Rock fracture as a precursor to lava dome eruptions at Mount St Helens from June 1980 to October 1986

Following its plinian eruption on 18 May 1980, Mount St Helens (Washington State, USA) entered a period of intermittent lava-dome extrusion until 1986. Renewed extrusion was frequently preceded by accelerating rates of seismicity, with more precursory seismicity observed prior to eruptions later in the sequence. Here the failure forecasting method (FFM) is used to investigate changes in the observed rate of volcano–tectonic (VT) seismicity. The analysis indicates that: (1) all VT crises resulted in an eruption within 3 weeks (usually less than 10 days), (2) the majority of eruptions had VT precursors, and (3) patterns of precursory seismicity showed fluctuations about the ideal model trend. Thus, although these seismic events could be used to warn of an impending eruption, specific forecasts were subject to an uncertainty of weeks or more. It is proposed that: (1) increased seismicity prior to later eruptions is a result of a larger and more solidified dome acting as a greater impediment to magma ascent; (2) the consistency of seismic swarms resulting in an eruption indicates that stresses high enough to initiate fracturing in the country rock and lava dome carapace were only achieved once the approach to an eruption had already begun; and (3) discrepancies between models of accelerating rock fracture and the observed seismicity may arise due to a significant amount of the rocks deforming through ductile mechanisms rather than seismogenic fracture.     

ALEXANDER THE GREAT'S MOUNTAIN*

ABSTRACT. The metaphorical power of natural landscapes and geographical objects has attracted an increasing amount of critical interest. The myth of Dinocrates planning to carve Mount Athos into the figure of Alexander the Great epitomizes the complex relationship between the insistent materiality of prominent geographical features such as mountains and imagination. Through an iconographic reading of different renderings of the Dinocratic myth in western Europe, this article explores the way Mount Athos turned into a powerful emblem circulating across space and time. While considering the continuity of a Classical vision projected on rock, the article focuses on its constant reappropriations and transformations in different historical and geographical contexts. Dinocratic Athos becomes a metaphor of egotism, power, and desire but also a free‐floating referent, reflecting a shifting relationship between the microcosm of the human body and the natural macrocosm.     

A Rhizocarpon geographicum growth curve for the Cascade Range of Washington and northern Oregon, usa

Lichen thallus measurements from 22 surfaces of known age on Mount Baker, Mount Hood, and Mount Rainier are used to construct a regional Rhizocarpon geographicum growth curve for the Cascade Range of Washington and northern Oregon. Growth rates determined by measuring the largest thallus diameters on the same surfaces at Mount Rainier in 1976 and 2002 are used for comparison with lichenometric data from Mount Baker and Mount Hood. Similar lichen thallus diameter vs age relationships identified in the data from the three mountains suggest the presence of uniform growth rates over the 400-km range. A regional growth curve developed during our study shows three growth phases of successively slower growth: a rapid phase from 8 to 20 yr, a linear phase from 20 to 145 yr, and a slow phase of unknown duration beyond ca. 145 yr. Uncertainty in lichen growth rates beyond 145 yr limits projection of the curve beyond that age; however, the age range of the constrained growth curve covers an important period of recent climate variability. When applied in appropriate settings, our growth curve can be used to determine numeric ages to ±10 yr for surfaces between 20 and 145 years old in areas where other techniques are not applicable or do not provide unique or well-constrained ages.     

Tephra-fall deposits from the 1992 eruption of Crater Peak, Alaska: implications of clast textures for eruptive processes

