Geology Reports⌕ Search

SEARCH · Geology Reports

Results for “Journal of Volcanology and Geothermal Research”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 73 records · Page 4Linked to original sources
USGS1994

Precursory swarms of long-period events at Redoubt Volcano (1989-1990), Alaska: Their origin and use as a forecasting tool

During the eruption of Redoubt Volcano from December 1989 through April 1990, the Alaska Volcano Observatory issued advance warnings of several tephra eruptions based on changes in seismic activity related to the occurrence of precursory swarms of long-period (LP) seismic events (dominant period of about 0.5 s). The initial eruption on December 14 occurred after 23 years of quiescence and was heralded by a 23-hour swarm of LP events that ended abruptly with the eruption. After a series of vent-clearing explosions over the next few days, dome growth began on December 21. Another swarm, with LP events similar to those of the first, began on the 26th and ended in a major tephra eruption on January 2. Eruptions continued over the next two weeks and then ceased until February 15, when a large eruption initiated a long phase of repetitive dome-building and dome-destroying episodes that continued into April. Warnings were issued before the major events on December 14 and January 2, but as the eruptive sequence continued after January 2, the energy of the swarms decreased and forecasting became more difficult. A significant but less intense swarm preceded the February 15 eruption, which was not forecast. This eruption destroyed the only seismograph on the volcanic edifice and stymied forecasting until March 4, when the first of three new stations was installed within 3 km of the active vent. From March 4 to the end of the sequence on April 21, there were eight eruptions, six of which were preceded by detectable swarms of LP events. Although weak, these swarms provided the basis for warnings issued before the eruptions on March 23 and April 6. The initial swarm on December 13 had the following features: (1) short duration (23 hours); (2) a rapidly accelerating rate of seismic energy release over the first 18 hours of the swarm, followed by a decline of activity during the 5 hours preceding the eruption; (3) a magnitude range from −0.4 to 1.6; (4) nearly identical LP signatures with a dominant period near 0.5 s; (5) dilatational first motions everywhere; and (6) a stationary source location at a depth of 1.4 km beneath the crater. This occurrence of long-period events suggests a model involving the interaction of magma with groundwater in which magmatic gases, steam and water drive a fixed conduit at a stationary point throughout the swarm. The initiation of that sequence of events is analogous to the failure of a pressure-relief valve connecting a lower, supercharged magma-dominated reservoir to a shallow hydrothermal system. A three-dimensional model of a vibrating fluid-filled crack recently developed by Chouet is found to be compatible with the seismic data and yields the following parameters for the LP source: crack length, 280–380 m; crack width, 140–190 m; crack thickness, 0.05–0.20 m; crack stiffness, 100–200; sound speed of fluid, 0.8–1.3 km/s; compressional-wave speed of rock, 5.1 km/s; density ratio of fluid to rock, ≈0.4; and ratio of bulk modulus of fluid to rigidity of rock, 0.03–0.07. The fluid-filled crack is excited intermittently by an impulsive pressure drop that varies in magnitude within the range of 0.4 to 40 bar. Such disturbance appears to be consistent with a triggering mechanism associated with choked flow conditions in the crack.

Alaska↗
USGS1994

The 1989-1990 eruption of Redoubt Volcano, Alaska: impacts on aircraft operations

The December 1989–June 1990 eruption of Redoubt Volcano affected commercial and military air operations in the vicinity of Anchorage, Alaska. These effects were due to the direct impact of volcanic ash on jet aircraft, as well as to the rerouting and cancellations of flight operations owing to eruptive activity. Between December and February, five commercial jetliners were damaged from ash encounters. The most serious incident took place on December 15, 1989 when a Boeing 747-400 aircraft temporarily lost power of all four engines after encountering an ash cloud as the airplane descended for a landing in Anchorage. While there were no injuries to passengers, the damage to engines, avionics, and aircraft structure from this encounter is estimated at $80 million. Four additional encounters between jet aircraft and Redoubt ash clouds occurred in the Anchorage area on December 15 and 16, 1989 and February 21, 1990; none resulted in engine failure. Two additional encounters took place on December 17, 1989 when jet airliners encountered the Redoubt cloud over west Texas. At the time of these encounters, the cloud was up to 55 hours old and had traveled in excess of 2,900 nautical miles (5,300 km). Following the December 15 encounters, Anchorage International Airport remained open, however, most airline companies canceled operations for up to several days. As communications between Federal agencies and airlines improved, and as a better understanding of the nature and behavior of ash-rich eruption clouds was achieved, most airlines resumed normal service by early January 1990. The resulting loss of revenue at Anchorage International Airport during several months following the eruption is estimated to total $2.6 million. The impact on general aviation and military operations consisted mostly of cancellation and rerouting of flights.

