The last time the Eyjafjallajokull volcano on the southern flank of Iceland erupted was in 1821 and that event took almost two years to subside.
Volcanologists are unsure whether the current eruption is going to be a similar, long-lived affair with months or even years of air-traffic disruption.
The 1600m-high volcano lies just west of the Katla volcano and one fear is that this second volcano could be disturbed by Eyjafjallajokull's activity, which initially began in March with the eruption of magma from a small vent or fissure on its lower eastern slopes.
The present eruption emerged from fissures within the summit's 2.5km-wide caldera, a huge crater that sits at the base of a 150m-thick glacier which is now melting, causing local flooding.
Volcanic ash from the eruption has reached a height of 18km and is heading mainly towards Scandinavia, but with substantial amounts spreading across Britain from the north.
Volcanic ash, formed when bubbles of molten magma collapse as they erupt from the ground, is like highly abrasive talcum powder. It is made of sharp fragments of rock that are capable of eroding plastic, metal and glass. The fine particles can block fuel nozzles and stick to the turbine blades of aircraft engines, causing them to stall.
The current ash cloud is too high to be a risk to people on the ground and too thin to be seen, although it is likely to make sunsets redder because of the way the volcanic particles diffract the sunlight.
Experts have predicted that the ash cloud will drift over southern England, but where it eventually goes depends on the winds at that altitude.
"At the moment there is a north-westerly airflow from Iceland into northern parts of the North Sea, thus most of the ash will be transported to Norway and central Sweden. Britain will be affected as well, though I don't expect it to be as strongly affected as Norway," said Anja Schmidt of the University of Leeds.
This eruption is not particularly big, but there is a possibility that it might continue for weeks or months, making it difficult for air-traffic controllers who have to take every precaution to prevent aircraft from flying into a cloud of ash, which cannot be detected by an aircraft's on-board radar.
Dave McGarvie, a volcanologist at the Open University, said there are several possible scenarios for what happens next. One is that the eruption could continue for a bit longer and then fizzle out. Another is that it could stimulate an even larger eruption.
"In this instance, the resulting ash cloud would disrupt international travel with flight paths being diverted," Dr McGarvie said.
Andrew Bell, a geologist at the University of Edinburgh, agreed: "There is a possibility that the current eruption could evolve into a more explosive event, involving a different composition of magma, or could lead to activity at the neighbouring volcano, Katla. Both scenarios could result in further ash production and disruption to air traffic."