Explosive eruptions from the interaction of magmatic and hydrothermal systems during flank extension: the Bellecombe Tephra of Piton de La Fournaise (La Réunion Island)

Piton de la Fournaise (La Réunion Island) is a very active, primarily effusive ocean-island volcano. The Bellecombe Tephra represents at least three explosive eruptions that occurred between about 5465 and 2971 calendar years BP. Near the margin of the present-day Enclos Fouqué caldera margin, two Bellecombe eruptions produced a sequence of two tuff breccias interbedded with tuff. The tuff breccias only reach a few hundred meters outside the current caldera margin. At Petite Carrière, an old scoria cone ~1 km from the Enclos Fouqué margin, these two deposits (the “lower Bellecombe Tephra”) are represented by two tuffs with incipient soil formation at the top of each. They are overlain by a third unit (the upper Bellecombe Tephra) made of bedded lapilli tuff and tuff, some reworked in small debris flows off the scoria cone. The lapilli increase in size and the beds in thickness southeastward, toward Chisny volcano and away from the Enclos Fouqué caldera. Deposits from the upper Bellecombe tephra are correlated to sites 5 km northwest of Petite Carrière and 6 km north of a postulated vent location on the north side of Chisny volcano. Distribution patterns of all Bellecombe tephra are consistent with eruption columns that did not rise above 8 km asl. The ash fraction of the Bellecombe Tephra contains three juvenile components: a dominant gray vitric basaltic ash, an oceanitic (olivine-rich basalt) ash, and pyroxene-bearing gabbro with a few percent glass. It also contains doubly terminated clear quartz grains, and olivine and rarer clinopyroxene crystals. The lower Bellecombe Tephra contains an altered brown ash, whereas a tan-yellow clay-rich ash is common in the upper unit. Lava flows of gray aphyric basalt and oceanite are exposed at the surface and preceded the Bellecombe eruptions, but the gabbro, quartz crystals, and hydrothermally altered grains indicate the involvement of the magma/hydrothermal system from 0.5- to 2-km depth. We propose that the three eruptions of the Bellecombe tephra were preceded by voluminous eruptions of lava flows that led to seaward-sliding mass movement of the volcano. This opened fractures that explosively depressurized the hydrothermal system and incorporated magma and gabbro, from the magma system and probably from rock along the fissure, in the ejecta. This model is consistent with the small erupted volume (150 Mm³ for the lower two combined and about the same for the upper tephra) and presence of components from a variety of depths. If the eruption had excavated downward, more surficial components would be expected. Instead, the fissures allowed material from many depths to erupt simultaneously. Similar eruptions may occur after voluminous eruptions and/or when lateral seaward sliding of the volcano occurs.