Relative Dating vs. Absolute Dating: What’s the Difference? – Difference Wiki
To find their age, two major geological dating methods are used. These are called relative and absolute dating techniques. Absolute dating. Using relative and radiometric dating methods, geologists are able to answer the question: how old is this fossil?. Relative dating by definition does not produce actual dates, but it does allow us to put an order on the rocks, and so if absolute dating is to rocks as deduced from stratigraphic methods — other.
Let us therefore turn to the evidence. Sea-floor spreading[ edit ] You will recall from our discussion of sea floor spreading that the sea floor spreads out from mid-ocean riftsand so ought to be younger nearer the rifts and progressively older further away from them.
This means that if we didn't have any other way of doing absolute datingwe would as a first approximation take the age of basalt on a spreading sea floor to be the distance from the rift divided by the rate of spreading. Now if we estimate the age of the sea floor like that, then we get a good agreement with the dates produced by radiometric methods. It is hard to think that this is a coincidence; it is also hard to think of any mechanism that could produce this agreement other than that the rocks are as old as radiometric methods tell us.
Marine sediment[ edit ] We began our discussion of absolute dating by saying that sedimentation rates could not be relied on for absolute dating. If there is one possible exception to this, it would be the deposition of marine sediment, since it is not subject to erosion, and since we would expect the rates of deposition of the various sediments to be, if not actually constant, then not subject to such a degree of variation as for example glacial till.
Based on the known rates of deposition, we may therefore at least say that the depths of marine sediment found on the sea floor are consistent with the ages of the igneous rocks beneath them as produced by radiometric dating. Radiometric dating and paleomagnetism[ edit ] The polarity of the Earth's magnetic field is a global phenomenon: So if our methods of radiometric dating are correct, then we would predict that rocks dated to the same age would have the same polarity, which they do.
If this does not completely prove that radiometric dating is correct, it does at least show that barring a wildly improbable coincidence there is at least a one-to-one relationship between the dates produced by radiometric methods and the true dates, and so it must be taken as an argument in favor of these methods. Comparison with historical dates[ edit ] It is possible to test radiocarbon dating by using it to put a date on historical artifacts of known date, and to show that it is usually very accurate.
It has also been possible to test Ar-Ar dating against the historical record, since it is sufficiently sensitive to date rocks formed since the inception of the historical record.Radiometric or Absolute Rock Dating
For example, Ar-Ar dating has been used to give an accurate date for the eruption of Vesuvius in 79 A. D, as recorded by Roman historians at the time. See Lanphere et al. Radiocarbon dating, varves, and dendrochronology[ edit ] Because varves contain organic material, it is possible to compare the dates from varves with the dates produced by radiocarbon datingand see that they are in good agreement.
We also see close agreement between dendrochronology and uncalibrated radiocarbon dates. I specify uncalibrated dates because as radiocarbon dating is calibrated against dendrochronologythe agreement of calibrated radiocarbon dates with dendrochronology is inevitable. Now, each of these three methods relies on a different underlying physical process: We can hardly suppose that there is some single mechanism which would interfere with all three of these very different processes in such a way as to leave the dates derived from them still concordant.
Absolute Dating As dating technology has progressed, it has become increasingly possible for scientists to provide absolute dates for specimens. One such method, introduced in the s, is amino-acid racimization. Amino acids exist in two forms, designated L -forms and D -forms, which are stereoisomers, or mirror images of each other.
Virtually all living organisms except some microbes incorporate only the L-forms, but once the organism dies the L-amino acids gradually convert to D-amino acids. Several factors influence the rate of conversion, and though amino-acid racimization was popular in the s, these uncertainties have led scientists to treat it with increasing disfavor.
The principles that undergird amino-acid racimization, however, are essential to most forms of absolute dating. Generally, absolute dating uses ratios between the quantities of a particular substance let us call it Substance A and the quantities of a mirror substance Substance B to which it is converted over a period of time.
Historical Geology/Absolute dating: an overview - Wikibooks, open books for an open world
The greater the ratio of Substance B to Substance A, the longer the time that has elapsed. The scale of time for various substances, however, differs greatly.
Carbon decay, for instance, takes place over a few thousand years, making it useful for measuring the age of human artifacts. On the other hand, uranium decay takes billions of years, and thus it is used for dating rocks. Cation-ratio dating, for instance, measures the amount of cations, or positively charged ions, that have formed on an exposed rock surface. An ion is an atom or group of atoms that have lost or gained electrons, thus acquiring a net electric charge.
Electron loss creates a cation, as opposed to a negatively charged anion, created when an atom or atoms gain electrons. Cation-ratio dating is based on the idea that the ratio of potassium and calcium cations to titanium cations decreases with age. It is applicable only to rocks in desert areas, where the dry air stabilizes the cation "varnish. Various forms of radiometric dating employ ratios as well.
Relative Vs. Absolute Dating: The Ultimate Face-off
Every element has a particular number of protons, or positively charged particles, in its nucleus, but it may have varying numbers of neutrons, particles with a neutral electric charge but relatively great mass. Neutrons and protons have approximately the same mass, which is more than 1, times greater than that of an electron.
When two or more atoms of the same element have a differing number of neutrons, they are called isotopes. Some types of isotopes "fit" better with a particular element and tend to be most abundant.
Relative Vs. Absolute Dating: The Ultimate Face-off
For instance, carbon has six protons, and it so happens that the most abundant carbon isotope has six neutrons. Because there are six protons and six neutrons, totaling 12, this carbon isotope is designated carbon, which accounts for Generally speaking, the most abundant isotope is also the most stable one, or the one least likely to release particles and thus change into something else.
- Geologic Time - How it works
- Relative and absolute ages in the histories of Earth and the Moon: The Geologic Time Scale
- Absolute dating
This release of particles is known as radioactive decay. In the context of radioactivity, "to decay" does not mean "to rot" rather, the isotope expels alpha particles positively charged helium nucleibeta particles either electrons or subatomic particles called positronsor gamma rays, which occupy the highest energy level in the electromagnetic spectrum.
Historical Geology/Absolute dating: an overview
Unsourced material may be challenged and removed. July Thermoluminescence[ edit ] Thermoluminescence testing also dates items to the last time they were heated. This technique is based on the principle that all objects absorb radiation from the environment. This process frees electrons within minerals that remain caught within the item.
Heating an item to degrees Celsius or higher releases the trapped electronsproducing light. This light can be measured to determine the last time the item was heated. Radiation levels do not remain constant over time. Fluctuating levels can skew results — for example, if an item went through several high radiation eras, thermoluminescence will return an older date for the item.
Many factors can spoil the sample before testing as well, exposing the sample to heat or direct light may cause some of the electrons to dissipate, causing the item to date younger. It cannot be used to accurately date a site on its own.
However, it can be used to confirm the antiquity of an item.