A cloud of volcanic ash that wafted in from Iceland shut down airports across Northern Europe on Thursday, causing a ripple effect that is expected to snarl air traffic around the globe for days to come.
The paralysis that began in London and Paris soon spread to Hong Kong and Tokyo, to Nairobi and Buenos Aires, and to New York and Washington. It was, by all accounts, one of the most bizarre acts of nature ever to constrict world travel, and it grounded tens of thousands of passengers internationally.
Until the eruption, which began last month, Iceland's long volcanic history was little known beyond trivia games, but as that legacy surfaced Thursday, so did news that the last time the Eyjafjallajokull volcano exploded 187 years ago the eruptions went on for more than a year.
Officials were at a loss to predict how long it would take for the ash to dissipate or for flights to resume.
If the particle-laden cloud lingers through the weekend and there were predictions it would take two days to clear once the eruptions stop it could disrupt plans of President Barack Obama and other world leaders to attend Sunday's state funeral for Polish President Lech Kaczynski, who died in a plane crash. Experts said that the weather radar on aircraft cannot detect the cloud of ash and that the particles could choke jet engines enough to shut them down.
As the cloud moved across Europe on Thursday, officials closed the airspace in Britain, Ireland, Denmark, Norway, Sweden, Belgium, the Netherlands, Finland, and parts of France, Germany and Poland.
Eurocontrol, the European aviation agency, anticipated that 50 percent of the flights between the continent and North America there are about 600 a day would be delayed or canceled Friday.
Many airlines were issuing waivers for travel to destinations affected by the clouds of volcanic ash. American Airlines, British Airways, Delta, Lufthansa, SAS, United Airlines and US Airways said travelers would be able to rebook without penalty or obtain refunds depending on the destination.
Some flights that which had taken off for Europe on Thursday were returned to the United States, said Federal Aviation Administration spokeswoman Laura Brown.
Although it stifled air travel, the cloud that hung over Europe was so high between 20,000 and 36,000 feet that it wasn't visible from the ground and health authorities said it posed no immediate danger.
The restriction of flights from Heathrow, which, with 1,200 flights carrying 180,000 passengers each day, is one of the world's busiest airports, will affect dozens of flights to and from the United States and other hubs around the world. British Airways said on its Web site that customers booked on a canceled flight could rebook or claim a full refund.
The restrictions on British airspace were imposed by Britain's National Air Traffic Services (NATS) and could be extended, a spokeswoman said.
"There are few historical precedents here," said NATS spokeswoman Taylor Samuelson. "It's nothing we're going to take any risks on."
She added: "No one knows when the ash is going to go. The volcano is still going."
As thousands of people scrambled to rearrange their travel plans, some decided the disruption was a good excuse to extend their vacations.
In London, Martin Gergely, 27, a traffic engineer from Vienna who was flying home from Los Angeles, said he and his girlfriend would extend their trip a little longer and take in some of London's tourist attractions.
"Well, I guess we will make the most of it," he said.
Mel Wilkinson, 28, a receptionist from Newcastle, was one of the thousands of passengers on London's subway system who heard an announcement that all flights had been canceled. Wilkinson had planned to fly from Heathrow to Hong Kong on Thursday to visit family.
She said the ash cloud was "very bizarre," adding, "You couldn't make this up."
Civil aviation experts cite two famous examples of volcanic ash disrupting flights. In both, fatalities were narrowly averted.
In 1982, a British Airways jumbo jet flew through an ash cloud over Indonesia, causing all four engines to flame out.
After plummeting thousands of feet, the flight crew managed to restart the engines and land the plane safely in Jakarta.
In 1989, a KLM jet flew through an ash cloud from Alaska's Redoubt volcano, causing all of the engines to fail. After a five-minute descent, the engines were restarted and the plane landed safely; however, the plane was severely damaged.
Although the Eyjafjallajokull volcano began erupting last month, Wednesday's cloud sent hundreds of people in southwestern Iceland fleeing from their homes, fearing flash floods. The volcano, which is under a glacier, is one of several in the northern nation of 320,000 people, which sits on a volcanic hot spot in the Atlantic.
Halsey reported from Washington. Staff writers Hamil R. Harris, Lisa Rein and Ovetta Wiggins in Washington contributed to this report.