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Volcano Watch — The 1969 Maunaulu eruption: 12 lava fountaining episodes

Volcano Watch — The 1969 Maunaulu eruption: 12 lava fountaining episodes

Photo & Video Chronology — January 12, 2026 — Kīlauea episode 40

Photo & Video Chronology — January 12, 2026 — Kīlauea episode 40

Volcano Watch — Hau’oli Makahiki Hou: a round-up of fireworks from Kīlauea’s fountains

Volcano Watch — Hau’oli Makahiki Hou: a round-up of fireworks from Kīlauea’s fountains

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The magmatic-hydrothermal system of the Three Sisters volcanic cluster, Oregon, imaged from field gravity measurements The magmatic-hydrothermal system of the Three Sisters volcanic cluster, Oregon, imaged from field gravity measurements

From 2019 to 2024, gravity surveys were conducted at the Three Sisters volcanic cluster (TSVC), measuring 246 gravity sites using a spring relative gravimeter. We calculated the residual Bouguer anomaly and identified three main zones with negative anomalies, ranging from −4 to −8 mGal, located southwest and west of South Sister, within an area that has been uplifting for the past two...
Authors
Helene Le Mevel, Nathan Lee Andersen, Annika E. Dechert, Josef Dufek
By
Volcano Hazards Program, Volcano Science Center

The anatomy and lethality of the Siberian Traps large igneous province The anatomy and lethality of the Siberian Traps large igneous province

Emplacement of the Siberian Traps large igneous province (LIP) around 252 Ma coincided with the most profound environmental disruption of the past 500 million years. The enormous volume of the Siberian Traps, its ability to generate greenhouse gases and other volatiles, and a temporal coincidence with extinction all suggest a causal link. Patterns of marine and terrestrial extinction...
Authors
Seth D. Burgess, Benjamin A. Black
By
Volcano Hazards Program, Volcano Science Center

Mitigation of human cognitive bias in volcanic eruption forecasting Mitigation of human cognitive bias in volcanic eruption forecasting

Modern operational eruption forecasting methods rely heavily on human judgment in the face of uncertainty and are thus susceptible to myriad cognitive biases and errors by the scientist-forecasters. Recent developments in the behavioral sciences have elucidated cognitive biases across a wide spectrum of human behaviors and found ways to mitigate them. These insights have led to...
Authors
Heather M. Wright, J. D. Pesicek, Stephen A. Spiller
By
Volcano Hazards Program, Volcano Science Center
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Evidence for Fluid Saturation and Degassing

One of the important conclusions from several studies of MI has been that silicic magmas are often saturated with a free fluid (volatile) phase prior to eruption. Three primary lines of evidence from MI have supported this conclusion; 1) mass-balance constraints, 2) buffered of dissolved volatile concentrations and 3) inclusions of non-silicate fluids such as vapors and hypersaline melts.

MASS-BALANCE CONSTRAINTS

Commonly, volcanologists have estimated volatile fluxes during eruptions by determining the difference in the concentrations of elements such as Cl and S in early-formed MI and the outgassed matrix glass adhering to the outside of erupted phenocrysts (e.g., Palais & Sigurdsson 1989). This technique (commonly called the 'petrologic method') implicitly assumes that the only source of volatiles is from degassing silicate melt and that all volatiles are dissolved in the melt at the time of MI entrapment. Combined with an estimate for the volume of erupted magma, this allows calculation of the mass of volatiles released from degassing melt. The development of satellite-based monitors of atmospheric chemistry permits an independent (and more direct) means of estimating volcanic gas flux (Symonds et al. 1994). During the 1991 eruption of Mt. Pinatubo, Philippines, satellites recorded a S flux orders of magnitude greater than that predicted by analysis of MI. Westrich and Gerlach (1992) found that the S concentrations in MI from Pinatubo were nearly identical to those in outgassed matrix, and that only a trivial amount of the erupted S could be attributed to degassing of melt subsequent to MI entrapment. They concluded that accumulated vapor accounted for most of the erupted S and estimated that it composed 2 to 5 vol% of the magma at the time the MI were trapped. Other potential S sources such as geothermal fluids, S-bearing crystals and unerupted magma were assessed as unlikely (also see Wallace & Gerlach 1994, Gerlach et al., accepted). Similar findings were reported for Redoubt Volcano, Alaska and Mt. St. Helens, Washington (Gerlach et al. 1994, Gerlach & McGee 1994). The studies of Gerlach and colleagues highlight the fact that MI provide direct information on the melt phase, but not the entire magma (Table 1; item II.1).

