Regularities in Growth of the Mount St. Helens Dacite Dome, 1980–1986

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The dacite dome at Mount St. Helens grew episodically between October 18, 1980, and October 22, 1986, chiefly by extrusion of thick flows but also by endogenous growth resulting from intrusion into its molten core. Typical growth episodes lasted several days and produced volumes of 1.2–4.5×106m3, but growth was continuous from February 1983 to February 1984. By the end of October 1986, the volume of the dome and its talus apron was about 74.1×106 m3, and the volume of all erupted material (including tephra and debris removed from the dome by explosions and rockfalls) was about 77.1×106m3.

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: 1.8×106 m3/mo between October 18, 1980 and the end of 1981, 1.3×106m3/mo between March 1982 and March 1984, and 0.62×106 m3/mo 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) - 83.79 and h = 23.22t 0.32, where 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 0.22, where 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 2, where 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 Vol.) and Denlinger (this Vol.) suggests that the outer shell is the most important of these factors.