Part 4

Time-scale in millions of years PERIODS Since beginning of period Duration of period PLEISTOCENE _ca_ 1 .... _ca_ 1 .... PLIOCENE 10 .... 11 .... MIOCENE 14 .... 25 .... OLIGOCENE 15 .... 40 .... EOCENE 20 .... 60 .... PALEOCENE 10 .... 70 ± 2 .... Upper } CRETACEOUS 65 Lower } .... 135 ± 5 .... Upper } JURASSIC 45 Mid. & Lower } .... 180± 5 .... TRIASSIC 45 .... 225 ± 5 .... PERMIAN 45 .... 270 ± 5 .... Upper } CARBONIFEROUS 80 Lower } .... 350 ± 10 .... Upper } DEVONIAN 50 Lower } .... 400 ± 10 .... SILURIAN 40 .... 440 ± 10 .... ORDOVICIAN 60 .... 500 ± 15 .... CAMBRIAN 100 .... 600 ± 20 ....

Using measurements made in many laboratories, and interpolating between them by using the relative thicknesses of sediments, the great British geologist, Arthur Holmes, established the time scale that is in general use today. His original scale is shown on the preceding page. Small changes can be expected to be made in this scale from time to time, but major alterations are not likely, except perhaps in the Cambrian Epoch where the present data are unreliable because they are not complete. (A scale showing the epochs or periods as often given now is on page 4.)

Precambrian Stratigraphy

So far we have talked only about rocks that are of the Cambrian Epoch or younger—rocks that may contain fossils. Yet there are vast areas (most of Canada, for example) that are covered with rocks older than the Cambrian formation. Some Precambrian fossils have been found, but they are so rare that they are useless for dating the strata containing them. Long-range correlation of Precambrian rocks must rely on nuclear measurements. Therefore it has been only in the last dozen or so years that some order could be established for the Precambrian rock sequences. The elaborate Precambrian stratigraphies (arrangements of strata in sequence) proposed in the past, most of them based on superficial similarities of the rocks in one place to those in another place, now have been drastically altered and in some cases completely overturned by nuclear measurements. We are still far from understanding the sequence of all the events in that vast span of time we call the Precambrian. Many thousands of nuclear age determinations will have to be made to lighten the dark corners of our ignorance.

AND WHERE DO WE GO FROM HERE?

Perhaps we must first realize that we really haven’t come very far yet. Granted that the age of rocks in many parts of the world is now suddenly known—and that this was a total mystery some dozen years ago. Granted that enormous strides forward have been made. It’s only a beginning.

Vast areas of the world are still geologically unexplored. The geologic time scale is still fragmentary and crude. Thousands of important geologic questions remain to be defined, explored, and answered by nuclear age determination. And—as is inevitable in science—many of them will lead to new questions. It is apparent that dating techniques have barely begun to be used and understood by geologists.

But apart from geologic work, what else is in store? It is difficult to predict, but probably the most important advance in the next decade or two will come when we obtain samples of rock from the moon. Will there be young rocks there or will they all be 4550 million years old? Or will they perhaps be some other age? The chemical composition and nuclear ages of the first moon samples will probably be the most important information we can hope to obtain from them. These results from the moon will contribute enormously to our understanding of the processes that formed the earth, made the continents, and determined the major features of our world.

We have a long way to go.

GLOSSARY

AEON One billion (10⁹) years.

ALPHA DECAY Radioactive decay with emission of an alpha particle.

ALPHA PARTICLE Essentially the nucleus of helium, composed of two neutrons and two protons with double positive charge.

ANTICOINCIDENCE RING A ring of counters connected to exclude outside radiation.

BACKGROUND COUNT The number of impulses per unit time registered on a counting instrument when no sample is present.

BETA DECAY Radioactive decay with emission of a beta particle.

BETA PARTICLE An electron emitted by a nucleus.

BRACKETED INTRUSIVE Igneous rock extending into sedimentary rocks that are datable by their fossils.

CLOSED SYSTEM A system in which the parent material radioactively decays into its daughter products and nothing is added or removed.

