Volcanic eruptions and rock formation

We started off our third Marrickville science session by watching a short animation from the BBC about the structure of the Earth and how this relates to volcanoes. The children were not all paying attention at this point, but the YouTube clips of volcanoes went down well and prompted some discussion. We talked about different types of eruption and how hot the lava gets.


Essentially, volcanoes (and earthquakes) occur near the edges of continental plates. New rocks are formed when magma cools. Even with the same minerals in the magma, the rocks that form may look very different depending on how they have formed. If the magma cools slowly you can get very large crystals forming in the rock. In general, the faster the cooling, the smaller the crystals. (This is something that you can test out yourself with more crystal growing experiments. I will try to post some more instructions in the next week.)

If magma cools really quickly you get obsidian, which is one of the rocks featured in Minecraft and which is also actually a glass. In a glass the particles are not held together in regular repeated units, but in varying orientations relative to each other.

There is also pumice, which is quite a unique rock that actually floats in water. This contains many holes (it is porous) due to the gases from the volcanic eruption.

My middle son remembered a game he’d played with a friend where you can vary the conditions of a volcano to see what kind of eruption you can get. One of the other mums found the link, and I’d encourage everyone to have a go at this:

Discovery Kids Volcano Explorer

I had planned to split the group into younger and older children but they all wanted to come out into the garden together. We put out some different rocks on the table and tried to identify which were igneous (formed from magma), sedimentary or metamorphic. I hadn’t really talked about the other two types of rock but we had a few pictures of the rock cycle and an A4 page with some descriptions and pictures of typical rocks to help us. I can e-mail copies of these to anyone interested. The New Zealand website Science Kids has some information and pictures. You might also like to look at Rock HoundsKids Love Rocks or KidsGeo.com. The last one is more wordy.

We had samples of pumice, granite, basalt, sandstone, limestone (with fossil crinoids), mica schist and several that I hadn’t been able to identify myself! You can find pictures of all these rocks on Geology.com, and crinoids are explained here with loads of good pictures. (If you do an internet search for Australian crinoids you can find out about ones that are still alive today.)

Personally, I find the metamorphic rocks the hardest to identify. Sometimes it’s hard to tell the difference between them and igneous rocks that have very small particle size. Hand lenses (magnifying glasses) can help but in some rocks, the crystals can only be seen with a microscope.

I had brought along some colouring sheets for the younger children but by this point they were all more interested in playing in the garden.

A couple of them, however, were enticed back when I produced an obsidian spearhead. They then wrote down a list of all the rocks in Minecraft and identified that all but about two existed in real life.

Next time we plan to make fake ‘fossils’ from modelling clay and plaster of Paris. We will also look at chocolate versions of the rocks we were looking at in the last session. I found a good rock identification game on KidsGeo.com which I suggested my students tried out in preparation for the next class. (Watch out for the typo where they wrote calcium instead of carbon!)

We might also try making sugar crystals and honeycomb. It won’t be a good day for our teeth but I hope it will be a fun way to end the classes.

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