
- The loss of ice from a glacier by melting, sublimation, calving, or wind erosion.
- Moraine
- A ridge or pile of unsorted sediment deposited directly by a glacier.
- Ice Age
- A long interval of Earth's history when global temperatures are cooler and ice sheets cover large areas of land.
- Ice-Albedo Feedback
- A climate feedback in which bright ice reflects sunlight, cooling Earth and allowing more ice to form.
Common Mistakes to Avoid
- Confusing glaciers with sea ice is wrong because glaciers form on land, while sea ice forms when ocean water freezes.
- Thinking glaciers do not move is wrong because glacial ice flows slowly downhill or outward due to gravity and pressure.
- Assuming all glacier sediment is sorted is wrong because till is unsorted, while meltwater outwash is usually sorted by particle size.
- Mixing up U-shaped and V-shaped valleys is wrong because rivers usually cut V-shaped valleys, while glaciers carve wider U-shaped valleys.
- Saying ice ages are caused by one factor only is wrong because orbital changes, greenhouse gases, ocean circulation, and feedbacks work together.
Practice Questions
- 1 A glacier gains 80 cm of snow and ice in a year and loses 55 cm by melting and calving. What is its mass balance, and is it gaining or losing ice?
- 2 A glacier advances 18 meters in 6 days. What is its average rate of movement in meters per day?
- 3 A field sample contains mixed clay, sand, gravel, and boulders with no clear layers. Is it more likely till or outwash, and why?
- 4 Explain how ice-albedo feedback can make a cooling climate become even colder during the start of an ice age.
Understanding Glaciers & Ice Ages
A glacier has different zones that respond to climate in different ways. Higher parts usually receive fresh snow, while lower parts lose ice through melting, evaporation, and pieces breaking away into water. The boundary between these zones is called the equilibrium line.
Its height is useful because it shifts uphill in warmer conditions and downhill in cooler conditions. Scientists track this line over many years. A single snowy winter may help a glacier briefly, but a long-term change needs many years of measurements.
Ice does not move like a rigid block. Deep inside a glacier, the enormous weight of overlying ice makes individual ice crystals change shape and slide past one another. This slow deformation is strongest near the center and bottom.
The surface can move at a different speed from the ice beside valley walls, where friction slows it down. These speed differences pull the ice apart and form crevasses.
Crevasses can be hidden by thin snow bridges, making glacier travel dangerous. Some glaciers speed up suddenly in events called surges, often because water pressure changes beneath the ice.
Glacial landscapes contain clues that help geologists reconstruct former ice flow. Long scratches on exposed bedrock show the direction rocks were dragged across the ground. Large boulders found far from their original rock source are called erratics.
Their rock type can reveal where the ice began. Streamlined hills called drumlins point in the direction of moving ice. Winding ridges of sand and gravel, called eskers, formed in meltwater tunnels beneath glaciers.
Kettle lakes formed where buried blocks of ice melted after the surrounding sediment was left behind. A landscape with several of these features gives stronger evidence than one feature alone.
Ice ages are not simply periods when every place on Earth is equally cold. During the most recent major glaciations, huge ice sheets covered northern North America and northern Europe, while many other regions became cooler or drier. Small changes in Earth’s orbit can alter how much summer sunlight reaches high northern latitudes.
Cool summers matter because winter snow can survive into the next year. Growing ice reflects more sunlight, which can increase cooling. Lower carbon dioxide levels can strengthen the change.
Ocean currents can move heat around the planet, so they influence where ice grows. These processes work over thousands of years, not over a single season.
Scientists test ideas about past ice ages using several kinds of evidence. Ice cores contain tiny air bubbles that preserve samples of ancient atmospheres. Layers in deep ocean mud contain shells from organisms that lived in water with different temperatures and ice volumes.
Pollen trapped in lake sediment shows which plants grew nearby. Sea level records matter too, because large ice sheets store water on land and lower ocean level. When studying this topic, pay attention to timescales.
Weather changes over days, climate patterns shift over decades, and ice sheets can respond over centuries or longer. These different timescales explain why glacier change is important for water supplies, flooding risks, and coastal communities.