REYKJAVIK – A violent volcanic eruption in Iceland spewed clouds of ash into the air for a second day Thursday, blanketing large parts of the Nordic country in the potentially toxic dust and disrupting air traffic across northern Europe.

(Zuma Press) "This is an explosive eruption. That means there's lots of volcanic ash," volcanologist Armann Hoeskuldsson of the University of Iceland told AFP.

"The situation is critical," he said, pointing to massive flooding and the danger of ash poisoning for animals in the surrounding area.

Iceland's second eruption in less than a month under the Eyjafjallajokull glacier in the south of the country began at around 1:00 am (0100 GMT) Wednesday.

Between 700 and 800 people were evacuated from their homes in the remote area 125 kilometres (75 miles) east of Reykjavik, as melted glacier water caused severe flooding.

All evacuees had been allowed to return home by Thursday, local police chief Kjartan Thorkelsson told AFP, adding that authorities had however "encourage people who live near the eruption to wear masks to prevent them from breathing in volcanic ash and dust."

"Everyone is now back home and the rescue centre has been closed," he said, adding that no people or animals had been hurt.

As the ice and water mixed with the hot magma, plumes of ash and smoke stacked more than 20,000 feet (6,000 metres) into the sky.

Strong winds then swept a massive cloud of ash over the southeast of Iceland and onto norther Europe, forcing most of northern Europe to shut its airspace Thursday because of the risk from volcanic ash, which can damage aircraft engines as well as cut visibility.

"I have never seen anything like this!" 86-year-old retired farmer Vilhjalmur Eyjolfsson said of the thick layer of ash that now covers his home near the small village of Vik, to the southeast of the volcano.

"There is gray ash all over and it is like a heavy snow of ash," Eyjolfsson told AFP.

Farmer tells of Iceland volcano blast

People calmly left their homes.

"They said they had practiced so many times leaving their homes, that their suitcases were always packed, due to the volcano risk," said an AFP photographer at the scene, describing how a "humungous waterfall gushed from a hole in the glacier" when the flooding began.

According to experts the eruption under Eyjafjallajokull could last anywhere from a few days to over a year.

"It is very variable how long these eruptions last," Magnus Tumi Gudmundsson, a professor of geophysics and civil protection advisor, told AFP.

"Judging from the intensity of this one, it could last a long time," he added.

Thorsteinn Jonsson of the Iceland Meteorological Office said one eruption about 100 years ago lasted for a whole year and the latest one could be the same, while adding: "It could also stop in two or three weeks, like a few other similar eruptions have."

Kjetil Toerseth, who heads up the regional and global pollution division at the Norwegian Institute for Air Research, agreed.

"Historically, most eruptions (in Iceland) don't last forever," he told AFP.

"This one had a very slow start and had a stronger eruption and I would assume that in the days to come it would fall back to a lower level," he said, stressing that he was not a geologist.

Last month, the first eruption at the Eyjafjallajokull glacier forced 600 people from their homes in the same area.

That eruption, the first under that glacier since 1823 and Iceland's first since 2004, gushed lava for more than three weeks and ended Tuesday, hours before the second one occurred.

The eruption in March "was an effusive one. It was just lava flow and beautiful to watch, and steady," volcanologist Hoeskuldsson said.

"This one is explosive, spewing ash and practically impossible to watch ... since it's in the ice and you can't approach it ... It's heavily dangerous," he explained.

On the bright side, he said the eruption was "stable" and would last "hopefully not more than two or three days."

Ice and fire: recent eruptions in Iceland

Jonsson of the Iceland Meteorological Office meanwhile said flight disruptions across Europe would likely last "at least 48 hours."

"The winds will be turning from the North during the weekend, so that should bring the ash clouds further south, so it will probably be better in Scandinavia but the problems might persist in Britain and Ireland Saturday and Sunday," he told AFP.

However, Iceland's main airport Keflavik, to the west, "and all other airports in Iceland are open today," Hjordis Gudmundsdottir of the Icelandic Airport Authority told AFP.

"It's amazing really," she said.

Volcanoes are notorious hazard for air travel

Iceland Air spokesman Gudjon Arngrimsson said that at least "afternoon flights to the US will be on schedule."

Road traffic around the volcano remained heavily disrupted.

"The main road to Reykjavik is cracked and there is no traffic at the moment, but hopefully it will be reopened today," Gudmundsson said. Roads to the east were "all blocked due to thick ash. You can't see."