![]() ![]() Calculations of lava viscosity were made by Moore using measured flow velocity with channel width and depth data from the same sample stations as used by Lipman and Banks. ![]() By 29 March to 9 April crystallinities were 15–25%, increasing to 25–30% during 10–15 April. At the same time, crystallinities increased from 0–5% on 25 March to 10–15% by 26–28 March. For a dense rock density of 2600 kg m −3, this equates to an increase in vesicularity from 55–60% to 70–85%. Lava densities, however, decreased from 1000 to 1200 kg m −3 during the first 12 days to 400–800 kg m −3 thereafter. Measurements by Lipman and Banks give eruption temperatures within a fairly narrow range (1140 ± 3☌) throughout the eruption. The 1984 eruption of Mauna Loa began on 25 March, continued for 21 days until 14 April, and was characterized by emplacement of channel-fed flows extending up to 27 km from the vent at discharge rates of up to 280 m 3 s −1. Our two cases are taken from the 1984 eruption of Mauna Loa (Hawaii), for which viscosity measurements are available from Moore, and lava at Mount Etna (Italy) measured using a viscometer by Pinkerton and Sparks in 1975. These allow for appraisal of the applicability of each method to basaltic mixtures. In assessing one-, two-, and three-phase treatments of lava flow viscosity we consider two cases for which independent measurements of viscosity are available. This formulation has good agreement with experimental data and is comparable to established formulae for two-phase systems currently in use for lava, i.e., treatments based on the work by Einstein and Roscoe. This rigorously derived treatment reduces, in a limiting case, to a viscosity treatment of rigid spheres and fluid. ![]() In this paper, we examine the use of the three-phase treatment of Phan-Thien and Pham for a basaltic lava mixture. Three-phase treatments have been developed in other contexts such as in the study of suspensions and multiphase flow. ![]() Nevertheless, three-phase treatments have received less attention in the context of lava flows. Dobran and Papale and Dobran, consider the three-phase viscosity for magma ascending a conduit during explosive Plinian eruptions. In addition, two-phase treatments have been applied to mixtures of fluid and crystals or fluid and bubbles. Single-phase treatments obtain fluid viscosity from composition, water content, and/or temperature. Many rheological treatments exist for one- and two-phase magma and/or lava mixtures. Derivation of viscosity is crucial in applications that seek to use field data and measurements such as lava flow temperature, crystallinity, and vesicularity for calculation of the bulk rheology of different suspensions, and/or to model lava flow dynamics. This mixture plays an influential role in determining the bulk rheology of the flow. The ability of the three-phase treatment to characterize the full range of melt-crystal-bubble mixture viscosities in both settings indicates the potential of this method in characterizing basaltic lava mixture viscosity.Ī basaltic lava is a three-phase mixture comprising a fluid component (basalt) plus voids (bubbles and/or space opening along shear lines) and solids (crystals, both phenocrysts and microphenocrysts). At Mauna Loa, the three-phase treatment provides a fit with the full range of field measured viscosities, giving three-phase mixture viscosities, upon eruption, of 110–140 Pa s (5% crystals, no bubble effect due to sheared vesicles) to 850–1400 Pa s (25–30% crystals, 40–60% spherical vesicles). This compares with a measured value for Etnean lava of 9400 ± 1500 Pa s. Application of a three-phase model yields mixture viscosities (45% crystals, 25–35% vesicles) in the range 5600–12,500 Pa s. % H 2O), and 135 ± 10 Pa s for less well-degassed (0.4 wt % H 2O), melt at 1080☌. At Etna, melt viscosities of 425 ± 30 Pa s are expected for well-degassed (0.1 w. We investigate existing one- and two-phase treatments using lava flow case studies from Mauna Loa (Hawaii) and Mount Etna (Italy) and compare these with a three-phase treatment that has not been applied previously to basaltic mixtures. However, three-phase treatments, allowing for the effects of coexisting crystallinity and vesicularity, are not well understood. While existing one-phase treatments allow melt phase viscosity to be assessed on the basis of composition, water content, and/or temperature, two-phase treatments constrain the effects of crystallinity or vesicularity on mixture viscosity. Lava flows comprise three-phase mixtures of melt, crystals, and bubbles. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |