LONDON - Ash clouds from <NOBR id=itxt_nobr_0_0 style="FONT-WEIGHT: normal; FONT-SIZE: 100%; COLOR: darkgreen; FONT-FAMILY: Verdana, Arial, Helvetica, sans-serif">Iceland's</NOBR> spewing volcano disrupted air traffic across Europe on Thursday as authorities closed air space over Britain, Ireland and the Nordic countries. Tens of thousands of passengers were stranded as flights were canceled and it was not clear when it would be safe enough to fly again. Britain's Civil Aviation Authority said non-emergency flights would be banned in all airports until 1 a.m. EDT Friday (6 a.m. local time). Irish authorities also closed their air space for at least eight hours, along with closures by aviation authorities in Denmark, Norway, Sweden and Finland. Shutdowns and cancellations spread to France, Belgium, the Netherlands, Denmark, Ireland, Sweden, Finland and Switzerland. London's Heathrow <NOBR id=itxt_nobr_4_0 style="FONT-WEIGHT: normal; FONT-SIZE: 100%; COLOR: darkgreen; FONT-FAMILY: Verdana, Arial, Helvetica, sans-serif">airport</NOBR>, Europe's busiest, handles upwards of 1,200 flights and 180,000 passengers per day. The closure also affected London's second- and third-largest airports, Gatwick and Stansted. 'Significant threat' The volcano was sending up smoke and ash that posed "a significant safety threat to aircraft," Britain's National Air Traffic Service said, as visibility is compromised and debris can get sucked into airplane engines. "I just wish I was on a beach in Mexico," said Ann Cochrane, 58, of Toronto, a passenger stranded in Glasgow. <TABLE style="PADDING-RIGHT: 15px; PADDING-LEFT: 0px; PADDING-BOTTOM: 0px; PADDING-TOP: 5px" cellSpacing=0 cellPadding=0 width="1%" align=left border=0 itxtvisited="1"><TBODY itxtvisited="1"><TR itxtvisited="1"><TD itxtvisited="1"></TD></TR><TR itxtvisited="1"><TD itxtvisited="1">Ingolfur Juliusson / Reuters Smoke billows from a volcano in Eyjafjallajokull April 14, 2010. <HR color=#c0c0c0 noShade SIZE=1 itxtvisited="1"></TD></TR></TBODY></TABLE> In Iceland, hundreds of people have fled rising floodwaters since the volcano under the <NOBR id=itxt_nobr_7_0 style="FONT-WEIGHT: normal; FONT-SIZE: 100%; COLOR: darkgreen; FONT-FAMILY: Verdana, Arial, Helvetica, sans-serif">Eyjafjallajokull</NOBR> glacier erupted Wednesday for the second time in less than a month. As water gushed down the mountainside, rivers rose up to 10 feet by Wednesday night. 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. 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. Volcanic ash is formed from explosive eruptions. Particles as hard as a knife blade range in size from as small as 1/25,000 inch to 1/12 inch, the Geological Survey says. Ash can melt in the heat of an aircraft engine and then solidify again, disrupting operation. No fatal accidents 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. <TABLE style="PADDING-RIGHT: 15px; PADDING-LEFT: 0px; PADDING-BOTTOM: 0px; PADDING-TOP: 5px" cellSpacing=0 cellPadding=0 width="1%" align=left border=0 itxtvisited="1"><TBODY itxtvisited="1"><TR itxtvisited="1"><TD itxtvisited="1"></TD></TR></TBODY></TABLE>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 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. If the volcano keeps erupting it could cause massive flight disruptions. "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," he added. 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 <NOBR id=itxt_nobr_19_0 style="FONT-WEIGHT: normal; FONT-SIZE: 100%; COLOR: darkgreen; FONT-FAMILY: Verdana, Arial, Helvetica, sans-serif">Airlines</NOBR> 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.