Evaluating the tephra input into Pacific Ocean sediments: distribution in space and time     

S. M. Straub  H. U. Schmincke 《International Journal of Earth Sciences》1998,87(3):461-476

We studied the volcanic contribution to the global sediment budget in the Pacific Ocean basin. It is the world's oldest (174 m.y.) and largest (≈49% of Earth's surface area) ocean basin and has had a high and continuous tephra influx from intraplate and convergent margin volcanoes through time. Computerized shipboard data from 65 legs of the Deep Sea Drilling Project (DSDP) and the Ocean Drilling Program (ODP) were screened for the presence of volcaniclastic components. Tephra-bearing and tephra-free core sections (standard 1.5- and 0.30-m core catcher sections) were separated, regardless of the mass fraction of tephra present. The percentage of tephra-bearing core sections ("tephra frequency") per site and time span ("age unit") was calculated. The age units were the Quaternary, the subepochs of the Tertiary, and the stages of the Cretaceous. A total of 424 drill sites yielded 1433 usable stratigraphic units. Fifty percent are younger than 13 m.y., corresponding to only approximately 10% of the total interval studied (124.5 m.y.). The percentage of tephra-bearing age units is high throughout (83±6%) and correlates linearly with the total number of age units (R ² =0.998; n=17). The average tephra frequency (30–50%) fluctuates, because the abundance of age units of different tephra frequency classes (0, 1–33, 34–67, 68–100% tephra frequency) varies with time. This indicates that the Cenozoic increase in tephra production results from increase in volcanicity and not spatial extension of volcanic source areas. The Cenozoic sediments that were recovered are dominated by distal tephra from explosive arc volcanism. Pulses of arc volcanism occurred in the Pliocene–Quaternary (since ≈5 m.y.) and mid-Miocene (≈12–15 m.y.). However, the record of explosive arc volcanism in Paleogene and Cretaceous sediments was either not drilled or has been destroyed by subduction. Except for the Cretaceous (≈70–110 m.y.) volcanic pulse, intraplate volcanism is poorly represented in the tephra record because the drill sites are outside the proximal range (>500–1000 km) of the sources. Thus, the tephra record drilled contains significant gaps that bias the estimate of tephra volume towards the less voluminous distal deposits. Most of the volcaniclastic volume accumulated by mass wasting as volcaniclastic aprons surrounding ocean island volcanoes. Volcaniclastic production rates range from 10,000 to 41,800 km³/m.y. for large intraplate volcanoes and approximately 10–13 km³/km arc length per million years for oceanic island arcs. Extrapolation over the lifetime of major Pacific arcs and hotspot chains, combined with a volume estimate of the distal tephra component, indicates a minimum of 9.3×10⁶ km³ of tephra, corresponding to 23 vol.% of the existing Pacific oceanic sediments. At least two thirds of the tephra volume was deposited in the proximal range and at least half of it is derived from intraplate sources. The large proportion of tephra, its composition, and its localized accumulation causes significant spatial and temporal variation in Pacific oceanic sediments that should have a perceptible impact on the elemental fluxes between ocean, crust, and mantle.  相似文献   

The lateral facies of ignimbrites at Laacher See volcano     

Rolf Schumacher  Hans-Ulrich Schmincke 《Bulletin of Volcanology》1990,52(4):271-285

Valley-fill ignimbrites of the Middle Laacher See Tephra (MLST) in the proximal depositional fan south of Laacher See volcano are laterally continuous with fine-grained overbank-facies deposits, 0.5–1.5 m thick, covering higher elevations and interfluves between the paleovalleys. The overbank deposits consist of up to 12 ash layers, each 4–10 cm thick, which show internal structures typical of ash-flow transport, such as poor sorting, reverse size-grading of pumice, local normal grading of coarse ash-sized lithics above a fine-grained basal layer, cross-stratification behind obstacles, and erosional unconformities. Thickness, median grain-size, and number of individual layers decrease systematically with distance from the vent. Overbank ash layers thicken at the valley slopes and form discrete valley-fill ignimbrite flow units in the paleochannels with median grain size increasing from 63 m to 350 m. Toward the center of paleochannels, however, the well-defined overbank facies is obscured by mutual erosion of individual flow units. Overlapping data fields in ternary grain-size variation diagrams indicate the overbank facies to have evolved from ash flows chiefly through depletion of lapilli and coarse ash. Overbank-facies ash layers do not represent dust layers resulting from elutriation clouds of ash flows (co-ignimbrite ash) or surge deposits developed on higher ground due to low concentration of solids. They are similar in some parameters to Taupo-type ignimbrite veneer deposits, but are interpreted differently. The thin, fine-grained, Laacher See ash layers are thought to have been deposited from diluted portions of the flow proceeding directly from the eruption column while the main pyroclastic flows were confined to the paleovalleys radiating away from the eruptive center. The wide distribution of the thin ash layers is attributed to the balance of deposition from various flow parts and turbulent entraining and heating of ambient air that sustained sufficient mobility of the diluted flows to spread across hills and level ground.  相似文献   

