In the 20th century, geologists made a timely discovery: Rocks contain atomic
clocks. They enable geologists to calculate when a rock formedits
absolute ageby measuring its radioactive elements. At last,
geologists were able to attach years to the fossil-based, relative
time scale and figure out, for example, exactly when the dinosaurs
ruled the Earth.
Man: Most of the life we know about, including these old guys [referring to
dinosaur fossils], is young compared to some of the rocks in this place.
Woman: Rocks. Old. This rock is nearly 1.7 billion years old. And this one:
this rock is nearly 4 billion years old. Its one of the oldest pieces of the
earths crust ever found.
Man: This rock is 1.1 billion years old.
Woman: You know how we know that? There are clocks in this rock.
Man: Ah, right, I can hear it ticking.
Woman: No, no… whats ticking is this Geiger Counter.
Man: Its counting geigers?
Woman: Counting events.
Man: What kind of events?
Woman: Radio-active events.
Man: What was that?
Woman: A radio-active event.
Man: Youre going to explain?
Woman: I am. Most of the elements in the world, the oxygen we breathe,
carbon, the silicon in glass, are atomically stable.
Woman: But not all. That one, thats uranium. Its naturally unstable.
And uranium atoms, every so often, something happens.
Man: An atom will decay.
Man: Spontaneously - all by itself.
Woman: Youre going to…
Man: I am. Decay. Some of the atoms in the nucleus will suddenly just
escape - burst of energy - get away!
Woman: Outta there!
Man: Whats left is a lighter element, truly different stuff. Uranium
decays, through different unstable steps and becomes lead.
Woman: Now this takes awhile. In fact it takes nearly 4 1/2 billion years
for half of the atoms in any given quantity of uranium to decay into lead.
Man: But theres jillions of atoms there, so theres plenty of
radio-active events happening.
Woman: Ok well, what about this one? This is a piece of granite
from Half Stone in Yosemite.
Man: Theres uranium decaying in there too.
Man: Actually, theres uranium in all rocks, just way less than the
Geiger counter can pick up.
Woman: Well then how do we find out how old this is?
Man: Inside this granite is a mineral called zircon.
Woman: Those are the grains of the rocks. Lot of different minerals.
And way down inside, that teeny white crystal is a zircon.
Man: Needle in a haystack.
Woman: Those are the atoms that make up the zircon. Crystal. Very neat.
Man: Heres the really neat part. When a zircon forms it incorporates uranium
atoms into it self, here and there, like this yellow one in place of zirconium
atoms these green ones.
Woman: And from the time the crystal forms uranium atoms are decaying into
lead. They are the clocks in the rocks.
Man: The longer the time, the more lead there is and we know how long
it take uranium to decay, so we figure out how old the rock is by comparing
how many uranium atoms and how many lead atoms there are trapped in the
zircon today. Of course, piece of cake. Just counting
Woman: Ah huh.
Man: Oh, right. It might be a little difficult.
Woman: A lot! But geologists have figured out how to do it. It takes a lot
of steps and some terrific technology.
1. Crush the Rock!
2. Separate the minerals.
3. Select the zircon.
4. Isolate the uranium and lead.
5. Paint these elements onto a filament.
6. Place the filament into the mass spectrometer.
Man: Ok, thats the many steps. Heres the nifty technology.
Woman: The Mass Spectrometer is an instrument researcher use to count atoms.
Man: Only uranium and lead?
Woman: Oh no, no other radio-active elements give off different decay
clocks. Some clocks are faster.
Man: Hey, here come the results. Now just a little math and were done.
Woman: This piece of half-domed granite formed 86 million years ago.
Man: But why do we care how old rocks are?
Woman: Well, as a result of dating rocks, we know how old the moon is.
Man: How young the Hawaiian islands are.
Woman: When the Atlantic Ocean began to open up.
Man: And when the dinosaurs went extinct.
Woman: The decay of atomic nuclei has been recording the passage of
time since long before there was anybody to care.
Man: Now we can read that age-old log book of the earth and the
timeline of ancient life that proceeded us. We can tell geologic time by
the clocks in the rocks.