Now, back to the subject of this blog, but still following the global theme. In the course of researching and writing the book, I learned a lot and came across a wide range of topics that were new to me, many of them thought-provoking. Here's one particularly provocative example.
Sand, strictly, is defined only by its size range, its composition being of secondary importance; the size of sand grains determines how they behave and that behavior is unique and sometimes downright bizarre. Size is important.
Although born in Scotland, Philip H. Kuenen was an internationally-renowned Dutch sedimentologist in the last century, a great geologist with a passion for sand. Thanks to Kuenen, our understanding of turbidity currents, submarine canyons, and their associated depositional systems was set firmly on its feet. I came, essentially by accident, across the publication* of a talk that he had given in 1959 when he was invited by the Geological Society of South Africa to give their annual lecture in memory of Alex du Toit, a South African geologist who gave early support to the ideas of continental drift. Kuenen chose for his title “Sand—its origin, transportation, abrasion and accumulation.” In the talk, he strode boldly across the landscapes of sand on all scales, from how long it takes a grain to become round to mass transport in the desert, his "back of the envelope" calculations illustrating the dramatic scales and frequencies of our planet's processes. He concluded his talk with a statement that I find particularly thought-provoking:
The size of the sand grains determines the mode of transportation. If the particles were much bigger they could not be moved by the wind and there would be no coastal dunes or sand deserts…The denudation of mountain chains would be a more laborious process…The all-important part played by sand in protecting coasts from wave attack would be largely suppressed, because rivers would not carry the pebbly quartz to the coast. Not much quartz would reach positions where turbidity currents could carry them to the deep-sea floor, and the few that were carried by such flows would travel less far. In fact dry land would be smaller, higher, and more siliceous.
On the other hand, if quartz left the parent rocks in much smaller particles than actually happens, these would be carried more easily by rivers and marine currents and also suspended in the air. A higher percentage would be lost from the continents to the deep sea. The continents would be less siliceous and thinner…and in general the protective cover of sand shielding more vulnerable material from weathering would be absent. Marine planation of the continents would be more active, because protective sands would have been wasted into the deep-sea. It thus appears probable that if quartz grains in granites were significantly bigger than they actually are, the continents would be smaller and steeper than we now find them. If granite quartzes were much smaller, the land would also be smaller but lower than at present.
This seems to me very much an anticipation of the yet-to-be-coined "earth systems" approach. Putting aside biogenic sands, the majority of sand grains in the world owe their origins to igneous activity and lithospheric process, their size range determined by plate tectonics, uplift and cooling rates, geothermal gradients and so on. Yet, as Kuenen suggests, there is feedback at work here, lithospheric processes being themselves influenced by the games that sand plays across the Earth's surface, games whose rules are determined by the size range of the grains. The size of sand grains makes the Earth look the way it does. Interesting and thought-provoking?
*Kuenen, Ph. H. Sand: Its Origin, Transportation, Abrasion and Accumulation. Annexure to vol. 62. Johannesburg: Geological Society of South Africa, 1959.