Video 1.6: Using radioactivity to date rocks - Dr. Ray Burgess

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Our Earth: Its Climate, History, and Processes

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University of Manchester

Our Earth: Its Climate, History, and Processes

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Develop a greater appreciation for how the air, water, land, and life formed and have interacted over the last 4.5 billion years.

À partir de la leçon

Building Blocks of Earth’s Climate System

Video 1.0: Introduction and Philosophy7:45

Video 1.1: How does science work?6:21

Optional Video: How do scientific papers get published?10:50

Video 1.2: Introduction to the Earth's climate system8:49

Video 1.3: How do we measure geologic time?9:50

Video 1.4: Geological Time Scale Song2:50

Video 1.5: Minerals and Rocks2:59

Video 1.5.1: Igneous Rock3:03

Video 1.5.2: Sedimentary Rock6:15

Video 1.5.3: Metamorphic Rock5:39

Video 1.6: Using radioactivity to date rocks - Dr. Ray Burgess7:48

Video 1.7: Using stable isotopes to understand Earth processes - Dr. Ray Burgess6:15

Video 1.8: How do we know how old the Earth is?5:46

Video 1.9: What are those rocks doing lying around?10:48

Rencontrer les enseignants

Prof. David M. Schultz
Professor of Synoptic Meteorology
School of Earth, Atmospheric and Environmental Sciences
Dr Rochelle Taylor
Postdoctoral Research Associate
School of Earth, Atmospheric & Environmental Sciences
Dr Jonathan Fairman
Postdoctoral Research Associate
School of Earth, Atmospheric and Environmental Sciences

So, we're here at the University of Manchester, and

I'm with Professor Ray Burgess, who's professor of isotope geochemistry,

and want to learn about isotopes, radioactive dating.

So, to start out with, what's an isotope?

>> Well the basic building blocks of atoms are protons, neutrons, and electrons.

And it's the protons and neutrons that form the nuclei of atoms and

also give them their mass.

And the chemical identity of them is defined by its number of protons.

So for example, a carbon atom always has have to have six protons in its nucleus.

If you vary the number of neutrons than you change the isotopic

form of your element.

So for another example for carbon again.

We know that carbon has six protons.

There are two stable configurations of carbon atoms.

One containing six neutrons and another containing seven neutrons.

So if we add together the number of protons and

neutrons in each of those, we end up with a carbon 12 atom, and a carbon 13 atom.

>> And then some of these, you said those were stable,

there are some that are radioactive as well, what does that mean?

>> Well, yes that's right, so

some isotopes will undergo spontaneous disintegration.

And they do this because they either got too few or

too many neutrons in their nuclei.

And they're called radioactive isotopes because they emit ionizing radiations.

That includes things like alpha particles, beta particles and gamma rays.

>> And what are these alpha, beta and

the other particles that you are >> Yeah, so

they're bits of the nuclei that are broken off from the parent isotope.

So some of them are quite massive.

Like alpha particles, are quite heavy.

And beta particles, some of those are just things like electrons.

>> Mmm, okay.

Now, if the particles are protons.

[CROSSTALK] >> Yeah, two protons, two neutrons.

>> Oh, okay. >> Yeah, so essentially, a helium nucleus.

>> Oh, okay.

All right.

And so these isotopes that are radioactive will emit these particles.

What determines how or when these isotopes decide to emit these particles?

>> Yes, so it's difficult to predict when they're going to break up.

So we talk about them in terms of probability.

And this is where the idea of the half-life comes in.

>> So you can't necessarily predict that particular atom will release particles.

In other words decay.

But, if you look at a sample as a whole then the probability of a certain

number of them being emitting these types of radiation becomes easier to define.

>> That's right.

So you need to, when you're dealing with a rock or a million,

you're usually dealing with millions of atoms, so knowing whether a particular one

is going to decay on a particular time scale doesn't really matter.

You're dealing with a very large number of particles.

>> So these radioactive isotopes then,

we can't consider each one and determine whether it's going to decay.

But we can consider the bulk and determine the probability then,

of a certain number of these decaying.

What's the best way that we can measure this decay rate?

>> Well, the easiest way of measuring it is in terms of the half life.

And this is the time it takes for

half the number of parent radioactive isotopes to decay.

So let's consider an example where you have 100 atoms,

of a parent radioactive isotope.

And you anticipate the probability that 50% of these will decay in one hour.

So after one hour has elapsed you will have 50 of the original parent atoms and

50 of the daughter isotope that's formed.

And then if you allow another hour to elapse after that,

you'll add another 25 daughter atoms through the decay process.

And we can think of this in terms of a half life.

Where the half life is the original number of radioactive atoms that are remaining.

>> Okay, so by looking then at the ratio of these daughter atoms

to the number of parent atoms still remaining in the rock,

we can then determine something about how old the rock is.

>> That's right.

Yes. So this forms the basis of geological age

dating using radioactive isotopes.

So I'll give you some examples first perhaps.

So some common ones that we use in geology are the decay of potassium 40,

a radioactive isotope of potassium, that decays to stable argon 40.

And this occurs with the half life of about 1.2 billion years.

>> Hm. >> Another example would be the decay of

rubidium 87 to Strontium 87.

And this has a much longer half life at 48.8 billion years.

>> Wow. >> So, we can still use these

decay processes, even though they occur over extremely long time scales.

>> Hm, and so, maybe,

what some people watching this may have heard of is Carbon 14 dating.

>> Yeah.

>> What's, how does that compare to those two, which have billion-year half-lives?

>> Well, Carbon-14 is an isotope that's formed in the Earth's atmosphere,

and it decays over a time scale of about 10,000 years.

So it's useful for dating

processes that are on the time scale of maybe 50,000 years or less.

But in terms of over geological time, it's a relatively short indicator I see.

>> And so if we were to take a rock sample and

you were to run it through your lab and determine this ratio then of parent to

daughter isotopes and came up with an age of this rock is two billion years old.

What would that represent?

What exactly happened to that rock two billion years ago that we are dating,

what does it mean?

>> Okay so there are a couple of things here.

Firstly it's the sort of concept of what an age actually represents.

So for some rocks that's very easy to think about so

if your considering a lava for example that could have been the point

when the rock crystallized and actually traps that moment in time.

Whereas if you're thinking about a metamorphic rock,

well a metamorphic rock forms over a much longer time period.

And it could take millions of years for that rock to form and cool down.

So there the age is a much more less well defined moment in time.

But essentially what you're doing is you're capturing the moment that

the daughter and parent isotopes became frozen in that sample.

>> Mm. >> So the point where they

can no longer be lost from the sample itself.

>> And for sedimentary rocks, that would be the time at which those grains

were formed in the parent rock, not necessarily the sediment itself.

>> Absolutely, yes.

>> So you can't really use radioactive

decay as a measure of when these sediments were put down.

>> Not so easily, and if there are new minerals which are formed during

the sedimentation process then they are amenable to dating.

So that can give the age of the sediment.

But you're quite right.

Very often what you're measuring are the ages of the parent rocks

that were eroded to form sediments.

>> Okay. Well thank you Ray.

And thank you for you're time.

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