An ash cloud from Iceland's spewing volcano halted air traffic across a wide swath of Europe on Thursday, grounding planes on a scale unseen since the 2001 terror attacks as authorities stopped all flights over Britain, Ireland and the Nordic countries.
Thousands of flights were cancelled, stranding tens of thousands of passengers, and officials said it was not clear when it would be safe enough to fly again.
An aviation expert said it was the first time in living memory that an ash cloud had affected some of the most congested airspace in the world, while a scientist in Iceland said the ejection of volcanic ash - and therefore disruptions in air travel - could continue for days or even weeks.
Can this eruption cause more than just flight delays? To answer this question and more about the unique volcanic event, Dr. John Stix of McGill University was online to answer your questions.
Dr. Stix is the chair of Earth and Planetary Studies at McGill and editor of the Bulletin of Volcanology.