Absolute Time
In contrast to relative time, absolute time systems attempt to establish
the age of an object or event in actual numbers of years. Theoretically,
the principle behind absolute age determinations is simple. All one has
to do, is to find some non-cyclic system which changes at a constant known
rate, determine how much change has taken place and, if one knows the rate
of change, one can calculate how much time has elapsed.
Over time, many people have taxed their ingenuity to find ways of determining
the age of the earth. From the time when it became obvious that Bishop
Ussher's 6000 year chronology of the earth simply ran counter to evidence,
many calculations were attempted. They included attempts to calculate the
age of the earth on the basis of the salinity of the oceans, as well as
the rate at which the earth lost heat. None of these was entirely successful,
not because they were conceptually flawed, but because they lacked sufficient
information. Even so, they all clearly pointed to the fact that the earth
is extremely old.
Several techniques have been used with some success within the recent
past. They include dendrochronology (aging based on tree rings), lichenology
(based on the growth pattern of lichens), and the study of warves, annual
layers deposited by the meltwaters of glaciers. All these methods leave
something to be desired, being limited to relatively recent materials,
but they can nevertheless give reasonable ages.
Radiometric techniques
The most successful group of techniques for dating are radiometric techniques.
They all involve the spontaneous change or decay of from unstable parent
radioactive nuclei into stable daughter materials by fission, splitting.
Because nuclear decay is unaffected by ambient conditions, this technique
is less dependent on climatic, chemical or geologic conditions for the
accuracy of results than others.
Every radioactive isotope decays exponentially, but in a predictable
fashion. The rate of decay can be expressed in half-lives, which is the
amount of time that it takes for one half the parent atoms to decay into
daughter atoms. For any given parent/daughter pair, this half life is constant
and is not affected by physical or chemical change. Some commonly used
pairs and their half-lives are listed below:
Parent/ Daughter |
Half-Life |
Thorium 232/ Lead 208 |
13.9 B.Y. billion years) |
Rubidium 87/ Strontium 87 |
4.7 B.Y. |
Uranium 238/ Lead 206 |
4.5 B.Y. |
Uranium 235/ Lead 207 |
0.7 B.Y.. |
Potassium 40/ Argon 40 |
1.3 B.Y. |
Carbon 14/ Nitrogen 14 |
5710 yrs. |
. . .
If we use, for example, U235/Pb206 we note that the half-life for this
decay series is 0.7 BY. What this means is that after 0.7 BY. half of the
atoms that started off as U have now changed into Pb. If we wait another
half-life (another .7 BY.) half of the remaining U atoms will again have
changed, leaving one quarter of the original U atoms. After one more half
life, there will be 1/8 U atoms and 7/8 Pb atoms, and so on indefinitely.
Thus, for any moment in time, there will be a unique ratio of parent/daughter
material.
In practice, calculation of the age of a sample is done in the following
fashion. A minute portion of the sample is introduced into a mass spectrometer
to get an accurate analysis of the composition of the sample. From this
we obtain a ratio of parent/daughter material. Using this ratio we can
calculate the number of half lives which must have elapsed since the formation
of this material. Having calculated the number of half-lives we then multiply
this number by the length of a half-life and obtain a date. Once the sample
is introduced into the mass spectrometer, all subsequent steps are carried
out automatically by machines and computers.
Although absolute dates seem correct to the last decimal place, the
entire system has some degree of error built into it. Analytical techniques
are never sufficiently precise to count every last atom, and the smaller
the sample, the greater the uncertainty. Computers which do the calculations
also calculate the standard deviation of the sample. Suppose that a lab
analysis indicated that the age of the sample was 50 million years. (Such
a date is meaningless without the degree of variation around the mean,
the standard deviation expressed as a + or - value). Suppose further that
the standard deviation was given as +or-3 M.Y.. What this means is that
there is approximately a 66% chance that the sample that was between 47
and 53 million years old. If you wanted to be more secure about the date
you may assume that there is a 95% chance that the date lies between 44
and 56 million years ago and you can be 99% confident that the event took
place between 41 and 59 million years ago. Thus, even the best of absolute
dates incorporates some degree of uncertainty.
Aside from laboratory constraints, there may also be geochemical problems
such as leakage or addition of parent or daughter material which may have
occurred in the past. In fact in some ways, absolute dates are less useful
than relative dates. At least the latter will give you the correct sequence
of events even if it does not tell you how long ago the events took place.
The above comments should be in no way construed as a dismissal of the
value of absolute dating. The elements used for radiometric dates are found
more commonly in igneous and metamorphic rocks. These rocks rarely contain
fossils and could otherwise be dated only with the greatest difficulty,
if at all. Because of the general lack of fossils in rocks of Pre-Paleozoic
age, absolute dates are of great use there. Absolute dates have helped
us unravel the history of the early earth, and even though world wide correlations
of that earlier time still involve much uncertainty, radiometric dates
have given us a fairly coherent picture of what happened then. |