Geoscience Reference
In-Depth Information
with the known geological ages of the specimens. Strutt concluded that “fossilized
bones and other materials do not always contain as much helium as would be
had leaked helium, as Rutherford had suspected. Strutt searched for a common
uranium-bearing mineral that would better retain helium, soon settling on zircon, a
refractory mineral that remains the primary one used today for age measurements.
Right away Strutt found that the uranium-helium ratio in zircons “stands in very
several hundred million years and the oldest, from Precambrian rocks in Ontario,
dated to 715 million years. But the ease with which helium can escape crystal lat-
tices, even those of zircon, meant that uranium-helium dating would always be
suspect. Other methods soon supplanted it.
While Strutt was experimenting with the uranium-helium method, the Yale
chemist Boltwood was following up on the possibility that, as he and Rutherford
suspected, lead is the end product of uranium decay. For forty-three specimens of
uranium minerals, Boltwood found that the older the mineral, the more lead it con-
tained. Using a method suggested by Rutherford, which required an estimate of the
decay rate of the intermediate product radium, Boltwood calculated ages ranging
from 410 million years to the then amazing 2.2 billion. He recognized that these
were only “rough” calculations and that the import of his study was that the ratio
of lead to uranium “is greatest in minerals from the locality which, on the basis
of geological data, is the oldest.” This he considered “proof that lead is the final
decay and its end products than in the practical use of the process.
But even these first, rough estimates of geologic ages using radioactivity gave
results well beyond Kelvin's limit. And because at least some of the minerals must
have lost daughter atoms, the true ages were even greater.
Biologists and geologists now had plenty of time; indeed, some had more than
they wanted. Kelvin's mathematical mill had ground down geologists and forced
them to accede to an age of the Earth of no more than 100 million years and pos-
sibly to as few as 20—even 10. Now a new group of physicists, armed not with
Fourier's mathematics but with startling new discoveries that seemed to border on
alchemy, were demanding that the geologists reverse themselves, throw out their
hourglasses, and capitulate to ages of at least several hundred million years, and
still these were only minimums. Estimates of geological time had now swung back
in Lyell's direction, possibly measuring in the thousands of millions, with the up-
per limit unknown. Geologists were not going to be whipsawed in this fashion.
None resisted more than John Joly in defense of his salt clock.
