The 1991 eruption fit a pattern defined by Hekla eruptions. The length of the preceding repose period is positively related to the volume and initial silica concentration of the lava and tephra and the explosivity and effusion rate of early phase of the eruption. The previous eruption was in 1981. Note the lava flows draping the slopes of the volcano and compare to the above map. Fissures and vents directly on Hekla are fed more silica rich magmas.
Eruptions fed by the margins of the magma chamber tend to be lower in silica.
One interesting observation was an increase in the concentration of fluorine in river water on the fifth day of the eruption. This was Hekla's third eruption in 20 years. Since 1104 there have been 55 eruptions, suggesting an average repose period of 55 years. Volcanol., v. 36, p. 269-288.