MO 2.2 Explain how magma composition (silica content) and temperature affect the formation of different types of igneous landforms and volcanic hazards.

The CSK volcanic field is made up of a series of volcanoes, seamounts, and calderas that formed an arc due to oceanic-oceanic subduction. The subduction zone itself has moved since the beginning of formation here, but that's not the main idea to talk about. The magmas coming out of these volcanoes are typically going to be more mafic or intermediate, but subduction zones are complicated so there could very well be some felsic magmas in the mix as well. In the past, this area has had explosive eruptions with lots of tephra and pumice deposited. These hazards align with what we see in the rocks: there's several seamounts and large calderas that show past explosive eruptions that lean more towards the felsic magma. There's deposition of volcanic felsic rocks like rhyolite that also support this conclusion, as well as submarine lava domes.

MO4.? Comparison of magma series at different tectonic settings and associated major features.

So this builds a lot on the concepts we were talking about earlier in the course and also very recently. Both isotopes of helium gas are considered volatiles, which have a big impact on the ways that magmas behave before and during an eruption. In this case there is no eruption, but as we see in Figure 5C there is significant degassing of these volatiles in the hydrothermal field. They are similar to the degassing composition of mid-ocean ridge basalts, which come from magma degassing deeper in the mantle. The MORBs are mafic, while the magma here could be any combination of felsic to intermediate to mafic. The magma still in the mantle that is degassing here is likely mafic, as that silica concentration doesn't hold onto gases as much as a felsic melt would. Previous eruptions have generated felsic rocks, so it is possible that the degassing is just fighting its way through a felsic magma. But either way, this shows that the mantle in this area is rising similar to the way it would at a mid-ocean ridge. The melting process is different, but the upwelling is similar and contains similar helium isotope signatures.

MO 2.1 Distinguish between different types of igneous structures.

I was never the best at this learning objective, so this explanation of how the Santorini caldera came to look the way it does today resonates with all the studying I did earlier in the course. I honestly just spent a lot of time staring at examples of igneous structures, and the caldera at Santorini is a great example of both a caldera and an island volcano. you can see from Figure 3 that it used to be a much larger volcano, and eruptions and other following seismic events have caused different collapses over time. It's a great example of how calderas can evolve and how they don't always look like the best examples of uniform circles or rock layers.

MO 1.3 Identify major igneous rock types on the basis of their texture and chemical or mineral make-up, using appropriate classification diagrams.

What's interesting to be about the description of this rhyolite is that they specify biotite-bearing. Rhyolite is the name for the extrusive crystallization of felsic magma, and it's on the diagram that compares alkali to silica content. I believe it's more common to see biotite in hand sample in granites, as when it's spread out in smaller crystals in rhyolite it becomes more of a glittery sheen than defined crystals. To specify this rhyolite as biotite-bearing, I wonder how high the biotite content actually is. Biotite is a phyllosilicate mineral that does contain potassium, so it's possible that an increased amount of biotite could impact the placement of the rhyolite on the TAS diagram.

MO4.1 Explain why melting occurs in different plate tectonic settings.

Interestingly, these magmas, if we think of the subduction zone as uniform, should be more similar than they are. However, there are marked differences in the incompatible elements found at each of these magma sources. We know that the melting that occurs at subduction zones creates a magma that is a combination of mantle wedge and partial melted oceanic crust from above. It's the same oceanic crust, so there has to be two different currents of magmas with different incompatible element enrichment relatively close together. It sounds like one is rising from further in the mantle thatn the other, as one is more enriched.

MO4.2 Distinguish between alkaline, tholeiitic, and calc-alkaline magma series on geochemical diagrams and identify tectonic settings in which you might find each.

The three eroded submarine edifices referenced here are all made of calc-alkaline magma series. This is a great example of how this magma series appears only in subduction zones, which is what's happening here. This is actually a reminder that the Mediterranean is very tectonically complex, as for the longest time I didn't know it was a true convergent boundary. These edifices show that there was active subduction and melt of the mantle wedge, which generated the calc-alkaline series seen here.