Rio Tinto | Minerals and Metals for the world

But why should some diamonds have had to wait over three billion years before they could hitch a ride towards the Earth's surface? Is the rarity of ancient diamond pipes an illusion? Can it be explained by erosion, which would tend to reduce old pipes to their narrow, hard to find roots, or does it reflect long term changes in natural systems that determined the present whereabouts of diamond pipes?

The same question can be asked about deposits of certain metals, or at least about certain types of metal deposit. With them biological systems have to be considered – bacteria played a crucial role in some ore forming processes, for instance by providing sulphur for massive sulphide deposits and promoting metal enrichment during weathering.

The late Stephen Jay Gould, a biologist distinguished by his interest in geological ideas, has written about a similar dichotomy in the evolution of life. He contrasts changes that work continuously in the same direction ("time's arrow") with those that vary in direction and may come to a standstill ("time's cycle"). There were suggestions of an "arrow" in Lord Kelvin's discredited model of planetary cooling (which some people used to argue that everything in the past had been directed towards setting the scene for human life).

Geological events commonly run in cycles, but it is hard to see that geological processes have been working inexorably towards some definite outcome. The history of our planet has been one of unceasing and sometimes dramatic change, but none of the inorganic changes have been constantly in the same direction. People see a different pattern in the organic world, especially where human influences are concerned, though if they are familiar with the annals of geological change they will probably have difficulty with apocalyptic scenarios.



But why should some diamonds have had to wait over three billion years before they could hitch a ride towards the Earth's surface? Is the rarity of ancient diamond pipes an illusion? Can it be explained by erosion, which would tend to reduce old pipes to their narrow, hard to find roots, or does it reflect long term changes in natural systems that determined the present whereabouts of diamond pipes? The same question can be asked about deposits of certain metals, or at least about certain types of metal deposit. With them biological systems have to be considered – bacteria played a crucial role in some ore forming processes, for instance by providing sulphur for massive sulphide deposits and promoting metal enrichment during weathering. The late Stephen Jay Gould, a biologist distinguished by his interest in geological ideas, has written about a similar dichotomy in the evolution of life. He contrasts changes that work continuously in the same direction ("time's arrow") with those that vary in direction and may come to a standstill ("time's cycle"). There were suggestions of an "arrow" in Lord Kelvin's discredited model of planetary cooling (which some people used to argue that everything in the past had been directed towards setting the scene for human life). Geological events commonly run in cycles, but it is hard to see that geological processes have been working inexorably towards some definite outcome. The history of our planet has been one of unceasing and sometimes dramatic change, but none of the inorganic changes have been constantly in the same direction. People see a different pattern in the organic world, especially where human influences are concerned, though if they are familiar with the annals of geological change they will probably have difficulty with apocalyptic scenarios.
PAGE 7 OF 9

Review is published by Rio Tinto, 6 St James' Square, London SW1Y 4LD, England Telephone +44 (0)20 7930 2399 Editor: Cherry DeGeer