The final Galileo SSI observations of Io: orbits G28-I33     

Elizabeth P. Turtle  Laszlo P. Keszthelyi  Jani Radebaugh  Damon P. Simonelli  David A. Williams  Windy L. Jaeger  H.Herbert Breneman  Cynthia B. Phillips  the Galileo SSI Team 《Icarus》2004,169(1):3-28

We present the observations of Io acquired by the Solid State Imaging (SSI) experiment during the Galileo Millennium Mission (GMM) and the strategy we used to plan the exploration of Io. Despite Galileo's tight restrictions on data volume and downlink capability and several spacecraft and camera anomalies due to the intense radiation close to Jupiter, there were many successful SSI observations during GMM. Four giant, high-latitude plumes, including the largest plume ever observed on Io, were documented over a period of eight months; only faint evidence of such plumes had been seen since the Voyager 2 encounter, despite monitoring by Galileo during the previous five years. Moreover, the source of one of the plumes was Tvashtar Catena, demonstrating that a single site can exhibit remarkably diverse eruption styles—from a curtain of lava fountains, to extensive surface flows, and finally a ∼400 km high plume—over a relatively short period of time (∼13 months between orbits I25 and G29). Despite this substantial activity, no evidence of any truly new volcanic center was seen during the six years of Galileo observations. The recent observations also revealed details of mass wasting processes acting on Io. Slumping and landsliding dominate and occur in close proximity to each other, demonstrating spatial variation in material properties over distances of several kilometers. However, despite the ubiquitous evidence for mass wasting, the rate of volcanic resurfacing seems to dominate; the floors of paterae in proximity to mountains are generally free of debris. Finally, the highest resolution observations obtained during Galileo's final encounters with Io provided further evidence for a wide diversity of surface processes at work on Io.  相似文献   

Explosive volcanic eruptions — V. Observations of plume dynamics during the 1979 Soufrière eruption, St Vincent     

R. S. J. Sparks  L. Wilson 《Geophysical Journal International》1982,69(2):551-570

Summary During the 1979 eruption of the Soufrière of St Vincent, West Indies, a 14 min period of explosive activity occurred on April 22. The first three minutes of this activity were filmed. Five individual explosions formed distinct plumes which fed an eruption column which eventually ascended to a height of over 18 km. The ascent velocities of the fronts of these plumes ranged from 8.5 to 61.7 ms^-1. The half-angle of spreading of the plume fronts ranged from 21.5 to 24°. One of the plumes was observed to 8 km height and decelerated steadily from 51.5 to 23 ms^-1. The main column fed by individual explosions was, however, observed to accelerate from 8.5 to 58 m s^-1 in the same height interval.
A theoretical treatment of volcanic plume motion is presented. Measurements of dimensions, velocities and spreading rates from the film are used to estimate plume parameters such as temperature, particle content and volume discharge rate of magma from the theoretical relationships. These calculations show that the individual plumes became hotter and richer in juvenile ash with time. The acceleration of the main eruption column was the result of being fed by increasingly hotter and more ash-rich explosions. An average volume discharge rate of 12600m³ s^-1 is estimated from the analysis of the plume motions. This value agrees closely with an estimate of discharge rate based on the heat flux required to form an 18 km high column. This agreement suggests that theories of convective motions in plumes can be successfully adapted to the volcanic case, as long as the effect of hot entrained ash particles is considered.  相似文献   

Petrology and geochemistry of the Loch Ba ring-dyke,Mull (N.W. Scotland): an example of the extreme differentiation of tholeiitic magmas     

