Kids' Paleontology: Rocks and Time

 Paleontology!  Old stuff! Realllly old stuff!

We're looking at traces of life from the recent (10,00 years ago), to the distant past (a billion years ago).  Yes, there was life for a couple billion years before that, but it was all single celled so... not as interesting?  Harder to show kids at any rate.

But I did create a one billion year time line!


If one side of a sheet of copy paper is 100,000 years, then the top sheet looks like this.

That bit at the bottom is recorder human history (5,000 years), and the line at the edge is all living people (about a hundred years).

That means two sides are 200,000 years, right? Using both sides, a ream of copy paper is 100 million years, and a case of copy paper is 1 billion years!

I was sorely tempted to get 3.5 more cases of paper so I could do the whole history of earth, but I contented myself with marking  where the cases would be.

Conveniently, the whole 4.5 billion year history of earth would be about the height of my ceiling.

But how do we know where things fit on that timeline?

To understand that, we need to know the rock cycle and radiometric dating.

 The rock cycle is easy: gather up all the broken bits of crayon in the house and peel off the paper.  First we talk about what breaks rocks down: people, wind, water, ice, earth movements, and chemical and biological weathering.

Then we put the crayons in a bag and "weather" them by taking turns whacking them with hammers.

When you are left with crumbly bits of sediment, fold them up in foil and have the kids squish them together as hard as they can. If you are doing this with younger kids, use newer crayons since they stick together more easily- you want them to stick, but still crumble if pressed. This is your "sedimentary rock." Note that you can still see the little pieces, and show them the same effect on sandstone.

Next, rewrap them in foil and hit them with a hammer.  You can also (with tongs) pass them through a flame.  These stick together much better, and represent metamorphic rock. Show them that the little pieces have started to change shape, and compare it to marble.  At this point, you might want to check the foil and rewrap it if the foil tore during the hammering phase.

Lastly, hold the foil packets over a low flame (stove top or candle) until they melt to form "igneous rock."  I compared this to pumice but any igneous rock works.

Once they understand how rock is recycled, you can explain why fossils are only formed in the
sedimentary layers.  We looked at a number of different rocks and fossils.

 I happen to have a large piece of mudstone filled with shell casts and molds (on the right in the picture), and it was helpful to show the softness of the rock (you can crumble it, it's really mud on the way to becoming rock), and how perfectly undistorted the fossils were.

But how do we know how old they are?

The layers the fossils are found in (or between) are dated by using radioactive minerals found naturally everywhere.  For fossils, it's mostly radioactive potassium which decays into argon very slowly.  As in, half of it decays (half life) in 1.3 billion years! That's long enough to date anything from before the planet was formed.  As opposed to carbon 14 which has a half life of  5,700 years - good for dating the last 60,000 years, but nothing older because it's all gone!

I gave the kids a quick overview of atomic structure so they could see that when the nucleus gets too big, having all those positive charges pushes it apart, breaking it into smaller atoms of completely different elements.  Magnets are a good way to show this, and it helps to have a periodic chart.

The important thing is that potassium is a solid, and argon is a gas.  Comparing how much you have of each tells you how long that rock has existed. Since they are inside a rock, the argon is trapped until the rock itself is melted down and the gas escapes, resetting the clock.

To show this, I made a giant candy bar! It started out about double this size.  I had the kids count to 3 repeatedly, and each time they got to 3, I snapped off another piece.

After a while, I stopped and asked them how long we had been doing it. No one knew, so we counted up the pieces and multiplied by 3.

That's basically how this form of dating works.  In real life instead of a straight count of argon (which would vary by sample size), it's the ratio of potassium to argon.

Also, geologists can't eat their sample!