Geology Reference
In-Depth Information
the rate of 'one foot in 8,616 years', he then applied his princi-
ple and worked out just how long it had taken to deposit the
total thickness of rock in the world. The enormity of the figure
he arrived at forced him to conclude, 'which I am by no means
willing to do', that even if 'the manufacture of strata in geologi-
cal times proceeded at ten times this rate . . . this gives for the
whole duration of geological time a minimum of 200 millions
of years'. Now we know that he would have been much closer
to the truth with his original estimate of 2000 million years, but
we can also sympathise with his caution in those Kelvin-days of
a young Earth.
But imagine trying to estimate how much rock there is in the
world - and without computers! Not only was that an enor-
mously di~cult task, but it was also impossible to measure the
rates of erosion and deposition accurately since they were dif-
ferent in different places and at different times. The values given
for rates at which sediments were deposited ranged from one
foot in a hundred years to one foot in nine thousand years, so
inevitably the ages calculated using these deposition rates pro-
duced a similarly broad range: from 3 million years for the total
age of the Earth, to 2400 million years just to the Base Cambrian.
In fact it was a method that could produce any age you liked
depending on how the parameters were adjusted, and thus the
majority of ages determined for the age of the Earth fell, fortu-
itously, within Kelvin's estimate of a hundred million years.
Then in 1893 the results were published of some experiments
that had been done in America that allowed Kelvin to refine his
calculations on the age of the Earth even further. These experi-
ments involved the heating of various rock types in a laboratory
to discover what temperatures they melted at. The results dra-
matically reduced the rather generous value of nearly 4000 ºC
that Kelvin had assumed for the melting point of rocks to an
actual value of 1200 ºC. This considerable drop in temperature
from which his molten globe had to cool resulted in an equally
dramatic drop in the age of the Earth.
