Thursday, May 16, 2013
How many grains of sand are there on earth? That is a good question. But a ball-park estimate is fairly simple.
We will look at fine sand (grain size = 100 microns), and coarse sand (grain size = 1 mm).
So a cubic mm can hold 1000 grains of fine sand, or 1 grain of course sand. Obviously grain size is important.
There are 1x1018 cubic millimeters in a cubic km.
How many cubic km of sand, sandstone, etc we have Is a tricky question. But if we say the average thickness of all sand for the globe is 200m (a thin number in any sedimentary basin, but most of the Earth is not a basin in the traditional sense. The surface area of earth is 5x10 8 km2, so a 0.2 km layer gives 10 8 cubic km of sand.
This brings the total grain count to somewhere between 103 x 10 18 x 10 8 = 10 29 for the fine sand, and a thousand times less than that, or 10 26 grains of the coarse sand. If you want to know how that compares to the number of stars in the sky, ask an astronomer.