 

FLUID SOLUBILITY AND BUFFERING PATHS

The concentrations of volatiles in MI can be used to estimate the depth at which fluid should exsolve from a magma and to estimate the composition of the evolved fluid. A saturation pressure is the minimum pressure at which a melt could have been trapped without being fluid-saturated. It can be calculated by incorporating volatile solubility models for the composition of interest as a function of temperature and pressure (Holloway & Blank 1994). Figure 8 illustrates the use of MI to gain insight into an ore-bearing system, the Pine Grove porphyry Mo system in Utah (Lowenstern, 1994b). Extrusive rhyolites were used as a proxy for coeval and comagmatic porphyries that host high-grade MoS2 ore (Keith et al. 1986). Lowenstern (1994b) analyzed thirty MI (black dots) for CO2 and H2O by FTIR spectroscopy. Isobars represent the range of compositions of melts in equilibrium with H2O-CO2 fluids at a given pressure. For each isobar, the mole fraction (X) of H2O in the fluid phase decreases from one (H2O saturation) on the x axis to zero (CO2 saturation) on the y axis (as H2O and CO2 are completely miscible under these conditions). As indicated in Figure 8, all analyzed melts would be saturated with fluid if allowed to ascend to pressures less than ~250 MPa, and several would have been fluid-saturated at pressures over 400 MPa. The data set were explained by entrapment of inclusions during decompression degassing, wherein H2O and CO2 vary due to partitioning of volatiles from melt to vapor during magma ascent. The high saturation pressures implied that the tuff of Pine Grove began degassing at ~16 km depth (430 MPa), about 12 km below the depth at which the associated porphyries were emplaced. The magma therefore must have been quite vesicular by the time it reached the level of porphyry emplacement (20 to 40% vesicularity). Lowenstern (1994b) suggested that the decrease in magma density and increase in porosity associated with ascent and degassing are critical elements of the mineralizing process in porphyry Mo systems (see also Shinohara & Kazahaya, 1995). Moreover, the low Mo (<5 ppm) and high H2O and CO2 concentrations in the MI were used as evidence that porphyry Mo deposits are not formed by shallow, crystallization-induced degassing of metal-rich magmas but by streaming of exsolved volatiles from a deep chamber of more normal magma (Lowenstern 1994b, Keith & Shanks 1988).

FIG. 8. Solubility plot for the system H2O-CO2-rhyolite at 675 °C. Small filled circles (with 2s errors) denote compositions of analyzed melt inclusions from Plinian unit of tuff of Pine Grove (Lowenstern 1994b). Lines labeled in units of pressure (MPa) show isobaric solubility of H2O and CO2 as a function of fluid composition-- their intersection with the y and x axes give CO2 and H2O solubility, respectively, all other points being mixtures (isopleths not plotted). Labeled trends display the effects of open and closed-system decompressional degassing, and isobaric fluid-saturated degassing (starting from the filled square), as well as fluid-absent crystallization (at P >440 MPa: ~16 km lithostatic) starting from the large filled circle. Data are most consistent with entrapment of MI along a degassing trend, during ascent of the magma, prior to eruption. Figure from Lowenstern (1994b).