Alaska↗
USGS1994

Vapor saturation and accumulation in magmas of the 1989-1990 eruption of Redoubt Volcano, Alaska

The 1989–1990 eruption of Redoubt Volcano, Alaska, provided an opportunity to compare petrologic estimates of SO 2 and Cl emissions with estimates of SO 2 emissions based on remote sensing data and estimates of Cl emissions based on plume sampling. In this study, we measure the sulfur and chlorine contents of melt inclusions and matrix glasses in the eruption products to determine petrologic estimates of SO 2 and Cl emissions. We compare the results with emission estimates based on COSPEC and TOMS data for SO 2 and data for Cl/SO 2 in plume samples. For the explosive vent clearing period (December 14–22, 1989), the petrologic estimate for SO 2 emission is 21,000 tons, or ~12% of a TOMS estimate of 175,000 tons. For the dome growth period (December 22, 1989 to mid-June 1990), the petrologic estimate for SO 2 emission is 18,000 tons, or ~3% of COSPEC-based estimates of 572,000–680,000 tons. The petrologic estimates give a total SO 2 emission of only 39,000 tons compared to an integrated TOMS/COSPEC emission estimate of ~1,000,000 tons for the whole eruption, including quiescent degassing after mid-June 1990. Petrologic estimates also appear to underestimate Cl emissions, but apparent HCl scavenging in the plume complicates Cl emission comparisons. Several potential sources of ‘excess sulfur’ often invoked to explain petrologic SO 2 deficits are concluded to be unlikely for the 1989–1990 Redoubt eruption — e.g., breakdown of sulfides, breakdown of anhydrite, release of SO 2 from a hydrothermal system, degassing of commingled infusions of basalt in the magma chamber, and syn-eruptive degassing of sulfur from melt present in non-erupted magma. Leakage and/or diffusion of sulfur from melt inclusions do not provide convincing explanations for the petrologic SO 2 deficits either. The main cause of low petrologic estimates for SO 2 is that melt inclusions do not represent the total sulfur content of the Redoubt magmas, which were vapor-saturated magmas carrying most of their sulfur in an accumulated vapor phase. Almost all the sulfur of the SO 2 emissions was present prior to emission as accumulated magmatic vapor at 6–10 km depth in the magma that supplied the eruption; whole-rock normalized concentrations of gaseous excess S in these magmas remained at ~0.2 wt.% throughout the eruption, equivalent to ~0.7 vol.% at depth. Data for CO 2 emissions during the eruption indicate that CO 2 at whole-rock concentrations of ~0.6 wt.% in the erupted magma was a key factor in creating the vapor saturation and accumulation condition making a vapor phase source of excess sulfur possible at depth. When explosive volcanism involves magma with accumulated vapor, melt inclusions do not provide a sufficient basis for predicting SO 2 emissions. Thus, petrologic estimates made for SO 2 emissions during explosive eruptions of the past may be too low and may significantly underestimate impacts on climate and the chemistry of the atmosphere.

Journal of Volcanology and Geothermal Research↗
USGS1994

Disruption of Drift glacier and origin of floods during the 1989-1990 eruptions of Redoubt Volcano, Alaska

Melting of snow and glacier ice during the 1989–1990 eruption of Redoubt Volcano caused winter flooding of the Drift River. Drift glacier was beheaded when 113 to 121 × 10 6 m 3 of perennial snow and ice were mechanically entrained in hot-rock avalanches and pyroclastic flows initiated by the four largest eruptions between 14 December 1989 and 14 March 1990. The disruption of Drift glacier was dominated by mechanical disaggregation and entrainment of snow and glacier ice. Hot-rock avalanches, debris flows, and pyroclastic flows incised deep canyons in the glacier ice thereby maintaining a large ice-surface area available for scour by subsequent flows. Downvalley flow rheologies were transformed by the melting of snow and ice entrained along the upper and middle reaches of the glacier and by seasonal snowpack incorporated from the surface of the lower glacier and from the river valley. The seasonal snowpack in the Drift River valley contributed to lahars and floods a cumulative volume equivalent to about 35 × 10 6 m 3 of water, which amounts to nearly 30% of the cumulative flow volume 22 km downstream from the volcano. The absence of high-water marks in depressions and of ice-collapse features in the glacier indicated that no large quantities of meltwater that could potentially generate lahars were stored on or under the glacier; the water that generated the lahars that swept Drift River valley was produced from the proximal, eruption-induced volcaniclastic flows by melting of snow and ice.

