Volume 17, numéro 3, september 1990

EN :

The May 18, 1980, eruption of Mount St.Helens comprised a complex series ofevents that started with a magnitude 5.1 earthquake. Studies of eye witness photographs and observations provided important information in determining the chronology of events. The eruption has been subdivided into six phases, based on evidence provided by this chronology and detailed stratigraphiework. The first phase, which included thelateral blast, has come under the closest scrutiny, but even here observations of the eye witnesses have provided useful information that must be accounted for in any model of the phenomena.

EN :

The 40,000-year eruptive history of Mount St.Helens reveals an overall compositional trend from rhyodacite to andésite, with basalt at -1.9and -1.6 ka. A cyclic eruption pattern is super-imposed on this trend. Cycles comprised arepose interval, when compositional and thermal gradients developed in the underlying magma body, followed by an eruption intervalin which progressive tapping of magma be-headed these gradients. Recovery of gradients varied with duration of the ensuing repose period. Eruption sequences follow the pattern: (1) eruptive progression from Pli-nian eruptions to dome growth accompanied by pyroclastic flows and tephra, followed (insome cases) by lava flows punctuated bypyroclastic outbursts; (2) a minéralogie pro-gression from hydrous Fe-Mg phenocrysts(hb, cm, bi) toward pyroxenes; (3) a magmatlc compositional progression from rhyodaciteor dacite to andésite. Progressions 1 and 2 stem mainly from volatile gradients in the magma reservoir whereas progression 3 (andto some extent 2) reflects gradients of melt composition and crystal content. Three eruption cycles within the last 4,000 years follow this pattern. Earlier cycles are probable but only dimly perceived, mainly from the partial record of tephras and pyroclastic flows.

EN :

in the crater walls and deep canyons on the north flank of Mount St. Helens, dacitic volcaniclastic rocks and domes of Pine Creekage (2.5-3.0 ka and possibly older) are per-vasively deformed and contain deposits ofpossibly two debris avalanches. The base ofthe younger avalanche deposit contains numerous logs, one of which yielded an age of 2590±120 14C years B.R The Pine Creeksection is capped by slightly faulted and ésite and basalt of Castle Creek age (1.7-2.2 ka). In the northeast and northwest walls of the crater, dacite domes of Pine Creek age and older are pervasively fractured. North-dipping normal faults and low-angle thrusts cut the domes. We postulate that forceful intrusion caused the deformation and slope failure in late Pine Creek time, in a manner similar to emplacement of the bulging crypto-dome in 1980.

EN :

Mount St. Helens has been seismically monitored since the summer of 1972. Seismic activity recorded during the 1970s by a single station located on the west flank of the volcano was limited to small swarms of high-frequency earthquakes and low-frequency transient signals usually attributed to glacier motion. The first seismic activity that was recognized as being unusual and with possible volcanic significance was a magnitude 4.2 earthquake on the afternoon of March 20, 1980. In retrospect, the initial unusual seismic activity began several days earlier as small, low-frequency earthquakes with seismogram character similar to, but not exactly the same as, the glacier events. Following the March 20 event, earthquake activity rapidly increased over the next five days untilon March 26 as many as eight magnitude 4+ earthquakes per hour were occurring. Onthe next day, phreatic eruptions began. From then until the climactic eruption on May 18, seismic energy release rates remained fairly constant though the number of earthquakes per unit time decreased. During this period, intermittent phreatic eruptions took place as well as continuous deformation of the northflank. The earthquakes were located in a limited volume directly under the volcano at shallow depths and now are interpreted to have been caused by the fracturing and faulting of shallow volcanic rocks as magma was injected into the base of the volcano. The major eruption on May 16 was triggered byone of these earthquakes, which caused aslope failure of the north flank. As this eruption progressed, shallow earthquake activity declined and deeper activity began. These deeper earthquakes outline the magma conduit system indicating the presence of a small crustal magma reservoir at a depth of 7to 12 km. Shallow seismicity preceding subsequent eruptions provided data to help with the prediction of most of these eruptions. Deeper seismicity has not been as obviously related to individual eruptions. We speculate that it reflects adjustments in the magma chamber and conduit system, due, in different cases, to a reduction or increase in magma pressure.

EN :

The massive rockslide-debris avalanche ofthe May 18, 1980, eruption of Mount St.Helens began with a retrogressive failure triggered by the 08:32 PDT earthquake. It depressurized the volcano's magmatic and hydrothermal system and produced a hummocky deposit with a volume of 2.5 km3. Detailed work provides a comprehensive understanding of a previously poorly understood type of event.

The deposit consists of relatively intactpieces (block faciès) of the pre-1980 mountain and mixed material (mixed facies) that is primarily rocks from the pre-1980 Mount St.Helens and the 1980 cryptodome. Travelpaths of rockslide blocks are interpreted from a geologic map of the deposit. The material was fractured and dilated during the rockslide, after which grain-to-grain dispersive stress facilitated flow. During transport, the dilated material mixed but significant finematerial was not produced.

EN :

The growth of the dacite dome at Mount St.Helens between 1980 and 1986 has been more intensively studied than that of any other dome-building eruption. The growth has been complex in detail, but remarkably regular overall. This paper summarizes some of what has been learned and provides many references to additional information. Whether dome building has ended is an open question, particularly in view of the renewed, though minor, explosive activity of late 1989and early 1990.