Saturation plots such as Fig. 8 were first used by Anderson et al. (1989) and Skirius (1990), who measured H2O and CO2 concentrations in MI from the Bishop tuff and found that their compositions were consistent with isobaric, vapor-saturated crystallization prior to eruption (similar trend shown on Fig.8). Wallace et al. (1994) plotted CO2 versus the incompatible element U to calculate the amount of isobaric fluid-saturated crystallization prior to eruption of the Bishop tuff. This allowed them to estimate the mass of exsolved fluid in the pre-eruptive magma chamber. For the 1991 Pinatubo eruption, Wallace & Gerlach (1994) analyzed MI and found that the amounts of dissolved H2O and CO2 were sufficiently high that the magma had to crystallize at pressures � 250 MPa. This pressure agrees with that estimated for the Pinatubo magma reservoir by a variety of experimental, geophysical, and mineralogical criteria (Gerlach et al., accepted).

Besides H2O and CO2, other volatiles have been used as evidence for saturation with a fluid phase. Lowenstern (1993) analyzed MI and matrix glass from the 1912 eruption at the Valley of 10,000 Smokes and found that Cl, S, and Cu appeared to have their concentrations buffered by non-crystalline phases (i.e., exsolved fluid). Metrich & Clocchiatti (1989) found evidence for two-stage degassing of the magma erupted in 1763 at Etna volcano in Sicily. An earlier degassing stage recorded in the varying compositions of silicate MI, and a later one that resulted in loss of S (though little Cl) from the bulk rocks during eruption. Stix et al. (1993) showed that MI compositions recorded a loss of H2O, S, and Cl during shallow degassing of magma from Galeras Volcano in Colombia. The degassing was accompanied by continued crystallization of the magma, fluid release into a confined space and resulting explosive activity observed from 1988 to 1993.

Sometimes, the fluid phase can unmix to form two fluids in equilibrium with the melt phase (e.g., vapor and hypersaline liquid for the NaCl-H2O system). This can cause the concentration of Cl to be fixed for a given pressure and temperature (Shinohara 1994; Candela & Piccoli, 1995). Layne & Stix (1991) noted that the concentrations of Cl in melt inclusions could be used to deduce which lavas from the Valles caldera had been saturated with both a vapor and hypersaline brine. Similarly, Lowenstern (1994a) observed that peralkaline rhyolites (pantellerites) from Pantelleria, Italy have melt Cl concentrations consistent with saturation with both vapor and a hypersaline phase.

 

TRAPPED VAPORS AND HYPERSALINE LIQUIDS

Melt inclusions also yield direct evidence on fluids in magmatic systems. Mixed inclusions may contain any combination of immiscible phases present at the time of crystal growth. Along with silicate melt, these inclusions may contain sulfide and hypersaline liquids, vapor or other crystals. Some criteria for identifying mixed inclusions are introduced above (subsection on Other Origins of Bubbles). If the different phases can be identified and differentiated, they can provide direct information on the compositions of immiscible phases within the magmatic system. Roedder (1992) gave a lengthy discussion of the significance of immiscibility in magmatic systems and reviewed recent works that describe mixed inclusions in volcanic and intrusive rocks.

Primarily two kinds of non-silicate magmatic fluid are found in mixed MI: hypersaline liquid (>~40 wt.% NaCl equiv.) and low-density, low-salinity vapors. Roedder & Coombs (1967) and Roedder (1972) found the former in granitic nodules ejected during eruptions of peralkaline trachytes from Ascension Island. More recently, several authors have described salt clusters in MI within quartz and feldspar from the peralkaline rhyolites from Pantelleria, Italy. When heated to magmatic temperatures, these clusters melt to form hydrosaline liquid that is immiscible with the silicate melt (Clocchiatti et al. 1990; Solovova et al. 1991; Lowenstern 1994a) and coexisting vapor (Solovova et al. 1991; Lowenstern 1994a). Remnants of the hypersaline fluid were found in outgassed matrix glass (Kovalenko et al. 1993; Lowenstern 1994a). De Vivo & Frezzotti (1994) summarized evidence for immiscible fluid phases in Italian subvolcanic systems and discussed their significance.

