# Radiometric dating - creation/evolution question - need all the help i can get!!?

I was having a discussion with a friend about radiometric dating and he asked a question I really didn't have an answer too.

"When one picks up an object (say a bone/rock etc.) how does one know which dating technique to use. (Carbon dating is only good for about 50,000 yrs and several other dating techniques have varying upper and lower bounds)"

To that I said, "O well we know that the earth is 4.7 billion years old and so we would use (say) x dating method and a dino bone is some million years old so we would use (say) y dating methods."

To that my friend argued that thiss unfair because we are already assuming that the earth is 4.7 billion years old and hence using a dating method that supports that claim is unfair.

So I want to know how do geologists determine what radiometric dating system to use on objects.

Also I would love it if someone could also please explain how these radiometric datig methods are calibrated.

### 9 個解答

• 10 年前
最佳解答

It would indeed be unfair IF YOU WERE TRYING TO DETERMINE THE AGE OF THE EARTH. Then yes, starting with a prior assumption about the age of the earth, and choosing a method that supports that claim would indeed be unfair.

But scientists do not do that.

Radiometric dating is based on the physics of radioactivity. Physicists can *measure* the rate of decay of a radioactive isotope. So by using *ratios* (not absolute amounts) between the isotope and its decay results, they can compute the age of the rock.

Note that one technique would technically be sufficient, because the technique is based purely on physics. But the fact that several technique using several isotopes all produce consistent results, just confirms that the basic concept of radiometry is sound.

No prior assumptions about the age of the earth are required to compute the age of a rock or fossil.

It is only after many labs, using many techniques, using many different isotopes, that all produced ages of rocks and meteors that converge on a maximum age of 4.56 billion years old, that geologists have reached a consensus that this is a good estimate of the age of the solar system (and thus the earth).

Then AFTER the age of the earth is established, THEN you can use that age as an expected maximum. But that's all. The only relevance of the age of the earth to the age of a rock or fossil is as a sanity check *based on all prior measurements taken to date*. You don't need to know the age of the earth to know the age of a fossil or rock.

One thing that is typically misunderstood by creationists is the concept of *index fossils*. Once we have various techniques for dating rocks, we can use this for dating fossils found in the same layers as those rocks. And again, after hundreds of samples of this, we start to get a good idea of what fossils are found in layers of certain age.

So AFTER we have used the radiometric ages of many rocks to date many fossil species, we can then start to estimate the ages of rocks based on the fossils. If we find a rock in the same layer as a trilobite fossil, that establishes the layer as a Cambrian layer, and we can be pretty sure the rock is about 540 million years old.

Creationists often accuse this of being "circular reasoning", but they are utterly missing the point! It is only AFTER we have dated many trilobites using radiometric dating of the rocks in the same layers, can we call them an "index fossil". All this is doing is saving us some time and expense of verifying the result in a lab. We can still send the rock in to get a radiometric reading. But if all we want to know is whether we're digging in a Cambrian or an Ordovician layer, then the index fossils are sufficient.

As far as "choosing the right technique", that is NOT to achieve a certain result, but just to avoid wasting time and expense. If it's a dinosaur bone, using carbon dating would simply be a waste of time and money, because no dinosaur bone has ever been found to be within the 50,000-year range of carbon dating. You are of course free to send it to a lab and insist on carbon dating ... but the value you would get back is "inconclusive" ... because there would be so little Carbon-14 in the sample that it would be unmeasurable.

An analogy.

It's a bit like deciding whether to use a 12-inch ruler, or a tape measure, or a surveyor's clickwheel. We can estimate the approximate length of the thing being measured, to know which one will get us the quickest result. But this does NOT mean that we are pre-deciding its length! If you are measuring, say, the length of a carpet, you could try to measure it with all three techniques, but the 12-inch ruler is too short, and the surveyor's clickwheel is too imprecise. Your ability to *estimate* the length lets you choose the tape measure as the fastest technique for this particular job.

