When you start to study volcanoes you inevitably start by studying the top of the volcano and you inevitably model your understanding about the volcano in reference to the cone or mountain that you see. Some volcanoes are even referred to as “perfect volcanoes” due to them being almost perfectly symmetrical. But, as we will see, nothing could be more wrong than that point of view.
First of all, do not feel bad about your fascination with the mountain part of the volcano. After all even the scientists who study volcanoes spend 90 percent of their efforts upon understanding the cone part of the volcano and that quite often produces errors in the modeling of how the volcano works.
The digestive tract model
Now, put on your imagination hat and try to really believe that you are a volcano and that your digestive tract is the plumbing of the volcano. Like you a volcano needs to eat to produce things. So, by now you need to imagine that you are upside down eating in a barrel or something like that. Uncomfortable for a human, but volcanoes indeed have their mouth at the deepest end.
There are 3 common versions of how volcanoes are feeding. The most common form of volcanism is spread-center volcanism; it is a mild form of volcanism that runs all along every spreading center where tectonic plates are pulled apart. All along those there is spread center-volcanism on and off. As the plates are pulled apart the pressure of mantle is lowered and decompression melt occurs and lava oozes up.
The next version is subduction volcanism where a tectonic plate is slowly sliding in under another dragging down water and other volatiles. As it slides down there is heat being produced by friction and at a certain depth melt will start to pool in an area called the acreation zone.
The third version is mantleplume or hotspot volcanism where the mantle is either pushed up or sucked upwards for reasons that are not very well understood.
There are two ways a volcano can form, if we disregard the spread center volcanism, the most common version of formation is that magma starts to exert pressure upwards into the crust and it start to slowly crack and fracture, and magma continuously flow into those cracks creating more cracks as time goes. Imagine that your hamburger is violently forcing its way into you creating the mouth and gullet as it goes.
The other way is what is happening at many volcanoes in Iceland. The magma upwelling from the mantleplume creates a pressure, but here the mouth and gullet is formed as Iceland is pulled apart by the MAR. This pulling apart causes the magma to be sucked upwards. In many respects this is like you eating a hamburger while hanging upside down.
Now it will be the same for most volcanoes after this. The magma will soon reach an area that is in many ways the equivalent of the stomach. Here the magma will rest for a while and start to change over time and take on different properties. The longer the magma sits in your stomach, the more it will change (digest).
The entire process so far has taken years, in some cases thousands or even millions of years. And still there will be no sign at all on the surface of all this fantastic activity. Now we just lack one thing before you are born as a fully fledged volcano. And that are the bowels leading up to your…
Now imagine that you have been eating a really large meal and your stomach is really full and now the waiter comes with the desserts. It would by now be handy if the food could leave your stomach in some expedient way wouldn’t it?
The increasing pressure will start to crack the lining on top of the magma chamber and magma will start to go upwards driven by both buoyancy and pressure. This magma conduit works as the bowels of a human and leads inevitably towards the surface, but now always in a straight path.
If the pressure is large enough the magma will arrive at the surface. And depending of what the volcano has been eating and how long it has been digested what comes out will differ quite a lot. I imagine that you can easily understand this without me explaining it further. Instead just imagine what happened the last time you had Mexican food and you will understand explosive volcanism to the fullest.
Now that we have understood that the beautiful cones that entice us are only the butt ends of the true volcanoes we can learn and understand a lot better.
Yes, the cones can tell us quite a bit about the volcanoes when they function as normal. You can take samples and see what lava has been produced over time. You can see where lava is likely to come out of the volcano.
You will learn were and what of the volcano by studying it’s butt, but not the why and how of its processes. To do that you need to study the other 90 percent that you can’t see without using instrumentation.
If you wish to understand when and how a volcano will erupt you need to study and understand the entire digestive tract of the volcano and also study what that particular volcano feeds on. Otherwise you will never understand why a particular volcano can change behavior over time and all of a sudden do the unexpected.
Let us say that you only studied the cone of Volcán Atitlán, then you would never understand why it in the deep future will be a part of a caldera forming eruption. But if you studied the entire plumbing of the volcano you would see how it repeatedly has grown an over-sized digestive tract that will explode like Mr. Creosote in Monty Pythons Meaning of Life again.
Next time you feel like you are sliding back into a cone-centrist view of volcanism, just remember that cone is volcanese for butt and you should be safe again.