R. S. J. Sparks 《Contributions to Mineralogy and Petrology》1988,100(4):446-461

The Loch Ba ring-dyke in the Tertiary igneous central complex of Mull, N.W. Scotland is composed predominantly of a banded rhyolitic welded tuff. The rhyolite contains numerous inclusions of dark aphanitic rock. The textural relationships between the different rocks indicate rapid, violent and intimate mixing during emplacement of the dyke. The dark glassy component varies continuously from basaltic andesite to andesite, dacite and rhyolite. These glasses are enriched in FeO and depleted in MgO at a given SiO₂ content in comparison to other tholeiitic highly differentiated volcanic rocks. The rhyolite contains an average of 4% phenocrysts and is associated with the mineral assemblage plagioclase (An₃₂ to An₂₁)-sanidine(Or_50–60)-hedenbergite-fayalite-magnetite-ilmenite-apatite-zircon. Mineral aggregates involving either plagioclase-hedenbergite-ilmenite or plagioclase-fayalite-magnetite are common, but aggregates containing fayalite and hedenbergite together are scarce. The dark glassy components are either phenocryst free or contain less than 0.2% phenocrysts. The main phenocrysts associated with the dark glasses are plagioclase (An₆₅-An₃₀), high calcium clinopyroxene ranging continuously from augite to pure hedenbergite, pigeonite, magnetite, ilmenite and rare apatite. Zoning in minerals is generally weak or absent. The plagioclase feldspar, high calcium clinopyroxenes and pigeonites have similar compositional ranges to the minerals observed in the Middle and Upper Zones of the Skaergaard Intrusion. The mineral compositions are systematically related to SiO₂ content and Mg number of the glasses. The data demonstrate that mineral compositions and assemblages similar to the Skaergaard form from silica-rich andesitic to rhyolitic liquids. The various mafic glasses are interpreted to have been derived from a zoned magma chamber underlying an upper layer of rhyolitic magma. Differentiation is attributed to fractional crystallization of the observed mineral assemblages causing SiO₂ enrichment and FeO depletion. However, glasses with less than 57% SiO₂ have unusual compositions with very low MgO and P₂O₅ as well as variable Al₂O₃ and TiO₂. Their peculiarities could be explained by andesitic magmas assimilating cumulate mineral aggregates precipitated from more differentiated dacite and rhyolite magmas. The bulk compositions of these cumulates have high FeO, low SiO₂ and negligible MgO and P₂O₅. It is suggested that the high density of the mineral aggregates containing fayalite-hedenbergite-magnetite and ilmenite caused them to settle through the zoned chamber to be assimilated by high temperature, less differentiated magmas.  相似文献   

The 1975 sub-terminal lavas, mount etna: a case history of the formation of a compound lava field     

H. Pinkerton  R.S.J. Sparks 《Journal of Volcanology and Geothermal Research》1976,1(2):167-182

The 1975 sub-terminal activity was characterised by low effusion rates (0.3–0.5 m³ s⁻¹) and the formation of a compound lava field composed of many thousands of flow units. Several boccas were active simultaneously and effusion rates from individual boccas varied from about 10⁻⁴ to 0.25 m³s⁻¹. The morphology of lava flows was determined by effusion rate (E): aa flows with well-developed channels and levees formed when E > 2 × 10⁻³ m³ s⁻¹, small pahoehoe flows formed when 2 × 10⁻³ m³ s⁻¹ >E > 5 > 10⁻⁴ m³ s⁻¹ and pahoehoe toes formed when E < 5 × 10⁻⁴ m³ s⁻¹. There was very little variation with time in the effusion temperature, composition or phenocryst content of the lava.New boccas were commonly formed at the fronts of mature lava flows which had either ceased to flow or were moving slowly. These secondary boccas developed when fluid lava in the interior of mature aa flows either found a weakness in the flow front or was exposed by avalanching of the moving flow front. The resulting release of fluid lava was accompanied by either partial drainage of the mature flow or by the formation of a lava tube in the parent flow. The temperature of the lava forming the new bocca decreased with increasing distance from the source bocca (0.035°C m⁻¹). It is demonstrated from the rate of temperature decrease and from theoretical considerations that many of the Etna lavas still contained a substantial proportion of uncooled material in their interior as they came to rest. The formation of secondary boccas is postulated to be one reason why direct measurements of effusion rates tend, in general, to overestimate the total effusion rates of sub-terminal Etna lava fields.  相似文献   

Laboratory investigations of viscous effects in replenished magma chambers     

Herbert E. Huppert  R. Stephen J. Sparks  J. Stewart Turner 《Earth and Planetary Science Letters》1983,65(2):377-381

Some laboratory experiments are described which investigate the dynamical effects of replenishment of a magma chamber containing high viscosity magma by hotter, denser and much more fluid magma. In the experiments a layer of hot KNO₃ solution is emplaced beneath cold glycerine, which has a viscosity 3000 times greater. Less dense fluid is released immediately and continuously from the interface as a result of crystallization in the lower layer and rises as plumes through the overlying glycerine. Further crystallization occurs in the plumes, and the crystals fall out; but there is little mixing between the two fluids and a layer of depleted KNO₃ solution forms at the top. The experiments demonstrate that interfacial processes begin to dominate where there are large viscosity differences between adjacent fluid layers as would be the case in a rhyolitic magma chamber replenished by basaltic magma.  相似文献   

