Long-term instability and retreat of lava deltas of Kilauea volcano, Hawai'i
Résumé
Subsidence and GPS measurements have been done on lava deltas formed on the coast during the Pu‘u ‘Ō‘ō - Kūpa‘ianahā eruptions of Kīlauea Volcano to see the evolution of these forms and the eventual hazards for tourists. When the flow of lava stops, benches formed below the old sea cliff continue to be unstable during several years. Landslides of the pyroclastic basement involve subsidence and collapse, partial or complete, of the delta. For example, the East Kupapa‘u bench, built between April 2001 and January 2002, has been subsided of 12 cm/year afterwards. Progressively, by compaction, the fan of pyroclasts stabilizes and so the delta. The bench can be qualified as mesastable when there is a relative stability. For instance, between 2000 and 2002, the subsidence only reaches from 0 to 3 mm/year on the 1995 bench. Movements are mostly concentrated on the outer part of the delta. Thus, on the benches we surveyed, the external margin subsides from 0,6 to 8,6 cm/year and gently moves seaward from 5 to 27 cm/year, with opening of cracks more or less parallel to the coastline. This movement certainly traduces underlying slides. Paroxysmic events cause collapses of the outward margin, building new benches, which can sometimes entirely disappear into the sea. Underlying slides, pressure of breaking waves, decompression and gravity create slow decollement of the cliff along the cracks too, up to breakpoint with toppling. The delta stabilization explains the decreasing retreat of the coast, shown by the GPS measurements done on the coast of the Pu‘u ‘Ō‘ō - Kūpa‘ianahā eruptions between 1997 and 2002. Firstly, numerous collapses of the bench result in a quick retreat of the coast. From approximately 100 m/year, the rate quickly lessens to 10 m/year or so, two years after the end of the bench building. Then, the rate stays high but reduces slowly during more or less 10 years to reach about 2,5-5 m/year. The stability of the major part of the delta prevents from large collapses, which only occur sporadically and to a lesser degree in the unstable outer part. When the delta is totally stable, the retreat of the coast is lower and only made by wave attack, which provokes topple and rockslides of the strongly fissured rock. Instability of inactive lava deltas, mostly on the external margin, causes hazards for tourists who walk on them. The dangerousness is especially high because of the absence of hazard perception, contrary to active lava deltas.