Obsidian Dome Research Articles

Obsidian Sources of Prehispanic Artefacts from Cultures near Ceboruco Volcano, Nayarit, Mexico

Ceboruco, located in the western part of the Trans-Mexican Volcanic Belt is an active stratovolcano in Nayarit, Mexico, with the last historic eruption in 1870–1875. The fertile valleys around this volcano have been occupied for several thousands of years. The recognised pre-Hispanic human settlements include the Shaft tomb and Aztatlán cultures, which were present in the vicinity of the volcano before and after the well-studied Plinian Jala eruption in 890 AD, respectively. Both cultures left abundant archaeological materials in housing units and cemeteries, including obsidian tools. In this study, we compare archaeological samples (poorly preserved tools or parts of them) with obsidian samples from local outcrops, as well as other known regional obsidian mine sites. We use hand sample and chemical analyses of main and trace elements. The results are statistically evaluated by using the Mahalanobis distance-method in order to assign probable sources. We show that obsidian has been locally extracted from long-known obsidian mines near Ixtlán del Río, but also from obsidian layers at the nearby Sierra Madre Occidental Mountain Range and the Pochotero obsidian dome. Nevertheless, clear evidence for obsidian trade since the earliest occupations has also been found, as we identified sources from the neighbouring states such as Jalisco.

Obsidian clasts as sintered remnants of agglutination processes in volcanic conduits, evidence from the Pepom tephras (Sete Cidades), São Miguel, Azores

The youngest explosive eruptions of the Sete Cidades volcano, São Miguel, Azores, are recorded by a series of relatively small-volume (<0.03 to 0.13 km3 DRE) trachytic pyroclastic deposits termed the Pepom tephra deposits. While dominated by crystal-poor to crystal-moderate (<10%) pumice clasts, these deposits also contain a suite of dense glassy clasts of broadly similar crystallinity. The obsidian clasts from a single deposit vary in texture from entirely dense to those that are moderately vesicular and typically single clasts will contain multiple textural domains. The majority (∼71%) of these dense clasts have compositions both from bulk rock and in-situ glass measurements that are identical to those of the pumice clasts within the same deposit. We interpret these dense clasts to reflect sintering of previously fragmented magma at shallow levels in the conduit prior to being re-entrained and erupted with the vesicular magma, in agreement with recent studies focussing on textural observations. Notably, across the exposed volcanic stratigraphy of São Miguel obsidian domes, flows/coulees are not preserved, arguing against the idea that the dense glass clasts within the Pepom tephras are sourced from existing surficial rocks. In contrast, the neighbouring island of Terceira exhibits domes and coulees with large obsidian bands that cut through the crystalline groundmass. Most silicic rocks of Santa Bárbara and Pico Alto volcanoes on Terceira are peralkaline, comenditic to pantelleritic in composition, and at similar conditions (e.g., temperature and water content) have lower viscosities than the trachytic Pepom obsidian clasts. However, the Santa Bárbara silicic lavas on Terceira (the less peralkaline suite) have more obsidian than the more peralkaline Pico Alto domes and coulees indicating that while peralkalinity, developed during magmatic evolution in the crust, may play a role, sintering occurring at shallow levels within the conduit likely is more important in producing obsidian.

Making sense of brittle deformation in rhyolitic lavas: Insights from Obsidian Dome, California, USA

