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W. Hildreth

Publications and source records attributed to W. Hildreth.

At least 19 recordsLinked to original sources
USGS2012

Catastrophic isotopic modification of rhyolitic magma at times of caldera subsidence, Yellowstone Plateau Volcanic Field

The Yellowstone Plateau volcanic field has undergone repeated eruption of rhyolitic magma strongly depleted in 18 O. Large calderas subsided 2.0, 1.3, and 0.6 Ma ago, on eruption of ash flow sheets that represent at least 2500, 280, and 1000 km 3 of zoned magma. More than 60 other rhyolite lavas and tuffs permit reconstruction of the long-term chemical and isotopic evolution of the silicic system. Narrow δ 18 O ranges in the ash flow sheets contrast with wide δ 18 O variations in postcaldera lavas of the first and third caldera cycles. Earliest postcollapse lavas are 3 to 6‰ lighter than the preceding ash flow sheets. The O 18 depletions were short-lived events that immediately followed caldera subsidence; hundreds of cubic kilometers of magma were drastically 18 O depleted and thousands were depleted by 1–2‰. Sequences of postcaldera lavas record partial recovery toward precaldera δ 18 O values; secular trends between collapse events thus reflect gradual reenrichment of the roofmost magma in δ 18 O. Much of the subcaldera reservoir was affected, because lavas that erupted as far apart as 115 km reflect the same pattern of depletion and partial recovery. Contemporaneous extracaldera rhyolites have the highest δ 18 O values in the volcanic field and show no effects of the repeated depletions. Sr and Pb isotope ratios of intracaldera rhyolites jump to more radiogenic values at times of caldera formation and show a longterm zigzag pattern like that of δ 18 O. Although some contamination by foundering roof rocks seenis to be required, water was probably the predominant contaminant. Even if roof rocks had been strongly depleted in O 18 before engulfment, their assimilation would have been far from sufficient to account for the large O 18 shift. The low- O 18 lavas contain no xenocrysts and show no trace element or phenocryst evidence of massive contamination. Their Fe-Ti-oxide temperatures indicate no cooling relative to the caldera-forming ash flow magma, and their whole-rock, glass, and phenoeryst chemistry suggests compositional continuity with the ash flow sequence. Oxygen exchange between the magma and a mass of low-O 18 water greatly exceeding solubility limits may require (1) recurrent explosive activity to sustain access and mixing of water with the magma and (2) convection of the magma reservoir to prevent local saturation.

Journal of Geophysical Research Solid Earth↗
USGS2011

Geophysics at Katmai: Geophysical expedition to Novarupta Volcano, Katmai National Park, Alaska

The great eruption of 1912 in the Aleutian Range of Alaska (Figure 1) is exceptional for both its size and relative simplicity. It was the largest eruption of this century and the largest rhyolitic outburst in almost 20 centuries. The 60-hour, 30-km 3 (ejecta volume) eruption produced extensive fallout deposits, an ash-flow sheet that gave rise to the Valley of Ten Thousand Smokes, and the 3-km-diameter Mt. Katmai caldera. Because magma reached the surface through uniform nonvolcanic basement and the vent underwent little or no collapse (unusual for such a large event), the site provides an ideal target for surface geophysical and subsurface coring exploration of structures and conditions produced by explosive volcanism [Panel on Volcanic Studies at Katmai, 1989]. A project to investigate upper crustal magmatic processes at Katmai is part of the U.S. Continental Scientific Drilling Program [Eichelberger and Hildreth, 1986]. The surface studies phase of this project was undertaken last summer and preliminary results are now emerging.

Alaska↗
USGS2011

Eruptive history of South Sister, Oregon Cascades

South Sister is southernmost and highest of the Three Sisters, three geologically dissimilar stratovolcanoes that together form a spectacular 20km reach along the Cascade crest in Oregon. North Sister is a monotonously mafic edifice as old as middle Pleistocene, Middle Sister a basalt-andesite-dacite cone built between 48 and 14ka, and South Sister is a basalt-free edifice that alternated rhyolitic and intermediate modes from 50ka to 2ka (largely contemporaneous with Middle Sister). Detailed mapping, 330 chemical analyses, and 42 radioisotopic ages show that the oldest exposed South Sister lavas were initially rhyolitic ~50ka. By ~37ka, rhyolitic lava flows and domes (72-74% SiO 2 ) began alternating with radially emplaced dacite (63-68% SiO 2 ) and andesite (59-63% SiO 2 ) lava flows. Construction of a broad cone of silicic andesite-dacite (61-64% SiO 2 ) culminated ~30ka in a dominantly explosive sequence that began with crater-forming andesitic eruptions that left fragmental deposits at least 200m thick. This was followed at ~27ka by growth of a steeply dipping summit cone of agglutinate-dominated andesite (56-60.5% SiO 2 ) and formation of a summit crater ~800m wide. This crater was soon filled and overtopped by a thick dacite lava flow and then by >150m of dacitic pyroclastic ejecta. Small-volume dacite lavas (63-67% SiO 2 ) locally cap the pyroclastic pile. A final sheet of mafic agglutinate (54-56% SiO 2 ) - the most mafic product of South Sister - erupted from and drapes the small (300-m-wide) present-day summit crater, ending a summit-building sequence that lasted until ~22ka. A 20kyr-long-hiatus was broken by rhyolite eruptions that produced (1) the Rock Mesa coulee, tephra, and satellite domelets (73.5% SiO 2 ) and (2) the Devils Chain of ~20 domes and short coulees (72.3-72.8% SiO 2 ) from N-S vent alignments on South Sister's flanks. The compositional reversal from mafic summit agglutinate to recent rhyolites epitomizes the frequently changing compositional modes of the South Sister locus throughout its lifetime. South Sister is part of a reach of the Cascades unusually active in the last 50kyr, characterized by high vent density, N-S vent alignments, and numerous eruptive units of true rhyolite (≥ 72% SiO 2 ) that distinguishes it from much of the Quaternary Cascade arc; these are eruptive expressions of the complex confluence of arc and intraplate magmatic-tectonic regimes.

Journal of Volcanology and Geothermal Research↗
USGS2008

Kaguyak dome field and its Holocene caldera, Alaska Peninsula

Kaguyak Caldera lies in a remote corner of Katmai National Park, 375 km SW of Anchorage, Alaska. The 2.5-by-3-km caldera collapsed ~ 5.8 ± 0.2 ka ( 14 C age) during emplacement of a radial apron of poorly pumiceous crystal-rich dacitic pyroclastic flows (61–67% SiO 2 ). Proximal pumice-fall deposits are thin and sparsely preserved, but an oxidized coignimbrite ash is found as far as the Valley of Ten Thousand Smokes, 80 km southwest. Postcaldera events include filling the 150-m-deep caldera lake, emplacement of two intracaldera domes (61.5–64.5% SiO 2 ), and phreatic ejection of lakefloor sediments onto the caldera rim. CO 2 and H 2 S bubble up through the lake, weakly but widely. Geochemical analyses ( n = 148), including pre-and post-caldera lavas (53–74% SiO 2 ), define one of the lowest-K arc suites in Alaska. The precaldera edifice was not a stratocone but was, instead, nine contiguous but discrete clusters of lava domes, themselves stacks of rhyolite to basalt exogenous lobes and flows. Four extra caldera clusters are mid-to-late Pleistocene, but the other five are younger than 60 ka, were truncated by the collapse, and now make up the steep inner walls. The climactic ignimbrite was preceded by ~ 200 years by radial emplacement of a 100-m-thick sheet of block-rich glassy lava breccia (62–65.5% SiO 2 ). Filling the notches between the truncated dome clusters, the breccia now makes up three segments of the steep caldera wall, which beheads gullies incised into the breccia deposit prior to caldera formation. They were probably shed by a large lava dome extruding where the lake is today.

Alaska↗
USGS2007

Compositional zoning of the Bishop Tuff

Compositional data for >400 pumice clasts, organized according to eruptive sequence, crystal content, and texture, provide new perspectives on eruption and pre-eruptive evolution of the >600 km 3 of zoned rhyolitic magma ejected as the Bishop Tuff during formation of Long Valley caldera. Proportions and compositions of different pumice types are given for each ignimbrite package and for the intercalated plinian pumice-fall layers that erupted synchronously. Although withdrawal of the zoned magma was less systematic than previously realized, the overall sequence displays trends toward greater proportions of less evolved pumice, more crystals (0·5–24 wt %), and higher FeTi-oxide temperatures (714–818°C). No significant hiatus took place during the 6 day eruption of the Bishop Tuff, nearly all of which issued from an integrated, zoned, unitary reservoir. Shortly before eruption, however, the zoned melt-dominant portion of the chamber was invaded by batches of disparate lower-silica rhyolite magma, poorer in crystals than most of the resident magma but slightly hotter and richer in Ba, Sr, and Ti. Interaction with resident magma at the deepest levels tapped promoted growth of Ti-rich rims on quartz, Ba-rich rims on sanidine, and entrapment of near-rim melt inclusions relatively enriched in Ba and CO 2 . Varied amounts of mingling, even in higher parts of the chamber, led to the dark gray and swirly crystal-poor pumices sparsely present in all ash-flow packages. As shown by FeTi-oxide geothermometry, the zoned rhyolitic chamber was hottest where crystal-richest, rendering any model of solidification fronts at the walls or roof unlikely. The main compositional gradient (75–195 ppm Rb; 0·8–2·2 ppm Ta; 71–154 ppm Zr; 0·40–1·73% FeO*) existed in the melt, prior to crystallization of the phenocryst suite observed, which included zircon as much as 100 kyr older than the eruption. The compositions of crystals, though themselves largely unzoned, generally reflect magma temperature and the bulk compositional gradient, implying both that few crystals settled or were transported far and that the observed crystals contributed little to establishing that gradient. Upward increases in aqueous gas and dissolved water, combined with the adiabatic gradient (for the ∼ 5 km depth range tapped) and the roofward decline in liquidus temperature of the zoned melt, prevented significant crystallization against the roof, consistent with dominance of crystal-poor magma early in the eruption and lack of any roof-rind fragments among the Bishop ejecta, before or after onset of caldera collapse. A model of secular incremental zoning is advanced wherein numerous batches of crystal-poor melt were released from a mush zone (many kilometers thick) that floored the accumulating rhyolitic melt-rich body. Each batch rose to its own appropriate level in the melt-buoyancy gradient, which was self-sustaining against wholesale convective re-homogenization, while the thick mush zone below buffered it against disruption by the deeper (non-rhyolitic) recharge that augmented the mush zone and thermally sustained the whole magma chamber. Crystal–melt fractionation was the dominant zoning process, but it took place not principally in the shallow melt-rich body but mostly in the pluton-scale mush zone before and during batchwise melt extraction.

California↗
USGS2007

Pre-eruption recharge of the Bishop magma system

The 650 km 3 rhyolitic Bishop Tuff (eastern California, USA), which is stratigraphically zoned with respect to temperatures of mineral equilibration, reflects a corresponding thermal gradient in the source magma chamber. Consistent with previous work, application of the new TitaniQ (Ti-in-quartz) thermometer to quartz phenocryst rims documents an ∼100 °C temperature increase with chamber depth at the time of eruption. Application of TitaniQ to quartz phenocryst cores, however, reveals lower temperatures and an earlier gradient that was less steep, with temperature increasing with depth by only ∼30 °C. In many late-erupted crystals, sharp boundaries that separate low-temperature cores from high-temperature rims cut internal cathodoluminescent growth zoning, indicating partial phenocryst dissolution prior to crystallization of the high-temperature rims. Rimward jumps in Ti concentration across these boundaries are too abrupt (e.g., 40 ppm across a distance of <10 µm) to have survived magmatic temperatures for more than ∼100 yr. We interpret these observations to indicate heating-induced partial dissolution of quartz, followed by growth of high-temperature rims (made possible by lowering of water activity due to addition of CO 2 ) within 100 yr of the climactic 760 ka eruption. Hot mafic melts injected into deeper parts of the magma system were the likely source of heat and CO 2 , raising the possibility that eruption and caldera collapse owe their origin to a recharge event.

