The exceptional magnitude and intensity of the Toba eruption,sumatra: An example of the use of deep-sea tephra layers as a geological tool     

D. Ninkovich  R. S. J. Sparks  M. T. Ledbetter 《Bulletin of Volcanology》1978,41(3):286-298

The eruption of Toba (75,000 years BP), Sumatra, is the largest magnitude eruption documented from the Quaternary. The eruption produced the largest-known caldera the dimensions of which are 100 × 30 km and which is surrounded by rhyolitic ignimbrite covering an area of over 20,000 km². The associated deep-sea tephra layer is found in piston cores in the north-eastern Indian Ocean covering a minimum area of 5 × 10⁶ km². We have investigated the thickness, grain size and texture of the Toba deep-sea tephra layer in order to demonstrate the use of deep-sea tephra layers as a volcanological tool. The exceptional magnitude and intensity of the Toba eruption is demonstrated by comparison of these data with the deep-sea tephra layers associated with the eruptions of the Campanian ignimbrite, Italy and of Santorini, Greece in Minoan time. The volume of ignimbrite and distal tephra fall deposit produced in the Toba eruption are comparable, a total of at least 1000 km³ of dense rhyolitic magma. In contrast the volume of dense magma produced by the Campanian and Santorini eruptions are approximately 70 and 13 km³ respectively. Thickness versus distance data on the three deep-sea tephra layers show that eruptions of smaller magnitude than Santorini are unlikely to be preserved as distinct tephra layers in most deep-sea cores. In proximal cores all three tephra layers show two distinct units: a lower coarse-grained unit and an upper fine-grained unit. We interpret the lower unit as a plinian deposit and the upper unit as a co-ignimbrite ash-fall deposit, indicating two major eruptive phases. The Toba tephra layer is coarser both in maximum and median grain size than the Campanian and Santorini layers at a given distance from source. These data are interpreted to indicate a very high cruption column, estimated to be at least 45 km. We have applied a method for estimating the duration of the Toba eruption from the style of graded-bedding in deep-sea tephra layers. Studies of two cores yield estimates of 9 and 14 days. The eruption column height and duration estimates both indicate an average volume discharge rate of approximately 10⁶ m³/sec. The Toba eruption therefore was not only of exceptional magnitude, but also of exceptional intensity.  相似文献   

The significance of vitric-enriched air-fall ashes associated with crystal-enriched ignimbrites     

R.S.J. Sparks  G.P.L. Walker   《Journal of Volcanology and Geothermal Research》1977,2(4):329-341

A distinctive type of fine-grained air-fall ash is found intimately associated with many ignimbrites. They have crystal/glass ratios systematically lower than artificially crushed pumice from the same ignimbrites. The crystal enrichment found in crystal-bearing ignimbrites indicates substantial losses of the vitric component, amounting to an average of at least 35% by weight of the original juvenile material, and this lost material is believed to occur in the ash-fall deposits. These ashes thus complement ignimbrite, and are here called “co-ignimbrite ashes”. The loss is believed to take place during ignimbrite eruptions as a result of: (1) the escape of fine ash and gas above a collapsing eruptive column; (2) the preferential entry of fine vitric ash into an upper turbulent cloud when (immediately following column collapse) the segregation of a dense pyroclastic flow from an initially highly turbulent, low-concentation density flow takes place; (3) the elutriation of fine vitric ash (generated in part within the pyroclastic flow) from the fluidised flow. Ash from all three mechanisms would be expected to rise to a great height in convective plumes and be dispersed by winds to produce extensive, vitric-enriched ash-fall deposits.The data indicate that the co-ignimbrite ashes must have volumes comparable with those of ignimbrites, and examples are given of particularly large ash-fall deposits (including some found in deep-sea cores) associated with large ignimbrites which may be of this type rather than fall-out from a preceding plinian phase as hitherto thought.  相似文献   

The transport of xenoliths in magmas     

R.S.J. Sparks  H. Pinkerton  R. Macdonald 《Earth and Planetary Science Letters》1977,35(2):234-238

