Geology Reference
In-Depth Information
inevitably occurred elsewhere. Strutt claimed that as soon as he
read of Rutherford's discovery that helium was a product of
radioactive decay, he realised at once that it could be used to
measure geological time and had in fact done just that when
Rutherford published his first 'age'. Like most other areas of
life, it is the first person past the post who gets the prize and
second place rarely gets a look in. But although Rutherford beat
Strutt to the publication of results and so his name goes down
in history as the first person to date a rock by radioactivity, it
was Strutt who recognised the flaw in the method. Helium is a
gas, and Strutt soon realised that much of the helium contained
in the rocks was escaping into the atmosphere as the rocks
were crushed up for analysis. This meant that only some of
the helium produced by radioactive decay was being measured,
and hence the age obtained was not truly representative of the
age of the rock, it was only a
minimum
age. Strutt recognised
that a new dating method was needed and encouraged Arthur
Holmes to look for it. An aloof man with a retiring disposition,
Strutt was nevertheless the perfect tutor for Holmes, leaving him
very much to himself to figure out how to improve things.
In 1907 Bertram Boltwood, an American chemist, made a
systematic analysis of rocks containing uranium. He noticed
that along with helium, unusually large amounts of lead were
present, and postulated that in fact it was lead that was the sta-
ble end product in the decay chain from uranium. If Boltwood
was right, Holmes realised, then it should be possible to obtain
an age by measuring the amount of lead present in the miner-
al, rather than the amount of helium. He decided to try. In the
winter of 1910 he cut short his Christmas holidays in Gateshead,
despite having to tear himself away from the comforts of home,
his friendship with Bob and access to a good piano. Arriving
back in London in the early hours of a cold, dark and rainy
January morning, having been obliged by the conditions of his
cheap ticket to leave Gateshead at midnight, he set up his appa-
ratus in the cold, dark and quiet laboratory feeling homesick
