As far as I understand it, in the mantle there is radioactive uranium mixed in with all of the other elements that make up the liquid rock down there. When this is spewed out onto the surface the breakdown of the uranium into lead that occurs happens at a standard half life rate and from the amount of breakdown we can work out how old the rock is. I have a couple questions about the intricasies of this.
1 - I heard that radioactive breakdown occurs in the mantle and that's why it's still hot, so wouldn't the ratio of Uranium and lead in the rocks that are spewed out by volcanoes change over time?
2 - What about regional differences within the magma?
Cheers
2006-10-25 02:57:13 · 6 answers · asked by schming2005 1 in Science & Mathematics ➔ Earth Sciences & Geology
1. You are correct, the breakdown has been occurring all along, so the amount of available radioactivity has indeed been decreasing. However, the key factor is the ratio of the radiocative product to the daughter product. If a rock has been sitting around for billions of years undisturbed there will be a lot of the non-radiocative daughter isotope compared with Uranium (for example). When fresh magma is formed, or older rocks are "re-cooked", new minerals grow incorporating any radioactive isotopes. These then continue breaking down at their usual half-life rate, creating daughter products from scratch.
2. Yes, the composition of magmas vary; some will have much more Uranium than others. Once again, it is the ratio that matters, not total amounts, though rocks with a lot of isotope are easier to date.
Rock ages can be made from individual minerals (zircon is a good one) or from the "whole rock". The latter is unreliable as a stand-alone if the the rock has been re-melted, because daughter product from earlier breakdown may be still present. But in conjunction with single-mineral dates it provides a back-up, or evidence of disturbance.
2006-10-25 03:18:08 · answer #1 · answered by Paul FB 3 · 1⤊ 0⤋
It is a little more complicated than that. Although I remember only the general scheme, depending on which isotope of uranium you start with you eventually end up with a different isotope of lead. There are many steps inbetween, some lasting microseconds while others lasting much longer (hundreds of thousands of years). Physicists have determined how to measure the various isotopes of uranium and lead to determine the age of the rock from isotope ratios. Although I am not completely sure on the next point, I believe most heat is generated in the core and is convected to the mantle. Regional differences within the magma are hard to deduce because it takes geophysics to learn most anything about the mantle, and even then they have to wait for earthquakes to occur.
2006-10-25 03:07:40 · answer #2 · answered by Amphibolite 7 · 2⤊ 0⤋
Now child you are begining to understand just how little geomorphologists, petrologists and vulcanologists really do understand. Sorry I can't help with the research becaust research methods and measuring equipment has changed so much since my active days.
My answer is (probably unhelpfully) is to resort to the old trusty scientific cornerstone :Make a hypothesis and see if it stands up.
2006-10-25 03:11:07 · answer #3 · answered by Anonymous · 0⤊ 2⤋
i might say the respond is close to to B. The carbon-14 relationship approach is used to estimate the age of carbon containing aspects of roughly fifty 8,000 years to 62,000 years. while the Uranium-Lead approach is used to estimate age of older supplies of a few million million years to 4 and a 0.5 billion years.
2016-12-08 20:58:31 · answer #4 · answered by Anonymous · 0⤊ 0⤋
I'm afraid i haven't a clue but will definitely be checking the answers
2006-10-25 03:07:23 · answer #5 · answered by Anonymous · 0⤊ 3⤋
wow you sound like a really interesting guy
2006-10-25 03:05:36 · answer #6 · answered by Anonymous · 0⤊ 4⤋