Glaciers: Ice with an attitude!

We’ve already spent a bit of time looking at streams (click here for a listing of posts related to moving water). And while many of us may assume that glaciers work just the same — and there are a multitude of similarities — there are some pretty awesome differences as well.

So… let’s start with a general introduction to the cryosphere. We’ll explore the details of the various types of glaciers in later posts.

For the most part, glaciers come in two flavors: those that form in the mountains (usually called alpine or valley glaciers), and those that don’t (often referred to as continental ice sheets, ice caps, or A.B.F.P.O.I.). Valley glaciers are generally the easiest to understand and usually flow downhill, while ice caps pretty much go wherever they want.

To form a glacier there are several non-negotiable requirements. First, it has to be cold enough so that any precipitation falls as snow, and doesn’t all melt during the spring and summer. After that, all it takes are years of accumulating snowfall to pile up enough of the white stuff — usually a hundred feet will set the process in motion — so that the layers on the bottom start to compress, drive out the air, and increase in density. This is a true metamorphic change.

(The short version on a true metamorphic change: I promise a more complete discussion later, but to get us past now, what you start with on an elemental basis is what you end up with (in this case H₂O), and the material never completely converts from the solid to the liquid phase. The rules may be simple, but they are absolutely etched in stone — or ice, as the case may be.)

Converting newly fallen snow to glacial ice is actually a fairly simple process. It’s all about depth of burial, pressure, and density. Freshly fallen snow generally has a density well less than that of water (which is, by definition, 1.0 gr/cm³). As the air is driven out, the snow (starting at 0.1 gr/cm³) changes to granular snow (now with a density of approx. 0.3 gr/cm³), then firn (0.6 gr/cm³), and finally to true glacial ice with a density of 0.9 gr/cm³. So yes, ice floats… as do icebergs, as the folks on the Titanic learned to their dismay.

One more requirement: a glacier can only form on land — there has to be a hard bottom so that the pressure can increase, drive out the air, and increase the density. So… floating sea ice doesn’t count.

(BTW: Check out the snow-white porcelain throne in this image of the North Pole, above. It may be hard to see — it’s in front of the left side of the submarine — but definitely worth looking for. WTF! Not sure of the nationality here, but I’m not convinced that the submarine is from my part of the world.)

Anyway, all glaciers have several parts, or zones, and the good news is that they have the same names and functions, no matter where the glacier forms. Since alpine glaciers are usually the easiest to understand, we’ll use them in our discussion.

Any glacier is divided into two distinct (but highly variable) zones by what is called the “snowline” — that location that marks the elevation (in the case of valley glaciers) where the annual temperature either stays below freezing year-round, or doesn’t.

The area above the snowline is called the Zone of Accumulation: that region where snow, and ultimately ice, is added to the glacier. By definition, this is the portion of the glacier where the year-round temperatures remain below freezing, some of the snow survives the summer sun, and new ice is added to the glacier.

The mass of ice will flow — always away from the Zone of Accumulation — into the Zone of Wastage, where the ice ultimately melts and the glacier ends at what is called the terminus. While the Zone of Accumulation is covered year-round with fresh, dazzlingly white, newly fallen snow, the Zone of Wastage is commonly where we find exposed dirty ice, crevasses, and very dangerous hiking conditions — all the season’s snow has melted.

This image of an alpine glacier, above, shows all three of these critical portions of a glacier.

In order to be a glacier, the ice has to move. There are two types of motion: the entire block of ice sliding over the bedrock it sits upon (called (“basal slip”), but more importantly inside itself. This is called “internal plastic deformation” and — just like any other metamorphic process —is an iron-clad requirement: if it isn’t deforming internally, it’s just a big ice cube.

I never drilled through a glacier during all my years as an exploration geologist, but have had several run-ins with kindred souls who did. None of them were very enthusiastic about the experience. Along with being cold all the time with frozen fingers and toes, they had to complete the drill hole within a week or so — any longer than that, and either the basal slip would pinch the drill stem and shear it off at the ice/bedrock interface, or the internal plastic deformation would get them. Both would be considered bad, and could cost the geologist not only the drill hole, but possibly their job.

Glaciers share their water with the water cycle, and since the vast majority of the earth’s water lives in the ocean, it is clear that sea level will rise and fall with the advance and retreat of an ice age. The northern Puget Sound clearly shows what can be left behind as the ice melts, returns its water to the sea, and the glacially carved valleys are drowned by the rising seas.

We are in an “interglacial” period now, but a mere 15,000 years ago there was a fully developed ice age just ending. The image above of the Gulf of Mexico is an excellent example. The submerged continental shelf surrounding Florida was surely exposed during the ice age, and the beach — and the location of most human communities, then as much as now — was hundreds of miles from where it is today. (I absolutely promise a much more detailed post about sea level fall — and rise — in response to fluctuations of the cryosphere.)

Well, this has been a very brief introduction to the cryosphere. There is so much more, but we’ll get to the rest in later posts when I will hopefully detail the various types of glaciers.