The Observatoire Volcanologique du Piton de la Fournaise reported a sharp rise in sulphur dioxide emissions from Piton de la Fournaise, a shield volcano which forms much of the eastern part of Réunion Island, an island in the western Indian Ocean which forms a department of France, on 3 May 2015, followed by a similar rise in hydrogen sulphide emissions on 5 May. From 4 May onwards a rise in Earthquake activity beneath the mountain, which often signifies magma moving through chambers beneath a volcano, was detected, combined with inflation of the base of the summit cone. This seismic activity climbed steadily till 17 May, when about 200 events were recorded over a period of about 90 minutes, between 11.00 am local time and 12.30 pm. A second burst of intense seismic activity began at about 12.50 pm, followed by an eruption from a new fissure to the southeast of Dolomieu Crater. A total of three new fissures were detected in the area that afternoon, all producing lava fountains, and tw lava flows were also observed, and a gas plume which rose about 4 km over the summit of the volcano and drifted to the northwest. The most westerly of the new fissures stopped emitting lava before midnight, and by morning on 18 may only a single fissure was active, producing a lava fountain reaching 40-50 m in height and a flow that travelled 4 km from the opening, as well as a smaller gas plume, with a high hydrogen sulphide and carbon dioxide content. On 19 May the fissure activity had subsided, with fountains reaching at most 20-30 m in height, and the lava flow proceeding only another 750 m during the day.
Lava fountain and flow emerging from a new fissure on Piton de la Fournaise. Clicanoo.
Piton de la Fournaise is believed to have been active for about 530 000 years, though its geology is complicated to unravel as lava flows are interbedded with those from Piton des Neiges, a larger, older and now extinct volcano to the northwest, which is responsible for the formation of about two thirds of the island. The island sits on the Réunion Hotspot, a deep mantle plume which is thought to have been active for about 66 million years, originally forming under what is now northeastern India, where it was responsible for the Deccan Traps flood basalts, then moving southward across the Indian Ocean (or more precisely sitting still while the continental plate upon which India and the Indian Ocean sit moves to the north), over time forming the Laccadive Islands, the Maldives, the Seychelles, Rodrigues Island, Mauritius and Réunion.
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