SAND PARTICLE DEPOSITION

Sand particles on a beach or dunes seem to reach a certain grain size and then reduce no further. After millions of years, shouldn’t most sand have become dust?

THE GRAINS that we see in desert sand dunes have been deposited mainly by wind action. These will generally have originated in other parts of the desert where there are bare rock surfaces that are constantly being weathered by exposure to the sun, wind and water – the last of which is a surprisingly powerful weathering agent in deserts.

The result is a build-up of fragments of various sizes: boulders, pebbles, sand grains and dust. The last two, being smaller, can be removed by the wind and transported hundreds of kilometres, either in suspension high in the atmosphere, or by saltation – the process of bouncing along the ground.

The maximum grain size that can be transported by the wind is proportional to the wind speed – faster winds will move larger particles. This means that the large particles are deposited when and where the wind speed drops, which is often in low or flat terrain. So sand grains of around a certain size can accumulate in great masses in lowland basins, while the smaller fragments can be carried further; dust from the Sahara Desert quite frequently falls on the UK, for example. The result is that dunes are made up of grains mainly of the same size.

Similar principles apply on beaches, although the movement of particles is also affected by a variety of additional processes such as wave action, tides, offshore currents and long-shore drift – sand creep caused by waves approaching the beach obliquely. How effective each mechanism is at moving particles depends on its energy, so each will deposit particles in a different location. For example, wave action can sought beach material so that shingle will accumulate as a ridge high up the beach, while sand will only be exposed at low tide. Or long-shore drift may carry sand to one end of a beach, leaving shingle at the other.

Of course, all these fragments – boulders pebbles and sand – may gradually be broken down into finer particles, so that we might suppose all of the world’s rocks should by now have been reduced to a mass of dust blanketing the continents. But this does not happen because deposits of sand and dust gradually get compressed and cemented together to form new rock – the sandstones and mudstones. Nor does the planet run out of sand and dust, because bare rock surfaces are constantly exposed to weathering processes, and there will always be new rock exposed as a result of tectonic movement.

IF the sand in a coastal system is too fine relative to the energy of the waves then it will stay in suspension in the water and will not be deposited. So for a beach of dust to exist, the environment would have to be profoundly calm, and the dust-like sand would have to be kept wet in order to prevent the wind from claiming it. Most beaches are not like this.

Dunes are deposits of wind-blown sand, and for the sand to be deposited the size of the grains must exceed the carrying capacity of the wind. Sand dunes are innately dry places and there is no way that dust-sized particles could hope to stay put in these areas, however weak the wind may be.

Desert dunes exist in gigantic systems, whereas beach dunes form only a narrow band running along the back of some sea beaches, and are created by gusts from the sea that transport sand up from the beach. Yet both systems result from the same key processes of wind-borne matter being deposited when the wind becomes too weak to keep it aloft. Of course, even the tiniest sand grains will be deposited somewhere, but they will be highly dispersed and will not form dunes.

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. Appendage:

Science-in-motion: a series of short articles following topics in science.

. Newtonian gravity

Isaac Newton’s law of universal gravitation, published in 1687, was the first clear mathematical description of how bodies such as planets and stars attract each other under their mutual gravitational pull.

Newton’s inspiration for the theory came from watching an apple falling from a tree. A falling apple accelerates towards the ground, so Newton reasoned from his laws of motion that there must be a force, which he called gravity, acting on the apple. This force might have a huge range and could also be responsible for the orbit of the Moon around the Earth, if the Moon had just the right speed to remain in orbit despite constantly ‘falling’ towards the Earth.

He went on to show that the gravitational force between two massive objects is directly proportional to the product of their masses and weakens with the square of the distance between them. But troubling, the theory didn’t explain why the force was transmitted across empty space. This problem is resolved in Einstein’s general relativity theory.