AbstractThe scarcity of observed active extrusive rhyolitic lava flows has skewed research to extensively focus on prehistoric lavas for information about their eruptive and emplacement dynamics. The first ever witnessed silicic lava eruptive events, Chaitén (2008) and Cordón Caulle (2011–2012) in Chile, were illuminating to the volcanology community because they featured a range of emplacement processes (endogenous versus exogenous), movement limiting modes, and eruptive behaviors (explosive versus effusive) that were often regarded as acting independently throughout an eruptive event. In this study, we documented evidence of a continuum of brittle and brittle-ductile deformation and fracture-induced outgassing during the emplacement of the ~600-yr-old silicic lava from Obsidian Dome, California, USA. This study focused on mapping the textural-structural relationships of the upper surface of the lava onto high-resolution (&amp;lt;10 cm2/pixel) orthorectified color base maps. We found that the upper surface is characterized by small (&amp;lt;1 m) mode 1 tensile fractures that grew and initiated new cracks, which linked together to form larger tensile fractures (1–5 m), which in turn penetrated deeper into the lava. We recorded ornamentations on these fracture surfaces that allow snapshot views into the rheological and outgassing conditions during the lava’s effusion. The largest fractures developed during single, large fracture events in the final stages of the lava’s emplacement. Ornamentations preserved on the fractured surfaces record degassing and explosive fragmentation away from the vent throughout the lava’s emplacement, suggesting explosive activity was occurring during the effusive emplacement. Field-based cataloguing of the complexities of fracture surfaces provides qualitative constraints for the future mechanical modeling of effusive lavas.

Rhyolite lava emplacement dynamics inferred from surface morphology

The morphology of a silicic lava is created by the fluid dynamic conditions that operated during flow. Careful analysis of these features can thus be used to reconstruct emplacement processes. We used unoccupied aerial system and structure-from-motion photogrammetry techniques to collect centimeter-scale spatial resolution imagery of the surfaces of South Coulee and Obsidian Dome rhyolite lavas (Mono-Inyo Craters, USA). We supplement the newly acquired orthomosaics and digital elevation models with existing imagery of other Holocene rhyolite lavas from across the western USA, including Rock Mesa Dome (South Sister, OR), Newberry Flow (South Sister, OR), Obsidian Flow (Newberry, OR), Interlake Flow (Newberry, OR), and Banco Bonito Flow (Valles Caldera, NM). Although many morphologic features exist, we restrict our quantitative analyses to the recurrent ridges and blocks of loose rubble that are common to the flows. Ridge spacing and ridge vergence indicate the ridges are likely folds produced by compression during emplacement. Pressure ridge spacings range from 27 to 45 m at South Coulee and 18 to 34 m on Obsidian Dome. Spacing generally decreases with distance from the vent across both lavas. Ridge amplitudes range from 4 to 17 m at both lavas and show little correlation with distance from the vent. The vergence of the crest of the ridges in the flow interiors points back to the source vent, which provides strong evidence for endogenous flow and the backward rotation of folds produced by undertow of the underlying lava. Ridges near flow margins verge towards the flow front, demonstrating that exogenous flow becomes increasingly important at the point of advance. In both South Coulee and Obsidian Dome block sizes are largest near the vent, and gradually decrease towards the flow front. The gradual decrease in block size with increasing distance from the vent likely reflects decreasing effusion rates from the conduit. We interpret our high-resolution field measurements under the lens of analytical solutions to fluid dynamic models to estimate emplacement timescales and associated rates. We calculate relatively abbreviated eruptive timescales ranging from 1 month to <2 years. Such values predict eruptive fluxes <135 m3 s−1 and velocities <100 m day−1, providing helpful criterion for volcanic monitoring or hazard forecasting efforts.

