Early attempts: Initially, three lines of evidence were pursued:
- Hutton attempted to estimate age based on the application of observed rates of sedimentation to the known thickness of the sedimentary rock column, achieving an approximation of 36 million years. This invoked three assumptions:
- Constant rates of sedimentation over time
- Thickness of newly deposited sediments similar to that of resulting sedimentary rocks
- There are no gaps or missing intervals in the rock record.
In fact, each of these is a source of concern. The big problem is with the last assumption. The rock record preserves erosional surfaces that record intervals in which not only is deposition of sediment not occurring, but sediment that was already there (who knows how much) was removed.
- Conformable strata: Strata which were deposited on top of one another without interruption.
- Unconformity: An erosional surface that marks an interval of non-deposition or removal of deposits - a break in the stratigraphic sequence.
- Sequence: Group of conformable layers lying between unconformities.
Unconformities are so common that today that sequence stratigraphy - the mapping and correlation of conformable sequences - is a major field in Geology. With unconformities factored in, the age of the Earth would have to be much greater than 36 million years. Similar attempts yielded results that varied widely between 3 million and 1.5 billion years.
- Evolution stokes the fire: By the 1860s century, the controversy surrounding evolution prompted new attention. After all, if the Earth were too young for there to have been time for evolution, the evolution debate would be over.
- Ocean salinity: In 1889 John Joly, acting on suggestion of Edmund Halley, attempted estimate based on the salinity of the ocean. He calculated the amount of salt being transported into the oceans by rivers and compared this to the salinity of sea water, obtaining an age of 90 million years.
- 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 a much older Earth, but the quantitative rigor of Thomson's approach made his the most prestigeous estimate of his day.
- As it developed, both Joly and Tomson were leaving vital (but unknown) information out of their equations.
- Joly missed that salt is removed from the oceans by various processes.
- Kelvin could not have know that new heat is generated inside the Earth by radioactive decay (nuclear fission), because the process had not been discovered.
The discovery of radioactivity: Ironically, radioactive decay, which frustrated Kelvin's purpose, ended up providing the true key to the absolute dating of rocks.
- Antoine Becquerel (1852-1908): Discovered natural radioactivity (1896). In the following years, a large number of radioactive isotopes and their daughter products became known.
- Pierre (1859-1906) and Marie (1867-1934) Curie: Discovered that the radioactive element radium continuously releases newly generated heat - radiogenic heat. With this discovery, it became clear that the decay of radioactive substances provided a continuous source of new heat that Thomson hadn't accounted for. The Earth might, indeed, be much older than his calculations indicated. But how old?
- At the beginning of the 20th century, Ernest Rutherford and Frederick Soddy developed the concept of the half-life - For any radioactive substance, there is a specific period of time in which half of a sample will decay to a daughter substance. E.G., if we have a newly created 1 kg. sample of a substance whose half-life is 10 years, then ten years from its creation, half of the radioactive material will remain in the sample. The other half will be the daughter product. After twenty years, 0.25 kg. will remain (with the rest being daughter product), and after thirty years, 0.125 kg. of the original radioactive substance will remain in the sample.
- In 1904, Rutherford made the first attempt to use this principle to estimate the age of a rock. His analysis was technically problematic because of his choice of a gas, helium as a radioactive product (gasses have a way of migrating out of rocks), but it was a start.
- In 1905, Bertram Boltwood noted a specific parent-daughter relationship between an isotope of uranium, 235U, a radioactive isotope, and lead (Pb) suggesting that one decayed into the other - the uranium-lead system. Because lead is usually found as a solid, this method was more promising. Like Rutherford's, Boltwood's attempt to apply the principle to the dating of rocks was technically flawed but a step forward.
- Beginning in 1911, Arthur Holmes began a long career of applying the concept of radiometric dating to rocks, and is given credit for ironing out the technical issues that hampered earlier attempts.
After a century of applying the method we now know that thet oldest known Earth rocks are aprox 4.2 billion years old (abbreviated "ga"). The oldest in the Solar System are 4.56 ga.
