Factbox: How did the Eyjafjallajokull ash cloud form?

(Reuters) - The eruption under the Eyjafjallajokull glacier in Iceland that has thrown up a six-km (3.7-mile) high plume of ash and disrupted air traffic across northern Europe shows no signs of abating after 40 hours of activity.

Located under Iceland's fifth largest glacier, the volcano has erupted five times since the area was settled in the 9th century.

Eyjafjallajokull has a 2.5 km-wide volcanic crater, covered in ice. Fissure-fed lava flows occur on its eastern and western flanks of the so-called stratovolcano, which is built up from alternating layers of ash, lava and rocks ejected by earlier eruptions.

When the volcano began erupting in late March it opened a 500-m fissure producing lava fountains along the vent.

The ash cloud has been formed through a process called fragmentation which occurs in several stages. First, magma traveling under pressure through underground conduits is broken up into pieces by expanding gases.

As pressure decreases closer to the surface, the magma turns into fine volcanic ash which breaks into even smaller particles when it makes contact with glacial ice on the surface of the crater. The fine dust melds with steam rising from the crater to form a dark, billowing plume.

"It's like a soda bottle when you take the top off," said Icelandic vulcanologist Armann Hoskuldsson, describing what happens to magma as it travels to the surface.

"At the same time there is little wind in Europe to disperse the plume," said Freystein Sigmundsson, a researcher at the Institute of Earth Sciences at the University of Iceland in Reykjavik.

(Writing by London Editorial Reference Unit; Editing by Michael Roddy)