EXPANDING HORIZONS 101
of magnitude. By 1946, British
geologist Arthur Holmes had made
some isotope measurements from
lead-bearing rocks from Greenland,
which gave an age of 3.015 billion
years. This was one of the first
reliable minimum ages for Earth.
Holmes went on to estimate the
age of the uranium from which the
lead was derived, obtaining a date
of 4.46 billion years, but he thought
that must be the age of the gas
cloud from which Earth formed.
Finally, in 1953, American
geochemist Clair Patterson
obtained the first generally
accepted radiometric age of 4.55
billion years for Earth’s formation.
There are no known minerals or
rocks dating from Earth’s origin,
but many meteorites are thought
to originate from the same event
in the solar system. Patterson
calculated the radiometric date for
lead minerals in the Canyon Diablo
meteorite at 4.51 billion years.
Comparing it with the average
radiometric age of 4.56 billion
years for granite and basalt igneous
rocks in Earth’s crust, he concluded
that the similarity of dates was
indicative of the age of Earth’s
formation. By 1956, he had made
further measurements, which
increased his confidence in the
accuracy of the date of 4.55 billion
years. This remains the figure
accepted by scientists today. ■James Hutton
Born in 1726 to a respected
merchant in Edinburgh,
Scotland, James Hutton
studied humanities at
Edinburgh University. He
became interested in
chemistry and then medicine,
but did not practice as a
doctor. Instead, he studied
the new agrarian techniques
being used in East Anglia,
England, where his exposure
to soils and the rocks they
were derived from led to an
interest in geology. This took
him on field expeditions all
over England and Scotland.
Returning to Edinburgh
in 1768, Hutton became
acquainted with some of
the major figures of the
Scottish Enlightenment,
including the engineer
James Watt and the moral
philosopher Adam Smith.
Over the next 20 years, Hutton
developed his famous theory
of Earth’s age and discussed it
with his friends before finally
publishing a long outline in
1788 and a much longer book
in 1795. He died in 1797.Key work1795 Theory of the Earth
with Proofs and IllustrationsAn uncomformity is a buried surface separating two rock
strata of different ages. This diagram shows an angular
unconformity, similar to those discovered by James Hutton
on the east coast of Scotland. Here, layers of rock strata have
been tilted by volcanic activity or movements in Earth’s
crust, producing an angular discordance with overlying,
younger layers.
The past history of our
globe must be explained by
what can be seen to be
happening now.
James HuttonAngular
discordance
Older, tilted
rock strata