Icelandic Volcanoes

A cloud of volcanic ash will force all airports in Berlin to cancel all flights in and out starting at 11 a.m., the German aviation safety agency said Wednesday.

Two other German airports, in Hamburg and Bremen, are already closed, following the eruption Saturday of the Grimsvotn volcano in Iceland.

The closures will affect at least 600 flights in Germany, but could be lifted by afternoon if current weather forecasts hold, German authorities said.

Poland could be affected later in the day, the European air traffic control agency Eurocontrol said

But the ash cloud is dissipating and much of the rest of European air travel is expected to be running normally Wednesday, the agency said.

A portion of the ash from Saturday's eruption in Iceland had spread over Britain by Tuesday afternoon, with the cloud reaching London's Heathrow airport -- the world's busiest international air travel hub -- around lunchtime, a computer model indicated.

Eurocontrol reported about 500 flights in British airspace were canceled Tuesday.

Heathrow alone normally operates about 1,300 flights a day, and Europe as a whole has about 29,000 daily, according to Eurocontrol.

The Grimsvotn eruption came about 13 months after Iceland's Eyjafjallajokull volcano belched smoke and ash into the skies over the continent, forcing the cancellation of thousands of flights per day at the peak of the problem. The ash can be a serious hazard to aircraft, reducing visibility, damaging flight controls and ultimately causing jet engines to fail.

Grimsvotn's eruption was more than 10 times larger and put more ash into the air in 36 hours than last year's burst did in a month, University of Iceland geophysicist Magnus Tumi Gudmundsson told CNN -- but the main eruption is now over, he said.

"We expect it to behave and slowly decline," Gudmundsson told CNN. "It will, however, last for several more days. The ash does continue to cause huge problems on the ground, but no new ash is coming into the high altitudes."

Gillian Foulger, a geophysicist at Britain's Durham University, told CNN that the disruptions from the larger eruption are likely to be smaller than they were in 2010. Most of the ash is blowing northwest, toward sparsely populated Greenland, and the European aviation industry collected valuable safety data during last year's event.

"It's much better able to set the safety thresholds accurately, so I think things are probably going to turn out very well for us," Foulger said.

Airlines have been making the case that it is safe to fly through ash clouds of medium density, Britain's Civil Aviation Authority said Tuesday. Carriers including British Airways, Virgin and EasyJet are now free to fly through clouds of up to 4,000 micrograms per cubic meter if they feel it is safe to do so, Richard Taylor of the CAA told CNN.

Grimsvotn lies beneath Iceland's Vatnajokull glacier, a sheet of ice more than three times the size of the U.S. state of Rhode Island -- larger than any on mainland Europe.

Grimsvotn is the country's most active volcano and last erupted in 2004, according to the Icelandic Meteorological Office.

In 1783, a 16.7-mile fissure system from the volcano produced the world's largest known historical lava flow over a seven-month period, damaging crops and livestock, according to the Smithsonian National Museum of Natural History. A resulting famine led to the death of one-fifth of Iceland's population, according to the museum.