The dome grew in a complex series of extrusions preceded, accompanied, and at times supplanted by periods of endogenous growth (Swanson et.al., 1987). The extrusions produced short (200-400m), thick (20-40m) flows, which we term lobes, that piled atop one another and generally did not reach the crater floor before crumbling into talus. The lobes were erupted in an overlapping, seemingly haphazard, manner that eventually built the composite dome. Most of the lobes were fed from the summit region of the dome, but a few issued from eccentric vents high on the flanks. Seventeen episodes of dome growth occurred between October 18, 1980, and October 22, 1986, inclusive. Fourteen episodes produced one lobe each, and three produced two lobes each (December 1980, March-April 1982, and February 1983-February 1984), when the dome ruptured at two different locations.
Endogenous growth began slowly 1-3 weeks before each extrusion. The rate of endogenous growth, determined by geodetic measurements of displacement of the surface of the dome, accelerated almost exponentially to the time of extrusion. The slow, pre-extrusive rise of magma up the conduit and into the dome caused radial cracking and thrust faulting of the crater floor and expansion of the dome itself (Chadwick et.al., 1983, 1988; Dzurisin et.al., 1983); such deformation was useful in predicting the start of each extrusion (Swanson et.al., 1983). Endogenous growth generally affected only a relatively small sector of the dome, typically half or less. Commonly the oldest exposed part of the dome was the site of greatest endogenous growth, possibly because cooling and alteration had decreased the tensile strength of the crust, but many exceptions occurred. Some periods of endogenous growth caused sever fracturing, faulting, and distension of the dome. In May 1985 (Swanson 1985) and May and October 1986, sector grabens tens of meters deep and hundreds of meters long resulted from endogenous growth, and outward-directed radial displacements of as much as 70m were measured. Endogenous growth was essentially continuous for one full year (February 1983 to February 1984) and became increasingly important during later episodes of growth as the volume of the dome and consequently its holding capacity enlarged. Overall, endogenous growth probably accounts for 30-40% of the volume of the dome.
Talus occurs as extensive aprons mantling the flanks of the dome and in irregular patches high on the dome. The talus accumulations comprise one of the most conspicuous features of the dome. Most of the talus formed from hot rockfalls during extrusion and rapid endogenous growth; only a minor amount was generated by cold rockfalls during periods of quiet. Hot talus blocks developed radial prismatic jointing during cooling. Renewed movement (slumping, rockfalls) broke the fragile, jointed blocks into several joint-bounded pieces and further contributed to the talus accumulation.
The dome slowly subsided and spread outward between episodes of growth, apparently as its hot, relatively ductile core yielded under gravitational stress. Typical maximum rates of spreading and subsidence during quiet periods were 2-5mm/day.
Several small explosions excavated pits and wedge-shaped sectors from the dome. The total volume of rock removed from the dome by these explosions was small, probably less than 2.5x10^6 cubic meters, but the surface morphology of the dome was significantly modified until later extrusion and endogenous growth filled or disrupted the depressions.
The first lobe was confined to a 300-m-wide shallow depression in the crater floor. Later lobes filled the depression and spread onto the surrounding floor, burying older lobes in the process. The crater floor was nearly flat except north of the depression, where the first 120m sloped northward about 12 degrees, the next 140m (an area often called "the rampart") about 19 degrees, and the next 200m about 11 degrees (slopes determined from Map 1, Table 3). -- (Web note: not available) -- These slopes may have influenced the shape of the dome to some degree, but we do not discuss them further.
By October 31, 1986, the dome stood about 267m above its vent and about 350m above its northern base, which rests on the northward-sloping crater floor 550-600m from the vent. At that time the slightly elliptical dome had an east-west diameter of 860m, a north-south diameter of 1060m, and a volume of about 74.1x10^6 cubic meters, including the volume of talus mantling its lower flanks.
The dacitic chemical composition of successive lobes showed little net change during growth of the 1980-86 dome (Table 1); -- (Web note: not available) -- on average, however, lobes extruded after 1981 may be slightly more silicic than earlier ones (Fig. 1). -- (Web note: not available) -- The combined content of plagioclase, orthopyroxene, hornblende, Fe-Ti oxides, and minor clinopyroxene is 40-45 vol% and may have increased very slightly with time (Cashman and Taggart 1983; KV Cashman, personal communication, 1988). The relatively small changes in SiO2 and crystallinity imply that the effective viscosity (liquid plus crystals) and yield strength of magma entering the dome probably remained almost constant during growth of the dome, although possible gas loss with time could have increased the effective viscosity slightly. We calculated an effective viscosity of 10^10-11 poise for several lobes on the basis of flow rates down the flanks of the dome (Chadwick et.al., 1988).
The growth of the dome was complex (Swanson et.al., 1987), and it would be difficult to reconstruct the detailed history of the dome from its present characteristics. Taking account of this complexity will ultimately lead to much greater understanding of the processes involved in its formation.
In this chapter, however, we disregard short-term complexities and search for regularities in the growth process. To do this, we must in general consider longer time periods than those involved in one or two growth events, and we must be content with averaging out observations that may be important to the detailed history of growth. Ultimately we seek commonalities that bear not only on the growth of the Mount St. Helens dome but perhaps on the growth of other dacite domes.
We recognized several regularities in the growth pattern that are easily overlooked if only the details are considered. We outline in this chapter some of these regularities in order to enable other workers to formulate realistic mechanistic and rheologic models of the observed behavior. However, we do not want to leave readers with an oversimplified view of how the dome grew, and so we point out that the narrative in Swanson et.al., (1987) stresses some of the realistic complexities of dome growth that may dampen exaggerated enthusiasm engendered by consideration of the regularities alone.
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