Author: Colin Gooch, Resene technical Director.
Late afternoon in what has been quite a beautiful autumn. The setting sun streams through the lounge windows, bathing everything in a roseate glow and highlighting – every particle of dust that sits on every surface! My wife hates this time of day!!
Where does it come from?
Well, even inside of a modern, almost hermetically sealed, home there are plenty of sources of these motes. Human hair and that from our pets, human skin, pollens and spores from house plants and (a big one) fibres from textiles and paper all contribute – but it is when one ventures outside that the story of dust becomes mind blowing.
The largest source and type of dusts are those derived from minerals. Mineral dusts are produced by glaciers grinding moraine under their vast, slow moving weight, the constant comminution of particles ground by the ever-moving deserts, the silt deposited by the seasonal flooding and the receding of river systems and human tilling practices are but a few means of generation. When the dust particles become fine enough and winds become strong enough, these particles get lifted into the air to be transported over vast areas.
To give some idea of size, these eolian dusts range from 0.001 microns to 40 microns – a micron is a millionth of a metre and the human eye cannot detect anything smaller than 40 microns. So, wind-blown dusts can be seriously small and, of course, the smaller the particle, the further the wind can carry them. All the loess plateaus of the world have been built by the deposition of wind-blown dusts. New Zealand’s South Island has its own loess plateaus, over 1 million hectares has been identified in central Canterbury, thought to have been carried from glacial valleys and braided river beds by the nor’westerly Foehn winds.
N.A.S.A. satellites have studied the dust movement from the earth’s largest source – the Sahara Desert. They report that an average of 188 million tonnes per year are blown away by the easterly trade winds, enriching the Canary Islands, some of the islands of the Caribbean, and ultimately dropping 27.7 million tonnes onto the Amazon basin. The Eastern Sahara is a rich source of phosphorous (as New Zealand importers know) and 22,000 tonnes of phosphorous is contained within the dust dropped in the naturally phosphate deprived Amazon basin.
The Intercontinental symbiosis where one of the most arid areas on earth fertilises one of the lushest is almost magical.
It is prevailing Westerlies which transports 64 million tonnes of dust from the deserts of Northern Asia to North America and these same winds stripped 850 million tonnes from the great American dust bowl in 1935 alone. Indeed, one storm on the 9th May in 1934 is estimated to have stripped away 359 million tonnes in one day!