A pulse of mid-Pleistocene rift volcanism in Ethiopia at the dawn of modern humans

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

Using lake sediment cores to improve records of volcanism at Aluto volcano in the main Ethiopian rift

info:eu-repo/semantics/publishe

Geochemical variations within the Upper Oligocene-Lower Miocene lava succession of Úhošt' Hill (NE margin of Doupovské hory Mts., Czech Republic)

Purely effusive activity built up a sequence of nine lavas on Úhoš' Hill. The oldest erupted lavas are tephrites/basanites (28.66 ± 1.06 Ma) and are of slightly evolved character. These are overlain by more differentiated alkali basalt. The succession continues with picrobasaltic lavas rich in phenocrysts (ankaramites 26.38 ± 1.04 Ma). The entire succession is crowned by alkali ba alt lavas (22.09 ± 0.73 Ma). All lavas appear to be emitted from a common conduit. With respect to geochemical and isotopic data, how ever, the studied sequence could not be interpreted in terms of the exhausting of a simple magma chamber. We propose the role of a more complex reservoir or, of two independent reservoirs attached to a single conduit system. The tephrites/basanites and picrobasalts (ankaramites) are closely related, the latter being derived by accumulation of mafic phenocrysts, namely clinopyroxene. The bas alts form an other group differing from the tephrites/basanites and picrobasalts in more evolved isotopic compositions of Sr and Nd as well as in in compatible-elementratios. The isotopic composition of the lavas seems to reflect mixed man tle sources derived from the European Asthenospheric Reservoir through the addition of components similar to Enriched Mantle (EM 1 and EM 2). The basalts originated by fractionation of different magma batches that contained a higher proportion of the enriched mantle component(s) or that were more contaminated with crustal material

Emplacement History of the Miocene Zebín Tuff Cone (Czech Republic) Revealed From Ground Geophysics, Anisotropy of Magnetic Susceptibility, Paleomagnetic, and 40

Abstract The Zebín Volcano, Czech Republic, is a quarried tuff cone that offers the opportunity to understand emplacement processes responsible for magma transport through a pyroclastic cone and magmatic processes involved in its development. A high‐resolution ground magnetometry survey, electrical resistivity tomography, and refraction seismic were conducted to supplement field mapping of the size, shape, and inner structure of the volcanic system. The Zebín Volcano yields normal polarity rocks, and the cone is underlain by a complex and branching magma feeder system with several diverging dikes. Samples were collected at 24 sites from feeder dikes and the main conduit for anisotropy of magnetic susceptibility and paleomagnetic analysis. Anisotropy of magnetic susceptibility results yield inferred magma flow directions indicating subhorizontal magma flow toward and away from the axial conduit as well as both upward and downward flow. Paleomagnetic data from all sites are normal polarity with statistically distinct remanence directions between some sites that indicate that significant time passed or deformation occurred during the growth of this volcanic system. 40Ar/39Ar isochron ages of groundmass provide emplacement ages of 18.38 ± 0.03 and 18.45 ± 0.03 Ma, whereas a weighted mean date of 18.52 ± 0.03 Ma for hornblende provides an eruption age. We argue that the Zebín Volcano evolved from a polyphase feeding system and through a complex feeder network. This detailed study shows that many monogenetic volcanoes deserve highly detailed study, as their subtleties can provide insight into broader crustal and magmatic environment during magmatism

A Permian andesitic tuff ring at Rožmitál (the Intra-Sudetic Basin, Czech Republic) : evolution from explosive to effusive and high-level intrusive activity

Mafic, monogenetic volcanism is increasingly recognized as a common manifestation of post-collisional volcanism in late Variscan, Permo-Carboniferous intramontane basins of Central Europe. Although identification of individual eruptive centres is not easy in these ancient successions, the Permian Rožmitál andesites in the Intra-Sudetic Basin (NE Bohemian Massif) provide an exceptionally detailed record of explosive, effusive and high-level intrusive activity. Based on field study and petrographic and geochemical data on pyroclastic and coherent rocks, the Rožmitál succession is interpreted as the proximal part of a tuff ring several hundred metres in diameter. Initial accumulation of pyroclastic fall and surge deposits occurred during phreatomagmatic eruptions, with transitions towards Strombolian eruptions. Gullies filled with reworked tephra document periods of erosion and redeposition. Andesitic blocky lavas capped the volcaniclastic succession. Invasion of lavas into unconsolidated sediments and emplacement of shallow-level intrusions in near-vent sections resulted in the formation of jigsaw- and randomly-textured peperites. Most geochemical differences between coherent andesites and pyroclastic rocks can be linked to incorporation of quartz-rich sediments during the explosive eruptive processes and to later cementation of the volcaniclastic deposits by dolomite. The Rožmitál tuff ring could have been one of several phreatomagmatic centres in a monogenetic volcanic field located on an alluvial plain