Science Questions

How are fossils dated?

Sun, 11th Apr 2010

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Andrew Cottrell asked:

Dear Chris,


Something I should like to know: How do scientists know the age of the earth? In so many texts (or in the BBC series, "Walking with dinosaurs, for instance), one hears or reads: "T-Rex lived 165 million years ago"

How do they know this for a fact?




Andrew Cottrell


Diana -  Well, actually, as I mentioned in the news story earlier, the Sediba skeletons were dated using paleomagnetism and uranium-lead, which is an isotope.  With paleomagnetism, certain rocks they will have magnetic polarity which tells you what the magnetic polarity of the Earth was at the time that they were exuded from the crust.  And this changes over time.  So, you can work out when this rock was made.  With uranium isotope dating, there’s uranium-238 and uranium-235,which both decay to lead over time.  So, you can check the relative proportions of uranium and lead and work out how much has decayed into lead.  These isotopes have half-lives of a million to even 4.5 billion years, so you can actually go quite far back with your fossils. These are absolute dating methods, but there's also relative dating, which is the simplest method really.  You just look at the layers of the rock and say, "Well, this layer is below that one and we know that layer is so old.  So this must be even older."  And that's how a lot of fossils are dated.

Chris -   So, I guess what you're aiming to do is to use a number of different methods.  And it's a bit like drawing a series of lines to see where they all converge.  And you take the best agreement between all the different methods and say, well, that one seems to agree with everything and, therefore, it's likely to be that old.

Diana -   Yeah, that's right.  And the uranium-lead method is quite popular and it is actually quite accurate.  They can get down to 0.1% with the actual date, which is pretty good.

Dave -   The original way which doesn't give you an absolute date but does give you a relative date is that you look at the other fossils which are around it, because if you've got something like a dinosaur bone, around it there's also tiny things like beetles and little tiny snails. If you know these fossils only appeared at a certain date and went extinct another date, you know that the fossil must be from within those two dates.  You've got 40, 50 different fossils there, so you can get a really quite accurate date.

Chris -   Yeah.  As long as you get the other ones right, you're right.


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Andrew Cottrell asked the Naked Scientists: Dear Chris, Something I should like to know: How do scientists know the age of the earth? In so many texts (or in the BBC series, "Walking with dinosaurs, for instance), one hears or reads: "T-Rex lived 165 million years ago" How do they know this for a fact, or what measuring instrument do they use? Regards, Andrew Cottrell What do you think? Andrew Cottrell, Fri, 7th Aug 2009

I believe that the primary means of dating a fossil is by the particular geological stratum from which it was found.  The age of that particular stratum can be established by a variety of means ranging from inferential dating to radiometric dating. LeeE, Fri, 7th Aug 2009

Nearly all geological dates are given with an error (e.g. 400 million +/- 2 million years old) Often the error figure is omitted for clarity (and what difference does a few million years here or there make?) 

Different things are measured in different ways - Igneous (and metamorphic) rocks can be dated radiometrically - i.e. by measuring the difference in the ratios of particular radioactive isotopes.  This will give an answer with a defined level of error/ uncertainty.  More samples and ongoing improvements in techniques will tend to reduce the level of error to 10's of thousands of years  rather than millions of years.

The radiometric "clock" can be reset during metamorphic events.

Sedimentary rocks, (the ones containing fossils) are a lot harder to date and historically have been dated relative to each other.  This is called stratigraphy.  In the simplest terms, beds of the same rock can be found in different places. These different beds of rock can be correlated across large areas using the fossils within them (or by other methods) - and absolutely dated using for example a lava that overlies the sedimentary rock in one or two places.        Mazurka, Mon, 10th Aug 2009

Fossils can be dated using index fossils. An index fossil is one which is known to have existed at a particular place in time. Finding an index fossil elsewhere gives a date to the other place.

Here's a good place to lok. stereologist, Mon, 10th Aug 2009

C(14) decay element. (CARBON 14 element, decay to neutrons of level 14 weight over massive levels of time, but reasonably predictable p/100years or thereabout). nicephotog, Sat, 15th Aug 2009

C14 dating is only good for things up to about 10 times the half life of C14 (5730 years). That rules out most fossils. Bored chemist, Sat, 15th Aug 2009

And even further than that, C14 is really only accepted in the paleo community at about 26,000 years because we do not have high resolution independent dating such as tree rings and corals to calibrate C14-years in to calendar years. frethack, Sat, 15th Aug 2009

As much as I hate to admit this, it is NOT sedimentary rocks that yield age dates. The dating of metamorphic and volcanic rocks that yield dates for the dates used to scale geologic time.

This is how it works:

"Oh, look at that rock, how old is it?"

First - What is it's relationship to other rock? Is it above or below a volcanic rock? if it is above then it is younger than the volcanic rock; if below, then it is older.

Then - You take a sample of the volcanic rock and use radiometric dating to find out how old it is. This is normlly done with the Potassium-Argon measurments or Uranium-Lead ratio evaluation (See Wikipedia - I don't need to reproduce it here - ) 

Once you have a lot of dates you look at the stratigraphy of the rock you want to date and figure this out using the principals of stratigraphy to place the rock in a time-constrained period.
JimBob, Mon, 17th Aug 2009

Unless of course there is still viable biologic material of late pleistocene age or younger contained within the fossil that is in the sedimentary rock.    (I love being nitpicky with JimBob) frethack, Tue, 18th Aug 2009

Pleistocene or younger?

That is just dirt !!

JimBob, Wed, 19th Aug 2009

Sorry JimBob.  Of course I use "rock" in the loosest sense of the term.  Maybe what I should have said is "consolidated sediment and dirt" frethack, Thu, 20th Aug 2009

You have a lot to learn, Grasshopper.

Obviously, you are yet not evil - as per your sig. JimBob, Fri, 21st Aug 2009

how does someone know the half life of uranium when no one has ever lived that long, or even have documents that old. Some materials break down at a slow rate but when they get to a certain point, the breakdown increases dramatically. How is this figured when no one has lived a million years Charles, Fri, 28th Mar 2014

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