UPDATE 2/19/2009 9:45 AM:Well, it seems that my hunch was at least partially right. Reuters (and Paula Narvaez, special envoy to the Chilean president) is calling the eruption as result of "what appeared to be a partial collapse of its cone." So, we might have seen the oversteepening of the dome growing in the Chaiten caldera that lead to a collapse, producing (likely) a pyroclastic flow and either an accompanying plinian eruption as the pressure was released or an ash column associated with the pyroclastic flow itself. Now, I might not take Reuters word for it, but it makes sense considering the suddenness of the event.
10:00 AM:More details for Reuters, including the ominous "Our security team have observed an increase in the size of a column of ash and smoke, with a deformation to one side" from the Deputy Interior Minister. My guess is he is referring to deformation ...
