Have you ever thought about how liquids flow but solids don’t? Well, glaciers are here to surprise you! These massive ice formations seem to break the rules of physics by flowing like a liquid, even though they’re solid. Let’s dive into the world of glaciers and uncover their secrets.
To understand how glaciers move, we need to start at the beginning. Imagine being on top of a glacier, surrounded by ice that originally came from the ocean. Warm, moist air from the Pacific Ocean rises over coastal mountains, cools down, and falls as snow and rain. Each year, about 30 meters of snow blankets the Juneau Icefield, where Mendenhall Glacier is located.
But snow alone doesn’t make a glacier. The climate plays a crucial role. Even in summer, temperatures here hover near freezing, allowing more snow to accumulate in winter than melts in summer. Over time, layers of snow build up, each one pressing down on the one below.
Fresh snow is light and fluffy, with lots of air pockets. A cubic meter of it weighs about as much as an adult human. As more snow piles up, the pressure compresses the snowflakes into smaller crystals, like grains of sugar. This process squeezes out the air, making the snow denser.
After about two years, the snow turns into “firn,” a dense form that’s about two-thirds the density of water. It can take decades for firn to become solid glacial ice, which is dense and bubble-free. At Mendenhall Glacier, this ice moves forward more than half a meter every day!
Glaciers are often called “rivers of ice” because they flow, but not because they melt. Glacier ice can flow without melting. Although it seems solid, the ice can bend and deform under long-term pressure, like its own weight.
In solids, atoms and molecules are tightly bonded and don’t move past each other. But in glacial ice, the water molecules are arranged in a way that allows them to flow under certain conditions. This is related to the pressure melting point: as pressure increases, the melting point of ice decreases. When glacier ice is close to this point, it becomes flexible, like heated metal.
The deepest layers of a glacier experience the most pressure, known as the “zone of plastic flow.” Here, the ice crystals can stretch and slide past each other, similar to a deck of cards. When the glacier encounters obstacles like boulders, the ice melts under pressure, flows around the obstacle, and refreezes on the other side.
At the top of the glacier, the “zone of brittle flow” doesn’t have as much pressure, so it cracks under stress. This is why you see deep crevasses near the surface. Imagine a candy bar bending around a curve: the top cracks while the bottom stretches.
Glaciers are constantly moving, but they never move backward. They are always melting, and when they lose more mass at the bottom than they gain at the top, they recede. Mendenhall Glacier is on a 13-mile journey to Mendenhall Lake, taking about 200 years for new ice to travel from the icefield to the lake.
However, warmer summers and less snow in winter are speeding up this process. The glacier is melting faster than it’s growing, and it has already retreated miles. Just a few decades ago, the spot where you might stand was covered in ice.
Understanding how glaciers move is fascinating, and it’s important for them to remain stable. Keep exploring and stay curious about these incredible natural wonders!
Create your own mini glacier using a plastic container, water, and sand. Freeze the mixture to simulate a glacier. Once frozen, tilt the container and observe how the “glacier” flows over time. Discuss how this model represents the movement of real glaciers and the factors that influence their flow.
Use a block of ice and a sloped surface to explore how glaciers move. Place the ice on the slope and measure how far it travels over a set period. Experiment with different slopes and temperatures to see how these variables affect the ice’s movement, simulating the pressure melting point concept.
Research different glaciers around the world and create an interactive map. Mark each glacier’s location and include information about its size, movement, and current status. Present your findings to the class and discuss how climate change is impacting these glaciers.
Engage in a vocabulary challenge where you match glacier-related terms with their definitions. Use terms like “firn,” “zone of plastic flow,” and “crevasse.” This activity will help reinforce your understanding of the key concepts and terminology related to glaciers.
Participate in a debate about the impact of climate change on glaciers. Divide into groups and research arguments for and against the idea that human activity is accelerating glacier retreat. Present your arguments and engage in a discussion about the importance of preserving these natural wonders.
Here’s something we all take for granted: liquid substances flow like liquids, and solid substances don’t flow, right? Not so fast. Glaciers are fascinating, and not just because they’re made of ice. I’m here at Mendenhall Glacier, one of about 40 glaciers in the Juneau Icefield. Glaciers are intriguing because the way they move seems to defy physics. A solid structure that flows like a liquid—how does that happen? That is a question of glacial proportions.
To understand how a glacier moves, let’s go back to the beginning—the beginning of the glacier. First, we need to get up to the top of a glacier. I’m excited; I’ve never been in a helicopter before, and we’re going to take this up to the glacier. This ice originates in the ocean. Warm, moist air from the Pacific rises up coastal mountains, where it cools, condenses, and falls as snow and rain. Each year, around 30 meters of snow, sometimes more, falls on this icefield.
