geology question?

Question

The age of granite can be determined using radiometric dating. Explain the basis for the determination of a U238 Pb radiometric date of 1120 ma for granite.

I have to explain what *the basic principles of radiometric
dating * the link between granite and the isotopes given(why these are relevant to granite of that age) * the basic principles of how you determine the age.

Can someone please help!!

Answer 1

Basically, radiometric dating relies on what nuclear physicists know about the decay of radioactive elements. They compare the amount of the parent element (the original element trapped in the rock when it cooled) to the amount of daughter element (the element the parent turns into as it decays). In this case, half of the Uranium turns into Lead in about 4.5 billion years. By comparing the percentage of Uranium to Lead in the sample, they can tell when the rock it came from was formed.
Here is a very good web page on the subject from our friends at the United States Geological Survey: http://pubs.usgs.gov/gip/geotime/radiometric.html

BTW, Chas_chas_123 obviously has some hidden agenda that has little to do with Christianity.... he's probably just trying to abuse religious belief to scam someone for money, but then again he could have been treated badly by a Nuclear Physicist or expert Mathemetician when he was a child. Who knows? Anyway, to counter Chas's attempt to confuse science and religion with his personal psychosis, here is a Christian perspective (not to be confused with Creationism) on radiometric dating: http://www.asa3.org/aSA/resources/Wiens.html It seems to have been written by some Christian who feels Creationists actually deserve a serious rebutal.

Answer 2

Igneous rocks are good candidates for dating. Recall that for igneous rocks the event being dated is when the rock was formed from magma or lava. When the molten material cools and hardens, the atoms are no longer free to move about. Daughter atoms that result from radioactive decays occurring after the rock cools are frozen in the place where they were made within the rock. These atoms are like the sand grains accumulating in the bottom of the hourglass. Determining the age of a rock is a two-step process. First one needs to measure the number of daughter atoms and the number of remaining parent atoms and calculate the ratio between them. Then the half-life is used to calculate the time it took to produce that ratio of parent atoms to daughter atoms.

However, there is one complication. One cannot always assume that there were no daughter atoms to begin with. It turns out that there are some cases where one can make that assumption quite reliably. But in most cases the initial amount of the daughter product must be accurately determined. Most of the time one can use the different amounts of parent and daughter present in different minerals within the rock to tell how much daughter was originally present. Each dating mechanism deals with this problem in its own way. Some types of dating work better in some rocks; others are better in other rocks, depending on the rock composition and its age.

The uranium-lead method is the longest-used dating method. It was first used in 1907, about a century ago. The uranium-lead system is more complicated than other parent-daughter systems; it is actually several dating methods put together. Natural uranium consists primarily of two isotopes, U-235 and U-238, and these isotopes decay with different half-lives to produce lead-207 and lead-206, respectively. In addition, lead-208 is produced by thorium-232. Only one isotope of lead, lead-204, is not radiogenic. The uranium-lead system has an interesting complication: none of the lead isotopes is produced directly from the uranium and thorium. Each decays through a series of relatively short-lived radioactive elements that each decay to a lighter element, finally ending up at lead. Since these half-lives are so short compared to U-238, U-235, and thorium-232, they generally do not affect the overall dating scheme. The result is that one can obtain three independent estimates of the age of a rock by measuring the lead isotopes and their parent isotopes. Long-term dating based on the U-238, U-235, and thorium-232 will be discussed briefly here; dating based on some of the shorter-lived intermediate isotopes is discussed later.

The uranium-lead system in its simpler forms, using U-238, U-235, and thorium-232, has proved to be less reliable than many of the other dating systems. This is because both uranium and lead are less easily retained in many of the minerals in which they are found. Yet the fact that there are three dating systems all in one allows scientists to easily determine whether the system has been disturbed or not. Using slightly more complicated mathematics, different combinations of the lead isotopes and parent isotopes can be plotted in such a way as to minimize the effects of lead loss. One of these techniques is called the lead-lead technique because it determines the ages from the lead isotopes alone. Some of these techniques allow scientists to chart at what points in time metamorphic heating events have occurred, which is also of significant interest to geologists.

Lots more info at the link!

Answer 3

The last person who answered (no name visible?) is absolutely right and gave a really good description. The only thing i can add is to say that the Uranium lead dating is used because of the relatively long half life. Other elements decay much quicker or are much slower or the elements aren't present in granite. (can't recall which but potassium - argon and carbon 12 to carbon 14 are other dating agents)
So Uranium - lead is used because the half life decay allows for good measurements in the time period expected.
You don't use a stop watch to measure the change of seasons or a calendar to time a 100m sprint Hope that helps.

Answer 4

In case you follow the first answerer's link, you will see that it is more claptrap from the relentless creationists. One should be advised that some of them have acquired scientific credentials and they now use those credentials to preach creationism through the guise of science. Note than none of them got those credentials for work contradicting current scientific notions of ancient geology or evolution, yet this is almost exclusively what they preach. (Well, that's the right word, since you will see the word "ministry" numerous times on the creation web site referenced here.)
I did a couple hours of web research on some of the prominent contributors to the creationist link in the first answer. The one theme that consistently comes through is dishonesty. I'm not talking about errors, but deliberate deception. Why? Because they believe in the literal interpretation of the bible and this is utterly unscientific. They believe their misrepresentations and fallacious assertions are all in God's name. They will preach young earth, Noah's flood, and anti-evolution regardless of the evidence against them. Not a very credible premise for the evaluation of radiometric dating methods.
Do not rely on ANY creationist argument if you want to know the facts about anything.

Answer 5

Some good articles about radiometric dating here
http://www.creationontheweb.com/content/view/3059/