Well, neither have I. A discussion with Julie, the campground chief, cook, and bottle washer enlightened me on this 40 degree barely sunny afternoon. Not believing when she told me that the ice only melts about an inch or so off the very top then sinks, I had to walk another 1/4 of a mile to get internet access so I could verify this phenomena. At the expense of erring by doing a simply cut and paste of an article address this I did just that. To be honest, after reading it 3 times my chemistry acumen continues to baffle me such that I am in no position to paraphrase, re-write or otherwise try and explain. For your edification here is the article from the Star Tribune, presumably written by Karen Youso as copied from the internet on 5/5/2016.
Fixit: Lake ice sinks then disappears each spring
AIce floats on a lake’s surface until it is melted. Although it sometimes floats low in the water, it does not sink to the bottom, as some mistakenly believe. Water is heaviest at 39 degrees, lighter at higher or lower temperatures. Falling air temperatures in autumn and lower sun angles, in conjunction with wind and wave action, result in the lowering of lake temperatures. Because the cooler water is heavier, it will sink, displacing and forcing warmer water to the surface. This continues until the entire lake reaches a temperature of about 39 degrees.Following this, surface temperatures will fall below 39 degrees and the cooler and lighter water will remain at the surface. As the surface water cools further, it will eventually change into ice.
Because the ice is colder and less dense than the water below it, it floats. In the spring, the reverse occurs. Rising air temperature and higher sun angles cause melting to begin on top of the ice layer. If there are no cracks or fissures, the melting water will accumulate on top of the ice. If this happens over an entire lake (which is unlikely) or over parts of the lake, one might get the impression that the ice is sinking. But it isn’t.
Further warming will cause the ice to become rotted or honeycombed, with water and air filling the void. The dark color of ice in late winter is because of this honeycombing. As the honeycombing process continues, the ice mass floats lower and lower in the water until it is completely melted, but it never sinks to the bottom.
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