Sunday, March 18, 2012

Sedimentary Rocks: Geology Lesson #4 for 8-9 year olds

We had a lot of fun with this class. Continuing with my geology rock type theme, we needed to discuss sedimentary rocks and how they get to be sedimentary rocks (erosion). A great visual resource on erosion can be found here: (The USGS, by the way, has some great resources.)

We talked about the different ways rocks could be eroded: wind, rain, ice, glaciers, gravity, and trees. I found several pictures around the internet to demonstrate each of these. I had pictures of glaciers, river deltas, land slides, hoodoos and more.

We then talked about sedimentation and how layers are put down over long periods of time. Those layers are eventually buried and put under more and more pressure. A great demonstration of this is showing a piece of paper and comparing that to the weight of a large book. The cumulative effect can be tremendous weight and pressure that squeeze the moisture out and eventually compact the particles into a rock, a sedimentary rock. Also, the size of the particles can be anything from boulders to silt and this determines the kind of rock.

I found a fun little activity online that was doable in a group like this. The lesson plan can be found here:
Each kid got their own jar. We placed pebbles, gravel, sand, and garden soil in each (enough of each to make a layer). We added water to cover it all and then some and sealed them tight. I asked them to write down predictions of what they thought would happen when they shook the jar and let everything settle. When they were done with their predictions, we let them sit. After they had sat a while, we looked at them again and wrote down what they saw. In many cases it was not what we would have predicted, but that is the nature of science. You make a hypothesis and then test it. The result may agree with your original hypotheses or it might not. While they were settling, I went on with the lesson.

I explained to them the various kinds of sedimentary rock using as many samples as I could get my hands on. has a great break down with pictures:

The first and most common type of sedimentary rock is Clastic. Clastic rocks have bits a pieces of rock all jumbled together. Breccia has fragments that are angled and Conglomerate has bits that are rounded (hence probably rolled down a stream for a while before solidifying). Shale is a clastic rock that is made up mostly of clay and Sandstone is made up of sand of course. Siltstone is made up of silt.

I have a marvelous friend who is also a naturalist. She gave me a large number of samples of shale she has collected in Rhode Island. These bits had small fossils of ferns and other plants. She urged me to hand them out, and so each kid was able to go home with a bit of rock from the Carboniferous era. They thought this was the coolest thing ever! I am so in debt to her as she provided me with a great many samples to share. We looked at some fossils and talked again about how fossils are formed, why many of them are in sedimentary rocks and how we can read rock layers (stratification or the Law of Superposition).

The second type of sedimentary rock is Chemical. These rocks are formed by chemical means (evaporates or precipitates). The best examples of this are rock salt or gypsum. Limestone can also sometimes be formed this way.

Finally, Organic sedimentary rocks are formed from remains of plants and animals. Limestone (calcium carbonate shells of clams, corals, bones, teeth, etc.), and coal are great examples of this.

I finished up with their FAVORITE thing, the Sedimentary Layer Cake! The whole lesson plan plus the cake can be found in this pdf.
Basically, you make a layer cake. I bought a couple of mixes and cooked them in small pans so that I had six layers in the end held together by a small amount of frosting. Food coloring was used to make each a different color. The edges can be cut so they are even, but don't frost it. You need to see the layers. A blob of dark colored frosting is then injected into the middle as unobtrusively as possible. I had some trouble with this. It might be necessary to hollow out a space in the middle. You also need some sort of clear tube to take "cores" with. I had some trouble finding something appropriate. I ended up cutting off the ends of a plastic packaging tube (like what those plastic animals come in), but you could certainly use something smaller. We took "core samples" predicting each time what we thought the cores would look like. The blob of frosting is like a hidden oil deposit. In real life, you can't crack open the earth to see where it all is, you have to take cores. We took a number of cores and the kids got to eat them.
There are other things we could have done, like make fossils, but we ran out of time.

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