DESCRIPTION:
How a Lava Dome Grows

[Schematic,14K,GIF]
Aerial View: Schematic drawings of lobe development in the growth of a lava dome, as traced off aerial photographs of Mount St. Helens Lava Dome, 1980-1981.
-- Modified from: Swanson, et.al., 1987

[Schematic,12K,GIF]
Emerging New Lobe on Mount St. Helens' Lava Dome, August 1982, taken from tracings of photographs from photo station on crater floor, showing 1-day and 2-week intervals.
-- Modified from: Topinka, 1992, IN: USGS Bulletin 1966

[Schematic,22K,GIF]
Ground Profile: Schematic drawing of the growth of Mount St. Helens Lava Dome, 1980-1983, as traced from photographs taken from a fixed photo station approximately one kilometer north of the center of the dome.
-- Modified from: Swanson, et.al., 1987

The dome at Mount St. Helens is termed a composite dome by scientists, because it represents the net result of many eruptive events, not just one event. The dome-building process may be pictured as the periodic squeezing of an upward-pointing tube of toothpaste or caulking compound. The process is dynamic, involving the upward movement of new material, cracking and pushing aside of old material, sloughing of material from steep surfaces of the dome, and occasional, small but violent explosions that blast out pieces of the dome.

A dacite dome began to form in the crater of Mount St. Helens on October 18, 1980, 5 months after the catastrophic events of May 18. ... The dome grew in a complex series of extrusions preceded, accompanied, and at times supplanted by periods of endogenous growth. The extrusions produced short (200-400 meters), thick (20-40 meters) 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 (growing from within) 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; such deformation was useful in predicting the start of each extrusion. 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 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 70 meters 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 2-5 millimeters per day. ...

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