Explosive Volcanism Inception, Evolution, and Hazards (1984) / Chapter Skim
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BACKGROUND
Pages 13-181

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From page 13...
... . If such temporal and spatial constraints can be established for explosive alkaline volcanoes, such as kimberlites and alkali basalts, this will place restrictions on the origin of alkaline magmas and shed light on the mantle source regions.
From page 14...
... observed that these regions in which kimberlites occur are also those of epeirogenic crustal doming. Kimberlites are not the only products of igneous activity associated with epeirogeny, but in many cratonic settings they are the only intracratonic igneous rocks indubitably associated with this process.
From page 15...
... , for example, meet these critejria but are apparently barren and are low in total alkalis. Alkali basalts commonly occur along offsets or transform faults; this contrasts with the preponderance of tholeiites along major rifts.
From page 16...
... . They show no alkali basalts older than ~250 m.y., and their production rate of tholeiites has progressively declined by about an order of magnitude since 3.5 b.y.
From page 17...
... . Metasomatism of Magma Source Regions Kimberlites and alkali basalts are relatively rich in K, Na, Ti, volatile components, and large-ion-lithophile (LIL)
From page 18...
... The wide range of 8D, S"* O, and H2O values for the hydrous minerals in alkali basalts may reflect complex interaction with aqueous fluids diluted with O, F, Cl, and components containing C and N in the source regions (<~70 km)
From page 19...
... , as is commonly accepted, they probably are richer in volatiles and LIL elements than the bulk composition of the upper mantle. We must also remember that alkali basalts and kimberlites sample only a restricted range of mantle environments (Figures 1.1 and 1.2)
From page 20...
... . Stable isotope, chemical and petrographic studies of high-pressure amphiboles and micas: Evidence for metasomatism in the mantle source regions of alkali basalts and kimberlites, Am.]
From page 21...
... . The Cima volcanic field, in Tectonic Framework of the Mojave and Sonoran Deserts, California and Arizona, South Coast, Geol.
From page 22...
... . Petrology and geochemistry of alkali basalts and ultramafic inclusions from the Palei-Aike volcanic field in southern Chile and the origin of the Patagonian Plateau lavas, /.
From page 23...
... Cenozoic volcanism in the northwestern United States shows a wide variety of eruptive styles, from the relatively quiescent fissure eruptions of the Columbia Plateau basalts to the highly explosive calc-alkaline Cascade volcanics and rhyolitic ash flows of the Oregon Plateau. The link between magma composition, eruptive style, and tectonic character of the eruption area of these lavas is believed to provide important information regarding the process of magma generation and eruption.
From page 24...
... ; farther to the east, Snake River Plain basalts have much higher "Sr/^Sr (>0.7060) (Leeman and Manton, 1971)
From page 25...
... isotopic composition of igneous rocks from the northwestern United States. The question remains, however, as to whether the isotopic variability in these rocks is best explained by mixing between primary melts and materials of the continental crust or by systematically variable isotopic composition in the mantle source of these lavas.
From page 26...
... To evaluate the two possibilities of crustal contamination or heterogeneous mantle sources to explain the isotopic variability of Cenozoic volcanics in the northwestern United States, consideration of the chemical composition of these lavas in league with the isotopic data is vital. The following discussion considers this information for the Columbia Plateau and OregonModoc Plateau basalts.
From page 27...
... . This difference in isotopic composition between the northeast Oregon Saddle Mountain flows and the majority of Saddle Mountain members erupted farther north might be indicative of the general correlation between isotopic composition and geographic position of igneous rocks in the northwestern United States.
From page 28...
... On the other hand, if instead of using the chemical and isotopic compositions of a primitive Imnaha or Picture Gorge basalt as the primary magma for all the Columbia Plateau lavas, the high "Sr/^Sr Saddle Mountain basalts are assumed to be derived from a parental magma similar in composition to the Pomona chemical unit, then the composition of other Saddle Mountain basalts can be explained by using a crystallizationcrustal assimilation model similar to that described earlier. Such a model of mixing between sialic crustal materials and a primary magma with "Sr/^Sr = 0.7075 and 143Nd/144Nd = 0.51245 can in fact account for much of the major- and traceelement and isotopic variations seen for the remainder of the
From page 29...
... Texturally, the majority of these low-K, high-Al olivine tholeiites are diktytaxitic equigranular and lack abundant phenocrysts -- in support of their chemically unfractionated nature. To explain the variable isotopic composition of the high-Al basalts it is necessary to consider the possibility of long-standing chemical heterogeneities in their mantle sources or perhaps mixing between a "depleted" or low 87Sr/86Sr mantle region and the isotopically evolved mantle source suggested for Snake River Plain basalts (Leeman and Manton, 1971)
From page 30...
... If chemical heterogeneities in the mantle result from the migration of partial melts, examination of the possibility of "enriching" an area in the mantle by adding to it melt derived from some other region in the mantle is necessary. In such a model a fluid originating by the partial melting of a deeper portion of the mantle ascends and either solidifies within or equilibrates with overlying mantle (e.g., Lloyd and Bailey, 1975)
From page 31...
... Presumably in both the Columbia Plateau and the Oregon-Modoc Plateau regions, the intrusion of primary basic magmas into the crust led to the production of rhyolitic melts through anatectic melting of the lower crust. After this point, however, the evolutionary paths followed by the Columbia Plateau and Oregon-Modoc Plateau lavas must have diverged.
