Got into a debate over Christmas about how much world knowledge people had in the middle ages. Specifically it was if people studying in an university in somewhere like England would know what a volcano is even though there aren't any in that country. To me it seems obvious that they would have to read history, especially by Greek and Romans that would reference such things. It was argued that while such books would be in a university library, "they wouldn't have read ALL the books in the library!" Basically no one sided with me and they all looked at me like I was an idiot. Finally I got through that volcanos were mentioned in the Bible and was countered by, "no they say mountains shooting fire," and they probably thought it was something apocalyptic that would only happen in end times. I agree the average person might not have been literate and possibly never heard of a volcano, but someone educated? What do you think? Am I way off or just need smarter friends?
The short answer is mostly yes but also a bit no.
Yes, the fires of Mount Etna, in particular, were a very well known natural phenomenon.
Alongside earthquakes, the cause of Etna's eruptions was a standard feature of common natural historical texts, like Bede's or Isidore's De rerum natura, and of many medieval encyclopedias, most importantly Isidore's Etymologies, but it is also mentioned by Honorius Augustodunensis in the geographical section of his Imago mundi (another important 12th century encyclopedia) and by Rabanus Maurus in his De universo (an important caroliginian encyclopedia). It is also mentioned in common geographical and historical compilations. So it is mentioned by Solinus in his very popular De mirabilibus mundi and it comes up in two of the most important and widely read late-antique histories. Orosius compares Sicily to Etna, explaining that like Etna used to erupt frequently so Sicily used to suffer from frequent war, but now both lie relatively quiet. Likewise in the chronicle of Eusebius-Jerome, a number of volcanic eruptions are recorded (one each of Etna and Vesuvius, as well as the creation of an island through a volcanic eruption). Finally it is also mentioned sometimes in theological and exegetical writings, since the fire and sulphur of Etna serve as a figure of hell. Thus the twelfth century author Gerhoh of Reichersberg explains Psalm 104 (103 in the vulgate), verse 32: "he who looks at the earth, and it trembles, who touches the mountains, and they smoke", by noting that:
Many mountains exhale fire and smoke, like Etna, that humans, thoroughly terrified by evils, may be revived.
Multi etiam montes flammam et fumum, ut Aetna efflant, ut homines perterriti a malis resipiscant
Finally it also turns up in the Responsorium curiosorum of Conrad of Halberstadt, essentially a handbook of interesting 'scientific' facts that a preacher could use to engage the laity in conversation, has a question about "What is the cause of the fire in mount Etna and similar mountains" (Que sit causa vulcani in monte Ethna et similibus).
But at the same time, sort of no, since medieval authors didn't really know about "volcanos".
They had no specific term for volcano, instead they normally refer to them as "mountains like Etna", and even then, in learned latin writings on the matter, this is pretty much associated with a couple of canonical Italian and Sicilian examples. So it's not clear that they would have thought about this as a general category in quite the same way that we do. Likewise, of course, they wouldn't have thought about it in terms of something like plate tectonics.
That said, they seem to have a decent notion of the phenomenon. Solinus, for example, explains that:
On the peak of Mt Etna there are two fissures, called craters, through which violent steam erupts, preceded by a roaring noise, which tumbles about through hidden caverns seething within the bowels of the earth with a prolonged rumbling, and not before it do globes of fire arise, which the internal noises precede.
In Aetnae uertice hiatus duo sunt, crateres nominati, per quos eructatus erumpit uapor, praemisso prius fremitu, qui per aestuantes cauernarum latebras longo mugitu intra terrae uiscera diuoluitur, nec ante se flammarum globi attollunt quam interni strepitus antecedant.
It's causes are also associated with earthquakes, which were (as I have explained in a previous thread) thought to be caused by wind blowing through underground caverns. So, for example, in the Etymologies, Isidore explains about Etna that:
Mount Etna is so called from fire and sulfur; Gehenna likewise. It is well known that, in the part in which the southeast wind or the south-west wind blows, it has caves that are filled with sulfur and that lead to the sea. When the waves rush into these caves, they create a wind that, when stirred up, ignites a fire in the sulfur; this creates the flames that one sees.
So, to conclude, I can't say that every latin literate person in the middle ages would have known what a volcano was. But it appears to be relatively common knowledge that Etna and some mountains like it emit fire. And anyone who was interested to know would have little difficulty finding some standing natural philosophical explanations, although it is unclear how much they would have understood it as a distinct class of natural phenomenon or whether it would have been understood more as a natural marvel occurring in only a few specific places.