LAB 4: ABSOLUTE AGE DATING AND THE GEOLOGIC TIME SCALE
Absolute age dating
You have learned how to place geologic events in sequential order and, also, that fossils succeed
each other the same way upwards in a stratigraphic column no matter where on earth that
column is located.
Guided by the same ideas, early geologists correlated stratigraphic sections
across distances, and saw fit to name time periods in which these sections were deposited.
names for the geologic time periods are still used today.
However, figuring out how old pieces of
the Earth’s surface were was not nearly so useful or accurate as it is today when you can find a
rock and take it to the laboratory to test how old it absolutely is.
The kind of technology needed to
do this was unavailable until advances in our understanding of the atom were made.
Not all atoms remain the same forever.
In some atoms an electron is captured by the nucleus
(electron capture decay), or perhaps a couple of protons and neutrons are emitted from the
nucleus (alpha decay), or a neutron emits an electron, changing it to a proton (beta decay).
These atoms are radioactive.
Several of the elements on the periodic table are radioactive or
have other isotopes (different number of neutrons) that are.
The original atom of a radioactive
isotope becomes parent to a new, or daughter, isotope by decaying.
Radioactive decay rates
have been experimentally determined and are measured, for convenience, in half-lives.
The half-life of a radioactive isotope is the amount of time it takes for one half of a sample of
parent isotopes to become daughter isotopes.
Therefore, if the decay rate is known, and the ratio
of parent to daughter isotopes for a rock is known, then the age of the rock can be calculated.
Say there are 1,000,000 atoms of a radioactive isotope.
=> After one half-life, there will be 500,000 parent isotopes, and 500,000 daughter
=> After two half-lives, there will be 250,000 parent isotopes, and 750,000 daughter
How many of each isotope will the sample have after four half-lives?
Table 1 shows a variety of radioactive isotopes that are used for absolute dating.
The metals in
the table are found in mineral structures and begin their decay at the cooling of the magma/lava
that formed an igneous rock.
The potassium-argon pair can record metamorphic events, as the
clock is reset when argon in the original structure is allowed to escape as a gas.
In the example
of volcanic ash, the age of the igneous rock is similar to age of its deposition, and the
sedimentary layer it creates can be dated.