Meydan Kalderası ve Gürgürbaba Domu Çevresinin Jeomorfolojik Özellikleri

Bu çalışmada, Van Gölü’nün kuzeyinde Van ili sınırları içinde yer alan Meydan Kalderası ve Gürgürbaba Domu çevresinin jeomorfolojik özelliklerinin incelenmesi hedeflenmiştir. İnceleme alanının jeomorfolojik evriminde etkili olan tektonik, volkanik ve hidrografik faktörler araştırılmış, volkanik kayaçların yaşlandırılması amacıyla radyometrik yaşlandırma(40Ar/39Ar) yöntemleri uygulanmış ve tematik haritalama çalışmaları için Coğrafi Bilgi Sistemleri kullanılmıştır. Volkanik kayaçlardan (obsidyen) alınan örnekler üzerinde radyometrik yaşlandırmalar yapılmış ve sonuçlar literatürde verilen yaşlar ile karşılaştırılmıştır. Jeokimyasal analizler (LA-ICP-MS) yapılarak volkanik ürünlerin kökenleri ve özellikleri belirlenmiştir. Araştırma sonucunda Meydan Kalderası’nın, Gürgürbaba obsidyen domunun oluşumundan (420±24 ka) önceki bir dönemde çökerek oluştuğu ve Gürgürbaba Domu’nun oluşumu sırasında Meydan Kalderası’nın doğu yamacını tahrip ettiği sonucuna varılmıştır. Meydan volkanik alanı içinde yayılan tüm obsidyenlerin kaynağının Gürgürbaba Domu olduğu anlaşılmıştır. Ayrıca, Meydan volkanizması drenaj ağında da değişikliğe neden olmuştur. Eski topografyaya göre şekillenen akarsuların drenaj tipi değişerek yeni volkanik topografyaya uyumlu radyal tipte bir drenaj ağı gelişmiştir.

Pre-eruptive storage conditions and eruption dynamics of a small rhyolite dome: Douglas Knob, Yellowstone volcanic field, USA

The properties and processes that control the size, duration, and style of eruption of rhyolite magma are poorly constrained because of a paucity of direct observations. Here, we investigate the small-volume, nonexplosive end-member. In particular, we determine the pre-eruptive storage conditions and eruption dynamics of Douglas Knob, a 0.011-km3 obsidian dome that erupted from a 500-m-long fissure in the Yellowstone volcanic system. To determine pre-eruptive storage conditions, we analyzed compositions of phenocrysts, matrix glass, and quartz-hosted glass inclusions by electron microprobe and Fourier-transform infrared analyses. The pre-eruptive melt is a high-silica rhyolite (∼75 wt.% SiO2) and was stored at 760 ± 30 °C and 50 ± 25 MPa prior to eruption, assuming vapor saturation at depth. To investigate emplacement dynamics and kinematics, we measured number densities and orientations of microlites at various locations across the lava dome. Microlites in samples closest to the inferred fissure vent are the most aligned. Alignment does not increase with distance traveled away from the vent, suggesting microlites record conduit processes. Strains of <5 accumulated in the conduit during ascent after microlite formation, imparted by a combination of pure and simple shear. Average microlite number density in samples varies from 104.9 to 105.7 mm−3. Using the magma ascent model of Toramaru et al. (J Volcanol Geotherm Res 175:156–157, 2008), microlite number densities imply decompression rates ranging from 0.03 to 0.11 MPa h−1 (∼0.4–1.3 mm s−1 ascent rates). Such slow ascent would allow time for passive degassing at depth in the conduit, thus resulting in an effusive eruption. Using calculated melt viscosity, we infer that the dike that fed the eruption was 4–8 m in width. Magma flux through this dike, assuming fissure dimensions at the surface represent its geometry at depth, implies an eruption duration of 17–210 days. That duration is also consistent with the shape of the dome if produced by gravitational spreading, as well as the ascent time of magma from its storage depth.

Obsidian Subsource Identification in the Sierra de Pachuca and Otumba Volcanic Regions, Central Mexico, by ICP‐MS and DBSCAN Statistical Analysis

Otumba and Sierra de Pachuca obsidian deposits in Central Mexico have been important sources of raw material since pre‐Hispanic times. Numerous archaeological investigations have suggested that the economical and political expansion of major Mesoamerican societies were linked to the control of obsidian sources and distribution of quarried material. Sierra de Pachuca contains several obsidian flows and numerous quarries throughout the region that were preferentially exploited by different cultures. The Otumba Volcanic Complex has four important obsidian domes, but three of them have not been studied in detail. A geochemical characterization of subsources from the Sierra de Pachuca and Otumba Volcanic Complex is an important step toward future sourcing of obsidian artifacts that would help provide insight into spheres of influence and trade by past cultures in Central Mexico. Having this purpose in mind, inductively coupled plasma mass spectrometry (ICP‐MS) was used to analyze obsidian samples collected from five separated locations at Sierra de Pachuca and four at Otumba, followed by statistical analysis (density‐based spatial clustering of applications with noise, DBSCAN). We were able to distinguish three chemically distinctive subsources in Sierra de Pachuca and three in Otumba. This study illustrates the importance of accurate characterization of obsidian raw material when attempting to define subsource usage.

