date buildings ( from the wood in say the doors ) and even Viking longships ! By overlapping the patterns of wood , that can be older than those from any living trees , the changing thickness of the tree rings can even be used to reconstruct climate and ecological patterns from long before meteorological records were kept .
But tree growth is not the only process that accelerates and slows on some regular cycle . One of the most interesting is paleomagnetic dating , which is used to date not trees but … rocks ! In his brilliant book The Ancestor ’ s Tale , Richard Dawkins describes the technique .
The earth ’ s magnetic field reverses from time to time . What had been magnetic north then suddenly becomes magnetic south for a few thousand or even a million years , and then it flips again . This has apparently happened 282 times during the last 10 million years . You can imagine how Hollywood can have fun with this one – navigation systems going rogue while planes are in midair , ships arriving at the wrong ports . But a flip can take a few thousand years to happen and then , when the dust has settled , it may perhaps be a million years until the next flip . A few thousand years may sound very long , but it is but a fleeting moment in geological time . The magnetic North Pole , as we know , seldom coincides with the true North Pole , it wanders around the polar region over time . Scientists know these paths . And nature also keeps an automatic record of the “ flips ”. In molten volcanic rock , certain minerals behave like little compass needles .
When the molten rock solidifies , these mineral needles constitute a frozen record of the earth ’ s magnetic field at the moment of solidification . After a “ flip ”, the miniature compass needles in the rocks point in the opposite direction than before the flip . It is like tree rings all over again , except that the stripes are not a year apart , but of the order of a couple of thousand or even around a million years . Once again , patterns of stripes can be matched up with other patterns and a continuous chronology of patterns can be chained together . And these are used to date rocks .
But back to tree rings – it is all about growth . As trees age , they do not stop growing . We have a giant old oak tree in our garden at home , probably around 90 years old , but this tree is still a dwarf compared to the biggest trees in the world , the Pacific redwoods in California . The largest single living creature in the world is the General Sherman Tree , a giant of over 30 meters in circumference which soars about 84 meters into the Sierra sky . And it is still growing . It has an estimated weight of more than 1 260 tonnes . Its age is not known for certain , but it is probably more than 3 000 years old . That means it existed at the time of King David , or of the Trojan war . The bark alone is more than a metre thick .
Growth is of course one of the seven life functions . The others are respiration , movement , reproduction , nutrition , excretion and
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