Against the grain
Why do sand particles on a beach or dunes seem to reach a certain grain size and then reduce no further? After millions of years, shouldn鈥檛 most sand have become dust?
鈥� 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 sun, wind and water 鈥� the last of which is a surprisingly powerful weathering agent in deserts.
The result is a build-up of fragments of various sizes 鈥� boulders, pebbles, sand grains and dust. These last two, being smaller, can be removed by the wind and transported hundreds of kilometres, both in suspension high in the atmosphere, or by saltation 鈥� the process of bouncing along the ground.
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The maximum grain size that can be transported by the wind is proportional to the wind speed 鈥� faster winds will move larger particles. This means that the larger 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 accumulate in great masses in lowland basins, while the smaller fragments can be carried further 鈥� dust from the Sahara 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 鈥� 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 sort 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 鈥� boulders, pebbles and sand 鈥� may gradually be broken down into finer particles, so that we might suppose all the world鈥檚 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 鈥� 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.
Michael Ghirelli
Hillesden, Buckinghamshire, UK
鈥� 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 process 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.
Edward Davies
Fareham, Hampshire, UK
Living fence
If I put a wooden fence post into soil it will eventually rot and collapse, sometimes within months. What prevents trees, which can live for hundreds of years, doing the same?
鈥� The reason for the difference in the rate of decay between a living tree and a wooden fence post is the same as the reason why a living person can live for 100 years, whereas a corpse decomposes quickly. Decay and rot are caused by bacteria, fungi, insects and related organisms. Living organisms, whether plant or animal, have defence mechanisms to ward off these agents of decay, and also repair mechanisms that act when damage does occur.
Only the outer layers of tree trunks and branches are alive. This is where the sap is. The inner parts are dead, and these do often start to decay. The hollow spaces that result provide homes for a range of birds and other arboreal animals. This is one of the reasons why 鈥渙ld growth鈥� forests, with trees that are many hundreds of years old, support a much richer diversity of animal species than 鈥渘ew growth鈥� forests that are a result of replanting after logging.
Simon Iveson
Cleveland, Queensland, Australia
This week鈥檚 questions
Icarus returns
I filmed a bird taking off from the ground with my new video camera. When I played it back in slow motion I saw the bird crouch down, then begin to open its wings and take off into the air with a combination of a thrust from its legs and a powerful wing beat. This got me thinking. First, are birds so amazingly light that they can do this? The average chicken I eat does not seem so light. And secondly, given that I weigh 75 kilograms, how big would my wings have to be for me to take off like a bird? Or is wing size not the only issue?
Polly Wadworth
Berlin, Germany
Heavy weather
Is it possible for a ground-based observer to calculate how much water is in a particular cloud? Do size and colour have any bearing? If this is possible, how is it done 鈥� I want to impress my friends. If it isn鈥檛, can it be calculated by more scientific methods?
James Down
Hemel Hempstead, Hertfordshire, UK