No abstract

The Ethiopian Rift Valley hosts the longest record of human co-existence with volcanoes on Earth, however, current understanding of the magnitude and timing of large explosive eruptions in this region is poor. Detailed records of volcanism are essential for interpreting the palaeoenvironments occupied by our hominin ancestors; and also for evaluating the volcanic hazards posed to the 10 million people currently living within this active rift zone. Here we use new geochronological evidence to suggest that a 200 km-long segment of rift experienced a major pulse of explosive volcanic activity between 320 and 170 ka. During this period, at least four distinct volcanic centres underwent large-volume (>10 km3) caldera-forming eruptions, and eruptive fluxes were elevated five times above the average eruption rate for the past 700 ka. We propose that such pulses of episodic silicic volcanism would have drastically remodelled landscapes and ecosystems occupied by early hominin populations.

Integrated field-based volcanology and petrologic studies can provide relevant clues about the way in which structural features, magma replenishment of a shallow sub-caldera reservoir, and magma evolution exert control on eruption behavior during post-caldera volcanism. Post-caldera activity of the past 1000 yrs at the La Fossa caldera (Island of Vulcano, Italy), occurred at two vents: the dominantly explosive La Fossa vent located at the center of the caldera and the lava-dominated Vulcanello vent located close to the northern ring fault of the caldera. Revised chrono-stratigraphic data indicate that the activity occurred in two clusters of eruptions: in the 11 th -12 th centuries and during the 17 th century. The activity was in part contemporaneous with La Fossa vent and led to the formation of three, partially overlapped strombolian cones. Each cone-building episode was accompanied by the outpouring of lava in subaerial and submarine environments. The erupted volumes of the pyroclastic cones vary between 2x10 -3 km 3 and 3x10 -6 km 3 , while the volumes of the three lavas span between 0.3 km 3 (a submarine lava field) and 3x10 -3 km 3 . Petrology data indicate that the activity of Vulcanello was fed by three different magma batches: Vulcanello 1 was fed by a slightly zoned reservoir of shoshonitic composition, Vulcanello 2 was fed by a slightly more evolved magma whereas Vulcanello 3 was fed by latitic magma. The compositions of melt inclusions (major elements and volatile content) trapped in olivine separated from pyroclastic materials record the entire differentiation history and suggest that all Vulcanello magmas underwent volatile loss during pre-eruption equilibration at ≤1 km depth. Integration of all available information also suggests that in the last 1000 yrs the northern caldera fault acted as a preferential duct for the rise of degassed magma from the sub-caldera magma reservoir whereas volatile release namely took place separately at La Fossa cone where activity was almost entirely explosive in nature.