Other workers have discovered evidence for entrapment of mixed inclusions of vapor and silicate melt. Roedder (1965) described CO2-rich bubbles (with liquid CO2 at room temperature) coexisting with basaltic glass in melt inclusions from phenocrysts in basalts (19 different localities) and their associated ultramafic xenoliths (72 localities). Densities of these inclusions indicated entrapment at depths from 10 to 15 km. Frezzotti et al. (1991) corroborated the work of Metrich & Clocchiatti (1989; described above) by finding two generations of CO2-rich vapors trapped along with silicate melt in mixed inclusions within phenocrysts as well as ejected ultramafic xenoliths. Similar mixed inclusions of vapor and silicate melt have been described in silicic systems. Pasteris et al. (accepted) used Raman spectroscopy to study vapor bubbles in MI from Pinatubo that contained high CO2 pressures. The bubbles were too CO2-rich to have formed during cooling and shrinkage of the MI and the authors concluded that the inclusions trapped both silicate melt and an immiscible CO2-bearing vapor. Subsequent to entrapment, the inclusions partly leaked, but still retained significant CO2.

Immiscible fluids trapped in MI can contain evidence for the volatility of ore-forming metals in magmatic systems (see also Candela & Piccoli 1995; Bodnar 1995). Fig. 9 (A, B) shows an MI believed to have formed as a two-phase mixed inclusion (melt + fluid) within a quartz phenocryst of peralkaline rhyolite (Lowenstern et al. 1991). The inclusion contained a large bubble with abundant Cu, S and Cl mineralization on its wall (Fig. 9C, D). Only a minority of MI from these rhyolites contained the Cu-rich bubbles. Using IR spectroscopy on the same samples, Aines et al. (1990) found that large, Cu-bearing bubbles contained CO2 anomalies. Both studies concluded that the anomalous CO2 and Cu is some samples was due to coeval entrapment of silicate melt and coexisting Cu-bearing fluid. Plausibly, the Cu could have come from a small droplet of immiscible hydrosaline liquid (Lowenstern 1994a) trapped along with a CO2-bearing vapor bubble in the MI. For the 1912 eruption at the Valley of 10,000 Smokes, Lowenstern (1993) also concluded that MI with large Cu-bearing bubbles represented mixed inclusions of exsolved fluid and melt. He used the concentration of Cu in the bubbles and coexisting glass to estimate fluid/melt partition coefficients.

FIG. 9. Features within vapor bubble in a single pantellerite MI (from Lowenstern et al. 1991). (A) Transmitted-light photomicrograph of a large bubble (b) within pantellerite glass inclusion (g) in quartz (q). The sample surface is mottled due to the Ar-ion milling technique used to expose the bubble to air. (B) Bubble, now exposed, as imaged by backscattered-electron imaging. The glass is lighter in color because its mean atomic weight is higher than the quartz. Note the bright speck within the bubble. (C) Copper- (Cu) and Cl- (h) rich mineralization within the bubble, as seen by secondary-electron imaging. (D) Close-up of the Cu-mineralization within vapor bubble. Lowenstern et al. (1991) used a variety of criteria to interpret that the Cu-rich bubbles formed by heterogeneous entrapment of pantellerite melt and a Cu-bearing volatile phase (see text).

The origins of some inclusions have proved more difficult to explain. Naumov et al. (1991) described rhyolites with quartz crystals containing presumed coeval melt and fluid inclusions. The fluid inclusions were dense (specific gravity = ~0.9 g/cm3) and H2O-dominated, implying extremely high entrapment pressures (over 600 MPa) if the inclusions were trapped at magmatic temperatures. The silicate MI contained only 3 or 4 wt.% H2O, only one-third of that expected for a silicic melt saturated with a H2O-rich phase at over 600 MPa. Figure 1f shows another puzzling inclusion type found in quartz from intrusive and extrusive units of the Pine Grove system of Utah (Keith et al. 1986). These 'empty' inclusions are located within the same growth zones as MI, are not intersected by capillaries or fractures, yet contain no detectable gas, liquid H2O, or mineralization other than minor Fe and Ca-bearing minerals along the inclusion walls (J.B. Lowenstern, unpublished). Because they are so different from hourglass inclusions from the same deposit (they contain NO glass or crystallized melt), they do not seem to have formed from leakage of silicate MI. Though they lack obvious fractures or capillaries, they are interpreted to be leaked inclusions of magmatic fluid. Gutmann (1974) found similar tubular voids in labradorite phenocrysts from Sonora, Mexico.

