Dating Methods

B

Varve Analysis

One of the oldest methods employed for absolute age determination, varve analysis, was developed by Swedish scientists in the early 20th century. A varve is a sedimentary bed, or sequence of beds, deposited in a body of still water over the period of a year. Counting and correlation of varves have been used to measure the ages of Pleistocene glacial deposits. By dividing the rate of sedimentation in terms of units per year by the number of units deposited following a geological event, geologists can establish the age of the event in years.

C

Obsidian Hydration Dating

Also referred to as hydration rind dating or obsidian dating, this method is used to calculate ages in years by determining the thickness of rims (hydration rinds) produced by water vapour slowly diffusing into freshly chipped surfaces on artefacts made of obsidian, or recent volcanic, glass. The method is applicable to types of glass which are 200 to 200,000 years old.

D

Thermoluminescence (TL) Dating

This method is based on the phenomenon of natural ionizing radiation inducing free electrons in a mineral that can be trapped in defects of the mineral’s crystal lattice structure. These trapped electrons escape as thermoluminescence (TL) when heated to a temperature below incandescence, so that by recording the TL of a mineral such as quartz and assuming a constant natural radiation level, the last drainage of the trapped electrons can be dated back to several hundred thousand years. In TL dating of pottery, for example, the specimen is heated until it glows with energy that has been stored ever since it was fired.

E

Radiometric Dating

Radiometric techniques were developed after the discovery of radioactivity in 1896. The regular rates of decay for unstable, radioactive elements were found to constitute virtual “clocks” within the Earth’s rocks.

E

1

Basic Theory

Radioactive elements such as uranium (U) and thorium (Th) decay naturally to form different elements or isotopes of the same element. (Isotopes are atoms of any elements that differ in mass from that element, but possess the same general chemical and optical properties.)

This decay is accompanied by the emission of radiation or particles (alpha, beta, or gamma rays) from the nucleus, by nuclear capture, or by ejection of orbital electrons. A number of isotopes decay to a stable product, a so-called daughter isotope, in a single step (for example, carbon-14), whereas others involve many steps before a stable isotope is formed. Multistep radioactive decay series include, for example, the uranium-235, uranium-238, and thorium-232 families. If a daughter isotope is stable, it accumulates until the parent isotope has completely decayed. If a daughter isotope is also radioactive, however, equilibrium is reached when the daughter decays as fast as it is formed.

Radioactive decay may take different routes. Thus, if the isotope decays by alpha emission, it loses the two protons and two neutrons that make up an alpha particle; the atomic number (number of protons) is reduced by two and the atomic mass (number of nuclear particles, or nucleons) by four. In beta decay, or electron loss, a radioactive nucleus can gain or lose one unit of electric charge without changing the number of nucleons. More radioactive substances are beta-ray emitters than alpha-ray emitters.

A third important mode of decay involves electron capture; the nucleus of an atom absorbs an electron, which unites with a proton of the nucleus to form a neutron. Thus, the atomic number is reduced by one, but the mass of the nucleus remains unchanged. The fourth mode of decay, gamma radiation, consists of the emission of waves of electromagnetic energy.

Scientists describe the radioactivity of an element in terms of half-life—the time the element takes to lose half of its activity through decay. This covers an extraordinary range of time, from a few microseconds to billions of years. At the end of the period constituting one half-life, half of the original quantity of the radioactive element has decayed; after another half-life, half of what was left is halved again, leaving one quarter of the original quantity, and so on. Every radioactive element has its own half-life; for example, that of carbon-14 is 5,730 years and that of uranium-238 is 4.5 billion years.

Radiometric dating techniques are based on radio-decay series with constant rates of isotope decay. Once a quantity of a radioactive element becomes part of a growing mineral crystal, that quantity will begin to decay at a steady rate, with a definite percentage of daughter products in each time interval. These “clocks in rocks” are the geologists’ timekeepers.