London (AP) - Ash clouds from Iceland's spewing volcano halted air traffic across Europe on Thursday as authorities closed air spaces over Britain, Ireland and the Nordic countries. Tens of thousands of passengers were stranded in one of most disruptive events to hit air travel in years.
Authorities said it was not even clear when it would be safe enough to fly again. In one sobering prediction, a scientist in Iceland said the ejection of volcanic ash -- and therefore the disruptions in air travel -- could continue for days or even weeks.
Britain's Civil Aviation Authority said non-emergency flights would be banned in all airports until at least 6 p.m. (1700 GMT, 1 p.m. EDT). Irish authorities also closed their air space for at least eight hours, as did aviation authorities in Denmark, Norway, Sweden and Finland.
The move shut down London's five major airports including Heathrow, a major trans-Atlantic hub that handles upwards of 1,200 flights and 180,000 passengers per day. Shutdowns and cancellations spread to France, Belgium, the Netherlands, Denmark, Ireland, Sweden, Finland and Switzerland.
In Iceland, hundreds of people have fled rising floodwaters since the volcano under the Eyjafjallajokull (ay-yah-FYAH'-plah-yer-kuh-duhl) 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.
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.
"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"
In Paris, all flights north were canceled until midnight. At Copenhagen's international airport, where spokesman Henrik Peter Joergensen said some 25,000 passengers would be affected there.
"At the present time it is impossible to say when we will resume flying," Joergensen said.
Passengers found themselves looking up at departure boards where every flight was canceled.
"I just wish I was on a beach in Mexico," said Ann Cochrane, 58, of Toronto, a passenger stranded in Glasgow.
Volcanic ash is formed from explosive eruptions. Particles as hard as a knife blade range in size from as small as 0.001 millimeters (1/25,000 inch) to 2 millimeters (1/12 inch), the Geological Survey says. Ash can melt in the heat of an aircraft engine and then solidify again, disrupting operation.
The U.S. Geological Survey said about 100 encounters of aircraft with volcanic ash were documented from 1983 to 2000. In some cases engines shut down briefly after sucking in volcanic debris, but there have been no fatal incidents.
The ash cloud has not disrupted 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 abrasive ash particles.
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.
"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. We're sitting in the north wind at the moment."
Norway's King Harald V and Queen Sonja -- who had planned to fly Thursday to Copenhagen for the Danish queen's 70th birthday -- were looking to take a "car, boat or train" after Norway shut its airspace.
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.
AP reporters Jennifer Quinn and Paisley Dodds in London, Ian MacDougall in Oslo, Shawn Pogatchnik in Dublin, Jan Olsen in Copenhagen, Gretchen Mahan in Brussels, Mike Corder in Amsterdam, Adam Schreck in Dubai, Frank Jordans in Geneva and Matti Huuhtanen in Helsinki contributed to this report.