Today I received three nice sand samples from Fuerteventura, Canary Islands. They were sent to me by Cathrene Rowell from England. One of the samples is taken from dunes which are supposedly made of sand from the Sahara Desert.
The sample is from the Corralejo Dunes (Dunas de Corralejo) which occupy large area of the NE part of Fuerteventura which is only 120 km away from the African coast. These dunes are a major tourist attraction of the island and as I can understand tourists are told that they are walking on a sand from the Sahara which is blown here by the wind.
There is one little problem with this nice story — it simply is not true. Why not? Dune sand of Sahara is composed of quartz grains mostly which are usually covered by fine hematitic powder that gives them reddish hue. The sand sample from the Corralejo Dunes, however, is composed of various fine-grained biogenic grains with no quartz at all. There are few silicate grains but these are mafic fragments of dark-colored volcanic rocks. This is the material the Canary Islands are made of.
I do not know whether locals genuinely believe the story of sand blown from the Sahara or are they just making it up to make the dunes more attractive to tourists. Corralejo Dunes are a remarkable place without this Sahara story. Approximately 15 square kilometers covered with biogenic grains of local marine origin on a relatively small volcanic island is a remarkable story in its own right.
This does not mean, however, that there is no sand blown there from the Sahara. Dry and dusty wind from the Sahara is well-known to the inhabitants of these islands but this is mostly very fine-grained dust, not sand. These tiny quartz grains from the Sahara are probably very common constituents of soil there but I am not aware of any mechanism that is able to separate this dust from the local material and pile it up into dunes.
But where is this sand coming from? It is believed that the source of the sand is seabed next to the island and between Lanzarote and Fuerteventura which are geologically the same structure. There is only a shallow strait separating them. The seabed was dry during the most recent glaciation and climate was very arid at the time around Fuerteventura which helped the prevailing trade winds to carry the sand near the present shoreline1. Important conclusion of it is that the sand dunes of Corralejo are essentially non-renewable resource (unless new glacial episode arrives).
Dunes of Corralejo. Photo: Cathrene Rowell.
Close-up photo of sand that makes up these dunes. It is composed of tiny biogenic calcareous fragments of forams, molluscs, sea urchins, etc. It is a coral sand in the loose sense of the word. Width of view 8 mm.
This is how the real Sahara sand looks like. It is almost pure quartz with fine hematite powder with no marine shells. The sample is from the Erg Murzuk, Libya.
1. Carracedo, Juan C. & Day, Simon (2002). Canary Islands (Classic Geology in Europe). Dunedin Academic Press Ltd.