Guide · The science

What is blue ice, and why do ice caves glow?

What is blue ice? It is ordinary glacier ice, dense enough and deep enough to filter out red light. Here is the physics in plain words, and what it means for the color you will see in a cave.

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Editorial team, blue-ice-cave-iceland.comUpdated October 25, 20267 minutes to read30 sources

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The short answer

01The cause: Ice absorbs the red part of light and lets blue through and scatter.¹
02Depth matters: The longer the path light travels in ice, the bluer it looks.²
03Why snow is white: Light leaves snow after passing through only about 20 cm of ice.³
04Glacier ice: Air closes into bubbles once density reaches about 830 kg per cubic meter.⁴
05Black bands: Volcanic ash trapped in the glacier shows up black in the caves.⁵

What is blue ice, in one paragraph

So, what is blue ice? It is glacier ice seen through enough thickness that its color shows. The US Geological Survey explains that glacier ice is blue because ice absorbs the red, long-wavelength part of white light, while the blue, short-wavelength light is transmitted and scattered.¹ Nothing is painted onto the ice; the color is a property of frozen water itself, revealed when light travels far enough through it.

Other public sources say the same in their own words. Vatnajökull National Park writes that ice is blue because it reflects blue light and absorbs the red and yellow part of the spectrum.⁶ NASA Earthdata puts it more bluntly: the dense ice of a glacier absorbs every other color of the spectrum except blue, and sometimes the ice looks almost turquoise.⁷ ⁸

Warren, in a 2019 review of the optics of ice and snow, describes ice simply as a weak filter for red light.⁹ That one phrase explains most of what you see in a glacier cave: thin ice looks clear, thick ice looks blue, and the deepest walls glow the most intense shade of it.

From snow to blue ice

How a snowflake becomes glacier ice

Each winter

Fresh snow falls

New layers bury and compress the snow below.¹⁰
First years

Sugar-sized grains

Compression makes the snow recrystallize into grains like sugar.¹⁰
About a year

Firn

About two-thirds as dense as water.¹¹
About 830 kg per cubic meter

Glacier ice

The air is sealed into bubbles.⁴
Deeper and older

Fewer bubbles

Bubbles shrink under pressure and dissolve into the ice.¹²
Centuries

Fist-sized crystals

Very old glacier ice can grow crystals the size of an adult fist.¹³

What causes blue ice: path length

If ice is blue, why is an ice cube clear? Because the effect is weak and needs distance. The USGS states that the longer the path light travels in ice, the more blue the ice appears.² Warren gives the absorption length of ice as approximately 2 m for red light at 700 nm, but approximately 200 m for violet light at 400 nm.¹⁴

In other words, red light is gone after a couple of meters of ice while blue light keeps going. Warren adds that photons of every visible color survive and are reflected or transmitted unless the path through ice is long enough to absorb the red.¹⁵ An ice cube is far too small for that; a cave wall lit from outside through meters of glacier is not.

This is what causes blue ice to look so different from one cave to the next, and even across a single cave. Where daylight has to pass through a thick ceiling, the light that reaches you is strongly blue. Near the entrance, or where the roof is thin, more light arrives unfiltered and the ice looks paler and greyer.

Density

From fluffy snow to dense glacier ice

kg per cubic meter

Typical densities as Warren (2019) gives them for snow, firn and the point where air is sealed into bubbles and the material becomes glacier ice.¹⁶ ⁴

Colors in the ice

What each color in a cave tells you

Color	What you see	Why
Deep blue	Thick, dense glacier ice	Red light is absorbed over a long path
Pale or clear	Thin ice near light	The path is too short to remove red
White	Snow or bubbly ice	Light escapes before it travels far
Black	Bands in the walls	Volcanic ash trapped in the glacier
Green	Occasional tints	In marine ice, organic matter or iron oxides

Based on the USGS, Warren (2019), NSIDC and Ice Explorers; what you see on a given day also depends on the weather and the light.¹ ² ³ ⁵ ¹⁷ ¹⁸

The colors

Blue, white and black in one cave

Long light path

Blue

Dense glacier ice

Red light gone after: About 2 m
Violet survives: About 200 m
Source: Warren 2019

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Short light path

White

Snow and firn

Light path in snow: About 20 cm
Snow density: 300-400 kg per cubic meter
Source: Warren 2019

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Buried eruptions

Black

Volcanic ash

Ash record back to: A.D. 1200
Ash in 2010: Almost all glaciers
Source: Icelandic research

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What to look for

Reading the ice on your tour

Use the science to see more in the cave

Look away from the entrance

The longer the light path through ice, the bluer it appears.²

Check the light

Colors vary with weather, snow cover and natural light.

That is volcanic ash

Ash trapped by the glacier shows black in the cave.⁵

That is air from old snow

Air is sealed into bubbles as firn becomes glacier ice.⁴

Short light path

In snow, light escapes after only about 20 cm of ice.³

Glacier ice is blue because the red (long wavelengths) part of white light is absorbed by ice and the blue (short wavelengths) light is transmitted and scattered.

