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At least 523 records · Page 29Linked to original sources
USGS2001

Progression of natural attenuation processes at a crude-oil spill site . I. Geochemical evolution of the plume

A 16-year study of a hydrocarbon plume shows that the extent of contaminant migration and compound-specific behavior have changed as redox reactions, most notably iron reduction, have progressed over time. Concentration changes at a small scale, determined from analysis of pore-water samples drained from aquifer cores, are compared with concentration changes at the plume scale, determined from analysis of water samples from an observation well network. The small-scale data show clearly that the hydrocarbon plume is growing slowly as sediment iron oxides are depleted. Contaminants, such as ortho -xylene that appeared not to be moving downgradient from the oil on the basis of observation well data, are migrating in thin layers as the aquifer evolves to methanogenic conditions. However, the plume-scale observation well data show that the downgradient extent of the Fe 2+ and BTEX plume did not change between 1992 and 1995. Instead, depletion of the unstable Fe (III) oxides near the subsurface crude-oil source has caused the maximum dissolved iron concentration zone within the plume to spread at a rate of approximately 3 m/year. The zone of maximum concentrations of benzene, toluene, ethylbenzene and xylene (BTEX) has also spread within the anoxic plume. In monitoring the remediation of hydrocarbon-contaminated ground water by natural attenuation, subtle concentration changes in observation well data from the anoxic zone may be diagnostic of depletion of the intrinsic electron-accepting capacity of the aquifer. Recognition of these subtle patterns may allow early prediction of growth of the hydrocarbon plume.

Journal of Contaminant Hydrology↗
USGS2004

Gas evolution in eruptive conduits: Combining insights from high temperature and pressure decompression experiments with steady-state flow modeling

In this paper we examine the consequences of bubble nucleation mechanism on eruptive degassing of rhyolite magma. We use the results of published high temperature and pressure decompression experiments as input to a modified version of CONFLOW, the numerical model of Mastin and Ghiorso [(2000) U.S.G.S. Open-File Rep. 00-209, 53 pp.] and Mastin [(2002) Geochem. Geophys. Geosyst. 3, 10.1029/2001GC000192] for steady, two-phase flow in vertical conduits. Synthesis of the available experimental data shows that heterogeneous nucleation is triggered at ΔP <5–20 MPa in water-saturated rhyolite and leads to equilibrium degassing through a discrete nucleation event. Typically 10 5 –10 7 bubbles/cm 3 are produced which evolve Gaussian bubble size distributions. Homogeneous nucleation requires ΔP >120–150 MPa, and leads to disequilibrium degassing at extreme H 2 O supersaturation. In this latter case, nucleation is an ongoing process controlled by changing supersaturation conditions. Exponential bubble size distributions are often produced with number densities of 10 6 –10 9 bubbles/cm 3 . Our numerical analysis adopts an end-member approach that specifically compares equilibrium degassing with delayed, disequilibrium degassing characteristic of homogeneously-nucleating systems. The disequilibrium simulations show that delaying nucleation until ΔP =150 MPa restricts degassing to within ∼1500 m of the surface. Fragmentation occurs at similar porosity in both the disequilibrium and equilibrium modes (∼80 vol%), but at the distinct depths of ∼500 m and ∼2300 m, respectively. The vesiculation delay leads to higher pressures at equivalent depths in the conduit, and the mass flux and exit pressure are each higher by a factor of ∼2.0. Residual water contents in the melt reaching the vent are between 0.5 and 1.0 wt%, roughly twice that of the equilibrium model.

Journal of Volcanology and Geothermal Research↗
USGS2020

Systematics, evolution, and genetics of bears

Molecular genetics are key to understanding current and historical relationships between isolated populations, including species’ colonizations during glacial–interglacial cycles, to determine viability of local populations, needs for habitat corridors, and other aspects of population management, especially where bears are harvested for sport, etc. As natural habitats shrink, some bear species will inevitably require high levels of management, perhaps combining captive and wild populations following the IUCN’s One Plan Approach. In this chapter we review the systematics of the Ursidae and its relationships with other Carnivora, the molecular phylogenetic of extant ursid species, the phylogeography of and morphological variation within each species, and the use of molecular genetics to monitor bear populations for management and conservation.

