Entitled +quot;Questions and the answer for chronologists+quot;, this is taken from the mo
Entitled "Questions and the answer for chronologists", this is
taken from the most recent issue of "New Scientist"
Review of "The Age of the Earth", by G. Brent Dalrymple, Stanford
Univ. Press, 474 pages. $49.50
(Typos are mine!)
How old is our planet? The answer, "four and one-half billion years",
is in the first sentence in Brent Dalrymple's story of how scientists have
deduced the age of the Earth. The events that led Dalrymple to write the
book are rooted in the creationist, pseudo-science controversies of the
American law courts, and this in itself makes an interesting tale.
Dalrymple is an accomplished geochronologist who was retained as a
witness by the State of California in 1980, defending against a civil suit
brought by creationists who objected to the teaching of evolution as fact
in public schools. The state had assembled a formidable panel of
scientific expertise, and although ultimately never called upon, Dalrymple
acquired a reputation as a scientist who was willing and able to debate the
creationist issue in legal proceedings, informal seminars, and the press.
Be aware, though, that this book is not a philosophical or
theological debate; it is a well-informed scientific textbook based on the
author's painstaking background research in a number of fields.
By starting with "The Answer", Dalrymple sets out his stall. This
was a wise move; it would take an extraordinary writer to maintain the
suspense of a whodunit through the development of theories and techniques
pertaining to a single question in earth sciences, however fundamental.
The answer has been hard-won down the centuries, however, and a wide
variety of disciplines have been brought to bear upon the question.
Early attempts to deduce or estimate the age of the Earth from the
first main section of the book, and provide some of the most entertaining
reading. James Ussher's celebrated biblical chronology of 1650, which
placed Creation in the evening of 22 October 4004 BC, was by no means the
earliest recorded "Answer". About 2000 years before Ussher, the Hindu
priesthood decided that creation had occurred 2 billion years ago. The
accuracy of this guess was unsurpassed until the 20th Century.
Biblical chronologies are all good fun, but the real entertainment
from a modern perspective comes with the work of 19th-century scientists,
such as Lord Kelvin, whose estimates were based on observational data
and sound physical principles. Physicists calculated the cooling rate of the
Earth and the effects of orbital forces, while geologists, who trusted their
own observations rather than the mathematical juggling of the establishment,
appealed to erosion and sedimentation rates.
They were all spectacularly wrong because they had misunderstood
processes, guessed starting conditions and overstated the certainty of
their assumptions. In some cases, they were soundly criticised on these
grounds by their contemporaries. Ironically, much modern scientific debate
revolves around the same criticisms. Perhaps we should take a lesson from
the authors who claimed their work to be no more than "wild speculation", yet
whose answer is often all that remains in the popular perception about the
age of the Earth.
Ultimately, the age of the Earth and everything in it can only be
addressed by radiometric dating. As a background to the main substance of
the story, modern radiometric methods are explained in a chapter that is
as clear and concise as any I have read.
Here the book stops being journalism and gets its teeth into the
subject; lead isotope systematics are tortuous enough for those of us who
deal with them daily, and Dalrymple has probably made this process as
accessible as possible. Although all the principal dating methods are
described, we find out that the whole story hinges on the decay of
uranium to lead. A chapter relates the oldest rocks found on the Earth,
and here the diversity of techniques available to the modern geochronologist
becomes invaluable, giving us a minimum age constraint that is bolstered by
data from lunar samples.
Another chapter introduces the meteorites, the oldest and most
primitive rocks available to scientists, which are crucial to the story
because the "age" of the Earth really means the time at which the matter
of the Solar System first segregated into discrete bodies. The chapter
devoted to lead isotopes just about wraps things up, but there follows
a brief account of astrophysical methods by which the age of the Universe
has been estimated, as well as a concluding chapter with the tantalizing
title of "What we know and do not know".
Although the author has an easy conversational style that allows
us to skip the mathematics if we wish, this book is by no means light
bedtime reading. A tremendous amount of information is presented or
cited between the covers, and as such it makes a useful source book.
A comprehensive glossary is included to help the non-specialist escape
the jargon trap. We all know "The Answer", but if you want to know
how the question was asked, this book will certainly tell you.
Reviewer: Simon Inger lectures in earth sciences at the
University of Leeds.
E-Mail Fredric L. Rice / The Skeptic Tank