what causes layering in glacial ice?

http://www.google.com/search?hl=en&rls=com.microso...

http://ice-glaces.ec.gc.ca/WsvPageDsp.cfm?ID=11911...

see photos about halfway down the page and a little more....

best pics....

http://www.waycooldogs.com/artistically-striped-an...

heck, I see that even in the snow over the course of the winter. Particularly obvious along roadsides where the banks have been cut away and removed.

For the most part, each snow event has its own compaction and grain characteristics, so there is distinctly visual changes between events. Often, the top of a layer is also marked by partial melting or recrystallization, making a distinct surface to the layer from each event.

Layer thickness is different due to different volumes of snowfall for any given annual event, but from layer to layer, there are rarely any defined changes, more of a gradual spectrum of blues depending on where and what you're looking at.

It's actually rather like answering, "Why is the sky blue?" As with water, this varying blue-green color is caused by the absorption of both red and yellow light wavelengths, leaving the blue wavelengths available to be seen by the human eye. The absorption spectrum of ice is similar to that of water (shouldn't be too surprising), except that hydrogen bonding causes all of the spectral peaks to shift to lower energy - making the color greener. This effect is augmented by scattering within snow, which causes the light to travel an indirect path, providing more opportunity for absorption.

Standing on the surface, snow and ice look uniformly white. This is because almost all of the visible light striking the snow or ice surface is reflected back, without any preference for a single color within the visible spectrum (just as white paper looks white - nearly the entire spectrum is bounce back to our eyes).

The situation is different for light that is not reflected, but penetrates or is transmitted into the snow and ice. As these light waves travel into the snow or ice, the ice grains scatter a large amount of light (similar to Rayleigh scattering in the atmosphere). If the light is to travel over any distance, it must survive many such scattering events, meaning that the light must keep scattering without being absorbed. But we usually see the light coming back from the near surface layers (less than 1 centimeter deep) after it has been scattered or bounced off other snow grains only a few times, which is why it still looks white.

In simplest of terms, think of the ice or snow layer as a filter. If it is only a centimeter thick, all the light makes it through; if it is a meter thick, mostly blue light makes it through. This is similar to the way coffee often appears light when poured, but much darker when it is in a cup.

But once you get deeper in the snow and ice, the preferential absorption of the red and yellow wavelengths begins to become noticeable. Again, just as with water, more red and yellow light is absorbed compared to blue. Not much more, but enough that over a considerable distance in the ice, perhaps a meter or more, the photons coming back to your eyes tend to be made up of more blue light than red light. This can be seen when poking a hole in the snow and looking down into the hole to see blue light, or far more famously, in the blue color associated with the depths of crevasses in glaciers. In each case the blue light is the product of a relatively long travel path through the snow or ice. This deep-ice spectral selection is related to absorption, and not to reflection, as with the shallower surfaces.

Water?