COMMON (strontium, lead, etc.) The ordinary element present in nature at any one time as distinguished from that produced by radioactive decay.

CONCORDIA ANALYSIS A mathematical technique to determine graphically the age of a material containing radiogenic lead by comparing its uranium-to-lead ratio with the similar ratio in a closed uranium-lead system.

CONTACT METAMORPHISM A metamorphism genetically related to the intrusion of molten masses of rock and taking place at or near the contact.

COSMIC RAYS High-energy particles moving in our galaxy.

CRYSTAL A periodic or regularly repeating arrangement of atoms, formed from a single element or compound.

DAUGHTER A nuclide formed from the radioactive decay of another nuclide.

DECAY CONSTANT The number of atoms decaying per atom per unit of time (0.693/half-life).

ELECTRON CAPTURE A nuclear process in which the nucleus of an atom captures an electron from one of the inner shells.

ELECTRONS Elementary particles with a unit negative electrical charge and a mass 1/1837 that of the proton, or 9.12 × 10⁻²⁷ gram. Electrons surround the atom’s positively charged nucleus and determine the atom’s chemical properties.

GAMMA RAYS Electromagnetic radiation from an atomic nucleus.

GEIGER COUNTERS Instruments that count pulses produced by radioactivity, consisting of a counting tube with a central wire anode, usually filled with a mixture of argon and organic vapor.

HALF-LIFE The time it takes for half the atoms in a radioactive substance to decay.

ION An atom or molecule that has lost or gained one or more electrons and is thus electrically charged.

ISOTOPE DILUTION An analytical technique involving addition of a known amount of an isotopic mixture of abnormal composition to the unknown amount of an element of normal or known isotopic composition.

ISOTOPES Nuclides of the same atomic number but different atomic weight. Isotopes of a given element have an identical number of protons but different numbers of neutrons in their nuclei.

LAW OF SUPERPOSITION Statement that overlying strata must be younger than underlying strata if there has been no inversion.

MASS SPECTROMETER An instrument for separation and measurement of isotopes by their mass.

NET COUNTING RATE Sample counting rate minus background counting rate.

NEUTRONS Elementary particles in the nucleus having no electric charge and the mass of one atomic mass unit.

NUCLIDE A species of atom characterized by the constitution of its nucleus.

ORIGINAL (strontium, lead, etc.) Common strontium, lead, etc., taken into a system at the time of its formation.

PALEONTOLOGY The study of fossil remains.

PARENT The radioactive element from which a daughter nuclide is produced by radioactive decay.

PLUMBOLOGY The study of the uranium and thorium-lead decay systems. The name is derived from the Latin name for lead, _plumbum_.

PRIMORDIAL Present at the time of the formation of the earth.

PROPORTIONAL COUNTER An instrument for detecting radiation by producing pulses of electrical charge that are proportional to the energy of the radiation being measured. The design permits use of radiation of a desired energy level (within limits), and discrimination against other radiation, especially background radiation.

PROTONS Elementary particles with a single positive electrical charge and a mass approximately 1837 times that of the electron. The atomic number of an atom is equal to the number of protons in its nucleus.

RADIOACTIVE DECAY The change of one nuclide to another by the emission of charged particles from the nucleus of its atom.

RADIOACTIVITY The property of some nuclides to decay by themselves into others.

RADIOGENIC Formed as the result of radioactive decay.

RARE EARTH Any of the elements from atomic number 57 (lanthanum) to 71 (lutetium).

SAMPLE COUNTER An instrument into which a sample of material can be placed to have its radiation measured.

SECULAR EQUILIBRIUM The production of a radioactive substance at a rate equal to its decay.

SPECIFIC ACTIVITY The number of atoms decaying per unit time per unit weight of the total amount being tested.

SPIKE A known amount of an element of unusual isotopic composition used in isotope-dilution analysis.

STATISTICAL ERROR The error associated with nuclear measurements and arising from the random distribution of nuclear events.

STRATA Plural of stratum. A sheet or mass of sedimentary rock (formed by deposits of sediments, as from ancient seas) of one kind, usually in layers between beds or layers of other kinds.

