geological terms?

Question

i was just reading a query re ,the age of rocks. And was reading about the uranium238 and thorium 230 test, but was puzzled, by reference to the term 'half life ' Can someone explain to me in simple terms what 'half life' means

Answer 1

Half-life is just a way to represent the exponential decay rate of a radionuclide. It is the amount of time that one-half of the atoms of the original number of atoms present of the parent nuclide will decay into a daughter nuclide.

Uranium 238 has a half-life of 4.468 x 10^9 years. If you begin with a quantity of 100 atoms of Uranium 238, after 4,468,000,000 years, you will only have 50 atoms remaining. After another 4,468,000,000 years, there will only be 25 atoms remaining, and so on until there is no Uranium 238 remaining.

It is not correct to say that it is the time needed for half of the Uranium 238 to decay to Thorium 230, because the daughter product of U238 is actually Thorium 234 which has a half-life of 24.1 days, and decays to Proactinium 234. Pa234 has a half-life of 6.69 hours, and in turn decays to Uranium 234. Uranium 234 has a half-life of 247,000 years and decays to Thorium 230. The process continues through about 10 more steps (each one called a daughter product) to end up as Lead 206, which is stable. The actual time needed to decay from U238 to Th230 is a little more than the actual half-life of U238.

Answer 2

Put simply, the half-life is the time taken for half of it to decay from one radio-nucleide to another, via the natural process of radioactive decay. Scientists have absolutely no evidence to assume that the rate of radioactive decay is anything other than constant.

Geologists choose radio-nucleides with extremely long half-lives (in millions of years) to measure geological processes, which also take place over similarly very long time periods. They measure the ratio of the "parent" radionucleide to that of its "daughter"... knowing the half-life of the parent, you can estimate with reasonable certainty the age of the rock.

You obviously have to be careful to sample correctly and avoid contamination. Geologists use igneous rocks which cooled directly from a molten state; you should not try it on sedimentary, metamorphic or re-deposited volcanic rocks as these are all derived from other (older) rocks. By dating the igneous rocks, one can build a framework by which to date all the other rocks which are interbedded with the igneous rocks, but which cannot be dated directly by radiometric means.

Beware the biblical arguments (chas chas) which refute radiometric dating. The argument about Mount St. Helens is always erroneously quoted. In the 1980 eruption of Mount St. Helens, barely any new molten lava was ejected. Most of the material came from the catastrophic destruction of the volcano itself (watch the video footage!), which having built up over a long period of time, was made up of considerably older rock. So you would not expect radiometric dating to give anything other than an older date for this material!!!

Hope this helps!

Answer 3

Here is a simple analagy. Say you have a bucket with a million balls in it, and every hour you threw out 5% of the balls. The actual number of balls discarded would be decreasing all the time as the number in the bucket got smaller. The half life would be the time it took you to get rid of half a million balls. I'm too old to remember how to do the maths. Probably calculus

Answer 4

Half life means, the time it takes for half of the U238 to decay into Th230. The U238 to Th230 ratio can give an approximate age of the rock, presuming that at formation the rock contained only U238. But today the U238 has decayed into Th230. Again, the amount of decay of U238 compared to the amount of the daughter isotope Th230 can give the approximate age of the specimen.

Answer 5

But note that radiometric dating methods *do not* necessarily give accurate dates for rocks.
Such methods rely on 3 assumptions:
That the original amount of parent and daughter isotope is known (usually assumed that there was no daughter isotope)
That the decay rate has been constant (quite an assumption when one is talking about half lives of millions of years and we have been measuring them for just decades)
That there has been no inflow or outflow of parent or daughter isotopes.

Just how wrong such dating can be is illustrated by the fact that volcanic rock from Mt St Helens (just decades old) is dated as millions of years old! When the methods fail for rocks of known age, how much credence should one give for rocks of unknown age.

Lots of info on such dating issues here
http://www.creationontheweb.com/content/view/3059/

Answer 6

Radio active isotope decay. The half life is the time they take to decay i think to the next isotope.

Answer 7

Yes.
These rocks are only half alive.
This is why they live so long.