IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

1W_2K-P3

Date/Time: July 21 Poster

Viscosity measurements of crystal-bearing natural lava samples from Unzen volcano,Japan
Akio Goto1 , Takehiko Hiraga2 , Hidemi Isibashi3
1Tohoku University, Japan, 2 University of Tokyo, Japan, 3 Shizuoka University, Japan
E-mail: ak-goto@cneas.tohoku.ac.jp

Goto (1999) pointed out the apparent viscosity of Unzen lobe lava from field observation, 0.9-4.2×1010 Pa s byFukui et al. (1991) and Suto et al. (1993), is much lower than predicted from matrix melt viscosity (8×1012 Pas at 800 °C) and about 50 vol.% lava crystallinity. This implies the mechanism of lobe lava displacement may bedifferent from the Newtonian flow.
To the contrary Cordonnier et al. (2009) measured the viscosity of Unzen dome lava samples and concluded that
their experimental results is in harmony with the observation if, based on the viscosity model by Hess and Dingwell(1997), the viscosity decrease by ≈0.2 wt.% water is considered. However, their experimental temperature ranges
between 940 and 1010 °C. If we linearly extrapolate their data down to 800 °C lava viscosity should be over 1016 Pas, which is difficult to be lowered to observed viscosity range by water effect. Thus far the viscosity of crystal-bearing
lava is still under debate.
We have started viscosity measurement of natural lava sample from Unzen 1991-1995 activity. The rock sample
used in our preliminary experiment was collected from 1991 Sep. 15 pyroclasitc flow deposit. The test piece wascored to 14.95 mm diameter and 30.55 mm high for parallel plate viscometry. The core bulk density was 2.24 g/cm3 .
The core was loaded 10 N (57 kPa) during the heating for 3 hours to desired temperature (980 °C) and additionallyheld for 30 minutes, then loaded up to 1800 N (10 MPa) within a minute and kept at this load for 30 minutes. Thetotal deformation was 47 %. Except the last 10 minutes viscosity was almost constant at each loads, 2×1010 Pa s
at 57 kPa and 6×109 Pa s at 10 MPa.
At the same temperature (980 °C) Cordonnier et al. (2009) obtained the apparent viscosity to be 2×1011 Pa s at 2.8
MPa and 3×1010 Pa s at 10 MPa. Our experimental result is in harmony with them in that shear thinning occurred
and time weakening did not occur at stresses less than 10 MPa, but differs in that the viscosity is lower at the samestress (10MPa) and the shear thinning is less prominent.
Avard and Whittington (2012) pointed out most laboratory studies are typically conducted at higher stresses andstrain rates than experienced by lava moving as surface flows, and showed their experiments at low stress (0.085
to 0.42 MPa) yielded apparent viscosities more than one order of magnitude lower than predicted by models basedon experiments at higher stress. Our experiment at 57 kPa actually yielded one order lower viscosity than at 2.8
MPa by Cordonnier et al. (2009), but the conclusion by Avard and Whittington (2012) conflicts with shear thinning.
We have to explore the rheological property of crystal-bearing lava more.

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