From page 32...
... . Radiogenic tracers applied to basalt genesis in the Snake River Plain-Yellowstone National Park region -- Evidence for 2.7 b.y.
From page 33...
... . The regional approach to studying the Columbia River basalt group, in International Symposium on Deccan Volcanism and Related Basalt Provinces in Other Parts of the World, pp.
From page 34...
... In particular, the anelasticity, a measure of partial melt, is rather uniform down to depths of approximately 250 km and is similar beneath Japan, northern Chile, and normal oceans. However, beneath central Peru (and central Chile)
From page 35...
... If the asthenosphere system (above the subduction zone) were closed, i.e., if there was a cooling of the asthenosphere in the wedge between the subducting and continental lithospheres without any involvement of the adjacent upper mantle material, the asthenospheric wedge would eventually cool so much that magma genesis would be inhibited, the viscosity would increase, and the subduction process would be modified.
From page 36...
... The geometry and seismicity of subduction zones in such anomalous regions are discussed below. ANOMALOUS REGIONS Central Peru and southwest Honshu, Japan, were studied to determine the geometry in anomalous subduction zones.
From page 37...
... Assuming that these earthquakes are in the subducted slab, the implication is that there is no low Q material (asthenosphere) between the subducted slab and the overlying continental lithosphere in central Peru.
From page 38...
... Schematic model of the descending Nazca plate near the boundary between the nearly horizontal subduction zone beneath central Peru and the more steeply dipping subduction zone beneath southern Peru. In both regions the initial subduction angle is 30° and persists to at least 100 km.
From page 39...
... . To sustain this deformation there must be an upward force of considerable magnitude on the subducted plate in the central Peru (anomalous)
From page 40...
... Note that the water depth does not increase following a square-root-of-age law beyond 80 m.y. or so because at greater ages the heat flow is essentially the background steady-state mantle heat flow.
From page 41...
... Thicker lines and negative signs indicate higher velocities, which are assumed to show the subducted plates. The younger East Philippine Sea plate dips down to about 60 km depth then flattens to horizontal (upper figure)
From page 42...
... Heat Flow My estimate of temperatures within the Earth is derived from observations of heat flow at the surface. Some results for western South America (Henry, 1981)
From page 43...
... subduction, above the 100-km contour, the mean heat flow in central Peru is -- 30 mW/m2, whereas in southern Peru the mean heat flow is about 60 mW/m2. To convert these surface values to temperature at depth, I follow the calculations (and assumptions)
From page 44...
... Open circles are sites established by the University of Michigan; open rectangle in central Peru represents five sites with questionable results but are believed to have low heat flow. Solid triangles are sites established by other investigators, which have been corrected for topography and uplift and erosional history (Henry, 1981)
From page 45...
... This is manifested by low heat flow above the region where the slab deforms to a near-horizontal aspect. Only in a cool environment will the basalt-eclogite transformation be retarded, so that the subducted plate can remain buoyant.
From page 46...
... . Precursors to ScS phases and dipping interface in the upper mantle beneath southwestern Japan, Tectonophysics 69, 1-35.
From page 47...
... Helens has confirmed this view. Maps showing the distribution of Upper Cenozoic volcanoes and volcanic rocks in the western United States provide abundant evidence for the existence of many potentially active volcanic loci outside the Cascades region.
From page 48...
... Time-space-composition patterns as shown on these composite maps were then interpreted for their relevance to future volcanic activity. From the map patterns displayed by our three arbitrarily chosen time increments, it was evident that in any discussion of future volcanism from existing volcanic fields the significant data base is primarily within the 0- to 5-m.y.
From page 49...
... For some areas the terms volcanic system and volcanic locus are synonymous; however, for others, loci, as we use the term, contain multiple, perhaps independent, volcanic systems (e.g., Snake River Plain)
From page 50...
... Black pattern includes all volcanic rock compositions. FIGURE 4.4 Volcanic loci 5 to 10 m.y.
From page 51...
... West Snake River Plain 4. Pocatello 5.
From page 52...
... Plates 1, 2. and 3 show the late Cenozoic volcanic fields in their entirety.
From page 53...
... Perhaps the system will ultimately evolve to this stage, but for the present we think it best to regard the Rio Grande rift as a major extensional boundary in the upper crust between the more stable continental interior and the unstable western part of the continent. If the Colorado Plateau is ultimately consumed by Basin and Range-type faulting and volcanism, the Rio Grande rift will simply become the eastern margin of the Basin and Range province, a position it occupies today at least in its southern reaches.
From page 54...
... The Jemez volcanic locus, which includes the Valles caldera, evolved at this intersection and is by far the most complex locus of either the Jemez zone or the Rio Grande rift and is, in fact, the most complex volcanic field in the entire southwestern United States within the last 15 m.y. During the early evolution of the Jemez field, there was a north-northeastward migration of the silicic foci comprising three volcanic cycles, but the vent distributions for nearly all Jemez precaldera vents were controlled by north-south fractures (Smith and Bailey, 1968; Smith et al., 1970)
From page 55...
... We have provisionally termed the eastern margin of the Great Basin and the southwestern margin of the Colorado Plateau the Sevier rift. This complex rift system is terminated on the north by the Snake River Plain and on the south by the Mogollon Rim tectonic zone.
From page 56...