APPENDIX Radioactive Decay

When a radioactive nucleus disintegrates or decays, the resultant remaining nucleus may still be radioactive, and sooner or later it also will disintegrate and become still another kind of atom. This process continues through a series of distinct steps until a stable atom—one that is not radioactive—is formed. All natural radioactivity in the heavy elements proceeds by such a series of steps, and the series finally ends with a stable form of lead as its end product. In other words, any naturally radioactive heavy element eventually becomes nonradioactive lead.

The nucleus of every atom (except hydrogen) contains one or more neutrons and one or more protons. The instability of the nuclei of the heavy atoms is related to the ratio of the number of neutrons to the number of protons in the nuclei. Radioactive decay is, in fact, a way of adjusting these ratios. The adjustment can occur in various ways. The most common is the emission of alpha particles or beta particles.

An alpha particle is identical with the nucleus of a helium atom and has two neutrons and two protons bundled together. Loss of an alpha particle from a nucleus lowers the mass number (the total of protons and neutrons) of the parent nucleus by four and the atomic number (the number of protons) by two; the number of neutrons also is reduced by two.

A beta particle is an electron and has a negative electric charge. When a beta particle is emitted from a nucleus, the nucleus is changed so that it has one more proton (which has a positive charge) and one less neutron (which has no charge); in effect, a neutron has changed into a proton as the nucleus lost a negative charge. Beta decay occurs in nuclei with a greater proportion of neutrons than is normal for the number of protons. Since beta emission increases the proportion of protons, the process raises the atomic number of the parent nucleus by one and leaves the mass number the same.

Gamma rays are a form of electromagnetic radiation. They are emitted when a nucleus shifts from one energy state to a lower energy state—the energy difference emerging as the gamma radiation. Gamma emission often accompanies alpha or beta emission, but the production of gamma rays does not itself alter the atomic number nor the mass number of the parent.

Nuclei also can decay by emission of a positron, which is a positively charged electron. When this occurs, the new nucleus has one more neutron and one less proton than its parent; in effect a proton has become a neutron as the nucleus loses a positive charge. Positrons usually are emitted by nuclei that have a greater proportion of protons than is normal for the number of neutrons.

Another process—internal electron conversion—sometimes occurs in connection with gamma-ray emission, usually in heavy elements when the gamma-ray energy is low. Instead of being emitted directly, the gamma ray strikes an orbital electron, knocking the electron out of the atom; the gamma ray then disappears. Another electron jumps into the “hole” in the orbit from which the first electron was emitted, and this jump—from a higher to a lower energy level—results in the emission of an X ray (which is similar to a gamma ray, but originates in the electron orbit region of the atom, not in the nucleus).

Finally, a nucleus may be altered by electron capture. In a nucleus with a low ratio of neutrons to protons, the nucleus captures one of its own orbital electrons. This immediately combines with a proton to form a new neutron and emit a neutrino (a high-energy particle with neither mass nor charge). The process increases the neutron-to-proton ratio of the nucleus; the daughter has the same mass number as the parent, but has an atomic number one less than the parent.

There are three series by which naturally radioactive nuclei decay to stable ones: The Uranium Series, the Thorium Series, and the Actinium Series. Man-made radioactive nuclei decay similarly, with bismuth as the end product, via the Neptunium Series. These can be illustrated in tabular form and diagrammatically. The Actinium Series (Uranium-235 Series), for example, proceeds like this:

THE URANIUM-235 SERIES Element Symbol Radiation Emitted Half-life Uranium ²³⁵U α 7.13 × 10⁸ years Thorium ²³¹Th β 25.6 hours Protactinium ²³¹Pa α 3.25 × 10⁴ years Actinium[16] ²²⁷Ac β (98.8%) and α (1.2%) 21.2 years Thorium ²²⁷Th α 18.17 days Francium ²²³Fr β 22 minutes Radium ²²³Ra α 11.7 days Radon ²¹⁹Rn α 4.0 seconds Polonium[16] ²¹⁵Po α (~100%) and β (~5 × 1.83 × 10⁻³ second 10⁻⁴%) Lead ²¹¹Pb β 36.1 minutes Astatine ²¹⁵At α ~10⁻⁴ second Bismuth[16] ²¹¹Bi α (99.7%) and β (0.3%) 2.15 minutes Polonium ²¹¹Po α 0.25 second Thallium ²⁰⁷Tl β 4.78 minutes Lead ²⁰⁷Pb — Stable

