Probabilistic volcanic hazard assessment for pyroclastic density currents from pumice cone eruptions at Aluto Volcano, Ethiopia

Aluto volcano, in the Main Ethiopian Rift, is a peralkaline caldera system, which comprises conglomerations of rhyolite (obsidian) lavas and enigmatic pumice cones. Recent work at Aluto has found that pumice cone eruptions are highly unsteady, and form convective eruption plumes that frequently collapse to generate pyroclastic density currents (PDCs). We develop a methodology and present results for the first probabilistic volcanic hazard assessment (PVHA) for PDCs at a pumice cone volcano. By doing so, we estimate the conditional probability of inundation by PDCs around Aluto volcano, incorporating the aleatory uncertainty in PDC hazard. We employ a Monte Carlo energy cone modeling approach, which benefits from parameterization informed by field investigations and volcanic plume modeling. We find that despite the relatively modest eruptions that are likely to occur, the wide distribution of past vent locations (and thus the high uncertainty of where future vents might open), results in a broad area being potentially at risk of inundation by PDCs. However, the aleatory uncertainty in vent opening means that the conditional probabilities are lower (≤ 0.12), and more homogeneous, over the hazard domain compared to central-vent volcanoes (where conditional probabilities are often ≤ 1 close to the vent). Despite this, numerous settlements, amenities, and economically valuable geothermal infrastructure, lie within the most hazardous (P(PDC|eruption) ≥ 0.05) regions of Aluto caldera. The Monte Carlo energy cone modeling approach provides a quantitative, accountable and defendable background and long-term PVHA for PDCs from Aluto. These results could be combined in the future with hazard assessments relating to tephra fall and/or lava to develop a comprehensive volcanic hazard map for the caldera. Following appropriate parameterization, the approach developed here can also be used to compute a PDC PVHA at other volcanoes where vent location is uncertain

Intrusive equivalents of flood volcanics: evidence from petrology of xenoliths in Quaternary Tana basanites

The Injibara Quaternary basanites enclose a variety of xenoliths spanning in composition from peridotite through pyroxenite and gabbro to granite. This study focuses on the pyroxenite, gabbro and granite xenoliths. The pyroxenite xenoliths (enstatite + diopside + olivine ± spinel ± plagioclase ± ilmenite ± paragasite) are diverse, including olivine-orthopyroxenite, olivine-clinopyroxenite and websterite. They represent a suite of crystal cumulates from basalts with tholeiitic affinity evolving by polybaric crystal fractionation processes, or alternatively they may be related to chemical diversification of parental magma. The gabbro xenoliths, containing widely varying modal proportions of plagioclase, augite, enstatite, olivine and ilmenite, appear to be fragments of cumulate plutonic rocks, fractionated from tholeiitic basalts at high-level. They often show reaction textures, with a vermicular intergrowth of smaller augite, plagioclase and a chemically complex opaque phase composition embedded in glass from orthopyroxene. The granitic xenoliths contain quartz and two distinct feldspars–a sodic plagioclase and a potassic alkali feldspar– coprecipitated from the melt; accompanying mafic minerals are hydrous biotite and muscovite. This suggests that such granite magmas crystallized under water-saturated condition (PH2O = 5 kbar). Keywords/phrases: Cumulate, Ethiopia, Injibara, tholeiitc basalt, xenolith SINET: Ethiopian Journal of Science Vol.26(2) 2003: 93-10

Structural controls on fluid pathways in an active rift system : a case study of the Aluto volcanic complex

Hutchison was funded by NERC studentship NE/J5000045/1.In volcanically and seismically active rift systems, preexisting faults may control the rise and eruption of magma, and direct the flow of hydrothermal fluids and gas in the subsurface. Using high-resolution airborne imagery, field observations, and CO2 degassing data on Aluto, a typical young silicic volcano in the Main Ethiopian Rift, we explore how preexisting tectonic and volcanic structures control fluid pathways and spatial patterns of volcanism, hydrothermal alteration and degassing. A new light detection and ranging (lidar) digital elevation model and evidence from deep geothermal wells show that the Aluto volcanic complex is dissected by rift-related extensional faults with throws of 50-100 m. Mapping of volcanic vent distributions reveals a structural control by either rift-aligned faults or an elliptical caldera ring fracture. Soil-gas CO2 degassing surveys show elevated fluxes (>>100 g m-2 d-1) along major faults and volcanic structures, but significant variations in CO2 flux along the fault zones reflect differences in near-surface permeability caused by changes in topography and surface lithology. The CO2 emission from an active geothermal area adjacent to the major fault scarp of Aluto amounted to similar to 60 t d-1; we estimate the total CO2 emission from Aluto to be 250-500 t d-1. Preexisting volcanic and tectonic structures have played a key role in the development of the Aluto volcanic complex and continue to facilitate the expulsion of gases and geothermal fluids. This case study emphasizes the importance of structural mapping on active rift volcanoes to understand the geothermal field as well as potential volcanic hazards.Publisher PDFPeer reviewe

