Peter Lobner, 1 September 2020
Why is glacial ice blue?
The US Geologic Survey (USGS) provides a basic explanation of why glacial ice is blue:
“The red-to-yellow (longer wavelength) parts of the visible spectrum are absorbed by ice more effectively than the blue (shorter wavelength) end of the spectrum. The longer the path light travels in ice, the more blue it appears. This is because the other colors are being preferentially absorbed and make up an ever smaller fraction of the light being transmitted and scattered through the ice.”
The key to blue ice is selective absorption, which occurs in a special kind of ice that is produced on land with the help of pressure and time. Becky Oskin provides the following general insights into how this process occurs in her 2015 article, “Why Are Some Glaciers Blue?”
- When glacial ice first freezes, it is filled with air bubbles that are effective in scattering light passing through the ice. As that ice gets buried and compressed by subsequent layers of younger ice, the air bubbles become smaller and smaller. With less scattering of light by the air bubbles, light can penetrate more deeply into the ice and the older ice starts to take on a blue tinge. Blue ice is old ice.
- Patches of blue-hued ice emerge on the surface of glaciers where wind and sublimation have scoured old glaciers clean of snow and young ice.
- Blue ice also may emerge at the edges of a glacial icepack, where fragments of glaciers tumble into the sea and reveal a fresh edge of the old ice.
Stephen Warren’s 2019 paper, “Optical properties of ice and snow,” provides the following more technical description of the selective absorption process in ice:
- “Ice is a weak filter for red light..….the absorption coefficient of ice increases with wavelength from blue to red (but the absorption spectrum is quite complex). The absorption length…… is approximately 2 meters at (a wavelength of ) λ = 700 nm (nanometers, red end of the visible spectrum) but approximately 200 meters at λ = 400 nm (blue-violet end of the visible spectrum). Photons at all wavelengths of visible light will survive without absorption, and be reflected or transmitted, unless the path length through ice is long enough to significantly absorb the red light.”…..”Ice develops a noticeable blue color in glacier crevasses and in icebergs, especially in marine ice (i.e., icebergs calved from glacial ice shelves), because of its lack of (air) bubbles (which would otherwise cause scattering and limit light transmission through the ice).”
- The absorption length is the distance into a material where the beam flux has dropped to 1/e (1/2.71828 = 0.368 = 37%) of its incident flux. For light at the red end of the spectrum, that is a relatively short distance of about 2 meters. This means that, in 2 meters, absorption decreases the red light component of beam flux by a factor of 1/e to about 37% of the original incident red light. In another 2 meters, the red light beam flux is reduced to about 14% of the original incident red light. At the same distances, the blue-violet end of the spectrum has hardly been attenuated at all.
You can see that even modest size pieces of glacial ice (several meters in length / diameter) should be able to attenuate the red-to-yellow end of the spectrum and appear with varying degrees of blue tints. Looking into an ice borehole in an Antarctic ice sheet shows how intensely blue the deeper part of the glacial ice appears to the viewer on the surface. The removed ice core is a slender cylinder of ice that looks like clear ice when viewed from the side.
So… why is snow white? Light does not penetrate into snow very far before being scattered back to the viewer by the many facets of uncompressed snow on the surface. Thus, there is almost no opportunity for light absorption by the snow, and hence very little selective absorption of the red-to-yellow part of the visible spectrum.
For the same reason, sea ice, which is formed by the seasonal freezing of the sea surface, appears white because of the high concentration of entrained air bubbles (relative to glacial ice) that causes rapid scattering of incident light. Sea ice does not go through the metamorphism that produces glacial ice on land.
What is glacial ice?
The USGS describes glacial ice as follows: “Glacier ice is actually a mono-mineralic rock (a rock made of only one mineral, like limestone which is composed of the mineral calcite). The mineral ice is the crystalline form of water (H2O). It forms through the metamorphism of tens of thousands of individual snowflakes into crystals of glacier ice. Each snowflake is a single, six-sided (hexagonal) crystal with a central core and six projecting arms. The metamorphism process is driven by the weight of overlying snow. During metamorphism, hundreds, if not thousands of individual snowflakes recrystallize into much larger and denser individual ice crystals. Some of the largest ice crystals observed at Alaska’s Mendenhall Glacier are nearly one foot in length.”
Where do glaciers exist?
The National Snow and Ice Data Center (NSIDC) reports that, “glaciers occupy about 10 percent of the world’s total land area, with most located in polar regions like Antarctica, Greenland, and the Canadian Arctic. Glaciers can be thought of as remnants from the last Ice Age, when ice covered nearly 32 percent of the land, and 30 percent of the oceans. Most glaciers lie within mountain ranges that show evidence of a much greater extent during the ice ages of the past two million years, and more recent indications of retreat in the past few centuries.”