California↗
USGS2006

Abrupt transitions during sustained explosive eruptions: Examples from the 1912 eruption of Novarupta, Alaska

Plinian/ignimbrite activity stopped briefly and abruptly 16 and 45 h after commencement of the 1912 Novarupta eruption defining three episodes of explosive volcanism before finally giving way after 60 h to effusion of lava domes. We focus here on the processes leading to the termination of the second and third of these three episodes. Early erupted pumice from both episodes show a very similar range in bulk vesicularity, but the modal values markedly decrease and the vesicularity range widens toward the end of Episode III. Clasts erupted at the end of each episode represent textural extremes; at the end of Episode II, clasts have very thin glass walls and a predominance of large bubbles, whereas at the end of Episode III, clasts have thick interstices and more small bubbles. Quantitatively, all clasts have very similar vesicle size distributions which show a division in the bubble population at 30 ??m vesicle diameter and cumulative number densities ranging from 107-109 cm-3. Patterns seen in histograms of volume fraction and the trends in the vesicle size data can be explained by coalescence signatures superimposed on an interval of prolonged nucleation and free growth of bubbles. Compared to experimental data for bubble growth in silicic melts, the high 1912 number densities suggest homogeneous nucleation was a significant if not dominant mechanism of bubble nucleation in the dacitic magma. The most distinct clast populations occurred toward the end of Plinian activity preceding effusive dome growth. Distributions skewed toward small sizes, thick walls, and teardrop vesicle shapes are indicative of bubble wall collapse marking maturation of the melt and onset of processes of outgassing. The data suggest that the superficially similar pauses in the 1912 eruption which marked the ends of episodes II and III had very different causes. Through Episode III, the trend in vesicle size data reflects a progressive shift in the degassing process from rapid magma ascent and coupled gas exsolution to slower ascent with partial open-system outgassing as a precursor to effusive dome growth. No such trend is visible in the Episode II clast assemblages; we suggest that external changes involving failure of the conduit/vent walls are more likely to have effected the break in explosive activity at 45 h. ?? Springer-Verlag 2006.

Bulletin of Volcanology↗
USGS2004

Rear-arc vs. arc-front volcanoes in the Katmai reach of the Alaska Peninsula: A critical appraisal of across-arc compositional variation

Physical and compositional data and K-Ar ages are reported for 14 rear-arc volcanoes that lic 11-22 km behind the narrowly linear volcanic front defined by the Mount Katmai-to-Devils Desk chain on the Alaska Peninsula. One is a 30-km3 stratocone (Mount Griggs; 51-63% SiO2) active intermittently from 292 ka to Holocene. The others are monogenetic cones, domes, lava flows, plugs, and maars, of which 12 were previously unnamed and unstudied; they include seven basalts (48-52% SiO2), four mafic andesites (53-55% SiO2), and three andesite-dacite units. Six erupted in the interval 500-88 ka, one historically in 1977, and five in the interval 3-2 Ma. No migration of the volcanic front is discernible since the late Miocene, so even the older units erupted well behind the front. Discussion explores the significance of the volcanic front and the processes that influence compositional overlaps and differences among mafic products of the rear-arc volcanoes and of the several arc-front edifices nearby. The latter have together erupted a magma volume of about 200 km3, at least four times that of all rear-arc products combined. Correlation of Sr-isotope ratios with indices of fractionation indicates crustal contributions in volcanic-front magmas (0.7033-0.7038), but lack of such trends among the rear-arc units (0.70298-0.70356) suggests weaker and less systematic crustal influence. Slab contributions and mantle partial-melt fractions both appear to decline behind the front, but neither trend is crisp and unambiguous. No intraplate mantle contribution is recognized nor is any systematic across-arc difference in intrinsic mantle-wedge source fertility discerned. Both rear-arc and arc-front basalts apparently issued from fluxing of typically fertile NMORB-source mantle beneath the Peninsular terrane, which docked here in the Mesozoic. Springer-Verlag 2004.