... However, in Idaho, where the rift merges with the Snake River Plain zone, 0- to 5-m.y.-old activity coincides with the major tectonic boundary; here the rift also appears to merge with the Rocky Mountain hinge zone, as suggested by the distribution of vents in the Blackfoot locus. In the northern part of the Blackfoot locus the distribution of vents seems to be controlled by stress of the Rocky Mountain hinge, whereas the central part of the locus seems to be dominated by Snake River Plain stress (as suggested by the alignment of the Blackfoot domes)
From page 57...
... Volcanism in this region is found both within the graben and outside it. In either place the dominant general arrangement of volcanic loci and vents within them is northsouth, and we interpret this to mean control by east-west extension, as in the Rio Grande and Sevier rifts.
From page 58...
... Vent alignments throughout the Yellowstone and the eastern Snake River Plain (Craters of the Moon and eastward) reflect the northwest trend of the Rocky Mountain hinge zone.
From page 59...
... Although most of these vents are related to multiple vent systems, even cursory examination of our maps suggests minimal loss by either burial or erosion during the last 5 m.y. Greatest obvious loss has been in the eastern Snake River Plain region, Idaho.
From page 60...
... active in the last 5 m.y. (the Snake River zone counted as 3)
From page 61...
... Maar-type eruptions, as have occurred in such loci as the Potrillo and Zuni-Bandera volcanic fields in New Mexico and the Bernardino field in Arizona, could be hazardous, but their probability is much lower than for more typical basaltic eruptions. Our maps record about 100 maars in the 0to 5-m.y.
From page 62...
... We think the waxing phase encompasses the present time. Given the motions described for the Mogollon Rim and Snake River zones, all of which appear real and have some documentary basis, we can look at the loci of other zones and, even in the absence of critical geochronological data, see loci, or groups of loci, that have vent distributions, suggesting northeastward or eastward migrations.
From page 63...
... In this region the mantle flow and migration of the magma-forming domains have acted on the upper crust to cause rifting and drifting of Basin and Range type. In the Cascade region the subducting Juan de Fuca Plate has probably acted to constrain the westward-migrating magma sources, possibly redirecting them northward at least as far as the region between Newberry Volcano and Mount Adams, where extensive recent basaltic volcanism appears anomalous.
From page 64...
... East-west extension in this region allows magma storage and the development of silicic nuclei, some of which, like the Mono-Long Valley complex, have high potential for future explosive eruptions. Available data for the area south of the Garlock fault do not indicate definitive volcanic migration motions, but the pattern is consistent with the general westward migration of the California zone, and very recent volcanism in this region suggests a continuation of the waxing phase evident in the 0- to 5-m.y.
From page 65...
... . Lead and strontium isotopes and related trace elements as genetic tracers in the Upper Cenozoic rhyolite-basalt association of the Yellowstone Plateau volcanic field, }.
From page 66...
... . The volcanic evolution of the Jemez Mountains and its relationship to the Rio Grande rift, in .2978 International Symposium on the Rio Grande Rift, K
From page 67...
... Heat transfer, coupled with studies of dynamics, has yielded several rules; for example, successful diapirism is multiple-diapirism, stoping magmas easily become congested with blocks, and dikes must travel about 10,000 times faster than diapirs. Generation of significant amounts of granitic magma from intrusion of basalt into continental crust is difficult, unless the volume of basalt is large or unless through repeated ascent of basalt it maintains its passageway at a high temperature.
From page 68...
... The movement of magma to the outer regions of the Earth is a form of penetrative convection and heat transfer that is important not only in forming both oceanic and continental crust but also in providing energy to remelt and allow reorganization of continental crust. The variety of igneous rocks found around volcanoes and plutons has led petrologists to concentrate on the chemical origins of these suites.
From page 69...
... Consider a layer of half-thickness L containing heat sources of strength e distributed as A~", where A and a are constants and x is the spatial coordinate normal to the layer. Placing the origin at the middle of the layer, the heating source will produce a symmetrical thermal disturbance in the absence of preexisting strong temperature gradients.
From page 70...
... The amount of heating depends on the magnitude of convective relative to conductive heat transfer, which is measured by Pe. For a unidirectional flow containing no heat sources (e = 0)
From page 71...
... The horizontal axis represents heat sources (positive portion) and heat sinks (negative portion)
From page 72...
... In alkali basalts the crystallinity may become slightly greater than 55 percent, and in rhyolite lavas it may be as low as 15 to 20 percent (Hildreth, 1981; Marsh, 1981)
From page 73...
... dikes in the field means of course that they are not far from their source and were injected quickly.] Some alkali basalts are thought, on chemical grounds, to represent small (<10 percent)
From page 74...
... At least in the upper crust, this places a useful constraint on the spatial relation of dikes to their magma chambers. To obtain an actual ascent time for each of the cooling curves in Figure 5.2, a value of Nu must be known.
From page 75...
... The block is larger because its residence time is longer, and this additional heat transfer must be offset by a reduction in the ratio of surface area to volume. An analytical treatment of this problem is given by Marsh (1982)
From page 76...
... Because stoped blocks present a large amount of surface area of the magma, the possibility of chemical contamination is great. Basaltic magma, having stoped its way through continental crust, should not only contain granitic xenoliths but also exhibit an unusual isotopic identity.
From page 77...
... Knowledge of this relation is also criticial to understanding the mechanical behavior of magma during both melt extraction and crystal fractionation. Sakuma's data would suggest a critical point where upon further crystallization the magma reaches a locking point and passes from liquid to solid.
From page 78...
