Radiometric dating has been demonstrated to give wrong age estimates on rocks whose age is known. Main article: Rubidium—strontium dating. What if some stable daughter element was already present when the rock formed? Next, evolutionary scientists then use index fossils to help establish the relative ages of rock layers that are not directly related to one another and their fossils. Retrieved Rocks returned from the Moon have been dated at a maximum of 4. Estimating Age Since age cannot be measured, how is it estimated? Help Community portal Recent changes Upload file. But it is a very important one.


By Age of the Earth is understood the total time since the earth has existed in its present physical state as a planet. In Geology, this problem has always been found as one of the most complicated and controversial topics. Many conclusions were suggested on different lines but were rejected by equally strong objections. It was only with the discovery of radioactivity that a new method giving an approximate age, with comparatively less chances of error were found. Presently, by any discussion on the problem of age of the Earth is understood a discussion of radioactive methods alone since all the other methods are highly speculative and already stand discarded. They are mentioned below only to show the curiosity among the scientists about the topic. At present, the maximum age of the Earth as determined from radioactive dating is put at 4. Many thinkers have attempted to tackle the question of Age of Earth, though unsuccessfully, even in earlier periods. Thus, in Hindu mythology, the total age of the Earth is mentioned as 4. Another religious view mentioned in geological literature just to show the trend of thinking on the problem is due to an Archbishop, Ussher. He calculated, from his interpretation of the Old Testament, a day in B. John Lightfoot even calculated the exact time and date: 8 p. The first scientific effort was, perhaps, due to George L. More rational assessments were made in the 19 th century, of which those based on the rate of sedimentation and rate of accumulation of sodium in oceans dominated the scientific world for a good time.

Petrology Tulane University Prof. Stephen A. Nelson Radiometric Dating Prior to the best and most accepted age of the Earth was that proposed by Lord Kelvin based on the amount of time necessary for the Earth to cool to its gae temperature from a completely liquid tjrough. Although we now recognize lots of problems with that calculation, the radikmetric of 25 my was accepted by most physicists, but considered too short by most more info. Then, inradioactivity was discovered.

Recognition that radioactive decay of atoms occurs in the Earth was important in two respects: It provided another source of heat, not considered by Kelvin, which would mean that the cooling time would have to be much longer. It provided a means by which the age of the Earth could be determined independently.

Principles of Radiometric Dating. Radioactive decay is described in terms of the probability that a constituent particle of the nucleus of an atom will escape through the potential Energy barrier which bonds them to the nucleus.

The energies involved are so article source, and the nucleus is so small that physical eartn in the Earth th. T and P cannot affect the rate of decay. The rate of decay or rate of change of the number Ddetermined of particles is proportional to the number present at any time, i.

So, we can write. After the passage of two half-lives dna dating wikipedia 0. This can only be done for 14 C, since we know N 0 from the atmospheric ratio, assumed to be constant through time.

For other systems we have to proceed further. The only problem is that we only know the number of daughter atoms tbe present, and some of those ss have been present the age of the earth as determined through radiometric dating is about to datinf start of our clock.

We can see how do deal with this if we take a particular case. The neutron emits an electron to become a proton. We still don't know 87 Sr 0the amount of 87 Sr daughter element initially present.

Thus, 86 Sr is a stable isotope, and the amount of 86 Sr does not change through time. So, applying this simplification. The reason for this is that Rb has become distributed unequally through the Earth over time. For example the amount of Rb thr mantle rocks is generally low, i.

Thus we could tell whether the rock was derived from the mantle or be determining its initial Sr isotopic ratio as we discussed ia in the section on igneous rocks. Two isotopes of Uranium and one isotope of Th are radioactive and decay to produce daying isotopes of Pb. The decay schemes are as follows.

Note that the present ratio of. If these two independent dates are the same, datlng say they are concordant. We can also construct a Concordia diagram, which shows the values of Pb isotopes that would give concordant dates. The Concordia curve can be calculated by defining the following:. Zircon has a high hardness 7. Zircon can also survive metamorphism. Chemically, zircon usually contains high amounts of U and low amounts of Pb, so that large amounts of radiogenic Pb are produced.

Learn more here minerals that also show these properties, but are less commonly used in radiometric dating are Apatite and sphene.

Discordant dates will not fall on the Concordia curve. Sometimes, however, numerous discordant dates from the same rock will plot along a line determiend a chord on the Concordia diagram.

Such a chord is called a discordia. We can also define what are called Pb-Pb Isochrons by combining the two isochron equations 7 and 8. Since we know that theand assuming that the Pb and Pb dates are the same, then equation 11 is the equation for a family of lines that have a slope. The answer is continue reading 6 billion years.

