Sand – Witness of Time

When you relax on the beach and get a sand grain blown into your eyes and are trying to get it out, do you realise that you have a geological “antiquity” on your finger? Most probably not…

Most of the sand is generated by physical and chemical erosion of rock that is of magmatic or metamorphic origin (granite) from where tiny crystals are being separated. Gravity, wind and water will transport, mix and break these crystals and their edges are being rounded during their journey. The longer the way the more they get rounded. Research has proven that it can take thousands of miles (!) to round an average sized sand grain.

Erosion of continents is the main supplier of sediments and of dissolved minerals that will be deposited later by the action of micro organisms. Another source of dissolved minerals is the hydrothermal reactions at the oceanic reef where the young hot basalt gets leached out upon contact with the sea water. Volcanic particles are mostly found in the oceans as layers of ash.

A part of the dissolved minerals are being used by micro organisms to build up their organic components and will return as sediment after their death. The major part is being used as encasing for microscopic animals and plants. They account for about 40% of the marine sediments.

There are different types of sand. The fine white sand at the coral islands originates from crushed coral skeletons and is calcium carbonate (CaCO3). Famous are the green beaches from Hawaii, coming from the volcanic material Olivine (also Lanzarote has some of these beaches). Eroded basalt will give black sand and there are some very rare beaches with pink sand. The vast majority of sand is of Quartz (Siliceous dioxide) due to its abundance, its hardness (7 on the Moh’s scale) and its chemical resistance.

The majority of the sand comes from sand stone and has gone through several erosion cycles: sand gets deposited and then covered by other sediments; it compacts due to its own weight and the grains get clustered by some adhesive component (diagenesis). When tectonic movements raise the compacted material it will be exposed to erosion forces which will free up again the grains so that they can be rounded further during their next transport. Even when one considers a cycle time of 200 million years then a nicely rounded grain of Quartz-sand (like the one you have on your finger), easily could have passed through 10 cycles and thus have witnessed half the history of Earth (2 billion years)!

Dr. A. Lüdeking