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The Encyclopedia of World History.
Potassium-Argon Dating
 
So far, we lack accurate methods for dating prehistoric time between about 50,000 and 500,000 years ago. The potassium-argon method is sometimes useful for dating very early prehistory. Geologists use it to date rocks as early as 4 to 5 billion years old and some as recent as 100,000 years old.

Potassium (K), one of the most abundant elements in the earth's crust, is present in nearly every mineral. In its natural form, it contains a small proportion of radioactive 40K atoms. For every 100 40K atoms that decay, 11 percent become 40Ar, an inactive gas that escapes from its parent material by diffusion when lava and other molten rocks are formed. By using a spectrometer, it is possible to measure the concentration of 40Ar that has accumulated since the rock formed.

Many early archaeological sites like Koobi Fora and Olduvai Gorge in East Africa were formed during periods of intense volcanic activity. Potassium-argon dates from lavas and volcanic ashes stratified above and below places where early human artifacts and bones occur date early human activity between 2.5 million and 500,000 years ago.

Radiocarbon and potassium-argon dating techniques are especially important, for they enable archaeologists to date widely separated archaeological sites and to compare, say, the dates for early agriculture in the Peruvian Andes with those in sub-Saharan Africa. They have made possible the writing of a truly global prehistory of humankind.

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