Vinsamlegast notið þetta auðkenni þegar þið vitnið til verksins eða tengið í það: http://hdl.handle.net/1946/24917
The Laki 1783-4 fissure eruption was a devastating time for Iceland. It featured 10 separate eruption episodes and two of them were phreatomagmatic. The phreatomagmatic (P1) eruption from fissure four produced tuff cone 1 on the SW section of the Laki cone row. Samples collected from the NW and SE side of the cone display a rich abundance of ash and gold pumice and it produced mean vesiculation values between 84-88% with some values as high as 97%. This very high vesiculation mimic values seen in the magmatic phases of the eruption, meaning that the magma, during the P1 eruption, was able to reside near the surface and fragment fully to magmatic foam before external water quenched the already vesiculated clasts. External water had no effect on the explosivity of this eruption phase and its main role was to quench the magma and granulate the fragile clasts. Fuel-coolant interaction processes cannot occur at such shallow conditions because the fragmented foam has a high yield strength making this kind of environment unsuitable to host such an interaction. Grain size data lead to the conclusion that the P1 eruption began on June 27th 1783 where a westerly wind deposited samples on the SE side of the cone and then an easterly wind on June 30th deposited the rest of the samples on the NW side. The SE side of the tuff cone has higher ash (~60-90%) content and golden pumice concentration. This increased ash content is due to cooling-contraction granulation of the vesiculated clasts and melt leading to an additional fragmentation. The availability of external water dwindled leading to the deposit of coarser-grained samples on the NW side of the tuff cone. High effusive rate (107 kg/s) and high bubble number densities (107 cm3) consisting of a rich population of small bubbles and bimodal tendencies indicate increased magma ascent rate due to early degassing of CO2 and burst nucleation of H2O, meaning Laki was most likely of subPlinian-Plinian intensity.
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