Alaska↗
USGS2004

Complex proximal deposition during the Plinian eruptions of 1912 at Novarupta, Alaska

Proximal (<3 km) deposits from episodes II and III of the 60-h-long Novarupta 1912 eruption exhibit a very complex stratigraphy, the result of at least four transport regimes and diverse depositional mechanisms. They contrast with the relatively simple stratigraphy (and inferred emplacement mechanisms) for the previously documented, better known, medial-distal fall deposits and the Valley of Ten Thousand Smokes ignimbrite. The proximal products include alternations and mixtures of both locally and regionally dispersed fall ejecta, and numerous thin complex deposits of pyroclastic density currents (PDCs) with no regional analogs. The locally dispersed component of the fall deposits forms sector-confined wedges of material whose thicknesses halve radially from and concentrically about the vent over distances of 100-300 m (cf. several kilometers for the medial-distal fall deposits). This locally dispersed fall material (and many of the associated PDC deposits) is rich in andesitic and banded pumices and richer in shallow-derived wall-rock lithics in comparison with the coeval medial fall units of almost entirely dacitic composition. There are no marked contrasts in grain size in the near-vent deposits, however, between locally and widely dispersed beds, and all samples of the proximal fall deposits plot as a simple continuation of grain size trends for medial-distal samples. Associated PDC deposits form a spectrum of facies from fines-poor, avalanched beds through thin-bedded, landscape-mantling beds to channelized lobes of pumice-block-rich ignimbrite. The origins of the Novarupta near-vent deposits are considered within a spectrum of four transport regimes: (1) sustained buoyant plume, (2) fountaining with co-current flow, (3) fountaining with counter-current flow, and (4) direct lateral ejection. The Novarupta deposits suggest a model where buoyant, stable, regime-1 plumes characterized most of episodes II and III, but were accompanied by transient and variable partitioning of clasts into the other three regimes. Only one short period of vent blockage and cessation of the Plinian plume occurred, separating episodes II and III, which was followed by a single PDC interpreted as an overpressured "blast" involving direct lateral ejection. In contrast, regimes 2 and 3 were reflected by spasmodic sedimentation from the margins of the jet and perhaps lower plume, which were being strongly affected by short-lived instabilities. These instabilities in turn are inferred to be associated with heterogeneities in the mixture of gas and pyroclasts emerging from the vent. Of the parameters that control explosive eruptive behavior, only such sudden and asymmetrical changes in the particle concentration could operate on time scales sufficiently short to explain the rapid changes in the proximal 1912 products. Springer-Verlag 2003.

Alaska↗
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

Rhyodacites of Kulshan caldera, North Cascades of Washington: Postcaldera lavas that span the Jaramillo

Kulshan caldera (4.5×8 km), at the northeast foot of Mount Baker, is filled with rhyodacite ignimbrite (1.15 Ma) and postcaldera lavas and is only the third Quaternary caldera identified in the Cascade arc. A gravity traverse across the caldera yields a steep-sided, symmetrical, complete Bouguer anomaly of −16 mGal centered over the caldera. Density considerations suggest that the caldera fill, which is incised to an observed thickness of 1 km, may be about 1.5 km thick and is flat-floored, overlying a cylindrical piston of subsided metamorphic rocks. Outflow sheets have been stripped by advances of the Cordilleran Ice Sheet, but the climactic fallout (Lake Tapps tephra) is as thick as 30 cm some 200 km south of the caldera. Ten precaldera units, which range in 40Ar/39Ar age from 1.29 to 1.15 Ma, are dikes and erosional scraps that probably never amounted to a large edifice. A dozen postcaldera rhyodacite lavas and dikes range in age from 1.15 to 0.99 Ma; rhyodacites have subsequently been absent, the silicic reservoir having finally crystallized. At least 60 early Pleistocene intermediate dikes next intruded the caldera fill, helping energize an acid–sulfate hydrothermal system and constituting the main surviving record of an early postcaldera andesite–dacite pile presumed to have been large. Most of the pre- and postcaldera rhyodacites were dated by 40Ar/39Ar or K–Ar methods, and 13 were drilled for remanent magnetic directions. In agreement with the radiometric ages, the paleomagnetic data indicate that eruptions took place before, during, and after the Jaramillo Normal Polarity Subchron, and that one rhyodacite with transitional polarity may represent the termination of the Jaramillo. Most of the biotite–hornblende–orthopyroxene–plagioclase rhyodacite lavas, dikes, and tuffs are in the range 68–73% SiO2, but there were large compositional fluctuations during the 300-kyr duration of the rhyodacite episode. The rhyodacitic magma reservoir was wider (11 km) than the caldera that collapsed into it (8 km).

