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Originally these flat pieces get created as the top layer on the beach where water pulls back (low tide), leaving the sand to dry on the Sun. The top layer dries and creates a separate layer, making these flat solid pieces.

enter image description here

Sometimes these flat crumbles are extremely thin, with a thickness of only around 4 grains. I cannot understand how these stay in one piece after they dry completely:

  1. first I thought it was simply moisture, but if I let them dry completely on the Sun, they become dry throughout. When I brake them apart, even the crack looks and feels completely dry throughout.

  2. then I thought maybe it is salt, somehow the salty ocean water sticks the grains together, and after they dry, the salt stays in between the grains as some kind of glue

  3. the grains are themselves edgy crystals that can somehow lock together because there might be vacuum

  4. Same thing happens in microwave, the sand piece stays solid, like a rock, and won't fall apart. This makes me think it shouldn't be water that keeps it in one piece.

Will a microwave heat sand?

Question:

  1. What keeps dry flat pieces of sand crumbles together (even if you microwave them)?
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  • $\begingroup$ This was a rather unusual observation and it is amazing that there is such an obvious answer. has anyone done any research on this and is there a source? $\endgroup$ Mar 11 at 5:13
  • $\begingroup$ @HolgerFiedler thank you so much! I have not found anything about this, but I just did my own research. $\endgroup$ Mar 11 at 5:21
  • $\begingroup$ Than more it is astonishing that Niels has an answer :-) $\endgroup$ Mar 11 at 5:23
  • $\begingroup$ I have never seen this on beeches in Greece, and I have been on many. Dry sand is just dry sand that your shoes fall in when walking on long beeches.. So it must be either the content of the ocean water as Niels guesses, or a chemical property of the sand itself, depending of it origin before becoming sand. see for example here en.wikipedia.org/wiki/Limestone $\endgroup$
    – anna v
    Mar 11 at 6:34
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The water which was mixed with the sand contained not only salt but lots of microorganisms called plankton. Taken together, they make a simple sort of glue that tends to absorb moisture and retain it even when heated, and keep the grains stuck together (weakly).

This is testable, by taking a sand sample and scrubbing it around with dish detergent and rinsing it several times so the sand particles are as clean as possible, and then drying the sample in the sun to see if a crust forms.

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