[Andrew Cottrell]

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?

[LeeE]

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.

[Mazurka]

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.         

[stereologist]

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.

http://en.wikipedia.org/wiki/Index_fossil

Here's a good place to lok.

[nicephotog]

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).

[Bored chemist]

C14 dating is only good for things up to about 10 times the half life of C14 (5730 years). That rules out most fossils.

[frethack]

C14 dating is only good for things up to about 10 times the half life of C14 (5730 years). That rules out most fossils.

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.

[JimBob]

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 - http://en.wikipedia.org/wiki/Radiometric_dating ) 

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.
 

[frethack]

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.

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)

[JimBob]

Pleistocene or younger?

That is just dirt !!

[frethack]

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"

[JimBob]

You have a lot to learn, Grasshopper.

Obviously, you are yet not evil - as per your sig.

[thedoc]

Go to the podcast page.
or [chapter podcast=2549 track=10.04.11/Naked_Scientists_Show_10.04.11_6044.mp3] Listen to the Answer Now[/chapter] or [download as MP3]

[Charles]

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