A vesicularity index for pyroclastic deposits

The vesicularity of juvenile clasts in pyroclastic deposits gives information on the relative timing of vesiculation and fragmentation, and on the role of magmatic volatiles versus external water in driving explosive eruptions. The vesicularity index and range are defined as the arithmetic mean and total spread of vesicularity values, respectively. Clast densities are measured for the 16–32 mm size fraction by water immersion techniques and converted to vesicularities using measured dense-rock equivalent densities. The techniques used are applied to four case studies involving magmas of widely varying viscosities and discharge rates: Kilauea Iki 1959 (basalt), Eifel tuff rings (basanite), Mayor Island cone-forming deposits (peralkaline rhyolite) and Taupo 1800 B.P. (calc-alkaline rhyolite). Previous theoretical studies suggested that a spectrum of clast vesicularities should be seen, depending on the magma viscosity, eruption rate, and the presence and timing of magma: water interaction. The new data are consistent with these predictions. In magmatic dry eruptions the vesicularity index lies uniformly in the range 70%–80% regardless of magma viscosity. For high viscosities and eruption rates the vesicularity ranges are narrow (< 25%), but broaden to between 30% and 50% as the viscosity and eruption rates are lowered and the volatiles and magma can de-couple. In phreatomagmatic wet eruptions, widely varying clast vesicularities reflect complex variations in the relative timing of vesiculation and water-induced fragmentation. Magma:water interaction at an early stage greatly reduces the vesicularity indices (< 40%) and broadens the ranges (as high as 80%), whereas late-stage interaction has only a minor effect on the index and broadens the range to a limited extent. Clast vesicularity represents a useful third parameter in addition to dispersal and fragmentation to characterise pyroclastic deposits.     

Ritz vectors and generation criteria for mode superposition analysis

A generation procedure of Ritz vectors to control the inclusion of static effect and the number of vectors in mode superposition dynamic analysis is presented. The original algorithm of the Ritz vectors¹⁵ is modified to improve stability in the generation procedure and to include the use of static residual. To reject unimportant Ritz vectors, cut-off criteria, which are based on the participation of mass distribution and spatial load distribution, are proposed. Numerical examples are presented to illustrate the effectiveness of the derived Ritz vectors over the eigenvectors and the performance of the cutoff criteria in the mode superposition dynamic analysis.     

The role of fluidization in the emplacement of pyroclastic claws: An experimental approach

A series of experiments was carried out to review the process of fluidization for a number of particulate materials having various sorting and grain shape characteristics. The up (increasing gas velocity) curve on a gas velocity/bed-pressure drop plot for a poorly sorted mixture of irregularly shaped particles is divided into three sections; non-expanded, expanded, and segregating. These sections are used to define a threefold genetic classification of pyroclastic flows which can be directly linked to conditions within a semifluidized parent flow. Type 1 flows are ungraded and mostly result from hot-avalanche flows and pyroclastic flows formed of relatively dense material. Type 2 and type 3 flows are mainly pumiceous ignimbrites and are distinguished by expansion induced coarse-tail grading, coupled with segregation structures in the latter. The implications of this classification are discussed with reference to the flow regimes, deposition slope angles, crystal concentration and “fossil fumaroles” variously developed in the flows. The relevance of the source of fluidizing gas is discussed in relation to the zoning of flow types within a single flow unit and it is shown how this, and its attendant structures, can be used to estimate the relative importance of each gas source during and after flow emplacement.     

Complex unmixed spinels in layered intrusions within an obducted ophiolite in the Natal-Namaqua mobile belt

Spinellids showing unmixed intergrowths of chromite or chromian spinel (sensu stricto) and magnetite or chromian magnetite are not known in mafic or ultramafic igneous rocks. They do occur within metamorphosed rocks that attained temperatures sufficiently high (upper amphibolite facies) for the formation of homogeneous Al-Cr-Fe³⁺-Ti spinel phases with compositions not matched in slowly cooled igneous rocks. In the Tugela Rand intrusion complex intergrowths of chromian spinel, chromian magnetite, ulvöspinel, ilmenite and a transparent aluminous spinel are observed and interpreted in terms of the thermal history of the rocks. Compositional differences between the separate areas of chromian spinel and chromian magnetite in complex intergrowths exhibited by the metamorphosed Tugela Rand and Mambulu Complexes confirm the extension of the magnetite-hercynite solvus (Turnock and Eugster 1962) towards magnesium- and chromium-rich compositions. The Tugela Rand spinellids are compared with those from the Carr Boyd Complex (Purvis et al. 1972) and the ultramafic rocks of the Giant Nickel Mine (Muir and Naldrett 1973) and the Red Lodge district (Loferski and Lipin 1983). Significant differences between the spinels from the Red Lodge district compared to the other three occurrences may reflect the different metamorphic histories of these areas.     

Detection of tannins in modern and fossil barks and in plant residues by high-resolution solid-state C nuclear magnetic resonance

Bark samples isolated from brown coal deposits in Victoria, Australia, and buried wood from Rhizophora mangle have been studies by high-resolution solid-state nuclear magnetic resonance (NMR) techniques. Dipolar dephasing ¹³C NMR appears to be a useful method of detecting the presence of tannins in geochemical samples including barks, buried woods, peats and leaf litter. It is shown that tannins are selectively preserved in bark during coalification to the brown coal stage.     

