Mechanism of the 1991 eruption of Hekla from continuous borehole strain monitoring

VOLCANOES erupt when the pressure in a magma chamber several kilometres below the edifice overcomes the strength of the intervening rock. Seismic activity may accompany and precede eruptions, allowing (in favourable circumstances) the location and movement of magma to be traced. Ground deformation near volcanoes can provide more direct evidence for magma movement, but continuous monitoring is necessary to ensure that all the essential aspects of an eruption are recorded. Here we report dilatational strain data collected continuously during the January 1991 eruption of Hekla volcano by five borehole strainmeters located 15-45 km from the volcano. The data record the upward propagation of magma, as well as the deflation of a deep reservoir. In only 30 minutes the magma forced open a conduit to the surface from a depth of ∼4km. Although other volcanoes might behave differently, our results suggest the possibility of using continuous deformation measurements to monitor conduit formation at other sites, perhaps providing short-term warnings of impending eruptions.