Alaska↗
USGS1994

Unusual ice diamicts emplaced during the December 15, 1989 eruption of Redoubt volcano, Alaska

Ice diamict comprising clasts of glacier ice and subordinate rock debris in a matrix of ice (snow) grains, coarse ash, and frozen pore water was deposited during the eruption of Redoubt Volcano on December 15, 1989. Rounded clasts of glacier ice and snowpack are as large as 2.5 m, clasts of Redoubt andesite and basement crystalline rocks reach 1 m, and tabular clasts of entrained snowpack are as long as 10 m. Ice diamict was deposited on both the north and south volcano flanks. On Redoubt's north flank along the east side of Drift piedmont glacier and outwash valley, ice diamict accumulated as at least 3 units, each 1–5 m thick. Two ice-diamict layers underlie a pumice-lithic fall tephra that accumulated on December 15 from 10:15 to 11:45 AST. A third ice diamict overlies the pumiceous tephra. Some of the ice diamicts have a basal ‘ice-sandstone’ layer. The north side icy flows reached as far as 14 km laterally over an altitude drop of 2.3 km and covered an area of about 5.7 km 2 . On Crescent Glacier on the south volcano flank, a composite ice diamict is locally as thick as 20 m. It travelled 4.3 km over an altitude drop of 1.7 km, covering about 1 km 2 . The much higher mobility of the northside flows was influenced by their much higher water contents than the southside flow(s). Erupting hot juvenile andesite triggered and turbulently mixed with snow avalanches at snow-covered glacier heads. These flows rapidly entrained more snow, firn, and ice blocks from the crevassed glacier. On the north flank, a trailing watery phase of each ice-diamict flow swept over and terraced the new icy deposits. The last (and perhaps each) flood reworked valley-floor snowpack and swept 35 km downvalley to the sea. Ice diamict did not form during eruptions after December 15 despite intervening snowfalls. These later pyroclastic flows swept mainly over glacier ice rather than snowpack and generated laharic floods rather than snowflows. Similar flows of mixed ice grains and pyroclastic debris resulted from the November 13, 1985 eruption of Nevado del Ruiz volcano and from eruptions of snowclad Mount St. Helens in 1982–1984. Such deposits at snowclad volcanoes are initially broad and geomorphically distinct, but they soon become extensively reworked and hard to recognize in the geologic record.

Alaska↗
USGS1994

An experiment to detect and locate lightning associated with eruptions of Redoubt Volcano

A commercially-available lightning-detection system was temporarily deployed near Cook Inlet, Alaska in an attempt to remotely monitor volcanogenic lightning associated with eruptions of Redoubt Volcano. The system became operational on February 14, 1990; lightning was detected in 11 and located in 9 of the 13 subsequent eruptions. The lightning was generated by ash clouds rising from pyroclastic density currents produced by collapse of a lava dome emplaced near Redoubt's summit. Lightning discharge (flash) location was controlled by topography, which channeled the density currents, and by wind direction. In individual eruptions, early flashes tended to have a negative polarity (negative charge is lowered to ground) while late flashes tended to have a positive polarity (positive charge is lowered to ground), perhaps because the charge-separation process caused coarse, rapid-settling particles to be negatively charged and fine, slow-settling particles to be positively charged. Results indicate that lightning detection and location is a useful adjunct to seismic volcano monitoring, particularly when poor weather or darkness prevents visual observation. The simultaneity of seismicity and lightning near a volcano provides the virtual certainty that an ash cloud is present. This information is crucial for aircraft safety and to warn threatened communities of impending tephra falls. The Alaska Volcano Observatory has now deployed a permanent lightning-detection network around Cook Inlet.

