THE OPENING SUBPLINIAN PHASE OF THE HEKLA 1991 ERUPTION: PROPERTIES OF THE TEPHRA FALL DEPOSIT

The 1991 Hekla eruption started on 17 January with an intense 50-minute-long explosive phase that transitioned into fire fountain activity lasting for 2 days. The opening phase eruption column rose to its maximum height of 11.5 km in about 10 minutes and produced 8.6 x 10⁹ kg of tephra, which makes this VEI 3 event. Isomass and isopleth contours are weakly bilobate in the proximal region, with the principal dispersal axis to the north-northeast.

Grain-size measurements reveal systematic decrease in grain-size away from Hekla. The grain-size distribution of the tephra at any one sample site is typically unimodal, although a weakly bimodal distribution is present at a few sites where the second mode is minor and peaking in the very fine ash fraction. The calculated total grain-size distribution (TGSD) is bimodal, with a primary peak in the lapilli size fraction at -3.5 to -2.5 φ and a subordinate fine ash peak at 2.5 to 3.5 φ. The lapilli was deposited within the first 25 km of transport, whereas the fine ash peak fits well with the modal grain-size of samples deposited >65 km from Hekla.

The bimodality of the TGSD for the opening phase tephra emphasizes the importance of capturing the distal, thin and fine grained parts of the fallout, because otherwise we may not obtain important information concerning magma fragmentation as well as modes of tephra transport. Rate of magma ascent and conditions for vesiculation in the shallow conduit became increasingly uniform with time throughout the 1991 opening phase.