Book chapter↗
USGS2023

Climate-driven mid- to late Holocene hydrologic evolution of arid wetlands documented by strontium, uranium, and oxygen isotopes from Lower Pahranagat Lake, southern Nevada, USA

Lacustrine carbonates in a 12.4-m-long core from Lower Pahranagat Lake (LPAH), southern Nevada, indicate that radiogenic isotopes of Sr and U ( 87 Sr/ 86 Sr and 234 U/ 238 U) preserve evidence of past variations in water sources and evolving hydrologic conditions. Sr and U isotope compositions in LPAH carbonates fall within the range defined by the three primary groundwater sources in Pahranagat Valley and reflect variable mixtures of those sources since the mid-Holocene. Compositions in the oldest sample (5.78 ka) closely match modern compositions of modern discharge from nearby springs, indicating that LPAH water was derived almost exclusively from the local volcanic aquifer. By ca. 5.3–5.2 ka, LPAH water compositions shifted sharply towards isotopic compositions observed in groundwater from the regional carbonate aquifer, indicating a marked increase in surface flow from high-volume springs discharging from the carbonate aquifer to the north. Sediments deposited between 3.08–1.06 ka indicate reduced contributions from the regional aquifer. A comparison of uranium- and oxygen-isotope values in LPAH carbonates suggests that wetter climate conditions favor increased supply from deeper, regional carbonate aquifers compared to drier conditions when contributions from shallower, local volcanic aquifers were more important.

Nevada↗
USGS2017

Late Cretaceous inoceramid bivalves of the Kuskokwim Basin, southwestern Alaska, and their implications for basin evolution

Upper Cretaceous rocks of the Kuskokwim Group are exposed in a large region of southwestern Alaska and are mainly composed of deformed turbidite deposits that contain few fossils other than inoceramid bivalves. This paper documents the taxonomy of the inoceramids in the Kuskokwim Group, develops an inoceramid biostratigraphy based on known ranges in other regions, and analyzes biogeographic patterns, paleoecology, and depositional history of the Kuskokwim Group. Most of the inoceramid bivalves present in the Kuskokwim Group are of Cenomanian and Turonian age, and an assemblage of species typical of late Turonian age rocks is particularly well developed. Only two localities appear to be as young as Santonian age. The following 16 species or subspecies are discussed and illustrated in detail: Birostrina tamurai Matsumoto and Noda, Inoceramus virgatus Schlüter, I. pennatulus Pergament, I. pictus minus Matsumoto, I. cf. I. yabei Nagao and Matsumoto, I. ? sp. aff. I. costatus Nagao and Matsumoto, I. hobetsensis Nagao and Matsumoto, I. longealatus Tröger, I. frechi Flegel, I. waltersdorfensis waltersdorfensis Andert, I. cf. I. waltersdorfensis hannovrensis Heinz, I. kuskokwimensis n. sp., Mytiloides cf. M. opalensis (Böse), M. teraokai (Matsumoto and Noda), M. cf. M. incertus (Jimbo), and Sphenoceramus naumanni (Yokoyama). In addition, a specimen with affinities to Mytiloides striatoconcentricus carpathicus (Simionescu) and a specimen that may belong to the I. (Cremnoceramus?) rotundatus–I. ( C .) erectus lineage are illustrated. Most of the taxa present in the Kuskokwim region are found in other regions of the North Pacific, particularly Japan and eastern Siberia, or are found throughout the Northern Hemisphere. Only one species, I. kuskokwimensis n. sp., is new and may be endemic. North Pacific taxa are predominant in the Kuskokwim region, but intervals near the Cenomanian–Turonian Stage boundary and in the upper Turonian contain taxa characteristic of Europe and the Western Interior basin of North America; some of these taxa have not been recorded previously in the North Pacific region. Turonian heteromorph ammonite assemblages associated with inoceramids in the finer grained facies of the Kuskokwim region are similar to those found in coeval rocks of Japan and Germany. The depositional area of the Kuskokwim Group can be broken into two northeast-trending subbasins, the Kuskokwim River subbasin to the northwest and the Mulchatna River subbasin to the southeast, connected by the Nushagak Hills corridor. Within the Kuskokwim River subbasin, deposition apparently started earlier in the north (middle Cenomanian) than in the south (late Cenomanian to early Turonian), and prograding deltaic sedimentation along the western margin also appears to have started earlier in the north. No marine fossils younger than latest Turonian to earliest Coniacian are known from the Kuskokwim River subbasin. The youngest fossils identified are Santonian in age and are from deep-water deposits in the Nushagak Hills corridor. Few fossils are known from the Mulchatna River subbasin and age control is limited.

