Rootless littoral hydrovolcanic explosions and deposits, 6 April 2007, Piton de la Fournaise, La Réunion Island

Abstract

The April 2007 basaltic eruption of Piton de la Fournaise began on 2 April at Piton Tremblet on the eastern flank of the volcano, 7 km from the summit. Vigorous fountains fed aphyric lavas that flowed another 3 km then entered the ocean. During the next few days, the olivine content of the lavas increased and by 5 April, the lavas were oceanite with ∼30 vol% olivine crystals. On 6 April, following a significant increase in lava discharge, spectacular hydrovolcanic explosions occurred where the lava in the main channel entered the ocean. Throughout the day, a sustained steam plume, vertical pyroclastic plumes, base surges and tephra jets were generated by the explosions. The products formed a nested littoral cone at the coast that was partly destroyed and almost entirely covered by subsequent lavas. Although lavas of oceanite composition continued to enter the ocean for the next 24 days, the hydrovolcanic explosions ceased after one day. One 7-m-thick exposure of the littoral cone studied in 2013 comprises bedded, poorly sorted combinations of glassy ash and vesicular lapilli and bombs. Three facies have been identified and interpreted to be deposits from base surges, fall and tephra jets. The explosive lava-water interaction coincided perfectly with a period of elevated discharge at Piton Tremblet on 6 April; the discharge rate subsequently declined. The Piton de la Fournaise 2007 littoral cone resembles historic ‘a‘ā-fed littoral cones on Hawaii. On Piton de la Fournaise, lava entering the ocean at high flow rate or following an increase in discharge ought to be considered potentially hazardous for rootless littoral hydrovolcanic explosions.