Abstract
In the discussion of lava dome formation, viscosity of magma plays an important role. Measurements of viscosity of magmas in field and laboratory are briefly summarized. The types of lava dome emplacements are classified into two, squeeze- and spine-type, by kinetic processes. The squeeze-type is the formation of a dome as a result of squeezes of magma through conduits and the latter is solidified magma forced to ascend by underlying fluid magma. An important parameter in the formation of such lava domes is their growth rates. Lava domes of squeeze-type are governed by the Hagen-Poiseuille Law which involves their viscosoties and other eruption parameters. At present, the real viscosity of magmas at the site of lava dome is still inaccessible. In order to avoid uncertainty in viscosity of magmas, a conception of «macroscopic viscosity» is proposed, which involves effects of chemical components, mainly SiO2 and volatile material, crystals and temperature, and their changes with time. Lava dome formations during the 20th century are briefly examined and their growth rates are estimated. The relationship between the growth rates and the SiO2 content of the magma is statistically studied, and the macroscopic viscosity is empirically expressed as a function of SiO2 content. The linearity between the two parameters is reasonably interpreted. This means that formation processes of lava domes are dominantly controlled by macroscopic viscosity of magma.
Highlights
Lava domes are one of the significant features of volcanism at the earth surface and their definitions and classifications have been published by several researchers
If we know the growth rates of lava domes, it is possible to find a relationship among the physical parameters of lava dome formation, such as viscosity of magmas, conduit dimension and driving pressure, by Hagen-Poiseuille Law which is conditioned for fluids to flow through a narrow space
Growth rates of lava domes are expected to undergo the strongest effect by viscosity of magmas
Summary
Lava domes are one of the significant features of volcanism at the earth surface and their definitions and classifications have been published by several researchers. Yokoyama (2004) interpreted the formation processes of the 1909 Tarumai lava dome (andesite) and the 1944 Usu lava dome (dacite), both in Hokkaido, Japan, as being squeezed from magma forced to flow through the vents. Ascending magmas became lower in temperature, gas and water contents, with increased degrees of crystallization. Their rheologic behavior changed from Newtonian viscous flows to Bingham plastic flows. These lava domes assumed different features of effusions such as growth process, its rate and final configurations, due to differences in viscosities of their magmas and other eruption parameters. Izumi Yokoyama various lava domes to estimate effective viscosity of their magmas from the macroscopic and statistic viewpoint
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