Processes and timescales in the evolution of a chemically zoned trachyte: Fogo A,Sao Miguel,Azores     

E. Widom  H. -U. Schmincke  J. B. Gill 《Contributions to Mineralogy and Petrology》1992,111(3):311-328

U-series disequilibria analyses have been combined with chemical and petrographic analyses in order to assess both the timescales and processes involved in the formation of the chemically zoned Fogo A trachytes. Least squares major element modelling demonstrates that the mafic trachytes could have evolved from a parental alkali basalt via trachybasalt with 70% fractionation of augite (35–36%), plagioclase (23%), magnetite (16%), kaersutite (15%), olivine (8%) and apatite (2–3%). Derivation of the mafic trachytes from a basanite parent is inconsistent with calculated fractionation paths. Major and trace element variations in 25 pumice samples collected from throughout the stratigraphic extent of the Fogo A deposit show that the trachytes represent the inverted, extrusive equivalent of a strongly chemically zoned magma chamber. The zonation is attributed to 70–75% Rayleigh fractional crystallization of the observed phenocryst phases. Wallrock assimilation and magma mixing did not contribute significantly to the observed chemical trends. The maximum age of the Fogo A trachytic magma based on radioactive disequilibria between ²³⁰Th and ²³⁸U is 300000 years. However, a calculated model age suggests that the time of evolution of the Fogo A trachytes from a parent alkali basalt is only 90000 years. Constant element variations and Th-isotopic ratios in Fogo C, Fogo A and 1563 A.D. trachytes suggest that a single long-lived trachytic magma chamber has been the source of at least the past 15.2 Ka of trachytic volcanism from Agua de Pao. After each eruption an evolved cupola reformed and became zoned prior to the next eruption. The maximum time necessary to form the zonation is 4600 years, the time between the Fogo A and 1563 A.D. eruptions. Low (²²⁶Ra)/(²³⁰Th)_i ratios in the Fogo A and 1563 A.D. trachytes suggest that alkali feldspar fractionation continued up to the time of the respective eruptions.  相似文献   

Formation of submarine canyons on the flanks of the Canary Islands     

S. Krastel  H.-U. Schmincke  C. Jacobs 《Geo-Marine Letters》2001,20(3):160-167

Detailed morphological data collected from the submarine flanks of the Canary Islands have revealed numerous submarine canyons down to water depths of >3,000 m. These canyons are interpreted to have formed by submarine erosion. We postulate formation of proto-canyons by downslope-eroding mass flows which originate on land, enter the sea, and continue below sea level for several tens of kilometers. Once proto-canyons have been formed, they become deepened by further erosion and failures of the canyon walls and/or floor. Large amounts of sediments, funnelled through the canyons from the islands into the adjacent deep-ocean sedimentary basins, play an important role in the evolution of volcanic aprons surrounding ocean islands. Some major canyon systems appear to have persisted for at least 14 million years.  相似文献   

Iblean diatremes 3: volcanic processes on a Miocene carbonate platform (Iblean Mountains,SE-Sicily): a comparison of deep vs. shallow marine eruptive processes     

Ines Suiting  H.-U. Schmincke 《Bulletin of Volcanology》2012,74(1):207-230

Evolution and magma fragmentation processes of two contrasting, well-exposed diatreme complexes interbedded with Late Miocene calcareous marine sediments in distinct sedimentary environments of a carbonate platform (Iblean Plateau, Sicily) are compared with each other. The nephelinitic Cozzo Molino diatreme (CMD) to the east developed in shallow water (0–80 m water depth); the alkali basaltic Valle Guffari seamount (VGS) to the west grew on a deeper water carbonate ramp (150–200 m water depth). We focus on the dominant boundary conditions inferred to have governed depth of magma fragmentation and subaqueous emplacement mechanisms: water depth, physical nature of host rocks, magma composition, and inferred differences in initial volatile concentrations. There are gross similarities in the composition of the two moderately evolved magmas. The low-viscosity magmas in both diatremes were laden with xenoliths originating from mantle to lower crustal sites. Although similar, the eastern shallow water CMD was likely more volatile-rich, with magma fragmented prior to reaching the surface and the surrounding tephra cone was partly emergent. The eruptions of the entirely submarine VGS diatreme complex in the deeper water environment were dominated by interaction of soft sediment and alkali basaltic magma or a pre-fragmented volatile-particle mixture. Eruption columns were, thus, strongly damped and the submarine complex never pierced the water surface.  相似文献   

