Saturday, March 03, 2007

Ice Thunder

I confess to having read Thoreau's WALDEN POND at an impressionable age, so when I heard ice thunder on Mohican Lake I had some vague knowledge of it. That may not describe the sound exactly, though it vibrates the ribcage the same way a deep clap of thunder does. Maybe "boom" or "resonant crack" expresses it better.

However, no description can convey the way it startles. From the living room it sounded as if something had knocked the house off its foundation.

I kept hearing it, too, so I went outside to listen. Sometimes the boom would start across the lake, sounding ever louder at irregular intervals until it reached the cove. Then it seemed to start in the cove, and roll to the opposite side. At other times I'd hear a single boom.

I assume I heard it because the lake had no snow cover. I don't remember hearing it the winter before when snow covered the ice, and I haven't heard it since a snowstorm blanketed the ice in February.

When I examined the ice, I noticed cracks of various lengths and depths, with some cracks seeming to run the full thickness of the ice, from 8-12 inches, according to an ice fisherman who'd drilled holes in several places. The boom probably occurs at the moment of cracking, or perhaps when these cracked sections of ice rub against or collide into each other.

Expansion and contraction must be the root cause of it. If you were to put a glass of water in the freezer, the water would freeze, expand and break the glass. Wind also might play a part. It seemed loudest during a very windy two day period.

A walk in Central Park that same week further illuminated the mystery. Ice on the pond by Bathesda Fountain had heaved, due to expansion, onto the concrete steps.

Thoreau's observations also help. Here's how he described hearing it on Walden Pond:

"The pond began to boom about an hour after sunrise, when it felt the influence of the sun's rays slanted upon it from over the hills; it stretched itself and yawned like a waking man with a gradually increasing tumult, which was kept up three or four hours. It took a short siesta at noon, and boomed once more toward night, as the sun was withdrawing his influence. In the right stage of the weather a pond fires its evening gun with great regularity. But in the middle of the day, being full of cracks, and the air also being less elastic, it had completely lost its resonance, and probably fishes and muskrats could not then have been stunned by a blow on it. The fishermen say that the "thundering of the pond" scares the fishes and prevents their biting. The pond does not thunder every evening, and I cannot tell surely when to expect its thundering; but though I may perceive no difference in the weather, it does. Who would have suspected so large and cold and thick-skinned a thing to be so sensitive? Yet it has its law to which it thunders obedience when it should as surely as the buds expand in the spring. The earth is all alive and covered with papillae. The largest pond is as sensitive to atmospheric changes as the globule of mercury in its tube."