Geoscience Reference
In-Depth Information
about twenty million years
2
would have made any such calculations seem quite
unrealistic to those involved in the debate. Soon after this lecture, Rutherford and
his colleague Boltwood developed radioactive dating, which was first applied to
show that even the geological estimates of the age of the Earth were too mod-
est. Rutherford's first dating method was to measure the accumulation of helium
(
particles) from the radioactive decay of uranium but he later suggested that
measuring the accumulation of lead would be a better method. In 1907, Boltwood
published the first uranium-lead dates.
The geological tools of stratigraphy and palaeontology provide a very
precise
3
method of measuring
relative ages
.Ingood cases, a palaeontological resolution
of better than 0.25 Ma can be attained. Stratigraphy and palaeontology enable us
to determine the order in which rocks were laid down and the order in which tec-
tonic events and sea transgressions occurred. The geological timescale is made up
of stratigraphic divisions based on observed rock sequences. However, palaeon-
tology is unable to give any
accurate
4
estimate of the
absolute ages
of geological
events. The use of radioactive isotopes to date rocks, being a measurement of
physical properties, is quite separate from any intuitive method and thus can pro-
vide an independent, fairly accurate and sometimes very precise date. There are,
of course, many assumptions and inherent problems in these dating methods, as
in any other, of which anyone using such dates needs to be aware. For details of
the practical uses of the various dating methods the reader is referred to textbooks
on geochronology (e.g., Faure 1986).
Table 6.1 shows a standard geological timescale, which is based on stratigra-
phy, palaeontology, geochronology and reversals of geomagnetic polarity.
6.2 General theory
The disintegration of any radioactive atom is a random event, which occurs
independently of neighbouring atoms, physical conditions and the chemical state
of the atom. Disintegration depends only on the structure of the nucleus. This
means that every atom of a given type has the same probability of disintegrating
in unit time. This probability is called the
decay constant
,
,adifferent constant
for each isotope. Suppose that at time
t
there are
P
atoms and that at a time δ
t
later,
δ
P
of these atoms have disintegrated. Then δ
P
is the product of the probability
that any one of these atoms will decay in a unit of time (
λ
) multiplied by the
number of atoms present (
P
) multiplied by the length of time (δ
t
):
λ
δ
P
=−
λ
P
δ
t
(6.1)
2
The estimate of the Sun's age was based on assumptions that its energy was of gravitational origin.
It was not until the 1930s that thermonuclear fusion reactions were discovered and understood to
be fuelling the Sun. Only then were the apparently young ages for the Sun shown to be unfounded.
3
Precision
describes the reproducibility with which measurements are made.
4
Accuracy
describes the truth of these measurements.
