I haven't yet figured out the other two clues, but Italy is not correct because the first clue indicates that the "arms" meet the Pacific Ocean. Pacific means peaceful. Italy reaches into the Mediterranean Sea. Mediterranean would mean "between the lands".
The second clue seems to apply to the layers of the Earth itself. I see no indication that the "arms" are necessarily rivers (although they may be), and no indication that a volcano is involved. Were you given more information than what you put in your question? Were you told that there is a volcano?
The assumption that it is a poetic civilization is also incorrect. Prose is ordinary writing as opposed to verse.
This gives you something to work with anyway. I will give it more thought to see if I can resolve it.
Edit: I am leaning toward the civilization being the Aztecs and the location being the southern coast of Mexico (Oaxaca state). There are several large rivers there meeting the Pacific Ocean and it was part of the Aztec Empire. Acapulco is located there, and Oaxaca is further from the coast. You don't actually say that you will be where the arms meet the Pacific, only that you will visit the two great arms.
