DESCRIPTION:
Mount St. Helens Lava Dome

MSH81_new_lobe_spreading_center_from_west_06-26-81.jpg
Betweem 1980 and 1986, Mount St. Helens' dome grew in different ways. From 1980 through 1982 the dome grew in periodic extrusions of stubby lava flows, called lobes. This pattern changed in February 1983, when growth became continuous and mostly endogenous (internal). Perioidic lobe growth, along with endogenous growth, resumed in early 1984. Pictured here is an aerial view of Mount St. Helens' dome and the June 1981 lobe with its "spreading center". A spreading center is the area from which new lava slowly emerges during lobe growth. During this time frame Mount St. Helens' lobes grew at a rate of 3 to 10 feet per hour (1-3 meters/hour)
USGS Photograph taken on June 26, 1981, by Dan Dzurisin.
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Mount St. Helens Lava Dome
Elevation of top of dome: 7,155 feet
Height: 876 feet above 1980 crater floor
Diameter: About 3,500 feet
Volume: 97 million cubic yards

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.

At the start of 1990, the composite dome was about 3,480 feet by 2,820 feet in diameter and rose about 1,150 feet above the low point on the adjacent crater floor. It has a volume of about 97 million cubic yards, less than 3 percent of the volume of the volcano (about 3.5 billion cubic yards) removed during the landslide and lateral blast on May 18, 1980. If the dome resumes growth at its average rate of the 1980s (about 17 million cubic yards per year), it would take nearly a century to fill in the summit crater and more than 200 years to rebuild Mount St. Helens to its pre-1980 size.

Following the 1980 explosive eruption (of Mount St. Helens), more than a dozen extrusions of thick, pasty lava built a mound-shaped lava dome in the new crater. The dome is about 1,100 meters in diameter and 250 meters tall.

Mount St. Helens remains a potentially active and dangerous volcano, even though it is now (1995) quiescent. In the last 515 years, it is known to have produced 4 major explosive eruptions (each with at least 1 cubic kilometer of eruption deposits) and dozens of lesser eruptions. Two of the major eruptions were separated by only 2 years. One of those, in 1480 A.D., was about 5 times larger than the May 18, 1980 eruption, and even larger eruptions are known to have occurred during Mount St. Helens' brief but very active 50,000-year lifetime. Following the most recent major eruption, on May 18, 1980, there were 5 smaller explosive eruptions over a period of 5 months. Thereafter, a series of 16 dome-building eruptions through October 1986 constructed the new, 270-meter- (880- feet) high, lava dome in the crater formed by the May 18, 1980 eruption.

MSH84_st_helens_crater_dome_from_NNW_09-13-84.jpg
Since December 1980, eruptions of Mount St. Helens have added material to a dacitic lava dome with the crater, as seen here in this 1984 view from the north-northwest.
USGS Photograph taken on September 13, 1984, by Lyn Topinka.
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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 1.2-4.5x10^6 cubic meters, 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 74.1x10^6 cubic meters, and the volume of all erupted material (including tephra and debris removed from the dome by explosions and rockfalls) was about 77.1x10^6 cubic meters.

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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