Wednesday, January 12, 2011


Modern scientific techniques help scientists measure the age of ancient rocks such as these pegmatite with quartz and feldspar ones in Wyoming’s Teton Range. Dr. Marli Miller/Visuals Unlimited/Getty Images

Geochronology is the field of scientific investigation concerned with determining the age and history of the Earth’s rocks and rock assemblages. Such time determinations are made and the record of past geologic events is deciphered by studying the distribution and succession of rock strata, as well as the character of the fossil organisms preserved within the strata.

The Earth’s surface is a complex mosaic of exposures of different rock types that are assembled in an astonishing array of geometries and sequences. Individual rocks in the myriad of rock outcroppings (or in some instances shallow subsurface occurrences) contain certain materials or mineralogic information that can provide insight as to their “age.”

For years investigators determined the relative ages of sedimentary rock strata on the basis of their positions in an outcrop and their fossil content. According to a longstanding principle of the geosciences, that of superposition, the oldest layer within a sequence of strata is at the base and the layers are progressively younger with ascending order. The relative ages of the rock strata deduced in this manner can be corroborated and at times refined by the examination of the fossil forms present. The tracing and matching of the fossil content of separate rock outcrops (that is, the correlation process) eventually enabled investigators to integrate rock sequences in many areas of the world and construct a relative geologic time scale.

Scientific knowledge of the Earth’s geologic history has advanced significantly since the development of radiometric dating, a method of age determination based on the principle that radioactive atoms in geologic materials decay at constant, known rates to daughter atoms. Radiometric dating has provided not only a means of numerically quantifying geologic time but also a tool for determining the age of various rocks that predate the appearance of life-forms.

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