Prominence mass ejections and their effects on the corona

Skylab soft X-ray observations of two lower coronal limb events and corresponding H observations (Skylab and ground-based) are analyzed. We discuss the morphology and evolution of an eruptive prominence occurring on 21 August 1973, beginning (in H) at about 1300 UT and of a surge on 4 December 1973, beginning at about 1758 UT. For the eruptive prominence, measured X-ray flux is used in the determination of line-of-sight temperatures, emission measures, and electron densities. A peak temperature of 8.5 × l0⁶ K and densities to 3.5 × l0⁹ cm^-3 are derived. A time-dependent, two-dimensional, single-fluid magnetohydrodynamic computer code has been used to simulate the coronal response to these prominences. We find that the coronal response to the observed eruptive prominence may be simulated with a density-dominated pressure pulse at the base of the corona ( 30000 km above photosphere), while a temperature pulse of short duration will simulate the coronal response to the surge. Approximately 10³¹ ergs and 10⁴⁰ particles (or 10¹⁶ g) were deposited into the corona during the eruptive prominence event, while about 10²⁹ ergs and 10³⁸ particles (or 10¹⁴ g) were injected during the surge event. A shock wave formed ahead of the ejected material at about 70000 km above the photosphere in the eruptive prominence event and had a velocity of 275 km s^-1 at 1.5 r above the limb.Presently at NASA / Marshall Space Flight Center.     

Factors influencing the height of Hawaiian lava fountains: implications for the use of fountain height as an indicator of magma gas content

The heights of lava fountains formed in Hawaiian-style eruptions are controlled by magma gas content, volume flux and the amounts of lava re-entrainment and gas bubble coalescence. Theoretical models of lava fountaining are used to analyse data on lava fountain height variations collected during the 1983–1986 Pu'u 'O'o vent of Kilauea volcano, Hawaii. The results show that the variable fountain heights can be largely explained by the impact of variations in volume flux and amount of lava re-entrainment on erupting magmas with a constant gas content of 0.32 wt.% H₂O. However, the gas content of the magma apparently declined by 0.05 wt.% during the last 10 episodes of the eruption series and this decline is attributed to more extensive pre-eruption degassing due to a shallowing of the sub-vent feeder dike. It is concluded that variations in lava fountain height cannot be simply interpreted as variations in gas content, as has previously been suggested, but that fountain height can still be a useful guide to minimum gas contents. Where sufficient data are available on eruptive volume fluxes and extent of lava entrainment, greatly improved estimates can be made of magma gas content from lava fountain height.     

Faculae,filigree and calcium bright points

Simultaneous observations of fine structure photospheric features at several wavelengths are described. The observations, which include regions near disk center and at the limb, were obtained using a narrow band calcium filter and selected wavelengths in Mg b₁, the red wing of H and the H continuum using the Universal Birefringent Filter of the Sacramento Peak Observatory.From the disk-center series it is concluded that bright point structures seen in calcium are cospatial with the H filigree and are related in origin.The limb photographs confirm that the calcium network is cospatial with the white light faculae. However, in regions of good seeing it is clear that even at the limb the calcium network patterns consist of individual bright points which are essentially the same as those seen near disk center.Thus it is concluded that the white light faculae, the H filigree and the calcium bright points are different manifestations of the same photospheric features.On leave from Department of Applied Mathematics, University of Sydney, Australia.The Sacramento Peak Observatory is operated by the Association of Universities for Research in Astronomy, Inc. under contract AST 78-17292 with the National Science Foundation.     

Antifouling effect of bioactive compounds from selected marine organisms in the Obhur Creek,Red Sea

Three species of sponges and a tunicate were collected from Obhur creek of Jeddah coast for this bioactivity study. In order to assess the antifouling efficacy of selected marine organisms, methanolic extracts of these organisms were tested against different fouling bacterial forms and II-instar stage of the barnacle, Balanus amphitrite. Antibiosis, bioactivity and followed by multivariate analyses were carried out to check the efficacy of antifouling effect of the selected marine organisms. Principal component analysis revealed the exemplary antifouling efficacy of the sponge extracts of Stylissa sp. observed followed by Hyrtios sp. against bacterial forms in the laboratory study. De-trended correspondence analysis confirmed that the contribution of antifouling efficacy of the selected sponge extracts was observed to be more towards Bacillus sp., Vibrio sp. and Alteromonas sp. Moreover, the efficacy of Hyrtios sp. extract(20.430 μg m L~(-1)) followed by Stylissa sp.(30.945 μg m L~(-1)) showed higher against barnacle instar compared with other extracts in the bioactivity assay. Bray-Curtis cluster analysis under paired linkage categorized all the sponge extracts into one major cluster with 75% similarity, and one outlier tunicate. More than 80% similarity observed between Hyrtios sp. and Stylissa sp. Fourier transform infrared spectroscopy(FTIR) showed that the contribution of major peaks found in the marine organisms were towards sulfones, sulfoxides, cyanates and ketones.