People make classification schemes and Nature might not follow the rules

March 2, 2020 12:05 PM

Most of the world’s volcanoes can be explained with just two conditions for magma production: pressure, and the mixture of water. Water-laden seafloor plates can trigger melt as they sink down into the Earth at subduction zones. These two facts generally explain both volcanoes along plate boundaries—like the Pacific Ring of Fire or the mid-ocean ridges—and those at hot spots like Hawaii and Yellowstone. But when looking back through Earth's history, there are plenty of volcanic weirdos that don’t seem to line up with the figures in a textbook. Volcanoes where they shouldn’t be? Maybe it’s a mantle sponge -- Hypothesis explains eruptions in China and off the coast of Japan. (Ars Technica; abstract at Nature) Also, those volcano figures in textbooks are bad, oversimplifying the wonderful world of volcanoes (Oregon State University, Volcano World).

More from Volcano World's page on types of volcanoes:

Many people are interested in ways to classify volcanoes. There is probably a natural human instinct to try and give labels to all things. This is not a bad instinct and many times it makes it easier to understand the particular thing that is being classified. For example, you start to identify patterns when you classify things and these patterns may lead to a better understanding of whatever it is you are classifying. However (and that is a big "however"), when you are classifying natural things (they might be fish, plants, birds, oceans, minerals, volcanoes, or whatever), you MUST remember that the classification scheme is made up by human beings and Nature might decide to not follow the rules exactly. There will ALWAYS be exceptions to your classification scheme and there will ALWAYS be things that fall into more than one category. As long as you realize this and it doesn't bother you, you'll be just fine. Certainly there are different ways to classify volcanoes and all of them have particular benefits and drawbacks. These include classifying by lava chemistry, tectonic setting, size, eruptive character, geographic location, present activity, and morphology. As an example of how these can get mixed together, note that there are basaltic strato volcanoes (i.e. Mt. Fuji), big basaltic calderas (i.e. Taal), big gradual-sloped basaltic shields (i.e. Mauna Loa) and big steep-sloped basaltic shields (i.e. Fernandina). Additionally, although most volcanoes associated withsubduction zones are steep-sided andesite or dacite cones, there are a few basaltic shields along these zones as well (i.e. Masaya, Westdahl, Tolbachik). These examples highlight the above-mentioned hurdle that any student of the Earth needs to get over - Nature makes exceptions to human rules.

Unfortunately, there is one particular volcano classification system that many people think is the only system. Not only is it not the only system, it is not a very good system. This is the famous "3 types of volcanoes" (shield volcanoes, strato volcanoes, and cinder cones), and it is found in many textbooks from elementary school to college. Why is this 3-types scheme so bad? First, it has no place in it for large caldera complexes (such as Yellowstone), flood basalts, monogenetic fields, or mid-ocean spreading centers. These are important types of volcanoes that you would never hear about if you thought there were only 3 types. Second, although you can occasionally find a cinder cone sitting somewhere all by itself, it is way more common for a cinder cone to either be one of many vents on a large (polygenetic) volcano or a member of a monogenetic field. Finally, if you actually think about the system you run into logical problems, as a teacher from Pittsburgh pointedly complained to VolcanoWorld about: She wanted to know how Pu'u 'O'o could be a cinder cone on Kilauea if cinder cones are a type of volcano and Kilauea is a shield volcano. The answer is that Pu'u 'O'o is one of hundreds of vents on Kilauea, and it happens to be a cinder cone.

posted by filthy light thief (4 comments total)

ooooh thanks so much FLT!!! my birthday isn't for a few more weeks but I feel like this post is just for me :D

I look forward to digging in once this pesky work stuff is taken care of!
posted by supermedusa at 12:53 PM on March 2, 2020

Vulcanology is one of those subjects which really makes me regret not choosing a science path back during late schooldom. It's a rabbit hole of technical depth combined with geographical and historical breadth, and it generates passionate debate that's (at least relatively) accessible to interested lay readers.

For more in this vein, I'd wholeheartedly recommend checking out the fantastic blog Volcano Cafe.
posted by protorp at 1:26 PM on March 2, 2020 [4 favorites]

It IS my birthday today and I am so happy to have this new information. My sister and I were informally studying plate tectonics and got totally volcano crazed during the the 2018 Kilauea eruption. These articles address some of the questions we have about unexpected locations of volcanoes. Thanks filthy light thief!
posted by a humble nudibranch at 4:29 AM on March 3, 2020 [1 favorite]

So disappointed that mantle sponges are not actual living sponges that can fake looking like a volcano.
posted by dances_with_sneetches at 9:35 AM on March 3, 2020