The entrance of pyroclastic flows into the sea, II. theoretical considerations on subaqueous emplacement and welding     

R.S.J. Sparks  H. Sigurdsson  S.N. Carey 《Journal of Volcanology and Geothermal Research》1980,7(1-2)

The submarine counterparts of late Quaternary subaerial pyroclastic flow deposits off the western flanks of Dominica, Lesser Antilles, have been investigated by 3.5 kHz seismic profiling and dredging (cruise EN20 of R/V “Endeavor”). Block-and-ash flow deposits formed by dome collapse and a welded ignimbrite from a prominent fan at Grande Savanne, Dominica. This fan can be traced underwater as a major constructional ridge (2–4 km wide and 200–400 m thick) to over 13 km offshore at a water depth of 1800 m. The submarine ridge has a volume of 14 km³ and has the characteristic morphology of a debris flow apron composed of several individual units. The evidence suggests that pyroclastic flows can move underwater without losing their essential character.  相似文献   

Stratigraphy and geology of the ignimbrites of Vulsini Volcano,central Italy     

R. Stephen J. Sparks 《International Journal of Earth Sciences》1975,64(1):497-523

Vulsini Volcano, Bolsena District, central Italy is a Quaternary centre eruptions of which have been dominated by the production of ignimbrites. Stratigraphic studies show that there are six major ignimbrite units with individual minimum volumes of 0.5 to 3 km³ and a large number of smaller ignimbrite units, together with pyroclastic surge and pyroclastic fall deposits. It is estimated that at least 90% of the volcanic products are pyroclastic producing a low profile volcano with 1 to 5 slopes.Lithic clasts in the ignimbrite units decrease systematically in size away from the source, but pumice clasts have an irregular size distribution. Grain size characteristics of the ignimbrites are typical of pyroclastic flow deposits with clasts ranging from a metre to a few microns in size. Reverse grading of large pumice clasts and normal grading of large lithic clasts are common in individual flow units. Each ignimbrite is composed of a large number of individual flow units each having a finer grained basal layer. Both sillar and non-welded facies occur. Coherent ignimbrite (sillar) is the result of vapour phase crystallisation in the pore spaces with no deformation of the clasts. The ignimbrites are associated with fine grained air fall ash beds, cross-bedded pyroclastic surge deposits and pumice fall deposits formed in the same eruptions.Chemical analyses show that the ignimbrites have a wide range of composition (50 to 62% SiO₂). Many chemical studies of volcanoes exclude pyroclastic rocks in favour of lavas. The implications and problems of such biased sampling are discussed.Transportation of the ignimbrites is believed to be that of a semi-fluidised debris flow. Viscosities of the pyroclastic flows, estimated from their capacity to transport lithic debris, must have been between 10¹ and 10³ poises. The previously proposed hypothesis that these ignimbrites have formed from a gas super-saturated lava or froth flow is incompatible with their field and grain size characteristics and is rejected.

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Zusammenfassung Der Vulkan Vulsini in Mittelitalien entstand im Quartär und wird von pyroklastischen Lavadecken geprägt. Sechs ausgedehnte Ignimbritmassen (Mengen 0,5 bis 4 km³) und eine gro\e Anzahl kleinerer Ignimbritkörper wurden festgestellt. Die Ignimbrite kommen zusammen mit Pyroklastiten, feinen Aschen und einigen hochtemperierten pyroklastischen surge-Absätzen vor. Die Ignimbrite sind schlecht sortiert, verschwei\t und unverschwei\t. Jeder Ignimbrit besteht aus vielen Lagen, und der Boden jeder Lage ist durch eine feinkörnige Grundlage markiert. Inverse Gradierung gro\er Bimsstein-Fragmente und normale Gradierung gro\er Klastite kommen häufig vor. Die grö\ten Fremdgesteins-Fragmente nehmen mit der Entfernung von der Caldera di Latera ab, nur Ignimbrit C verringert sich mit der Entfernung von der Caldera di Bolsena. Analysen von Bimsstein-Fragmenten der Ignimbrite zeigen eine wechselnde Zusammensetzung (SiO₂ 49–62%). Ein Vergleich der Zusammensetzung der Lava und der Pyroklastika zeigt jedoch, da\ die Magmen, die das pyroklastische Gestein bilden, allgemein kieselsäurereicher entwickelt sind. Die Korngrö\e und die Morphologie der Ignimbrite deuten darauf hin, da\ sie aus dichten Gas/Feststoff-Dispersionen deponiert worden sind, und pyroklastische Lagen werden als durch Gas transportierte Gesteinstrümmer gedeutet.