Field measurements of the rheology of Hawaiian and Etnean lavas have shown that, on eruption, they behave as Bingham liquids with yield strengths in the range 70–450 N/m². Ultramafic nodules entrained in a Bingham liquid cannot settle unless the stress they impose on the liquid exceeds approximately 5–7 times its yield strength. Consequently magmas with yield strengths of 10–1000 N/m² can transport xenoliths up to 30 cm diameter without settling occurring. The size of nodules commonly observed rarely exceeds 30 cm. A review of experimental data shows that, when conditions are appropriate for settling, the terminal velocities of nodules in magmas are substantially slower if a Bingham rather than Newtonian Model is assumed.The view that large nodules imply fast rates of magma ascent is rejected. A case is presented for slow rates of ascent being more suitable for nodule transport as there is more opportunity for cooling, crystallisation and hence development of a yield strength. The relative abundance of nodules in the alkaline suite may be a consequence of their slow rates of ascent, whereas their absence in tholeiitic melts may be a consequence of rapid ascent rates. This interpretation is compatible with deductions on their relative rates of ascent based on other geological evidence.  相似文献   

The fluid dynamics of a basaltic magma chamber replenished by influx of hot,dense ultrabasic magma     

Herbert E. Huppert  R. Stephen J. Sparks 《Contributions to Mineralogy and Petrology》1980,75(3):279-289

This paper describes a fluid dynamical investigation of the influx of hot, dense ultrabasic magma into a reservoir containing lighter, fractionated basaltic magma. This situation is compared with that which develops when hot salty water is introduced under cold fresh water. Theoretical and empirical models for salt/water systems are adapted to develop a model for magmatic systems. A feature of the model is that the ultrabasic melt does not immediately mix with the basalt, but spreads out over the floor of the chamber, forming an independent layer. A non-turbulent interface forms between this layer and the overlying magma layer across which heat and mass are transferred by the process of molecular diffusion. Both layers convect vigorously as heat is transferred to the upper layer at a rate which greatly exceeds the heat lost to the surrounding country rock. The convection continues until the two layers have almost the same temperature. The compositions of the layers remain distinct due to the low diffusivity of mass compared to heat. The temperatures of the layers as functions of time and their cooling rate depend on their viscosities, their thermal properties, the density difference between the layers and their thicknesses. For a layer of ultrabasic melt (18% MgO) a few tens of metres thick at the base of a basaltic (10% MgO) magma chamber a few kilometres thick, the temperature of the layers will become nearly identical over a period of between a few months and a few years. During this time the turbulent convective velocities in the ultrabasic layer are far larger than the settling velocity of olivines which crystallise within the layer during cooling. Olivines only settle after the two layers have nearly reached thermal equilibrium. At this stage residual basaltic melt segregates as the olivines sediment in the lower layer. Depending on its density, the released basalt can either mix convectively with the overlying basalt layer, or can continue as a separate layer. The model provides an explanation for large-scale cyclic layering in basic and ultrabasic intrusions. The model also suggests reasons for the restriction of erupted basaltic liquids to compositions with MgO<10% and the formation of some quench textures in layered igneous rocks.  相似文献   

Discharge of pyroclastic flows into the sea during the 1996–1998 eruptions of the Soufrière Hills volcano, Montserrat     

Karen Hart  Steven Carey  Haraldur Sigurdsson  R. Stephen J. Sparks  Richard E. A. Robertson 《Bulletin of Volcanology》2004,66(7):599-614

Explosive eruptions of the Soufrière Hills volcano on the island of Montserrat in the West Indies generated pyroclastic flows that reached the sea on the east and southwest coasts between November 1995 and July 1998. Discharge of the flows produced two pyroclastic deltas off of the Tar and White River valleys. A marine geological survey was conducted in July 1998 to study the submarine extensions of both deltas. Detailed profiles of depth and sub-bottom structure were obtained using a CHIRP II/bubble pulser system. These profiles were compared with pre-eruption bathymetric data in order to identify areas of recent deposition and erosion. Deposition off the Tar and White River valleys was thickest nearest the coastline and deltas, and extended into deeper water up to 5 km from shore. The total volume of submarine pyroclastic deposits as of July 1998 was 73×10⁶ m³ DRE. Submarine pyroclastic deposits off the Tar River valley made up more than two-thirds of the total volume (55×10⁶ m³ DRE) and covered an area of approximately 5.0 km², which included the delta. The volume of submarine pyroclastic deposits in the White River area (18×10⁶ m³ DRE) is probably underestimated due to the lack of precise pre-eruption bathymetric data in areas greater than 2 km from shore. Growth of pyroclastic deltas at the mouths of the Tar and White River valleys continued to the edge of the submarine shelf where there was a steep break in slope. In the Tar River area pyroclastic material was distributed down the steep shelf break and into deeper water at least a few kilometers from shore. The material spread out radially, forming a submarine fan, where distribution was primarily controlled by bathymetry and slope.Editorial responsibility; J. Stix  相似文献   

Kinetics of zinc and arsenate co-sorption at the goethite-water interface     

Markus Gräfe  Donald L. Sparks 《Geochimica et cosmochimica acta》2005,69(19):4573-4595

Little or no information is available in the literature about reaction processes of co-sorbing metals and arsenate [As(V)] on variable-charged surfaces or factors influencing these reactions. Arsenic and metal contamination are, however, a common co-occurrence in many contaminated environments. In this study, we investigated the co-sorption kinetics of 250 μM As(V) and zinc [Zn(II)] in 10, 100, and 1000 mg goethite L⁻¹ 0.01 M NaCl solution at pH 7, collected complementary As and Zn K-edge extended X-ray absorption fine structure (EXAFS) data after various aging times, and performed a replenishment desorption/dissolution study at pH 4 and 5.5 after 6 months of aging time. Arsenate and Zn(II) formed adamite-like and koritnigite-like precipitates on goethite in 100- and 10-ppm goethite suspensions, respectively, whereas in 1000-ppm goethite suspensions, As(V) formed mostly double-corner sharing complexes and Zn(II) formed a solid solution on goethite according to EXAFS spectroscopic analyses. In all goethite suspension densities, surface adsorption reactions were part of the initial reaction processes. In 10- and 100-ppm goethite suspensions, a heterogeneous nucleation reaction occurred in which adamite-like precipitates began to form 48 h earlier than koritnigite-like surface precipitates. Arsenate and Zn(II) uptake from solution decreased after 4 weeks. Replenishment desorption studies showed that the precipitates and surface adsorbed complexes on goethite were susceptible to proton-promoted dissolution resulting in many cases in more than 80% loss of Zn(II) and ∼ 60% to 70% loss of arsenate. The molar Zn:As dissolution ratio was dependent on the structure of the precipitate and was cyclic for the adamite and koritnigite-like surface precipitates, reflecting the concentric and plane-layered structures of adamite and koritnigite, respectively.  相似文献   

Thermodynamics and fluid dynamics of effusive subglacial eruptions     

A Höskuldsson  R. S. J. Sparks 《Bulletin of Volcanology》1997,59(3):219-230

We consider the thermodynamic and fluid dynamic processes that occur during subglacial effusive eruptions. Subglacial eruptions typically generate catastrophic floods (jökulhlaups) due to melting of ice by lava and generation of a large water cavity. We consider the heat transfer from basaltic and rhyolitic lava eruptions to the ice for typical ranges of magma discharge and geometry of subglacial lavas in Iceland. Our analysis shows that the heat flux out of cooling lava is large enough to sustain vigorous natural convection in the surrounding meltwater. In subglacial eruptions the temperature difference driving convection is in the range 10–100??°C. Average temperature of the meltwater must exceed 4??°C and is usually substantially greater. We calculate melting rates of the walls of the ice cavity in the range 1–40?m/day, indicating that large subglacial lakes can form rapidly as observed in the 1918 eruption of Katla and the 1996 eruption of Gjálp fissure in Vatnajökull. The volume changes associated with subglacial eruptions can cause large pressure changes in the developing ice cavity. These pressure changes can be much larger than those associated with variation of bedrock and glacier surface topography. Previous models of water-cavity stability based on hydrostatic and equilibrium conditions may not be applicable to water cavities produced rapidly in volcanic eruptions. Energy released by cooling of basaltic lava at the temperature of 1200??°C results in a volume deficiency due to volume difference between ice and water, provided that heat exchange efficiency is greater than approximately 80%. A negative pressure change inhibits escape of water, allowing large cavities to build up. Rhyolitic eruptions and basaltic eruptions, with less than approximately 80% heat exchange efficiency, cause positive pressure changes promoting continual escape of meltwater. The pressure changes in the water cavity can cause surface deformation of the ice. Laboratory experiments were carried out to investigate the development of a water cavity by melting ice from a finite source area at its base. The results confirm that the water cavity develops by convective heat transfer.  相似文献   

