Geology Reference
In-Depth Information
Nier's three geological clocks.
Clock 1, uranium 238 Æ lead 206. Clock 2, uranium 235 Æ lead 207.
Clock 3, the growth of lead 206 and 207, relative to 204.
the two clocks were ticking at di¬erent rates (the decay rate of
uranium 235 is more than six times faster than that of 238) it
was possible to determine the age of a rock simply by compar-
ing the growth over time of the two resulting lead isotopes,
relative to something else that did not change with time.
So when Alfred Nier finally identified 'ordinary' lead (lead 204)
it became that 'something else' because it was not derived
from radioactive decay and therefore did not change in value
over time. In other words a
third
clock was found simply
by measuring the amount of lead left by each of the two uranium
isotopes and then comparing them with 'ordinary' lead to see
how much they had increased over time. This new 'lead-lead'
method, as it was called, was a major step forward and is still