Glaciers exist on every continent except Australia. The approximate distribution of glaciers is:
- 91% in Antarctica
- 8% in Greenland
- Less than 0.5% in North America (about 0.1% in Alaska)
- 0.2% in Asia
- Less than 0.1% is in South America, Europe, Africa, New Zealand, and New Guinea (Irian Jaya).
There are several schemes for classifying glaciers; some are described in the references at the end of this article. For simplicity, let’s consider two basic types.
- A polar glacier is defined as one that is below the freezing temperature throughout its mass for the entire year. Polar glaciers exist in Antarctica and Greenland as continental scale ice sheets and smaller scale ice caps and ice fields.
- A temperate glacier is a glacier that’s essentially at the melting point, so liquid water coexists with glacier ice. A small change in temperature can have a major impact on temperate glacier melting, area, and volume. Glaciers not in Antarctica or Greenland are temperate glaciers. In addition, some of the glaciers on the Antarctic Peninsula and some of Greenland’s southern outlet glaciers are temperate glaciers.
How old is glacier ice?
Some glacial ice is extremely old, while in many areas of the world, it is much younger than you might have expected.
USGS reports: “Parts of the Antarctic Continent have had continuous glacier cover for perhaps as long as 20 million years. Other areas, such as valley glaciers of the Antarctic Peninsula and glaciers of the Transantarctic Mountains may date from the early Pleistocene (starting about 2.6 million years ago and lasting until about 11,700 years ago). For Greenland, ice cores and related data suggest that all of southern Greenland and most of northern Greenland were ice-free during the last interglacial period, approximately 125,000 years ago. Then, climate (in Greenland) was as much as 3-5o F warmer than the interglacial period we currently live in.”
“Although the higher mountains of Alaska have hosted glaciers for as much as the past 4 million years, most of Alaska temperate glaciers are generally much, much younger. Many formed as recently as the start of the Little Ice Age, approximately 1,000 years ago. Others may date from other post-Pleistocene (younger than 11,700 years ago) colder climate events.”
- The age of the oldest glacier ice in Antarctica may approach 20,000,000 years old.
- The age of the oldest glacier ice in Greenland may be more than 100,000 years old, but less than 125,000 years old.
- The age of the oldest Alaskan glacier ice ever recovered was about 30,000 years old.
Blue glacial ice along the coast of the West Antarctic Peninsula
In February 2020, my wife and I made a well-timed visit to the West Antarctic Peninsula. One particularly amazing spot was Pléneau Bay, which easily could earn the title “Antarctic Museum of Modern Art” because of the many fanciful iceberg shapes floating gently in this quiet bay. Following is a short photo essay highlighting several of the beautiful blue glacial ice features we saw on this trip.
Examples of blue glacial ice in Switzerland & New Zealand
In previous years, my wife and I visited a temperate glacier and ice cave in Grindelwald, Switzerland and hiked on the temperate Franz Josef Glacier on the South Island of New Zealand. Following is a short photo essay that should give you an idea of the complex terrain of these glaciers and the smaller scale blue ice features visible on the surface. In contrast, the ice cave was a unique, immersive, very blue experience. The blue color inside the cave looked like the eerie blue light from Cherenkov radiation, like you’d see in an operating pool-type nuclear research reactor.
For more information:
- “What is a glacier?” US Geologic Survey (USGS) website: https://www.usgs.gov/faqs/what-a-glacier?qt-news_science_products=0#qt-news_science_products
- “Why Glacier Ice is Blue,” USGS website: https://www.usgs.gov/faqs/why-glacier-ice-blue?qt-news_science_products=0#qt-news_science_products
- “Common Questions and Myths About Glaciers,” National Park Service (NPS) website: https://www.nps.gov/glba/learn/nature/common-questions-and-myths-about-glaciers.htm
- Becky Oskin, “Why Are Some Glaciers Blue?” LiveScience website: https://www.livescience.com/51019-why-is-antarctica-ice-blue.html
- Stephen Warren, “Optical properties of ice and snow,” Philosophical Transactions of the Royal Society, 15 April 2019: https://royalsocietypublishing.org/doi/full/10.1098/rsta.2018.0161
- “About Glaciers,” National Snow and Ice Data Center (NSIDC) website: https://nsidc.org/cryosphere/glaciers/information.html
- Robin George Andrews, “Icebergs can be emerald green. Now we know why,” National Geographic, 15 March 2019: https://www.nationalgeographic.com/science/2019/03/icebergs-can-be-emerald-green-now-we-know-why