... This effect maintains higher temperatures near the contact and may increase the melt zones to about 30 to 40 percent of the intrusive's width. This increase varies with depth, since the lower crust is much nearer to its solidus temperature than is the upper crust where the melt zones would be very thin.
From page 79...
... Because crustal temperature increases with depth, deep in the crust this heating will produce a zone of partial melting that will shrink in volume with its approach to the surface. That is, whereas at depth the partially melted zone may span the distance between conduits, with decreasing depth it will be very local only around each conduit.
From page 80...
... Later partial melting in the upper granitic crust produces a sluggish (small-density contrast) diapir that may not ascend far enough to allow melt extraction, yet this body may intrude the others, forming a zoned pluton.
From page 81...
... This span of conditions is much narrower than for basalt, and because of this we see that granitic magmas have a much higher probability of becoming plutons. Basaltic magmas, on the other hand, have a much higher probability of being erupted as lava.
From page 82...
... . Gradients in silicic magma chambers: Implications for lithospheric magmatism, }.
From page 83...
... . Coalescence of magma pockets into large pools in the upper mantle, Geol.
From page 84...
... CHRISTIANSEN U.S. Geological Survey ABSTRACT The Yellowstone Plateau volcanic field of Wyoming, Idaho, and Montana evolved through three cycles during the past 2 m.y.
From page 85...
... , or basalts, containing about 48 to 52 wt.% SiO2; intermediate compositions are absent except for a few small-volume lavas that represent locally mixed rhyolitic and basaltic magmas. Intensive study of the Quaternary volcanism, tectonism, and hydrothermal activity of the Yellowstone Plateau volcanic field reveals a major active system of energy transfer in the crust and upper mantle.
From page 86...
... Centennial fault HF, Hebgen fault TF. Teton fault 100 km FIGURE 6.1 Yellowstone Plateau, Island Park, and the eastern Snake River Plain, showing locations of late Cenozoic faults (heavy lines)
From page 87...
... First-cycle caldera I Second cycle caldera I, lima cycle caldera Fault Dotted where concealed Bar and ball on downthrown side Inner limn ol ring fracture zone FIGURE 6.2 Geologic map of the Yellowstone Plateau volcanic field (modified from Christiansen, 1979)
From page 88...
... B.P. For about 600,000 yr, rhyolitic lavas erupted intermittently on the Yellowstone Plateau from a slowly forming set of arcuate fractures that eventually outlined the area that would later collapse as the third-cycle Yellowstone caldera.
From page 89...
... In the last 630,000 yr, both segments of the Yellowstone caldera have filled with sediments and with rhyolitic lavas that have continued to erupt intermittently; much of the caldera basin is covered by rhyolitic flows erupted during the past 150,000 yr (Figures 6.2 and 6.3)
From page 90...
... . Large rhyolitic systems more or less comparable to the Yellowstone Plateau volcanic field have evolved, solidified, and eventually cooled in areas situated successively farther northeastward over time along the axis of the eastern Snake River Plain.
From page 91...
... COMPARISON WITH OTHER VOLUMINOUS PYROCLASTIC SYSTEMS Although the emphasis of this paper is on Yellowstone as an example of a magmatic system that has evolved to produce superexplosive eruptions, it is important to compare it, at least briefly, with a few other systems to delineate elements that are common in their various courses of development. Such systems, though not well represented in the written historical record, are geologically common in several regions of the Earth, such as Japan, New Zealand, the Central Andes, Mexico, and parts of the western United States.
From page 92...
... , whose evolution climaxed with that voluminous pyroclastic eruption. A very different tectonomagmatic setting is evident in the evolution of the San Juan Mountains volcanic field of southwestern Colorado, although its evolutionary pattern reveals important similarities to Yellowstone and the other two systems just noted.
From page 93...
... A regional zone of seismicity extends eastward north of the eastern Snake River Plain to the north rim of the Yellowstone caldera (Figure 6.1)
From page 94...
... . The Yellowstone-Snake River Plain seismic profiling experiment: Crustal structure of the eastern Snake River Plain, /.
From page 95...
... . The 1978 Yellowstone-Eastern Snake River Plain seismic profiling experiment: Crustal structure of the Yellowstone region and experiment design, /.
From page 96...
... Taken together with the geologic data this crustal model is interpreted to reflect the structure and properties of a thermally deforming Archean crust and the initial stages of the bimodal rhyolitic/basaltic volcanism of the Yellowstone-Snake River Plain volcano-tectonic system. While the interpretations are not unique, the youthfulness and volume of Quaternary volcanism, the high heat flow, the high rates of contemporary uplift, and the upper-crustal low-velocity layers infer the presence of hot crustal material and possible partial melts that underlie the Yellowstone Plateau.
From page 97...
... . While the young volcanism and high heat flow reflect a major thermal episode, they are but the primary phase of a temporal-spatial propagating silicic system -- the Yellowstone-Snake River Plain (Y-SRP)
From page 98...
... These data and inferences made from the geologic record suggest that crustal sources of heat are driving the Yellowstone hydrothermal systems and that they provide much of the energy for the contemporary tectonic deformation. In 1978 a major seismic refraction/reflection experiment was conducted in the eastern Snake River Plain-Yellowstone region as a means of evaluating the lithospheric structure of this continental tectono-volcanic province (Braile et al., 1982; Smith et al., 1982)
From page 99...
