Geology Reference
In-Depth Information
The Age of Uranium
Because the pathway from uranium to lead was peculiarly
complicated, others had abandoned their researches, leaving
the 21 year old research student to become the world
authority on a technique that was finally to provide the
planet with its authentic, scientifically determined birthday.
Robert Muir Wood on Arthur Holmes
Half a lifetime had gone by since Arthur Holmes had lain in his
tent in Mozambique, racked with fever, dreaming of developing
a geological time scale and wondering how he could reconcile
the age of the Earth as determined by radioactivity with that
calculated by the old established methods of sedimentation rates.
While progress on a geological time scale had been made over
the following years, it had largely been in the physics arena:
improved understanding about the atom; the discovery of iso-
topes; development of the mass spectrometer; and recognition
of the four stable isotopes of lead. The geological side, however,
lagged far behind. A rock assigned an age of 300 million years,
for example, still could not be classified as 'Carboniferous' with
any confidence because it was still not known how long, in
geological time, the Carboniferous ranged. So, as we saw with
the helium results from the Whin Sill, extreme errors could be
accepted as reasonable values because no limits could be placed
on the extent of the Carboniferous. Clearly, what was needed was
a time scale that said 'the Carboniferous starts here at this age
and ends there at that age, therefore any age in between must be
Carboniferous'. But that was still a long way o¬.
