MECHANISM OF THE 1991 ERUPTION OF HEKLA FROM CONTINUOUS BOREHOLE STRAIN MONITORING

VOLCANOES erupt when the pressure in a magma chamber several kilometre s below the edifice overcomes the strength of the intervening rock. Se ismic activity may accompany and precede eruptions, allowing (in favou rable 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 19 91 eruption of Hekla volcano by five borehole strainmeters located 15- 45 km from the volcano. The data record the upward propagation of magm a, as well as the deflation of a deep reservoir. In only 30. minutes t he magma forced open a conduit to the surface from a depth of approxim ately 4 km. Although other volcanoes might behave differently, our res ults suggest the possibility of using continuous deformation measureme nts to monitor conduit formation at other sites, perhaps providing sho rt-term warnings of impending eruptions.