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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Bulletin of volcanology 58 (1996), S. 298-309 
    ISSN: 1432-0819
    Keywords: Key words Microlites ; Rhyolite ; Obsidians ; Viscosity ; Glass transition ; Lava flow rheology
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract  To investigate the influence of microlites on lava flow rheology, the viscosity of natural microlite-bearing rhyolitic obsidians of calc-alkaline and peralkaline compositions containing 0.1–0.4 wt.% water was measured at volcanologically relevant temperatures (650–950  °C), stresses (103–105 Pa) and strain rates (10–5 to 10–7 s–1). The glass transition temperatures (T g ) were determined from scanning calorimetric measurements on the melts for a range of cooling/heating rates. Based on the equivalence of enthalpic (calorimetric) and shear (viscosity) relaxation, we calculated the viscosity of the melt in crystal-bearing samples from the T g data. The difference between the calculated viscosity of the melt phase and the measured viscosity for the crystal-bearing samples is interpreted to be the physical effect of microlites on the measured viscosity. The effect of 〈5 vol.% rod-like microlites on the melt rheology is negligible. Microlite-rich and microlite-poor samples from the same lava flow and with identical bulk chemistry show a difference of 0.6 log10 units viscosity (Pa s), interpreted to be due to differences in melt chemistry caused by the presence of microlites. The only major differences between measured and calculated viscosities were for two samples: a calc-alkaline rhyolite with 1 vol.% branching crystals, and a peralkaline rhyolite containing crystal-rich bands with 〉45 vol.% crystals. For both of these samples a connectivity factor is apparent, with, for the latter, a close packing framework of crystals which is interpreted to influence the apparent viscosity.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Bulletin of volcanology 59 (1997), S. 103-111 
    ISSN: 1432-0819
    Keywords: Key words Magma ; Mount St. Helens ; Cryptodome ; Viscosity ; Physical ; Rheological ; Experimental
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract  Physical properties of cryptodome and remelted samples of the Mount St. Helens grey dacite have been measured in the laboratory. The viscosity of cryptodome dacite measured by parallel–plate viscometry ranges from 10.82 to 9.94 log10 η (Pa s) (T=900–982  °C), and shrinkage effects were dilatometrically observed at T〉900  °C. The viscosity of remelted dacite samples measured by the micropenetration method is 10.60–9.25 log10 η (Pa s) (T=736–802  °C) and viscosities measured by rotational viscometry are 3.22–1.66 log10 η (Pa s) (T=1298–1594  °C). Comparison of the measured viscosity of cryptodome dacitic samples with the calculated viscosity of corresponding water-bearing melt demonstrates significant deviations between measured and calculated values. This difference reflects a combination of the effect of crystals and vesicles on the viscosity of dacite as well as the insufficient experimental basis for the calculation of crystal-bearing vesicular melt viscosities at low temperature. Assuming that the cryptodome magma of the 18 May 1980 Mount St. Helens eruption was residing at 900  °C with a phenocryst content of 30 vol.%, a vesicularity of 36 vol.% and a bulk water content of 0.6 wt.%, we estimate the magma viscosity to be 1010.8 Pa s.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1432-1459
    Keywords: Key words Multi-parametric ; Multiple sclerosis ; Magnetic ; resonance imaging ; Lesion volume ; Clinico-radiological correlation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract Magnetic resonance imaging (MRI) is frequently used to monitor new treatments in multiple sclerosis (MS), but its role is limited by the uncertain relationship between MRI parameters and clinical disability. A brain MRI study using nine MRI parameters was undertaken in 15 MS patients with a wide spectrum of disability to evaluate the relationship between each parameter and disability. A strong correlation was found between disability (measured using Kurtzke’s EDSS) and total lesion load on both proton density (PD; r = 0.79) and T1 (r = 0.71) weighted sequences. There was also a strong correlation of disability with average lesion magnetisation transfer ratio (MTR; r = –0.74) and calculated T1 (r = 0.71) but not with calculated T2 or the average signal intensity of lesions on the conventional T1-weighted, PD-weighted and heavily T2-weighted images. Thus, four parameters which measured either the extent of lesions (PD lesion load) or their pathological severity (MTR, calculated T1, hypointense T1-lesion load) were correlated significantly with disability. While this suggests that such parameters will be useful in treatment trial monitoring, further multi-parameter MRI studies, of larger cohorts and using a wider range of techniques, are indicated.
    Type of Medium: Electronic Resource
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