Washington↗
USGS2003

Eruptive history and geochronology of the Mount Baker volcanic field, Washington

Mount Baker, a steaming, ice-mantled, andesitic stratovolcano, is the most conspicuous component of a multivent Quaternary volcanic field active almost continuously since 1.3 Ma. More than 70 packages of lava flows and ~110 dikes have been mapped, ???500 samples chemically analyzed, and ~80 K-Ar and 40Ar/39Ar ages determined. Principal components are (1) the ignimbrite-filled Kulshan caldera (1.15 Ma) and its precaldera and postcaldera rhyodacite lavas and dikes (1.29-0.99 Ma); (2)~60 intracaldera, hydrothermally altered, andesite-dacite dikes and pods-remnants of a substantial early-postcaldera volcanic center (1.1-0.6 Ma); (3) unaltered intracaldera andesite lavas and dikes, including those capping Ptarmigan and Lasiocarpa Ridges and Table Mountain (0.5-0.2 Ma); (4) the long-lived Chowder Ridge focus (1.29-0.1 Ma)-an andesite to rhyodacite eruptive complex now glacially reduced to ~50 dikes and remnants of ~10 lava flows; (5) Black Buttes stratocone, basaltic to dacitic, and several contemporaneous peripheral volcanoes (0.5-0.2 Ma); and (6) Mount Baker stratocone and contemporaneous peripheral volcanoes (0.1 Ma to Holocene). Glacial ice has influenced eruptions and amplified erosion throughout the lifetime of the volcanic field. Although more than half the material erupted has been eroded, liberal and conservative volume estimates for 77 increments of known age yield cumulative curves of volume erupted vs. time that indicate eruption rates in the range 0.17-0.43 km3/k.y. for major episodes and longterm background rates of 0.02-0.07 km3/k.y. Andesite and rhyodacite each make up nearly half of the 161 ?? 56 km3 of products erupted, whereas basalt and dacite represent only a few cubic kilometers, each representing 1%-3% the total. During the past 4 m.y., the principal magmatic focus has migrated stepwise 25 km southwestward, from the edge of the Chilliwack batholith to present-day Mount Baker.

Geological Society of America Bulletin↗
USGS2002

Magma storage prior to the 1912 eruption at Novarupta, Alaska

New analytical and experimental data constrain the storage and equilibration conditions of the magmas erupted in 1912 from Novarupta in the 20th century's largest volcanic event. Phase relations at H 2 O+CO 2 fluid saturation were determined for an andesite (58.7 wt% SiO 2 ) and a dacite (67.7 wt%) from the compositional extremes of intermediate magmas erupted. The phase assemblages, matrix melt composition and modes of natural andesite were reproduced experimentally under H 2 O-saturated conditions (i.e., P H2O =P TOT ) in a negatively sloping region in T-P space from 930 ° C/100 MPa to 960 ° C/75 MPa with f O 2 ~N NO + 1. The H 2 O-saturated equilibration conditions of the dacite are constrained to a T-P region from 850 ° C/ 50 MPa to 880 ° C/25 MPa. If H 2 O-saturated, these magmas equilibrated at (and above) the level where coerupted rhyolite equilibrated (~100 MPa), suggesting that the andesite-dacite magma reservoir was displaced laterally rather than vertically from the rhyolite magma body. Natural mineral and melt compositions of intermediate magmas were also reproduced experimentally under saturation conditions with a mixed (H 2 O + CO 2 ) fluid for the same range in P H2O . Thus, a storage model in which vertically stratified mafic to silicic intermediate magmas underlay H 2 O-saturated rhyolite is consistent with experimental findings only if the intermediates have X H2O fl=0.7 and 0.9 for the extreme compositions, respectively. Disequilibrium features in natural pumice and scoria include pristine minerals existing outside their stability fields, and compositional zoning of titanomagnetite in contact with ilmenite. Variable rates of chemical equilibration which would eliminate these features constrain the apparent thermal excursion and re-distribution of minerals to the time scale of days.