Physical Volcanology of Obsidian Dome, California: A Complex Record of Emplacement of a Youthful Lava Dome

Physical Volcanology of Obsidian Dome, California: A Complex Record of Emplacement of a Youthful Lava Dome

Cristobalite in a rhyolitic lava dome: evolution of ash hazard

Prolonged and heavy exposure to particles of respirable, crystalline silica-rich volcanic ash could potentially cause chronic, fibrotic disease, such as silicosis, in individuals living in areas of frequent ash fall. Here, we show that the rhyolitic ash erupted from Chaiten volcano, Chile, in its dome-forming phase, contains increased levels of the silica polymorph cristobalite, compared to its initial plinian eruption. Ash erupted during the initial, explosive phase (2–5 May 2008) contained approximately 2 wt.% cristobalite, whereas ash generated after dome growth began (from 21 May 2008) contains 13–19 wt.%. The work suggests that active obsidian domes crystallise substantial quantities of cristobalite on time-scales of days to months, probably through vapour-phase crystallisation on the walls of degassing pathways, rather than through spherulitic growth in glassy obsidian. The ash is fine-grained (9.7–17.7 vol.% <4 µm in diameter, the respirable range) and the particles are mostly angular. Sparse, fibre-like particles were confirmed to be feldspar or glass.

阿蘇カルデラ西部に分布する栃ノ木溶岩中にみられる組成不均質とその成因

A composite lava flow is found in core samples of Tochinoki lava unit, one of the post-caldera volcanic products distributed in the western part of Aso caldera. Including core and surface outcrop samples, Tochinoki lava shows two compositionally different lava types. One is silicic (SiO2 63-66 wt%) and phenocryst-rich (8-16 vol%), and the other is mafic (SiO2 60-62 wt%) and phenocryst-poor (< 7 vol%). Mass balance calculation and trace element modeling show that the compositional variation found among Tochinoki core samples are explained by the fractional crystallization of plagioclase, clinopyroxene, orthopyroxene and magnetite phenocrysts from the most mafic lava. Tochinoki composite lava flow is probably produced by the viscous segregation of compositionally distinct magmas ascending simultaneously in the conduit as proposed by Inyo Obsidian Dome drilling project. Bulk rock composition and mineral chemistry indicate that mafic-silicic pairs observed in Tochinoki core samples are different from those in outcrop samples. This difference probably suggests the existence of contemporaneous multiple flow units in the Tochinoki lava unit.

Vesiculation rates of obsidian domes inferred from H2O concentration profiles

Explosivity of rhyolite domes depends on bubble nucleation and growth. To understand these processes, we measure millimeter‐scale variations in H2O by synchrotron Fourier transform infrared (FTIR) spectromicroscopy on interlayered obsidian‐pumice samples. The H2O contents of all samples are above the 1‐atm solubility value (∼0.10 wt.%) and decrease systematically towards vesicular zones, indicating that gas bubbles were growing and that degassing of melt to atmospheric pressure was incomplete. H2O profiles are compared with models for water diffusion in order to constrain the temporal scale of vesiculation. Diffusion timescales range from 0.4 to 15 days, and represent the time between bubble nucleation and quenching. We use these results to estimate the duration of volatile exsolution and evaluate the timing of the formation of explosion pits frequently found on the surfaces of obsidian domes, though never witnessed.