The eruptive history and magmatic evolution of Aluto volcano: new insights into silicic peralkaline volcanism in the Ethiopian rift

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 km2 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 × 106 m3

Mixing and crystal scavenging in the Main Ethiopian Rift revealed by trace element systematics in feldspars and glasses

For many magmatic systems, crystal compositions preserve a complex and protracted history which may be largely decoupled from their carrier melts. The crystal cargo may hold clues to the physical distribution of melt and crystals in a magma reservoir and how magmas are assembled prior to eruptions. Here we present a geochemical study of a suite of samples from three peralkaline volcanoes in the Main Ethiopian Rift. Whilst whole-rock data shows strong fractional crystallisation signatures, the trace element systematics of feldspars, and their relationship to their host glasses, reveals complexity. Alkali feldspars, particularly those erupted during caldera forming episodes, have variable Ba concentrations, extending to high values that are not in equilibrium with the carrier liquids. Some of the feldspars are antecrysts, which we suggest are scavenged from a crystal-rich mush. The antecrysts crystallised from a Ba-enriched (more primitive) melt, before later entrainment into a Ba-depleted residual liquid. Crystal-melt segregation can occur on fast timescales in these magma reservoirs, owing to the low viscosity nature of peralkaline liquids. The separation of enough residual melt to feed a crystal-poor post caldera rhyolitic eruption may take as little as months to tens of years (much shorter than typical repose periods of 300-400 years). Our observations are consistent with these magmatic systems spending significant portions of their life cycle dominated by crystalline mushes containing ephemeral, small (< 1 km3) segregations of melt. This interpretation helps to reconcile observations of high crustal electrical resistivity beneath Aluto, despite seismicity and ground deformation consistent with a magma body.This project is funded by the Natural Environment Research Council grant NE/L013932/1 (RiftVolc)

Structural controls on magma pathways in Bora-Baricha-Tullu Moye (BBTM) volcanic system, Main Ethiopian Rift

The Bora-Baricha-Tullu Moye (BBTM) volcanic complex is located at a transitional zone in the Main Ethiopian Rift where tectonic and volcanic features show complex interplays. We mapped and characterised volcanic and tectonic features using high-resolution digital elevation models and performed morphometric and vent spatial distribution analyses. Structural analysis reveals NNE–SSW, NE–SW, and NW–SE trending faults in the region. The dominant post-caldera volcanic landforms are lava domes, pumice cones, scoria cones, maars, obsidian coulees and lava flows, which have distinct morphological characteristics. Vent elongation and alignment highlight close association between these landforms and the caldera(s) as well as with tectonic structures, suggesting these structures acted as the main magma pathways during the BBTM recent eruptions. We estimate that during the entire BBTM post-caldera phase a total bulk volume of 10.9 km3 of material was erupted. This would represent a time-averaged magma flux of 0.05 km3 ky-1 in the BBTM

Contrasting styles of post-caldera volcanism along the Main Ethiopian Rift : implications for contemporary volcanic hazards

This work was funded by the Natural Environment Research Council grant NE/L013932/1 (RiftVolc) and a Boise Fund grant from the Department of Zoology, University of Oxford.The Main Ethiopian Rift (MER, ~7–9°N) is the type example of a magma-assisted continental rift. The rift axis is populated with regularly spaced silicic caldera complexes and central stratovolcanoes, interspersed with large fields of small mafic scoria cones. The recent (latest Pleistocene to Holocene) history of volcanism in the MER is poorly known, and no eruptions have occurred in the living memory of the local population. Assessment of contemporary volcanic hazards and associated risk is primarily based on the study of the most recent eruptive products, typically those emplaced within the last 10–20 ky. We integrate new and published field observations and geochemical data on tephra deposits from the main Late Quaternary volcanic centres in the central MER to assess contemporary volcanic hazards. Most central volcanoes in the MER host large mid-Pleistocene calderas, with typical diameters of 5–15 km, and associated ignimbrites of trachyte and peralkaline rhyolite composition. In contrast, post-caldera activity at most centres comprises eruptions of peralkaline rhyolitic magmas as obsidian flows, domes and pumice cones. The frequency and magnitude of events varies between individual volcanoes. Some volcanoes have predominantly erupted obsidian lava flows in their most recent post-caldera stage (Fentale), whereas other have had up to 3 moderate-scale (VEI 3–4) explosive eruptions per millennium (Aluto). At some volcanoes we find evidence for multiple large explosive eruptions (Corbetti, Bora-Baricha, Boset-Bericha) which have deposited several centimeters to meters of pumice and ash in currently densely populated regions. This new overview has important implications when assessing the present-day volcanic hazard in this rapidly developing region.PostprintPeer reviewe