... The seismicity south of the east-west-trending Hebgen Lake fault zone extends eastward toward the Yellowstone caldera, then turns southeastward along zones that continue 2 to 4 km into the caldera.
From page 100...
... Gravity Field and Teleseismic P-Wave Delays Excess heat and advective magma transport from deep lithospheric sources can generate zones of low density and lowseismic velocities, parameters that produce measurable gravity anomalies and delays in the travel times of seismic waves. The Earth's gravitational field in the Yellowstone region is shown in Figure 7.4, which is a complete Bouguer gravity map contoured from data compiled by Blank and Gettings (1974)
From page 101...
... (1981) interpret these delays to represent velocity decreases averaging 10 to 20 percent throughout the upper crust and 5 to 10 percent in the lower crust and upper mantle to depths as great as 250 km.
From page 102...
... demonstrate anomalously high uplift rates over the Yellowstone Plateau with uplift contours that generally parallel the outline of the Yellowstone caldera. Uplift rates averaged greater than about 5 mm/yr within the outer edge of the ring-fracture zone and are greatest, up to 15 mm/yr, in the northeast caldera area adjacent to the Sour Creek resurgent dome.
From page 103...
... Two additional low-velocity zones of 30 percent decrease to ~4.0 km/sec were identified in the southwestern and the northeastern caldera areas. The crystalline velocity layering is a diagnostic parameter of the upper crust beneath the Yellowstone caldera.
From page 104...
... Madison Valley, Hebgen Lake NW -3 Park Boundary 'Yellowstone Caldera Yellowstone Lake FIGURE 7.7 Generalized northwest-southeast cross section of crustal parameters across the Yellowstone-Hebgen Lake region. Teleseismic P-wave delays from Iyer et al.
From page 105...
... The data on this profile do not directly map magma bodies, but they show that the upper 10 km of the crust beneath the Yellowstone caldera are marked by a low Pwave velocity and apparently a hot crust that restricts brittle fracture. In Figure 7.8 a southwest-northeast upper-crustal cross section parallels the axis of the Yellowstone caldera.
From page 106...
... I 109* 30' I NE I5_ x -SxLA BORHYOLITE YELLOWSTONE 3 8 "ALLAHD LAKE SOUR CREEK •3:8 DOME DOME COOKE BEARTOOTH Sp, cITY UPLIFT BASALT HOT EOCENE LOW VELOCITY VOLCANICS S CRUST FIGURE 7.9 Idealized NE-SW geologic-seismic velocity model for the crustal structure of the Yellowstone-Island Park-Snake River Plain region.
From page 107...
... On ascent into the upper crust, partial melts transport heat advectively upward and in turn could produce additional silicic components that further ascend toward the surface. The final stage produces tumescence and thermal expansion prior to explosive eruptions.
From page 108...
... . The Yellowstone-Snake River Plain seismic profiling experiment: Crustal structure of the eastern Snake River Plain, /.
From page 109...
... . The 1978 Yellowstone-eastern Snake River Plain seismic profiling experiment: Crustal structure of the Yellowstone region and experiment design, /.
From page 110...
... Another index of recent vigor is the number of recognized Holocene volcanoes in each belt divided by its length. A third index is the number of large explosive eruptions (VEI a 3)
From page 111...
... should be completed in the near future. In vigorous volcanic regions, however, such compilations are swiftly outdated, and in 1960 the Volcanological Society of Japan began recording the new eruptions of the world by publishing a yearly Bulletin of Volcanic Eruptions.
From page 112...
... Where data are available, half of these first-day paroxysms came within the eruption's first hour. History's largest documented explosive eruption, the devastating 1815 event at Tambora, Indonesia, culminated nearly 3 yr after its start and killed 92,000 people.
From page 113...
... This shows in a striking way the shifting proportion of eruptions in each interval group with progressively more explosive eruptions. The human significance of these more explosive eruptions is underlined by the fatality statistics.
From page 114...
... Volcanoes with no historical activity are often termed inactive, and many are not even recognized to be volcanoes by people who live on their flanks. But the chronological record of explosive volcanism shows that a volcano's long repose is more likely to be a cause for concern than reassurance.
From page 115...
... Their plate thicknesses of less than 20 km (IzuMarianas, Tonga, Kennadec, and South Sandwich Islands) are all marked by relatively quiet volcanism, and the most vigorous belts of New Zealand, Kamchatka, and Central America are all on thick (25 to 35 km)
From page 116...
... 116 TOM Sl^JT/Aand LEE SEIBERT 80 12(7 16(7 40' FIGURE 8.6 Map of the world's volcanoes. Volcanoes with known eruptions since A.D.
From page 117...
... Explosive Eruptions in Space and Time 117 vbcr 160' 40' Volcanoes with uncertain or only solfataric activity are shown by a small x. Volcanic belts are marked and numbered corresponding to Table 8.1.
From page 118...
... 118 «h t u o A 5 '€ u < I s o 0 CO u oa '8 8,8 8 (O ^ (O O> CM 1O CO CO O -^OO ^H^OO t~tN<6 t~ oin IN -- < fl I sox o o I I gee*
From page 119...
... 119 o -- ! • r -- i -< -- -- no'oioooic'iooo' -- C4(boQG3OO4r~^ O4 51 I US «H -- -- rt -- OO
From page 120...
... The number of Holocene volcanoes, number of years in which volcanoes have been active since 1880, and number of large explosive eruptions (VEI & 3) since 1880, all normalized per 100 km of belt length, are plotted for 45 volcanic belts.
