How are future apocalyptic events detailed so greatly?
From Wikipedia article on Ragnarök (emphasis mine):
In Norse mythology, Ragnarök is a series of future events, including a great battle foretold to ultimately result in the death of a number of major figures (including the gods Odin, Thor, Týr, Freyr, Heimdallr, and Loki), the occurrence of various natural disasters, and the subsequent submersion of the world in water. Afterward, the world will resurface anew and fertile, the surviving and returning gods will meet, and the world will be repopulated by two human survivors . Ragnarök is an important event in the Norse canon, and has been the subject of scholarly discourse and theory.
Not wanting to make the question broad, I'm putting it in scope of the Norse mythology and it becomes: If the events of Ragnarok are set in the future, how is it that it is detailed so greatly at present?
In the Völuspa (part of the poetic Edda), the tale is told to Odin as a prophecy by a völva who tell him both the story of earth creation and destruction.
In the Gylfaginning (part of the prose Edda), which quotes extensively the latter, the tale is told by three characters in Ásgard (Hárr, the king, Janhárr and Thridi).
Presumably in the first case it is a vision that the völva had while in the second case the characters are, I think, meant to be omniscient.
Alternatively, an interesting theory is the one argued by Rudolf Simek (in Lexikon der germanischen Mythologie), according to which the norse timeline is a palingenesis (i.e. time is cyclical): Ragnarokr being, according to him, a prelude to the creation myth. In this case these events would be set both in the future and in the past, which would explain how the völva and the three characters from Ásgard can recount it as if it already happened.
Simek's theory is interesting, but sounds a lot more like the cyclical nature of Hindu myths, rather than the linear nature of the myths found in the Northern European area. It would be quite unique if the Voluspa was meant to indicate a cyclic creation and destruction, rather than a linear creation, then destruction, then possible rebirth/renewal.