RADIOMETRIC TIME SCALE

The mathematical expression that relates radioactive decay to geologic time is called the age equation and is:

Dating rocks by these radioactive timekeepers is simple in theory, but the laboratory procedures are complex. The numbers of parent and daughter isotopes in each specimen are determined by various kinds of analytical methods. The principal difficulty lies in measuring precisely very small amounts of isotopes.

The potassium-argon method can be used on rocks as young as a few thousand years as well as on the oldest rocks known. Potassium is found in most rock-forming minerals, the half-life of its radioactive isotope potassium-40 is such that measurable quantities of argon (daughter) have accumulated in potassium-bearing minerals of nearly all ages, and the amounts of potassium and argon isotopes can be measured accurately, even in very small quantities. Where feasible, two or more methods of analysis are used on the same specimen of rock to confirm the results.

Another important atomic clock used for dating purposes is based on the radioactive decay of the isotope carbon-14, which has a half-life of 5,730 years. Carbon-14 is produced continuously in the Earth's upper atmosphere as a result of the bombardment of nitrogen by neutrons from cosmic rays. This newly formed radiocarbon becomes uniformly mixed with the nonradioactive carbon in the carbon dioxide of the air, and it eventually finds its way into all living plants and animals. In effect, all carbon in living organisms contains a constant proportion of radiocarbon to nonradioactive carbon. After the death of the organism, the amount of radiocarbon gradually decreases as it reverts to nitrogen-14 by radioactive decay. By measuring the amount of radioactivity remaining in organic materials, the amount of carbon-14 in the materials can be calculated and the time of death can be determined. For example, if carbon from a sample of wood is found to contain only half as much carbon-14 as that from a living plant, the estimated age of the old wood would be 5,730 years.

The radiocarbon clock has become an extremely useful and efficient tool in dating the important episodes in the recent prehistory and history of man, but because of the relatively short half-life of carbon-14, the clock can be used for dating events that have taken place only within the past 50,000 years.

The following is a group of rocks and materials that have dated by various atomic clock methods:

Sample	Approximate Age in Years
Cloth wrappings from a mummified bull Samples taken from a pyramid in Dashur, Egypt. This date agrees with the age of the pyramid as estimated from historical records	2,050
Charcoal Sample, recovered from bed of ash near Crater Lake, Oregon, is from a tree burned in the violent eruption of Mount Mazama which created Crater Lake. This eruption blanketed several States with ash, providing geologists with an excellent time zone.	6,640
Charcoal Sample collected from the "Marmes Man" site in southeastern Washington. This rock shelter is believed to be among the oldest known inhabited sites in North America	10,130
Spruce wood Sample from the Two Creeks forest bed near Milwaukee, Wisconsin, dates one of the last advances of the continental ice sheet into the United States.	11,640
Bishop Tuff Samples collected from volcanic ash and pumice that overlie glacial debris in Owens Valley, California. This volcanic episode provides an important reference datum in the glacial history of North America.	700,000
Volcanic ash Samples collected from strata in Olduvai Gorge, East Africa, which sandwich the fossil remains of Zinjanthropus and Homo habilis -- possible precursors of modern man.	1,750,000
Monzonite Samples of copper-bearing rock from vast open-pit mine at Bingham Canyon. Utah.	37,500,000
Quartz monzonite Samples collected from Half Dome, Yosemite National Park, California.	80,000,000
Conway Granite Samples collected from Redstone Quarry in the White Mountains of New Hampshire.	180,000,000
Rhyolite Samples collected from Mount Rogers, the highest point in Virginia.	820,000,000
Pikes Peak Granite Samples collected on top of Pikes Peak, Colorado.	1,030,000,000
Gneiss Samples from outcrops in the Karelian area of eastern Finland are believed to represent the oldest rocks in the Baltic region.	2,700,000,000
The Old Granite Samples from outcrops in the Transvaal, South Africa. These rocks intrude even older rocks that have not been dated.	3,200,000,000
Morton Gneiss [see Editor's Note] Samples from outcrops in southwestern Minnesota are believed to represent some of the oldest rocks in North America.	3,600,000,000

Carbon samples are converted to acetylene gas by combustion in a vacuum line. The acetylene gas is then analyzed in a mass spectrometer to determine its carbon isotopic composition.

Interweaving the relative time scale with the atomic time scale poses certain problems because only certain types of rocks, chiefly the igneous variety, can be dated directly by radiometric methods; but these rocks do not ordinarily contain fossils. Igneous rocks are those such as granite and basalt which crystallize from molten material called "magma".

When igneous rocks crystallize, the newly formed minerals contain various amounts of chemical elements, some of which have radioactive isotopes. These isotopes decay within the rocks according to their half-life rates, and by selecting the appropriate minerals (those that contain potassium, for instance) and measuring the relative amounts of parent and daughter isotopes in them, the date at which the rock crystallized can be determined. Most of the large igneous rock masses of the world have been dated in this manner.