This needs a comedy sketch

So funny you should mention this! I am almost finished re-reading one of my childhood favorite comics, Rahan [1] , where authors imagined live in prehistoric times, and where almost one third of the albums bring up the topic, one way or another, attempting to imagine what kind of feelings and beliefs such events induced.

[1] https://en.m.wikipedia.org/wiki/Rahan_(comics)

There is a Doctor Who episode where they went to Pompeii.

No need to guess. There is almost a thousand year old story in Iceland which describes such a thing in Brennunjálssaga. The story goes something along the lines that they were discussing whether to adopt Christianity at the Parliament. but as they were debating somebody told them that a new eruption had started. Some assembly members wanted to blame this on the debate, that the gods were insulted that they were considering being abandoned, however others argued: “Then what angered the gods that produced the lave which we stand on now?”

In short, people kind of knew what was going on. They may not have known the exact mechanism, but they did know that this was how land formed, and was a normal part of nature, just like the northern lights, earthquakes and rain.