All about the WAIS (not the treble)!
Stratigraphy
In the summer at WAIS Divide, temperatures hover around -10C/14F, more or less. In the winter here, temperatures drop down to -50C/-68F. At temperatures that low, it is actually too cold to snow!
So most of the snow that falls in Antarctica actually falls during the summer months here. The solar radiation on the top layer of snow during the summer and over the fall (as the sun rises and sets) causes a metamorphosis in the ice crystals just under the surface. This, along with wind, temperature changes, the loss of sunlight over the winter, and various other natural processes, creates a visible difference in the density of the snow. Therefore, the snow that is on the surface as winter starts is visibly different from the snow on the surface as summer starts.
Since this pattern repeats itself each year (now for 68,000 years at WAIS Divide), you can see a structure of layers in the ice. Scientists can use these layers to identify each year's snowfall within the ice.
Within the walls of a back-lit snow pit, you can see the many layers of snow and ice in the top few meters of the ice sheet. Photo credit: UNH
This is similar to a tree, whose rings you can count to know its age.
For each new year of growth, the tree adds a layer in the form of a ring. The youngest layers are on the outside. In ice, the layers get added atop one another. Photo credit: Albert Bridge
The term scientists use to describe this layered structure of annual snowfall is "stratigraphy." It is also used by geologists who study layers of rocks and soils in a similar way.
To be continued...
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