eli5:How are rocks dated through radiometric dating when all of the matter that Earth is made of is of the same age and should decay at the same rate despite the the time it was deposited at the surface of the Earth?

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How are rocks dated through radiometric dating when all of the matter that Earth is made of is of the same age and should decay at the same rate despite the the time it was deposited at the surface of the Earth, it shouldn’t matter if a volcanic rock was deposited a billion years ago or a thousand years ago if the rate of radioactive decay is the same ?

In: Geology

A common dating method for igneous rocks is K-Ar dating (Potassium-Argon.)

Radioactive potassium breaks down into argon. When the rock is molten, the argon boils away. When the rock cools down and solidifies, the argon gets trapped in the rock.

Measuring the proportion of argon in the rock, relative to the proportion of potassium, tells us how long ago the rock solidified.

Not just the Earth. Technically, all matter in the universe is the same age.

However, matter can be transformed. You know how Lego bricks can be attached together in different ways to make different things? Well, matter is made out of something called particles, which are like tiny little Lego bricks. You can make lots of different things with Legos, and similarly, you can make lots of different things with particles. When you take Lego bricks apart and stick them back together as something new, they’re still the same Legos. Likewise, when you take particles apart and rearrange them as something new, it’s still the same matter. However, the structure is different. The Legos will look and behave differently. Matter is very similar.

Like Legos, particles can sometimes be arranged in ways that are unstable. Look at the Legos below. These might stand for a little while, but eventually they’re definitely going to topple down.

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Compare that to this Lego structure. This one looks a lot more stable!

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Okay. Now go back to the unstable structure. It’s tipsy, right? But the thing is, we don’t know how long it will take for it to fall. We just know that it’s probably going to happen some time or another.

But what if we had a thousand of those tipsy Lego structures! Over time, more of the Lego structures will have fallen down. This is something that we can know for sure.

Now imagine there are two rooms, each filled with a thousand of those tipsy Lego towers. You walk into one room, and see that only ten of the Lego structures have fallen down. You walk into the other room, and see that nine hundred have fallen. Which group of Lego structures do you think has been around for longer?

The answer is the second room, with the 900 toppled structures. Why? Well, it makes sense that the older room would be the one with more toppled structures, because the structures have had more time to topple.

That’s how radioisotope dating works. We’re looking at unstable STRUCTURES of particles, and seeing how many are still left. So what we’re actually measuring isn’t how old the particles (ie Lego bricks) themselves are, but how long ago those particles were stuck together in that particular structure.