Permeability of vesicular silicic magma: inertial and hysteresis effects

Abstract The permeability of crystal-poor obsidian flow and pumice samples from Medicine Lake Volcano, USA, are measured to assess (1) the existence of a critical vesicularity (porosity) below which low crystallinity magma is effectively impermeable, (2) the effects of bubble texture on permeability, and (3) the importance of inertial effects in resisting fluid flow through magma. Consistent with prior studies, the Medicine Lake data indicate that fluids can readily percolate through magma with porosities greater than 65%. However, we find no abrupt decrease in permeability below 60% porosity, as found previously for low crystallinity samples of similar origin from Obsidian Dome, USA. Rather, the permeabilities of Medicine Lake samples show a gradual increase with increased vesicularity similar to that observed in highly crystalline samples from Soufriere Hills Volcano, Montserrat, and Mount Saint Helens, USA. We suggest that both vesicle microstructure and resulting porosity–permeability relationships depend on the deformation, decompression and degassing history of the magma. In particular, bubble deformation by shear and/or partial bubble collapse allows open-system degassing of magma with vesicularity of 20%, and perhaps lower. Permeability determines the rate at which samples can degas during decompression. Air flow rates through lava and pumice samples are not proportional to the pressure gradients driving flow, indicating that inertial effects are significant at laboratory conditions. Flow resistance from both inertial and viscous effects generally decreases with increasing porosity, and inertial effects are smaller for coarsely vesicular lava than for finely vesicular lava or pumice (tephra) samples with similar viscous (Darcian) permeabilities. For an H2O fluid at 800 °C and 25 MPa, the critical average fluid speed at which inertial and viscous effects are predicted to be comparable is between 10−4 and 10−1 m/s for all the Medicine Lake samples. As flow rates in this range are anticipated for volcanic eruption conditions, inertial effects should be considered in models of magma degassing at depth.

AMS measurements on obsidian from the Inyo Domes, CA: a comparison of magnetic and mineral preferred orientation fabrics

Measurements of the anisotropy of magnetic susceptibility (AMS) on lavas can yield information concerning flow directions, deformation regimes, and relative shear rates. Here we report results of measurements made on samples of rhyolitic obsidian collected from Obsidian Dome, CA. Comparisons of AMS measurements with previously determined 3D microlite orientation distributions in these samples serve as a calibration of the AMS technique. Despite the low values of bulk susceptibility (1.06–7.04×10 −3 SI) found in the dome relative to typical values in basalts (10 −2 SI), the degree of anisotropy is extremely high in all specimens (3.4–10.7). Furthermore, susceptibility tensors exhibit a wide range of shapes varying from prolate to triaxial and flattened ellipsoids ( B parameter from −8.6 to +6.4) corresponding to various degrees of microlite dispersion. Our results also indicate that despite the tight clustering of principal susceptibility axes obtained on each site, bulk flow directions inferred from the orientations of k max axes may be misleading because the orientation of the local extension changes from place to place within the dome. However, it is shown that the AMS of each specimen provides a good measure of the direction of local extension. Consequently, it is possible to interpret spatial variations in flow regime from systematic changes in the orientation of principal susceptibility axes. As the proposed association between the local deformation regime and AMS is strongly dependent on the aspect ratio of the crystals embedded in the flowing lava, some caution must be exerted when interpreting AMS results from other flows, and a direct comparison between different flows may not be granted in many occasions.

Volcanic stratigraphy and phase chemistry of the 11 900 yr BP Waiohau eruptive episode, Tarawera Volcanic Complex, New Zealand

