Experts have warned that Iceland's Eyjafjallajokull volcanic activity, which is still pumping ash into the sky and disrupting European air traffic, could last for months.
When asked if it is possible to predict when the eruptions might cease, Dr. Pall Einarsson, a geophysicist of the Institute of Earth Sciences at the University of Iceland, said: "There is really no way to know that. We have quite good measurements to know what's going on."
"We can see where the stresses are changing due to earthquakes, and so on. But how it will develop is a very difficult thing to say. The activity is quite variable; it goes up and down a bit. But on the whole the vigour of the eruption seems very little changed," The Scotsman quoted Dr. Einarsson, as saying.
Although the eruption is small in relative terms, the location, close to a very busy airspace, and the fact it is beneath an icecap, have produced a great deal of ash.
"It's the interaction of the molten rock, the magma and the glacial ice which is causing the magma to cool very quickly and to be pulverised into tiny fragments of rock," said Dr. Matthew Roberts, a British glaciologist.
"These updrafts of fine volcanic ash are being lifted into the sky by the enormous steam plumes that have been created by the vast quantities of ice that's been melted," he added.
Almost two-thirds of all transatlantic flights into European airports were cancelled and authorities shut down airspace over France, Denmark, Norway, Sweden, Finland, Belgium, Italy, Switzerland, Slovakia, Croatia and Hungary.
As the cloud moved east, flights were halted at Frankfurt, Europe's third-busiest airport, and at 10 other German airports.