LONDON - Volcanic ash blanketed parts of rural Iceland on Friday and left a widening arc of grounded aircraft across Europe, as thousands of planes stayed on the tarmac to avoid the hazardous cloud.

Eurocontrol, the European air traffic agency, said the flight disruptions that upended travel in Europe and reverberated throughout the world Thursday were even worse on Friday. Half a dozen European nations have closed their airspaces, the cloud was drifting east, about 60 percent of European flights were not operating and delays will continue into Saturday, it said.

"We expect around 11,000 flights to take place today in European airspace. On a normal day, we would expect 28,000," said Kyla Evans, a spokeswoman for Eurocontrol. "The cloud of volcanic ash is continuing to move east and southeast."

Ice chunks the size of houses tumbled down from a volcano beneath Iceland's Eyjafjallajokull (ay-yah-FYAH'-plah-yer-kuh-duhl) glacier Thursday, as hot gases melted the ice. The volcano began erupting Wednesday for the second time in less than a month.

As torrents of water roared down the steep slopes of the volcano, about 40 people nearby were evacuated because of flash flooding. More floods from melting waters are expected as long as the volcano keeps erupting, said Rognvaldur Olafsson of the Civil Protection Department.

The ash cloud, drifting between 20,000 to 30,000 feet (6,000 to 9,000 meters) high and invisible from the ground, left tens of thousands of travelers stranded around the globe and blocked the main air flight path between the U.S. east coast and Europe.

Trains and hotels in key European cities were packed as people scrambled to make alternate travel plans.

Fearing that microscopic particles of highly abrasive ash could endanger passengers by causing aircraft engines to fail, authorities shut down air space over Britain, Ireland, France, Denmark, Norway, Sweden, Finland and Belgium. That halted flights at Europe's two busiest airports - Heathrow in London and Charles de Gaulle airport in Paris - as well as dozens of other airports, 25 in France alone.

The volcanic ash cloud also could turn into a long, costly headache for businesses, airlines and tourists in the United States.

The ash spewed by an eruption in Iceland forced airlines to cancel flights and redirect planes around the ash. Those diversions caused jetliners to burn more fuel and created delays in the air-cargo business that could quickly run into tens of millions of dollars.

The slowdown could affect everything from package shipments to business meetings and long-planned vacations.

"The costs could be extraordinary," said Jeffrey Price, an aviation professor at Metropolitan State College of Denver.

Many in the travel industry on Thursday weren't asking if they would be affected - but how badly.

"This is the ultimate act of God," said Chicago-based transportation expert Joseph Schwieterman. "It's hard to imagine a weather scenario that would disrupt the entire Atlantic flight system like this."

As the cloud moved east, flights were halted Friday at Frankfurt airport, Europe's third-busiest terminal, and at 10 other German airports including Duesseldorf, Berlin, Hamburg and Cologne. No flights were allowed at the Ramstein Air Base, a key U.S. military hub in southwestern Germany.

No more than 120 trans-Atlantic flights reached European airports Friday morning, compared to 300 on a normal day, said Eurocontrol's Evans. About 60 flights between Asia and Europe were canceled Friday, stranding several thousand passengers.

As the cloud moved to the south and east, some European countries reported a slight easing of conditions. The French Civil Aviation said it will allow some planes to land at the three Paris airports during a four-hour window starting at noon Friday.

Sweden and Norway declared skies in the far north to be safe again for travel even as flights in both capitals - Stockholm and Oslo - were still on a lockdown. Aviation authorities in Ireland reopened airports in Dublin and Cork and lifted most restrictions on the country's airspace.

Poland expanded its no-fly zone Friday to most of the country, excluding the southern cities of Krakow and Rzeszow. Anxious Polish officials worried that the ash cloud could threaten the arrival of many world leaders for Sunday's state funeral of President Lech Kaczynski and his wife, Maria, in the southern city of Krakow.

Among those coming are President Barack Obama, Russian President Dmitry Medvedev and German Chancellor Angela Merkel. Officials have said a postponement of the funeral would be an absolute last resort.

The White House says Obama still planned to fly to Poland on Saturday for the funeral.

NATS, the private company which controls British air space, said the air over England would remain closed at least until 1 a.m. Saturday (0000 GMT) but that some international flights might be allowed into Northern Ireland and western Scotland later in the day.

One Toronto-bound flight departed from Glasgow in Scotland on Friday morning and three flights landed.

Professor Jon Davidson of the Department of Earth Sciences at Durham University in England said the dispersal of the cloud is dependent on the weather. The cloud can be blown elsewhere, the ash could all fall to Earth, or the ash particles spread and become diluted, no longer posing a threat to aircraft, he said.

Britain's Meteorological Office said the wind was expected to blow form the north, which would bring further ash across parts of Britain. Small amounts of ash settled in northern Scotland and Norway, but officials said there was little risk to health.

"It's not toxic or poisonous, it's not radioactive ... and shouldn't pose any danger to general health," Scottish first minister Alex Salmond told the BBC.

Officials at the World Health Organization in Geneva disagreed, saying Europeans should try to stay indoors if ash from Iceland's volcano starts raining down from the sky.

WHO spokesman David Epstein says the agency doesn't know the exact health risks from the ash cloud. However, he said the ash is potentially dangerous if it starts to "settle" on the earth because inhaling the particles can cause respiratory problems, especially for those suffering from asthma and respiratory diseases.

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.

Explosive volcanic eruptions inject large amounts of highly abrasive ash - essentially very small rock fragments - into the upper atmosphere, the cruising altitude of most jet airliners. It can cause significant damage to both airframes and engines.

The U.S. Geological Survey said 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. Still, authorities are very wary, because ash cannot be detected by a plane's normal weather radar.

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 windshield. The pilot had to stand and look out a side window to land safely.