Cauldron and the volcano

From AJS.COM

Spoilers for Shackled City below!!!

Cauldron is a fictional city located in a dormant volcano in the Shackled City Adventure Path. Recently I came across someone who was quite upset about this city for a number of reasons:

• People were living in a volcano
• The volcano goes off, and yet most of the city remains intact
• The volcano goes off, and yet after the story, the people move back in

Let me address these one at a time, as I think each is easy to understand in the context of the modules.

The dormant volcano

Cauldron is built into a dormant volcano. This volcano has no activity at all. If it did, the fish in the central lake would be long dead, and yet we're told that people fish in this lake. Because of this, it was a perfectly safe place to found a city, and the conditions of that founding may be read in the opening chapter of the Adventure Path hardcover.

The eruption

So, how does a volcano that is long-dormant erupt? The simple answer is that it doesn't. During the course of the module the volcano "erupts", but this is an act of massive magical ritual which has been under way for a decade before the game even starts! This ritual involves the construction of a major artifact whose sole purpose is to create an erruption and use that to fuel a gateway to the outer planes.

The lava that is under the city is likely partially a result of this artifact and partially a result of the Pyroclastic Dragon that lives there (who requires such a setting for its home). Pulling magma up from the depths of Oerth's crust to serve this purpose would certainly be dangerous, but as in the real world, if you were to poke a hold into a magma pocket, the magma would come out under enormous pressure, but due to the conductive qualities of the rock that it would move through, it would seal itself back up almost immediately. In order to create a dangerous opening to such a pocket would require a breach in its rock dome of a size which mortals are not likely to be able to create.

Return to the city

After the "eruption" the people return to the town! How can they be safe, and how can the city not be destroyed by an explosion in the mountains caldera?!

Well, for this, we have to look at the physics. About 1/3rd of the town is destroyed. How large an explosion would that be? Certainly far, far below the power of the true eruption that created the caldera in the first place. This detonation is also off-center, which makes a fair amount of sense. The water that is in the caldera is very deep, and we know that the tunnels under the city are extensive (presumably sealed from the water by hundreds of feet of rock, or they would have long been flooded). This makes for a high-density, low-compressability (water) and a low-density, high-compressability (pumice, earth and stone with many tunnels) path through which the explosion can move, and we can presume that this was the reason for the off-center detonation.

So, in the end, there's a huge amount of property damage from an explosion that was probably measured in the mere 10s to 100s of tons of TNT range (small when measured on the scale of a what a full eruption would be like).

As for the state of the volcano after the detonation, it is likely still dormant. Unless the ritual that caused the explosion significantly cracked the dome of a deep magma pocket, such a small erpution would be unlikely to re-awake the volcano.