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Volcanic Pyroclastic flows: What are some good analogies to understand the full spectrum of causes?

I’m trying to wrap my head around something that appears far more complex than I first thought. I don’t understand the explosive elements and chemistry that drives ash production and the heat to create the eruption column. I’m aware of how molten metal behaves in a foundry crucible with flux and degassing required. So I understand how magmas can have a tremendous amount of dissolved elements that can release like CO2 from a shaken soda bottle being opened. I can picture this kind of pyroclastic flow easily; the “shaken soda bottle” type.

I don’t have a very good grasp of how the ash column can reach enormous heights and then collapse, or how composition impacts this kind of collapse. I just can’t picture in my mind how this type of collapse results in a flow, like some kind of avalanche or pronounced river of material as opposed to more of a micro-regional rain like ash fall event that is very intense and superheated. What is the trigger and how does it overcome any hotter material lower in the column?

en.m.wikipedia.org/wiki/Pyroclastic_flow

weariedfae ,

Here are some resources to clear up the difference between eruptions and flows and provide more context to answer your question.

Background on explosive eruptions: youtu.be/tQzaQd72DJI?si=wmhLU3TjlRuiKvJW

Part 2 (more interesting from my pov): youtu.be/umNWZOTHFXo?si=ZfmHJCEnQ_eil87I

Timestamp ~30:10 on the second video (Lesson 10) talks about the origin of pyroclastic flows.

j4k3 OP ,

Thanks. I watched both of them. That cleared up the missing pieces.

Senshi ,

I’m keeping your soda bottle analogy:

In this case, a very strong eruption ejects kids of super hot gas and rock upwards, like when you open a shaken bottle. After some time, pressure will decrease, and gravity will start dragging things down again.

Unlike a regular soda bottle, heat is significant. Hot gas rises in the atmosphere against gravity. During this rise, it loses energy ( so it cools down). When it reaches a high enough temperature where the lifting momentum is overcome by gravity, it starts falling again.

As the top starts to fall while there still is more material below in the column, the column gets compressed. As the center of the column is the hottest part, it still pushes material upwards. So the colder material falling from the top is pushed outwards, widening the column a bit. It also encounters the cold air outside and starts cooling even more itself, falling ever faster in the outside “ring” of the column. It still is only “cool” compared to the rising inner column, still thousands of degrees. Also, all the light glasses will have moved further up the atmosphere and either fall slower or not at all. This is where the long term effects such as your mentioned ash fall/ rain comes from. So most of the rapidly falling material that then form pyroclastic flows are actually fairly heavy liquids/solids and heavier-than-air gasses. They only seem so light and fast inside a pyroclastic flow because if their immense temperature and contained energy.

However, sooner or later the falling material encounters the ground, a solid obstacle. As the inner column is densely filled with super hot, probably still rising fresh material, the only possible way is outwards. And with continuous pressure from above from all the falling material, the material needs to move out of the way very rapidly. This is not dissimilar from how water behaves that flows from a bottle or faucet and hits solid ground. But a pyroclastic flow is a bit more viscous, and still very hot. While moving outwards, it quickly has to push away the cool, resting atmosphere. The only way for the air is to step aside upwards. Now, as the cold air likes to stay close to the ground and was compressed, it forms a seemingly paradoxical barrier layer of cold, dense air above the pyroclastic flow, pressing down on it, even squeezing it further outwards. This together with it’s own viscosity means there’s surprisingly little turbulence between the two layers, with the hot flow continuing to rush along below the cold barrier layer instead of mixing and rising through it upwards. If this interests you, look up inversion layers: they are a normal phenomenon in regular weather as well, especially winter time, and can sometimes even last many days.

Consider that ash columns reach many km in altitude, filled with many tons of material. It doesn’t all fall slowly at the same time. It’s literally rock falling from high atmosphere to the ground, carried by heavier-than-air gasses that also want to sit below the atmosphere.

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