Open University – Ice Cubes and Igneous Rocks

Dear Readers, as I mentioned last week I have just started my second year at the Open University, and for this module we actually get to do some science stuff. This week we are looking at different kinds of rocks, and, believe it or not, you can simulate what happens when volcanoes throw out lava and igneous rocks are created simply by looking at the effects of cooling on different solutions.

Another aspect of our work this year has been that we will be collaborating with one another on our results, so we all got different experiments to do. I fear that I might have drawn the short straw this time, because I had different solutions of bicarbonate of soda to work with, and I fear that they don’t do anything interesting at the temperature of a household freezer. Alas, if only I’d had the kind of freezer that we used to have when I was a student, with an icebox – these have an average temperature of -12 degrees Centigrade, and I suspect that this would have produced some more interesting results. My freezer runs at -18 degrees Centigrade and what I ended up with was three ‘ice’ cubes that looked identical, in spite of the different concentrations, plus one standard ice cube as a control.

The ‘ice’ cubes containing bicarbonate of soda were all interesting in one way, however. They had all gone white (the solutions were clear when they went in), and they were all extremely fragile, breaking into shards when I extracted them, so clearly something has happened. Alas, in the groups who used sugar and salt solutions, I imagine there might have been some crystals – when the molten rock is thrown out of a volcano it can cool to produce all sorts of effects, depending on the speed at which it cools. For example, two different samples of magma that cool at different rates can produce two very different rocks. Granite is normally the result of magma that has cooled slowly deep within the earth, allowing lots of different kinds of crystals to grow to a perceptible size. No wonder it’s so popular for kitchen work surfaces.

Photo One by By I, Friman, CC BY-SA 3.0,

Granite with lots of different mineral crystals that have developed as the magma cooled slowly (Photo One)


This is a sample with exactly the same mineral composition, but where the lava cooled so quickly that crystals didn’t have a chance to form – not surprisingly, this kind of rock is created where the magma is exposed suddenly to air or to water, lowering its temperature swiftly. Those who watched Game of Thrones might recognise this as obsidian, and it is completely clear, with no visible crystals at all.

Photo Two by By Ji-ElleIt feels nice and warmIt feels like a ________ - Own work, CC BY-SA 3.0,

Obsidian (Photo Two)

And so, in our experiment we wanted to see what would happen to our mineral solutions when they were cooled to a set temperature, just as would happen if some volcano shot out some magma (not likely in East Finchley but you can never tell!) Later, we will pool our results, and I’m sure that some people would have had more interesting results. Maybe I’ll cheekily have a go at one of the other experiments later on in the week just to see what happens. Until then, I’m off to find out about metamorphic and sedimentary rocks.

What I find extremely interesting, even at this early stage, is how the earth is continually recycling itself – rocks come to the surface, are eroded, deposited and in the fullness of time become other kinds of rocks. I never realised how fascinating geology was! So let me hear your geology stories if you have any – I feel as if I’m entering a whole new world, and am already excited about how rocks, and the things that come from them such as soil and sand, impact on the entire ecosystem.

Photo Credits

Photo One By I, Friman, CC BY-SA 3.0,

Photo Two by By Ji-ElleI Own work, CC BY-SA 3.0,

4 thoughts on “Open University – Ice Cubes and Igneous Rocks

  1. Claire

    This Open University course sounds very interesting. This experiment reminds me of our physics courses at school.
    Geology was another course, Natural Sciences .At the time, we studied geology for one year( between ages 12 and 13). I loved it, especially because, at the time, we lived in Auvergne, with a whole horizon of ( extinct) volcanoes in front of us. Every trimester, we went on a class excursion: traces of ancient volcanic activity, a bituminous source, metamorphic rocks on site… I regret not remembering the name of this excellent teacher…My parents were interested in science too, so we went looking for amethyst and other minerals which are common in the mountains ( at the time, people were not so aware of the necessity to protect natural resources).

  2. Anne

    Geology has always fascinated me. My father, who worked in a gold mine, introduced us to all sorts of different rocks found all over the country and explained how the scenery we take for granted is shaped by what lies underneath. I thus travel through the country constantly amazed at the history of what I see.


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