Why Is Glacier Ice Blue? The Science Behind the Color That Doesn’t Look Real

A journal entry from the ice.

She was standing at the edge of the melt pool and she hadn’t moved in about two minutes.

Her family was a few steps behind her, chatting, taking photos, doing what people do when they’ve just landed on a glacier for the first time. But she was just standing there, looking at the water. Our guide waited. He’s been doing this long enough to know when someone needs a moment.

Finally she turned around and said, very seriously: “Someone edited this.”

Nobody had edited anything. That’s just what the water looks like.

We get some version of this reaction on almost every tour. People crouch down and look at the melt pools up close as if getting closer will reveal the trick. They hold their phones over the water to make sure the screen isn’t adding something. They say it looks fake, looks like a swimming pool, looks like a screensaver, looks like someone dropped dye in it while they weren’t watching.

It’s none of those things. So, why is glacier ice blue? The color is real and the explanation for it is one of the more beautiful pieces of physics we know of. Here’s what’s actually happening.

It Starts with Light: How Glacier Ice Blue Gets Its Color

White light, the kind that comes from the sun, is not actually a single color. It’s every color at once: the full visible spectrum from red through orange, yellow, green, blue, and violet, all traveling together. When that light hits most surfaces, the surface absorbs some wavelengths and reflects others. The wavelengths it reflects are the color you see. A red apple reflects red and absorbs the rest. A green leaf reflects green. You know this.

Ice is different because it doesn’t just reflect light at the surface. Light actually enters the ice and travels through it. And as it does, something interesting happens.

Ice molecules absorb red wavelengths. Not all at once, not dramatically, but consistently and cumulatively. Every inch of ice the light passes through, a little more red is absorbed. The blue wavelengths, sitting at the other end of the visible spectrum, pass through much more freely.

In a thin layer of ice, this effect is invisible. The ice just looks clear, or white if there are air bubbles scattering the light. But in deep, dense, ancient ice, the kind that has been compressed over hundreds or thousands of years under the weight of everything above it, the light travels a long path through the crystal structure before it finds a way back out. By the time it does, the reds are largely gone. What emerges is blue.

“Glacier ice is blue because the red wavelengths of white light are absorbed as light passes through the dense ice, while blue wavelengths pass through more readily and are scattered back to the observer.” — National Snow and Ice Data Center, University of Colorado Boulder

The deeper and older the ice, the more saturated the blue. That’s not a coincidence. It’s physics.

Why the Melt Pools Are a Different Blue Entirely

The melt pools on Knik Glacier are not the same blue as the glacier walls. They are not the same blue as the sky reflecting in still water. They are something else, and guests notice immediately that they can’t quite place what they’re looking at.

There are a few things happening at once.

First, the water in a glacial melt pool is extraordinarily pure. Over the course of centuries, pressure and time have expelled almost all of the air and impurities from the ice. When that ice melts, the resulting water carries almost none of the particulates, minerals, or organic material that give most natural water its color or cloudiness. It is essentially distilled water, and optically pure water has a natural blue tint that you only see clearly when there’s enough of it in one place.

Second, the pool sits in a basin of blue ice. The light that enters the water doesn’t just interact with the water itself. It bounces off the glacial walls and floor beneath and around the pool, which are absorbing red wavelengths the same way the ice above does. The blue that reflects back is amplified by its surroundings.

Third, and most dramatically, the glacial flour.

Glacial Flour: The Secret Ingredient in Blue Glacier Ice

As a glacier moves, and glaciers are always moving, the immense weight of the ice grinds the bedrock beneath it. This grinding produces an ultra-fine rock powder called glacial flour, or glacial silt, with particles so small they remain suspended in water almost indefinitely rather than sinking to the bottom.

These suspended particles scatter light. Specifically they scatter shorter wavelengths, which are the blue and green end of the spectrum, more efficiently than longer wavelengths. This is the same principle that makes the sky blue: tiny particles in the atmosphere scatter blue light in all directions, so when you look up, blue is what you see from every angle.

In a glacial melt pool, glacial flour does the same thing to the water. The blue wavelengths scatter throughout the pool and back toward your eye from every direction at once. The result is a color that seems to glow from within, a color that has no real equivalent in ordinary experience, which is why people reach for words like “CGI” and “edit” and “not real.”

