Can I harness a volcano as a weapon?

Question

^{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 capable of massive eruptions. A cutaway view would look something like this:

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.

Answer 1

Yes, you could weaponize it.

However, releasing the magma (which then becomes lava to burn pedantic people) will not be an easy thing to turn on (or off), especially for medieval tech. A more practical approach would be to use steam.

By flushing water over a large area very near the magma it can be superheated and directed much more easily to anywhere outside using pipes. The steam will:

• Move much more quickly, catching enemies unaware/unable to dodge.
• Be more easily reset. The rocks just need to heat up sufficiently.
• Be used for powering the internal workings of the base when not used for defense.

As a last ditch effort you can attempt to rig the side tunnels to blow up and release the pressurized magma. It's a crapshoot and you might be able to get it to blow out one side or another, but without precision mapping of the inside of the volcano (you won't have a cross-sectional view after all), engineering such a controlled explosion is nigh impossible. Essentially, this is a base self-destruct, it will be irreversible.

Answer 2

Yes. And it's surprisingly simple.

Two words: Sea water.

When lava hits sea water, you get clouds of hydrochloric acid because the sea salt and hydrogen reacts. When it turns into steam, you get clouds of acidic steam. By digging trenches filled with sea water (easily done using medieval technology), you can poison everyone near those trenches to death when the lava hits it.

Regarding your idea: Unlikely to be possible using medieval tech.

It might actually be easier to just dig out a line in the side of the volcano and pray that it spurts out of that vertical line like a hose with a hole...

Answer 3

Maybe, but it won't be easy.

Mt. Saint Helens incurred a directed blast (not by mankind), which was the result of a landslide triggered by an earthquake. Here's how I would approach repeating this effect.

The tunnels you dig don't have to deal directly with the side channels, rather under them. Gently sloped tunnels like the qanat dug in desert climates (for water), would go underneath the side channels. How you would know where the side channels are, is beyond me.

Then, when the enemy is advancing, some brave soul detonates a mass of gunpowder, which we've known about since the first century AD. This detonation triggers a similar landslide / drop of part of the mountain releasing the above-described directed blast and hope for the best.

I think your volcanologists would be better tasked at using the volcano to build your fortress rather than defend it. Good luck!