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Riassunto Il vulcano di Vulsini, nell'Italia Centrale, è un centro di epoca quaternaria nel quale la produzione di colate piroclastiche è stata prédominante. Vi si possono riconoscere sei maggiori ignimbriti di larga mole (volumi 0.5–3 Km³) oltre a un gran numéro di ignimbriti di minor volume, associate a depositi di pyroclastic fall, di ceneri minute e a depositi di spinta piroclastica (base surge) ad alta temperatura, formatesi nelle stesse eruzioni.Le ignimbriti si presentano sia saldate o non saldate e sono scarsemente separate. Ognuno di esse è composto da un insieme di colate individuali la cui base è segnata da uno strato basilare a grana più fine, e nelle quali sono comuni larghi frammenti di pomice a gradazione inversa e larghi frammenti litici di gradazione normale. Per quanto riguarda tutti le maggiori ignimbriti, la mole massima dei frammenti di roccia estranea decresce con l'allontanarsi dalla caldera di Latera, ad eccezione dell'ignimbrite C, per la quale questo avviene allontanandosi dalla caldera di Bolsena. Analisi di frammenti di pomice proveniente dalle ignimbriti mostrano la presenza di una vasta gamma die elementi (SiO₂ 49%–62%), tuttavia dal confronta fra lave e materiali piroclastici risulta che i magmi che formano le rocce piroclastiche sono, in generale, molto più evoluti. La grossezza della grana e le caratteristiche morfologiche delle ignimbriti fanno ritenere che la loro origine sia dovuta al depositarsi di dispersioni di gas densi, poco espansi e di solidi, e che le colate piroclastiche siano da considerarsi come colate di detriti, lubrificati dal gas.

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. 6 , 0,5 4 « . , «surge» . . , . . Caldera di Latra, Caldera di Bilsena. (SiO₂ 49 62%). , , , . , - , , .

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Geological and geochemical studies of the sintra alkaline igneous complex,portugal     

R. S. J. Sparks  G. Wadge 《Bulletin of Volcanology》1975,39(3):385-406

The Sintra igneous complex, Portugal was an important centre of activity in late Cretaceous times. The great proportion of thealkaline rocks are felsic and include five large quartz syenite intrusions and trachyandesite, trachyte and alkali rhyolite lavas and dykes, most of which are oversaturated. Mafic rocks are sparse, but vary widely from alkaline and highly undersaturated types containing high K₂O, TiO₂ and Ba, similar to the contemporaneous Lisbon lavas, to hypersthene normative trachybasalts and one hypersthene normative basalt. The various magma types are intimately associated and a well-developed netveined complex of alkali gabbro, monzonite and syenite is recognised at Cabo da Roca. A study of the dyke distributions, intersections and orientations suggest a close propinquity of both oversaturated and undersaturated and of both felsic and matic magmas. The basic magmas of Sintra and Lisbon show a continuous range in undersaturation (0 to 16% normative nepheline) and rare hypersthene normative basalts. Derivation of the hypersthene normative and mildly undersaturated basalts from the more undersaturated melts by low pressure fractionation or contamination by siliceous crust is shown to be unlikely. High pressure eclogite fractionation of a hypersthene normative basalt or variations in the percentage partial melting of a mantle under conditions where titanphlogopite is a low melting fraction are both processes compatible with the variations in undersaturation and proportions of TiO₂, K₂O and Ba. The quartz syenites and over satured felsic lavas of Sintra are thought to be derived from hypersthene nor mative parents.  相似文献   

Ignimbrites of the Cerro Galan caldera, NW Argentina     

R.S.J. Sparks  P.W. Francis  R.D. Hamer  R.J. Pankhurst  L.O. O'Callaghan  R.S. Thorpe  R. Page 《Journal of Volcanology and Geothermal Research》1985,24(3-4)

