Clocks in the Rocks

Geologic Time: The science of Geology didn't take shape until the discovery of geologic time (deep time).

Using these, it began to be possible to say what order the separate rock layers had formed in, provided they could be seen in association. More importantly, Steno was the first to grasp that rocks could be formed from fragments of preexisting rocks that had been transported as sediments and that different rocks formed at different times. Of course, Steno thought that all of this deposition had occurred during the 40 days and nights of Noah's flood.


Prologue: In the next stage of the discovery of geologic time, natural historians took up the question of whether the Earth has a long or short history.

  • Catastrophism - maintains that the rocks were deposited in a series of unusual upheavals. This was consistent with the story of the Flood and even after naturalistsbegan to speculate that Noah's Flood may have been only the last in a long sequence of catastrophic changes, remained by default the prevalent mind set. During the 18th century, the naturalists began to question catastrophism.

  • James Hutton made two significant breakthroughs:

    Proposed "Uniformitarianism" in his paper The Theory of the Earth 1795. Simply paraphrased, uniformitarianism holds that

    "The present is the key to the past."

    This meant that the processes acting on the Earth's surface today were the same ones that had been acting upon it for all of history. This was a momentous and liberating idea because it meant that geologists could reconstruct the history of the Earth by observing processes in the modern world. They didn't have to invoke catastrophes in which the normal rules don't apply. In some ways, Hutton and his intellectual successor Charles Lyell went too far.

    Nevertheless, in the 19th century, the effects were mostly liberating. The biggest revelation was simply that the Earth had to be millions, not thousands of years old. (Scroll to the bottom of this link for and example.)

    Relative Dating: Thus the notion of Geological time was born. Now the problem was how to measure it. There are now two complimentary approaches:

    During the 19th century, however, geologists could only establish the relative ages of rock units. The means to do so had already been provided by Steno and Hutton, but only provided rocks could be seen in direct association.

    Once again, the study of fossils provided the stimulus for a major innovation:

    Faunal succession: Our understanding of fossils has a varied history:

    The Geologic Time Scale: Using Steno's and Smith's principles, Geologists gradually developed, a standardized a Geologic Time scale developed. It's major features:

    Absolute dating

    Several early attempts were made to come up with absolute dates, based on things like thickness of sediments and ocean salinity. The attempt with the most influence - for good or ill - was based on .....

  • Thermodynamics: Sir William Thomson, Lord Kelvin, during the late 19th century, assumed that the Earth had originally been molten then, using averge melting point of rocks and the laws of thermodynamics, determined that the Earth would completely solidify within 20 million years. Both uniformitarians and evolutionists were uncomfortable, since their notions required an older Earth.

  • As it developed, Kelvin was leaving an important factors out of his equations - new heat is generated inside the Earth by radioactive decay (nuclear fission).
  • The discovery of radioactivity: Ironically, radioactive decay, which frustrated Kelvin's purpose, ended up providing the true key to the absolute dating of rocks.

    Radiometric dating

  • In 1905, Bertram Boltwood noted a parent-daughter relationship between 235U, a radioactive isotope, and Pb suggesting that one decayed into the other. It develops that each radioactive isotope has a characteristic decay rate, transforming from a parent radioactive substance to a daughter decay product product.--__> half life - the amount of time it takes for half of a given sample to decay. If we know a substance's half-life and can measure the proportions of parent and daughter substances, we can calculate the time at which the crystal containing the substances solidified from a melt.

  • Ernest Rutherford calculated decay rate from U to Pb. This enabled the first radiometric dating.

  • Arthur Holmes: First used radioactive decay as a means of dating rocks (1911).

    Result is that oldest known Earth rocks are aprox 3.8 g.a. Oldest in Solar System 4.56 g.a.

  • Some commonly used substances:

    Radioactive isotopeDaughter substanceHalf LifeApplicable range
    Uranium 238Lead 2064.5 g.a.10 m.y. to 4.6 g.a.
    Potassium 40Argon 401.3 g.a.100,000 to 4.6 g.a.
    Carbon 14*Nitrogen 145,730to 70,000

    * used in plant material. Note that the effective range of these dating systems is limited by the degree of error in measurement.

  • Which rocks are useful? The aim of radiometric dating is to determine how long ago the minerals in the rock crystallized from magma. Thus, sedimentary and metamorphic rocks can't be radiometrically dated. Note: relatively young plant material can be dated with 14C.

    Although only igneous rocks can be radiometrically dated, ages of other rock types can be constrained by the ages of igneous rocks with which they are interbedded.


    The Earth generates a magnetic field that encompasses the entire planet. In the last fifty years, a new dating method has emerged that exploits two aspects of rocks' interactions with the Earth's magnetic field. It is, in essence a form of relative dating.

    Result: We can hang a numerical age on these rocks if their paleomagnetic "fingerprint" can be matched with that of a sequence of igneous rocks that can be radiometrically dated.

  • Magnetostratigraphy, the study of paleomagnetic polarity of rockshas led to the development of a paleomagnetic time scale that is correlated with the regular time scale.

    The utility of paleomagnetism:

    Currently, the paleomagnetic record has been worked out as far back as the Jurassic period.

    Fission Track Dating

    A clastic sedimentary rock is made of grains of preexisting sediment that are all older than the rock they make up. What if you want to know the age of the grains (and therefore the age of the preexisting rock they came from?) The microscopic crystal lattice of some minerals (notably apatite, zircon and monazite) is damaged by the particles that fly off when a radioactive nucleus decays. This damage can be seen as fission tracks. The density of fission tracks in a mineral grain are a good indicator of its age, thus fission track dating has emerged as a means of determining the age of individual crystal grains. The winner so far, a zircon grain from the Jack Hills of Western Australia dated to 4.4 billion years. (As compared to 3.8 billion for the oldest whole rocks.)