Geology Reference
In-Depth Information
one of the fundamental age dating techniques used in modern
geochronology.
To demonstrate the validity of using these three geological
clocks now available to him, in the late 1930s and early 1940s
Alfred Nier performed a series of very precise age determi-
nations on twenty-five di¬erent rocks of varying geological ages,
from di¬erent parts of the world, using the most up to date mass
spectrometer then available to him. While in most cases the
three ages for each 'clock' agreed within reason, one rock, the
Manitoba pegmatite from the Huron Mine in Canada, gave a
wide range of age values, the most reliable of which was thought
to be the oldest one at 2200 million years. (A pegmatite is a
coarse grained igneous rock that frequently contains the world's
largest crystals and choicest mineral specimens.)
The implications of finding this extremely old mineral meant
that once again a large question mark hung over the age of the
Earth, which, by then, was considered to be fairly well estab-
lished at 2000 million years. Such was Nier's concern about the
age of this mineral at a time when astronomers were insistent
that the Universe could not be more than 2000 million years old
(two billion), that a year later he analysed another rock from the
same area in an attempt perhaps to
disprove
his first age. But
instead he obtained an even older age - this rock was 2570 mil-
lion years old! Even more worried, Nier was reluctant to trum-
pet his results and modestly claimed that 'One of the samples
was the oldest so far studied and appears to have an age close
to two billion years.' Actually, if he were being strictly scientific,
an age of 2570 million years is closer to three billion than two
billion years. Unfortunately, intervention of the Second World
War prevented Nier from investigating this problem any further
when he was required to assist in development of the atom
bomb.
