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The silicic peralkaline volcanoes of the East African Rift are some of the least studied volcanoes on Earth. Here we bring together new constraints from fieldwork, remote sensing, geochronology and geochemistry to present the first detailed account of the eruptive history of Aluto, a restless silicic volcano located in a densely populated section of the Main Ethiopian Rift. Prior to the growth of the Aluto volcanic complex (before 500 ka) the region was characterized by a significant period of fault development and mafic fissure eruptions. The earliest volcanism at Aluto built up a trachytic complex over 8 km in diameter. Aluto then underwent large-volume ignimbrite eruptions at 316 ± 19 ka and 306 ± 12 ka developing a ~ 42 km 2 collapse structure. After a hiatus of ~ 250 ka, a phase of post-caldera volcanism initiated at 55 ± 19 ka and the most recent eruption of Aluto has a radiocarbon age of 0.40 ± 0.05 cal. ka BP. During this post-caldera phase highly-evolved peralkaline rhyolite lavas, ignimbrites and pumice fall deposits have erupted from vents across the complex. Geochemical modelling is consistent with rhyolite genesis from protracted fractionation (> 80%) of basalt that is compositionally similar to rift-related basalts found east of the complex. Based on the style and volume of recent eruptions we suggest that silicic eruptions occur at an average rate of 1 per 1000 years, and that future eruptions of Aluto will involve explosive emplacement of localised pumice cones and effusive obsidian coulees of volumes in the range 1–100 × 10 6 m 3 . [ABSTRACT FROM AUTHOR]

The East African Rift (EAR) hosts the highest density of peralkaline volcanoes of any region globally, making it an ideal location to study the subaerial and magmatic processes of peralkaline volcanism. Corbetti Caldera is one such peralkaline centre found within the southern part of the Main Ethiopian Rift (MER), a segment of the EAR. Corbetti is actively deforming and has previously undergone large-scale Plinian eruptions. However, our broader understanding of Corbetti's evolution is limited. Here we present a detailed study of the pre-, syn-and post-caldera eruptive deposits and compile them into a composite stratigraphy of the volcanic sequence. We find evidence for multiple previously undocumented large-scale eruptions, including the deposition of two pre-caldera lithic lag breccias, and we identify three additional post-caldera obsidian lavas. We constrain the age of a young Tuff Cone, Biftu, which sits outside the caldera walls, to <7375 ± 54 cal BP through 14C dating of shells within an associated PDC deposit. The Wendo Koshe Cone, the most recent cone within the complex, is interpreted as the remains of two cones that opened in close proximity to each other and formed the vents for two pyroclastic eruptions: Bedded Pumice and Wendo Koshe Younger Pumice (WKYP). There is abundant evidence for the occurrence of pyroclastic density currents (PDC) within the caldera, usually associated with pumice cone-forming eruptions. These flow deposits rarely extend beyond the caldera walls. From our compiled stratigraphy, we estimate a recurrence rate of one eruption per 300–400 years over the last 2.3 ky. These findings are especially significant as Corbetti is earmarked for potential geothermal exploration, and an understanding of eruptive frequency and style provides vital context to assess the potential hazards associated with this future infrastructure. • We present a detailed composite stratigraphy of eruptive deposits from Corbetti Caldera, Ethiopia • Previously undocumented deposits are identified, indicating Corbetti commonly generates large-scale, dilute PDCs • The Wendo Koshe Cone comprises of two overlapping vents from which the Bedded Pumice and WKYP eruptions were sourced • The age of the Biftu Tuff Cone has been constrained to <7375 ± 54 cal BP • We estimate Corbetti has a recurrence rate of one eruption per 300–400 years over the last 2.3 ka [ABSTRACT FROM AUTHOR]

