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I noticed a strange phenomenon observing a lava lamp. We all know the complex flow of bubbles when the lamp is "hot" and running, but I would like to have some explanation about the startup behavior of the lamp:

  • When the lamp is cold and just turned on the wax is at the bottom;

  • The lamp starts heating, small long and narrow wax "sticks" start to form;

  • In a few minutes inside the lamp we can see a big almost stable formation of was starting from the bottom of the lamp and rising to the top (with a lot of wax on the top);

  • That formation stays almost stable for several minutes, with minor movements, then it collapses quite quickly:

  • some "mountains" start to form at the bottom, from there bubbles start to rise and fall;

  • more and more wax is put on movement, the lamp is hot.

Of course there are variations, but the process is mostly as described. Here is a (not mine) timelapse, at about 1 minute and 45 seconds you can see the "superstructure" that quickly rises then stays there for a lot of time.

I understand why the wax goes up and down when the lamp is hot, what I do not understand is why most times during the startup inside the lamp there is a huge wax "superstructure" that can stay almost motionless for several minutes (the step in bold). When the lamp is hotter huge formations are not stable at all and it is possible to see ascending and descending flow of wax inside big formations, that change shape quickly. For some reason this stability is instead possible when the temperature is not yet at the top.

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    $\begingroup$ The stalagmites that form initially are because the wax wants to rise due it's lighter density, but the water cools the exposed surface of the wax which forms the column whose exterior sets solid. However, the wax internally remains molten. The top of this structure continues to burst upwards because of the pressure from molten wax underneath which wants to rise, and because the hottest wax continues to flow to the peak of the structure. Wax which is cooler but remains molten, can simply sink down the column and be replaced by newly-heated wax from the bottom of the column. $\endgroup$ – Steve Jun 2 at 21:52

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