Geology Reference
In-Depth Information
appear much older than it actually was, in the same way that the
escape of helium had made all the ages analysed by that method
appear much too young. Opposition to a great antiquity for the
Earth was still prevalent amongst most geologists, who were
firmly entrenched in their models of estimating an age by meas-
uring either the rate that sediments accumulated, or the amount
of salt in the oceans. The majority of results from these meth-
ods produced an age around one hundred million years which,
following Kelvin's early lead, had become widely accepted as the
age for the Earth - hence the outcry when Kelvin suddenly low-
ered it to twenty million - but equally problematic was that most
geologists now seemed unable to adapt to suddenly having more
than a billion years to play with. Where, for example, were all
the extra sediments to come from to fill all that time? The possi-
bility that one or two billion years might be available seemed
to many geologists to be as embarrassing as the former limi-
tation of twenty to forty million years. No, clearly there must be
something wrong with this new method of radioactivity. It
therefore must have been di~cult for such a young man to stand
against this strong tide of opposition without some feelings of
intimidation, so instead of idling away the time in Mozambique,
Holmes set out to try and marry the ages obtained from the
'hour-glass' methods - sedimentation and salinity - with those
obtained from radiometric dates, by taking a new approach to
estimating the total amount of sedimentary rocks present in the
world.
Igneous rocks are the precursors of sedimentary rocks. They
are the result of molten material that has forced its way up
towards the surface from deep within the Earth. Originally the
Earth's crust would have been comprised entirely of igneous
rocks, but as soon as these were exposed at the surface they
would have started to erode to form the first sediments.
Consequently Holmes considered that a more realistic means of
determining the age of the Earth by the 'hour-glass' method was
to first deduce the amount of igneous rocks in the world from