Journal of Paleontology↗
USGS2017

Systematics, biostratigraphy, and dental evolution of the Palaeothentidae, later Oligocene to early-middle Miocene (Deseadan- Santacrucian) caenolestoid marsupials of South America

The family Palaeothentidae contains some of the dentally more specialized of the small-bodied marsupials of South America and was a clade almost equivalent with the Abderitidae in having been the most abundant caenolestoids. They were unquestionably the most diverse, containing two subfamilies, nine genera, and 19 species, with a distribution ranging from Colombia to Tierra del Fuego. The best and most continuous record of the Palaeothentidae is from Patagonian Argentina where eight genera and 17 species are recognized. There, the Palaeothentidae ranged in age from the Deseadan (later Oligocene) through the late Santacrucian (middle Miocene—the Santacrucian record lasting from about 19.4 m.y. to considerably less than 16.05 m.y. before the present). The family appears to have survived longer in Colombia. The palaeothentine Palaeothentes boliviensis (Bolivia) and the incertae sedis genus and species Hondathentes cazador (Colombia) are the only taxa restricted to an extra-Argentine distribution.

Journal of Paleontology↗
USGS2011

Evolution of silicic magma in the upper crust: the mid-Tertiary Latir volcanic field and its cogenetic granitic batholith, northern New Mexico, USA

Structural and topographic relief along the eastern margin of the Rio Grande rift, northern New Mexico, provides a remarkable cross-section through the 26-Ma Questa caldera and cogenetic volcanic and plutonic rocks of the Latir field. Exposed levels increase in depth from mid-Tertiary depositional surfaces in northern parts of the igneous complex to plutonic rocks originally at 3-5 km depths in the S. Erosional remnants of an ash-flow sheet of weakly peralkaline rhyolite (Amalia Tuff) and andesitic to dactitic precursor lavas, disrupted by rift-related faults, are preserved as far as 45 km beyond their sources at the Questa caldera. Broadly comagmatic 26 Ma batholithic granitic rocks, exposed over an area of 20 by 35 km, range from mesozonal granodiorite to epizonal porphyritic granite and aplite; shallower and more silicic phases are mostly within the caldera. Compositionally and texturally distinct granites defined resurgent intrusions within the caldera and discontinuous ring dikes along its margins: a batholithic mass of granodiorite extends 20 km S of the caldera and locally grades vertically to granite below its flat-lying roof. A negative Bouguer gravity anomaly (15-20 mgal), which encloses exposed granitic rocks and coincides with boundaries of the Questa caldera, defined boundaries of the shallow batholith, emplaced low in the volcanic sequence and in underlying Precambrian rocks. Paleomagnetic pole positions indicate that successively crystallised granitic plutons cooled through Curie temperatures during the time of caldera formation, initial regional extension, and rotational tilting of the volcanic rocks. Isotopic ages for most intrusions are indistinguishable from the volcanic rocks. These relations indicate that the batholithic complex broadly represents the source magma for the volcanic rocks, into which the Questa caldera collapsed, and that the magma was largely liquid during regional tectonic disruption. -from Author

Transactions of the Royal Society of Edinburgh, Ea↗
USGS2000

The origin and evolution of safe-yield policies in the Kansas groundwater management districts

The management of groundwater resources in Kansas continues to evolve. Declines in the High Plains aquifer led to the establishment of groundwater management districts in the mid-1970s and reduced streamflows prompted the enactment of minimum desirable streamflow standards in the mid-1980s. Nonetheless, groundwater levels and streamflows continued to decline, although at reduced rates compared to premid-1980s rates. As a result, "safe-yield" policies were revised to take into account natural groundwater discharge in the form of stream baseflow. These policies, although a step in the right direction, are deficient in several ways. In addition to the need for more accurate recharge data, pumping-induced streamflow depletion, natural stream losses, and groundwater evapotranspiration need to be accounted for in the revised safe-yield policies. Furthermore, the choice of the 90% flow-duration statistic as a measure of baseflow needs to be reevaluated, as it significantly underestimates mean baseflow estimated from baseflow separation computer programs; moreover, baseflow estimation needs to be refined and validated.

