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Thursday, 29 May 2014

Annual layers

I found this great photo of the annual layers in a snowpatch near a glacier edge in Iceland.  If you try, you can count these in exactly the same way as you count varves or tree rings to assess age.  The dark lines represent the melting or ablation surfaces at the end of each summer, when there is always a lot of dust and other debris on the snow or ice surface.

In Iceland, things can get complicated because of the proximity of volcanoes and the likelihood that in some years layers of ash will fall onto snow and ice surfaces.  If the ash layer is thin, it will absorb heat and accelerate melting; but if it is thick it can act as a protective layer and inhibit melting on the surface.  If the distribution of ash on a cold surface is patchy -- affected by wind turbulence, for example -- then serious complications might set in, because in some places older layers might be ablated away entirely, while in other areas they will be preserved.  So when it comes to interpretations, care is of the essence........

These layers are often transformed into ogives in glacier ice -- sometimes very much distorted by the complexities of ice flow around obstacles.

2 comments:

ND Wiseman said...

Hi Brian -
Please clarify: Is the process Ablation or Sublimation?

Neil

BRIAN JOHN said...

Could be either, Neil. Sublimation occurs where the air is very dry, as in some high mountain areas -- the snow or ice is converted straight into gas without the intervention of a liquid phase. Most commonly we see ablation -- that's a broad term covering all melting processes. Normally snow or ice is converted to water, which then seeps or trickles away, maybe eventually ending up in roaring torrents on some braided river system or other.....