Before we get too far, I wanted to make sure that folks understand that I'm just making educated conjectures on the nature of the eruptions I read about and by no means do I have any extra insight over those scientists on the ground at the eruption. I have a very limited set of data to examine - whatever the media reports - so I am just speculating based on what I know about the eruption style, volcano in question and whatever other variables might come into play. So, please, don't think that I know exactly what is going on or what will happen better than the scientists tackling the volcano firsthand.
That being said, I might not be a big fan of Donald Rumsfeld, but he does divide things up nicely when it comes to assessing a situation, so here we go.
For Volcan Chaiten:
Known knowns: The volcano has been erupting for the past week; large ash column up to possibly as high as 90,000 feet (!); thick ash deposits (>30 cm) out 10s of km from the vent; potentially lava extrusion at the vent; no pyroclastic flows, no lahars (so far); two discrete vents have coalesced into one vent; upwards of 2 cubic kilometers of erupted material (thus far); last known eruption at Chaiten was 7450 BC, or roughly 9,500 years ago.
Unknown knowns: How long will the eruption last?; how long until the supply rate of pyroclastic material from the vent drops below the rate needed to keep the ash column aloft?; when the ash column collapses, how big will the pyroclastic flows be?; will lahars begin to be generated?; will the volcanic edifice collapse due to the eruption? if so, will there be pyroclastic flows generated by the collapse and/or ring vents forming?; is this the main eruption or will things get bigger? how much lava will be extruded at the main vents? will more vents open?; how long until people can get near the vent to see what has occured firsthand?
Unknown unknowns: Well, I can't really speculate of these, now can I?
At least we know that Chaiten spared the berry crops!