EN :

Modern monitoring of seismic activity atCascade Range volcanoes began at Longmire on Mount Rainier in 1958. Since then, there has been an expansion of the regional seismic networks in Washington, northern Oregon and northern California. Now, the Cascade Range from Lassen Peak to Mount Shasta in the south and Newberry Volcano to Mount Baker in the north is being monitored for earthquakes as small as magnitude 2.0, and many of the strato volcanoes are monitored for non-earthquake seismic activity. This monitoring has yielded three major observations. First, tectonic earthquakes are concentrated in two segments of the Cascade Range between Mount Rainier and Mount Hood and between Mount Shasta and Lassen Peak, where as little seismicity occurs between Mount Hood and MountShasta. Second, the volcanic activity and associated phenomena at Mount St. Helens have produced intense and widely varied seismicity. And third, at the northern strato-volcanoes, signals generated by surficral events such as debris flows, icequakes, steam emissions, rockfalls and icefalls are seismically recorded. Such records have been used to alert authorities of dangerous events in progress.

EN :

The cataclysmic eruption of Mount St.Helens on May 18,1980, made an enormous impact on the science of volcanology. The eruption was in daylight in clear weather, which provided an unprecedented opportunity to investigate relations among observations, products, and effects of a large explosive eruption. The May 18 events and subsequent activity stimulated perhaps the most intensive studies ever made at an active composite volcano, leading to greatly enhanced insights into both geologic and hydrologie processes operative in explosive volcanism. The eruption also disrupted much of the social and economic fabric of the Pacific Northwest. Volcanologists were called upon to explain the activity, in lay-man's terms, to government and corporate officials, the news media, schools, and the public at large. People eventually learned to live with the volcano and its uncertainties, and volcanologists better learned their role in helping society deal with a major natural disaster. Difficulties encountered at volcanicc rises elsewhere in the world in the 1980s demonstrate that these are hard lessons. In future years, a paramount challenge for scientists will be to help society apply what has been learned at Mount St. Helens to crises both nearby and far away.

EN :

The Mount Meager Complex [recent publications have misnamed the complex Meager Mountain; Mount Meager is the accepted name of the mountain] of Pliocene to Recentage contains volcanic rocks ranging in composition from basalt to rhyolite. Products of three periods of volcanism dominate the complex; early and late periods of rhyodacite bound a middle episode of and ésite. In the early episode rhyodacite tephra and flows, from <1.9±0.2 Ma to > 1.0±0.1 Ma, covered remnants of a basal breccia and overlying dacite flows on the southwestern edge of the complex. Products from the middle episode of and ésite volcanism, from 1.0±0.1 Ma to0.5±0.1 Ma, underlie the southern and central parts of the complex; The Devastator was their principal source. The late episode of rhyodacite volcanism, from 0.1 ±0.02 Ma to2340±50 years B.P., produced rhyodacite flows, tephra and lava domes from vents in the northeastern part of the complex. Thevent at the 1650 m (5400 foot) level on the northeast flank of Plinth Peak is the source of the Bridge River tephra. Meager and Pebble Creek hot springs issue from the Mesozoic basement near vents.

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At least twelve Pleistocene-Holocene calc-alkaline eruptive complexes were formed in the Mount Garibaldi and Garibaldi Lake volcanic fields during the intervals 1.1-1.3 Ma, 0.4-0.7 Ma, 0.2-0.3 Ma, and 0.10 Ma to present. Mildly alkalic basalts, which resemble extensional magma types, were erupted only during the last 100,000 years.

Evolution of basaltic and ésite and an-désite magmas can be explained by polybaric crystal fractionation of more mafic parental magmas. The dacitic and rhyodacitic magmas probably originated by continued fractionation of andesitic liquids accompanied by extensive assimilation of heterogeneous crustal contaminants. Erupted basalts and calc-alkaline rocks, however, cannot be related by fractionation processes.

EN :

On May 25, 1980,there sort town of Mammoth Lakes. California, was shaken by a remarkable 48-hour-long earthquake sequence that included four M=6, two M=5 and 300 M=3 quakes. The nature of the precursory seismicity plus the unusual character of the May 25-27 sequence itself suggested that it was not typical of tectonic earthquakes in the region. Discovery of 25 cm of domical uplift centred on the resurgent dome within LongValley caldera strongly implied that this activity was accompanied, if not caused, by influx of magma into the Long Valley magma chamber.

EN :

About 110 well-dated and 70 poorly date deruptive periods less than 15,000 years old at individual volcanoes in the Cascade arc constitute a data set for identifying spatial and temporal patterns of eruptive activity. Key features of the record include: (1) the mean frequency of eruptive periods during the past 4,000 years is approximately two per century, however, the variance about the mean may belarge; (2) at most major centres, episodes of activity lasting several thousand years are defined by groups of eruptive periods separated by apparent dormant intervals of roughly similar duration, (3) arc-wide clustering of eruptive activity may exist at 0-4 ka, 6-8 ka, and 10-14 ka. Such clustering would be remarkable in light of significant along-arc changes in crustal structure, stress field, and subduct Ion-zone geometry.

EN :

The Cascade Range stretches from south-western British Columbia to northern Califor-nia; the Range consists of major composite volcanic centres, most of which have been active during late Pleistocene and Holocenetime. In addition, thousands of smaller basaltic or basaltic-andesite volcanoes have been active during the past few million years. Flow-age and tephra hazards associated with future eruptions of composite volcanoes in theRange will endanger communities located within about 50 km of erupting volcanoes.Significant effects will extend to still greater distances down wind from the volcanoes and along stream valleys that head at the volcanoes. Volcanic-hazard assessments and hazard-zonation maps developed for volcanoes in the Range can be used by authorities for long-range land-use planning and provide information to help mitigate the effects of future eruptions.