Journal of Volcanology and Geothermal Research↗
USGS1995

Volcanic and structural evolution of Taupo Volcanic Zone, New Zealand: A review

The Taupo Volcanic Zone (TVZ) in the central North Island is the main focus of young volcanism in New Zealand. Andesitic activity started at c. 2 Ma, joined by voluminous rhyolitic (plus minor basaltic and dacitic) activity from c. 1.6 Ma. The TVZ is c. 300 km long (200 km on land) and up to 60 km wide, as defined by vent positions and caldera structural boundaries. The total volume of TVZ volcanic deposits is uncertain because a sub-volcanic basement has not been identified, but present data suggest bulk volumes of 15–20,000 km 3 , and that faulted metasediments form most of the immediate subvolcanic basement. Rhyolite (≥15,000 km 3 bulk volume, typically 70–77% SiO 2 ) is the dominant magma erupted in the TVZ (mostly as calderaforming ignimbrite eruptions), andesite is an order of magnitude less abundant, and basalt and dacite are minor in volume (< 100 km 3 each). The history of the TVZ is here divided into ‘old TVZ’ from 2.0 Ma to 0.34 Ma, and ‘young TVZ’ from 0.34 Ma onwards, separated by the Whakamaru eruptions, which obscured much of the evidence for older activity within the zone. The TVZ shows a pronounced segmentation into northeastern and southwestern andesite-dominated extremities with composite cones and no calderas, and a central 125-km-long rhyolite-dominated segment. Eight rhyolitic caldera centres have so far been identified in the central segment, of which two (Mangakino and Kapenga) are composite features, and more centres will probably be delineated as further data accumulate. These centres account for 34 inferred caldera-forming ignimbrite eruptions, in the c. 1.6-Ma lifetime of the central TVZ. The modern central TVZ is the most frequently active and productive silicic volcanic system on Earth, erupting rhyolite at c. 0.28 m 3 s −1 , and available information suggests this has been so for at least the past 0.34 Ma. The rhyolites show no major compositional changes with time, though the extent of magma chamber zonation may have changed with the incoming of rifting and crustal extension in the past c. 0.9 Ma. Within the central TVZ, non-rhyolitic compositions have been erupted apparently irregularly in time and space; in particular there is no evidence for a geographic separation of basalts from andesites. Between 0.9 and 0.34 Ma, a major episode of uplift affected areas around the TVZ, while at the same time the main focus of activity may have migrated eastwards within the TVZ accompanying rifting along the axis of the zone. The modern TVZ is rifting at rates between 7 and 18 mm a −1 and restoration of the thin (15km) ‘crust’ ( V p ≤ 6.1 km s −1 ) beneath the central TVZ to its pre-rifting thickness (25 km) implies that rifting at such rates may have begun only at c. 0.9 Ma. The TVZ is a rifted arc, but its longitudinally segmented nature, high thermal flux and voluminous rhyolitic volcanism make it unique on Earth.

Taupo Volcanic Field↗
USGS1995

Roof-rock contamination of magma along the top of the reservoir for the Bishop Tuff

The Bishop Tuff, a Quaternary high-silica rhyolite in east-central California, is widely considered the type example of a vertically and monotonically zoned pyroclastic deposit that represents zoning in the source magma reservoir, inverted during the process of pyroclastic emplacement. However, the deposit of plinian pumice, which forms the base of the Bishop Tuff and represents the initial 10% or so of all magma erupted during the event that produced the Bishop Tuff, contains features at odds with monotonic zoning for the reservoir. Relative to overlying ignimbrite, the plinian deposit contains a reversal in trace-element zoning. The data have been previously interpreted as due to processes of chemical fractionation and evolution operating within a magma system closed to chemical interactions with its roof rocks. However, it is suggested here that the reversed zoning and other above-noted features can be explained equally well as consequences of minor assimilation of roof rocks into a magma reservoir that was erupted from the top down. -from Authors

Journal of Volcanology and Geothermal Research↗
USGS1996

Mount St. Augustine volcano fumarole wall rock alteration: Mineralogy, zoning, composition and numerical models of its formation process