From page 121...
... . Subduction zones and back arc basins -- a review, Geol.
From page 122...
... Pyroclastic deposits are present in the Hilina Formation on the south flank of Kilauea near the coast; about six of these deposits are estimated to be 40,000 to 50,000 yr old, and others are both younger and older. None of the deposits older than 2000 yr is well dated, but if we assume generally uniform growth rates for Kilauea's shield during the past 100,000 yr, an average, but not periodic, recurrence of major explosive eruptions is about every 2000 yr; minor explosive eruptions may be more frequent.
From page 123...
... For some island-arc volcanic provinces it exceeds 90 percent; the 1 percent in Hawaii is nearly the minimum. Why, then, is there any concern at all about explosive eruptions on Hawaii?
From page 124...
... Assuming that similar conditions existed near Halemaumau in 1924 to those at the well in 1973, these explosions probably originated near a depth of 500 m below the rim of Halemaumau. Though unusual and spectacular, the explosive eruptions of 1924 produced so little ejecta that they will probably not be evident in the eventual stratigraphic record of Kilauea, in sharp contrast to the extensive hydromagmatic deposits produced from the major explosive eruption in 1790.
From page 125...
... Units 1 and 6, which are wholly to partially reworked, clearly preceded and succeeded, respectively, the explosive eruptions that deposited the bulk of the formation. Distinctive nonplanar bedding characteristics of many layers in the Keanakakoi Formation demonstrate that these layers were deposited by pyroclastic surges.
From page 126...
... Progressively more lithic material was erupted along with the vitric ash, and pyroclastic-surge deposition became prevalent. FIGURE 9.5 Pyroclastic deposits of the Keanakakoi Formation in sand wash about 1 km southwest of Kilauea caldera.
From page 127...
... Geological Survey) indicates that the Uwekahuna Ash, which is comparable to or greater in volume and extent to the Keanakakoi Formation, was formed by similar explosive eruptions.
From page 128...
... DECKER and ROBERT L CHRISTIANSEN 2B Post-1823 lavas and depos1ts \ Pum1ce, ret1cul1te, and Pele's ha1r; well sorted, partly reworked L1th1c Iap11l1 and blocks; moderately to poorly sorted ,L1th1c f1ne ash and dust; abundant accret1onary Iap1ll1 L1th1c coarse ash, Iap1ll1, and blocks; moderately sorted '/L1th1c f1ne ash and dust; abundant accret1onary lap1lH /L1th1c coarse ash, lap-Ill 1, and blocks; moderately sorted LHh1c f1ne ash and dust; abundant accret1onary lapHH L1th1c coarse ash and lap1lH; poorly to moderately sorted -- Lava, spatter, pum1ce, Pele's ha1r and v1tr1c coarse ash; wel 1 sorted L1th1c-v1tr1c f1ne to coarse ash; moderately to poorly sorted; lent1cular beds -- V1tr1c to I1th1c-v1tr1c coarse to f1ne ash; well sorted; well sorted; planar beds -- V1tr1c f1ne to med1um ash; well sorted; generally planar beds V1tr1c coarse ash; well sorted; planar bed -- V1tr1c f1ne to med1um ash; well sorted; generally planar beds V1tr1c pum1ce, c1nders, and coarse ash; well sorted; planar beds -- V1tr1c f1ne to med1um ash; well sorted; generally planar beds -- V1tr1c to crystal-v1tr1c coarse ash and pum1ce; well-sorted; planar beds -- V1tr1c alternat1ng f1ne and coarse ash, generally coarser 1n basal part; well sorted; beds generally planar, locally wavy -- Pum1ce, ret1cul1te, and Pele's ha1r; generally well sorted; reworked on 1rregular surface to f1ll low areas and cracks Pre-1790 lavas, depos1ts, and so1ls FIGURE 9.7 Idealized stratigraphic section of the Keanakakoi Formation.
From page 129...
... If this interpretation is correct, the eruptions that resulted in the Pahala Ash may have deposited material at distances considerably farther than did the eruptions responsible for the Keanakakoi Formation and Uwekahuna Ash. PYROCLASTICS IN THE HILINA FORMATION The previously named Hilina Volcanic Series, now renamed Hilina Formation, is an approximately 250-m-thick sequence of Kilauea lava flows and pyroclastic deposits that occurs beneath the Pahala Ash.
From page 130...
... It is difficult, however, to establish a recurrence interval with any precision. The total number of preserved and exposed pyroclastic deposits is about 15, but the Pahala Ash and some of the pyroclastic units within the Hilina Formation probably result from the accumulation of several episodes of major explosive eruptions.
From page 131...
... Steam-blast eruptions from the summit area, such as the one in 1924, seem best explained by a sudden lowering of the magma column beneath Kilauea, which would cause intermittent explosions of steam from movement of subsurface water into the zone evacuated by the shallow magma. A larger version of this same process probably caused the 1790 Keanakakoi, the 1500 yr ago Uwekahuna, and perhaps earlier explosive eruptions.
From page 132...
... . Tragic base surge in 1790 at Kilauea Volcano, Geology 1, 83-86.
From page 133...
... RICE The Aerospace Corporation ABSTRACT A study of satellite infrared sensor data and survivor photographs provides an extensive record of the events of the first few minutes of the May 18, 1980, explosive eruptions of Mount St. Helens.
From page 134...
