Geoscience Reference
In-Depth Information
caused by the faster decay of uranium in the past, or by the decay of a
uranium isotope that was no longer present. Similarly, other isotopes
might have been present which have now disappeared, and so the
decay rates of these would be unknown. The fact that thorium halos
of all ages were constant in size was difficult to explain. Arthur
Holmes argued that the inconsistencies of the uranium halos were
due not to time but to other factors including the presence of the
recently discovered rare uranium daughter isotope actinium.
Nevertheless he accepted that Joly's scheme of correlating halo radii
with time would give a scale against which the ages of other halos
could be determined.
PUTTING AGES ON THE GEOLOGICAL COLUMN
Boltwood's method is, in essence, the fundamental basis of radio-
metric dating, which ultimately allowed geologists to fix absolute
dates to the geological column. Today geologists use one or a combi-
nation of various decay sequences (which begin with a radioactive
'parent' isotope and end with a stable 'daughter' isotope) to arrive at
an absolute date for a particular rock (Table
13.1
). The last three decay
series have been developed in the past 25 years following improved
analytical instrumentation.
In 1911 the only methods available to Arthur Holmes, who at
the time was a young research student working in Strutt's laboratory
in London, were the uranium
!
lead and the helium accumulation
methods. Holmes recognised the value of the new methodologies as
tools which he could use to provide absolute dates to rocks from
known portions of the geological column. He wanted to provide an
absolute timescale for geological history, and throughout his life he
remained at the cutting edge of this research and was its prime mover
and shaker.
Holmes' peregrinations
Holmes was born in 1890 in the village of Hebburn, near Newcastle in
northeast England, but spent his childhood at the nearby town of