The 11 900 yr BP Waiohau eruptive episode is a moderately sized rhyolitic event of the Tarawera Volcanic Complex. Subdivision of the pyroclastic fall deposits previously mapped as Waiohau Tephra has allowed the chronology of the eruption to be deciphered. At least 15 events (units A‐O) with volumes in the range <0.01–1.7 km3 can be recognised in proximal to medial settings. The eruption commenced with a series of minor pyroclastic falls and surges, which rapidly changed to a major phase of pyroclastic surges (unit D) that reached up to 14 km from the vent with ash clouds recognisable up to 30 km. Eastern Dome is considered to have been extruded early in the episode. Several prominent plinian phases (unit E, L, M) followed, two of which are estimated to have had column heights of c. 30 km. One event was associated with the destruction of an obsidian dome and dispersal of obsidian lapilli (unit F). These plinian events were accompanied by thin pyroclastic flows punctuated by minor magmatic and phreatomagmatic fall eruptions. There is no evidence for prolonged quiescence during these eruptions. The climactic pyroclastic fall phase (unit M) was followed by a major failure of the northeast flanks of the Tarawera massif, either by a series of block and ash flows or debris avalanche. Effusive activity resulted in the emplacement of Waikakareao lava flows, and voluminous and widespread pyroclastic flows (unit N) to the north and northeast. The Waiohau episode concluded with effusive activity producing the Pokuhu flow and Kanakana Dome. A new volume estimate for the combined units A‐O is 4.5 km3. This is a minimum because it does not properly incorporate distal ash (>50 km) poorly preserved in paleosols. Previous volume calculations are considered overestimates because they included tephric loess. Effusive activity represents 3.9 km3, and pyroclastic debris in the northern sector adds another 2.4 km3, but much of the latter is xenolithic. Deposits from the entire Waiohau eruptive episode were derived from a hypersthene (+ minor hornblende) rhyolite magma that lacked compositional (SiO2 = 78 ± 0.2 wt%) and mineralogic gradients. Electron microprobe analyses reveal that phenocrystic phases lack compositional variation. The bulk of the magma also lacked physical gradients (T= 780–790°C; logfO2 = c. ‐15) as determined by Fe‐Ti oxide geothermometry. Eastern Dome and the Pokohu lava display lower temperature (<740°C) and log fO2 (c. ‐16). There is no evidence for mafic triggering of the Waiohau eruption, unlike other eruptive episodes of Tarawera, and the magma represents a separate batch, unrelated to earlier and later biotite‐bearing magmas. The Waiohau eruptive products appear to have been derived from a rapidly convecting, low aspect ratio magma body that did not reside in the crust for a prolonged time.

Dynamics of obsidian flows inferred from microstructures: insights from microlite preferred orientations

The flow of obsidian lava leads to crystal alignments that reflect both the accumulated strain and the type of flow across the surface. Microlite preferred orientations are used to investigate the emplacement dynamics, strain history, and structural evolution of Obsidian Dome, eastern California. Measurements of three-dimensional microlite trend and plunge in samples from the near-vent region, distal flow, and flow front show: (1) the flow directions along the dome margins, (2) the deformation style (e.g., pure versus simple shear) at the dome margins, and (3) the variation in strain as a function of position within the flow. Microlites form well-developed lineations in the plane of flow banding in all samples. Stereographic projections indicate that lineations trend normal to the western flow front and plunge shallowly away from the margin. The radial flow pattern indicated by measurements made along the western margin suggests that extrusion was from a roughly elliptical vent. These results highlight a strong correlation between microlite trend and the bulk flow direction inferred from the geometry of the flow. Along most of the eastern periphery, lineations trend parallel to the margin and likely reflect the local flow direction as influenced by compression against the thickening flow crust, marginal talus piles, and topography. Orientation distributions imply that radial spreading accompanied by flattening was the dominant mechanism for flow emplacement. Comparisons of measured orientation distributions with theoretical predictions suggest that microlite fabrics in flow front and near-vent samples developed in a pure shear flow. Microstructures in a sample from near the distal flow base records a component of simple shear. Variance in microlite trend provides a measure of the amount of strain acquired during flow. Standard deviation in trend decreases from the near-vent region to the flow margins, reflecting progressive alignment of microlites during transport. Pure shear strain inferred from orientation distributions increases from approximately 0.3 near the vent to about 1.1 at the flow front. The difference between these strains (0.8) is an estimate of the strain associated with flow emplacement. Such strain is similar in size to that estimated from mesoscopic structures (∼1) and for horizontal spreading of a fluid whose volume is equal to that of Obsidian Dome (∼1.6). This suggests that flow on the surface was sufficient to produce observed microlite preferred orientations in the flow front. These techniques can be applied to interpret older dissected lavas where erosion has erased much of the original flow front or where larger-scale structures indicative of flow directions are poorly preserved.

