A little fun with diagrams

There are two diagrams which are widely used to classify most igneous (lava) rocks. I have written about both of them before: QAPF diagram (for plutonic) and TAS diagram (for volcanic rocks).

Rock types like granite, gabbro, diorite, syenite, granodiorite, etc. can be defined with the QAPF diagram (shown below). These rocks have volcanic equivalents: rhyolite, basalt, andesite, trachyte, and dacite, respectively. It must be mentioned that the QAPF diagram exists for them too (same diagram with “granite” replaced by “rhyolite” and so on) but it is used not nearly as widely.

QAPF diagram for plutonic rocks.

TAS diagram for volcanic rocks.

We use the TAS diagram instead for volcanic rocks. I am not going into details why we use it instead of QAPF but what is really important to understand now is the fact that these two diagrams are completely different. QAPF uses minerals in the studied rocks while TAS is based on the chemical composition. And here comes the obvious and unavoidable problem. How do we make sure that rhyolite according to the QAPF diagram is indeed still rhyolite according to the TAS diagram and not dacite, trachyte or something else.

I am sure the creators of these diagrams have put considerable effort into solving this problem but they surely could not have avoided the inevitable outcome that one particular rock sample may be rhyolite according to one and dacite according to another classification diagram.

So I thought that it could be fun and even somewhat educational to try to put these two diagrams together. I can not say that they match perfectly, far from that. But I am still quite positively surprised that it is possible to do. I was sceptical about that before I started.

Here is the result. QAPF double triangle below (red) and TAS (blue) placed on top of it so that basalt is together with gabbro, rhyolite is with granite, and syenite is with trachyte. I had to squeeze the TAS diagram a bit for that and I had to rotate it about 125 degrees counter-clockwise to get the result.

It was educational for me and I’d even say that there is a certain amount of beauty how these so much different diagrams actually do match each other if you just tinker a bit.

QAPF (red) + TAS (blue). Plutonic rock types are red and volcanic rock types blue. Volcanic equivalents of each plutonic rock are below them and each label is more or less in the correct place on both diagrams.


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