Magma plays a vital role in the break-up of continental lithosphere. However, significant uncertainty remains about how magma-crust interactions and melt evolution vary during the development of a rift system. Ethiopia captures the transition from continental rifting to incipient sea-floor spreading and has witnessed the eruption of large volumes of silicic volcanic rocks across the region over ∼45 Ma. The petrogenesis of these silicic rocks sheds light on the role of magmatism in rift development, by providing information on crustal interactions, melt fluxes and magmatic differentiation. We report new trace element and Sr–Nd–O isotopic data for volcanic rocks, glasses and minerals along and across active segments of the Main Ethiopian (MER) and Afar Rifts. Most δ 18 O data for mineral and glass separates from these active rift zones fall within the bounds of modelled fractional crystallization trajectories from basaltic parent magmas (i.e., 5.5–6.5‰) with scant evidence for assimilation of Pan-African Precambrian crustal material ( δ 18 O of 7–18‰). Radiogenic isotopes ( ε Nd = 0.92 – 6.52 ; 87 Sr/ 86 Sr = 0.7037–0.7072) and incompatible trace element ratios (Rb/Nb <1.5) are consistent with δ 18 O data and emphasize limited interaction with Pan-African crust. However, there are important regional variations in melt evolution revealed by incompatible elements (e.g., Th and Zr) and peralkalinity (molar Na 2 O + K 2 O / Al 2 O 3 ). The most chemically-evolved peralkaline compositions are associated with the MER volcanoes (Aluto, Gedemsa and Kone) and an off-axis volcano of the Afar Rift (Badi). On-axis silicic volcanoes of the Afar Rift (e.g., Dabbahu) generate less-evolved melts. While at Erta Ale, the most mature rift setting, peralkaline magmas are rare. We find that melt evolution is enhanced in less mature continental rifts (where parental magmas are of transitional rather than tholeiitic composition) and regions of low magma flux (due to reduced mantle melt productivity or where crustal structure inhibits magma ascent). This has important implications for understanding the geotectonic settings that promote extreme melt evolution and, potentially, genesis of economically-valuable mineral deposits in ancient rift-settings. The limited isotopic evidence for assimilation of Pan-African crustal material in Ethiopia suggests that the pre-rift crust beneath the magmatic segments has been substantially modified by rift-related magmatism over the past ∼45 Ma; consistent with geophysical observations. We argue that considerable volumes of crystal cumulate are stored beneath silicic volcanic systems (>100 km 3 ), and estimate that crystal cumulates fill at least 16–30% of the volume generated by crustal extension under the axial volcanoes of the MER and Manda Hararo Rift Segment (MHRS) of Afar. At Erta Ale only ∼1% of the volume generated due to rift extension is filled by cumulates, supporting previous seismic evidence for a greater role of plate stretching in mature rifts at the onset of sea-floor spreading. We infer that ∼45 Ma of magmatism has left little fusible Pan-African material to be assimilated beneath the magmatic segments and the active segments are predominantly composed of magmatic cumulates with δ 18 O indistinguishable from mantle-derived melts. We predict that the δ 18 O of silicic magmas should converge to mantle values as the rift continues to evolve. Although current data are limited, a comparison with ∼30 Ma ignimbrites (with δ 18 O up to 8.9‰) supports this inference, evidencing greater crustal assimilation during initial stages of rifting and at times of heightened magmatic flux. [ABSTRACT FROM AUTHOR]

Highlights: [•] The study unit is an alkali rhyolite phreatomagmatic ash and pumice fall deposit. [•] It has a minimum DRE volume of ∼0.8km3 (equivalent to an eruption magnitude of 5.3). [•] Deposition style changed over time from broadly dispersed to near-vent cone-building. [•] The maximum column height was ∼23km; neutral buoyancy height around the tropopause. [•] The 1-cm isopach contour likely extended for hundreds of km from the vent. [Copyright &y& Elsevier]

Abstract: Quaternary volcanism in the northern Main Ethiopian Rift is characterized by rift-axis eruptive centres with complicated collapse structures. Despite their association with major explosive eruptions, few modern studies have been carried out on them. We present the results of fieldwork and laboratory analyses of one of the largest of these centres, the Kone Volcanic Complex (KVC). It consists of nested calderas, trachyte lava flows, trachyte and alkali rhyolite pyroclastic deposits, alkali rhyolite lava domes and numerous small transitional to alkali basalt lava vents and flows. The largest subsidence structure, Birenti Caldera, has a diameter of ∼11km and an area of ∼95km2. Post-collapse volcanism in the interior of Birenti Caldera produced a predominantly trachytic cone that was partially destroyed by a further eruption, forming the well-preserved but irregularly-shaped Kone Caldera (diameter of ∼8km and an area of ∼24km2). Rhyolite lava dome eruptions and tectonically-controlled basalt lava effusion represent the most recent volcanism in the KVC. We report here the first detailed geologic map, along with petrologic and stratigraphic analyses, in order to reconstruct the eruptive history of this large but previously little-known volcanic complex. The findings have implications for understanding the relationships between rifting, tectonics and volcanism in the northern Main Ethiopian Rift, and for future efforts to trace widespread tephra horizons for regional stratigraphic correlation. [Copyright &y& Elsevier]

