Anticipating that a massive and potentially deadly torrent of water could be unleashed, Icelanders reinforced freshly built barriers along river banks, and prepared to dig channels through roads in the threatened region. The aim is to limit any damage to bridges, power stations and telephone lines.
The eruption of the volcano, which was last active in 1938, has continued for a week, while the level of water that has been melted beneath the glacier by the hot magma has reached its highest level this century. If a fresh eruption breaks up the glacier cap, the water will run to the sea, devastating anything in its path.
This could be more dangerous than standard volcanic eruptions because the column of water will move much faster than molten rock, or lava, thrown out in normal incidents.
Pall Imsland, a geologist at the University of Iceland, said: "As soon as we know that the water has begun to flow, we'll start to dig through the road to try and save the bridges on the central plain in front of the glacier."
The uninhabited Vatnajokull glacier lies in south-east Iceland, 120 miles east of Reykjavik and roughly the same distance south of the Arctic Circle. It is separated from the coast by a fringe of farmland, where farmers have been warned to take their livestock into shelter to reduce the danger from toxic gases thrown off by the eruption.
Scientists monitoring the eruption at the Vatnajokull ice sheet reckon that the present activity probably began on Tuesday, and that about two cubic kilometres of water and debris had collected under the glacier.
Professor Imsland said: "The reservoir is filling very fast. We're just waiting for the starting mechanism of the flood, which is a process we don't really understand." Seismic monitors showed a steady pattern of tremors.
The original eruption at the Grimsvotn crater, which started on Wednesday, burst through some 600m of ice, spewing a column of steam and ash up to 10km high. The column has since subsided to between one and 3km. Aircraft are being diverted around it.
The Vatnajokull glacier stretches over 8,300sq kms and in parts reaches a depth of 1,000m. Its name, which means "water glacier", originates from the large lakes and rivers under its ice sheet, which are created from the ice melted by the heat of the underlying volcano. In 1938 an eruption at exactly the same site as the present one caused massive flooding.
Iceland is a comparatively active volcanic area of northern Europe, and has not escaped the consequences. In 1973, the eruption of a volcano near the sea town of Heimaey opened a fissure 1,600m long fissure on the east side of the island, spouting lava in glowing fountains up to 150m high. The activity continued for six months, by the end of which 417 houses had been destroyed by lava and the remainder of the town was subsumed in 250 cubic metres of material from the eruption.
Volcanoes are the unpredictable results of weak points in the earth's crust. Where the pressure caused by movement of the molten magma becomes too great, it breaks through the upper layers to produce volcanic vents. In most cases these are relatively harmless, though sometimes the volcanic gases can be poisonous, and the pyroclastic flows - combinations of heated earth, mud and rock - can demolish anything in their path. Europe has a number of such volcanic hot spots, including a number in Italy, but it is the sea bed which provides the majority of volcanic weaknesses.
One of the most active areas for volcanoes is Hawaii, which has a number of active and extinct volcanos in its immediate area.
One of the biggest problems faced by vulcanologists, and echoing that of seismologists attempting to deal with earthquakes, is prediction. Volcanoes, like seismic faults, may lie quiescent for years before abruptly bursting into life again.
Hundreds flee threatened port in New Guinea
Government officials in Papua New Guinea said yesterday that about 300 people had left the port town of Rabaul since Friday after one of two active volcanoes erupted, spewing ash 4,000 metres into the air and opening a new lava chasm. The eruption lasted three hours.
Rabaul, on New Britain island - 800 kilometres north-east of the Papua New Guinea capital Port Moresby - was devastated in September 1994, when the two volcanoes on either side of the town erupted, forcing the evacuation of its 30,000 residents. Five people died during evacuation and subsequent attempts to return to the town over the next few days to rescue belongings from gangs of looters. An earthquake measuring 6.2 on the Richter scale shook Rabaul in August last year and there was another eruption last May which was more severe than Friday's rumbling.