RADIUM AND THE AGE OF THE EARTH
One of the important consequences of the discovery of radioactivity was
to afford the scientist a means for solving the problem of the earth’s
age. By “age of the earth” we mean here the time which has elapsed
since the earth’s surface became fitted for the habitation of living
beings. By means of radioactivity we can form an approximate estimate
of the time which has passed since the formation of any given series of
geological strata. Radium is our geological time clock.
It is now known that all the common rocks and soils of which the
earth’s crust is built up contain measurable amounts of radium.
According to the computation made by Prof. John Joly, the total
quantity of radioactive matter may be as much as one 500 billionth part
of the whole volume of the globe, or something over half a cubic mile.
All of the 36 known radio-elements are disintegration products of the
primary radio-elements uranium and thorium--_i.e._, they are produced
from one or the other of these in their long sequence of changes. And
the rate at which the radioactive products change--their average life
period,--from the first transmutation to the final product, radium
lead, an isotrope of common lead, is accurately known. (Helium atoms
are “the debris shed at the various stages of the transformation.”)
It is now well established that a gram of uranium as found along with
its products in rocks and minerals is changing at a rate represented by
the production of 1.88 x 10^{-11} grams of helium and 1.22 x 10^{-10}
grams of lead (isotrope) _per annum_. We do not know for a certainty,
of course, that this rate of production has been maintained throughout
geological time. In the opinion of Lord Rayleigh, we may safely assume
that the rate of transformation has not changed, so that “it would seem
that in the disintegration of a gram of uranium we have a process the
rate of which can be relied upon to have been the same in the past as
we now observe it to be” (_Nature_, October 27, 1921).
Acting on Rutherford’s suggestion, the Hon. R. J. Strutt (later
Lord Rayleigh) made a determination of the amount of radium in the
superficial parts of the earth--which are alone accessible; and he
also determined the ratio of the lead (isotope) to the uranium, which
was found to be 1.3 (specifically, in the broggerite found in the
Pre-Cambrian rocks at Moss, Norway). Now, if we assume--as the evidence
seems to warrant--that the lead of this atomic weight (206.06) was
all produced by uranium at the rate given above, we get an age of 925
million years for these rocks. Some minerals from other Archaean rocks
in Norway give a rather larger figure.
“In other cases,” says Lord Rayleigh, “there is some complication,
owing to the fact that thorium is associated with uranium in the
mineral and that it, too, produces helium and an isotrope of lead of
atomic weight probably 208 exactly, about one unit higher than common
lead.”
Sir Ernest Rutherford estimated the time required for the accumulation
of the radium content of a uranium mineral in the Glastonbury granitic
gneiss of the early Cambrian as no less than 500,000,000 years. Later
investigations give some of the Pre-Cambrian rocks an antiquity of
1,640 millions of years! The zoologist may now have all the time he
wants for the slowly evolving organisms revealed by the sedimentary
strata.
Prof. John W. Gruner, of the geology department of the University
of Minnesota, discovered (in 1925) microscopic forms of plant life
(algae) embedded in iron formations of the Vermillion Range near Lake
Armstrong, Minnesota. Most of Minnesota’s iron deposits are due to the
algae, Dr. Gruner thinks. The growth has the property of extracting
iron from sea water and making of it a solid shell with which to
surround itself. Accumulations of these iron shells through millions of
years have been embedded in rock formations forming the iron ore.
Slices of rock a thousandth of an inch thick were examined under
microscopes in the search for the algae. Algae began to flourish
immediately after the earth, in cooling (according to one cosmological
theory), got below the boiling point. Their form is much like seaweed,
and they thrive at a temperature of 95° C. Dr. Gruner estimates the
age of these algae-bearing deposits at 200,000,000 years, ten million
years earlier than previous evidence showed.
If we employ the radioactivity test as a measure of geological
time, the age of these fossil algae would have to be placed much
higher--older by hundreds of millions of years. And the same must be
said of the amphibian footprints recently (1925) discovered in the
sandstone slabs of the Grand Canyon, by the caretaker on Hermit’s
Trail, a thousand feet below the rim of the canyon. On the older
geological time scale, these deposits date back some 50,000,000 years
(lower Carboniferous period--the so-called “Mississippian” system). On
the radium time schedule, these figures would need to be multiplied
considerably (according to Boltwood and Holmes, by a multiple of six or
more). It should be said, however, that on the time deposits of Walcott
and Schuchert, based on the rate of deposition of sediments, the
lower Carboniferous (Mississippian) deposits are not older than some
18,000,000 years.
But amphibian footprints are known from the far older Devonian period,
whose strata are, on the radium basis, some 370 million years old.
Prof. Charles Schuchert, of Yale, regards the estimates of geological
time based upon the rate of disintegration of radioactive minerals as,
on the whole, far more reliable than estimates based upon the rate of
deposition of sediments. No scientist pretends to be able to state
exactly the age of strata by the amount of radium lead contained in
them.
“In a third class of cases,” Lord Rayleigh points out, “the uranium
mineral, pitchblende, occurs in a metalliferous vein, and the lead
isotope produced in the mineral is diluted with common lead which
entered into its original composition, ... but the complications
cannot, I think, be considered to modify the broad result.
“A determination of the amount of helium in minerals gives an
alternative method of estimating geological age; but helium, unlike
lead, is liable to leak away, hence the estimate gives a minimum only.
