Sunday, January 16, 2011

Mickey Mouse and the Erosive Power of Frozen Water

Frost erosion, wedging or shattering is an effective agent of erosion in mountainous areas with characteristically low temperatures and numerous frost cycles. Typically, eroded, angular debris called scree or talus falls and collects in great quantities at the base of steep mountain slopes. This can be seen in the photo on the lower slopes of Tuckerman Ravine on Mount Washington in the White Mountains of New Hampshire, the second tallest summit east of the Mississippi River. The talus to the left of center in the photo is a landslide deposit derived from Lion's Head.

Several Quaternary continental glaciations and at least two episodes of cirque glaciation (Illinoian and Wisconsinan) have produced the peaks and cirques (locally referred to as ravines or gulfs) of the range. Note the classic U-shaped valley where glaciation has scoured the slopes of the mountain, also contributing rock litter to the valley floor. Note that the glaciers were a powerful erosional force that modified the existing landscape, but they did not build the mountains of the Presidential Range. That occurred 400 to 360 million years ago during the Acadian Orogeny.

Let's return to our discussion about erosion.

Wind, water and gravity. These are the prime agents of erosion. Of these elements, it is water in the form of glaciers, rivers, streams and tides that mechanically scours, abrades and cleanses the landscape in time bringing even the largest mountains to the sea. But water also acts in a way we seldom think of, by prying rocks apart by freezing. But how?

Water is a substance with some very unique behavioral properties. One property in particular makes water’s behavior one of a kind. Unlike most other liquids, when it freezes, it expands rather than contracts. How can this be, and what implications does this have for the landscape?

If you greatly magnified a molecule of water, it would look like Mickey Mouse’s head with two hydrogen-ears and an oxygen-head. This Disney-esque configuration makes a molecule of water highly charged like a magnet. We say that it possesses polarity. If you tossed a bunch of magnets into a bowl, the magnets would try to rearrange themselves based on their mutual attraction and repulsion. Once in the bowl, the magnets, being charged, might occupy a somewhat larger space, a different geometry if you will, than say similarly-shaped metal without a charge. The polarity of water affects its behavior in the same way when it freezes. Here’s how it works.

When water approaches its freezing point, its molecules draw closer together, as with all liquids. But at 4˚C (40˚F), the molecules become so close to each other that their polarity forces them to rearrange into a new spatial configuration, like magnets behave when they come close together. This is related to water's tendency to form a strong network of hydrogen bonds, each hydrogen-atom bonding with two oxygen-atoms. As the water gets colder, hydrogen bonding becomes stronger and more pervasive, as more water molecules come into closer proximity. The new arrangement occupies slightly more space than before, because the water molecules have assumed a more favorable polarity-relationship based on their Mickey Mouse-configuration. This is why water expands just above its freezing point, and by about 9%, thereby increasing its volume.


The first diagram depicts liquid water. The second illustrates frozen water, ice.
The ice occupies more volume, about 9%, and has more open spaces.
Pictures from the MathMol Project at the NYU/ACF Scientific Visulaization Laboratory.

This is why glass bottles containing water crack apart in your freezer and why pipes burst when the water in them freezes (a problem that southerners aren't concerned with but northerners potentially face every winter). And, this is how water pries rocks apart, when it flows into the pore spaces of rocks, joints and crevices, freezing and thawing repeatedly. During every freezing cycle, the water expands creating an outward pressure, enough to shatter rocks into smaller and smaller fragments. During every thawing cycle, more water enters each new available void. And so on.

Interestingly, it is the open structure that causes ice to be less dense than water. Water reaches its density maximum at 4˚C. This allows ice to float on water, as we all know. Therefore, in winter, water freezes from top to bottom. This helps to prevent water in the subsurface from freezing which protects life within lakes, rivers and oceans.

And lastly, as we all know, in the world of science there are always other potentially valid explanations........

From Calvin and Hobbes by cartoonist Bill Watterson

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