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. The big problem is with the last assumption. Discovered natural radioactivity In John Joly , acting on suggestion of Edmund Halley , attempted estimate based on the salinity of the ocean. Similar attempts yielded results that varied widely between 3 million and 1. 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. After twenty years, 0. Recalling the principle of included fragments that you may have learned early on - an inclusion in a metamorphic rock is similar, it must have formed before or at the same time as the mineral enclosing it. By the mid 19th century it was obvious that Earth was much older than years, but how old? With unconformities factored in, the age of the Earth would have to be much greater than 36 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. Because lead is usually found as a solid, this method was more promising. There are lots of caveats with this, but it's a viable if still indirect method. Garnet usually contains Lu and Sm which decay to Lu and Nd, respectively ; micas often contain K and Rb which decay to Ar and Sr, respectively ; and so on. Unconformities are so common that today that sequence stratigraphy - the mapping and correlation of conformable sequences - is a major field in Geology. 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. There are a few ways to go about that.
The oldest in the Solar System are 4. Ti is especially useful because it serves as a thermometer in zircon - when zircon grows in the presence of certain other minerals rutile and quartz in particular , the Ti content of zircon is a direct function of the temperature that the zircon grows at. The discovery of radioactivity: 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 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. Discovered natural radioactivity After all, if the Earth were too young for there to have been time for evolution, the evolution debate would be over. Group of conformable layers lying between unconformities. Initially, three lines of evidence were pursued: Sometimes these radioactive-element-bearing minerals will also be zoned, and you can see how these minor and trace elements evolve with time and perhaps get a sense of timing for a larger window of the metamorphic path. Finally, lots of metamorphic minerals do have radioactive elements in them and can be dated directly. Similar attempts yielded results that varied widely between 3 million and 1. 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. There are lots of caveats with this, but it's a viable if still indirect method. The big problem is with the last assumption. This problem attracted the attention of capable scholars but ultimately depended on serendipitous discoveries. The first is the most indirect. 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. 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. This invoked three assumptions: Discovered that the radioactive element radium continuously releases newly generated heat - radiogenic heat. Unconformities are so common that today that sequence stratigraphy - the mapping and correlation of conformable sequences - is a major field in Geology. Recalling the principle of included fragments that you may have learned early on - an inclusion in a metamorphic rock is similar, it must have formed before or at the same time as the mineral enclosing it. Like Rutherford's, Boltwood's attempt to apply the principle to the dating of rocks was technically flawed but a step forward. The Earth might, indeed, be much older than his calculations indicated. Because lead is usually found as a solid, this method was more promising.
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