Emma L. Horn , Danielle McLean , Adriano Pimentel , José M. Pacheco , Simone Aguiar , Christina J. Manning , CAVES Project Team, Nick Barton , Victoria C. Smith
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引用次数: 0
Abstract
Volcanoes in the Azores have experienced large, silicic explosive eruptions, and these events have been concentrated mostly on the islands of São Miguel and Terceira. It is evident these eruptions are capable of dispersing tephra thousands of kilometers, owing to distal sedimentary sequences in Europe and Africa recording tephra sourced from the Azores, but the lack of detailed glass geochemical datasets precluded robust distal correlations to specific eruptions. Establishing the glass compositions of these large pyroclastic deposits will allow these Azores tephra to serve as chronostratigraphic markers. Here we present the major and trace element glass geochemical compositions of the largest known eruptions from Sete Cidades, Fogo, and Furnas volcanoes on São Miguel, and Pico Alto/Guilherme Moniz on Terceira. The glasses are predominately trachytic with 60.5–67.5 wt% SiO2, 12–19 wt% Al2O3, 1–7 wt% FeOt, Eu <6.5 ppm, Zr <2400 ppm, and Yb <14 ppm. A notable feature is that the glass compositions of individual deposits are diverse, and often show distinct populations that are associated with different eruption phases. The melts erupted from central volcanoes on both islands are distinctive - glasses erupted from São Miguel contain distinctively lower FeOt (<4.0 wt%), and higher Al2O3 (>16.8 wt%) and La/Y ratios (>1.6 ppm) relative to those erupted from Terceira. The dataset was used to establish that a cryptotephra layer in the Taforalt archaeological site in Morocco is linked to the 18-20 cal. ka BP Santa Bárbara eruption from Sete Cidades volcano >2000 km away. Interestingly, despite several tephra layers from the Azores being identified in Holocene sequences across Europe, there are no voluminous deposits on the Azores, except for Fogo A, within this timeframe. This confirms that the tephra is widely dispersed even in some of the smaller eruptions and this geochemical dataset provides a useful framework for identifying Azores tephra in Late Pleistocene distal records. However, the Azores volcanoes typically erupt compositionally diverse evolved melts that do not vary much between successive eruptions. Thus, it is often difficult to distinguish the eruption deposits based on glass compositions alone, and often chronostratigraphic information is also required to reliably correlate distal tephra to specific eruptions.
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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.