This may be the first of a few questions in a series about building a medieval mountain fortress. I'll wait and see what the reaction is.

In the middle of a series of mountains lies a [stratovolcano](http://en.wikipedia.org/wiki/Stratovolcano) capable of massive eruptions. A cutaway view would look something like this:

[![](https://upload.wikimedia.org/wikipedia/commons/thumb/9/97/Volcano_scheme.svg/574px-Volcano_scheme.svg.png)](https://upload.wikimedia.org/wikipedia/commons/thumb/9/97/Volcano_scheme.svg/574px-Volcano_scheme.svg.png)

I'd like to weaponize it.

I'm not interested so much in the top part, but in the side channels - those leading to the sides of the mountain. My idea - well, that of the commander in charge of the operation - is to create a series of artificial channels running deep into the volcano, ending at strategic points on the outside. The second part of the plan would be to cap off the top and unwanted side channels. An eruption could therefore be directed, to some extent.

Is this possible, using medieval technology? Can a medieval army use this to weaponize a volcano?

Some assumptions:

* Eruptions occur at predictable, periodic intervals.
* The army can *not* trigger eruptions.
* The channels are aimed at fixed locations, and material is ejected far into the air.

If you have a better idea - or think that my idea is pretty poor, which is just fine - feel free to make that an answer.