Alaska↗
USGS1997

The Bishop Tuff: New insights from eruptive stratigraphy

The 0.76 Ma Bishop Tuff, from Long Valley caldera in eastern California, consists of a widespread fall deposit and voluminous partly welded ignimbrite. The fall deposit (F), exposed over an easterly sector below and adjacent to the ignimbrite, is divided into nine units (F1‐F9), with no significant time breaks, except possibly between F8 and F9. Maximum clast sizes are compared with other deposits where accumulation rates are known or inferred to estimate an accumulation time for F1‐F8 as ca. 90 hrs. The ignimbrite (Ig) is divided into chronologically and/or geographically distinct packages of material. Earlier packages (Ig1) were emplaced mostly eastward, are wholly intraplinian (coeval with fall units F2‐F8), Lack phenocrystic pyroxenes, and contain few or no Glass Mountain‐derived rhyolite lithic fragments. Earlier packages (Ig2) were erupted mostly to the north and east, are at least partly intraplinian (interbedded with fall unit F9 to the east), contain pyroxenes, and have lithic fractions rich in Glass Mountain‐derived rhyolite or other lithologies exposed on the northern caldera rim. Recognition of the intraplinian nature of Ig1 east of the caldera and use of the fall deposit chronometry yields accumulation estimates of ca. 25 hrs for an earlier, less‐welded subpackage and ca. 36 hrs for a later, mostly welded subpackage. Average accumulation rates range up to ≥1 mm/s of dense‐welded massive ignimbrite, equivalent to ≥2.5 mm/s of non‐welded material. Comparisons of internal stratification in Ig1 and northern Ig2 lobes suggest the thickest northern ignimbrite accumulated in ≥35 hrs. Identifiable vent positions migrated from an initial site previously proposed in the south‐central part of the caldera (F1‐8, Ig1) in complex fashion; one vent set (for eastern Ig2) migrated east and north toward Glass Mountain, while another set (for northern Ig2) opened from west to east across the northern caldera margin. Vent locations for Ig1 and Ig2 southwest of the caldera have not been identified. The new stratigraphic framework shows that much of the Bishop ignimbrite is intraplinian in nature, and that fall deposits and ignimbrite units previously inferred to be sequential are largely or wholly coeval. Fundamental reassessment is therefore required of all existing models for the eruption dynamics and the nature and causes of pre‐eruptive zonations in trace elements, volatiles, and isotopes in the parental magma chamber.

California↗
USGS1997

Primitive magmas at five Cascade volcanic fields: Melts from hot, heterogeneous sub-arc mantle

Major and trace element concentrations, including REE by isotope dilution, and Sr, Nd, Pb, and O isotope ratios have been determined for 38 mafic lavas from the Mount Adams, Crater Lake, Mount Shasta, Medicine Lake, and Lassen volcanic fields, in the Cascade arc, northwestern part of the United States. Many of the samples have a high Mg# [100Mg/(Mg + FeT) > 60] and Ni content (>140 ppm) such that we consider them to be primitive. We recognize three end-member primitive magma groups in the Cascades, characterized mainly by their trace-element and alkali-metal abundances: (1) High-alumina olivine tholeiite (HAOT) has trace element abundances similar to N-MORB, except for slightly elevated LILE, and has Eu/Eu* > 1. (2) Arc basalt and basaltic andesite have notably higher LILE contents, generally have higher SiO2 contents, are more oxidized, and have higher Cr for a given Ni abundance than HAOT. These lavas show relative depletion in HFSE, have lower HREE and higher LREE than HAOT, and have smaller Eu/Eu* (0.94-1.06). (3) Alkali basalt from the Simcoe volcanic field east of Mount Adams represents the third end-member, which contributes an intraplate geochemical signature to magma compositions. Notable geochemical features among the volcanic fields are: (1) Mount Adams rocks are richest in Fe and most incompatible elements including HFSE; (2) the most incompatible-element depleted lavas occur at Medicine Lake; (3) all centers have relatively primitive lavas with high LILE/HFSE ratios but only the Mount Adams, Lassen, and Medicine Lake volcanic fields also have relatively primitive rocks with an intraplate geochemical signature; (4) there is a tendency for increasing 87Sr/86Sr, 207Pb/204Pb, and ??18O and decreasing 206Pb/204Pb and 143Nd/144Nd from north to south. The three end-member Cascade magma types reflect contributions from three mantle components: depleted sub-arc mantle modestly enriched in LILE during ancient subduction; a modern, hydrous subduction component; and OIB-source-like domains. Lavas with arc and intraplate (OIB) geochemical signatures were erupted close to HAOT, and many lavas are blends of two or more magma types. Pre-eruptive H2O contents of HAOT, coupled with phase-equilibrium studies, suggest that these magmas were relatively dry and last equilibrated in the mantle wedge at temperatures of ~1300 degrees C and depths of ~40 km, virtually at the base of the crust. Arc basalt and basaltic andesite represent greater extents of melting than HAOT, presumably in the same general thermal regime but at somewhat lower mantle separation temperatures, of domains of sub-arc mantle that have been enriched by a hydrous subduction component derived from the young, relatively hot Juan de Fuca plate. The primitive magmas originated by partial melting in response to adiabatic upwelling within the mantle wedge. Tectonic extension in this part of the Cascade arc, one characterized by slow oblique convergence, contributes to mantle upwelling and facilitates eruption of primitive magmas.

