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If you have ever seen a volcano you are aware of its destructive power you know that it can wipe out entire cities like it did with Pompeii. However there are examples of human towns and settlements that peacefully coexist with active volcanoes without much problem. So what is the difference? If you look at locations such as the Hawaiian Islands these volcanoes only produce lava. Lava normally moves at a relatively slow pace and if natives are familiar with the common channels they can be avoided or diverted. However there is one kind of volcanic eruption that could easily wipeout life in a surrounding area in an instant. It is called a pyroclastic flow.
You may be wondering,”what is a pyroclastic flow?” A pyroclastic flow is a fast current of hot gases, rock, and volcanic ash. They can move at extraordinary speeds catching people unaware. The worst volcanic eruptions in history have always had pyroclastic flows. So what is there to expect? The first is that a pyroclastic flow can either act a precipitate raining down from the sky or as a fast moving flow along the ground that can move as fast 700 km/hr.
There are several ways for pyroclastic flows to be formed and manifest. The first is fountain collapse. This is the way the pyroclastic flow that destroyed Pompeii was created. In this method the volcanic ash and rock is carried and mixed in by the convection currents that are created by the erupting volcano. However, the currents of air need to be hot enough.
Another kind of pyroclastic flow is the kind caused by part of a volcano exploding. This can be devastating in its powers as testified by the Mt. St. Helens eruption. This type can cause destruction and damage that goes on for miles.
So what makes these kinds of eruptions so powerful? To know the mechanics you have to see how moisture and lava interact. If you have ever used an iron or poured water on a heated stove you know what happens. The water immediately vaporizes with a lot of noise. However that is what happens with a solid heat source. The reaction is much more powerful if you mix cold water or ice with lava. The cold water makes the lava cool rapidly making the ash and volcanic rock that is expelled in a pyroclastic flow. It also releases the trapped gases that make such eruptions move so quickly.
We have written many articles about pyroclastic flow for Universe Today. Here’s an article about volcano dangers, and here’s an article about pyroclastic flow.
We’ve also recorded related episodes of Astronomy Cast about Volcanoes. Listen here, Episode 141: Volcanoes, Hot and Cold.