Peralkaline felsic magmatism at the Nemrut volcano,Turkey: impact of volcanism on the evolution of Lake Van (Anatolia) IV     

Ray?MacdonaldEmail author  Mari?Sumita  Hans-Ulrich?Schmincke  Bogus?aw?Bagiński  John?C.?White  S?awomir?S.?Ilnicki 《Contributions to Mineralogy and Petrology》2015,169(4):34

Nemrut volcano, adjacent to Lake Van (Turkey), is one of the most important peralkaline silicic centres in the world, where magmatism for ~570,000 years has been dominated by peralkaline trachytes and rhyolites. Using onshore and Lake Van drill site tephra samples, we document the phenocryst and glass matrix compositions, confirming a complete spectrum from very rare mafic to dominantly silicic magmas. Magma mixing has been common and, along with the multi-lineage nature of the magmas, indicates that Nemrut has been a very open system where, nevertheless, compositionally zoned caps developed during periods of relative eruptive quiescence. Geothermometry suggests that the intermediate-silicic magmas evolved in an upper crustal magma reservoir at temperatures between 1100 and 750 °C, at fO₂ close to the FMQ buffer. The silicic magmas either were halogen poor or exsolved a halogen-rich phase prior to or during eruption. An unusual Pb-rich phase, with up to 98.78 wt% PbO, is interpreted as having exsolved from the intermediate-rhyolitic magmas.  相似文献   

IAVCEI News     

H. -U. Schmincke 《Bulletin of Volcanology》1988,50(6):419-420

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IAVCEI news     

H -U Schmincke 《Bulletin of Volcanology》1990,53(1):72-72

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Models for the origin of accretionary lapilli   总被引：1，自引：0，他引：1  

R. Schumacher  H. -U. Schmincke 《Bulletin of Volcanology》1995,56(8):626-639

Binding between initially cohesionless ash particles to form concentric accretionary lapilli is provided primarily by the capillary forces of liquid bridges from condensed moisture and by electrostatic attraction. Capillary forces are strong bonds if the particles are in close contact, but they decrease rapidly with increasing particle spacing. Electrostatic attraction between charged ash particles is much weaker but effective over larger distances, increasing the frequency of collision between them.Experimental results of liquid film binding of volcanic ash showed that agglomeration was most successful between 15 and 25 wt.%, defining the agglomeration window for the formation of accretionary lapilli. Below 5–10 wt.% and above about 25–30 wt.% of water, concentric agglomeration was inhibited. Particles <350 m could be selected from a wider particle population in the experiments using only small amounts of water, which can explain the growth of accretionary lapilli in pyroclastic surges around agglomeration nuclei. Experiments testing the behavior of volcanic ash in electric fields showed that ash clusters formed instantaneously when the ash entered the field between a corona discharge gun and a grounded metal plate. The maximum grain size incorporated into the artificial clusters was about 180 m but >90 wt.% of ash was <45 m.Accretionary lapilli form in turbulent ash clouds when particles carrying liquid films of condensed moisture collide with each other and when the binding forces exceed the grain dispersive forces. Larger particles >500 m act as agglomeration nuclei in surges, accreting ash <350 m around them. In pyroclastic flows the aggregates are thought to originate from already size-sorted ash at the interface between the lower avalanche part of the flow and its overriding elutriation cloud. The fine-grained rims around accretionary lapilli found close to source are interpreted to be accreted dominantly by electrostatic attraction of very fine ash similar to clustering in elutriation clouds.  相似文献   

Land subsidence in Mashhad Valley, northeast Iran: results from InSAR, levelling and GPS     

Mahdi Motagh  Yahya Djamour  Thomas R. Walter  Hans-Ulrich Wetzel  Jochen Zschau  Siavash Arabi 《Geophysical Journal International》2007,168(2):518-526

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