Canadian Mineralogist↗
USGS1997

Complexities of plinian fall deposition at vent: An example from the 1912 Novarupta eruption (Alaska)

An extremely proximal ejecta ring, with exposures to within 100 m of vent, was deposited during later-stage plinian fall activity during the 1912 Novarupta eruption in Alaska. One bed in the ejecta ring (bed S) contains predominantly andesitic clasts which serve to delineate the striking contrast in thinning rates along dispersal axis of the ejecta ring [Pyle bt values of 70 m (bed S alone) or 190 m (whole ejecta ring)] and the coeval dacitic plinian fall deposits [Pyle bt, values of 4 km (proximal) and 37 km (medial-distal)]. The locally deposited andesitic and dacitic clasts of the ejecta ring are interpreted as products of an irregular 'collar' of low-fountaining ejecta partially sheathing the core of higher-velocity dacitic ejecta that fed the stable, convecting 23-km-high column. The presence of such an extremely proximal accumulation of ejecta appears to be a feature common to several other historic eruptions that generated widespread fall deposits. This feature in part accounts for conflicts between measured and calculated values for thickness maxima in plinian fall deposits and suggests that modifications may be required of existing models for plinian eruption columns.

Journal of Volcanology and Geothermal Research↗
USGS1996

Kulshan caldera: A quaternary subglacial caldera in the North Cascades, Washington

Calderas that collapse during large pyroclastic eruptions are anomalously rare in the Cascade arc. Recognition of the early Pleistocene 4.5 × 8 km Kulshan caldera, filled with rhyodacite ignimbrite at the northeast foot of Mount Baker, brings to only three the Quaternary calderas identified in the Cascades. A near-vertical ring fault cut in basement rocks of the North Cascades encloses 30 km 2 of intracaldera ignimbrite (and intermixed collapse breccia) >1 km thick but with no floor exposed. The Lake Tapps tephra in the Puget lowland is the correlative fallout; 200 km from the source, it is as thick as 30 cm. Features of the distal ash fall and the intracaldera tuff suggest large-scale phreatomagmatism during an eruption that may have started subglacially. Several advances of the Cordilleran ice sheet subsequently obliterated the topographic rim, removed every vestige of extracaldera ignimbrite and proximal fallout, and stripped any precaldera extrusive rocks-the former existence of which is suggested only by a few silicic intrusions that cut the circumcaldera basement. Although the caldera is not structurally resurgent, several early intracaldera rhyodacite lavas intrude and rest directly on ignimbrite or on ashy caldera-lake sediments reworked from the eruption products. Subsidence areas, pumice compositions, and volumes of magma erupted (>50 km 3 ) are similar for the Kulshan, Rockland, and Crater Lake (Mazama) events, the three Quaternary caldera-forming eruptions now recognized in the Cascades.

Geological Society of America Bulletin↗