Harmful gases emanating from the volcanic fissure near Reykjavik threaten to pollute the air in the village of Vogar, near the eruption, the Meteorological Institute of Iceland (IMO) said on Friday.
The gases could spread to Reykjavik, located 40 km from the volcano, by Saturday.
Sulfur dioxide concentrations could reach up to 2,600 micrograms per cubic meter, a level considered "harmful to the health of sensitive people", according to the Icelandic Environment Agency.
The Meteorological Office warns, however, that the models are uncertain "due to the very uneven flow of the eruption".
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The warning came after measurements showed the fissure's volcanic activity had halved and its length had shrunk by around 200m.
Its flow estimated at 32 cubic meters per second during the first hours was halved the next day and is only around 18 cubic meters per second on average, according to an assessment carried out on Thursday morning.
A risk of new cracks
“This behavior is very similar to what is usually observed during eruptions in the country: the eruption is powerful at the beginning and then subsides,” writes the Institute of Earth Sciences.
The lava field extended Thursday over 144,000 m2.
The volcanic fissure, a crack in the earth from which the viscous liquid spurts out at 1,200°C, is however no longer as long as it was originally and would now extend over around 130 m.
" It's so beautiful !
": in Iceland, the eruption of a volcanic fissure attracts the curious
The pressure in the dike – the magma tunnel feeding the eruption – would not be balanced, leading geophysicists to say the flow could increase or an eruption could start at a new location.
"New cracks can open in the immediate vicinity of the eruption site in a very short time," warns the Meteorological Office of Iceland.
Visitors flock to hear the ground rumble beneath their feet and roar as fountains of lava spurt from the depths.
According to counters installed for a year and a half by the authorities, more than 4,200 curious people traveled the 14 km round trip of the arduous path on Thursday to reach the site on the Reykjanes peninsula in south-west Iceland, at about two hours from the nearest parking lot.