LONDON — Ash clouds from Iceland's 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 nonemergency flights would be banned in all airports until at least 6 p.m. Irish authorities also closed their air space for at least eight hours, along with closures by aviation authorities in Denmark, Norway, Sweden and Finland.

The move shut down London's Heathrow Airport, a major trans-Atlantic hub which 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. Shutdowns and cancellations spread to France, Belgium, the Netherlands, Denmark, Ireland, Sweden, Finland and Switzerland.

In Iceland, hundreds of people have fled rising floodwaters since the volcano under the Eyjafjoll 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.

Volcanic ash is formed from explosive eruptions. Particles as hard as a knife blade range in size from as small as 0.001 millimeters to 2 millimeters, the Geological Survey says. Ash can melt in the heat of an aircraft engine and then solidify again, disrupting operation.