Increased levels of sulfur dioxide (SO2) gas in the town of Grindavik were detected this afternoon, while residents had been allowed to return in order to collect personal belongings from their abandoned homes. As a result, an immediate evacuation was ordered.
The gas meters had been installed recently in an effort to give as much warning as possible should an eruption begin while at the same time allowing a calculated-risk based management to allow people return and save as much as possible from homes.
Sulphur dioxide is contained in solution in magma at greater depths, but when magma is closer to the surface and under less pressure, most if it is released as its solubility decreases with decreasing pressure, thus leaves the magma and finds its way to the surface through cracks where fumaroles form or by diffuse degassing.
Since gas is much more mobile than the molten rock (magma) itself, it is typically arriving at the surface before an eruption starts. Thus, it could be a sign that an eruption is indeed imminent.