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TITLE: Time Detectives

Scientists have discovered several methods to determine a rock’s age. Geologists can work out the sequence of events recorded in a rock and its age relative to others around it. Paleontologists can look for fossils in sedimentary rocks to figure out the rocks’ ages based on the worldwide geological time scale defined by fossils. Geochemists can use dating techniques based on radioactive decay to determine a rock’s absolute age.

[Photo Montage: 1) geologic section, 2) closeup of fossil, and 3) closeup of geiger counter.]



[Illustration: Geologic sections in a quarry, hill and canal.]

Relative Time
With this method, geologists work out the sequence of events recorded in a rock and its age relative to others around it. In 1815, William Smith—an observant canal engineer—used fossils in rock exposures like the three below to create geologic sections like the one on the left. By the mid-1800s, geologists developed a relative time scale based on fossil correlations of distant rocks. But they could not determine the rocks’ actual ages until atomic clocks were discovered in the 20th century.



What time is it? If you found one of these three index fossils in a rock, you’d know. Each animal was once widespread but quickly became extinct. Only a small percentage of fossils meet these criteria. Paleontologists look for fossils in sedimentary rocks to figure out the rocks’ ages based on the worldwide geological time scale defined by fossils.

[PHOTO: Sandstone]

Sandstone with Gastropods
Paleocene Epoch

[PHOTO: Shale]

Shale with graptoilites
Ordovician Period

[PHOTO: Limestone]

Limestone with ammonites
Jurassic Period



[Photo: Geiger counter with zircon specimen.] Atomic Clocks
In the 20th century, geologists made a timely discovery: Rocks contain atomic clocks. They enable geologists to calculate when a rock formed—its absolute age—by 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.

Video Transcript
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. It’s one of the oldest pieces of the earth’s 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: “Really?”

Woman: “Really!”

Man: “Ah, right, I can hear it ticking.”

Woman: “No, no… what’s ticking is this Geiger Counter.”

Man: “It’s counting geigers?”

Woman: “Counting events.”

Man: “What kind of events?”

Woman: “Radio-active events.”

Man: “What was that?”

Woman: “A radio-active event.”

Man: “You’re 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.”

Man: “Stable.”

Woman: “But not all. That one, that’s uranium. It’s naturally unstable. And uranium atoms, every so often, something happens.”

Man: “An atom will decay.”

Woman: “Yes.”

Man: “Spontaneously - all by itself.”

Woman: “You’re 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: “What’s 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 there’s jillions of atoms there, so there’s 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: “There’s uranium decaying in there too.”

Woman: “Really?”

Man: “Actually, there’s 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: “Here’s 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, that’s the many steps. Here’s 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 we’re 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.”

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Smithsonian National Museum of Natural History Department of Mineral Sciences website Credits