Biology Reference
In-Depth Information
arctica, the Andean and Himalayan glaciers, and some stalactites; organic
material produced in annual cycles (e.g., tree rings); and many historical
records can be dated to the year. These and the other methods discussed
below are used both independently and collectively to check results from
different procedures in a process called cross-dating. Absolute chronologies
are based on the following techniques.
Fission-track dating
When decay products of the radioactive isotope uranium 238 move through
an enclosing mineral, a track is left in the form of destroyed or modifi ed
atomic latticeworks. The decay or fi ssion occurs at a constant rate, so the
number of tracks per area, called the spontaneous density (number of
tracks per cm
2
), is an indication of the age of the rock. It is applicable to
apatite, natural glass (obsidian), zircon, and other minerals. The tracks are
destroyed by heat, so the measurements indicate the last time the sample
cooled. It is used to date tiles, and to assess an area for tectonic hazards
such as earthquakes and the “suitability” of the site for storing nuclear
waste. It has also been used to cross-check the potassium-argon ages of
sediments from the anthropologically important Ulduvai Gorge in Africa.
Depending on the uranium concentration, fi ssion-track dating is applicable
to material from essentially modern historical and archeological records to
rocks billions of years old.
Thermoluminescent dating
The technique known as thermoluminescent dating is based on the occur-
rence of minute imperfections in minerals such as calcite, diamond, feld-
spar, or quartz, as well as in ceramics, bricks, and tiles, that trap electrons
emitted in low-level radioactivity from, for example, inclusions of thorium,
potassium, or uranium. If the sample is heated to about 500°C or exposed
to a laser beam, this energy is released as sparks of light proportional in
intensity to the energy stored, which, in turn, is a function of the time it
has been accumulating. Since lava originates in the molten state, and since
pottery, brick, and tile are fi red in the manufacturing process, the thermo-
luminescent clock, like the number of fi ssion tracks, is reset back to zero
and dates the time of the last cooling. Standards are set up by bombarding
material of known age and measuring the amount of emission. The method
is applicable to samples mostly several thousand years old and, depending
on the extent of the traps, as old as a few hundred thousand years.
