Dating Methods

I

Introduction

Dating Methods, in Earth sciences, 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—or chronology—of the Earth. The events of the geological 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.

II

Development of Relative and Absolute Methods

With the methods then available, 19th-century geologists could only construct a relative timescale. Thus, the actual age of the Earth and the duration, in millions of years, of the units of the timescale 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 geological 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 of rock are on top and the oldest on bottom (or, the higher beds are younger than the lower).

Based on the fossils they contain, rock 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 geological time into vast blocks. Thus, the history of the Earth was divided into four broad eras—Precambrian, Palaeozoic, Mesozoic, and Cenozoic; the eras were in turn divided into a number of periods. These divisions of time are fundamental to the study of Geology.

III

Absolute Dating Methods

The absolute dating of rocks and minerals enables geoscientists to attribute numerical ages to points on the relative geological timescale. Unfortunately, the practice is much more difficult than the principle. The first absolute (but incomplete) geological timescale was devised many decades ago, and since then at least a dozen others, all different, have been published. One reason for the continual change is that some absolute ages obtained in the past have subsequently been proved wrong (for example, contaminated rocks can give inaccurate ages) and these have had to be revised. A more fundamental reason is that there is often no agreement on precisely where a rock fits into the relative timescale, or views on the position may change. Or it may simply be difficult to find a datable rock that lies exactly at, say, a timescale boundary for which it would be useful to have an absolute age. Disagreements and uncertainties on such matters are continually being expressed in the scientific press.

Encarta uses the absolute ages published by W. B. Harland and others as A Geologic Time Scale 1989 (Cambridge University Press, 1990). However, this scale will not stand for all time, and some revisions have already been proposed. In the meantime, readers of works beyond Encarta will find that points on the relative geological timescale do not always have the same numerical ages attached to them. The ages quoted will depend upon when the works in question were written.

Although development of radiometric methods led to the first and principal breakthroughs in establishing an absolute timescale, 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 south-western 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 3,000 to 4,000 years.