DEFINITION OF GEOCHRONOLOGY

The study of time with respect to Earth

history, including both absolute and relative dating systems

as well as correlation methods. Absolute dating systems

include a variety of geochronometers such as radioactive

decay series in specific isotopic systems that yield a numerical

value for the age of a sample. Relative dating schemes include

cross-cutting features and discontinuities such as igneous

dikes and unconformities, with the younger units being the

cross-cutting features or those overlying the unconformity.

During the 19th and early 20th centuries, geochronologic

techniques were very crude. Many ages were estimated by the

supposed rate of deposition of rocks and correlation of units

with unconformities with other, more complete sequences.

With the development of radioactive dating it became possible

to refine precise or absolute ages for specific rock units.

Radiometric dating operates on the principle that certain

atoms and isotopes are unstable. These unstable atoms tend to

decay into stable ones by emitting a particle or several particles.

Alpha particles have a positive charge and consist of two

protons and two neutrons. Beta particles are physically equivalent

to electrons or positrons. These emissions are known as

radioactivity. The time it takes for half of a given amount of a

radioactive element to decay to a stable one is known as the

half-life. By matching the proportion of original unstable isotope

to stable decay product, and knowing the half-life of that

element, one can thus deduce the age of the rock. The precise

ratios of parent to daughter isotopes are measured in an

instrument known as a mass spectrometer.

Radiocarbon or carbon-14 dating techniques were developed

by Willard F. Libby (1908–80) at the University of

Chicago in 1946. This discovery represented a major breakthrough

in dating organic materials and is now widely used by

archaeologists, Quaternary geologists, oceanographers, hydrologists,

atmospheric scientists, and paleoclimatologists. Cosmic

rays entering Earth’s atmosphere transform regular carbon

(C12) to radioactive carbon (C14). Within about 12 minutes of

being struck by cosmic rays in the upper atmosphere, the carbon-

14 combines with oxygen to become carbon dioxide that

has carbon-14. It then diffuses through the atmosphere and is

absorbed by vegetation (plants need carbon dioxide in order to

make sugar by photosynthesis). Every living thing has carbon

in it. While it is alive, each plant or animal exchanges carbon

dioxide with the air. Animals also feed on the vegetation and

absorb its carbon dioxide. At death, the carbon-14 is no longer

exchanged with the atmosphere but continues to decay in the

material. Theoretically, analysis of this carbon-14 can reveal

the date when the object once lived by the percent of carbon-

14 atoms still remaining in the object. The radiocarbon

method has subsequently evolved into one of the most powerful

techniques to date late Pleistocene and Holocene artifacts

and geologic events up to about 50,000 years old.

Uranium, thorium, and lead isotopes form a variety of

geochronometers using different parent/daughter pairs. Uranium

238 decays to lead 206 with a half-life of 4.5 billion

years. Uranium 235 decays to lead 207 with a half-life of 0.7

billion years, and thorium 232 decays to lead 208 with a

half-life of 14.1 billion years. Uranium, thorium, and lead are

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