The Uranium (Uranium-238) Series proceeds like this:

THE URANIUM-238 SERIES Element Symbol Radiation Half-life Uranium ²³⁸U α 4.51 × 10⁹ years Thorium ²³⁴Th β 24.1 days Protactinium[17] ²³⁴Pa β 1.18 minutes Uranium ²³⁴U α 2.48 × 10⁵ years Thorium ²³⁰Th α 7.6 × 10⁴ years Radium ²²⁶Ra α 1.62 × 10³ years Radon ²²²Rn α 3.82 days Polonium[18] ²¹⁸Po α (99.98%) and β 3.05 minutes (0.02%) Lead ²¹⁴Pb β 26.8 minutes Astatine ²¹⁸At α 1.3 seconds Bismuth[18] ²¹⁴Bi β (99.96%) and α 19.7 minutes (0.04%) Polonium ²¹⁴Po α 1.6 × 10⁻⁴ second Thallium ²¹⁰Tl β 1.32 minutes Lead ²¹⁰Pb β 22 years Bismuth[18] ²¹⁰Bi β (~100%) and α (~2 × 5.0 days 10⁻⁴%) Polonium ²¹⁰Po α 138.4 days Thallium ²⁰⁶Tl β 4.30 minutes Lead ²⁰⁶Pb ... Stable

SUGGESTED REFERENCES

Books

_How Old Is the Earth?_, Patrick M. Hurley, Doubleday & Company, Inc., Garden City, New York, 1959, 160 pp., $1.25.

_Radiocarbon Dating_ (second edition), Willard F. Libby, University of Chicago Press, Chicago, Illinois, 1955, 175 pp., $5.00 (hardback); $1.95 (paperback).

_The Birth and Death of the Sun_, George Gamow, The Viking Press, New York, 1949, 238 pp., $4.75 (out of print but available through libraries), $0.60 (paperback) from the New American Library of World Literature, Inc., New York.

_Inside the Nucleus_, Irving Adler, The John Day Company, Inc., New York, 1963, 191 pp., $4.95 (hardback); $0.60 (paperback) from the New American Library of World Literature, Inc., New York.

_Ages of Rocks, Planets, and Stars_, Henry Faul, McGraw-Hill Book Company, New York, 1966, 109 pp., $2.45.

_Principles of Geochemistry_ (second edition), Brian Mason, John Wiley & Sons, Inc., New York, 1958, 310 pp., $7.50. (New edition due in 1967.)

_Potassium Argon Dating_, J. H. Zahringer and O. A. Schaeffer, Springer-Verlag New York, Inc., New York, 1966, 250 pp., $12.50.

Articles

A Clock for the Ages: Potassium Argon, Garniss H. Curtis, _National Geographic Magazine_, 120: 590 (October 1961).

Exploring 1,750,000 Years into Man’s Past, L. S. B. Leakey, _National Geographic Magazine_, 120: 564 (October 1961).

Five-Billion-Year Clock, Patrick M. Hurley, _Saturday Evening Post_, 234: 26 (March 18, 1961).

Geologic Time Scale, J. Laurence Kulp, _Science_, 133: 1105 (April 14, 1961).

Geology, Reginald A. Daly, _Scientific American_, 183: 36 (September 1950).

Tracks of Charged Particles in Solids, R. L. Fleischer, P. B. Price, and R. M. Walker, _Science_, 149: 383 (July 23, 1965).

How Old Is It?, Lyman J. Briggs and Kenneth F. Weaver, _National Geographic Magazine_, 114: 234 (August 1958).

Moving Picture of the Last Ice Age, Richard Foster Flint, _Natural History_, 66: 188 (April 1957).