Intensely altered wall rock was collected from high-temperature (640 °C) and low-temperature (375 °C) vents at Augustine volcano in July 1989. The high-temperature altered rock exhibits distinct mineral zoning differentiated by color bands. In order of decreasing temperature, the color bands and their mineral assemblages are: (a) white to grey (tridymite-anhydrite); (b) pink to red (tridymite-hematite-Fe hydroxide-molysite (FeCl 3 ) with minor amounts of anhydrite and halite); and (c) dark green to green (anhydrite-halite-sylvite-tridymite with minor amounts of molysite, soda and potash alum, and other sodium and potassium sulfates). The alteration products around the low-temperature vents are dominantly cristobalite and amorphous silica with minor potash and soda alum, aphthitalite, alunogen and anhydrite. Compared to fresh 1986 Augustine lava, the altered rocks exhibit enrichments in silica, base metals, halogens and sulfur and show very strong depletions in Al in all alteration zones and in iron, alkali and alkaline earth elements in some of the alteration zones. To help understand the origins of the mineral assemblages in altered Augustine rocks, we applied the thermochemical modeling program, GASWORKS, in calculations of: (a) reaction of the 1987 and 1989 gases with wall rock at 640 and 375 °C; (b) cooling of the 1987 gas from 870 to 100 °C with and without mineral fractionation; (c) cooling of the 1989 gas from 757 to 100 °C with and without mineral fractionation; and (d) mixing of the 1987 and 1989 gases with air. The 640 °C gas-rock reaction produces an assemblage consisting of silicates (tridymite, albite, diopside, sanidine and andalusite), oxides (magnetite and hercynite) and sulfides (bornite, chalcocite, molybdenite and sphalerite). The 375 °C gas-rock reaction produces dominantly silicates (quartz, albite, andalusite, microcline, cordierite, anorthite and tremolite) and subordinate amounts of sulfides (pyrite, chalcocite and wurtzite), oxides (magnetite), sulfates (anhydrite) and halides (halite). The cooling calculations produce: (a) anhydrite, halite, sylvite; (b) Cu, Mo, Fe and Zn sulfides; (c) Mg fluoride at high temperature (> 370 °C); (d) chlorides, fluorides and sulfates of Mn, Fe, Zn, Cu and Al at intermediate temperature (170–370 °C); and (e) hydrated sulfates, liquid sulfur, crystalline sulfur, hydrated sulfuric acid and water at low temperature (< 170°C). The volcanic gas-air mixing calculation produces major amounts of Na and K sulfates, minor amounts of hematite and trace amounts (< 1%) of anhydrite at log gas/air (Ig/a) ratios > 0.41 (> 628°C). This is followed by precipitation of sulfates of Fe, Cu. Pb. Zn and Al at Ig/a ratios between 0.31 and -0.4 (>628-178°C). At a lg/r ratio of ≤ -0.4 (178°C). anhydrous sulfates are replaced by their hydrated forms and hygroscopic sulfuric acid forms. At these low g/a ratios. hydrated sulfuric acid becomes the dominant phase in the system. Comparison of the thermochemical modeling results with the natural samples suggests that the alteration assemblages include: (1) minerals that precipitate from direct cooling of the volcanic gas; (2) phases that form by volcanic gases mixing with air: and (3) phases that form by volcanic gas-air-rock reaction. A complex interplay of the three processes produces the observed mineral zoning. Another implication of the numerical simulation results is that most of the observed incrustation and sublimate minerals apparently formed below 700°C.

Journal of Volcanology and Geothermal Research↗
USGS2004

Dome growth behavior at Soufriere Hills Volcano, Montserrat, revealed by relocation of volcanic event swarms, 1995-1996

We have relocated a subset of events from the digital waveform catalogue of ???17,000 volcanic microearthquakes recorded between July 1995 and February 1996 at Soufriere Hills Volcano (SHV), Montserrat, using a cross-correlation-based phase repicking technique with a joint location method. Hypocenters were estimated for 3914 earthquakes having five or more corrected P-wave picks. The seismic source region collapsed to a volume of ???1 km3 from an initial ???100 km3. Relocated events represent 36 swarms, each containing nearly identical waveforms, having source dimensions of 10 to 100 m in diameter and spatial separations on the order of 500 m or less. Each swarm occurred over a span of several hours to a few days.Triggered data appear to miss between 65% and 98% of the events that occur within these swarms, based on review of helicorder records. Visual estimates of summit dome growth show a rough correspondence between episodes of intense swarming and increases in extruded magma, although dome observations are too sparse to make a direct comparison for this time period. The limited depth range over which dome-growth-related events occur is consistent with a dynamic model of cyclic plug extrusion behavior in the shallow conduit, governed by magma supply rate, overpressure buildup and physical properties of the magma and conduit geometry. Seismic sources may occur in locally overpressured regions that result from microlite formation in a zone of rapid decompression; we propose that this zone exists in the vicinity of a detachment plane associated with the cyclic plug extrusion. ?? 2004 Elsevier B.V. All rights reserved.

Journal of Volcanology and Geothermal Research↗
USGS2004

Acoustic measurements of the 1999 basaltic eruption of Shishaldin volcano, Alaska 1. Origin of Strombolian activity

The 1999 basaltic eruption of Shishaldin volcano (Alaska, USA) displayed both classical Strombolian activity and an explosive Subplinian plume. Strombolian activity at Shishaldin occurred in two major phases following the Subplinian activity. In this paper, we use acoustic measurements to interpret the Strombolian activity. Acoustic measurements of the two Strombolian phases show a series of explosions that are modeled by the vibration of a large overpressurised cylindrical bubble at the top of the magma column. Results show that the bubble does not burst at its maximum radius, as expected if the liquid film is stretched beyond its elasticity. But bursting occurs after one cycle of vibration, as a consequence of an instability of the air-magma interface close to the bubble minimum radius. During each Strombolian period, estimates of bubble length and overpressure are calculated. Using an alternate method based on acoustic power, we estimate gas velocity to be 30-60 m/s, in very good agreement with synthetic waveforms. Although there is some variation within these parameters, bubble length and overpressure for the first Strombolian phase are found to be ??? 82 ?? 11 m and 0.083 MPa. For the second Strombolian phase, bubble length and overpressure are estimated at 24 ?? 12 m and 0.15 MPa for the first 17 h after which bubble overpressure shows a constant increase, reaching a peak of 1.4 MPa, just prior to the end of the second Strombolian phase. This peak suggests that, at the time, the magma in the conduit may contain a relatively large concentration of small bubbles. Maximum total gas volume and gas fluxes at the surface are estimated to be 3.3 ?? 107 and 2.9 ?? 103 m3/s for the first phase and 1.0 ?? 108 and 2.2 ?? 103 m3/s for the second phase. This gives a mass flux of 1.2 ?? 103 and 8.7 ?? 102 kg/s, respectively, for the first and the second Strombolian phases. ?? 2004 Elsevier B.V. All rights reserved.

