The Age of the Earth

ABSTRACT

Professor Bob White, Department of Earth Sciences, University of Cambridge, Email: rwhite@esc.cam.ac.uk

Understanding the ages and sequences of rocks and correlating them across large areas has always been and remains one of the pre-eminent concerns of geologists. Even in the earliest days of geological study, it was clear that the relative ages of rock strata could be determined from the order in which they were deposited, and that rocks of the same age could be recognised worldwide. Since the fossil content of rocks changes throughout the earth's history, fossils were used initially simply as 'markers' of different aged strata.

Early in the development of geological study it became clear that the wide variety of rock types and their settings found in the geological record could be explained by the on-going processes observable at the present day: This, indeed, is what marked out geology as a scientific subject in its own right. It was also clear that extremely large time periods were required to build the variety and volumes of rocks observed.

By the second half of the twentieth century, the subject moved from relative dating (strata A is younger than strata B). to absolute dating (strata A is x million years old). Part, but not all, of the capability for deducing absolute ages arose from the recognition of a variety of radioactive decay sequences. By comparing the ratio of daughter to parent isotopes, and knowing the half-life, the age of the rock could be deduced. A wide variety of radioactive decay sequences with widely different half-lives have been utilised in dating rocks of different ages. But there are also many other techniques available for dating, which are used to cross-check and to calibrate the different methods. They include, for example, simply counting back through layers that have an annual signal: tree rings, deep ice cores, and fine-grained lake deposits have all been used in this way.

Other techniques include assessing damage to crystals from cosmic rays, using the irregular polarity flips of the earth's magnetic field as a clock and, recently, measuring the rates of continental drift directly using satellite navigation systems that have precisions in the range of centimetres. A striking recent discovery has been that the regular orbital changes in the positions of the planets drive a repetitive cycle – the Milankovitch cycle – that enables some sedimentary rocks hundreds of millions of years old to be dated to a precision of better than 100,000 years.

The geologists' view of the development of the earth, however, is far from the common perception of endless ages of infinitesimal changes. Rather, much of the rock record now preserved is the result of abrupt, short-lived catastrophic events: massive ash falls or lava flows from volcanic eruptions; landslides and stratal disruption from earthquakes; alluvial sediments laid down primarily by the one-in-a-hundred or thousand-year flood; beach deposits dominated by the largest storms; worldwide, instantaneous sea level changes caused by glaciations; and even global catastrophes caused by extra-terrestrial impacts. In the light of all these, it is perhaps even more remarkable that environmental conditions on earth have remained in the narrow range that has allowed life to flourish throughout almost all its history, from shortly after the earth was first formed to the present time.

Along with the longevity of life on earth the other striking consequence of an understanding of geological timescales is that humans have been around for such a short period: only for less than one- thousandth of one percent of the history of the earth. From a Christian perspective, this is cause for wonder and worship of the creator God whom the bible repeatedly proclaims has placed mankind at the pinnacle of his creation, for the purpose of relationship with himself. Furthermore, it underlines God's bountiful and generous creativity: from the initial formation of the stars which generated the elements from which our very bodies are made, through the long history of the earth prior to the arrival humankind, he has been preparing a place for us. And as Hebrews 1:3 proclaims, he has sustained, and continues to uphold his created universe throughout all its, and our history.