I have found in this way ages which, speaking generally, are about
one-third of the values which estimates of lead have given, and are,
therefore, generally confirmatory, having regard to leakage of helium.”
Dr. Homer P. Little, of the National Research Council, Washington,
D. C., tells us (_Scientific American Monthly_, August, 1921, p. 173)
that “from both calculation and experiment it is found that one gram
of uranium will produce helium at the rate of one cubic centimeter in
9,600,000 years. The ratio between the amount of radium in a mineral
and the amount of helium present therefore allows us to calculate the
age of the mineral. The amount of uranium originally present compared
to that left does not enter into the problem unless extreme lengths of
time are under consideration, because of the fact that it is calculated
to take 5,000 million years for one-half a given volume of uranium to
disintegrate.
“It is perfectly true that much of the helium generated may escape. The
assumption is, however, that in some minerals comparatively little
escapes: zircon, particularly, seems to be an effective retainer.
This mineral shows very effectively the increasing ratio of helium
to uranium as consecutively older rocks are examined. Recent or
Pleistocene specimens from Vesuvius show an apparent age of 1 million
years; Miocene specimens from the Auvergne, France, of 6.3 million. The
Devonian of Norway furnishes specimens 54 million years in age, and the
Upper Cambrian of Colorado specimens of 141 million years; the Archaean
of Ceylon, of the diamond-bearing rocks of South Africa, and of certain
rocks of Ontario furnish specimens aged 286, 321 and 715 million years,
respectively.”
The following table gives the mean of the results of Professors
Boltwood and Holmes’ careful studies, based upon the accumulation of
lead as a final product of the uranium series:
MILLIONS OF
YEARS
Carboniferous 340
Devonian 370
Pre-Carboniferous 410
Silurian or Ordovician 430
Pre-Cambrian:
Sweden 1,025
United States of America 1,310–1,435
Ceylon 1,640
These results, a total of 1,400,000,000 years, greatly transcend Lord
Rayleigh’s (Strutt’s) earlier calculations regarding the antiquity they
assign to Paleozoic and Pre-Cambrian times.
In 1918, Prof. Joseph Barrell reviewed the various methods employed
and the results obtained in the attempt to determine from geological,
chemical and physical evidences the time that has elapsed since the
beginning of the Cambrian Period (when abundant fossil invertebrates
are first met with), and reached the following time estimates for
the principal divisions of the geologic record (exclusive of the
Pre-Cambrian rocks):
Cenozoic time, 55,000,000 to 65,000,000 years long
Mesozoic time, 135,000,000 to 180,000,000 years long
Paleozoic time, 360,000,000 to 540,000,000 years long
The time thus established covers a period of from 550,000,000 to
700,000,000 years, or from ten to 15 times longer than has usually been
accepted by geologists. Pre-Cambrian time was found to have a similar
order of magnitude; but here the evidence rests largely upon the
radioactivity of the crystalline rocks formed during this vast period.
It is now universally accepted that the time required for the formation
of the Pre-Cambrian rocks was fully as long as, if not longer than,
that for the succeeding geological divisions. The Archaean deposits
have a vertical thickness, in the regions north of the Great Lakes,
estimated at about 65,000 feet, or 12 miles. Their base, as a matter
of fact, has never been reached. It is interesting to note that the
granites of Norway, Canada, Texas and East Africa have an indicated age
of 1,120,000,000 years, measured in terms of radium products. Prof.
Henry Norris Russell, of Princeton University, concludes, from his
careful investigations in radioactivity, that the age of the earth is
“a moderate multiple of 1000 million years.”
Professor Joly has computed that if there are two parts of radioactive
material for every million million parts of other matter throughout the
whole volume of the earth, and this is considerably less than he has
found on the average in the earth’s crust, then this earth, instead of
cooling off, is actually now heating up, so that in a hundred million
years the temperature of the core will have risen through 1,800 degrees
centigrade.
Dr. Millikan observes (_Science_, July 9, 1921) that this is a
temperature “which will melt almost all of our ordinary substances....
It means that a planet that seems to be dead, as this our earth seems
to be, may, a few eons hence, be a luminous body, and that it may go
through periods of expansion when it radiates enormously, and then of
contraction when it becomes like our present earth, a body which is
a heat insulator and holds in its interior the energy given off by
radioactive processes, until another period of luminosity ensues.”
Lord Rayleigh’s series of researches for the purpose of determining
the quantity of radium present in a number of representative rocks,
both igneous and sedimentary, seems to prove that the average amount of
radium in the earth’s crust is about 20 times larger than the amount
calculated by Rutherford to be necessary to retain its temperature
unaltered. Joly’s investigations revealed values in general agreement
with these, but in many cases he obtained a value several times greater
than the amount found by Lord Rayleigh. Further investigations showed
that thorium is as widely distributed as radium in the earth’s crust,
which is true also of uranium.
“Incredible as it may appear,” remarks Rutherford, “the radioactive
bodies must have been steadily radiating energy since the time of their
formation in the earth’s crust. While the activity of uranium itself
must decrease with the lapse of time, the variation is so slow that an
interval measured by millions of years would be required to show any
detectible change.”
In his 1921 address to the British Association for the Advancement of
Science, Lord Rayleigh said: “It appears certain that the radioactive
materials present in the earth are generating at least as much heat as
is now leaking out from the earth into space. If they are generating
more than this (and there is evidence to suggest that they are), the
temperature must, according to all received views, be rising.”