Journal of Volcanology and Geothermal Research↗
USGS2004

Acoustic measurements of the 1999 basaltic eruption of Shishaldin volcano, Alaska 2. Precursor to the Subplinian phase

The 1999 eruption of Shishaldin volcano (Alaska, USA) displayed both Strombolian and Subplinian basaltic activity. The Subplinian phase was preceded by a signal of low amplitude and constant frequency (??? 2 Hz) lasting 13 h. This "humming signal" is interpreted as the coalescence of the very shallow part of a foam building up in the conduit, which produces large gas bubbles before bursting. The acoustic waveform of the hum event is modelled by a Helmholtz resonator: gas is trapped into a rigid cavity and can only escape through a tiny upper hole producing sound waves. At Shishaldin, the radius of the hole (??? 5 m) is close to that of the conduit (??? 6 m), the cavity has a length of ??? 60 m, and gas presents only a small overpressure between (??? 1.2 ?? 10-3 and 4.5 ?? 10-3 MPa). Such an overpressure is obtained by the partial coalescence of a foam formed by bubbles with a diameter from ??? 2.3 mm at the beginning of the episode towards ??? 0.64 mm very close to the end of the phase. The intermittency between hum events is explained by the ripening of the foam induced by the H2O diffusion through the liquid films. The two extreme values, from 600 to 10 s, correspond to a bubble diameter from 2.2 to 0.3 mm at the beginning and end of the pre-Subplinian phase, respectively. The extremely good agreement between two independent estimates of bubble diameters in the shallow foam reinforces the validity of such an interpretation. The total gas volume lost at the surface during the humming events is at most 5.9 ?? 106 m3. At the very end of the pre-Subplinian phase, there is a single large bubble with an overpressure of ???0.42 MPa. The large overpressure suggests that it comes from significant depth, unlike other bubbles in the pre-Subplinian phase. This deep bubble may be responsible for the entire foam collapse, resulting in the Subplinian phase. ?? 2004 Elsevier B.V. All rights reserved.

Journal of Volcanology and Geothermal Research↗
USGS2004

What makes hydromagmatic eruptions violent? Some insights from the Keanakāko'i Ash, Kı̄lauea Volcano, Hawai'i

Volcanic eruptions at the summit of Kilauea Volcano, Hawai'i, are of two dramatically contrasting types: (1) benign lava flows and lava fountains; and (2) violent, mostly prehistoric eruptions that dispersed tephra over hundreds of square kilometers. The violence of the latter eruptions has been attributed to mixing of water and magma within a wet summit caldera; however, magma injection into water at other volcanoes does not consistently produce widespread tephras. To identify other factors that may have contributed to the violence of these eruptions, we sampled tephra from the Keanakako'i Ash, the most recent large hydromagmatic deposit, and measured vesicularity, bubble-number density and dissolved volatile content of juvenile matrix glass to constrain magma ascent rate and degree of degassing at the time of quenching. Bubble-number densities (9X10 (super 4) -1X10 (super 7) cm (super -3) ) of tephra fragments exceed those of most historically erupted Kilauean tephras (3X10 (super 3) -1.8X10 (super 5) cm (super -3) ), and suggest exceptionally high magma effusion rates. Dissolved sulfur (average=330 ppm) and water (0.15-0.45 wt.%) concentrations exceed equilibrium-saturation values at 1 atm pressure (100-150 ppm and approximately 0.09%, respectively), suggesting that clasts quenched before equilibrating to atmospheric pressure. We interpret these results to suggest rapid magma injection into a wet crater, perhaps similar to continuous-uprush jets at Surtsey. Estimates of Reynolds number suggest that the erupting magma was turbulent and would have mixed with surrounding water in vortices ranging downward in size to centimeters. Such fine-scale mixing would have ensured rapid heat exchange and extensive magma fragmentation, maximizing the violence of these eruptions.

