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.
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.Radioactive Dating
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.
Radiometric Dating: Methods, Uses & the Significance of Half-Life
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: 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. So, geologists prefer to work with igneous rocks. Useful to archaeologists, maybe, but system is not typically used on rocks at all. Thus, sedimentary and metamorphic rocks can't be radiometrically dated. 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. Magnetostratigraphy 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. 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. They found that after years, half the 14 C in the original sample will have decayed and after another years, half of that remaining material will have decayed, and so on. This became known as the Libby half-life. After 10 half-lives, there is a very small amount of radioactive carbon present in a sample. At about 50 to 60 years, the limit of the technique is reached beyond this time, other radiometric techniques must be used for dating.
By measuring the 14 C concentration or residual radioactivity of a sample whose age is not known, it is possible to obtain the number of decay events per gram of Carbon. By comparing this with modern levels of activity wood corrected for decay to AD and using the measured half-life it becomes possible to calculate a date for the death of the sample. As a result of atomic bomb usage, 14 C was added to the atmosphere artificially.
This affects the 14 C ages of objects younger than Any material which is composed of carbon may be dated. Herein lies the true advantage of the radiocarbon method. Potassium-Argon K-Ar dating is the most widely applied technique of radiometric dating.
Potassium is a component in many common minerals and can be used to determine the ages of igneous and metamorphic rocks. The Potassium-Argon dating method is the measurement of the accumulation of Argon in a mineral. It is based on the occurrence of a small fixed amount of the radioisotope 40 K in natural potassium that decays to the stable Argon isotope 40 Ar with a half-life of about 1, million years.
In contrast to a method such as Radiocarbon dating, which measures the disappearance of a substance, K-Ar dating measures the accumulation of Argon in a substance from the decomposition of potassium.
Argon, being an inert gas, usually does not leech out of a mineral and is easy to measure in small samples. This method dates the formation or time of crystallisation of the mineral that is being dated; it does not tell when the elements themselves were formed. It is best used with rocks that contain minerals that crystallised over a very short period, possibly at the same time the rock was formed.
This method should also be applied only to minerals that remained in a closed system with no loss or gain of the parent or daughter isotope. Uranium-Lead U-Pb dating is the most reliable method for dating Quaternary sedimentary carbonate and silica, and fossils particulary outside the range of radiocarbon.
Quaternary geology provides a record of climate change and geologically recent changes in environment. U-Pb geochronology of zircon , baddelyite , and monazite is used for determining the age of emplacement of igneous rocks of all compositions, ranging in age from Tertiary to Early Archean. U-Pb ages of metamorphic minerals, such as zircon or monazite are used to date thermal events, including terrestrial meteoritic impacts.
U-Pb ages of zircon in sediments are used to determine the provenance of the sediments. The Fission track analysis is based on radiation damage tracks due to the spontaneous fission of U.
What type of rock is used for radiometric dating
Fission-tracks are preserved in minerals that contain small amounts of uranium, such as apatite and zircon. Fission-track analysis is useful in determining the thermal history of a sample or region. By determining the number of tracks present on a polished surface of a grain and the amount of uranium present in the grain, it is possible to calculate how long it took to produce the number of tracks preserved.
As long as the mineral has remained cool, near the earth surface, the tracks will accumulate. If the rock containing these minerals is heated, the tracks will begin to disappear. The tracks will then begin to accumulate when the rock begins to cool. If a rock cools quickly as in the case of a volcanic rock or a shallow igneous intrusion, the fission-track ages will date this initial cooling.