“The blue-green color of glacial lakes is caused by light scattering off fine rock particles, called glacial flour, that are suspended in the meltwater. The particles are so fine they remain in suspension and scatter light similarly to the way the atmosphere scatters sunlight to make the sky appear blue.” — United States Geological Survey (USGS)

The concentration of glacial flour varies depending on the season, the recent movement of the glacier, and the specific location of the pool. Which is part of why the color shifts. Some pools are a deeper cobalt. Others are a vivid turquoise. Some are almost teal. The glacier is not painting with a single color. It has a full palette and it changes depending on the day.

The Age of the Ice Changes the Color

One of the more remarkable aspects of blue glacier ice is that it is partly a record of time.

Snow falls. More snow falls on top of it. The layers compress under their own weight, year after year, decade after decade, century after century. As compression increases, the air bubbles that give fresh snow and young ice their white appearance are gradually forced out. The ice becomes denser, more crystalline, more optically clear.

The older and denser the ice, the longer the path that light must travel through it before escaping. The longer the path, the more red is absorbed. The more red is absorbed, the deeper and more saturated the blue.

“In glaciers, the weight of accumulated snow compresses the ice crystals and eliminates air pockets. This dense glacial ice absorbs red light so effectively that even a small amount of it will appear blue.” — Dr. Mathis Jager, glaciologist, Alfred Wegener Institute for Polar and Marine Research

The blue ice you see on Knik Glacier is not decorative. It is a visual indicator of age and compression. When you look at the deepest blue in a crevasse wall or the floor of a Moulin, you are looking at ice that may be hundreds of years old. The color is the age made visible.

Why Crevasses and Moulins Show the Most Intense Blue

If you’ve stood at the edge of a crevasse or looked down into a Moulin on our glacier climbing tours, you’ve noticed that the blue intensifies as your eye travels deeper. The surface of the glacier might be white or pale. Twenty feet down, the walls are unmistakably blue. Deeper still and the color is almost electric.

This makes perfect sense given what we now know. The deeper you go, the older and denser the ice. The older and denser the ice, the more completely it absorbs red wavelengths. In the depths of a Moulin, where the ice has been under pressure for a very long time and where sunlight must travel a significant distance through glacial crystal before it reaches your eye, the reds are almost entirely gone. What remains is a blue that photographers routinely underexpose by a full stop just to keep it from blowing out their sensors.

It is, in the most literal sense, a color that the glacier earns over centuries.

A Color That Only Exists Here

There is a reason people have trouble describing glacier ice blue to someone who hasn’t seen it. There is no everyday reference for it. It is not the blue of the ocean, which carries sediment and organic material and reflects the sky. It is not the blue of a swimming pool, which is painted and chemically treated. It is not the blue of a clear sky.

It is the blue of ancient compressed ice filtering light that has traveled through thousands of years of snowfall to reach your eye. It is the blue of rock ground to powder by a moving glacier and suspended in meltwater so pure it barely qualifies as water in any sense we normally experience. It is a color produced by physics operating over geological time, and it exists in a handful of places on earth, none of them easily reached.

Knik Glacier is one of them.

“Glacier blue is one of the few colors in nature that genuinely has no analogue in ordinary human experience. People who see it for the first time almost universally describe it as unreal, because their visual memory has no category for it.” — Dr. Lonnie Thompson, glaciologist, Ohio State University Byrd Polar Research Center

The woman at the edge of the melt pool eventually stopped trying to figure out the trick. She took about forty photos, none of which she was satisfied with, and then she put her phone away and just looked at it for a while.

That’s the right move. The photos are worth taking. But the color is worth just seeing, too.

If you want to see glacier blue in person, on a glacier you can actually walk on, in melt pools you can kneel beside and touch, our tours out of Palmer and Seward put you there.

Browse our glacier tours here and see the color that doesn’t look real until it’s right in front of you.

Alaska Helicopter Tours operates glacier tours out of Palmer and Seward, Alaska. The ice is that blue. Nobody edited it. We promise.

Frequently Asked Questions About Blue Glacier Ice