How do you explain to kids how sedimentary rocks form?

Answer 1

If you have access to soil / dirt, get about a cup. If you can't get soil / dirt, you can buy a box of Baking Soda for 0.50 to $1.00 per pound box. Get an old small box and old newspapers. You can also get a tall black garbage bag, and cut it so you can open it up completely. Lastly, get a half cup of cool-cold water, and an eyedropper or teaspoon.

Lay the garbage bag on the floor to protect the rug/floor. Lay a couple sheets of an open newspaper in the middle; save the rest. Next, put your open small box on the newspaper.

Sedimentary rock is typically composed of sand. But explain to the child that it can take millions of years for sand to compress to make rock. So, we''re going to try with either dirt, or baking soda. I think Baking Soda will harden faster so try it first.

Put a thin layer of baking soda in the box -- just a couple inches long and a couple inches wide. Carefully add a verysmall amount of water across the soda, just to make it wet. Important to just use a small amount of water! You can use a Popsicle stick to mix it, or a knife, or even your finger.

Now, take another sheet of newspaper and fold it so it fits over the Soda mixture. PRESS down on the soda, REALLY hard. Explain that this pressure is what happens to sands and sand mixed with soil, which comes from rocks eroding.

You can put a lamp (100 watt bulb) above the mix and let it dry. Check in 30 minutes, and 1 hour, to see how much it has dried. Leave the newspaper square on top... add a new layer of Baking Soda and another small amount of water... and again, PRESS down. Let that layer dry.

Explain that water moves dirt particles from rocks, moving the dirt down hills and down streams, etc. Each layer adds pressure from above/top, which presses out the water. After many thousands of years, the grains of sand/dirt become stuck together and don't move as much when water comes across it. More & more layers get added, but they aren't all neat and even. After millions of years, there's been a lot of pressure and many hundreds of layers. If you could cut the rock, you would see layers with a mixture of colors-- tan, red, yellow, etc, depending on what elements were in the particles.

Looking at the Baking Soda "rock", it is likely crumbly. It breaks apart easily with only 2 layers. If you move a piece of it and add water, the material would easily "run"--- it has not dried for millions of years as more layers got added. The piece you moved will not go back into the rest of it and cling together again right away--- it would again need time and pressure.

*If you try the dirt, it will likely not cling together as easily. It would need days to dry.