_How old is a rock?_
_How old is man?_
_How old is the earth?_
It would be difficult to find a reason why anyone would really have to
know the answers to these questions, yet they have been asked over and
over again since the dawn of human society. The records of every
civilization disclose attempts to delve into the past beyond the memory
of the oldest man, beyond recorded history, beyond earliest legend.
Curiosity about the remote past may be very ancient, but the only
reliable method of measuring very long intervals of time is new. The
possibility of doing so became apparent only after the discovery of
RADIOACTIVITY[1] in 1896 by Henri Becquerel, when Marie and Pierre Curie
in 1898 recognized that some atoms are radioactive and change by
themselves into other atoms at regular and constant rates. If something
gradually transforms itself into something else, if this transformation
goes on at a known pace, and if all the products of the activity are
preserved in some kind of a CLOSED SYSTEM, then it is theoretically
possible to calculate the time that has elapsed since the process
started. The theory was clear for years; the only problem was how to
satisfy all those ifs.
By 1910 it was well established that the earth must be extremely
ancient. Analyses of some minerals containing uranium showed them to be
hundreds of millions of years old, even though the uranium came from
rocks that were known to be relatively young among geologic STRATA.
Measurements still were inaccurate, however, and only a few rare and
unusually rich radioactive minerals contained enough of the products of
RADIOACTIVE DECAY[2] to allow analysis of their age by the crude methods
then available.
Not much progress was made for about 30 years until A. O. C. Nier, a
Harvard University physicist, perfected an instrument called a MASS
SPECTROMETER (to be described later) just before World War II. The rapid
technological advances of the war years followed. The Manhattan
Project[3] made the atomic bombs that ended the fighting; it also
developed new scientific techniques that could be applied, when peace
returned, to the measurement of geologic time.
The next important advance was contributed in 1946 by Arthur Holmes in
England and by F. G. Houtermans in Germany. Each of these scientists had
seen Nier’s reports before the war and had realized that Nier’s
mass-spectrometer analyses of lead made it possible, for the first time,
to make rational calculations about the age of the earth. The two
scientists independently calculated that age at about 2 to 3 billion
years, using the handful of data available to them from Nier’s
measurements. It is interesting that today, thousands of analyses later,
our planet’s age usually is given as 4.5 billion years. The early
estimates were not far off.
Development of various methods for measuring the age of minerals
followed rapidly, and by 1955 many fundamental studies needed for
measuring the age of very old substances were complete. The basic
techniques are summarized in Table I. They will be explained later. The
new methods produced broad confirmation of the early rough estimates and
they also brought a few surprises.
Before we go into these discoveries, let us look at some theoretical
foundations.
Table I BASIC MEASUREMENT METHODS
Method Material Time Dated Useful Time Span
(years)
Carbon-14 Wood, peat, When plant died 1000-50,000
charcoal
Bone, shell Slightly before 2000-35,000
animal died
Potassium-argon Mica, some When rock last 100,000 and up
whole rocks cooled to about
300°C
Hornblende When rock last 10,000,000 and up
Sanidine cooled to about
500°C
Rubidium-strontium Mica When rock last 5,000,000 and up
cooled to about
300°C
Potash When rock last 50,000,000 and up
feldspar cooled to about
500°C
Whole rock Time of 100,000,000 and up
separation of
the rock as a
closed unit
Uranium-lead Zircon When crystals 200,000,000 and up
formed
Uranium-238 Many When rock last 100-1,000,000,000
fission cooled (Depending on
material)
[Illustration: _Time scale of the Earth, drawn to scale. See also the
Holmes time scale on page 46._]
Event Geologic Time Period Era
Man appears[4] (2 million years ago)
Quaternary CENOZOIC
Tertiary
Extinction of the dinosaurs (70 million
years ago)
Cretaceous MESOZOIC
Mammals appear (130 million years ago) Jurassic
Triassic
Oldest known reptiles (300 million years Permian PALEOZOIC
ago)
Pennsylvanian
Mississippian
Devonian
Silurian
Ordovician
Cambrian
First abundant life in the sea (animals
without backbones) (550 million years ago)
Algae and other microorganisms (1,900 Proterozoic PRECAMBRIAN
million years ago)
Archean
Oldest rocks in North America (2,800
million years ago)
First hint of life (bacteria?) (3,100
million years ago)
Oldest rocks (3,300 million years old)
Formation of Earth’s core (4,500 million
years ago)