A numerical (or "absolute") age is a specific number of years, like 150
million years ago. A relative age simply states whether one rock formation
is older or younger than another formation. The Geologic Time Scale was
originally laid out using relative dating principles.
The geological time scale is based on the the geological rock record,
which includes erosion, mountain building and other geological events.
Over hundreds to thousands of millions of years, continents, oceans and
mountain ranges have moved vast distances both vertically and
horizontally. For example, areas that were once deep oceans hundreds of
millions of years ago are now mountainous desert regions.
The earliest geological time scales simply used the order of rocks laid
down in a sedimentary rock sequence (stratum) with the oldest at the
bottom. However, a more powerful tool was the fossilised remains of
ancient animals and plants within the rock strata. After Charles Darwin's
publication Origin of Species (Darwin himself was also a geologist) in
1859, geologists realised that particular fossils were restricted to
particular layers of rock. This built up the first generalised geological
Once formations and stratigraphic sequences were mapped around the world,
sequences could be matched from the faunal successions. These sequences
apply from the beginning of the Cambrian period, which contains the first
evidence of macro-fossils. Fossil assemblages 'fingerprint' formations,
even though some species may range through several different formations.
This feature allowed William Smith (an engineer and surveyor who worked in
the coal mines of England in the late 1700s) to order the fossils he
started to collect in south-eastern England in 1793. He noted that
different formations contained different fossils and he could map one
formation from another by the differences in the fossils. As he mapped
across southern England, he drew up a stratigraphic succession of rocks
although they appeared in different places at different levels.
By matching similar fossils in different regions throughout the world,
correlations were built up over many years. Only when radioactive isotopes
were developed in the early 1900s did stratigraphic correlations become
less important as igneous and metamorphic rocks could be dated for the
Divisions in the geological time scales still use fossil evidence and mark
major changes in the dominance of particular life forms. For example, the
Devonian Period is known as the 'Age of Fishes', as fish began to flourish
at this stage. However, the end of the Devonian was marked by the
predominance of a different life form, plants, which in turn denotes the
beginning of the Carboniferous Period. The different periods can be
further subdivided (e.g. Early Cambrian, Middle Cambrian and Late
This is the latest version of the time scale, as revised and published in
The basic equation of radiometric dating requires that neither the parent
nuclide nor the daughter product can enter or leave the material after its
formation. The possible confounding effects of contamination of parent and
daughter isotopes have to be considered, as do the effects of any loss or
gain of such isotopes since the sample was created. It is therefore
essential to have as much information as possible about the material being
dated and to check for possible signs of alteration. Precision is
enhanced if measurements are taken on multiple samples from different
locations of the rock body. Alternatively, if several different minerals
can be dated from the same sample and are assumed to be formed by the same
event and were in equilibrium with the reservoir when they formed, they
should form an isochron. This can reduce the problem of contamination. In
uranium-lead dating, the concordia diagram is used which also decreases
the problem of nuclide loss. Finally, correlation between different
isotopic dating methods may be required to confirm the age of a sample.
For example, the age of the Amitsoq gneisses from western Greenland was
determined to be 3.6 ± 0.05 million years ago (MA) using
uranium-lead dating and 3.56 ± 0.10 Ma using lead-lead dating,
results that are consistent with each other.
Accurate radiometric dating generally requires that the parent has a long
enough half-life that it will be present in significant amounts at the
time of measurement (except as described below under "Dating with
short-lived extinct radionuclides"), the half-life of the parent is
accurately known, and enough of the daughter product is produced to be
accurately measured and distinguished from the initial amount of the
daughter present in the material. The procedures used to isolate and
analyze the parent and daughter nuclides must be precise and accurate.
This normally involves isotope ratio mass spectrometry.
The precision of a dating method depends in part on the half-life of the
radioactive isotope involved. For instance, carbon-14 has a half-life of
5,730 years. After an organism has been dead for 60,000 years, so little
carbon-14 is left that accurate dating can not be established. On the
other hand, the concentration of carbon-14 falls off so steeply that the
age of relatively young remains can be determined precisely to within a
Numerical dating takes advantage of the "clocks in rocks" -
radioactive isotopes ("parents") that spontaneously decay to form new
isotopes ("daughters") while releasing energy.
For example, decay of the parent isotope Rb-87 (Rubidium) produces a
stable daughter isotope, Sr-87 (Strontium), while releasing a beta
particle (an electron from the nucleus). ("87" is the atomic mass number =
protons + neutrons.
Numerical ages have been added to the Geologic Time Scale since the advent
of radioactive age-dating techniques.
