NATURE OF EXPLOSIVE ACTIVITY IN THE 10TH CENTURY ELDGJA ERUPTION, ICELAND

The 10^th century Eldgjá flood lava event is the largest eruption in Iceland in historical time (i.e. the last 1140 years) and erupted 21 km³ of magma, 19.7 km³ as lava and about 1.3 km³ as tephra (all volumes are given as dense rock equivalent). Activity began in the southwest beneath the Mýrdalsjökull glacier and progressed to the northeast with time to form a 70 km long vent system. The tephra was formed in at least 13 separate explosive episodes originating initially from subglacial and later subaerial fissure segments along the vent system, producing a sequence of phreatomagmatic and magmatic tephra deposits more than 2.5 m thick at distance of 10 km from the vents. Unit 7 (magmatic) and unit 8 (phreatomagmatic) were studied in detail. Both units were dispersed to the east-southeast from 13 to 17 km high eruption columns. Unit 7 deposit has a volume of 0.02 km³ and has a positively-skewed unimodal total deposit grain-size distribution with a mode at -2.5 φ and a median grain-size of -2.2 φ. Unit 8 has a volume of 0.03 km³ and a bimodal total deposit grain-size distribution with broad peaks at 0.0 φ and 5.0 φ. Hence, the total deposit grain size distribution for the magmatic versus the phreatomagmatic units are distinct and record different fragmentation histories. However, the high clast vesicularity (61-80%) along with identical vesicle-size distributions for both units show that the magma erupted in the phreatomagmatic phase featured identical vesiculation (i.e. nucleation and growth of bubbles) as the magma erupted in the magmatic phase. Hence, in the case of the phreatomagmatic phase the external water came into contact with the magma after it was fully vesiculated or in other words at the time of venting. We take this to indicate that the contribution of the external water to magma interaction was confined to enhancing fragmentation an already formed lapilli-size foam of the magma upon venting via quenched granulation and its contribution to enhancing the intensity of the explosive phases was minimal.