 The 1992 eruption of Crater Peak, Mount Spurr, Alaska, involved three subplinian tephra-producing events of similar volume and duration. The tephra consists of two dense juvenile clast types that are identified by color, one tan and one gray, of similar chemistry, mineral assemblage, and glass composition. In two of the eruptive events, the clast types are strongly stratified with tan clasts dominating the basal two thirds of the deposits and gray clasts the upper one third. Tan clasts have average densities between 1.5 and 1.7 g/cc and vesicularities (phenocryst free) of approximately 42%. Gray clasts have average densities between 2.1 and 2.3 g/cc, and vesicularities of approximately 20%; both contain abundant microlites. Average maximum plagioclase microlite lengths (13–15 μm) in gray clasts in the upper layer are similar regardless of eruptive event (and therefore the repose time between them) and are larger than average maximum plagioclase microlite lengths (9–11 μm) in the tan clasts in the lower layer. This suggests that microlite growth is a response to eruptive processes and not to magma reservoir heterogeneity or dynamics. Furthermore, we suggest that the low vesicularities of the clasts are due to syneruptive magmatic degassing resulting in microlitic growth prior to fragmentation and not to quenching of clasts by external groundwater. Received: 5 September 1997 / Accepted: 1 February 1998     

Paleo-environmental and volcano-tectonic evolution of the southeastern flank of Mt. Etna during the last 225 ka inferred from the volcanic succession of the ‘Timpe’, Acireale, Sicily

The tectonic escarpments locally known as ‘Timpe’ cut a large sector of the eastern flank of Etna, and allow an ancient volcanic succession dating back to 225 ka to be exposed. Geological and volcanological investigations carried out on this succession have allowed us to recognize relevant angular unconformities and volcanic features which are the remnants of eruptive fissures, as well as important changes in the nature, composition and magmatic affinity of the exposed volcanics. In particular, the recognition in the lower part of the succession of important and unequivocal evidence of ancient eruptive fissures led us to propose a local origin for these volcanics and to revise previous interpretations which attributed their westward-dipping to the progressive tectonic tilting of strata. These elements led us to reinterpret the main features of the volcanic activity occurring since 250 ka BP and their relationship with tectonic structures active in the eastern flank of Etna. We propose a complex paleo-environmental and volcano-tectonic evolution of the southeastern flank of Mt. Etna, in which the Timpe fault system played the role of the crustal structure that allowed the rise and eruption of magmas in the above considered time span.     

On some future tsunamis in the Pacific Ocean

Possible tsunamis in the Pacific Ocean, especially in its northeastern part, are discussed in relation to a predicted major earthquake in the Shumagin Seismic Gap (located in the eastern part of the Aleutian Island Chain) and to a major eruption of the St. Augustine volcano in Cook Inlet, Alaska. The deep-water propagation of the tsunami generated in the Shumagin Gap is simulated through the use of a spherical polar coordinate grid of the approximate size of 14km. The tsunami generated by the St. Augustine volcano is studied through the fine mesh grid confined to the Cook Inlet only. The numerical models were calibrated against historical tsunami data. The properties of the tsunami signal are described by the maximum amplitude which occurs in the tsunami record. This allows us to single out the direction along which a maximum tsunami is to be expected.Presented at the International Conference on Natural and Man-Made Hazards in Coastal Zones, held in Ensenada, Mexico, August 1988.     

Eruptive activity at Mount St Helens,Washington, USA, 1984–1988: a gas geochemistry perspective

The results from two different types of gas measurement, telemetered in situ monitoring of reducing gases on the dome and airborne measurements of sulfur dioxide emission rates in the plume by correlation spectrometry, suggest that the combination of these two methods is particularly effective in detecting periods of enhanced degassing that intermittently punctuate the normal background leakage of gaseous effluent from Mount St Helens to the atmosphere. Gas events were recorded before lava extrusion for each of the four dome-building episodes at Mount St Helens since mid-1984. For two of the episodes, precursory reducing gas peaks were detected, whereas during three of the episodes, COSPEC measurements recorded precursory degassing of sulfur dioxide. During one episode (October 1986), both reducing gas monitoring and SO₂ emission rate measurements simultaneously detected a large gas release several hours before lava extrusion. Had both types of gas measurements been operational during each of the dome-building episodes, it is thought that both would have recorded precursory signals for all four episodes. Evidence from the data presented herein suggests that increased degassing at Mount St Helens becomes detectable when fresh upward-moving magma is between 2 km and a few hundred meters below the base of the dome and between about 60 and 12 hours before the surface extrusion of lava.     

