Did you know that petrified rock is actually a fossil?

In this simple STEM lesson, we are going to learn a bit about petrified rock and make a petrified sponge!

Let's dig in...

Your Simple STEM Lesson on Petrified Rock

A brief explanation of petrified rock

Petrified rock is a type of fossil known as a replacement fossil. Replacement fossils are replicas of things that were once alive, such as trees, sea creatures, or the bones of an animal.

The living thing dies and is trapped in the ground. As it lays there, it is covered by mineral rich water. The process of decaying begins and as this happens, the organic matter from the living thing is replaced by a hard mineral called silica.

The minerals fill in the spaces and create a replacement, or replica, fossil of the living thing. Pure silica quartz crystals are colorless, but petrified rocks come in array of colors due to the presence of other elements, such as manganese, iron, and copper.

How to make a petrified sponge

You will need the following for this science activity:

• A pot
• An old tin can
• Tongs
• Popsicle stick
• Hot mitts
• Wax (either from a taper candle or a block of wax)
• Foil muffin cup liner (or aluminum foil to line a muffin tin)
• A muffin tin
• A cellulose sponge (a.k.a. a kitchen sponge with lots of holes of differing sizes)

Here are the steps to make your petrified sponge:

1. Fill the pot a little less than halfway with water and set it on the stove to bring the water to a low simmer.
2. Meanwhile, break up the wax into smaller chunks that will fit in the can and cut off a piece of the sponge so that it fits into one of the cups in the muffin tin. Line the muffin tin with the foil muffin cups (or aluminum foil) and set one sponge piece into each cup.
3. {Adults Only} Once the water is simmering, place the wax in the can and use the tongs to set the can into the water in the pot. As the wax melts, make sure that none of the water gets into the can and make sure that there is always water in the pot. You can use the popsicle stick as a stirrer, just make sure you have your hot mitts on when you do this.
4. {Adults Only} Once the wax has melted, put on the hot mitts and use the tongs to remove the can. Gently tip the can so that a bit of wax pours onto the sponge. Do not fully coat the sponge, just a little bit at a time so that the sponge will absorb the wax rather than float in it. To get the best results, take your time to pour the wax over the sponge and use the popsicle stick to press on the sponge, helping to massage the melted wax into the holes. You want the sponge to absorb as much of the melted wax as possible.
5. Wait for the wax to fully cool and harden before taking out your petrified sponge. 

The melted wax seeps into the holes in the sponge, filling up the spaces with liquid. As the wax cools it hardens and the sponge "petrifies," becoming hard as a rock.

In the same way, water rich with minerals seeps in the cracks and spaces found in dead organic matter, like trees. Over time it hardens, and the organic matter becomes a replacement fossil known as petrified rock.

Digging deeper into petrified rocks

Check out the following links to learn more about petrified rocks:

• Petrified wood colors
• A tour of the Petrified National Forest {Video}

Wrapping Up Your Simple STEM Lesson

So now you know that petrified rocks are really replacement fossils! I trust that you and your students enjoyed making your own petrified sponge.

If you want to learn more about fossils, check out one of our earth science programs:

• Earth Science & Astronomy for the Grammar Stage
• Earth Science & Astronomy for the Logic Stage
• The Sassafras Science Adventures Volume 5: Geology
• Lapbooking through Rocks and Fossils