On page 18, Tarshis writes, "There is not even a word for volcano in Latin." What purpose does this detail serve?

Answer: since this has been here for 5 hours, it requires an answer. When Mount Vesuvius erupted in 79 A. D., is was given the Title Magna Ignius", or "The Great Fire", and there were assorted similar names. Each mountain that "exploded into fire" was given its own name; With this detail, one can infer that at that time, the Romans had a limited understanding of Geology, and did not give these "mountains" A common name, like our "Volcano". That is the purpose served.