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參考資料： One of the best introductions to radiometric dating was actually written by a Christian geochronologist who wanted to correct all the misinformation that Christians are getting about radiometry from BAD creationist sites. "Radiometric Dating: A Christian Perspective" by Roger C. Wiens: http://www.asa3.org/ASA/resources/wiens.html
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• FA
Lv 5
10 年前

There's a fairly obvious point that nobody has mentioned yet, it depends on what you have. If you have a bone, then there is bugger all point in dating it using using U-Pb for the simple reasons that it is unlikely to have much uranium in it and you can't guarantee that it is a closed system with respect to uranium. Likewise, if the bone has been mineralised then there's no point in carbon dating it as it is clearly not a closed system (on account of the fact that it has changed) and there won't be much (or any) carbon left. You use as many dating techniques as you can based on what minerals a rock contains and what events you want to date (no isotopes used in radiometric dating are dominant in a mineral, they are trace elements which substitute into the mineral structure). An example of the first point is if a rock contains zircons then you will use U-Pb, if it contains garnets then you'd have a fair shot at getting Sm-Nd to work, Rb-Sr is good with feldspars etc although bear in mind that for the general age of a rock you can use whole rock analysis as well. If you want to date specific events (for example, in a metamorphic rock) then you will use the minerals which will not be altered unless subject to very high temperatures. So, for example you wouldn't analyse the feldspars in a high grade metamorphic rock because they alter far too easily, instead you would use zircons or garnets.

If you get agreement between several dating methods, then chances are it's right. If not, then you look for possible sources of inaccuracy or contamination in both to see which is the most reliable. Dating methods aren't chosen based on what they support, they are chosen based on how reliable the method is proven to be and some of them have very broad ranges.

As for assuming the age of the Earth, that's a completely different topic and it isn't an assumption. I would also like to know which dating methods your friend is proposing as just about all "young Earth" dating methods are a complete joke.

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• 匿名使用者
10 年前

For any particular age range there are usually several different methods that apply,

Carbon 14 has a half life of 5730 years, making the method good for anything from about 100 years old to about 70,000 years.

Other techniques that apply to this age range are dendrochronology and magnetic dating.

Dendrochronology uses tree rings. In any area there will be a pattern of tree rings - say three thick ones followed by two thin ones and another thick one - reflecting two dry years in the middle of a wet period. By correlating the rings its possible to work successively backwards - especially in dry areas where wood persists for a long time.

Magnetic methods depend on the fact that the direction of magnetic north has changed in a known way over time. Some event may occur that freezes magnetic particles in a particular direction. For example people may have built a camp fire. The heat of the fire exceeds the Curie temperature of magnetite particles in clay around the fire. When they cool they are aligned with the then magnetic north. The direction can be measured now and gives an age for the fire.

When several different methods give the same or very similar ages, then we know we are on the right track.

We can also date historic objects for which we have a known date. For example somewhat controversially. the Shroud of Turin made its first documented appearance in 1390. Well controlled radiocarbon testing gave dates between 1260 and 1390.

At the other end of the scale Potassium 40 has a half life of 1.29 billion years. When Potassium 40 undergoes positron emission or electron capture it transmutes into Argon 40. Argon is an inert substance, which means that it basically will not combine chemically with other elements. It is also a gas over an extremely wide range of temperatures, which means that any Argon 40 would escape while the rock was molten like carbon dioxide escaping from a glass of soda. After solidification, those Argon 40 nuclei that appeared as a result of radioactive decay would be trapped by the crystal structure and accumulate as the mineral aged. Measuring argon is an extremely good method for determining the crystallization age of old rocks. The creationists don't understand or don't want to understand the method, so they come up with irrelevant arguments like we don't know the initial radioactivity. With Potassium Argon no assumptions are made about initial radioactivity. Or they try and discredit the method by analyzing very young rocks, where the slightest bit of argon contamination alters the result. The method is no good for anything under 5 million years.

While on discrepancies, you might ask your friend about calculating the age of the earth by counting up the generations in the Bible. Luke 3:23–38 lists 42 generations between David and Jesus, whereas Mathew 1:1–17 comes up with only 27. That's a far bigger discrepancy than would be accepted for any of the scientific methods.

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• 10 年前

Usually, there would be clue around in the terrain where the fossil is found that would indicate the kind of age range it would be. If you pick up what seems to be a humanoid bone, in a site that features things like traces of fire or tool making, then you are pretty sure it is going to be less than one million year old.