The 35 × 20 km Cerro Galán resurgent caldera is the largest post-Miocene caldera so far identified in the Andes. The Cerro Galán complex developed on a late pre-Cambrian to late Palaeozoic basement of gneisses, amphibolites, mica schists and deformed phyllites and quartzites. The basement was uplifted in the early Miocene along large north-south reverse faults, producing a horst-and-graben topography. Volcanism began in the area prior to 15 Ma with the formation of several andesite to dacite composite volcanoes. The Cerro Galán complex developed along two prominent north-south regional faults about 20 km apart. Dacitic to rhyodacitic magma ascended along these faults and caused at least nine ignimbrite eruptions in the period 7-4 Ma (K-Ar determinations). These ignimbrites are named the Toconquis Ignimbrite Formation. They are characterised by the presence of basal plinian deposits, many individual flow units and proximal co-ignimbrite lag breccias. The ignimbrites also have moderate to high macroscopic pumice and lithic contents and moderate to low crystal contents. Compositionally banded pumice occurs near the top of some units. Many of the Toconquis eruptions occurred from vents along a north-south line on the western rim of the young caldera. However, two of the ignimbrites erupted from vents on the eastern margin. Lava extrusions occurred contemporaneously along these north-south lines. The total D.R.E. volume of Toconquis ignimbrite exceeds 500 km³.Following a 2-Ma dormant period a single major eruption of rhyodacitic magma formed the 1000-km³ Cerro Galán ignimbrite and the caldera. The ignimbrite (age 2.1 Ma on Rb-Sr determination) forms a 30–200-m-thick outflow sheet extending up to 100 km in all directions from the caldera rim. At least 1.4 km of welded intracaldera ignimbrite also accumulated. The ignimbrite is a pumice-poor, crystal-rich deposit which contains few lithic clasts. No basal plinian deposit has been identified and proximal lag breccias are absent. The composition of pumice clasts is a very uniform rhyodacite which has a higher SiO₂ content but a lower K₂O content than the Toconquis ignimbrites. Preliminary data indicate no evidence for compositional zonation in the magma chamber. The eruption is considered to have been caused by the catastrophic foundering of a cauldron block into the magma chamber.Post-caldera extrusions occurred shortly after eruption along both the northern extension of the eastern boundary fault and the western caldera margin. Resurgence also occurred, doming up the intracaldera ignimbrite and sedimentary fill to form the central mountain range. Resurgent doming was centred along the eastern fault and resulted in radial tilting of the ignimbrite and overlying lake sediments.  相似文献   

Density changes during the fractional crystallization of basaltic magmas: fluid dynamic implications   总被引：3，自引：1，他引：3  

R. Stephen J. Sparks  Herbert E. Huppert 《Contributions to Mineralogy and Petrology》1984,85(3):300-309

The dynamical behaviour of basaltic magma chambers is fundamentally controlled by the changes that occur in the density of magma as it crystallizes. In this paper the term fractionation density is introduced and defined as the ratio of the gram formula weight to molar volume of the chemical components in the liquid phase that are being removed by fractional crystallization. Removal of olivine and pyroxene, whose values of fractionation density are larger than the density of the magma, causes the density of residual liquid to decrease. Removal of plagioclase, with fractionation density less than the magma density, can cause the density of residual liquid to increase. During the progressive differentiation of basaltic magma, density decreases during fractionation of olivine, olivine-pyroxene, and pyroxene assemblages. When plagioclase joins these mafic phases magma density can sometimes increase leading to a density minimum. Calculations of melt density changes during fractionation show that compositional effects on density are usually greater than associated thermal effects.In the closed-system evolution of basaltic magma, several stages of distinctive fluid dynamical behaviour can be recognised that depend on the density changes which accompany crystallization, as well as on the geometry of the chamber. In an early stage of the evolution, where olivine and/or pyroxenes are the fractionating phases, compositional stratification can occur due to side-wall crystallization and replenishment by new magma, with the most differentiated magma tending to accumulate at the roof of the chamber. When plagioclase becomes a fractionating phase a zone of well-mixed magma with a composition close to the density minimum of the system can form in the chamber. The growth of a zone of constant composition destroys the stratification in the chamber. A chamber of well-mixed magma is maintained while further differentiation occurs, unless the walls of the chamber slope inwards, in which case dense boundary layer flows can lead to stable stratification of cool, differentiated magma at the floor of the chamber.In a basaltic magma chamber replenished by primitive magma, the new magma ponds at the base and evolves until it reaches the same density and composition as overlying magma. Successive cycles of replenishment of primitive magma can also form compositional zonation if successive cycles occur before internal thermal equilibrium is reached in a chamber. In a chamber containing well-mixed, plagioclase — saturated magma, the primitive magma can be either denser or lighter than the resident magma. In the first case, the new magma ponds at the base and fractionates until it reaches the same density as the evolved magma. Mixing then occurs between magmas of different temperatures and compositions. In the second case a turbulent plume is generated that causes the new magma to mix immediately with the resident magma.  相似文献