- Some commonly used radiometric systems:
Radioactive isotope Daughter substance Half Life Applicable range Uranium 238 Lead 206 4.5 Ga 10 m.y. to 4.6 Ga Potassium 40 Argon 40 1.3 Ga 100,000 to 4.6 Ga Carbon 14* Nitrogen 14 5,730 to 70,000
* used in plant material only, not rocks. Note that the effective range of these dating systems is limited by the degree of error in measurement.
- Which rocks are useful for radiometric dating? When you radiometrically date a mineral grain you are determining when it crystallized. Thus, you would like to use rocks whose crystals are roughly the same age.
- The easiest are igneous rocks in which all crystals are roughly the same age, having solidified at about the same time.
- The age of new minerals crystallizing in metamorphic rocks can also be determined by radiometric dating. The problem is that metamorphism - the pressure-cooking of rocks - can occur over long intervals. Thus, different crystal grains can yield different ages.
- With sedimentary rocks, one would end up dating the individual grains of sediment comprising the rock, not the rock as a whole. These grains could have radically different ages.
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.
- Paleomagnetism: Some magnetic minerals, such as magnetite occur naturally in igneous rocks. When their grains form, they align themselves with the Earth's magnetic field. The Earth's magnetic field changes quickly (i.e. on the scale of a human life.) Nevertheless, because of the orientation of their magnetic minerals, their intrinsic magnetic field records the orientation of the Earth's field as it existed when they formed. Such ancient magnetic fields are called remnant or paleomagnetism. ("Paleomag" in geological slang.)
- Magnetic reversals: The Earth's magnetic field has a north and south pole. For unknown reasons, at intervals of (very) roughly 500,000 years, the north and south poles trade places.
The result is that the paleomagnetic polarity of igneous rocks is either:
- Normal: Magnetic north coincides roughly with geographic north.
- Reversed: Magnetic north coincides roughly with geographic south.
What kinds of rocks retain paleomagnetism:
- Igneous, for reasons noted.
- Some sedimentary rocks retain paleomagentism when they contain minerals derived form earlier igneous rocks. Three requirements need to be met:
- Sediments consist of very small grains that settle slowly from water
- Sediments include magnetic minerals
- Sediments were deposited in very quiet body of water, like a lake.
The fact that sediments can record paleomagnetism is very useful. Remember, we have no means of directly measuring the radiometric age of sediments that aren't preserved in association with igneous rocks. We can, however, hang a numerical age on them if their paleomagnetic "fingerprint" can be matched with that of a sequence of igneous rocks that can be radiometrically dated.
- By studying paleomagnetic polarity of rocks of different ages, geologists have developed a paleomagnetic time scale that is correlated with the regular time scale. The scale consists of chrons (a.k.a. "epochs" - not to be confused with the epochs of the Cenozoic Era) periods between reversals. The study of the history of paleomagnetic reversals is called magnetostratigraphy.
- Radiometric dates are always subject to margins of error, whereas a rock's paleomagnetic polarity is absolute. Knowing the paleomagnetic polarity of a sample can, therefore, give an independent means of constraining its age.
- Most rocks that preserve paleomagnetism (igneous) can also be radiometrically dated. Because some sedimentary rocks can also retain paleomagnetism, then by knowing their polarity, we can assign them more reliable absolute dates by correlating them with igneous rocks of the same paleomagnetic chron.
- Since paleomagnetic chrons are not of the same duration, paleomagnetic time charts resemble sections of tree rings in which the differing thicknesses of adjacent rings provide a "fingerprint" of a time period. Similarly, the differing duration of adjacent chrons gives each period of Earth history a distinct paleomagnetic fingerprint.
Key concepts and vocabulary:
- Conformable strata
- Sequence stratigraphy
- John Joly's salinity approach to dating
- William Tomson (aka Lord Kelvin)
- Kelvin's thermodynamic approach to dating
- Antoine Becquerel
- Pierre and Marie Curie
- Radiogenic heat
- Ernest Rutherford
- Bertram Boltwood
- Uranium-lead system
- Arthur Holmes
- Oldest Earth rocks - 4.2 ga
- Potassium-argon system
- Igneous rocks best for radiometric dating
- Paleomagnetism (aka Remnant magnetism)
- Normal and reversed paleomagnetism
- Paleomagnetic chrons