Pahoehoe and aa in Hawaii: volumetric flow rate controls the lava structure

The historical records of Kilauea and Mauna Loa volcanoes reveal that the rough-surfaced variety of basalt lava called aa forms when lava flows at a high volumetric rate (>5–10 m³/s), and the smooth-surfaced variety called pahoehoe forms at a low volumetric rate (<5–10 m³/s). This relationship is well illustrated by the 1983–1990 and 1969–1974 eruptions of Kilauea and the recent eruptions of Mauna Loa. It is also illustrated by the eruptions that produced the remarkable paired flows of Mauna Loa, in which aa formed during an initial short period of high discharge rate (associated with high fountaining) and was followed by the eruption of pahoehoe over a sustained period at a low discharge rate (with little or no fountaining). The finest examples of paired lava flows are those of 1859 and 1880–1881. We attribute aa formation to rapid and concentrated flow in open channels. There, rapid heat loss causes an increase in viscosity to a threshold value (that varies depending on the actual flow velocity) at which, when surface crust is torn by differential flow, the underlying lava is unable to move sufficiently fast to heal the tear. We attribute pahoehoe formation to the flowage of lava at a low volumetric rate, commonly in tubes that minimize heat loss. Flow units of pahoehoe are small (usually <1 m thick), move slowly, develop a chilled skin, and become virtually static before the viscosity has risen, to the threshold value. We infer that the high-discharge-rate eruptions that generate aa flows result from the rapid emptying of major or subsidiary magma chambers. Rapid near-surface vesiculation of gas-rich magma leads to eruptions with high discharge rates, high lava fountains, and fast-moving channelized flows. We also infer that long periods of sustained flow at a low discharge rate, which favor pahoehoe, result from the development of a free and unimpeded pathway from the deep plumbing system of the volcano and the separation of gases from the magma before eruption. Achievement of this condition requires one or more episodes of rapid magma excursion through the rift zone to establish a stable magma pathway.     

The massive double-lined O-type binary HD 165052

We present a new optical spectroscopic study of the O-type binary HD 165052 based on high- and intermediate-resolution CCD observations. We re-investigated the spectral classification of the binary components, obtaining spectral types of O6.5 V and O7.5 V for the primary and secondary, respectively, finding that both stars display weak C  iii λ 5696 emission in their spectra. We also determined a radial-velocity orbit for HD 165052 with a period of  2.95510±0.00001 d  , and semi-amplitudes of 94.8 and  104.7±0.5 km s^-1  , resulting in a mass ratio   Q =0.9  . From a comparison with previous radial-velocity determinations, we found evidence of apsidal motion in the system. Several signatures of wind–wind collision, such as phase-locked variability of the X-ray flux and the Struve–Sahade effect, are also considered. It was also found that the reddening in the region should be normal, in contrast with previous determinations.     

Age verification,growth and life history of rubyfish Plagiogeneion rubiginosum

Age verification of rubyfish (Plagiogeneion rubiginosum) was sought using the bomb radiocarbon chronometer procedure. Stable isotopes were investigated for life history characteristics. Radiocarbon (¹⁴C) and stable isotope (δ¹⁸O and δ¹³C) levels were measured in micro-samples from five otoliths that had been aged using a zone count method. All the core ¹⁴C measurements were ‘pre-bomb’ indicating ages of at least 45 years, and the ¹⁴C measurements across the otolith sections suggested that the zone-count ageing method described herein is not biased. Maximum estimated age was 100 years. There was no significant between-sex difference in the von Bertalanffy growth curves. The δ¹⁸O values indicated that rubyfish are near-surface as juveniles, and move deeper with age. Adults appear to reside in 600–1000 m; this is deeper than most trawl-capture data suggest, but not implausible, and has stock assessment implications. The δ¹³C values reflect fish metabolic rates, trophic feeding levels and oceanographic conditions. The stable isotopes record the environmental life history of each fish, and have value in distinguishing stocks and/or indicating vertical and latitudinal migratory patterns.     

