Pliny the Younger wrote two letters about the eruption of Mount Vesuvius, which he witnessed. They're letters 6.16 and 6.20.

Though my Latin ability has faded, to the extent that it ever existed, and I'm therefore unable to tell you the quality of these translations, here are two translations, such as they are :). An excerpt:

Recalling his relation, Pliny the Elder, observing the eruption:

It was not clear at that distance from which mountain the cloud was rising (it was afterwards known to be Vesuvius); its general appearance can be best expressed as being like an umbrella pine, for it rose to a great height on a sort of trunk and then split off into branches, I imagine because it was thrust upwards by the first blast and then left unsupported as the pressure subsided, or else it was borne down by its own weight so that it spread out and gradually dispersed. Sometimes it looked white, sometimes blotched and dirty, according to the amount of soil and ashes it carried with it. My uncle's scholarly acumen saw at once that it was important enough for a closer inspection, and he ordered a boat to be made ready, telling me I could come with him if I wished. I replied that I preferred to go on with my studies, and as it happened he had himself given me some writing to do. [. . . .]

He hurried to the place which everyone else was hastily leaving, steering his course straight for the danger zone. He was entirely fearless, describing each new movement and phase of the portent to be noted down exactly as he observed them. Ashes were already falling, hotter and thicker as the ships drew near, followed by bits of pumice and blackened stones, charred and cracked by the flames: then suddenly they were in shallow water, and the shore was blocked by the debris from the mountain.

The account is fascinating and merits reading in whole.

I'm sure someone else can give a fuller idea of ancient notions of volcanoes, but this is certainly a start!