But you need more than a mountain of snow to create a glacier; you also need the right climate. Even in summer, temperatures in this region dip near freezing. This means that the accumulation of snow in winter exceeds snowmelt in summer, allowing snow to build up over time, with each layer landing on top of the previous one. A cubic meter of fresh snow typically weighs between 70 to 150 kilograms—about as much as an adult human or two. Most of that volume is air. However, as it continues to pile up, the collective forces on the fluffy snow begin a transformation.
First, those beautiful snowflake shapes are broken down into smaller crystals, about the size of grains of sugar. As they get compressed, the air pockets between them start to shrink, and the snow becomes denser. After about two years, the snow transforms into a new form called “firn,” which is an intermediate state between snow and glacial ice. Firn is about two-thirds the density of water, and it can take decades to fully transition into its final form: a dense, bubble-free ice mass. The ice in Mendenhall is flowing forward more than half a meter every day.
Glaciers are often compared to “rivers of ice,” and that’s not incorrect. These massive solid structures exhibit liquid-like tendencies. You might think, “ice flows because it melts,” but glacier ice can flow without melting. Mendenhall fits the definition of a solid: I can stand on it, and it will hold its shape. Short-term stress doesn’t affect it, but long-term stress, like bearing its own weight, causes it to deform and bend.
What defines something as a solid, rather than a liquid, is that its atoms and molecules are tightly bonded and cannot move past each other. However, this isn’t the case with glacial ice. Its water molecules are arranged in an orderly pattern, but under certain conditions, they can still flow. Much of this has to do with the pressure melting point. As pressure increases, the melting point of ice decreases. When glacier ice stays close to—but just below—that point, it becomes malleable, similar to how you can bend solid metal when it’s heated near its melting point.
The deepest layers of a glacier are exposed to the most pressure. This is known as the “zone of plastic flow,” where the bonds between the ice crystals can stretch rather than break. Here, the molecular bonds between ice crystals stretch and slide past each other, similar to how a deck of cards deforms as cards slide past one another. When the bottom of the glacier needs to navigate around large obstacles, like boulders, higher pressures on the uphill side cause the ice to melt, flow around the obstacle, and refreeze on the other side.
As these processes repeat, the glacier creeps along, propelled by gravity, like a giant conveyor belt. Up at the top of the glacier, things get a bit different. The upper 150 feet or so—the “zone of brittle flow”—doesn’t experience as much pressure, making it prone to cracking under stress, which is why you’ll find deep crevasses near the surface. Imagine a candy bar warping around a curved surface: the top has to stretch farther and faster, causing it to crack, while the gooey bottom bends and stretches.
Some glacier movement comes from slipping on sediment or a thin layer of water, but most glaciers not located at Earth’s poles move by this process of deforming. However, glaciers are products of climate, and they change with the climate. Mendenhall Glacier is currently making a 13-mile journey to its lowest point, Mendenhall Lake. The terminal edge of a glacier is one of the easiest places to observe its movement in action, but it’s also where we can see how much things are changing.
Glaciers never move backward, and they are always melting. However, when mass melts away at the bottom faster than new mass is added at the top, they can recede. It takes about 200 years for new ice on Mendenhall to move from the icefield to the lake. It’s a slow process, but warmer summers combined with less snow in winter are accelerating things. The glacier is melting faster than it’s growing—and it has already retreated miles. Just a few decades ago, where I’m standing was covered in ice.
Understanding how glaciers move is incredibly fascinating, but for that to continue, they need to remain stable. Stay curious!
Glacier – A large, slow-moving mass of ice formed from compacted layers of snow, found in cold regions or on mountains. – The glacier slowly carved out the valley over thousands of years.
Ice – Frozen water, a solid state of water that forms when the temperature drops below 0°C (32°F). – The ice on the lake was thick enough to walk on during the winter.
Snow – Precipitation in the form of ice crystals, forming a white layer on the ground when it accumulates. – The snow covered the trees, creating a beautiful winter landscape.
Flow – The movement of a liquid or gas in a particular direction. – The flow of the river was strong after the heavy rain.
Pressure – The force exerted on a surface per unit area. – The pressure inside the Earth can cause rocks to melt and form magma.
Climate – The average weather conditions in a region over a long period of time. – The climate in the Arctic is cold and dry, with long winters.
Melting – The process of changing from a solid to a liquid due to an increase in temperature. – The melting of the ice caps is a concern for scientists studying climate change.
Crystals – Solid materials whose atoms are arranged in a highly ordered, repeating pattern. – Snowflakes are made up of ice crystals that form in the atmosphere.
Density – The measure of how much mass is contained in a given volume. – Ice has a lower density than liquid water, which is why it floats.
Water – A transparent, tasteless, odorless, and nearly colorless chemical substance, essential for all known forms of life. – Water covers about 71% of the Earth’s surface, mostly in oceans.
