The Seven Coloured Earth(s) (Terres des Sept Couleurs in French) are a geological formation and prominent tourist attraction found in the Chamarel plain of the Rivière Noire District in south-western Mauritius. It is a relatively small area of sand dunes comprising sand of seven distinct colours (approximately red, brown, violet, green, blue, purple and yellow). The main feature of the place is that since these differently coloured sands spontaneously settle in different layers, dunes acquire a surrealistic, striped colouring. This phenomenon can also be observed, on a smaller scale, if one takes a handful of sands of different colours and mixes them together, as they'll eventually separate into a layered spectrum. Another interesting feature of Chamarel's Coloured Earths is that the dunes seemingly never erode, in spite of Mauritius' torrential, tropical rains.
The sands have formed from the decomposition of volcanic rock (basalt) gullies into clay, further transformed into ferralitic soil by total hydrolysis; the two main elements of the resulting soil, iron and aluminium, are responsible for red/anthracite and blue/purplish colours respectively. The different shades of colour are believed to be a consequence of the molten volcanic rock cooling down at different external temperatures (hence rates), but the causes of their consistent spontaneous separation are yet to be fully clarified.
source : http://en.m.wikipedia.org/wiki/Seven_Coloured_Earths