Kansas↗
USGS2013

Formation and evolution of valley-bottom and channel features, Lower Deschutes River, Oregon

Primary geologic and geomorphic processes that formed valley-bottom and channel features downstream from the Pelton-Round Butte dam complex are inferred from a canyon-long analysis of feature morphology, composition, location, and spatial distribution. Major controls on valley-bottom morphology are regional tectonics, large landslides, and outsized floods (floods with return periods greater than 1000 yrs), which include the late Holocene Outhouse Flood and several Quaternary landslide dam failures. Floods with a return period on the order of 100 yrs, including historical floods in 1996, 1964, and 1861, contribute to fan building and flood plain formation only within the resistant framework established by the major controls. Key processes in the formation of channel features, in particular the 153 islands and 23 large rapids, include long-term bedrock erosion, outsized floods, and century-scale floods. Historical analysis of channel conditions since 1911 indicates that the largest islands, which are cored by outsized-flood deposits, locally control channel location, although their margins are substantially modified during annual- to century-scale floods. Islands cored by bedrock have changed little. Islands formed by annual- to century-scale floods are more susceptible to dynamic interactions between tributary sediment inputs, mainstem flow hydraulics, and perhaps riparian vegetation. Temporal patterns of island change in response to the sequence of 20th century flooding indicate that many islands accreted sediment during annual- to decadal-scale floods, but eroded during larger century-scale floods. There is, however, no clear trend of long-term changes in patterns of island growth, movement, or erosion either spatially or temporally within the lower Deschutes River.

Oregon↗
USGS1999

Early evolution of a stratospheric volcanic eruption cloud as observed with TOMS and AVHRR

This paper is a detailed study of remote sensing data from the total ozone mapping spectrometer (TOMS) and the advanced very high resolution radiometer (AVHRR) satellite detectors, of the 1982 eruption of El Chichón, Mexico. The volcanic cloud/atmosphere interactions in the first four days of this eruption were investigated by combining ultraviolet retrievals to estimate the mass of sulfur dioxide in the volcanic cloud [ Krueger et al. , 1995] with thermal infrared retrievals of the size, optical depth, and mass of fine-grained (1–10 μm radius) volcanic ash [ Wen and Rose , 1994]. Our study provides the first direct evidence of gravitational separation of ash from a stratospheric, gas-rich, plinian eruption column and documents the marked differences in residence times of volcanic ash and sulfur dioxide in volcanic clouds. The eruption column reached as high as 32 km [ Carey and Sigurdsson , 1986] and was injected into an atmosphere with a strong wind shear, which allowed for an observation of the separation of sulfur dioxide and volcanic ash. The upper, more sulfur dioxide-rich part of the cloud was transported to the west in the stratosphere, while the fine-grained ash traveled to the south in the troposphere. The mass of sulfur dioxide released was estimated at 7.1 × 10 9 kg with the mass decreasing by approximately 4% 1 day after the peak. The mass of fine-grained volcanic ash detected was estimated at 6.5 × 10 9 kg, amounting to about 0.7% of the estimated mass of the ash which fell out in the mapped ash blanket close to the volcano. Over the following days, 98% of this remaining fine ash was removed from the volcanic cloud, and the effective radius of ash in the volcanic cloud decreased from about 8 μm to about 4 μm.

Journal of Geophysical Research D: Atmospheres↗
USGS2000

Geophysical evidence for the evolution of the California Inner Continental Borderland as a metamorphic core complex

We use new seismic and gravity data collected during the 1994 Los Angeles Region Seismic Experiment (LARSE) to discuss the origin of the California Inner Continental Borderland (ICB) as an extended terrain possibly in a metamorphic core complex mode. The data provide detailed crustal structure of the Borderland and its transition to mainland southern California. Using tomographic inversion as well as traditional forward ray tracing to model the wide-angle seismic data, we find little or no sediments, low (≤6.6 km/s) P wave velocity extending down to the crust-mantle boundary, and a thin crust (19 to 23 km thick). Coincident multichannel seismic reflection data show a reflective lower crust under Catalina Ridge. Contrary to other parts of coastal California, we do not find evidence for an underplated fossil oceanic layer at the base of the crust. Coincident gravity data suggest an abrupt increase in crustal thickness under the shelf edge, which represents the transition to the western Transverse Ranges. On the shelf the Palos Verdes Fault merges downward into a landward dipping surface which separates "basement" from low-velocity sediments, but interpretation of this surface as a detachment fault is inconclusive. The seismic velocity structure is interpreted to represent Catalina Schist rocks extending from top to bottom of the crust. This interpretation is compatible with a model for the origin of the ICB as an autochthonous formerly hot highly extended region that was filled with the exhumed metamorphic rocks. The basin and ridge topography and the protracted volcanism probably represent continued extension as a wide rift until ∼13 m.y. ago. Subduction of the young and hot Monterey and Arguello microplates under the Continental Borderland, followed by rotation and translation of the western Transverse Ranges, may have provided the necessary thermomechanical conditions for this extension and crustal inflow.

