Fragmentation of magma during Plinian volcanic eruptions   总被引：2，自引：0，他引：2  

James E. Gardner  Richard M. E. Thomas  Claude Jaupart  Steve Tait 《Bulletin of Volcanology》1996,58(2-3):144-162

 The ratio of the volume of vesicles (gas) to that of glass (liquid) in pumice clasts (V _G /V _L ) reflects the degassing and dynamic history experienced by a magma during an explosive eruption. V _G /V _L in pumices from a large number of Plinian eruption deposits is shown here to vary by two orders of magnitude, even between pumices at a given level in a deposit. These variations in V _G /V _L do not correlate with crystallinity or initial water content of the magmas or their eruptive intensities, despite large ranges in these variables. Gas volume ratios of pumices do, however, vary systematically with magma viscosity estimated at the point of fragmentation, and we infer that pumices do not quench at the level of fragmentation but undergo some post-fragmentary evolution. On the timescale of Plinian eruptions, pumices with viscosities <10⁹ Pa s can expand after fragmentation, as long as their bubbles retain gas, at a rate inversely proportional to their viscosity. Once the bubbles connect to form a permeable network and lose their gas, expansion halts and pumices with viscosities <10⁵ Pa s can collapse under the action of surface tension. Textural evidence from bubble sizes and shapes in pumices indicates that both expansion and collapse have taken place. The magnitudes of expansion and collapse, therefore, depend critically on the timing of bubble connectivity relative to the final moment of quenching. We propose that bubbles in different pumices become connected at different times throughout the time span between fragmentation and quenching. After accounting for these effects, we derive new information on the fragmentation process from two characteristics of pumices. The most important is a relatively constant minimum value of V _G /V _L of ∼1.78 (64 vol.% vesicularity) in all samples with viscosities >10⁵ Pa s. This value is independent of magma composition and thus reflects a property of the eruptive mechanism. The other characteristic is that highly expanded pumices (>85 vol.% vesicularities) are common, which argues against overpressure in bubbles as a mechanism for fragmenting magma. We suggest that magma fragments when it reaches a vesicularity of ∼64 vol.%, but only if sheared sufficiently strongly. The intensity of shear varies as a function of velocity in the conduit, which is related to overpressure in the chamber, so that changes in overpressure with time are important in controlling the common progression from explosive to effusive activity at volcanoes. Received: 19 April 1995 / Accepted: 3 April 1996  相似文献   

Quartz latite rheoignimbrite flows of the Etendeka Formation,north-western Namibia     

SC Milner  AR Duncan  A Ewart 《Bulletin of Volcanology》1992,54(3):200-219

The Etendeka Formation of north-western Namibia consists of a sequence of interbedded quartz latites and tholeiitic basalts and forms part of the Karoo Igneous Province in southern Africa. The age of the Etendeka Formation is approximately 130–135 Ma. The quartz latites make up a significant proportion of the stratigraphic succession (<25% of the total stratigraphic thickness) and form as much as 60% of the outcrop area in the southern Etendeka. Apart from some systematic differences between pitchstones and devitrified quartz latite, largely explained by alteration processes, individual quartz latite units exhibit remarkably uniform compositions with no significant vertical or lateral variation. Geochemistry can be used as a primary criterion for the correlation of major quartz latite units over much of the southern Etendeka area enabling the reconstruction of the Etendeka Formation stratigraphy in this region. Individual quartz latite units occur as voluminous (400–2600 km³), widespread (up to 8800 km²), sheet-like deposits typically between 40 and 300 m thick. Each unit consists of basal, main and upper zones. The main zone generally constitutes over 70% of the thickness of the unit and typically consists of texturally featureless devitrified quartz latite. In contrast the basal and upper zones of the flow are characterised by flow banding, pitchstone lenses and breccia, with rare occurrences of pyroclastic textures. The quartz latites are sparsely porphyritic (<10% phenocrysts) with glassy or devitrified groundmass textures. The phenocrysts consist of plagioclase, pyroxene, titanomagnetite and rare ilmenite. Pyroxene geothermometry indicates high (1000–1100°C) temperatures of crystallisation which, coupled with the absence or primary hydrous phases, indicates that the quartz latites were relatively hot, H₂O-undersaturated magmas. The quartz latites display features common to both rhyolite lavas and ignimbrites and are clearly the products of an unusual eruption style. The local preservation of pyroclastic textures and the broad areal extent of these units lead to the conclusion that the quartz latites are high-temperature rheomorphic ignimbrites (i.e. rheoignimbrites). A combination of high eruption temperature and relatively low viscosity helps to explain the often completely welded and homogeneous textures observed in most quartz latite outcrops in the Etendeka area.  相似文献   