Hawaii↗
USGS2004

Volcanological perspectives on Long Valley, Mammoth Mountain, and Mono Craters: Several contiguous but discrete systems

The volcanic history of the Long Valley region is examined within a framework of six successive (spatially discrete) foci of silicic magmatism, each driven by locally concentrated basaltic intrusion of the deep crust in response to extensional unloading and decompression melting of the upper mantle. A precaldera dacite field (3.5–2.5 Ma) northwest of the later site of Long Valley and the Glass Mountain locus of >60 high-silica rhyolite vents (2.2–0.79 Ma) northeast of it were spatially and temporally independent magmatic foci, both cold in postcaldera time. Shortly before the 760-ka caldera-forming eruption, the mantle-driven focus of crustal melting shifted ∼20 km westward, abandoning its long-stable position under Glass Mountain and energizing instead the central Long Valley system that released 600 km 3 of compositionally zoned rhyolitic Bishop Tuff (760 ka), followed by ∼100 km 3 of crystal-poor Early Rhyolite (760–650 ka) on the resurgent dome and later by three separate 5-unit clusters of varied Moat Rhyolites of small volume (527–101 ka). West of the caldera ring-fault zone, a fourth focus started up ∼160 ka, producing a 10×20-km array of at least 35 mafic vents that surround the trachydacite/alkalic rhyodacite Mammoth Mountain dome complex at its core. This young 70-vent system lies west of the structural caldera and (though it may have locally re-energized the western margin of the mushy moribund Long Valley reservoir) represents a thermally and compositionally independent focus. A fifth major discrete focus started up by ∼50 ka, 25–30 km north of Mammoth Mountain, beneath the center of what has become the Mono Craters chain. In the Holocene, this system advanced both north and south, producing ∼30 dike-fed domes of crystal-poor high-silica rhyolite, some as young as 650 years. The nearby chain of mid-to-late Holocene Inyo domes is a fault-influenced zone of mixing where magmas of at least four kinds are confluent. The sixth and youngest focus is at Mono Lake, where basalt, dacite, and low-silica rhyolite unrelated to the Mono Craters magma reservoir have erupted in the interval 14 to 0.25 ka. A compelling inference is that mantle-driven magmatic foci have moved repeatedly, allowing abandoned silicic reservoirs, including the formerly vigorous Long Valley magma chamber, to crystallize. A 100-fold decline of intracaldera eruption rate after 650 ka, lack of crystal-poor rhyolite since 300 ka, limited volumes of moat rhyolite (most of it crystal-rich), absence of postcaldera mafic volcanism inside the structural caldera (or north and south adjacent to it), low thermal gradients inside the caldera, and sourcing of hydrothermal underflow within the western array well outside the ring-fault zone all suggest that the Long Valley magma reservoir is moribund.

California↗
USGS2004

Evidence of crustal contamination, sediment, and fluid components in the campanian volcanic rocks

The Campanian Volcanic Subprovince is part of the classic western potassic volcanic province of the Italian Peninsula. The Campanian volcanic products show the effects of shallow assimilation and fractional crystallisation, and the contribution of regional crustal sources (e.g., Hercynian basement-Calabrian crust). The Roccamonfina, Campi Flegrei, and Ventotene volcanic rocks are characterised by wide isotopic and geochemical variations. Such variations appear to reflect both AFC processes and chemical heterogeneity in the upper mantle that may be linked to subduction processes. Mixing curves (Th/Ce-, Ba/K- and Eu/Eu*-143Nd/144Nd) linking sediments and mantle end-members account for the variations in the Campanian Subprovince volcanic rocks with a sediment contribution of 2-10%. The upper mantle sources for the low- and high-K rocks at Roccamonfina have been constrained on the basis of a multi-element normalised diagram. The two sources require different amounts of sediment in the mantle wedge (LK???2% versus HK???10%) and a fluid component probably from altered ocean crust to explain the fluid mobile elements. Low-K Roccamonfina rocks are geochemically similar to those from Campi Flegrei, Ventotene, and Somma-Vesuvius, suggesting a similar proportion of sediment in their upper mantle source regions. ?? 2004 B.V. All rights reserved.