California↗
USGS2000

Mars south polar spring and summer behavior observed by TES: seasonal cap evolution controlled by frost grain size

Thermal Emission Spectrometer (TES) observations of the recession phase of Mars' south polar cap are used to quantitatively map this recession in both thermal and visual appearance. Geographically nonuniform behavior interior to the cap is characterized by defining several small regions which exemplify the range of behavior. For most of the cap, while temperatures remain near the CO 2 frost point, albedos slowly increase with the seasonal rise of the Sun, then drop rapidly as frost patches disappear over a period of ∼20 days. A “Cryptic” region remains dark and mottled throughout its cold period. TES observations are compared with first-order theoretical spectra of solid CO 2 frost with admixtures of dust and H 2 O. The TES spectra indicate that the Cryptic region has much larger grained solid CO 2 than the rest of the cap and that the solid CO 2 here may be in the form of a slab. The Mountains of Mitchel remain cold and bright well after other areas at comparable latitude, apparently as a result of unusually small size of the CO 2 frost grains; we found little evidence for a significant presence of H 2 O. Although CO 2 grain size may be the major difference between these regions, incorporated dust is also required to match the observations; a self-cleaning process carries away the smaller dust grains. Comparisons with Viking observations indicate little difference in the seasonal cycle 12 Martian years later. The observed radiation balance indicates CO 2 sublimation budgets of up to 1250 kg m −2 . Regional atmospheric dust is common; localized dust clouds are seen near the edge of the cap prior to the onset of a regional dust storm and interior to the cap during the storm.

Journal of Geophysical Research E: Planets↗
USGS2000

Geochronology of the proterozoic basement of southwesternmost North America, and the origin and evolution of the Mojave crustal province

The Proterozoic Baldwin gneiss in the central Transverse Ranges of southern California, a part of the Mojave crustal province, is composed of quartzofeldspathic gneiss and schist, augen and granitic gneiss, trondhjemite gneiss, and minor quartzite, amphibolite, metagabbro, and metapyroxenite. Sensitive high resolution ion microprobe (SHRIMP) data indicate that augen and granitic gneisses comprise a magmatic arc intrusive suite emplaced between 1783 ± 12 and 1675 ± 19 Ma, adjacent to or through thinned Archean crust. High U/Th rims on zircons in most samples suggest an early metamorphic event at ∼1741 Ma, but peak amphibolite facies metamorphism and penetrative, west vergent deformation occurred after 1675 Ma. The Baldwin gneiss is part of a regional allochthon emplaced by west vergent deformation over a Proterozoic shelf-slope sequence (Joshua Tree terrane). We hypothesize that emplacement of this regional allochthon occurred during a late Early or Middle Proterozoic arc-continent collision along the western margin of Laurentia.

Tectonics↗
USGS2001

Thermal thickness and evolution of Precambrian lithosphere: A global study

The thermal thickness of Precambrian lithosphere is modeled and compared with estimates from seismic tomography and xenolith data. We use the steady state thermal conductivity equation with the same geothermal constraints for all of the Precambrian cratons (except Antarctica) to calculate the temperature distribution in the stable continental lithosphere. The modeling is based on the global compilation of heat flow data by Pollack et al. [1993] and more recent data. The depth distribution of heat‐producing elements is estimated using regional models for ∼300 blocks with sizes varying from 1°×1° to about 5°×5° in latitude and longitude and is constrained by laboratory, seismic and petrologic data and, where applicable, empirical heat flow/heat production relationships. Maps of the lateral temperature distribution at depths 50, 100, and 150 km are presented for all continents except Antarctica. The thermal thickness of the lithosphere is calculated assuming a conductive layer overlying the mantle with an adiabat of 1300°C. The Archean and early Proterozoic lithosphere is found to have two typical thicknesses, 200–220 km and 300–350 km. In general, thin (∼220 km) roots are found for Archean and early Proterozoic cratons in the Southern Hemisphere (South Africa, Western Australia, South America, and India) and thicker (>300 km) roots are found in the Northern Hemisphere (Baltic Shield, Siberian Platform, West Africa, and possibly the Canadian Shield). We find that the thickness of continental lithosphere generally decreases with age from >200 km beneath Archean cratons to intermediate values of 200±50 km in early Proterozoic lithosphere, to about 140±50 km in middle and late Proterozoic cratons. Using known crustal thickness, our calculated geotherms, and assuming that isostatic balance is achieved at the base of the lithosphere, we find that Archean and early Proterozoic mantle lithosphere is 1.5% less dense (chemically depleted) than the underlying asthenosphere, while middle and late Proterozoic subcrustal lithosphere should be depleted by ∼0.6–0.7%. Our results suggest three contrasting stages of lithosphere formation at the following ages: >2.5 Ga, 2.5–1.8 Ga, and <1.8 Ga. Ages of komatiites, greenstone belts, and giant dike swarms broadly define similar stages and apparently reflect secular changes in mantle temperature and, possibly, convection patterns.