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Volcano Hazards Program

Find U.S. Volcano

There are about 170 potentially active volcanoes in the U.S. The mission of the USGS Volcano Hazards Program is to enhance public safety and minimize social and economic disruption from volcanic unrest and eruption through our National Volcano Early Warning System. We deliver forecasts, warnings, and information about volcano hazards based on a scientific understanding of volcanic behavior.

Learn more about NVEWS

Volcano Activity Notifications

Ash-rich plume rises out of Halemaʻumaʻu Crater, Kilauea Volcano Hawaiʻi

USGS Volcano Observatories release regular updates and notifications to communicate increases or decreases in volcanic activity and explain unusual or hazardous circumstances. 

Get Latest Updates

Volcano Hazards Assessments

Mount Rainier seen from Puyallup, Washington

Long-term hazards assessments identify locations at risk from volcanic hazards. They are released as high-resolution hazards-zonation maps with accompanying explanations, which help guide eruption preparedness plans.

View Assessments

U.S. Volcano Observatories

Volcano Observatory staff monitor, research, and issue formal notices of activity for volcanoes in assigned geographic areas. Scientists also assess volcano hazards and work with communities to prepare for volcanic eruptions.

Learn More

News

Volcano Watch — The 1969 Maunaulu eruption: 12 lava fountaining episodes

Volcano Watch — The 1969 Maunaulu eruption: 12 lava fountaining episodes

Photo & Video Chronology — January 12, 2026 — Kīlauea episode 40

Photo & Video Chronology — January 12, 2026 — Kīlauea episode 40

Volcano Watch — Hau’oli Makahiki Hou: a round-up of fireworks from Kīlauea’s fountains

Volcano Watch — Hau’oli Makahiki Hou: a round-up of fireworks from Kīlauea’s fountains

View All

Publications

The magmatic-hydrothermal system of the Three Sisters volcanic cluster, Oregon, imaged from field gravity measurements The magmatic-hydrothermal system of the Three Sisters volcanic cluster, Oregon, imaged from field gravity measurements

From 2019 to 2024, gravity surveys were conducted at the Three Sisters volcanic cluster (TSVC), measuring 246 gravity sites using a spring relative gravimeter. We calculated the residual Bouguer anomaly and identified three main zones with negative anomalies, ranging from −4 to −8 mGal, located southwest and west of South Sister, within an area that has been uplifting for the past two...
Authors
Helene Le Mevel, Nathan Lee Andersen, Annika E. Dechert, Josef Dufek
By
Volcano Hazards Program, Volcano Science Center

The anatomy and lethality of the Siberian Traps large igneous province The anatomy and lethality of the Siberian Traps large igneous province

Emplacement of the Siberian Traps large igneous province (LIP) around 252 Ma coincided with the most profound environmental disruption of the past 500 million years. The enormous volume of the Siberian Traps, its ability to generate greenhouse gases and other volatiles, and a temporal coincidence with extinction all suggest a causal link. Patterns of marine and terrestrial extinction...
Authors
Seth D. Burgess, Benjamin A. Black
By
Volcano Hazards Program, Volcano Science Center

Mitigation of human cognitive bias in volcanic eruption forecasting Mitigation of human cognitive bias in volcanic eruption forecasting

Modern operational eruption forecasting methods rely heavily on human judgment in the face of uncertainty and are thus susceptible to myriad cognitive biases and errors by the scientist-forecasters. Recent developments in the behavioral sciences have elucidated cognitive biases across a wide spectrum of human behaviors and found ways to mitigate them. These insights have led to...
Authors
Heather M. Wright, J. D. Pesicek, Stephen A. Spiller
By
Volcano Hazards Program, Volcano Science Center
View All
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