... show the movement of the pyroclastic surge and the development of ash clouds rising from it. The surge moved north in excess of 90 m/sec (see Figure 10.5)
From page 135...
... The pressure disturbance that rapidly produced the horizontal atmospheric cloud is believed to have been generated by a northern explosion emanating from the moving landslide at about 15:34.3. Alternately, the horizontal cloud may have been produced by convective rise and condensation in a moisture-rich air column above the hot pyroclastic surge or by a negative pressure pulse rising above the moving landslide caused by displacement of air in front of the advancing avalanche and surge.
From page 136...
... B, 15:33.9: the two explosion clouds have coalesced, and both the avalanche and the pyroclastic surge have reached Toutle River and Spirit Lake. C, 15:34.5: surge has begun descending the south slope, and two white condensation clouds appear in the eastern surge.
From page 137...
... 10 37 36 35 ffi 34 2 33 32 1 SURGE FRONT / / ROCKSLIDE AVALANCHE * # EXPLOSION A PHOTOS -ON SITE O SATELLITE A PHOTOS-SATELLITE FK TOUTLE RIVER 10 KM 15 20 25 FIGURE 10.5 Time-travel plot showing north-south position of landslide blocks (I, II, and III)
From page 138...
... The timing and relative intensity of these three emissions are comparable for the two satellites, although one of the satellites recorded a lower intensity because of the greater signal attenuation of a much longer slant path through the atmosphere. The hot material ejected prior to 15:34.8 was confined primarily to the north face of the mountain and the Toutle River-Spirit Lake area.
From page 139...
... During transport in the avalanche the hot dacite of the crypotodome was, no doubt, continuously broken and fragmented, which triggered secondary explosions. After nearly 2 min and several kilometers of northward transport, a major explosion was emitted from the avalanche, which produced a secondary higher-velocity pyroclastic surge that spread outward in every direction.
From page 140...
... Whatever their origin, these later explosions accelerated the pyroclastic surge, which moved outward to the limit of the devastated zone, carrying with it a high proportion (about 50 percent according to Moore and Sisson, 1981) of hot dacite that served as the explosion heat source.
From page 141...
... and later by the hot pumice and ash flows. Pumice Pond, 2 km west of Spirit Lake in the Toutle River valley, may be a partially filled vestige of a crater produced by the northern explosions (Lipman, 1981)
From page 142...
... MOORE and CARL J RICE May 18 pyroclastic surge, in The 1980 Eruptions of Mount St.
From page 143...
... Overpressured jets may show a much greater diversity in structure than pressure-balanced jets because the jet structure depends on the ratio of jet pressure to atmospheric pressure and typical volcanic eruption conditions allow a wide range of ratios. Plinian eruption columns are typical terrestrial pressurebalanced jets because they commonly emerge through craters and thus enter the atmosphere at atmospheric pressure.
From page 144...
... , many misconceptions remain of processes occurring during explosive volcanism. This paper describes how jets from violently erupting volcanoes obtain their "explosive" velocities from the transformation of the initial enthalpy stored in magma into kinetic energy and how the transformation process depends on the interplay between the system geometry, fluid thermodynamic properties, reservoir initial conditions, and ambient atmospheric conditions.
From page 145...
... The devastated area is assumed to mimic the shape of the jet from the lateral blast over the area where the jet remained close to the ground. The jet actually extended beyond these boundaries but lifted from the ground when it expanded to ambient atmospheric density (Kieffer, 1982a)
From page 146...
... was not involved in the lateral blast, (c) Discharge of deep reservoir through conduit and, perhaps, the shallow surface crater into jet to form plinian eruption column at Mount St.
From page 147...
... phase diagrams, shown in Figure 11.4, because many eruptions are quasi-isentropic and analysis of the fluid flow is often restricted by the assumption of isentropic processes. Even if the expansion of the erupting volatile 200 400 600 800 1000 1200 1400 1600 T ( K)
From page 148...
... The left arrow begins at initial conditions plausible for the lower crust of the Earth. H ,O ascending from such conditions would boil upon decompression to 150 bars pressure.
From page 149...
... , or of SO2 or S from a very high temperature reservoir on lo (Figures 11.4c and d) , to ambient atmospheric pressure could occur without complications in the fluid flow due to phase changes.
From page 150...
... The most important reservoir variable influencing overall jet structure is the ratio of initial reservoir pressure to atmospheric pressure. On Earth or on lo the range of tens of bars to perhaps a thousand bars brackets most initial reservoir pressures; for rare kimberlite eruptions on Earth, initial reservoir pressures may even be on the order of tens of thousands of bars.
From page 151...
... The high fluid pressures that exist in fluid erupting directly from a straight conduit or fracture cause surface craters to form, either by direct erosion of the sharp lips of such features or indirectly by slumping of unstable near-surface material into the flow (which has a great capacity for material transport in this relatively high pressure state; see discussion in Kieffer, 1982b)
From page 152...
... Thus, in most cases of terrestrial volcanism where there are even minor surface craters, jets enter the atmosphere with a fluid pressure approximately equal to ambient atmospheric pressure, a condition that I will call pressure balanced. If they enter the atmosphere at a pressure significantly above ambient, I call them overpressured, although the commonly used description in fluid mechanics is under expanded.
From page 153...
... that causes the flow to expand laterally will be maintained through a fairly large crater traverse, and the flow streamlines will diverge to become parallel to the crater walls. As the flow traverses the surface crater, fluid pressures drop from several bars to atmospheric pressure.
