Synchrotron X-ray computed microtomography: studies on vesiculated basaltic rocks

Synchrotron X-ray computed microtomography (CMT) was applied to five basalt samples from different locations in order to determine the microgeometrical structures and physical properties of vesicles. Two vesiculated basaltic lavas and one calcite-filled vesiculated basaltic lava were collected from the Lesotho Highland, South Africa, and two basaltic scoria tephra were taken from the Lungkuan Volcano Group in northeast China. They were examined with CMT on the X27C beam line of the Brookhaven National Synchrotron Light Source. The voxel size varied from 10 to 20 µm depending on the chosen field of view of the tomographic instrument. A fast-filtered back-transform (FFBT) algorithm was employed in the tomographic reconstruction. A 3D image geometry analysis package was used to measure the vesicularity, specific surface area and interconnectivity of the vesicles. The results of the analyses showed that the bubbles have roughly spheroidal forms of different sizes. Vesicularity ranged from 45% for lavas to 80% for scoria. At least 90% of the vesicles are interconnected. Specific surface area was determined both by counting voxel faces and by use of two-point correlation functions.     

The 1999 eruption of Shishaldin Volcano, Alaska: monitoring a distant eruption

Shishaldin Volcano, in the central Aleutian volcanic arc, became seismically restless during the summer of 1998. Increasing unrest was monitored using a newly installed seismic network, weather satellites, and rare local visual observations. The unrest culminated in large eruptions on 19 April and 22-23 April 1999. The opening phase of the 19 April eruption produced a sub-Plinian column that rose to 16 km before rapidly dissipating. About 80 min into the 19 April event we infer that the eruption style transitioned to vigorous Strombolian fountaining. Exceptionally vigorous seismic tremor heralded the 23 April eruption, which produced a large thermal anomaly observable by satellite, but only a modest, 6-km-high plume. There are no ground-based visual observations of this eruption; however we infer that there was renewed, vigorous Strombolian fountaining. Smaller low-level ash-rich plumes were produced through the end of May 1999. The lava that erupted was evolved basalt with about 49% SiO₂. Subsequent field investigations have been unable to find a distinction between deposits from each of the two major eruptive episodes.     

Abstract databases as information sources in economic geography

The limitations of existing information sources on mergers/acquisitions within the EU are a serious obstacle to the study of the restructuring of the European production system. The experience of using a computerized abstracting service as a source of information on mergers/acquisitions in the chemical industry is reviewed. This experience informs an assessment of the strengths and weaknesses of similar databases as information sources in economic geography. The proliferation of such databases, together with advances in text management software, suggests that their potential has not been fully appreciated. This potential is considerable in providing easy access to detailed factual information on economic events and processes at continental and global scales. It is, however, strictly limited when it comes to interpreting the corporate strategies and motivations responsible for these developments.     

FLUORINE ABUNDANCES OF TWENTY NINE GEOLOGICAL AND OTHER REFERENCE SAMPLES USING FAST-NEUTRON ACTIVATION ANALYSIS

The fluorine content in twenty-nine reference samples of geological interest has been determined using a well tried and tested method which has been previously reported. The analytical method uses fast neutron activation involving the reaction ¹⁹F(n,a)¹⁶N and gamma spectrometry. Interferences have been found to be negligible for the experimental conditions used. The method involves minimal sample preparation, is rapid and can easily measure concentrations down to 5 parts per million in a 5 g sample.     

Dominance of Tilapia mossambica,an introduced fish species,in three Puerto Rican estuaries

We compared species presence, abundance, and size characteristics of fish in three brackish, coastal marshes at Humacao, Roosevelt Roads, and Boqueron, Puerto Rico, in February and March 1988. The three marsh ecosystems were similar with respect to the presence of large expanses of open water bordered by emergent vegetation, creeks, and mangroves, and all had some recreational use. We sampled fish using gill nets. Tilapia (Oreochromis) mossambica were the most abundant fish, accounting for 55–79% of the samples at all three marshes. Overall, tilapia were both the largest (North Lagoon) and the smallest (Frontera Creek) at Humacao. Tilapia were most common in open lagoons rather than creeks or bays (except for Mandri Creek), and their distribution seemed unrelated to salinity. Tarpon (Megalops atlantica) were more abundant at low salinities, whereas other fish were more abundant at higher salinities.     

Subsidence analysis on the Sardinian margin and the central Tyrrhenian Basin: Thermal modelling and heat flow control; deep structure implications

The subsidence of the Tyrrhenian Basin is analysed based on numerous MCS reflection data and related seismic velocities, and calibrated using subsidence curves versus time at ODP Leg 107 sites 654 and 652 on the Sardinian margin. Two main phases of rifting are clearly defined for the margin-basin evolution: the first, beginning at 7 Ma, split the Sardinian-Calabrian margins, the second, beginning at 5 Ma formed the Central oceanic subbasins.Thermal models, based on thinned crust geometry from reflection-refraction data, are used which consider both vertical and horizontal conduction over time along a NW-SE transect across the entire Tyrrhenian Basin from Sardinia to Calabria. In general, there is a close agrrement between the predicted and the observed surface heat flow and the rate of subsidence over time. However, within the oceanic domain an initial subsidence must be chosen about 1000 m greater than observed on typical spreading centers, i.e., at about 3500 m as postulated in numerous back-arc basins.Thermal models, constrained by subsidence analysis and gravity observations, imply a large difference for the Moho depth beneath the Sardinian margin as determined after refraction seismic experiments. The upper mantle, hot but with high density, would be situated much higher than is postulated by seismic data and probably includes 7.3–7.4 km/s velocity layers previously interpreted as deep crustal layers.