The effect of salinity on ammonium sorption in aquatic sediments: Implications for benthic nutrient recycling   总被引：1，自引：0，他引：1  

Sybil P. Seitzinger  Wayne S. Gardner  Anne K. Spratt 《Estuaries and Coasts》1991,14(2):167-174

Ambient exchangeable ammonium concentrations in freshwater sediments are generally considerably greater than those reported for marine sediments. Laboratory measurements indicate that competition for cation exchange sites by ions in seawater is a factor responsible for the lower exchangeable ammonium concentrations in marine sediments. Exchangeable ammonium concentrations were 3- to 6-fold higher when river and estuarine sediments were incubated with fresh water relative to the same sediments incubated with salt water (%.-23). A model was developed to explore the implications for benthic nitrogen cycling of this salinity effect on exchangeable ammonium concentrations. Ammonium diffusion, exchangeable and dissolved ammonium concentrations, and nitrification rates were components of the model formulation. The model output suggests that higher exchangeable ammonium concentrations predicted in fresh water relative to marine sediments can markedly increase the fraction of the ammonium produced in sediments that is nitrified (and subsequently denitrified). These results are consistent with field and experimental laboratory data which indicate that a larger percentage of net ammonium production in aerobic freshwater sediments is nitrified and denitrified (80–100%) relative to marine sediments (40–60%).  相似文献   

Observations on the distribution of dissolved mercury in the ocean     

Doris Gardner 《Marine pollution bulletin》1975,6(3):43-46

The concentration of dissolved mercury in sea water has been determined in a number of areas in the northern and southern hemispheres and an attempt has been made to correlate the values found for certain areas with natural physical phenomena. The average concentrations found ranged from 11.2 ng l^?1 in the southern hemisphere to 33.5 ng l^?1 for the northern hemisphere; an increase suspected to be the result of dispersion of industrial pollution by jet streams. Local effects including upwelling, volcanic action, land run-off and heavy suspended matter loads are also discussed.  相似文献   

Mercury in some surface waters of the world ocean     

R. Chester  D. Gardner  J.P. Riley  J. Stoner 《Marine pollution bulletin》1973,4(2):28-29

Mercury concentrations in mid-latitude eastern Atlantic surface waters is about twice as high as the oceanic average, perhaps due to pollution. Low concentrations in the tropical Atlantic may be due to uptake by plankton off the west coast of Africa.  相似文献   

Geology and petrology of ejecta from the 1999 eruption of Shishaldin Volcano,Alaska   总被引：1，自引：0，他引：1  

Stelling  P.  Beget  J.  Nye  C.  Gardner  J.  Devine  J.  George  R. 《Bulletin of Volcanology》2002,64(8):548-561

Bulletin of Volcanology - Shishaldin Volcano erupted repeatedly during April and May 1999, with major eruptive events on 19 April and 23 April. Tephra deposits &;gt;20&;nbsp;cm thick were...  相似文献   

Holocene geomagnetic secular variation recorded by volcanic deposits at Mount St. Helens,Washington     

Jonathan T. Hagstrum  Richard P. Hoblitt  Cynthia A. Gardner  Thomas E. Gray 《Bulletin of Volcanology》2002,63(8):545-556

A compilation of paleomagnetic data from volcanic deposits of Mount St. Helens is presented in this report. The database is used to determine signature paleomagnetic directions of products from its Holocene eruptive events, to assign sampled units to their proper eruptive period, and to begin the assembly of a much larger database of paleomagnetic directions from Holocene volcanic rocks in western North America. The paleomagnetic results from Mount St. Helens are mostly of high quality, and generally agree with the division of its volcanic deposits into eruptive episodes based on previous geologic mapping and radiocarbon dates. The Muddy River andesite's paleomagnetic direction, however, indicates that it is more likely part of the Pine Creek eruptive period rather than the Castle Creek period. In addition, the Two-Fingers andesite flow is more likely part of the Middle Kalama eruptive period and not part of the Goat Rocks period. The paleomagnetic data from Mount St. Helens and Mount Hood document variation in the geomagnetic field's pole position over the last ~2,500 years. A distinct feature of the new paleosecular variation (PSV) record, similar to the Fish Lake record (Oregon), indicates a sudden change from rapid clockwise movement of the pole about the Earth's spin axis to relatively slow counterclockwise movement at ~800 to 900 years B.P.  相似文献   