Journal of Geophysical Research B: Solid Earth↗
USGS2004

Holocene to Pliocene tectonic evolution of the region offshore of the Los Angeles urban corridor, southern California

Quaternary tectonism in the coastal belt of the Los Angeles urban corridor is diverse. In this paper we report the results of studies of multibeam bathymetry and a network of seismic reflection profiles that have been aimed at deciphering the diverse tectonism and at evaluating the relevance of published explanations of the region's tectonic history. Rapid uplift, subsidence in basins, folds and thrusts, extensional faulting, and strike-slip faulting have all been active at one place or another throughout the Quaternary Period. The tectonic strain is reflected in the modern physiography at all scales. Los Angeles (LA) Basin has filled from a deep submarine basin to its present condition with sediment impounded behind a large sill formed behind uplifts near the present shoreline. Newport trough to the south-southeast of LA Basin also accumulated a large volume of sediment, but remained at midbathyal depths throughout the Period. There is little or no evidence of Quaternary extensional tectonism in either basin although as much as 6 km of subsidence, which mainly occurred by sagging, has been recorded in places since the middle Miocene. The uplifts include folded and thrust faulted terranes in the Palos Verdes Hills and the shelves of Santa Monica and San Pedro Bays. The uplifted areas have been shortened in a southwest-northeast direction by 10% or slightly more, and some folds are reflected in the bathymetry. Two large adjacent midbathyal basins, Santa Monica and San Pedro, show strong evidence of subsidence and slight west-northwest extension (10%) during the same time folding was taking place in the uplifts. The tectonic boundaries between uplifts and basins are folded, normal faulted, reverse-faulted, and strike-slip faulted depending on location. The rapid Quaternary uplift and subsidence, along with the filling of LA Basin, have produced a reversal in the regional physiography. In the early Pliocene, LA Basin was a submarine deep, Palos Verdes and the shelves comprised a northeast basin slope, and the present offshore basins and Catalina Island formed an emergent or shallowly submerged shelf. Since extensional, compressional, and lateral strains are all locally in evidence, simple notions that this part of southern California underwent a change from Miocene transtension to Quaternary transpression fail to explain our observations.

Tectonics↗
USGS2005

Evolution of melt-vapor surface tension in silicic volcanic systems: Experiments with hydrous melts

We evaluate the melt‐vapor surface tension (σ) of natural, water‐saturated dacite melt at 200 MPa, 950–1055°C, and 4.8–5.7 wt % H 2 O. We experimentally determine the critical supersaturation pressure for bubble nucleation as a function of dissolved water and then solve for σ at those conditions using classical nucleation theory. The solutions obtained give dacite melt‐vapor surface tensions that vary inversely with dissolved water from 0.042 (±0.003) J m −2 at 5.7 wt % H 2 O to 0.060 (±0.007) J m −2 at 5.2 wt % H 2 O to 0.073 (±0.003) J m −2 at 4.8 wt % H 2 O. Combining our dacite results with data from published hydrous haplogranite and high‐silica rhyolite experiments reveals that melt‐vapor surface tension also varies inversely with the concentration of mafic melt components (e.g., CaO, FeO total , MgO). We develop a thermodynamic context for these observations in which melt‐vapor surface tension is represented by a balance of work terms controlled by melt structure. Overall, our results suggest that cooling, crystallization, and vapor exsolution cause systematic changes in σ that should be considered in dynamic modeling of magmatic processes.

Journal of Geophysical Research B: Solid Earth↗