Many minerals contain radioactive isotopes.
In theory, the age of any of these minerals can be determined by:
1) counting the number of daughter isotopes in the mineral, and
2) using the known decay rate to calculate the length of time
required to produce that number of daughters.
It illustrates how the amount of a radioactive parent isotope decreases
with time. This amount is a percentage of the original parent amount. Time
is expressed in half-lives. Experiment by dragging on the graph. For
example when 42% of the parent still remains, 1.23 Half-Lives of time has
Parent Decay and
Daughter Growth Curves
The half-life of U-235 decaying to Pb-207 is 713 million years. Note that
this half-life can be obtained from the graph at the point where the decay
and growth curves cross. Determine the half-lives for the other three
isotopes and enter your estimate into the text fields below each graph.
Note the differences in scale between the various graphs
If a material that selectively rejects the daughter nuclide is heated, any
daughter nuclides that have been accumulated over time will be lost
through diffusion, setting the isotopic "clock" to zero. The temperature
at which this happens is known as the closure temperature or blocking
temperature and is specific to a particular material and isotopic system.
These temperatures are experimentally determined in the lab by
artificially resetting sample minerals using a high-temperature furnace.
As the mineral cools, the crystal structure begins to form and diffusion
of isotopes is less easy. At a certain temperature, the crystal structure
has formed sufficiently to prevent diffusion of isotopes. This temperature
is what is known as closure temperature and represents the temperature
below which the mineral is a closed system to isotopes.
Thus an igneous or metamorphic rock or melt, which is slowly cooling, does
not begin to exhibit measurable radioactive decay until it cools below the
closure temperature. The age that can be calculated by radiometric dating
is thus the time at which the rock or mineral cooled to closure
temperature. Dating of different minerals and/or isotope systems (with
differing closure temperatures) within the same rock can therefore enable
the tracking of the thermal history of the rock in question with time, and
thus the history of metamorphic events may become known in detail.
This field is known as thermochronology or thermochronometry.
The radiocarbon dating method was developed in the 1940's by Willard F.
Libby and a team of scientists at the University of Chicago. It
subsequently evolved into the most powerful method of dating late
Pleistocene and Holocene artifacts and geologic events up to about 50,000
years in age. The radiocarbon method is applied in many different
scientific fields, including archeology, geology, oceanography, hydrology,
atmospheric science, and paleoclimatology. For his leadership, Libby
received the Nobel Prize in Chemistry in 1960. See..
our C14 dating calculator
Dating Rocks with the Rb-Sr "Isochron"
There are numerous radioactive isotopes that can be used for numeric
dating. All of the dating methods rely on the fundamental principles
of radioactive decay, but the specific materials that can be dated
and the exact procedures for calculating a date are very different from
one method to the next.
The Rb-Sr method.
Rubidium occurs in nature as two isotopes: radioactive Rb-87 and stable
Rb-85. Rb-87 decays with a half-life of 48.8 billion years to Sr-87. This
half-life is so long that the Rb-Sr method is normally only used to date
rocks that are older than about 100 million years.
Which minerals and rocks can be dated with the Rb-Sr method? The minerals
must contain Rb, which is a rather rare element. Fortunately, Rb
behaves chemically very much like the more common potassium (K), so that
most K-bearing minerals contain a small amount of Rb. Examples include the
mica family (biotite and muscovite) and the feldspar family (plagioclase
and orthoclase). These minerals are abundant in granite (an igneous rock)
and gneiss (a metamorphic rock).
1. Select a fresh, unweathered rock sample.
A geologist collects a fresh, unweathered hand sample for age dating.
Fresh is the key word here, and means that the chemistry of the sample
has NOT been changed since the sample formed. Weathering alters the
chemistry of rocks including their isotopic compositions. Therefore, a
highly weathered rock may yield unreliable age information.
2. Crush the rock and separate the Rb-bearing minerals.
Getting a Rock Sample Ready for the Mass
For reliable age determination, careful sample preparation is an important
and often tedious process. The rock is mechanically crushed into small
fragments. Fragments of the Rb-bearing minerals are then separated from
the whole rock using a variety of methods, such as a magnetic separator.
These materials are then used to prepare a "whole-rock" sample and several
"mineral separate" samples. The whole rock sample will yield the weighted
average isotopic composition of all the minerals in the rock. Each mineral
separate will yield the composition of that particular mineral.
There are other steps that must be carried out to prepare a sample for
analysis by a mass spectrometer, such as converting the sample to a
solution by dissolving the mineral separates in selected acids, using
techniques of column chemistry to increase the concentration of the small
amounts of Rb and Sr in the solution and then precipitating the
concentrated solution as a "salt" compound. It's this compound of Rb-Sr
salts that can be attached to a special filament and placed into the mass
spectrometer for analysis.