Effects of mechanical and chemical processes on the degradation of plastic beach debris on the island of Kauai, Hawaii

Plastic debris is accumulating on the beaches of Kauai at an alarming rate, averaging 484 pieces/day in one locality. Particles sampled were analyzed to determine the effects of mechanical and chemical processes on the breakdown of polymers in a subtropical setting. Scanning electron microscopy (SEM) indicates that plastic surfaces contain fractures, horizontal notches, flakes, pits, grooves, and vermiculate textures. The mechanically produced textures provide ideal loci for chemical weathering to occur which further weakens the polymer surface leading to embrittlement. Fourier transform infrared spectroscopy (FTIR) results show that some particles have highly oxidized surfaces as indicated by intense peaks in the lower wavenumber region of the spectra. Our textural analyses suggest that polyethylene has the potential to degrade more readily than polypropylene. Further evaluation of plastic degradation in the natural environment may lead to a shift away from the production and use of plastic materials with longer residence times.     

MOISTURE STRESS,MID-TROPOSPHERIC PRESSURE PATTERNS,AND THE FOREST/GRASSLAND TRANSITION IN THE SOUTH CENTRAL STATES

The climatic causes of the major forest/grassland ecotone in the south central United States (Kansas, Missouri, Oklahoma, Texas) are still poorly understood. Grassland and forest vegetation types differ markedly in their ability to withstand water stress induced by vapor pressure deficit (VPD), the difference between saturation vapor pressure and actual vapor pressure in the atmosphere. VPD is an airmass characteristic induced by ambient temperatures higher than dewpoint temperature. Mean summer airmass movement is from the Gulf of Mexico onto the continent in the central states area, but mean VPD displays a strong gradient approximately parallel to the ecotone. A subset of days having the strongest VPD gradients across the ecotone also had a 500 mb pressure height pattern identical to the long-term mean (modal) pattern. This 500 mb pattern, with a ridge over the Rocky Mountains and a trough over the Great Lakes, induces subsidence, stability, warming, and high VPD in Great Plains airmasses. Farther east, away from the zone of maximum subsidence, VPD is much lower. The grassland region coincides with the area of highest VPD. Because of the importance of the daily configuration of mid-tropospheric windflow patterns in controlling the distribution of VPD, moisture, and precipitation, more frequent occurrence of the modal 500 mb pattern is one mechanism for the occurrence of drought.     

ASCA spectroscopy of the hard X-ray emission from the colliding wind interaction in γ2 Velorum

We discuss an ASCA observation of the eccentric WC8+O7.5 III binary γ ² Velorum near apastron. The X-ray spectrum is compared with two previous observations obtained when the system was near periastron. All three spectra display a hard-emission component that undergoes strong variability over the orbital cycle. The properties of the hard X-ray emission of γ ² Vel are constrained by taking into account the contribution from contaminating soft X-ray sources in the vicinity of γ ² Vel. We find that the observed variations are in qualitative agreement with the predictions of colliding wind models. We investigate for the first time the effect of uncertainties in the chemical composition of the X-ray emitting plasma on our understanding of the high-energy properties of the wind interaction region. Our results indicate that these uncertainties significantly affect the derived shock temperature and absorption column, but play a smaller role in determining the intrinsic X-ray luminosity of the colliding wind zone. We further find that the intrinsic luminosity from the hard X-ray component in γ ² Vel does not follow the 1/ D distance relation expected from simple models of adiabatic shocks.     

Glacier Changes at Svartisen,Northern Norway,during the Last 125 Years:Influence of Climate and Other Factors

The two ice caps of Svartisen,at the latitude of the Arctic Circle in Norway,supply 60 glaciers,ranging in size from >50 to <1 km2.Until the last two decades of the 19th century,the glaciers remained close to their maximum recent(Little Ice Age) size.In response to the prevailing 20th century climate,they have become smaller,but the changes have varied between glaciers.Climatic factors have not been the sole control of the variations.The response times of small,steep glaciers are shorter than those of the l...