Temperature measurements in carbonatite lava lakes and flows from oldoinyo lengai, Tanzania

The petrogenesis of carbonatites has important implications for mantle processes and for the magmatic evolution of mantle melts rich in carbon dioxide. Oldoinyo Lengai, Tanzania, is the only active carbonatite volcano on Earth. Its highly alkalic, sodium-rich lava, although different in composition from the more common calcium-rich carbonatites, provides the opportunity for observations of the physical characteristics of carbonatite melts. Temperature measurements on active carbonatitic lava flows and from carbonatitic lava lakes were carried out during a period of effusive activity in June 1988. Temperatures ranged from 491 degrees to 519 degrees C. The highest temperature, measured from a carbonatitic lava lake, was 544 degrees C. These temperatures are several hundred degrees lower than measurements from any silicate lava. At the observed temperatures, the carbonatite melt had lower viscosities than the most fluid basaltic lavas. The unusually low magmatic temperatures were confirmed with 1-atmosphere melting experiments on natural samples.     

Venus volcanism: initial analysis from magellan data

Magellan images confirm that volcanism is widespread and has been fimdamentally important in the formation and evolution of the crust of Venus. High-resolution imaging data reveal evidence for intrusion (dike formation and cryptodomes) and extrusion (a wide range of lava flows). Also observed are thousands of small shield volcanoes, larger edifices up to several hundred kilometers in diameter, massive outpourings of lavas, and local pyroclastic deposits. Although most features are consistent with basaltic compositions, a number of large pancake-like domes are morphologically similar to rhyolite-dacite domes on Earth. Flows and sinuous channels with lengths of many hundreds of kilometers suggest that extremely high effusion rates or very fluid magmas (perhaps komatiites) may be present. Volcanism is evident in various tectonic settings (coronae, linear extensional and compressional zones, mountain belts, upland rises, highland plateaus, and tesserae). Volcanic resurfacing rates appear to be low (less than 2 Km(3)/yr) but the significance of dike formation and intrusions, and the mode of crustal formation and loss remain to be established.     

Sulfur and chlorine in late Cretaceous Deccan magmas and eruptive gas release

Large-volume pāhoehoe lava flows erupted 67 to 65 million years ago, forming the Deccan Traps, India. The impact of these flood basalt eruptions on the global atmosphere and the coeval end-Cretaceous mass extinction has been uncertain. To assess the potential gas release from this volcanism, we measured sulfur and chlorine concentrations in rare glass inclusions inside crystals and on glassy selvages preserved within lavas. Concentrations range from approximately 1400 parts per million of S and 900 parts per million of Cl in inclusions down to a few hundred parts per million in the lava. These data indicate that eruptions of Deccan lavas could have released at most 0.103 weight % of S, yielding up to 5.4 teragrams of SO2 per cubic kilometer of lava. A more conservative estimate is 0.07 weight % of S and 0.04 weight % of Cl, yielding 3.5 teragrams of SO2 and 1 teragram of HCl for every cubic kilometer of lava erupted. The flows were very large in volume, and these results imply that huge amounts of S and Cl gases were released. The environmental impact from even individual eruptions during past flood basalt activity was probably severe.     

Origin of espanola island and the age of terrestrial life on the galapagos islands

Geological studies of Espa?ola (Hood) Island, Galápagos, Ecuador, indicate that the island had a subaerial rather than a submarine origin. Because the younger lava flows are dated at 3 million years, Espa?ola has apparently existed as an island for at least that long. Thus terrestrial life may have existed or arrived on the Galápagos Islands at least 3 million years ago, more than twice as long as had been assumed.     

The crystallization age of eucrite zircon

Eucrites are a group of meteorites that represent the first planetary igneous activity following metal-silicate differentiation on an early planetesimal, similar to Asteroid 4 Vesta, and, thus, help date geophysical processes occurring on such bodies in the early solar system. Using the short-lived radionuclide (182)Hf as a relative chronometer, we demonstrate that eucrite zircon crystallized quickly within 6.8 million years of metal-silicate differentiation. This implies that mantle differentiation on the eucrite parent body occurred during a period when internal heat from the decay of (26)Al and (60)Fe was still available. Later metamorphism of eucrites took place at least 8.9 million years after the zircons crystallized and was likely caused by heating from impacts, or by burial under hot material excavated by impacts, rather than from lava flows. Thus, the timing of eucrite formation and of mantle differentiation is constrained.     

Fissure basalts and ocean-floor spreading on the East pacific rise

A basalt pavement outcrops almost continuously in a band along the crestal region of the East Pacific Rise from about 14 degrees S to 6 degrees S, that is, for more than 800 ikilometers; the outcrop may well extend beyond the above limits along the axis of the rise. The basalt band is generally between 40 and 60 kilometers wide and is replaced laterally by sediment. The lavas are fresh, "oceanic tholeiites" which were emplaced less than I million years ago by fissure eruptions. These findings, can be explained by the hypothesis of ocean-floor spreading; the basalts are the expression of material originating from the mantle and rising through fissures along the axis of the ridge. The absence of an axial rift valley on the East Pacific Rise may be explained by the fact that large volumes of lava are being outpoured along its crest.     

