Key concepts Absolute Dating The problem: By the mid 19th century it was obvious that Earth was much older than years, but how old? This problem attracted the attention of capable scholars but ultimately depended on serendipitous discoveries. 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.
Strata which were deposited on top of one another without interruption. An erosional surface that marks an interval of non-deposition or removal of deposits - a break in the stratigraphic 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. Evolution stokes the fire: By the s 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.
In 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. 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.
Discovered natural radioactivity In the following years, a large number of radioactive isotopes and their daughter products became known. Pierre and Marie 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.
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. The other half will be the daughter product. After twenty years, 0.
In , 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 , Bertram Boltwood noted a specific parent-daughter relationship between an isotope of uranium, U, 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 , 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. The oldest in the Solar System are 4.
Some commonly used radiometric systems: