Mechanisms of crustal uplift and subsidence at the Yellowstone caldera,Wyoming

Leveling surveys in 1923, 1976, and each year from 1983 to 1993 have shown that the east-central part of the Yellowstone caldera, near the base of the Sour Creek resurgent dome, rose at an average rate of 14±1 mm/year from 1923 to 1976 and 22±1 mm/year from 1976 to 1984. In contrast, no detectable movement occurred in the same area from 1984 to 1985 (-2±5 mm/year), and from 1985 to 1993 the area subsided at an average rate of 19±1 mm/year. We conclude that uplift from 1923 to 1984 was caused by: (1) pressurization of the deep hydrothermal system by fluids released from a crystallizing body of rhyolite magma beneath the caldera, then trapped beneath a self-sealed zone near the base of the hydrothermal system; and (2) aseismic intrusions of magma into the lower part of the sub-caldera magma body. Subsidence since 1985 is attributed to: (1) depressurization and fluid loss from the deep hydrothermal system, and (2) sagging of the caldera floor in response to regional crustal extension. Future intrusions might trigger renewed eruptive activity at Yellowstone, but most intrusions at large silicic calderas seem to be accommodated without eruptions. Overpressurization of the deep hydrothermal system could conceivably result in a phreatic or phreatomagmatic eruption, but this hazard is mitigated by episodic rupturing of the self-sealed zone during shallow earthquake swarms. Historical ground movements, although rapid by most geologic standards, seem to be typical of inter-eruption periods at large, mature, silicic magma systems like Yellowstone. The greatest short-term hazards posed by continuing unrest in the Yellowstone region are: (1) moderate to large earthquakes (magnitude 5.5–7.5), with a recurrence interval of a few decdes; and (2) small hydrothermal explosions, most of which affect only a small area (<0.01 km²), with a recurrence interval of a few years.     

Caldera subsidence in extensional tectonics

A 45-km-long regional dike was emplaced over a period of 2 weeks in August 2014 at the boundary between the East and North Volcanic Zones in Iceland. This is the first regional dike emplacement in Iceland monitored with modern geophysical networks, the importance of which relates to regional dikes feeding most of the large fissure (e.g., Eldgja 934 and Laki 1783) and lava shield (e.g. early Holocene Skjaldbreidur and Trölladyngja) eruptions. During this time, the dike generated some 17,000 earthquakes, more than produced in Iceland as a whole over a normal year. The dike initiated close to the Bardarbunga Volcano but gradually extended to the northeast until it crossed the boundary between the East Volcanic Zone (EVZ) and the North Volcanic Zone (NVZ). We infer that the strike of the dike changes abruptly at a point, from about N45°E (coinciding with the trend of the EVZ) to N15°E (coinciding with the trend of the NVZ). This change in strike occurs at latitude 64.7°, exactly the same latitude at which about 10 Ma dikes in East Iceland change strike in a similar way. This suggests that the change in the regional stress field from the southern to the northern part of Iceland has been maintained at this latitude for 10 million years. Analytical and numerical models indicate that the dike-induced stress field results in stress concentration around faults and particularly shallow magma chambers and calderas in its vicinity, such as Tungnafellsjökull, Kverkfjöll, and Askja. In particular, the dike has induced high compressive, shear, and tensile stresses at the location of the Bardarbunga shallow chamber and (caldera) ring-fault where numerous earthquakes occurred during the dike emplacement, many of which have exceeded M5 (the largest M5.7). The first segment of the dike induced high tensile stresses in the nearby part of the Bardarbunga magma chamber/ring-fault resulting in radially outward injection of a dike from the chamber at a high angle to the strike of the regional dike. The location of maximum stress at Bardarbunga fluctuates along the chamber/ring-fault boundary in harmony with dike size and/or pressure changes and encourages ring-dike formation and associated magma flow within the chamber. Caldera collapse and/or eruption in some of these volcanoes is possible, most likely in Bardarbunga, but depends largely on the future development of the regional dike.     

Renewed uplift at the Yellowstone Caldera measured by leveling surveys and satellite radar interferometry

 A first-order leveling survey across the northeast part of the Yellowstone caldera in September 1998 showed that the central caldera floor near Le Hardy Rapids rose 24±5 mm relative to the caldera rim at Lake Butte since the previous survey in September 1995. Annual surveys along the same traverse from 1985 to 1995 tracked progressive subsidence near Le Hardy Rapids at an average rate of –19±1 mm/year. Earlier, less frequent surveys measured net uplift in the same area during 1923–1976 (14±1 mm/year) and 1976–1984 (22±1 mm/year). The resumption of uplift following a decade of subsidence was first detected by satellite synthetic aperture radar interferometry, which revealed approximately 15 mm of uplift in the vicinity of Le Hardy Rapids from July 1995 to June 1997. Radar interferograms show that the center of subsidence shifted from the Sour Creek resurgent dome in the northeast part of the caldera during August 1992 to June 1993 to the Mallard Lake resurgent dome in the southwest part during June 1993 to August 1995. Uplift began at the Sour Creek dome during August 1995 to September 1996 and spread to the Mallard Lake dome by June 1997. The rapidity of these changes and the spatial pattern of surface deformation suggest that ground movements are caused at least in part by accumulation and migration of fluids in two sill-like bodies at 5–10 km depth, near the interface between Yellowstone's magmatic and deep hydrothermal systems. Received: 30 November 1998 / Accepted: 16 April 1999     

Neogene crustal emersion and subsidence at the Romanche fracture zone,equatorial Atlantic

Partly phosphatized, oolitic-biogenic limestones were recovered 950–1300 m below sea level from two sites near the crest of a transverse ridge running parallel and adjacent to the Romanche fracture valley (equatorial Atlantic). Some of the biogenic contituents of the limestones (the benthonic foraminiferal genusAmphistegina; corals of the genusStylophora sp., scaphopods, etc.); their paleofacies assemblage (including echinoderms, gastropods, calcareous algae, etc.); and the presence of a well-developed oolitic facies, indicate that the limestones formed in very shallow water close to sea level. In addition, several features of the limestones (including the presence of stromatolite-like laminae, and dissolution features typical of subaerial diagenesis) suggest that the summit of the transverse ridge might have undergone episodes of emersion. The limestones were formed on a shallow carbonate bank at around the Miocene-Pliocene boundary, i.e., 5 ± 1 Myr ago, as determined by the age of fossil planktonic foraminifera and corals. The transverse ridge must have subsided since that time at an average rate of 0.2 mm/yr. It is unlikely that the vertical motions of the Romanche transverse crustal block were caused solely by accreting plate margin- or mantle plume-related volcanic and/or tectonic processes. It is suggested instead that such motions are related to vertical tectonism typical of large oceanic fracture zones.     

Porosity, permeability, and fluid flow in the Yellowstone geothermal system, Wyoming

Cores from two of 13 U.S. Geological Survey research holes at Yellowstone National Park (Y-5 and Y-8) were evaluated to characterize lithology, texture, alteration, and the degree and nature of fracturing and veining. Porosity and matrix permeability measurements and petrographic examination of the cores were used to evaluate the effects of lithology and hydrothermal alteration on porosity and permeability. The intervals studied in these two core holes span the conductive zone and the upper portion of the convective geothermal reservoir. Variations in porosity and matrix permeability observed in the Y-5 and Y-8 cores are primarily controlled by lithology. Y-8 intersects three distinct lithologies: volcaniclastic sandstone, perlitic rhyolitic lava, and non-welded pumiceous ash-flow tuff. The sandstone typically has high permeability and porosity, and the tuff has very high porosity and moderate permeability, while the perlitic lava has very low porosity and is essentially impermeable. Hydrothermal self-sealing appears to have generated localized permeability barriers within the reservoir. Changes in pressure and temperature in Y-8 correspond to a zone of silicification in the volcaniclastic sandstone just above the contact with the perlitic rhyolite; this silicification has significantly reduced porosity and permeability. In rocks with inherently low matrix permeability (such as densely welded ash-flow tuff), fluid flow is controlled by the fracture network. The Y-5 core hole penetrates a thick intracaldera section of the 0.6-Ma Lava Creek ash-flow tuff. In this core, the degree of welding appears to be responsible for most of the variations in porosity, matrix permeability, and the frequency of fractures and veins. Fractures are most abundant within the more densely welded sections of the tuff. However, the most prominent zones of fracturing and mineralization are associated with hydrothermal breccias within densely welded portions of the tuff. These breccia zones represent transient conduits of high fluid flow that formed by the explosive release of overpressure in the underlying geothermal reservoir and that were subsequently sealed by supersaturated geothermal fluids. In addition to this fracture sealing, hydrothermal alteration at Yellowstone appears generally to reduce matrix permeability and focus flow along fractures, where multiple pulses of fluid flow and self-sealing have occurred.     

Chemical and stable isotopic models for boundary creek warm springs, southwestern Yellowstone National Park, Wyoming

Thermal springs of the Boundary Creek hydrothermal system in the southwestern part of Yellowstone Park outside the caldera boundary vary in chemical and isotopic composition, and temperature. The diversity may be accounted for by a combination of processes including boiling of a deep thermal water, mixing of the deep thermal water with cool meteoric water and/or with condensed steam or steam-heated meteoric water, and chemical reactions with surrounding rocks. Dissolved-silica, Na⁺, K⁺ and Ca²⁺ contents of the thermal springs could result from a thermal fluid with a temperature of 200 ± 20°C. Chloride-enthalpy and silica-enthalpy mixing models suggest mixing of 230°C, 220 mg/l Cl⁻ thermal water with cool, low-Cl⁻ components. A 350 to 390°C component with Cl⁻ ≥ 300 mg/l is possibly present in thermal springs inside the caldera but is not required to fit observed spring chemical and isotopic compositions. Irreversible mass transfer models in which a low-temperature water reacts with volcanic glass as it percolates downward and warms, can account for observed pH and dissolved-silica, K⁺, Na⁺, Ca²⁺ and Mg²⁺ concentrations, but produces insufficient Cl⁻ or F⁻ for measured concentrations in the warm springs. The ratio of a_Na/a_H, and Cl⁻ are best accounted for in mixing models. The water-rock interaction model fits compositions of acid-sulfate waters observed at Summit Lake and of low-Cl⁻ waters involved in mixing.The cold waters collected from southwestern Yellowstone Park have δD values ranging from −118 to −145 per mil and δ¹⁸O values of −15.9 to −19.4 per mil. Two samples from nearby Island Park have δD values of −112 and −114 per mil and δ¹⁸O values of −15.1 and −15.3 per mil. All samples of thermal water plot significantly to the right of the meteoric water line. The low Cl⁻ and variable δD values of the thermal waters indicate isotopic compositions are derived by extensive dilution with cold meteoric water and by steam separation on ascent to the surface. Many of the hot springs with higher δD values may contain in addition a significant amount of high-D, low-Cl⁻, acid-sulfate or steam-heated meteoric water. Mixing models, Cl⁻ content and isotopic compositions of thermal springs suggest that 30% or less of a deep thermal component is present. For example, the highest-temperature springs from Three Rivers, Silver Scarf and Upper Boundary Creek thermal areas contain up to 70% cool meteoric water and 30% hot water components, springs at Summit Lake and Middle Boundary Creek spring 57 are acid-sulfate or steam-heated meteoric water; springs 27 and 48 from Middle Boundary Creek and 49 from Mountain Ash contain in excess of 50% acid-sulfate water; and Three Rivers spring 46 and Phillips could result from mixing hot water with 55% cool meteoric water followed by mixing of acid-sulfate water. Extensive dilution by cool meteoric water increases the uncertainties in quantity and nature of the deep meteoric, thermal component.     

Complex Archean lower crustal structure revealed by COCORP crustal reflection profiling in the Wind River Range,Wyoming

A COCORP deep crustal reflection profile across the Wind River uplift crosses exposed Archean rocks and resolves an unusual complex deep crustal structure at a depth of 24–31 km in an area where depth relations in Precambrian rocks can be inferred. The different levels of exposure across the beveled plunge of the Wind River uplift reveal supracrustal rocks at shallower levels with migmatites and pyroxene granulites at deeper levels. For the first time, deep crustal structure from reflection profiling may be interpreted in terms of exposed basement geology. A folded, multilayered deep structure shown by relfection data resembles multiply folded pyroxene granulite interlayered with granitic gneiss exposed in the central Wind River uplift; isoclinal folding is suggested in the folded layered seismic structure. Earlier seismic reflection studies suggested a simpler lower crust. These data indicate that lower crustal structure may have a complexity similar to deeply eroded Precambrian granulite-facies rocks. If this seismic feature represents folded metamorphic rocks, it seems unlikely that this Archean crust could have been thickened by underplating after 2.7 b.y. B.P. and the crust would have to be at least 30 km thich when this structure was formed.     

Recent crustal movement studies at seiyal area, northwest of aswan Lake, Egypt

A local geodetic network of 20 points was established in 1988 around the Seiyal fault for monitoring lateral movements. Two levellinglines, crossing the fault, were prepared for monitoring vertical movements. The initial horizontal geodetic measurements were carried out in March 1989. The measurements were repeated twice a year. The initial vertical geodetic measurements (levelling) were carried out in November 1989 and repeated in November 1990.Four horizontal geodetic measurements and two levelling measurements were analysed. The results revealed remarkable horizontal and vertical changes during the period of measurement.     

Gravity profiles across the Cheyenne Belt, a precambrian crustal suture in southeastern Wyoming

Geologic discontinuities across the Cheyenne Belt of southeastern Wyoming have led to interpretations that this boundary is a major crustal suture separating the Archaean Wyoming Province to the north from accreted Proterozoic island arc terrains to the south. Gravity profiles across the Cheyenne Belt in three Precambrian-cored Laramide uplifts show a north to south decrease in gravity values of 50–100 mgal. These data indicate that the Proterozoic crust is more felsic (less dense) and/or thicker than Archaean crust. Seismic refraction data show thicker crust (48–54 km) in Colorado than in Wyoming (37–41 km). We model the gravity profiles in two ways: 1) thicker crust to the south and a south-dipping ramp in the Moho beneath and just south of the Cheyenne Belt; 2) thicker crust to the south combined with a mid-crustal density decrease of about 0.05 g/cm³. Differences in crustal thickness may have originated 1700 Ma ago because: 1) the gravity gradient is spatially related to the Cheyenne Belt which has been immobile since about 1650 Ma ago; 2) the N-S gradient is perpendicular to the trend of gravity gradients associated with local Laramide uplifs and sub-perpendicular to regional long-wavelength Laramide gradients and is therefore probably not a Laramide feature. Thus, gravity data support the interpretation that the Cheyenne Belt is a Proterozoic suture zone separating terrains of different crustal structure. The gravity “signature” of the Cheyenne Belt is different from “S”-shaped gravity anomalies associated with Proterozoic sutures of the Canadian Shield which suggests fundamental differences between continent-continent and island arc-continent collisional processes.     

Asymmetric,multiple-block collapse at Rotorua Caldera,Taupo Volcanic Zone,New Zealand

Calderas worldwide have been classified according to their dominant collapse styles, although there is a good deal of speculation about the processes involved. Recent laboratory experiments have tried to constrain these processes by modelling magma withdrawal and observing the effects on overlying materials. However, many other factors also contribute to final caldera morphology. Rotorua Caldera formed during the eruption of the Mamaku Ignimbrite. Collapse structure and evolution of Rotorua Caldera is interpreted based its geophysical response, geology and geomorphology, and the stratigraphy of the Mamaku Ignimbrite. Rotorua Caldera is situated at the edge of the extensional Taupo Volcanic Zone, in which major faults strike NE-SW. A second, less dominant fault set strikes NW-SE. These two fault sets have a strong influence on the morphology of Rotorua Caldera. No one style of collapse can be applied to Rotorua Caldera; it was formed during a single eruption, but subsided as many blocks and shows features of trapdoor, piecemeal and downsag types of collapse. Here Rotorua Caldera is described, according to its composition, activity and geometry, as a rhyolitic, single event, asymmetric, multiple-block, single locus collapse structure. The Mamaku Ignimbrite is the only ignimbrite to have erupted from Rotorua Caldera. Extracaldera thickness of the Mamaku Ignimbrite is up to 145 m, whereas inside the caldera it may be greater than 1 km thick. The Mamaku Ignimbrite can be separated into a basal tephra sequence and main ignimbrite sequence. The main ignimbrite sequence contains no observable flow unit boundaries but can be split into lower, middle and upper parts (LMI, mMI, uMI respectively) based on crystal content, welding, jointing, devitrification and vapour phase alteration. Juvenile clasts within the ignimbrite comprise three consanguineous silicic pumice types and andesitic fragments. Only the most evolved pumice type occurs in the basal tephra sequence. All three pumice types occur together throughout the main ignimbrite sequence, whereas the andesitic fragments are only present in uMI. Lithic lag breccias in uMI indicate a late stage of caldera collapse. Concentration of lithic fragments increases towards the middle of the ignimbrite, and may also reflect increased subsidence rate during an earlier stage. Collapse of Rotorua Caldera is thought to have occurred throughout the eruption of the main ignimbrite sequence of the Mamaku Ignimbrite, allowing simultaneous eruption of all the different pumice types and causing the abrupt transition from deposition of the basal tephra sequence to the main ignimbrite sequence.     

Vp/Vs ratios in the Yellowstone National Park region, Wyoming

In this paper we study the variation of V_p/V_s and Poisson's ratio (δ) in the Yellowstone National Park region, using earthquakes which were well recorded by a local seismic network. We find that the average V_p/V_s value within the geothermally active Yellowstone caldera is about 7% lower than in the area outside the caldera. Within the caldera itself there may be a further 2–7% reduction of V_p/V_s in the hydrothermally active Norris Geyser Basin, the Upper and Lower Geyser Basins, and the Yellowstone Lake and Mud Volcano regions. After considering various possible causes for V_p/V_s changes, such as geologic and structural differences, thermal effects, partial melting, and hydrothermal activity, we conclude that the most plausible explanation for the observed V_p/V_s reduction is the presence of hot-water at temperatures and pore-pressures near the water steam transition in the caldera geothermal reservoirs.     

Recent crustal movement studies in Aswan region, Egypt: introduction

After the November 1981 earthquake in the Aswan area along the Kalabsha fault, Nubian Desert, studies on its origin were initiated in the Helwan Institute of Astronomy and Geophysics [now National Research Institute of Astronomy and Geophysics (NRIAG) in Helwan]. As a first step an array of seismic stations were established in cooperation with the Lamont-Doherty Geological Observatory and the correlation between the number of seismic events and the water level in Lake Nasser was investigated. In order to understand the eartqquake mechanism, additional information on the crustal deformation due to water load was required, and the International Center on Recent Crustal Movements (ICRCM) was invited to consult in the necessary procedures aimed at recent crustal movement studies around part of the western bank of Lake Nasser. The first local network of horizontal and vertical geodetic measurements was established and measured in 1984. Simultaneously, with respect to the extreme meteorological conditions in the desert, additional investigations as refraction studies, stability of bench marks etc. were initiated. The extension of the network and an increase of repeated measurements first brought results during the last few years. The results of these studies will be reported in the following papers in detail as a homogenous block of scientific communication.     

Implications of tectonic subsidence models for crustal structure beneath the Mid-Polish Trough

Summary Tectonic subsidence analysis of the preserved and reconstructed stratigraphy of the Polish Basin indicates an initial Late Permian-Early Triassic (255-241 Ma) syn-rift phase of development with a subsequent extensional rejuvenation during the Late Jurassic (ca. 157-152 Ma). Forward modelling of the subsidence data, in view of existing geophysical interpretations which show the presence of a deep Moho and a very high seismic velocity lower crustal layer beneath the Mid-Polish Trough (MPT), suggest that Permo-Mesozoic basin development may be related at least in part to the intrusion of mantle material into and densification of the lower crust rather than exclusively to crustal extension and thinning.     

Recent crustal horizontal movement in the Chinese mainland

IntroductionSupportedbytheNationaIClimbingProject"RecentCrustalMovementandGeodynamicResearch",acrustalmovementmonitoringnetworkdistributedinChinesemainlandhasbeenmeasuredfortWotimesinl994andl996.Thenetconsistsof22stationsthatarelocatedonsev-eralmajortectonicblocksinChinesemainland.ExceptNanchongstationwhichwasdestroyedatsometimebetWeenl994and1996andre-settledinl996,alltheother2lstationswereoccupiedfortwotimes.BasedontheresuItSobtainedfromcarefulpre-processingofGPSobservations,therecent…     

Sulfur geochemistry of hydrothermal waters in Yellowstone National Park, Wyoming, USA. II. Formation and decomposition of thiosulfate and polythionate in Cinder Pool

Cinder Pool is an acid-sulfate-chloride boiling spring in Norris Geyser Basin, Yellowstone National Park. The pool is unique in that its surface is partially covered with mm-size, black, hollow sulfur spherules, while a layer of molten sulfur resides at the bottom of the pool (18 m depth). The sulfur speciation in the pool was determined on four different days over a period of two years. Samples were taken to evaluate changes with depth and to evaluate the importance of the sulfur spherules on sulfur redox chemistry. All analyses were conducted on site using a combination of ion chromatography and colorimetric techniques.Dissolved sulfide (H₂S), thiosulfate (S₂O₃²⁻), polythionates (S_xO₆²⁻), and sulfate were detected. The polythionate concentration was highly variable in time and space. The highest concentrations were found in surficial samples taken from among the sulfur spherules. With depth, the polythionate concentrations dropped off. The maximum observed polythionate concentration was 8 μM. Thiosulfate was rather uniformly distributed throughout the pool and concentrations ranged from 35 to 45 μM. Total dissolved sulfide concentrations varied with time, concentrations ranged from 16 to 48 μM. Sulfate was relatively constant, with concentrations ranging from 1150 to 1300 μM. The sulfur speciation of Cinder Pool is unique in that the thiosulfate and polythionate concentrations are significantly higher than for any other acid-sulfate spring yet sampled in Yellowstone National Park. Complementary laboratory experiments show that thiosulfate is the intermediate sulfoxyanion formed from sulfur hydrolysis under conditions similar to those found in Cinder Pool and that polythionates are formed via the oxidation of thiosulfate by dissolved oxygen. This last reaction is catalyzed by pyrite that occurs as a minor constituent in the sulfur spherules floating on the pool's surface. Polythionate decomposition proceeds via two pathways: (1) a reaction with H₂S, yielding thiosulfate and elemental sulfur; and (2) by disproportionation to sulfate and thiosulfate.This study demonstrates that the presence of a subaqueous molten sulfur pool and sulfur spherules in Cinder Pool is of importance in controlling the pathways of aqueous sulfur redox reactions. Some of the insights gained at Cinder Pool may be relevant to acid crater lakes where sulfur spherules are observed and variations in polythionate concentrations are used to monitor and predict volcanic activity.     

Geochemistry of Los Humeros Caldera,Puebla, Mexico

Geochemistry of Pliocene to recent volcanic rocks from Los Humeros caldera (19°30′ N – 19°50′ N and 97°15° W – 97°35′ W) in East-Central mexico is described. The volcanic rocks from this area seem to represent both alkali and high-alumina basalt series, or both calcalkaline and high-K calc-alkaline sequences. The available bulk-chemical analyses (23 this study and 18 from unpublished literature) show that the entire sequence of rocks from basalts to rhyolites are present in this area. Different degrees of partial melting of the source region followed by extensive shallow-level crystal differentiation seem to have taken place before most volcanic eruptions. These processes are perhaps the most important mechanisms for magma genesis in Los Humeros caldera. Geophysical studies in this area are not sufficient and more detailed geophysical surveys and a better geological interpretation are needed in order to delimit the underlying magma chamber.     

Eruptive history of Fisher Caldera,Alaska, USA

New field, compositional, and geochronologic data from Fisher Caldera, the largest of 12 Holocene calderas in Alaska, provide insights into the eruptive history and formation of this volcanic system. Prior to the caldera-forming eruption (CFE) 9400 years ago, the volcanic system consisted of a cluster of several small (∼3 km³) stratocones, which were independently active between 66±144 and 9.4±0.2 ka. Fisher Caldera formed through a single eruption, which produced a thick dacitic fall deposit and two pyroclastic-flow deposits, a small dacitic flow and a compositionally mixed basaltic-dacitic flow. Thickness and grain-size data indicate that the fall deposit was dispersed primarily to the northeast, whereas the two flows were oppositely directed to the south and north. After the cataclysmic eruption, a lake filled much of the caldera during what may have been a significant quiescent period. Volcanic activity from intracaldera vents gradually resumed, producing thick successions of scoria fall interbedded with lake sediments. Several Holocene stratocones have developed; one of which has had a major collapse event. The caldera lake catastrophically drained when a phreatomagmatic eruption generated a large wave that overtopped and incised the southwestern caldera wall. Multiple accretionary-lapilli-bearing deposits inside and outside the caldera suggest significant Holocene phreatomagmatic activity. The most recent eruptive activity from the Fisher volcanic system was a small explosive eruption in 1826, and current activity is hydrothermal. Late Pleistocene to Holocene magma eruption rates range from 0.03 to 0.09 km³ ky⁻¹ km⁻¹, respectively. The Fisher volcanic system is chemically diverse, ∼48–72 wt.% SiO₂, with at least seven dacitic eruptions over the last 82±14 ka that may have become more frequent over time. Least squares calculations suggest that prior to the CFE, Fisher Volcano products were not derived from a single, large magma reservoir, and were likely erupted from multiple, compositionally independent magma reservoirs. After the CFE, the majority of products appear to have derived from a single reservoir in which magma mixing has occurred.