A close-up look at Io from Galileo's near-infrared mapping spectrometer

Infrared spectral images of Jupiter's volcanic moon Io, acquired during the October and November 1999 and February 2000 flybys of the Galileo spacecraft, were used to study the thermal structure and sulfur dioxide distribution of active volcanoes. Loki Patera, the solar system's most powerful known volcano, exhibits large expanses of dark, cooling lava on its caldera floor. Prometheus, the site of long-lived plume activity, has two major areas of thermal emission, which support ideas of plume migration. Sulfur dioxide deposits were mapped at local scales and show a more complex relationship to surface colors than previously thought, indicating the presence of other sulfur compounds.     

Paleomagnetic record of a geomagnetic field reversal from late miocene mafic intrusions, southern nevada

Late Miocene (about 8.65 million years ago) mafic intrusions and lava flows along with remagnetized host rocks from Paiute Ridge, southern Nevada, provide a high-quality paleomagnetic record of a geomagnetic field reversal. These rocks yield thermoremanent magnetizations with declinations of 227 degrees to 310 degrees and inclinations of -7 degrees to 49 degrees , defining a reasonably continuous virtual geomagnetic pole path over west-central Pacific longitudes. Conductive cooling estimates for the intrusions suggest that this field transition, and mafic magmatism, lasted only a few hundred years. Because this record comes principally from intrusive rocks, rather than sediments or lavas, it is important in demonstrating the longitudinal confinement of the geomagnetic field during a reversal.     

Cristobalite in volcanic ash of the soufriere hills volcano, montserrat, british west indies

Crystalline silica (mostly cristobalite) was produced by vapor-phase crystallization and devitrification in the andesite lava dome of the Soufriere Hills volcano, Montserrat. The sub-10-micrometer fraction of ash generated by pyroclastic flows formed by lava dome collapse contains 10 to 24 weight percent crystalline silica, an enrichment of 2 to 5 relative to the magma caused by selective crushing of the groundmass. The sub-10-micrometer fraction of ash generated by explosive eruptions has much lower contents (3 to 6 percent) of crystalline silica. High levels of cristobalite in respirable ash raise concerns about adverse health effects of long-term human exposure to ash from lava dome eruptions.     

Pacific geomagnetic secular variation

We have considered several different types of records of long-period geomagnetic secular variation: direct measurements made in geomagnetic observatories; paleomagnetic measurements on Hawaiian lava flows with accurately known ages in the interval 0 to 200 years; paleomagentic measurements on Hawaiian lava flows with loosely determined ages within the interval 200 to 10,000 years ago; and worldwide paleomagnetic measurements of the average geomagnetic angular dispersion recorded in lava flows that formed during the past 0.7 million years. All these magnetic records indicate that, during this time, the nondipole component of the earth's field was lower in the central Pacific than elsewhere, as it is today. This, in turn, indicates that there is some type of inhomogeneity in the lower mantle which is coupled to the earth's core in such a way as to suppress the generation of the nondipole field beneath the central Pacific. With the present incomplete state of knowledge about the processes that give rise to the earth's field, it is uncertain whether undulations in the core-mantle interface or lateral variations in the composition and physical state of the lower mantle are ultimately responsible for the pattern of secular variation seen at the earth's surface.     

Timing of hot spot--related volcanism and the breakup of madagascar and India

Widespread basalts and rhyolites were erupted in Madagascar during the Late Cretaceous. These are considered to be related to the Marion hot spot and the breakup of Madagascar and Greater India. Seventeen argon-40/argon-39 age determinations reveal that volcanic rocks and dikes from the 1500-kilometer-long rifted eastern margin of Madagascar were emplaced rapidly (mean age = 87.6 +/- 0.6 million years ago) and that the entire duration of Cretaceous volcanism on the island was no more than 6 million years. The evidence suggests that the thick lava pile at Volcan de l'Androy in the south of the island marks the focal point of the Marion hot spot at approximately 88 million years ago and that this mantle plume was instrumental in causing continental breakup.     

Sea floor spreading, topography, and the second layer

Local sea floor topography and also the thickness of the second layer of the oceanic rise-ridge system appear related to the spreading rate in the region. Slow spreading, away from the ridge center at 1 to 2 centimeters per year, is associated with a thick second layer, a central rift, and adjacent rift mountains. Fast spreading, 3 to 4.5 centimeters per year, is associated with a thin second layer and subdued topography lacking any central rift. The volume of lava discharged in this layer per unit time and per unit length along the crest of the whole active system is relatively constant regardless of the spreading rate. Total second layer discharge of the system has been about 5 to 6 cubic kilometers per year during the last several million years.     

