CHAPTER III
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.”