Eyjafjallajökull Eruption
When Eyjafjallajökull's glaciated summit cracked open in April 2010, the ash plume it sent into the jet stream grounded 95,000 flights and cost the airline industry $1.7 billion. It was the largest closure of European airspace since World War II.
The eruption was modest by Icelandic standards — VEI 4, similar to Mount St. Helens 1980. What made it consequential was geography: the volcano sat directly under the North Atlantic jet stream, and the molten lava meeting glacial ice produced unusually fine, glassy ash that stays airborne for days.
Between April 15 and April 23, almost the entire upper-airspace network from Iceland to Eastern Europe was closed to commercial aviation. 10 million passengers were stranded. Container shipping rates surged. Kenyan flower farmers — whose product depends on overnight air freight to European florists — lost $1.3 million per day.
The eruption also catalyzed a permanent change in how aviation authorities respond to ash. Pre-2010, the rule was zero ash tolerance. Post-2010, the EASA introduced a tiered system based on ash density — recognizing that the old rule, applied literally, would close European airspace far too often.
Eyjafjallajökull stopped erupting on June 22, 2010. Its larger neighbor Katla, which historically erupts in tandem with it, has not yet.