When catastrophic events occur, travel insurance carriers release position statements explaining how coverage is affected. Policies purchased after an event is considered foreseeable will not cover situations related to that event. The following is a statement from iTravelInsured on the volcanic ash in Europe.
The first day of quiet activity for the Eyjafjallajökull volcano was recorded as May 25, 2010. If no new or renewed activity occurs by September 23, 2010 it will be considered dormant. Any eruption or new/renewed activity after September 23, 2010 would be considered a new occurrence.