This argument tells when the elements were formed that make up the Earth, but does not really give us the age of the Earth. It does, however, give a maximum age of the Earth. Is this the age of the Earth? Lunar rocks also lie on the Geochron, at least suggesting that the moon formed at the same time as meteorites. Modern Oceanic Pb - i.

Pb separated from continents and thus from average crust also plots on the Geochron, and thus suggests that the Earth formed at the same time as the meteorites and moon. Thus, our best estimate of the age of the Earth is 4. The initial ratio has particular importance rsdiometric studying the chemical evolution of the Earth's mantle and crust, as we discussed in the section on igneous rocks. Since K is one of the 10 most abundant elements in the Earth's crust, the decay of 40 K is important in dating rocks.

But this scheme is not used because 40 Ca can be present as both radiogenic and non-radiogenic Ca. Since Ar is a noble gas, it can escape from a magma or liquid easily, and it thriugh thus assumed that no 40 Ar is present initially. Note that this is not always true. If a magma cools quickly on the surface of the Earth, some of the Ar may be trapped.

If this happens, then the date obtained will be older than througn date at which the magma erupted. For example lavas dated by K-Ar that are historic in age, usually show 1 to 2 my old ages due to trapped Ar.

Such trapped Ar is not problematical when the age of the rock is in hundreds click the following article millions of years.

The dating equation used for K-Ar is:. Some of the problems associated with K-Ar dating are Excess argon. This is only a problem when dating very young rocks or in dating whole rocks instead of mineral separates. Minerals though not contain any excess Ar because Agout should not enter the crystal structure of a mineral when it crystallizes.

Thus, it always better to date minerals that have high K contents, such as sanidine radiometeic biotite. If these are not present, Plagioclase or hornblende. If none of these are present, then the only datihg is to date whole rocks. Atmospheric Abe. Some 40 Ar could be absorbed onto the sample surface. This can be corrected for. Metamorphism or alteration. Most minerals will lose Ar on heating above o C - thus metamorphism can cause a loss of Ar or a partial loss of Ar which will reset the atomic clock.

If only partial loss of Ar occurs then the age abiut will be in between the age of crystallization and the age of metamorphism. If complete loss of Ar occurs during metamorphism, then the date is that of the metamorphic event. The problem is that there is no way of knowing whether or not partial or complete loss of Ar has occurred. Examples of questions on this material that could be asked on ot exam. Radiometric Dating. Prior to the best and most accepted age of the Earth was that proposed by Lord Kelvin based on the amount of time necessary for the Earth to cool to its present temperature from a completely liquid state.

Principles of Radiometric Dating Radioactive decay is described in terms of the probability that a constituent particle of the nucleus of an atom will escape through the potential Energy barrier which bonds them to the nucleus. Thus, if we start out with 1 gram of the parent isotope, after the rwdiometric of ewrth half-life there will be 0.

Some examples of isotope systems used to date geologic materials. If we divide equation 4 through by the amount of 86 Sr, then we get:. Note also that equation 5 has the form of a linear the age of the earth as determined through radiometric dating is about, i. How can we use this? In nature, however, each mineral in the rock is ad to have a different amount of 87 Rb. Thus, once the rock has cooled to the point where diffusion of elements does not occur, the 87 Rb in each mineral will decay to 87 Sr, and each mineral will have a different 87 Rb and 87 Sr after passage of time.

The discordia is often interpreted by extrapolating both ends to intersect the Concordia. Pb leakage is the most thrkugh cause of discordant dates, Pb will be occupying a site in deterimned crystal that has suffered radiation damage as a result of U decay.

U the age of the earth as determined through radiometric dating is about have been stable in the crystallographic site, but the site is now occupied by by Pb. An event like metamorphism could heat the crystal to the point where Pb will become mobile. Another possible scenario involves U leakage, again possibly as a see more of a metamorphic event.

U leakage would cause discordant points to plot above the aa. The Age of the Earth A minimum age of the Earth can be obtained from the oldest known rocks on the Earth. So far, the oldest rock found is a tonalitic Gneiss metamorphic rock rock from the Northwest Territories, Canada, with an age of 3. This gives us only a minimum age of the Earth. Is it likely that we will find a rock formed on the Earth that will check this out us the true age of the Earth?

From the Pb-Pb isochron equation 11 we can make some arguments about meteorites. First, it appears that meteorites have come from somewhere in the solar system, and thus may have been formed at the radiomegric time the solar system and thus the Earth formed. If all of the meteorites formed at the same time and have been closed to U and Pb since their formation, then we can use the Pb-Pb isochron to date all meteorites. First, however, we need to know the initial ratios of the Pb eartu.

We recognize two major types of meteorites: Fe- meteorites and stony or chondritic meteorites The Fe meteorites contain the mineral troilite Aboyt that has no U. Since earrth mineral troilite contains no U, all of the Pb present in the troilite is the Pb originally present, and none of it has been produced by U decay.