Thursday, December 31, 2009

Dihydrogen Monoxide in Winter

One crisp and perfectly calm morning, many winters ago, I found myself wandering along the icy north shore of Lake McDonald, a skinny 10-mile-long lake in Glacier Park. I had noticed ice-free waters at the far end, but from this vantage the silent lake was frozen over as far as one could see.

The silence broke wthout warning.

The single sheet of surface ice started heaving towards me, and the leading edge began crawling up the inclined beach. When the crackling and tinkling of so many ice cubes faded away 30 seconds later, almost a mile of beach was covered with a six-foot-wide pile of broken ice. The marching ice was probably more amazing to me than it was to the lone Chickadee perched nearby, but neither one of us felt the distant wind that pushed the tons of ice ashore.

Lake McDonald ice cubes (c) John Ashley
Such is winter in mysterious Montana.

All the glory and wonder of our landscapes take a backseat to the powers of ice. Dihydrogen monoxide (also known as "water") has some unusual properties that make magical things possible, like ice crystals, snowflakes and glaciers -- and maybe even life on Earth.

All life that we know of is water-based. And with the possible exception of a few antifreeze-producing insect cells, all cellular activity (i.e. "life") takes place in the narrow temperature range between the boiling and freezing points of water.

Water reaches its maximum density at 39F (4C). As it approaches its 32F (0C) freezing point, water molecules must spread out to line up into crystalline forms. More empty space (9% increase) between molecules means lower density. If not for that rare property, lakes and oceans would freeze solid from the bottom up, instead of forming a thin insulating layer of surface ice, and Earth might just be another frozen, lifeless planet.

We should celebrate the fact that ice floats in water -- while digging our cars out of the snow -- even if the stuff falls from the sky all winter.

Snowflake photomicroscope images by Kenneth G. Libbrecht, used by permission. Click to see his wonderful website.Inside a winter cloud, a freezing water molecule forms a tiny, six-sided crystal. As this new crystal gets blown about, more water molecules adhere and grow branches from the prism's six corners. Because the conditions are nearly identical on each corner, each branch might grow to look like the other five.

The result is a magical ice crystal that is usually less than one-quarter inch wide. Some are perfectly symmetrical, but most are not. The final form can take many shapes, including a star, plate, bullet, needle or prism. Some crystals look very similar, but no two are exactly alike.

Because they are now heavier than water vapor, these beautiful ice crystals begin falling towards Earth and colliding with each other to form snowflakes. The crystals stick to each other better when it is relatively warmer, and they are more fragile when it's colder. That 's why early spring snowflakes tend to be bigger than frigid midwinter snowflakes.

Snowflake after snowflake makes piles of snow, and piles and piles of snow make excellent insulation.

Within a few hours of landing, the uppermost layers of ice crystals interlock and form a crust. Once insulated from the cold air, the snow on the bottom is melted by the Earth's latent heat, and water vapor from the melting crystals migrates upward where it refreezes and reinforces the crust. Over time, a network of ice columns and air spaces about an inch tall forms below the snow insulation, and the temperature here will remain 1 or 2 degrees above freezing all winter, regardless of the air temperatures above the snow.

This "subnivean" world forms a warm winter habitat for short Montanans, like mice, voles and shrews. And these in turn form the winter diets of coyotes, foxes and some owls. These predator species have a keen sense of hearing, and can pinpoint the invisible rodents rustling around under the snow. Canids pounce feet first, and owls dive into and punch through the crust with clenched feet. They quickly sniff (canids) and feel (owls) through the pile of broken ice crystals and grab their stunned snack.

When a pile accumulates more snow and ice in winter than it looses in summer, it becomes a (non-moving) snowfield, which might eventually grow into a deep (moving) glacier.

Ice fractures easily if it's less than 160 feet (49 meters) deep. But if the ice grows deeper, the pressure of its own weight will make it act more like plastic. Flexible ice on flat ground begins to ooze out from underneath its own weight, while thick ice laying on a slope begins to flow downhill. Friction slows the glacier's bottom and edges, but the middle can move almost 100 feet (30 meters) per day.

These rivers of ice created many of the cool landscapes of northern Montana -- features like drumlins, moraines, erratics and aretes.

In the Tobacco Valley, the tadpole-shaped hills of the "Eureka Drumlin Field" are so many piles of leftover glacial till. The town of Polson sits atop a terminal moraine (a massive pile of rocky debris) that was carried down-valley by a glacier, and which also helped dam the waters of Flathead Lake.

"Erratics" are Canadian-born granite bolders that were delivered to and deposited in non-granitic eastern and central Montana by glaciers. In central Montana, a massive glacier also pushed a portion of the Missouri River many miles southward.

In southwestern Montana, crumbled ice from a massive glacier blocked the flow of the Clark Fork River and created "Glacial Lake Missoula," between 13,000 and 15,000 years ago. The lake was 2,000 feet (610 meters) deep at its highest level. The ice dam crumbled and reformed at least 41 times, sending scouring flash floods across Idaho and the eastern half of Washington. There were seven such glacial lakes in Montana.

Montana's Glacier National Park was named, not for todays' remnant glaciers, but for the massive, mile-deep glaciers from many thousands of years ago. When the most recent glacial period ended some 10,000 years ago, the melting glaciers left behind the famaliar U-shaped valleys and knife-edge ridges ("aretes") that we see today. In 1850, the park still harbored 150 smallish glaciers. Today, 26 glaciers remain, and these are predicted to melt away within the next 10 years.

Hoar frost on the hood of my blue truck (c) John AshleyIce is still hard at work today -- especially here in Montana. The repeated freeze/thaw cycles continue to create more of our most ubiquitous landforms, the highway "pothole." In spite of our warming climate, many of these potholes are predicted to grow larger in the years to come.

Behind the molecule: Long winters can also lead to unusual behaviors. Several college students created the "Dihydrogen Monoxide Research Division," to warn people about the dangers of water (and to illustrate the lack of scientific literacy). This in turn led to the creation of "Friends of Hydrogen Hydroxide."