From page 154...
... Across these shocks the flow velocities decrease, the pressure increases, and the flow changes direction. The intercepting shocks coalesce across the flow to form the strong Mach disk shock.
From page 155...
... The pressure drops to 4 percent of atmospheric pressure immediately in front of the Mach disk shock. When the fluid passes through the lateral intercepting shocks and the Mach disk shock, the pressure is brought back up toward atmospheric.
From page 156...
... This paper describes the role of compressibility, without considering the influences of other important effects, such as gravity; buoyancy; viscous effects at boundary layers; and, for lateral jets, topography. Accurate prediction of hazards from jets during explosive eruptions must account simultaneously for the influence of all of these effects.
From page 157...
... . Explosive volcanic eruptions -- IV.
From page 158...
... Although experimentation on hydromagmatic volcanism has just begun, considerable work on vapor explosions has been done in the field of nuclear reactor safety (Sandia Laboratories, 1975)
From page 159...
... Surtseyan tuff rings result from more highly explosive eruptions than do Surtseyan tuff cones, hence their lower profiles.
From page 160...
... Water Box Ignition wire -5 gallon bucket FIGURE 12.3 Three experiments for simulating volcanic eruptions. Design Four experimental setups have been used to simulate different volcanic environments (see Figure 12.3)
From page 161...
... Documentation High-speed cinematography was used as the primary means of documenting the experiments. Through photography it is possible both to compare the model qualitatively with volcanic eruptions and to measure quantitative features such as velocity, event timing, and ejecta trajectories by frame-to-frame analysis (Figures 12.5 and 12.6)
From page 162...
... As discussed earlier, the mechanism is basically the same as that in nature. The experimental explosions bear a remarkable resemblance to volcanic eruptions despite the following limitations: (1)
From page 163...
... Weak Surtseyan ejection of melt in less than 1 sec; eruption column 24 m high; horizontally moving base surge; 1-mm fragments; formed a tuff cone 2.5 m in diameter, 0.5 m high; continuous ejecta 5 m from rim. Strong Surtseyan blast lasting less than 1 sec; eruption column 40 m high; surge reaching 6 m from vent; submillimeter ejecta and accretionary lapilli; formed tuff ring 2.1 m in diameter, 0.27 m high; continuous ejecta 6 to 7 m from rim.
From page 164...
... (12.6) This calculation does not consider possible heat transfer to the steam during the expansion process, which would produce a greater thermal efficiency.
From page 165...
... Conductive heat transfer, which is primarily a function of surface area, dominates in this rapid process. The experiments indicate that the size of ejected melt fragments decreases with increasing explosive energy (efficiency)
From page 166...
... The curve demonstrates the added efficiency of superheated systems in which maximum efficiency may occur at mass ratios near 0.35 to 0.70. A tentative conclusion on the effect of confinement is that confinement appears to increase the efficiency of equilibrium systems.
From page 167...
... A spectrum of explosive phenomena may be experimentally produced and compared with different kinds of hydromagmatic activity, including Strombolian, Surtseyan, and submarine volcanic activity. Furthermore, experiments indicate that the nature of activity and its explosiveness is primarily controlled by the water-to-melt mass ratio and confining pressure.
From page 168...
... Mass ratios of water and melt near 0.3 result in highly explosive Surtseyan blasts of millimeter and micronsized material. Abundant amounts of water (greater than ratios of 2 or 3)
From page 169...
... . Atmospheric shock waves and condensation clouds from Ngauruhoe explosive eruptions, Nature 259, 190-192.
From page 170...
... Even more obvious, however, is the fact that explosive volcanism has always been of vital concern to the people and governments immediately affected by such activity. When a potentially explosive volcano enters a period of crisis, interested scientists and a concerned, if not terrified, local community are brought into what euphemistically might be called a challenging relationship.
From page 171...
... For all intents and purposes, fresh volcanic glass was not contained in the ash, and there was in fact no clear evidence that magma was about to erupt to the surface. The authorities on Guadeloupe, of course, did not realize this at the time, and so the volcano was thought to be far more dangerous than it actually was.
From page 172...
... However, for an hour or two after the event, and especially during the period when the extent of injuries sustained by the summit party was still uncertain, journalists sent out the news that the volcano had entered a serious episode of eruptive activity. In North America, due to garbling of the story after it had been transmitted from Guadeloupe, the American public was informed in front-page headlines that La Soufriere had at last delivered up the long-awaited eruption.
From page 173...
... Volcanologists, Journalists, and the Concerned Local Public 173 "A FIGURE 13.3 Deserted street in Basse-Terre, Guadeloupe, during the evacuation of 1976. The smoldering summit of La Soufriere is seen in the background.
From page 174...
... Because the hurriedly established volcano observatory was located only 9 km from the volcano, well within the area of evacuation, the journalists were not able to visit the observatory or to interview scientists in the field. The members of the press corps received information about the volcano from government officials in the capital city of Kingstown, who passed along informaton that had been supplied to them by the scientific team.
From page 175...
... Vincent. If more scientific information had been available on Guadeloupe, in the form of a geologic and geophysical data base, the anomalies observed there in 1976 could have been assessed in a better way.
From page 176...
... In conclusion, it is worth emphasizing that the two examples of volcanic crises cited here, Guadeloupe in 1976 and St. Vincent in 1979, clearly represent extremes with regard to scientist/journalist interactions.


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More information on Chapter Skim is available.