Abstract: Rift zones are the most common magmatic environment on Earth. However opportunities to observe active rifting are rare, and consequently the volcanological characteristics of rift systems are not well understood. An ongoing phase of magmatic rifting along a section of the Red Sea system in Afar, Ethiopia, presents an exceptional opportunity to constrain relationships between volcanism and crustal growth. Here, by integrating analyses of satellite images (i.e. MODIS, OMI, ASTER, and ALI) with field observations, we characterise two recent (August 2007 and June 2009) basaltic fissure eruptions in Afar and evaluate the role and significance of volcanism in the rifting process. Both events were brief (36–72h) and erupted 4.4–18×106 m3 of lava from a fissure system 4–6.5km in length. Data from the spaceborne Ozone Monitoring Instrument (OMI) suggests total SO2 emissions for each eruption of 26±5kt (2007) and 34±7kt (2009), consistent with complete degassing of the erupted magma volumes. Using geodetic models for the intrusive activity in Afar we estimate the partitioning of magma between intrusive and extrusive components, up to July 2009, to be ∼180:1. Comparing the first-order volcanic characteristics and the intrusive–extrusive volume balance for the Afar volcanism with data from the 1975–1984 Krafla rifting cycle (Iceland) suggests that the volcanic flux in Afar will rise significantly over the next few years as the stresses are increasingly relieved by dyking, and subsequent dykes are able to propagate more easily to the surface. As a consequence, basaltic fissure eruptions in this section of the Afar rift will become of increasing large magnitude as the rifting event matures over the next 5–10yr. Using available models of magmatic rifting we forecast the likely size and location of future eruptions in Afar. [Copyright &y& Elsevier]

Fumaroles are the surface manifestation of hydrothermal circulation and can be influenced by magmatic, hydrothermal, hydrological and tectonic processes. This study investigates the temporal changes in fumarole temperatures and spatial extent on Aluto, a restless volcano in the Main Ethiopian Rift (MER), in order to better understand the controls on fluid circulation and the interaction between the magmatic and hydrothermal systems. Thermal infrared (TIR) satellite images, acquired by the Advanced Spaceborne Thermal Emission and Reflection radiometer (ASTER) over the period of 2004 to 2016, are used to generate time series of the fumarole temperatures and areas. The thermal anomalies identified in the ASTER images coincide with known fumaroles with temperatures > 80 °C and are located on or close to fault structures, which provide a pathway for the rising fluids. Most of the fumaroles, including those along the major zone of hydrothermal upwelling, the Artu Jawe Fault Zone, have pixel-integrated temperature variations of only ~ 2 ± 1.5 °C. The exception are the Bobesa fumaroles located on a hypothesised caldera ring fault which show pixel-integrated temperature changes of up to 9 °C consistent with a delayed response of the hydrothermal system to precipitation. We conclude that fumaroles along major faults are strongly coupled to the magmatic-hydrothermal system and are relatively stable with time, whereas those along shallower structures close to the rift flank are more strongly influenced by seasonal variations in groundwater flow. The use of remote sensing data to monitor the thermal activity of Aluto provides an important contribution towards understanding the behaviour of this actively deforming volcano. This method could be used at other volcanoes around the world for monitoring and geothermal exploration. [ABSTRACT FROM AUTHOR]

The Main Ethiopian Rift (MER) is renowned as a focus of investigations into human origins. It is also the site of many large volcanic calderas, whose eruptions have spanned the timeframe of speciation, cultural innovation, and dispersal of our species. Yet, despite their significance for dating human fossils and cultural materials, the timing and geochemical signatures of some of the largest eruptions have remained poorly constrained at best. Here, through a programme of field surveys, geochemical analysis and 40Ar/39Ar dating, we report the ages of MER ignimbrites and link them to widespread tephra layers found in sequences of archaeological and paleoenvironmental significance. We date major eruptions of Fentale (76 ± 18 ka), Shala (ca. 145–155 ka), Kone (184 ± 42 ka and ca. 200 ± 12 ka) and Gedemsa (251 ± 47 ka) volcanoes, and correlate a suite of regionally important tephra horizons. Geochemical analysis highlights the predominantly peralkaline rhyolitic melt compositions (7.5–12 wt% Na 2 O + K 2 O, 70–76 wt% SiO 2) across the central MER and remarkable similarity in incompatible trace element ratios, limiting the correlation of deposits via glass composition alone. However, by integrating stratigraphic and geochronological evidence from proximal deposits, lake sediment cores and distal outcrops at archaeological sites, we have traced ash layers associated with the ca. 177 ka Corbetti, ca. 145–155 ka Shala and ca. 108 ka Bora-Baricha-Tullu-Moye eruptions across southern Ethiopia. In addition to strengthening the tephrochronological framework that supports paleoenvironmental and archaeological work in the region, our findings have wider implications for evaluating the hypothesis of a middle Pleistocene 'ignimbrite flare-up' in the MER, and for evaluating the impacts of these great eruptions on landscapes, hydrology, and human ecology. • Quaternary explosive eruptions in the Main Ethiopian Rift linked to palaeoanthropological and archaeological archives. • Review of the Pleistocene calderas of Main Ethiopian Rift. • An improved middle to late Pleistocene tephrostratigraphy for the Ethiopian Rift. [ABSTRACT FROM AUTHOR]