Dating Methods

I

Introduction

Dating Methods, in earth science, methods used to date the age of rocks and minerals. By applying this information, geologists are able to decipher the 4.6-billion-year history of the earth. The events of the geologic past—uplift of mountain ranges, opening and closing of seas, flooding of continental interiors, changes in climate—are all recorded in the strata of the earth’s crust (see Chronology).

II

Development of Relative and Absolute Methods

With the methods then available, 19th-century geologists could only construct a relative time scale. Thus, the actual age of the earth and the duration, in millions of years, of the units of the time scale remained unknown until the dawn of the 20th century. After radioactivity was discovered, radiometric dating methods were quickly developed. With these new methods geologists could calibrate the relative scale of geologic time, thereby creating an absolute one.

The relative scale was devised mainly by application of the principles of stratigraphy. An example of these is the law of superposition, which simply states that in an undisturbed succession of strata, the youngest beds are on top and the oldest on bottom (or, the higher beds are younger than the lower).

Based on the fossils they contain, strata in one area were correlated with those in other areas. As more and more such correlations were made, geologists began to make broad groupings of strata, which became the basis for dividing geologic time into vast blocks. Thus, the history of the earth was divided into four broad eras—Pre-cambrian, Paleozoic, Mesozoic, and Cenozoic; the eras were in turn divided into a number of periods (see Geology).

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III

Absolute Dating Methods

Although development of radiometric methods led to the first and principal breakthroughs in establishing an absolute time scale, other absolute methods were devised that have limited applications. Chief among these are dendrochronology, varve analysis, hydration dating, and TL dating.

A

Dendrochronology

This method of dating events and conditions of the recent past is based on the number, width, and density of annual growth rings of long-lived trees. In the southwestern United States, for example, a master tree-ring index has been constructed from the Douglas fir and bristlecone pine. This index enables dendrochronologists to date accurately events and climatic conditions of the past 3000 to 4000 years. See Tree.

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