Geoscience Reference
In-Depth Information
20
20
18
Increase of Pb
206
in Sial during
Geological Time
18
16
16
Date of
Origin of Sial
14
14
12
12
Increase of Pb
206
in
average Earth Material
10
10
8
8
0
4000
3500
3000
2500
2000
1500
1000
500
0
Millions of Years
Figure 13.7 Graph showing seven isochrons converging on a point 'E'
indicating an age of 3,400 million years (from Arthur Holmes, Geological
Magazine 84 (1947), p. 124). (Geological Museum, Trinity College
Dublin.)
formed over time by the radioactive decay of uranium and thorium
and that the rest was what he called 'Primeval Lead': lead that was
present at the origin of the Earth. This was a major breakthrough
that should not be understated, as it allowed others to make serious
refinements to our understanding of our planet's age. Arthur Holmes
tookNier's results on board and in 1946 established a dating procedure
based on this new information, which took into account the different
decay times of the two uranium isotopes. If the proportions of the two
uranium-to-lead systems are plotted on to a graph, the points form a
straight line, called an isochron, and the slope of this line represents
the age of the Earth (Figure
13.7
). The following year Holmes, using
the method, calculated that the Earth was 3,350 million years old.
Unknown to him some of his ideas had been considered earlier by the
Russian Erik Karlovich Gerling (1904-1985) in 1942, and the method
was developed almost simultaneously in Germany by Friedrich (Fritz)
Georg Houtermans (1903-1966). Houtermans was imprisoned in
