Temporal and spatial evolution of explosive silicic peralkaline eruptions at the Olkaria Volcanic Complex and Longonot volcano in the Southern Kenya Rift
P.A. Wallace , V. Otieno , P. Godec , R.W. Njoroge , M.S. Tubula , L. Cappelli , P.M. Kamau , S. Nomade , N.O. Mariita , K. Fontijn
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引用次数: 0
Abstract
The Olkaria Volcanic Complex and adjacent Longonot volcano in the Southern Kenya Rift host major geothermal resources yet also pose volcanic hazards owing to their long histories of explosive eruptions. Olkaria is a multicentred dome complex consisting of lava domes, craters and fissures produced following the eruption of peralkaline comenditic rhyolites. Longonot is a trachytic caldera hosting a central summit crater, situated <10 km east of Olkaria, with pumice and ash eruptions being a dominant feature. This study reconstructs their evolution through detailed field, geochemical and geochronological investigations of widespread pyroclastic deposits. Logging sequences of pumice and ash fallout and pyroclastic density current deposits enables establishing a regional stratigraphic framework spanning the past 42 ka. Ten key eruptive units record explosive phases at Olkaria and Longonot, with eight 40Ar/39Ar ages and a radiocarbon date providing the first accurate age constraints of both volcanoes. The oldest explosive deposits identified in our field area are associated with the large-magnitude Maiella Pumice (MP) eruptions (338–333 ka), representing one of the first explosive silicic peralkaline eruptions in the Southern Kenya Rift. Within Olkaria, the earliest preserved deposits in our tephrostratigraphic framework are 42–37 ka pyroclastic density currents (OP) and pumice fallouts (OFB) representing early rhyolitic dome eruptions. Overlapping deposits reveal Longonot concurrently experienced repetitive large explosive eruptions emplacing pumice fall deposits from 37 ka to 17 ka (LAP). After 17 ka, LAP eruptions were punctuated by a single eruption of mixed basaltic trachyandesite, trachyandesite and trachyte pumice (LMx), followed by persistent trachyte ash venting at Longonot (LA1). Simultaneously, Olkaria transitioned to localised lava dome construction and collapse generating block-and-ash flows from 17 ka to 14 ka (OD1–2). Younger Longonot ash eruptions (LA2) and recent 191 ± 23 cal yr BP effusive–explosive activity at Olkaria's Ololbutot centre (OD3) reveals both volcanoes have remained active throughout the Holocene. Minimum estimated magnitudes for widespread fall units at both systems range between 4 and 5 (DRE deposit volumes of 0.04–0.35 km3), representing substantial regional hazards, with eruption frequencies averaging up to one moderate–large eruption (magnitude 4–5) every ∼2000 years for Longonot and one small–moderate explosive eruption (magnitude ∼3) every ∼200 years for Olkaria until ca. 10 ka, after which eruptions are significantly smaller and more localised. Statistical analysis of pumice glass geochemistry enables the fingerprinting of Olkaria deposits to likely source vents, tracking spatial-temporal variability across the complex. The larger explosive eruptions at both Olkaria and Longonot dispersed voluminous pyroclastic material (pumice and/or ash), which generated up to 1-m-thick deposits located ∼20 km from its source vent in the case of Longonot, suggesting up to 200,000 people could be impacted from future similar magnitude eruptions, emphasising the necessity to combine eruptive history reconstructions with hazard analysis to advance preparedness and mitigation strategies.
期刊介绍:
An international research journal with focus on volcanic and geothermal processes and their impact on the environment and society.
Submission of papers covering the following aspects of volcanology and geothermal research are encouraged:
(1) Geological aspects of volcanic systems: volcano stratigraphy, structure and tectonic influence; eruptive history; evolution of volcanic landforms; eruption style and progress; dispersal patterns of lava and ash; analysis of real-time eruption observations.
(2) Geochemical and petrological aspects of volcanic rocks: magma genesis and evolution; crystallization; volatile compositions, solubility, and degassing; volcanic petrography and textural analysis.
(3) Hydrology, geochemistry and measurement of volcanic and hydrothermal fluids: volcanic gas emissions; fumaroles and springs; crater lakes; hydrothermal mineralization.
(4) Geophysical aspects of volcanic systems: physical properties of volcanic rocks and magmas; heat flow studies; volcano seismology, geodesy and remote sensing.
(5) Computational modeling and experimental simulation of magmatic and hydrothermal processes: eruption dynamics; magma transport and storage; plume dynamics and ash dispersal; lava flow dynamics; hydrothermal fluid flow; thermodynamics of aqueous fluids and melts.
(6) Volcano hazard and risk research: hazard zonation methodology, development of forecasting tools; assessment techniques for vulnerability and impact.