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(meteorobs) Age Dating of Rocks:The Earth, and Solar System Compared
The following was posted on the meteorite list by Frank Stroik. I thought
some of you all might find this interesting and informative. I will post the
other installments as he posts them.
George Zay
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> I would like to take a few minutes of your time to introduce the
>concept of age dating. By understanding what age dating is, and how it is
>applied, we can better understand the history of our planet, and it's
>place in the Solar System. This is the first of a three part posting,
>which will discuss the Earth, Mars and the Moon, and Meteorites and
>asteroids. I know that this is a sight for meteorite discussion, but an
>understanding of the process here on Earth, will lead to greater insight
>into the geologic processes of the Solar System.
> The Earth formed 4.6 billon yrs ago. Well how do we know this?
>What would lead scientists to make this statement, and what is the basis
>for it? Do we know how to accuratly age date anything, that we did not
>see form?
> The answers to the questions above lie in a field known as
>Geochronology. This field relates geologic principles with Nuclear
>Physics, to provide a scientificaly firm basis for theory.
> Geochronologists use many different ways to date geologic
>materials. The most effective is the decay of radioactive isotopes(isotope
>is a atom such a lead, that contains extra atomic particles as compared
>to a common atom of the same element). These isotopes form what is known
>as a system. The two most important systems are Potassium-Argon(k-Ar),
>and Niobium-Strontium(Nd-Sr).
> The concept is quite simple. The first thing to do is to find how
>much of the parent material was present when the rock formed(in the
>system Nb-Sr Nb is the parent isotope). This is found by measuring how
>much daughter material is in the rock(in Nb-Sr, Sr is the daughter isotope).
>This will give a pretty good age date for the rock.
> You may ask," How do we know how much of an element was present
>to begin with?" Thanks for asking, I will explain. You see there is a
>concept known as half-life of an element. This concept basicly says that
>if we know how fast an element decays, we can calculate it's original
>abundance.
>For example, I have just been given a meteorite to age date. I decide I
>will use K-Ar method. K has atomic particles that are in a stable
>configuration(neutral). It is known that K has some forms that have more
>atomic particles than can be unstable. It is also known that these unstable
>forms of K decay at a certain rate that never changes. If enough decay
>occurs K will, in a sense, turn into Ar(look at a periodic table of
>elements, and see that Ar is the element before K).
> I then measure how much Ar is found in the rock. Then I find out
>how much of the unstable K is in the rock. I compare the two, and develop
>a ratio. I know that the Ar was produced by the K decay, and I know the
>abundances of each in the rock. I apply those numbers to some
>mathimatical principles ,and get an age of the rock.
> Ex: Lets say we have 100 atoms of unstable K in a rock when it forms.
>We find the rock today, and date it. We find that we have 50 K atoms, and
>50 Ar atoms. We know that in order to get half of the K atoms it takes
>4.5 billon yrs(that is the Half life of the element K). So by the amount
>of the two elements we can safely conclude an age of the rock is 4.5 billon
>yrs.
> How do we know the age of the Earth? We have to look at rocks
>here on Earth to find out. In Greenland, there is a rock body called the
>Istaq Gniess(gniess is a pressure and temperature deformed granite).
>These are the oldest rocks on the planet, because they date to 3.9 billon
>yrs. They were the first rocks to solidify on our planet. They tell us
>about the atmosphere of the early Earth, composition of of early igneous
>processes, and what the Earth looked like before any continents formed.
> But these are only 3.9 billion yrs old, where did 4.6 billon come
>from as the date? We get the rest of the data from meteorites. The Earth
>had to be around at that time, because other bodies had already formed.
> I will continue this discussion over the next week or so. I
>wanted to stop here so as people can ask questions, and just take time to
>read this post with leisure. If any one has questions, please let me
>know. I will answer them before I post again.
>
>Frank Stroik