November 24, 2007 in Keepers Log, More Logbook, Questions for the Keeper

Question for the keeper: bubbles and sand-ice

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This morning, I stepped outside after breakfast to admire the day. Last night, the full moon shone brightly and a cold wind blew fiercely. At dawn, a thin fringe of ice was left on the beach by the retreating tide. Now, as the rising tide submerged the beach, I noticed steady streams of bubbles emerging from the sand. This was unusual. I’ve watched the tide rise and fall daily for over two years now at the lighthouse, but I’ve never seen this bubbling sand before today. I stood dumbfounded on the ramp to the dock watching the bubbles and trying to figure out their origin. Some visitors happened by and asked me, Where do those bubbles come from? I didn’t have an answer. I was as baffled as they were. Did they come from a buried pipe? they wondered. Not a pipe, I was certain. Determined to figure out this puzzle, I put on some waterproof boots and waded into the water to investigate. I pushed my finger into the sand. It was hard, but eventually I broke through the top layer. Immediately, a stream of bubbles emerged. Eureka! The top layer of sand was frozen for a depth of a 1/2 inch. The wet sand must have frozen overnight when exposed to the wind at low tide, trapping air underneath. Now that the sand was submerged, the trapped air escaped through cracks and holes in the frozen layer of sand-ice. Normally, when the sand is not frozen, air escapes uniformly, seeping through the porous sand. When the sand is frozen, the air escapes in visible streams of bubbles.

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