But even if someone was given a sample that is unidentified, with the challenge to determine its age, who is to say one cannot use several methods in succession? Carbon 14 does not destroy the complete specimen, just a tiny bit of it, so there is usually enough of the specimen left to try another method.

All radiometric dating method work on the principle that, while alive, a living organism will pick up some isotope of an element essential to life, say potassium for example, that will over time decay to an element that would NOT be found in living tissue (argon, in the case of potassium 40). So, how much argon would be found in the sample has to correlate with how old that specimen is.

Actually, one can even date rocks in the same manner, as a rock that contains potassium when it formed from lava would not be containing any argon, as argon is a gas.

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• 10 年前

You don't start with an assumption of the age of the earth. What you start with is the various techniques that will work with the geology of the area you are investigating. The ages of most of the terranes you will be working with are already known and therefore the recommended method of analysis. It will depend upon what you are trying to age and what the general age of the geology is for that area. We rely upon the radioactive decay of isotopes of elements which we believe decay at a given rate, called the half-life. The use of this technique has been correlated by several complementary methods. When one element decays in this manner, it will change, sometimes to another isotope of the same element, and sometimes to another element. So there will usually be a mixture of two isotopes or elements in the same mineral or other material. When the decay rate of the parent material is known, then the time it has decayed can be estimated. From the formation of material, some of the elements may have decayed and if you have a good specimen for analysis you are in luck. Not everything can be done this way though.

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• 10 年前

Hi there,

Great question. I work in paleontology. Samples are sent to radiography labs. It's the same physics as in CT scans and xrays. All physical elements begin to decay once they are formed and do so at a determinable rate. Radiometric dating is like a clock inside a rock which began ticking when that rock was formed and hence began to decay. The rate of decay is determined by the ratio between two isotopes called the parent and daughter. Their ratio determines the age of the rock.. A variety of elements can be used to date rocks. Only carbon can be used to date carbon based life forms and then it's only good on material from Present - 70,000 years. In the use of Carbon 14 (which a lot of people have heard of), it's the ratio between the carbon 14:carbon 12 which is 1/trillionth:1. Carbon 14 loses half its remaining total every 5,500years. A hugely accurate clock. For older material, other elements are tested like: Uranium 238/Lead 206, Uranium 235/Lead 207, Thorium 232/Lead 208, Potassium 40/Argon 40, and, for use with only young material, Carbon 14/Carbon 12.

參考資料： books here at home.
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• 10 年前

<<When one picks up an object (say a bone/rock etc.) how does one know which dating technique to use.>>

Easy. You use any and every technique that can be used with the available evidence.

<<To that my friend argued that thiss unfair because we are already assuming that the earth is 4.7 billion years old and hence using a dating method that supports that claim is unfair.>>

None of the dating techniques that may apply have any connection with the age of the Earth, as that happens to not be relevant to the age of the object that may be under examination.

PS: This supposed dialogue between you and your friend sounds suspiciously like faked.

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• 匿名使用者
10 年前

your friend clearly doesn't know much geology

there aren't any "random old rocks" to pick up.

where you are geographically and what type of rock is picked up tell A LOT about the rock before any dating is done

you pick up a bluestone or shale in up-state New York you're picking up a sedimentary rock from a 500 million year old mountain range (approximate age can be assessed (NOT guessed) by taking a look at plate tectonics)

so you'd already have a range of different molecules to date the rock sample, which would independently corroborate if they converge on a date

K-Ar and Pb-Po dating are not the same thing you can't say it's circular if they converge on one date

also, saying that plate tectonics and radiometric dating are used to confirm each other is blatantly false.

the two techniques corroborate... they are not used to confirm eachother

radiometric dating is confirmed with chemistry, math, physics

tectonic theory is confirmed with seismic geology, direct measurement and satellite analysis

the age of the earth is only an assumption in the sense that it has been so irrevocably proven.

the rate of gravity isn't an assumption either, but when you ask someone they'll say 9.8m/s/s. and if you say anything else they'll look at you funny

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• 匿名使用者
10 年前

The whole radiometric dating thing is fake. Let's get real, how can anyone know the date of a rock? No one knows what conditions were like when it was created or if perceived constants in effect now were like that even 5000 years ago. What were the starting levels of radioactivity, no one knows that either. I bet someone here will say they can know, but NO you cannot.

參考資料： Common sense
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