Andrew Cottrell asked:
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?
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.
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?)
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.
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
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.
Pleistocene or younger?
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.
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