Journal of Volcanology and Geothermal Research↗
USGS2004

Observations of deep long-period (DLP) seismic events beneath Aleutian arc volcanoes; 1989-2002

Between October 12, 1989 and December 31, 2002, the Alaska Volcano Observatory (AVO) located 162 deep long-period (DLP) events beneath 11 volcanic centers in the Aleutian arc. These events generally occur at mid- to lower-crustal depths (10-45 km) and are characterized by emergent phases, extended codas, and a strong spectral peak between 1.0 and 3.0 Hz. Observed wave velocities and particle motions indicate that the dominant phases are P- and S-waves. DLP epicenters often extend over broad areas (5-20 km) surrounding the active volcanoes. The average reduced displacement of Aleutian DLPs is 26.5 cm 2 and the largest event has a reduced displacement of 589 cm 2 (or M L 2.5). Aleutian DLP events occur both as solitary events and as sequences of events with several occurring over a period of 1-30 min. Within the sequences, individual DLPs are often separated by lower-amplitude volcanic tremor with a similar spectral character. Occasionally, volcano-tectonic earthquakes that locate at similar depths are contained within the DLP sequences. At most, Aleutian volcanoes DLPs appear to loosely surround the main volcanic vent and occur as part of background seismicity. A likely explanation is that they reflect a relatively steady-state process of magma ascent over broad areas in the lower and middle portions of the crust. At Mount Spurr, DLP seismicity was initiated by the 1992 eruptions and then slowly declined until 1997. At Shishaldin Volcano, a short-lived increase in DLP seismicity occurred about 10 months prior to the April 19, 1999 eruption. These observations suggest a link between eruptive activity and magma flux in the mid- to lower-crust and uppermost mantle.

Alaska↗
USGS2005

Petrologic constraints on the thermal structure of the Cascades arc

Primitive late Cenozoic basaltic lavas from the Cascades volcanic arc near latitude 46°N comprise two distinct compositional groups. Group I includes samples with low Ba/Nb (<20) and other compositional similarities to oceanic island and MORB lavas from within-plate settings. In contrast, Group II exhibits enrichment of Ba and large-ion lithophile elements (LILE) and depletion of Nb and high-field strength elements (HFSE) as seen commonly in calcalkalic lavas from other volcanic arcs. Lavas of both groups are widely distributed across the transect, and Group I lavas are found as much as 30–40 km trenchward of stratovolcanoes that define the High Cascades ‘volcanic front (VF)’. The most primitive lavas are sparsely porphyritic, have elevated Ni, Cr, and Mg#, high calculated magmatic temperatures (1200–1300 °C), and lack evidence of shallow (crustal level) storage and crystallization. Compositions of parental liquids were calculated for each primitive sample on the premise of Fe–Mg equilibrium with mantle peridotite. Assuming that such magmas ascended rapidly from accumulation zones in the mantle, we estimate P and T of segregation. We infer that (a) Group I magmas ascended from systematically greater depths (∼50–70 km) than Group II (∼30–50 km), implying the possible existence of compositional stratification in the mantle wedge; (b) Group I basalts show the least evidence for slab-derived contributions in their sources despite their apparently greater segregation depths (approaching the locus of the Cascadia slab beneath the frontal arc region); (c) Group II lavas with the strongest slab compositional signature have temperatures far exceeding the wet peridotite solidus at high pressure; and (d) the inferred thermal structure of the mantle wedge is very warm, implying a significant component of mantle upwelling and convection. Group I lavas are interpreted as decompression melts from this mantle, and their compositions suggest that their source was little modified by slab-derived contributions. We speculate that melting to produce Group II magmas occurs in the shallow mantle, possibly in response to heating by hot ascending Group I magmas. If true, it seems unlikely that the slab-like signal in Group II lavas can be attributed to modern slab inputs; rather, we postulate that this signature may reflect melting of lithospheric mantle domains containing a ‘stored’ slab-derived component inherited from earlier stages of Cascadia subduction. This scenario differs from the standard paradigm for subduction zones (SZs), and stresses the importance of convecting asthenospheric mantle in driving arc magmatism, particularly in warm subduction zones where slab fluid contributions likely are minimal. In contrast, because tectonic conditions in more typical volcanic arcs favor subduction of cooler, less dehydrated oceanic lithosphere, slab-derived fluids may promote extensive flux-melting in the wedge. Such melts may dominate the magmatic output and mask wedge contributions. The Cascade arc thus provides rarely afforded insights into arc magma genesis.

British Columbia, California, Oregon, Washington↗
USGS2005

The controlling effect of viscous dissipation on magma flow in silicic conduits

Nearly all volcanic conduit models assume that flow is Newtonian and isothermal. Such models predict that, during high-flux silicic eruptions, gradients in pressure with depth increase upward as magma accelerates and becomes more viscous, leading to extremely low pressure and fragmentation at a depth of kilometers below the surface. In this paper I show that shear heating, also known as viscous dissipation, dramatically reduces the pressure gradient required for flow and concentrates shear in narrow zones along the conduit margin. The reduction in friction may eliminate the zone of low pressure predicted by isothermal models and move the fragmentation level up to the surface.

Journal of Volcanology and Geothermal Research↗