O.K - I'm always shooting my mouth off in 'physics + maths' so let me show my ignorance of other subjects. How do geologists determine the age of rocks? I understand carbon dating for previously live material, but I don't understand rock dating.
They use things like uranium and stuff. Question I have is how they know how much was originally there.
This link has a good overall picture of radiometric dating, and what is used for what span of dates. Radioactive isotopes are found in igneous rock only, so dating other rocks where fossils might be found needs to rely on analysis of rocks above and below to give a range. From the reading I gather that upon solidification of the molten rock, the isotopes are trapped in their initial state (not sure about this), and slowly decay from there. How much percent of a sample of say Potassium vs Argon-40 determines when the lava cooled. There is also a cooling time period that needs to be taken into account. Plus there are many cross checks done to validate the findings. Simple in theory, but complex in the lab...
Thanks guys. I'll have a read at those links.
Radioactive dating = Atomic Girl and Uranium Guy go to the movies. :D
I'll restate my question: You compare how much if an isotope is in the sample(x), versus how much there was(y). for carbon dating, the isotope is continually being produced, and so a living organism will have a certain amount, so it works. But elements like uranium aren't, so how do we know what the value y was?
02-25-03, 06:44 PM
Perhaps 20-30 years ago, there was an article in Scientific American about developing a calibration system for carbon 14 dating, using Bristle Cone Pine trees, which grow in California or elsewhere near the West Coast of the USA.
Carbon 14 dating makes two assumptions. It is assumed that the atmosphere has the same mixture of gases all over the world. I think this assumption is considered to be a good one. It is assumed that the mixture does not vary with time. It is my understanding that this assumption is not terrible, but that it is not very good and is a potential source of error.Bristle Cone Pine trees live for about 2000 years, but the inner part of the trunk is not alive. Only the leaves, bark, and outermost part of the trunk lives. If you cut down a 2000 year old tree and find the stump of a 2000 year old tree that died 1500 years ago, you can establish a tree ring chronology going back 3500 years. If you find the stump of a tree that died 3000 years ago, you can extend the chronology back further.
The amount of growth each year is affected by temperature, amount of moisture, and perhaps a few other conditions. Thus the width and other features of the rings vary from year to year. This variation makes it possible to match up corresponding rings from two different trees. Thus, after a lot of patient analysis, it is possible to say that the first 500 years of this tree’s life corresponded to the last 500 years of that tree’s life.
The SciAm article claimed that some patient researchers used analysis these trees to get an 8000 year chronology of tree rings. They then used Carbon 14 dating against various rings. The Carbon 14 dating did not agree with the tree ring chronology, which is known (or considered) to be very accurate. The result of this analysis was a calibration chart for use in adjusting dates derived from the Carbon 14 method.
The article claimed that various European dates were pushed back, while the method had little effect on Egyptian and certain other dates. This is due to Carbon 14 dating not being used for the Egyptian and various other cultures. Those cultures had an unbroken written history with references to astronomical and other events with known dates, resulting in little or no use of Carbon 14 dating. In contrast, Carbon 14 dating was used to establish dates for Europe and various other parts of the world.
The SciAm article claimed that some historical theories had to be revised due to this calibration technique.
I assume that Carbon 14 dating has used this calibration technique for perhaps 20 years, but have never seen any reference to it since reading the SciAm article.
History and archaeology are interesting to me, but I am an amateur in those disciplines. Does some more knowledgeable person have any more data about the above?