The differentiation of the Skaergaard intrusion   总被引：6，自引：0，他引：6  

R. H. Hunter  R. S. J. Sparks 《Contributions to Mineralogy and Petrology》1990,104(2):248-251

Conclusions We find no support for the claim that the Skaergaard magma followed the trend of common tholeiitic volcanic magmas, such as those of Iceland and the Scottish Tertiary. The end product of differentiation was not a large mass of rhyolite but an iron-rich, silica-poor liquid not unlike that deduced by Wager in 1960.The proposal that a large mass of rhyolitic liquid occupied the upper levels of the intrusion finds no support in the field. The thick series of ferrogabbos, which became richer in iron and poorer in silica until they reached a field of immiscibility cannot be reconciled with crystallization of a large mass of felsic magma. Mass-balance calculations that indicate otherwise are invalid, because they fail to take into account large volumes of rocks that differ in composition from those assumed in the calculations.While ignoring the existence of major units of the intrusion, Hunter and Sparks propose that lavas in Scotland and Iceland are more relevant to the liquid compositions than rocks that are intimately associated with the intrusion. Their argument that the Skaergaard Intrusion followed a trend of silica enrichment that is universal to tholeiitic magmas is based on an incomplete knowledge of the rocks and faulty calculations of mass-balance relations.We agree that much remains to be learned about the Skaergaard Intrusion and the basic mechanisms of magmatic differentiation. In this case, however, we are ready to hang our case on well-established field relations and a mass of laboratory data for what must be the most intensely studied body of rock on Earth.  相似文献   

Rejoinder to “comment on ‘estimation of the generalized covariance function: II. A response surface approach’ by Thomas H. Starks and Allen R. Sparks”     

Thomas H. Starks  Allen R. Sparks 《Mathematical Geosciences》1987,19(8):789-792

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Chemical and physical studies of type 3 chondrites—III. Chondrules from the Dhajala H3.8 chondrite     

Derek W.G. Sears  Mary H. Sparks  Alan E. Rubin 《Geochimica et cosmochimica acta》1984,48(6):1189-1200

Fifty-eight chondrules were separated from the Dhajala H3.8 chondrite and their thermoluminescence properties were measured. Chips from 30 of the chondrules were examined petrographically and with electron-microprobe techniques; the bulk compositions of 30 chondrules were determined by the fused bead technique. Porphyritic chondrules, especially 5 which have particularly high contents of mesostasis, tend to have higher TL (mass-normalized) than non-porphyritic chondrules. Significant correlations between log(TL) and the bulk CaO, Al₂O₃ and MnO content of the chondrules, and between log(TL) and the CaO, Al₂O₃, SiO₂ and normative anorthite content of the chondrule glass, indicate an association between TL and the abundance and composition of mesostasis. Unequilibrated chondrules ( i.e. those whose olivine is compositionally heterogeneous and high in Ca) have low TL, whereas equilibrated chondrules have a wide range of TL, depending on their chemical and petrographic properties.We suggest that the TL level in a given chondrule is governed by its bulk composition (which largely determined the abundance and composition of constituent glass) and by metamorphism (which devitrfied the glass in those chondrules with high Ca glass to produce the TL phosphor). We also suggest that one reason why certain chondrules in type 3 ordinary chondrites are unequilibrated, while others are equilibrated, is that the mesostasis of the unequilibrated chondrules resisted the devitrification. This devitrification is necessary for the diffusive communication between chondrule grains and matrix that enables equilibration.  相似文献