Striped Icebergs: Why Antarctic Ice Shows Blue, Green and Brown Bands

Most icebergs are white, some glow blue, and a few in the Southern Ocean carry bold stripes of blue, green, brown or black. In 2008 the Norwegian sailor Øyvind Tangen photographed a series of these striped icebergs from a research ship about 1,700 miles (2,700 km) south of Cape Town, roughly 660 miles (1,060 km) north of the Antarctic coast.

Why most icebergs are white, and some are blue

An Antarctic iceberg starts as snow on the ice sheet. Over centuries the snow is buried and squeezed into glacier ice, but it keeps millions of tiny air bubbles. Those bubbles scatter light of every colour, which is why ordinary icebergs look white.

Blue comes from the ice itself. As the USGS explanation of blue glacier ice puts it, ice absorbs the red, long-wavelength part of white light, while blue light passes through and is scattered. The longer the path the light travels through ice, the bluer it looks, so dense, bubble-free ice shows the deepest colour.

How the blue stripes form

A blue stripe usually marks a crack that filled with meltwater. On the ice sheet, water runs into a crevasse and freezes fast, without trapping bubbles. The result is a vein of clear, dense ice inside the white glacier ice. When the glacier reaches the sea and calves, the iceberg carries that vein with it, and once the berg tilts or breaks, the vein shows on its face as a sharp band of blue.

Green, brown and black bands

Green stripes and the rare all-green “jade” icebergs come from marine ice. This is seawater that freezes onto the underside of an ice shelf. Like meltwater ice, it has no bubbles, so it is clear and dark.

Sailors have reported green icebergs near parts of Antarctica since the early 1900s. Glaciologist Stephen Warren of the University of Washington first sampled one near the Amery Ice Shelf in East Antarctica on an Australian expedition in 1988. Algae and dissolved organic matter were long the favourite explanation, but samples taken in 1996 showed green and blue marine ice holding similar amounts of organic carbon.

In a 2019 study in the Journal of Geophysical Research: Oceans, Warren and colleagues proposed that iron oxides do it. Rock dust ground off the continent by the ice gets trapped under the shelf, and a core from the Amery held nearly 500 times more iron in its marine ice than in the glacier ice above. Yellowish-brown iron mixed with the natural blue of ice would give green. If the idea holds, icebergs could be carrying iron out to sea, where it feeds phytoplankton at the base of the Southern Ocean food web.

Brown, yellow and black stripes are simpler: they are sediment. As a glacier grinds downhill over bedrock, it picks up dust, gravel and rock debris and folds them into its lower layers. When an iceberg rolls over in the water, those dirty layers can end up high above the waves.

Where to see striped icebergs

The best chance is on an Antarctic voyage. Most expedition cruises sail from Ushuaia in Argentina across the Drake Passage to the Antarctic Peninsula between November and March, the southern summer. Striped bergs turn up more often after storms, when icebergs have flipped and shown their undersides. Keep a telephoto lens ready: ships give large icebergs a wide berth, because a berg can roll or shed ice without warning.

For more Antarctic ice, read about the red colour at Blood Falls, or compare the blue lake-ice hummocks of Siberia’s Lake Baikal.

Related reading

• Antarctic Icebergs in Motion: Drift, Collapse and Meltwater
• Zaria Forman’s Greenland Icebergs in Pastel
• An Iceberg Rolls near Argentina’s Upsala Glacier
• The Ilulissat Calving Sequence in Chasing Ice

Sources