Endogenetic craters interpreted from crater counts on the inner wall of copernicus

Collapse craters formed in terrestrial basalt flows exhibit size-frequency distributions that are similar to distributions for craters located in terraces in the inner wall of the lunar crater Copernicus. These distributions and surface morphology suggest that the interior terraces are basalt lava flows containing collapse craters as well as impact craters.     

Surficial deposits at Gusev Crater along Spirit Rover traverses

The Mars Exploration Rover Spirit has traversed a fairly flat, rock-strewn terrain whose surface is shaped primarily by impact events, although some of the landscape has been altered by eolian processes. Impacts ejected basaltic rocks that probably were part of locally formed lava flows from at least 10 meters depth. Some rocks have been textured and/or partially buried by windblown sediments less than 2 millimeters in diameter that concentrate within shallow, partially filled, circular impact depressions referred to as hollows. The terrain traversed during the 90-sol (martian solar day) nominal mission shows no evidence for an ancient lake in Gusev crater.     

Cation-ratio and accelerator radiocarbon dating of rock varnish on mojave artifacts and landforms

The first accelerator radiocarbon dates of rock varnishes are reported along with potassium/argon ages of lava flows and conventional radiocarbon dates of pluvial lake shorelines, in an empirical calibration of rock varnish K(+) + Ca(2+)/Ti(4+) ratios with age in the Mojave Desert, eastern California. This calibration was used to determine the cation-ratio dates of 167 artifacts. Although cation-ratio dating is an experimental method, some dates suggest human occupation of the Mojave Desert in the late Pleistocene.     

Latest pleistocene and holocene geomagnetic paleointensity on hawaii

Geomagnetic paleointensity determinations from radiocarbon-dated lava flows on the island of Hawaii provide an estimate of broad trends in paleointensity for Holocene time and offer a glimpse of intensity variations near the end of the last glacial period. When the data from Hawaii are compared with others worldwide, the intensity of the geomagnetic field seems to have been reduced from the Holocene average by about 35 percent between 45,000 and 10,000 years ago. A long-term reduction of this magnitude is compatible with reported increases in the production rate of cosmogenic nuclides during the same interval.     

Outward-dipping ring-fault structure at rabaul caldera as shown by earthquake locations

The locations of a large number of earthquakes recorded at Rabaul caldera in Papua New Guinea from late 1983 to mid-1985 have produced a picture of this active caldera's structural boundary. The earthquake epicenters form an elliptical annulus about 10 kilometers long by 4 kilometers wide, centered in the southern part of the Rabaul volcanic complex. A set of events with well-constrained depth determinations shows a ring-fault structure that extends from the surface to a depth of about 4 kilometers and slopes steeply outward from the center of the caldera. This is the first geophysical data set that clearly outlines the orientation of an active caldera's bounding faults. This orientation, however, conflicts with the configuration of many other calderas and is not in keeping with currently preferred models of caldera formation.     

重庆农田土壤硫分布特征及其影响因素

为研究不同环境要素作用下重庆农田土壤硫分布特征及其与环境因素的关系,结合野外调查和室内分析,研究了重庆农田土壤全硫的分布特征及其与土壤剖面深度、土壤类型、地形、土地利用方式、土壤pH和有机质之间的关系。结果表明,表层土壤全硫的平均值为262 mg·kg^-1,处于我国土壤硫平均水平,高于作物缺硫临界值150 mg·kg^-1。全硫含量空间分布在水平方向上呈现渝西、渝东北大于渝东南、渝中的趋势;在垂直方向上,呈现出自上而下逐渐降低的分布特征。全硫含量在不同土壤类型中表现为黄棕壤土最高、潮土最低,在不同地形条件下表现为中山最高、台地最低,在不同土地利用方式下则表现为未开垦农地最高、旱地最低。土壤全硫含量与pH对数呈显著负相关（P<0.01）,与有机质含量对数呈显著正相关（P<0.01）。多元线性逐步回归分析表明,土壤类型和有机质含量是影响土壤全硫含量分布的主要因素,pH、地形、土地利用方式是次要因素。     

Shipboard geophysical indications of asymmetry and melt production beneath the east pacific rise near the MELT experiment

Near the Mantle Electromagnetic and Tomography (MELT) Experiment, seamounts form and off-axis lava flows occur in a zone that extends farther to the west of the East Pacific Rise than to the east, indicating a broad, asymmetric region of melt production. More seamounts, slower subsidence, and less dense mantle on the western flank suggest transport of hotter mantle toward the axis from the west. Variations in axial ridge shape, axial magma chamber continuity, off-axis volcanism, and apparent mantle density indicate that upwelling is probably faster and more melt is produced beneath 17 degrees15'S than beneath 15 degrees55'S. Recent volcanism occurs above mantle with the lowest seismic velocities.     

Fumarolic activity in marie byrd land, antarctica

Ice towers, probably formed by recent fumarolic activity, have been found around the summit calderas of two volcanoes in Marie Byrd Land. These active (?) volcanoes lie within a broad belt of Mesozoic intrusion and late Cenozoic extrusion that appears to be part of the circum-Pacific orogenic province.