Mixing and crystal scavenging in the Main Ethiopian Rift revealed by trace element systematics in feldspars and glasses

For many magmatic systems, crystal compositions preserve a complex and protracted history which may be largely decoupled from their carrier melts. The crystal cargo may hold clues to the physical distribution of melt and crystals in a magma reservoir and how magmas are assembled prior to eruptions. Here we present a geochemical study of a suite of samples from three peralkaline volcanoes in the Main Ethiopian Rift. Whilst whole‐rock data shows strong fractional crystallisation signatures, the trace element systematics of feldspars, and their relationship to their host glasses, reveals complexity. Alkali feldspars, particularly those erupted during caldera‐forming episodes, have variable Ba concentrations, extending to high values that are not in equilibrium with the carrier liquids. Some of the feldspars are antecrysts, which we suggest are scavenged from a crystal‐rich mush. The antecrysts crystallised from a Ba‐enriched (more primitive) melt, before later entrainment into a Ba‐depleted residual liquid. Crystal‐melt segregation can occur on fast timescales in these magma reservoirs, owing to the low viscosity nature of peralkaline liquids. The separation of enough residual melt to feed a crystal‐poor post‐caldera rhyolitic eruption may take as little as months to tens of years (much shorter than typical repose periods of 300‐400 years). Our observations are consistent with these magmatic systems spending significant portions of their life cycle dominated by crystalline mushes containing ephemeral, small (< 1 km3) segregations of melt. This interpretation helps to reconcile observations of high crustal electrical resistivity beneath Aluto, despite seismicity and ground deformation consistent with a magma body

Focused Mid‐Crustal Magma Intrusion During Continental Break‐Up in Ethiopia

Significant volumes of magma can be intruded into the crust during continental break‐up, influencing rift evolution by altering the thermo‐mechanical structure of the crust and its response to extensional stresses. Rift magmas additionally feed surface volcanic activity and can be globally significant sources of tectonic CO2 emissions. Understanding how magmatism may affect rift development requires knowledge on magma intrusion depths in the crust. Here, using data from olivine‐hosted melt inclusions, we investigate magma dynamics for basaltic intrusions in the Main Ethiopian Rift (MER). We find evidence for a spatially focused zone of magma intrusion at the MER upper‐lower crustal boundary (10–15 km depth), consistent with geophysical datasets. We propose that ascending melts in the MER are intruded over this depth range as discrete sills, likely creating a mechanically weak mid‐crustal layer. Our results have important implications for how magma addition can influence crustal rheology in a maturing continental rift

Age of the oldest known Homo sapiens from eastern Africa.

Efforts to date the oldest modern human fossils in eastern Africa, from Omo-Kibish1-3 and Herto4,5 in Ethiopia, have drawn on a variety of chronometric evidence, including 40Ar/39Ar ages of stratigraphically associated tuffs. The ages that are generally reported for these fossils are around 197 thousand years (kyr) for the Kibish Omo I3,6,7, and around 160-155 kyr for the Herto hominins5,8. However, the stratigraphic relationships and tephra correlations that underpin these estimates have been challenged6,8. Here we report geochemical analyses that link the Kamoya's Hominid Site (KHS) Tuff9, which conclusively overlies the member of the Omo-Kibish Formation that contains Omo I, with a major explosive eruption of Shala volcano in the Main Ethiopian Rift. By dating the proximal deposits of this eruption, we obtain a new minimum age for the Omo fossils of 233 ± 22 kyr. Contrary to previous arguments6,8, we also show that the KHS Tuff does not correlate with another widespread tephra layer, the Waidedo Vitric Tuff, and therefore cannot anchor a minimum age for the Herto fossils. Shifting the age of the oldest known Homo sapiens fossils in eastern Africa to before around 200 thousand years ago is consistent with independent evidence for greater antiquity of the modern human lineage10.Leverhulme Trust Cambridge-Africa ALBORADA Research Fund SFI award 13/RC/209