A few billion years ago, the particles which would come to make up our planet Earth were simply hanging around in what is called the solar nebula, or the protoplanetary disk. Gradually, over a few more million years, these particles started to stick together, and the gravitational pull of the larger particles, plus their sweep through the disk as they orbited the sun, meant even more bits stuck on. This process was very slow but eventually our Earth was formed, something like 4.4 billion years ago. It had an interior of molten metal and a thin rocky crust. The crust survived many bombardments of meteors and the interior started to rearrange itself into the core and mantle as we know now.
This is roughly the story which I told to the older group in their second class this term. I had made a model of the Earth out of modelling clay, with its inner and outer core, and inner and outer mantle, and a thin crust.
I thought it would be good for the children to be aware that the crust of the Earth covers a liquid mantle, and that the crust is relatively thin compared to the rest of the Earth. I hope they will remember this when we start to talk about plates moving over the surface of the Earth.
I presented them with a very rough map of the world with the continents cut out in cardboard. I wanted them to follow the thinking of Alfred Wegener in the 1920s when he was trying to put together his theory of continental drift, taking account of the evidence of the time.
I talked a bit about sedimentary rocks and how they can be evidence for the climate in that area at the time the rocks were formed. I also talked about fossils of the same type being found at great distances from each other, on different continents.
Each child designed a symbol for one of these pieces of evidence and they took turns sticking their symbols onto the world map. The aim was to use the map with symbols as a kind of jigsaw, trying to match up areas with the same sedimentary deposits, or the same fossils.
Everyone enjoyed their drawing and placing their symbols on the map. It was good to see the children coming to a kind of collaboration when they were rearranging the pieces. It is a good group to teach, although I feel I should still do more to involve the quieter ones.
I had also mentioned volcanoes, so most of them wanted to draw a volcano or an earthquake too. We will stick these on the map next week when it will become clear that these tend to occur at plate boundaries. We will also talk about the different types of plate boundaries, and sea-floor spreading. We will briefly talk about how to date rocks, and start to build our models of sedimentary rocks ready for testing the week after.
The younger group, in the meantime, had been having fun looking at pictures of fossils, at samples of sedimentary rocks (sandstone) and playing with dinosaurs and modelling clay.