Decadal‐scale change of infiltration characteristics of a tephra‐mantled hillslope at Mount St Helens,Washington

The cataclysmic 1980 eruption of Mount St Helens radically reduced the infiltration characteristics of ∼60 000 ha of rugged terrain and dramatically altered landscape hydrology. Two decades of erosional, biogenic, cryogenic, and anthropogenic activity have modified the infiltration characteristics of much of that devastated landscape and modulated the hydrological impact of the eruption. We assessed infiltration and runoff characteristics of a segment of hillslope thickly mantled with tephra, but now revegetated primarily with grasses and other plants, to evaluate hydrological modifications due to erosion and natural turbation. Eruptive disturbance reduced infiltration capacity of the hillslope by as much as 50‐fold. Between 1980 and 2000, apparent infiltration capacities of plots on the hillslope increased as much as ten fold, but remain approximately three to five times less than the probable pre‐eruption capacities. Common regional rainfall intensities and snowmelt rates presently produce little surface runoff; however, high‐magnitude, low‐frequency storms and unusually rapid snowmelt can still induce broad infiltration‐excess overland flow. After 20 years, erosion and natural mechanical turbation have modulated, but not effaced, the hydrological perturbation caused by the cataclysmic eruption. Copyright © 2005 John Wiley & Sons, Ltd.     

Palynology of Archean microfossils (c. 3.0 Ga) from the Mount Grant area, Pilbara Craton, Western Australia: Further evidence of biogenicity

Palynological techniques have only rarely been applied to Archean rocks, mainly because of concerns about contamination. However, where microstructures are abundant and well documented, palynology can add considerably to interpretations based on thin-section examination. A modified palynological preparation method, which avoids vigorous physical or chemical methods that might cause the fragmentation of fragile specimens, was applied to black chert from the c. 3.0 Ga Farrel Quartzite of the Mount Grant area, Pilbara Craton, Western Australia, known to contain abundant microstructures of putative biogenic origin.Four main morphological types have been reported from chert thin sections: threads, films, hollow spheres and spindles. These microstructures exhibit morphological and chemical signatures consistent with a biological origin as well as showing features indicative of taphonomic degradation. Nevertheless, the possibility remains that they are artefacts. Similar structures have been attributed to physical processes, such as the accretion of fine particulate matter that has subsequently been redistributed as a result of crystal growth.By extracting structures palynologically, we have increased the probability of a biogenic origin. Firstly, examination of the morphology free of the encompassing matrix demonstrates that they are indeed three-dimensional structures with complex morphology; secondly, the extraction of entire specimens militates against them being particulate matter aggregations, which would simply disintegrate during preparation and thirdly, the chemicals used remove silicates and most other minerals, confirming the organic composition of the structures. To date, we have extracted films, hollow spheres and a few poorly preserved spindles, but preparation continues. The fact that the specimens retain their morphological integrity during palynological extraction argues strongly in favour of a biogenic origin for the microstructures.     

Typology of Natural Hazards and Assessment of Associated Risks in the Mount Bambouto Caldera (Cameroon Line,West Cameroon)

Abstract: Mount Bambouto is a polygenic stratovolcano of the Cameroon Volcanic Line, built between 21?Ma and 4.5?Ma. It is situated approximately 200?km NE of Mount Cameroon, between 09°55′ and 10°15′ longitude east and, 05°25′ and 05°50′ latitude north. The volcano covers an area of 500?km2 and culminates at 2740?m at Mélétan dome and bears a collapsed caldera at the summit (13?×?8?km). Mount Bambouto is characterized by several natural hazards of different origins: meteorological, such as landslides and rock falls; anthropogenic, such as bushfires, tribal wars and deforestation; and volcanological, such as volcanic eruption. The thematic map shows that 55–60% of the caldera has high probability of occurrence of mass movement. The caldera has a high population density (3000 inhabitants), which increases the level of risk, evaluated at approximately $US3.8 million for patrimony, 3000 civilian deaths and destruction of biodiversity.