Modern Methods for Measurement of Geologic Time, E. J. Zeller, _Mineral Information Service_, 18: 9 (January 1965). Single copies are $0.10 from Division of Mines and Geology, Ferry-Building, San Francisco, California 94111.

Fluted Projectile Points: Their Age and Dispersion, C. Vance Haynes, Jr., _Science_, 145: 1408 (September 25, 1964).

Unraveling the Age of Earth and Man, E. L. Simons, _Natural History_, 76: 52 (February 1967).

Radiocarbon Dating and Archeology in North America, F. Johnson, _Science_, 155: 165 (January 13, 1967).

Fission-track Dating of Bed I, Olduvai Gorge, R. L. Fleischer and others, _Science_, 146: 72 (April 2, 1965).

Lead Isotopes and the Age of the Earth, G. R. Tilton and R. H. Steiger, _Science_, 150: 1805 (December 31, 1965).

Strontium-Rubidium Age of an Iron Meteorite, G. J. Wasserburg and others, _Science_, 150: 1814 (December 31, 1965).

U. S. Geological Survey scientists prepare acetylene gas, made from the carbon-14 in a geological specimen, in a vacuum line. This gas will be fed into a proportional counter to determine the age of the specimen by its ¹⁴C count. (See “Carbon-14 Counting” beginning on page 12.)

Geophysicist Henry Faul, an authority on nuclear dating, is professor of Geophysics and chairman of the Department of Geology at the University of Pennsylvania, Philadelphia. He holds a doctoral degree from the Massachusetts Institute of Technology, and has taught at the Universities of Strasbourg and Bern in Europe. Dr. Faul was a geophysicist with the Manhattan Project during World War II, and was formerly chief of the Radiation Laboratory, U. S. Geological Survey, Denver, Colorado. For many years he was engaged in geological age determination work with the Geological Survey and the Carnegie Institution of Washington in Washington, D. C.

PHOTO CREDITS

Frontispiece courtesy Anthropology Department, Southern Methodist University (SMU)

Cover photo courtesy U. S. Geological Survey (USGS)

Page 11 USGS 12 USGS 14, 16, 17 Willard F. Libby 18 University of Arizona 21 Massachusetts Institute of Technology 22 Graduate Research Center of the Southwest 23 Graduate Research Center of the Southwest 25 Robert L. Fleischer 26 Perkin-Elmer Corporation 30 University of Texas 32 Graduate Research Center of the Southwest 36 Massachusetts Institute of Technology 40 Des Bartlett, Armand Denis Productions 43 Anthropology Department, SMU 44 USGS 47 USGS 59 Author’s photo courtesy Graduate Research Center of the Southwest

Footnotes

[1]Words appearing in SMALL CAPITAL LETTERS are defined in the Glossary beginning on page 49.

[2]The process of natural radioactive decay is described in the Appendix beginning on page 52.

[3]The U. S. War Department program during World War II that developed the first nuclear weapons.

[4]Drawn to scale, the whole age of man is represented by less than the width of the line.

[5]For more information on the structure of atoms, see _Our Atomic World_, a companion booklet in this series.

[6]From the _Chart of the Nuclides_, prepared by David T. Goldman, Knolls Atomic Power Laboratory, August 1964.

[7]This decay process proceeds in a series of steps, during which 6 alpha particles and 4 beta particles are emitted. (See Appendix.)

[8]Named after their creator, John Napier, a Scottish mathematician (1550-1617), who also invented the decimal point.

[9]It is difficult to determine the half-life of ¹⁴C exactly. In the early days of ¹⁴C dating, in order not to delay continued work, an arbitrary value of 5568 years was chosen and this value is still used in calculations.

[10]This means that uranium decays through successive steps in which the entire series emits eight alpha particles. (See Appendix.)

[11]Remember, this enormous period of time is a measure of the _rate_ of spontaneous fission, _not_ of the age of ²³⁸U.

[12]The rhenium-osmium scheme is shown below the dotted line because the method is still in an early experimental stage and its general utility is not yet established.