Why a volcano and not an asteroid?
Simply because the descriptions given are not consistent with asteroids (not even remotely close).. And neither is the actual hard evidence outside scripture. All the evidence clearly points to a volcanic eruption. Even Psalms 18 depicts this very context, and even to the point where GOD is not only the spirit of the volcanic mountain, but the volcanic mountain itself.. When you read the bible in this context, it actually makes coherent sense.. Even revelations to which references back to Exodus makes complete sense to what they are actually inferring to.. This is anthropomorphism and animism of the most powerful eruption human kind has ever witnessed as far we are aware of. To them it would have been the most powerful thing in their era..
If you are looking for the time period of Exodus, I would strongly recommend reading the source
http//www.bibleandscience.com/archaeology/exodusdate.htm
You also need to consider that Exodus is written post event, and that could range 1 to a few hundred years.. But it's clear that the event in question is most assuredly in reference to the Thera eruption and the aftermath...