Eyjafjöll, located immediately west of Katla volcano, consists of an E-W-trending, elongated ice-covered basaltic-andesite stratovolcano with a 2.5-km-wide summit caldera. Fissure-fed lava flows occur on both the eastern and western flanks of the volcano, but are more prominent on the western side. Although the 1666-m-high volcano has erupted during historical time, it has been less active than other volcanoes of Iceland's eastern volcanic zone, and relatively few Holocene lava flows are known. The sole historical eruption of Eyjafjöll, during December 1821 to January 1823, produced intermediate-to-silicic tephra from the central caldera.
A volcanic eruption started yesterday in south Iceland at or near the Eyjafjallajökull glacier.  The first signs of the eruption were seen between 23:00 and midnight GMT. It is still not clear where the exact location of the eruption is but it appears to be on Fimmvörðuháls.
Eyjafjallajökull glacier 
People living in the area are  being evacuated. Police have closed the roads into the area and a state of emergency has been declared. The eruption is expected to result in flooding if it is under the glacier, but fortunately it appears to between Eyjafjallajökull and another glacier, Mýrdalsjökull, reducing the danger of flooding. It is reported that the eruption can be seen as far away as from Vestmannaeyjar. So far the lava flow is viscous and thick.

There is also a danger that this eruption could trigger another eruption on nearby Mt. Katla, which is an off-rift volcano beneath Mýrdalsjökull. In the past, eruptions at Eyjafjallajökull have triggered eruptions at Mt. Katla. If Katla does erupt, the effect would be far more significant than that of the current eruption, as lava could melt the ice at the top of the mountain, causing the potential for massive flooding.

The air carrier Icelandair has diverted flights which were destined for Iceland as a precaution, due to the risks to aircraft of flying into volcanic ash.  A notice to airlines prohibits flights within 120 nautical miles of the volcano.

Iceland is a volcanic island situated atop the Mid-Atlantic Ridge at the juncture of the North American and Eurasian tectonic plates.