Bay 12 Games Forum

When a volcano erupts, it will do one of two things, depending on the type of volcano.  There are various hybrids of these, and if you throw water in, well...  things get interesting.

You have to know a few basic things to understand these first:

* Magma has a lot of dissolved gasses.  The more pressure you put on it, the more it can hold.  As it rises, the pressure drops, and that gas is released.  Funny enough, this gas is mostly water and CO2.  Magma, however, is generally so thick that it cannot boil out, and will build up pressure.  Like a bomb.  Almost exactly like a bomb.  Thinner magmas make weaker bombs, however, because more gas can escape.  The magma cooling underground will diffuse this "bomb" via (really complex) chemical reactions and gas seepage.

1.  If it is an effusive eruption (thin magma, basalts - See: Hawaii) the lava will ooze out of the ground, bubbling.  If it's a lot, or under a lot of pressure, you may get a lava fountain.  In rare cases, a lava fountain may be a mile tall.  It then flows downhill, as a wall of fiery death - however, in most cases (not always), it's so thick that it moves no faster than 2 or 3 mph at the front of the flow.  You can outpace it with a typical walking speed.


2. If it's an explosive eruption (thick magma, so rhyolite - See: Krakatoa) then the lava gets a few unusual properties:
As the gas contained comes out of solution and forms bubbles, the bubbles then actually make the liquid thicker.  This makes it move up the vent slower, but there's still all that pressure behind it.
Eventually it reaches a point where it gets so thick that it stops flowing.  It then either will form a plug (and not erupt, but what fun is that?) or it will promptly detonate - it behaves like a solid and fractures, and this releases all some of that trapped gas.  Sort of.
Before it explodes, it might be 20% gas by volume.  40% if it's really really frothy.  After it explodes, it's 80%+ gas.
This bomb eruption cloud continues up the vent, at speeds that can be up to several hundred miles per hour, until it leaves the vent.  At that point, the gas expands the rest of the way.  It can, in a large eruption, breach the sound barrier while leaving the crater.
This evacuation of mass rapidly travels upwards - or, as was demonstrated with Mt. St. Helens, will travel whichever direction is most convenient to it. (Are you going to tell it no?)
This mixture of gas, lava and rock (the lava hasn't cooled yet, the rock is usually already solid rock torn from the vent on the way up), is much denser than air.  It's also very hot, and a gas.  And traveling upwards, very quickly.

The second one means it pulls in air from the surrounding atmosphere and entrains it in the ash column. 
It will either pull in so much air that it becomes less dense than the surrounding air and keeps rising, until it reaches a point of equilibrium where it is the same density of the surrounding atmosphere, at which point it will stop rising keep rising through sheer inertia (overshooting top, like a thunderstorm), fall back down, and then settle at the point of equilibrium.  This means that whoever is downwind gets a nice volcanic shower.  Be warned:  some of those chunks can be rather large and still hot if you're close enough.  Most of the time, it's just a mess.

Or, it won't.
It won't pull in enough air (or, more commonly, only part of it will pull in enough air), and fall back down.  Did I mention that it's in a gas state, even with big chunks of lava in it?  It then flows along the ground like a twisted demonic avalanche so hot you get to watch your skin burn off for a few seconds before your eyes boil, if it doesn't just flat out kill you from the sheer impact of it (you just got hit by a wall of flying, superheated gas and rock)
This is Real Fun.

That last part is the most fun.  I don't think I missed anything important.