Despite episodic activity, certain aspects of the 1980-1986 dome growth were quite regular. The long-term growth rate was approximately linear during three distinct periods: 1.8x10^6 cubic meters per month between October 18, 1980 and the end of 1981, 1.3x10^6 cubic meters per month between March 1982 and March 1984, and 0.62x10^6 cubic meters per month thereafter. The change from one period to the next coincides with distinct changes in style of eruption, magma composition, or associated seismicity. Long-term magma supply was approximately volume-predictable during each growth period and for certain episodes was also time-predictable. The height of the dome increased according to the equations
h = 43.44 (ln t) - 83.79and
h = 23.22t^0.32where h is height in meters and t is time in days since October 18, 1980. The average diameter (including talus apron) increased according to the power law
d = 176.16t^0.22where d is diameter in meters. the ratio h/d ranged from 0.227 to 0.292 (mean of 0.266) except for the initial period of growth, when the dome was flatter (h/d = 0.142) possibly owing to a weak, relatively thin crust and the lack of a significant mantle of talus. The h/d ratios fall within the field defined for Japanese domes by I.Moriya and are less than the empirical upper limit of 0.32. The general equation
C = V/hd^2where V is known and h and d are calculated from the above equations, yields a value for the shape factor C of 0.2583 (s.d.=0.0282) before the year of continuous growth and 0.3341 (s.d.=0.0196) thereafter. The shape is probably controlled by the net effective viscosity and tensile strength of the hot core, cool outer shell, and flanking talus. Modeling by Iverson (this volume) -- (Web note: not available) -- and Denlinger (this volume) -- (Web note: not available) -- suggests that the outer shell is the most important of these factors.
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