Ole Nielsen wrote an interesting post about the world’s highest volcanoes.
There are several candidates. Nevados Ojos del Salado and Llullaillaco being the most serious contenders depending on whether we count all of them or only the historically active ones. Which volcano claims the title is not of paramount importance to me. What is interesting is that they are all located in South America. This is nice to know that they are there but so what?
This is just a mere fact that gives us no further understanding of the mechanisms that allow these volcanoes to become so high. I thought that maybe there are some easy answers in the web but as it usually happens, there are tons of descriptions and trivial facts but very little really useful insight.
So I just tried to collect my pieces of thoughts together and come up with some ideas.
I know that the highest point on the surface of the Earth is the tip of Chimborazo volcano in Ecuador. Some of you may protest that at least yesterday Mt. Everest was higher. Yes, it most likely still is but Chimborazo is the highest point if measured from the center of the Earth. This is so because the Earth’s equatorial radius is longer than its polar radius (the difference is 22 kilometers). Chimborazo is definitely not the highest mountain if measured from the mean sea level (even not in South America) but it is nicely located almost at the equator. Its height from the mean sea level is 6268 meters.
How does that help us answer the question? Well, I just thought that if the volcano is farther away from the center of the Earth, then gravity should be weaker there which makes it easier for the volcano to grow little bit higher. Highest volcano in the Solar System is Olympus Mons. Its height (measured from the base) is 25 kilometers and it is located on Mars where gravity is much weaker that it is here.
We also have to consider the effect of centrifugal force because our Earth is rotating. This force gets stronger as we move higher. So it also helps to counteract the gravitational pull.
The value of gravitational acceleration (which takes into account both factors described above) is 9.78 (m/s2) at the equator and 9.83 at the poles. The difference is there but only about 0.5%. It means that we (and mountains) would weigh about 0.5% more at the poles than at the Equator. That is clearly not enough to explain why the volcanoes in the Andes are so high.
Maybe we have to blame the climate? Atacama desert that is right there next to the highest volcanoes is very dry. Perhaps weathering and erosion are not keeping pace with the uplift as successfully as it is doing in other, more humid regions? I think it might be one part of the answer.
The Andes are on the one hand compressional orogene that is situated at the border of Nazca and South American plates but on the other hand it is also a volcanic range that sits on top of the subduction zone. The highest peaks there are volcanic edifices. Perhaps they are so high because they sit on top of the orogenic belt.
Perhaps the subduction and mountain building in this region is particularly intense which allows the volcanoes to become so high? I don’t have good data for that but the sea floor spreading is very fast in the Southern Pacific, so the subduction should be intense as well.
Maybe the fact that the subduction angle there is very small helps to push the mountain range higher. It is small because the subducting slab is relatively young and therefore more buoyant than older slabs.
Last but not least, Earth’s surface is pushed higher in the regions where the crust is thicker. The crust beneath the Andes is very thick (even 60 kilometers) which is definitely one of the contributing factors.
So I found seven possible contributing factors:
1. Equatorial bulge
2. Centrifugal force
4. Orogene + subduction
5. Intensity of subduction
6. Subduction angle
7. Thickness of the crust
Let me know what else might influence the Andes or correct mistakes in my arguments if you found any.