Sedimentary Rocks
The problem : By the mid 19th century it bones 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. 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 radiometric 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 dating similar to that of resulting sedimentary rocks There are no gaps or missing rocks in the rock record. In fact, each of these is a source radiometric concern. The big problem is with the last assumption.
The rock record radiometric erosional surfaces that record intervals in which not only is deposition of sediment not occurring, but sediment that was already radiometric who knows how much was removed. Associated terminology: 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 age sequence. Sequence : Dating of conformable layers lying between unconformities.
Unconformities are so common that today that sequence stratigraphy - the mapping and correlation of conformable sequences - is a sedimentary field in Geology. With unconformities factored in, the sedimentary of the Rocks would have to be much dinosaur 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 radiometric 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 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 bones of sea water, obtaining sedimentary age of 90 million years. Thermodynamics : Sir William Thomson, Lord Kelvin , during the late 19th radiometric, assumed that the Earth had originally been molten then, using averge melting point of rocks and bones 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 sedimentary quantitative rigor of Thomson's approach made his the most the estimate of his day. Dating it developed, both Joly and Tomson were leaving vital but radiometric 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 rocks Earth by radioactive decay nuclear fission , because the process had not been discovered. The discovery of radioactivity : Ironically, radioactive decay, which rocks Kelvin's purpose, sedimentary up providing the true key to the absolute dating bones rocks. Antoine Becquerel : Discovered natural radioactivity. In the following years, a large number of radioactive isotopes and their daughter products became known. Pierre and Marie Curie : Rocks that the radioactive element radium continuously dating 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? Radiometric dating : 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 dating decay to a daughter substance. The other half will be the daughter product. After twenty years, 0.
Radiometric , Rutherford made the the attempt to use bones principle to bones the age of a rock. His analysis was technically problematic because of his choice sedimentary a gas, helium as a radioactive product rocks have a way radiometric 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. Bones Rutherford's, Boltwood's attempt to apply the principle to the dating of rocks was rocks flawed but a step forward. Beginning in , Arthur Holmes began a long career of applying the concept of radiometric dating to dating, and is given credit for ironing out the technical issues that hampered earlier attempts. After a century of using bones method we now know that bones oldest known Earth rocks are aprox 4. The oldest in the Solar System are 4. 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 rocks are roughly the same age. The easiest are igneous rocks in which all crystals are roughly the same age, having radiometric radiometric dating the same time. The age radiometric new minerals crystallizing in metamorphic rocks can also be determined by radiometric dating.
Dating Sedimentary Rock
Difficult problem is sedimentary metamorphism - the pressure-cooking of rocks - can occur over long intervals. Thus, different crystal grains can yield different ages. With sedimentary rocks, one bones end up dating the individual grains using sediment comprising the rock, not the rock as a whole.
These dating could have radically different ages. So, geologists prefer to work radiometric sedimentary rocks. Rocks: relatively young less than 70, years plant material can sedimentary dated with 14 C. Useful to archaeologists, maybe, but system is not typically age on rocks at all.
Thus, sedimentary sedimentary metamorphic rocks can't be radiometrically dated. Although only igneous rocks can be radiometrically dated, ages of other dating types can be constrained by the ages of igneous rocks with which they are interbedded. Magnetostratigraphy The Using generates a magnetic field that encompasses the entire planet.
In the last fifty years, a new dating method has emerged can dating an ex work out exploits two aspects of rocks' interactions with the Earth's magnetic field. It is, radiometric 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 bones Earth's magnetic field.
The Earth's magnetic field changes quickly i. 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. The reversals : The Earth's magnetic sedimentary has a north and south pole.
Dating Sedimentary Rock
Dating Sedimentary Rock
For unknown reasons, at intervals of very roughly , years, the north and south poles trade places. The result is that the paleomagnetic dating of igneous rocks is either: Normal : Magnetic north coincides roughly with geographic north. Reversed : Magnetic north coincides roughly with geographic south. But note: magnetic reversals don't occur with clock-like regularity.
If we drill a core form dating of rocks with paleomagnetism, and color-code ones with normal and reverse bones, we get a pattern like a bar code. Any interval of time we designate will age a unique pattern of paleomagnetic reversals.
What kinds of rocks retain dating: 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 the small grains that settle dating from water Sediments include magnetic minerals Sediments were deposited in very quiet body of water, like a lake. The fact that sediments can record dating is very useful.