3. Analyze the isotopic compositions of the whole rock and mineral
separates on a mass spectrometer.
A Mass Spectrometer is used to Measure
The gas source mass spectrometer includes three fundamental parts,
(1) a "source" of positively charged ions or molecular ions,
(2) a magnetic analyzer, and (3) ion collectors. The source is a
low-pressure chamber (~10-5 torr) into which sample or standard gas is
ionized, either by an electron beam produced by a hot filament, or by
a strong electrostatic field. Once formed, the ions are accelerated
and focused by charged plates into a beam that enters a flight tube.
The flight tube has a bend that coincides with an electromagnet that
alters the path of the ions according to their mass/charge ratio, thus
several beams leave the magnetic sector. Multiple ion detectors are
arranged to collect the ion beams of interest. These collectors
measure each beam as a current that can be amplified and determined
with high precision.
A Mass Spectrometer is a very powerful and sophisticated instrument. Many
types exist. Below is a simplified diagram of the electro-mechanical mass
spectrometer system and a picture of a modern instrument. Understanding
how a mass spectrometer functions is beyond the level of this activity.
But you should know that it measures the amounts of various isotopes
present in specially prepared samples of rocks and minerals as well as
Understanding the isochron diagram is the key to determining the age of a
rock using the Rb-Sr method.
Thermoluminescence (TL) dating is the determination, by means of measuring
the accumulated radiation dose, of the time elapsed since material
containing crystalline minerals was either heated (lava, ceramics) or
exposed to sunlight (sediments).
As a crystalline material is heated during measurements the process of
thermoluminescence starts. Thermoluminescence emits a weak light signal
that is proportional to the radiation dose absorbed by the material. It is
a type of luminescence dating.
Sediments are more expensive to date. The destruction of a relatively
significant amount of sample material is necessary, which can be a
limitation in the case of artworks.
The heating must have taken the object above 500° C, which covers most
ceramics, although very high-fired porcelain creates other difficulties.
It will often work well with stones that have been heated by fire. The
clay core of bronze sculptures made by lost wax casting can also be
Different materials vary considerably in their suitability for the
technique, depending on several factors. Subsequent irradiation, for
example if an x-ray is taken, can affect accuracy, as will the "annual
dose" of radiation a buried object has received from the surrounding soil.
Ideally this is assessed by measurements made at the precise findspot over
a long period. For artworks, it may be sufficient to confirm whether a
piece is broadly ancient or modern (that is, authentic or a fake), and
this may be possible even if a precise date cannot be estimated.
Natural crystalline materials contain imperfections: impurity ions, stress
dislocations, and other phenomena that disturb the regularity of the
electric field that holds the atoms in the crystalline lattice together.
These imperfections lead to local humps and dips in the crystalline
material's electric potential. Where there is a dip (a so-called "electron
trap"), a free electron may be attracted and trapped.
The flux of ionizing radiation—both from cosmic radiation and from natural
radioactivity excites electrons from atoms in the crystal lattice into the
conduction band where they can move freely. Most excited electrons will
soon recombine with lattice ions, but some will be trapped, storing part
of the energy of the radiation in the form of trapped electric charge
Depending on the depth of the traps (the energy required to free an
electron from them) the storage time of trapped electrons will vary as
some traps are sufficiently deep to store charge for hundreds of thousands
In thermoluminescence dating, these long-term traps are used to determine
the age of materials: When irradiated crystalline material is again heated
or exposed to strong light, the trapped electrons are given sufficient
energy to escape. In the process of recombining with a lattice ion, they
lose energy and emit photons (light quanta), detectable in the laboratory.
The amount of light produced is proportional to the number of trapped
electrons that have been freed which is in turn proportional to the
radiation dose accumulated. In order to relate the signal (the
thermoluminescence—light produced when the material is heated) to the
radiation dose that caused it, it is necessary tocalibrate the material
with known doses of radiation since the density of traps is highly
Thermoluminescence dating presupposes a "zeroing" event in the history of
the material, either heating (in the case of pottery or lava) or exposure
to sunlight (in the case of sediments), that removes the pre-existing
trapped electrons. Therefore, at that point the thermoluminescence signal
is zero. As time goes on, the ionizing radiation field around the material
causes the trapped electrons to accumulate. In the laboratory,
the accumulated radiation dose can be measured, but this by itself is
insufficient to determine the time since the zeroing event.