Sand dune
There are 4 sections. The first section: how did the sand dune be formed. The sand was blown by the wind and bounced when they stroke on hard thing (for example rock). If they met soft thing, they would piled gradually and formed a dune. When other sand passed by the top of the dune, the sand is easier to fall. So sand dune has been formed.
The second section: Why does the sand dune move? When the dune is huge enough (namely an angle of the slope face surpasses 30 degree), the sand on the top of slope will collapse. Then the entire sand dunes go forward along the collapsing direction.
The third section mentions an interesting phenomenon that the scientists resemble have not been able to explain--when the dune moved, it made a sound (Moaning). The faster it moves, the louder the sound. (A detail question is here). But some dune does not emit sound at all. Some dunes emits sound now and then (temperamental). (Another detail question, why the sand dune is said to be temperamental).
The fourth section: This kind of moaning has relationship with moisture, generally speaking, when moisture is very high, you cannot hear the dune moaning. But, the author has points out a counter-example, he said that in seashore, the moisture of the sand is high, but it still emits moaning. Why? The scientists explain that the facial interaction between the sand is determined by not only moisture but also the material of the sand. The main ingredient of common sand is quartz, but of seashore sand, is calcification. |