Multibeam bathymetric and sediment profiler evidence for ice grounding on the Chukchi Borderland, Arctic Ocean   总被引：1，自引：0，他引：1  

Martin Jakobsson  James V. Gardner  Larry A. Mayer  Jan Backman  Brian Calder  Barbara Kraft 《Quaternary Research》2005,63(2):150-160

Multibeam bathymetry and 3.5-kHz sub-bottom profiler data collected from the US icebreaker Healy in 2003 provide convincing evidence for grounded ice on the Chukchi Borderland off the northern Alaskan margin, Arctic Ocean. The data show parallel, glacially induced seafloor scours, or grooves, and intervening ridges that reach widths of 1000 m (rim to rim) and as much as 40 m relief. Following previous authors, we refer to these features as “megascale glacial lineations (MSGLs).” Additional support for ice grounding is apparent from stratigraphic unconformities, interpreted to have been caused by ice-induced erosion. Most likely, the observed sea-floor features represent evidence for massive ice-shelf grounding. The general ESE/WNW direction of the MSGLs, together with sediment, evidently bulldozed off the Chukchi Plateau, that is mapped on the western (Siberian) side of the plateau, suggests ice flow from the Canada Basin side of Chukchi Borderland. Two separate generations of glacially derived MSGLs are identified on the Chukchi Borderland from the Healy geophysical data. The deepest and oldest extensive MSGLs appear to be draped by sediments less than 5 m thick, whereas no sediment drape can be distinguished within the resolution of the sub-bottom profiles on the younger generation.  相似文献   

Shelf-edge deltas and drowned barrier–island complexes on the northwest Florida outer continental shelf     

J.V. Gardner  P. Dartnell  L.A. Mayer  J.E. Hughes Clarke  B.R. Calder  G. Duffy   《Geomorphology》2005,64(3-4):133-166

A high-resolution multibeam survey of the northwest Florida shelf mapped six relict shelf-edge deltas, each with a drowned barrier–island system developed on its south and southwestern rims. The deltas appear to have formed during periods of sea-level stasis that occurred between 58,000 and 28,000 years ago. The barrier islands formed on the deltas during periods of slow regression during this same time interval. Large fields of asymmetric dunes are found on the delta surfaces as well as on the south and southwestern flanks of the deltas. The asymmetry and orientation of the dunes suggest that a northward-flowing current was sheared by the presence of the delta topography, and as a result, the upper layer of the flow continued to the north, whereas the lower layer was steered by the topography. The topographic steering accelerated the northward flow around the south and southwestern flanks with speeds adequate to form large dunes. The flow slowed after rounding southwestern flank but accelerated again as it encountered the next delta flank to the north. The age of the dune formation is unknown, and no northward-flowing geostrophic flow has been reported in the literature from this area.  相似文献   

Modified HNO3 seasonality in volcanic layers of a polar ice core: Snow-pack effect or photochemical perturbation?     

Paolo Laj  Julie M. Palais  James E. Gardner  Haraldur Sigurdsson 《Journal of Atmospheric Chemistry》1993,16(3):219-230

Using the chemical composition of snow and ice of a central Greenland ice core, we have investigated changes in atmospheric HNO₃ chemistry following the large volcanic eruptions of Laki (1783), Tambora (1815) and Katmai (1912). The concentration of several cations and anions, including SO ₄ ^2– and NO ₃ ^– , were measured using ion chromatography. We found that following those eruptions, the ratio of the concentration of NO ₃ ^– deposited during winter to that deposited during summer was significantly higher than during nonvolcanic periods. Although we cannot rule out that this pattern originates from snow pack effects, we propose that increased concentrations of volcanic H₂SO₄ particles in the stratosphere may have favored condensation and removal of HNO₃ from the stratosphere during Arctic winter. In addition, this pattern might have been enhanced by slower formation of HNO₃ during summer, caused by direct consumption of OH through oxidation of volcanic SO₂.  相似文献