Blue Glacier Ice
December 21, 2010
From the outside, glaciers look as white as snow. After all, they start out as snow. But on the inside, glaciers are deep, pure blue. Why are some parts of glaciers white while others are blue? Two things control their color: the spectral absorption of ice and the amount of air trapped in the ice.
Ice, like water, behaves like a weak blue filter, absorbing red and orange light more strongly than it absorbs greens and blues. The thicker the ice, the bluer it looks as more and more reds and oranges are absorbed. The same is true of water: the deep end of the pool is bluer than the shallow end. Glacier ice is not pure frozen water -- it contains a lot of air. The myriad air-ice interfaces reflect white sunlight before it has a chance to penetrate and undergo absorption. Therefore, snow and the surface of glaciers appear bright white. However, some of the light trickles in and deeper in the glacier much of the air has been squeezed out of the ice, leaving fewer bubbles to reflect light back out. Here is where most of the absorption takes place. The result is deep blue light scattering around inside the glacier. This picture shows a view down a crevasse in the Mendenhall Glacier near Juneau, Alaska. Note the white surface ice and the progressively bluer color with depth. By the way, glaciers are not as pure as, well, the driven snow. In the ablation zone, as shown here, they're quite dirty. Photo taken on May 5, 2010.