U.S. Geological SurveySource 1

Why snow stays white

Snow is made of the same ice, so why is a snowfield white? Warren's answer lies in the size of the grains. In snow the grain radii are typically around 100 micrometers, so even after 1000 refraction events a photon re-emerges having passed through only 20 cm of ice, far too little to lose its red.³

Set that against the absorption length of about 2 m for red light and the answer is plain: 20 cm of ice is nowhere near enough to filter out the red.³ ¹⁴ Warren also notes that ice absorbs visible light so weakly that, in natural snow, the sunlight that is absorbed is dominated by tiny amounts of impurities such as dust and soot.¹⁹

That is why the fresh snow lying in a cave entrance looks brilliant white right next to deep blue walls. Ice Explorers describes colors on its tours that run from bright blue through emerald green to black, and the snow is the white that frames them all.¹⁸

Checklist

Photographing blue ice

Camera

Turn off the flash to keep the natural blue
Shoot toward thick, back-lit walls
Hold still in low light, or brace on the wall

Comfort

Gloves you can work a phone in
Keep the helmet on while you shoot

Video

The secret of why glacier ice is blue

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From snowflake to glacier ice

Blue ice starts as snow. NSIDC explains that each year new layers of snow bury and compress the old ones, forcing the snow to recrystallize, first into grains about the size and shape of sugar.¹⁰ After about a year the snow becomes firn, an intermediate state between snow and glacier ice, about two-thirds as dense as water.¹¹

Warren puts numbers on the squeeze. Snow with a density typically of 300-400 kg per cubic meter is compressed under its own weight into firn of 550-830 kg per cubic meter, and when the density reaches approximately 830, the air is sealed off into bubbles and the material is called glacier ice.¹⁶ ⁴ Deeper still, the bubbles shrink under pressure and eventually dissolve into the ice.¹²

The whole process is slow. NSIDC says that over time the crystals become so compressed that any air pockets between them are very tiny, that crystals in very old glacier ice can reach the size of an adult fist, and that for most glaciers the process takes more than a century.¹³ Warren notes that ice develops a noticeable blue in glacier crevasses and icebergs, especially marine ice with its lack of bubbles.²⁰

The black bands: volcanic ash in the ice

Iceland adds a third color. NSIDC describes the light in a cave at Breiðamerkurjökull as moving between the blue of pure ice, the white of fresh snow and the black of volcanic ash trapped by the glacier.⁵ NASA notes that some icebergs in Jökulsárlón are electric blue with black streaks of ash from past eruptions.²¹

Those streaks are buried history. Researchers have detected tephra layers of known age inside the Mýrdalsjökull and Vatnajökull ice caps, and a study of Vatnajökull's outlet glaciers, an ice core and written records built a volcanic record reaching back to A.D. 1200.²² ²³ In the summer of 2010 the Eyjafjallajökull eruption spread a thin layer of tephra onto almost all Icelandic glaciers.²⁴

Ash on the surface also speeds up melting. A 2020 study by Aðalgeirsdóttir and colleagues says tephra and dust on glacier surfaces enhance melt by lowering the albedo.²⁵ An experiment on Vatnajökull in 2015 found that thin ash layers, about 1.5 mm and 8.5 mm thick, substantially increased how fast the snow surface lowered, while a thick cover had some insulating effect.²⁶

Blue ice you can see without entering a cave

You do not need a cave to see blue ice in Iceland. Warren notes that ice develops a noticeable blue color in glacier crevasses and in icebergs.²⁰ At Jökulsárlón, NASA reports that a few icebergs are thought to be made of ice more than 1,000 years old, and that while many are white, some are electric blue.²¹

A walk on a glacier shows the same physics from the outside. On the lagoon side of Vatnajökull, Ice Explorers runs a summer glacier walk from May to September, when its cave tours are off, and the crevasses and moulins on such walks open windows into the ice.²⁷ Look into any deep crack and the blue appears exactly where the ice is thickest.

The man-made tunnel in Langjökull offers another view. Its operator gives 25 meters of ice above the tunnel and 200 meters below it.²⁸ It is a quieter, lit corridor rather than a cave shaped by water, but its walls are the same old snow turned to glacier ice.

Why no tour can promise the color

Because the color depends on thickness, light and what lies in the ice, it changes with every cave and every day. Lagoon tour listings say plainly that the ice in a cave can look bluer or paler as weather, snow cover and daylight change. A cave under fresh snow on a grey morning will look darker than the same cave on a bright day with thin cover.

The caves themselves also change. Glacier caves form within the ice and are often carved out by water running through or under it, NSIDC explains, and Ice Explorers says its caves change from season to season and even within a season.²⁹ ³⁰ Each winter's cave sits in different ice, with its own thickness, bubbles and ash bands.

So enjoy the color you get rather than chasing an old photograph. Look at how the blue deepens away from the entrance, at the white of snow drifted inside, at the black lines of ash, and at the clear, glassy ice where the cave wall is thin. All of it is the same frozen water, telling you how much glacier stands between you and the sky.

This winter's cave