In a sobering comment, one scientist in Iceland said the ejection of volcanic ash -- and therefore possible disruptions in air travel -- could continue for days or even weeks.
With the cloud drifting south and east across Britain, the country's air traffic service banned all non-emergency flights until at least 7 a.m. (0600GMT, 2 a.m. EDT) Friday. Irish authorities closed their air space for at least eight hours, and aviation authorities in Denmark, Norway, Sweden and Finland took similar precautions.
The move shut down London's five major airports including Heathrow, a major trans-Atlantic hub that handles over 1,200 flights and 180,000 passengers per day. Airport shutdowns and flight cancellations spread across Europe -- to France, Belgium, the Netherlands, Denmark, Ireland, Sweden, Finland and Switzerland -- and the effects reverberated worldwide.
Airlines in the United States were canceling some flights to Europe and delaying others. In Washington, the Federal Aviation Administration said it was working with airlines to try to reroute some flights around the massive ash cloud.
Flights from Asia, Africa and the Middle East to Heathrow and other top European hubs were also put on hold.
The volcano's smoke and ash poses a threat to aircraft because it can affect visibility, and microscopic debris can get sucked into airplane engines and can cause them to shut down. The plume, which rose to between 20,000 feet and 36,000 feet (6,000 meters and 11,000 meters), lies above the Atlantic Ocean close to the flight paths for most routes from the U.S. east coast to Europe.
It was not the first time air traffic has been halted by a volcano, but such widespread disruption has not been seen the Sept. 11, 2001 terror attacks.
"There hasn't been a bigger one," said William Voss, president of the U.S.-based Flight Safety Foundation, who praised aviation authorities and Eurocontrol, the European air traffic control organization, for closing down airspace. "This has prevented airliners wondering about, with their engines flaming out along the way."
At Heathrow, passengers milled around, looking at closed check-in desks and gazing up at departure boards listing rows of cancellations.
"It's so ridiculous it is almost amusing," said Cambridge University researcher Rachel Baker, 23, who had planned to meet her American boyfriend in Boston but got no farther than Heathrow.
"I just wish I was on a beach in Mexico," said Ann Cochrane, 58, of Toronto, a passenger stranded in Glasgow.
The National Air Traffic Service said Britain had not halted all flights in its space in living memory, although most flights were grounded after Sept. 11. Heathrow was also closed by fog for two days in 1952.
In Iceland, hundreds of people have fled rising floodwaters since the volcano under the Eyjafjallajokull glacier erupted Wednesday for the second time in less than a month. As water gushed down the mountainside, rivers rose up to 10 feet (3 meters) by Wednesday night, slicing the island nation's main road in half.
The volcano still spewed ash and steam Thursday, but the floods had subsided. Some ash was falling on uninhabited areas, but most was being blown by westerly winds toward northern Europe, including Britain, about 1,200 miles (2,000 kilometers) away.
"It is likely that the production of ash will continue at a comparable level for some days or weeks. But where it disrupts travel, that depends on the weather," said Einar Kjartansson, a geophysicist at the Icelandic Meteorological Office. "It depends how the wind carries the ash."
The ash cloud did not disrupt operations at Iceland's Keflavik airport or caused problems in the capital of Reykjavik, but has affected the southeastern part of the island, said meteorologist Thorsteinn Jonsson. In one area, visibility was reduced to 150 meters (yards) Thursday, he said, and farmers were advised to keep livestock indoors to protect them from eating the abrasive ash. In Paris, all flights north were canceled until midnight. In Copenhagen, spokesman Henrik Peter Joergensen said some 25,000 passengers were affected.
"At the present time it is impossible to say when we will resume flying," Joergensen said.
Eurostar train services to France and Belgium and cross-Channel ferries were packed as travelers sought ways out of Britain. P&O ferries said it had booked a passenger on its Dover-Calais route who was trying to get to Beijing -- he hoped to fly from Paris instead of London.
The U.S. Geological Survey says about 100 aircraft have run into volcanic ash from 1983 to 2000. In some cases engines shut down briefly after sucking in volcanic debris, but there have been no fatal incidents.
Kjartansson said until the 1980s, airlines were less cautious about flying through volcanic clouds.
"There were some close calls and now they are being more careful," he said.
In 1989, a KLM Royal Dutch Airlines Boeing 747 flew into an ash cloud from Alaska's Redoubt volcano and lost all power, dropping from 25,000 feet to 12,000 feet (7,500 meters to 3,600) before the crew could get the engines restarted. The plane landed safely.
In another incident in the 1980s, a British Airways 747 flew into a dust cloud and the grit sandblasted the windscreen. The pilot had to stand and look out a side window to land safely.
Last month's eruption at the same volcano occurred in an area where there was no glacial ice -- lessening the overall risk. Wednesday's eruption, however, occurred beneath a glacial cap. If the eruption continues, and there is a supply of cold water, the lava will chill quickly and fragment into glass.
If the volcano keeps erupting, there's no end to the flight disruptions it could cause.
"When there is lava erupting close to very cold water, the lava chills quickly and turns essentially into small glass particles that get carried into the eruption plume," said Colin Macpherson, a geologist with the University of Durham. "The risk to flights depends on a combination of factors -- namely whether the volcano keeps behaving the way it has and the weather patterns."
Iceland, a nation of 320,000 people, sits on a large volcanic hot spot in the Atlantic's mid-oceanic ridge, and has a history of devastating eruptions.
The worst was the 1783 eruption of the Laki volcano, which spewed a toxic cloud over Europe with devastating consequences. At least 9,000 people, a quarter of the population of Iceland, died, many from the famine caused by the eruption, and many more emigrated. The cloud may have killed more than 20,000 people in eastern England and an estimated 16,000 in France.