Fission track dating obsidians in Central and Northern Anatolia

Twenty obsidian samples from Central and North Anatolia were dated by the fission track method. Split samples analyzed in two different laboratories give consistent results. Data presented refer to three volcanic fields and are summarized as follows: Orta-Sakaeli (Northern Anatolia) obsidians with ages from 21–23 Ma; Gollu Dagi dome complex near Ciftlik (Central anatolia) has given ages between 1.33–0.98 Ma; Acigol caldera (Central Anatolia) shows three age groups: (1) Eruptions of precaldera (Bogazkoy) obsidians with ages 0.18–0.15 Ma; (2) Formation of the large intracaldera dome Kocadag-Taskesiktepe at about 0.08 Ma (uncorrected “apparent” age of 0.075 Ma); and (3) Acigol peripheral obsidian domes with plateau ages of 0.020 and 0.019 Ma, and an additional uncorrected age of 0.018 Ma form a distinct age group. This fission track parameters discriminate between the various obsidian sources of prehistoric obsidian artifacts.

Thermal and dynamical regimes of single‐ and two‐phase magmatic flow in dikes

Finite element calculations of magma flow in dikelike channels with length‐to‐width ratios of 1000:1 or more have been used to investigate the coupling between thermal and dynamical regimes due to temperature‐dependent viscosity and dissipation. Steady state solutions with realistic thermal and dynamical parameter values have been obtained. The models show that the onset of solidification on the boundaries of a basaltic or andesitic dike, as predicted by idealized laminar flow models, can be prevented or significantly delayed by a small amount of transverse flow induced by rising bubbles, boundary roughness, or turbulence. This effect will reduce the critical initial widths of dikes estimated by Bruce and Huppert (1989). In the absence of transverse flow, the bulk temperature of the magma may actually increase slightly with distance along the dike as a result of viscous dissipation even while solidification is occurring on the walls of the dike. With converging or necking dike walls it is found that boundary temperatures fall to a minimum and then increase with distance along a dike even if viscous heating is neglected. For viscous heating to offset significant rates of heat loss (2 kW/m2) in a plane‐parallel 1‐m‐wide basaltic dike, an average flow velocity of 2.7 m/s driven by a pressure gradient of 1.7 MPa/km is required. A 7% void fraction caused by exsolution of volatiles or chamber gas in the magma will produce this pressure gradient. The ease of producing such a gradient by reducing the density of the magmatic column with addition of a gas phase makes it likely that flows of basaltic magma could be maintained in dikes tens of kilometers long. Furthermore, a gas phase may be important for the propagation of the fracture prior to the initial injection of magma during dike emplacement. Rapid transport of more silicic magmas through dikes is inhibited by the requirement of large driving pressure gradients exceeding several hundred megapascals per kilometer. However, the pressure gradient can be substantially reduced if the magma is a heterogenous mixture of a predominantly silicic component with a more mafic component. Pipe flow experiments involving molten polymers that exhibit dynamical similarity to magmas strongly suggest that unmixing occurs when a two‐component magma mixture, in which the components have different viscosities, rises within a dike. Within a few dike widths of the inlet the less viscous mafic component encapsulates the more viscous silicic part, effectively lubricating the passage of the more viscous component. Compositional variations in the Obsidian Dome volcano support the occurrence of this pressure reduction or self‐lubrication mechanism. Such a self‐lubrication process often may be necessary to permit very viscous magmas to reach Earth's surface. If so, chemical and lithologic zoning would be anticipated as a common feature of near‐surface intrusions or lava flows that are characterized by a high silica and/or high crystal content such as at Deadman Dome in Long Valley, California.

Extended cooling and viscous flow of large, hot rhyolite lavas: implications of numerical modeling results

Abstract A common misconception about rhyolite lava flows is that they cannot advance far from their vents and are constrained to be small due to their high viscosities. Although a high-viscosity lava will indeed flow quite slowly, it may advance a great distance if the lava efficiently retains heat and thus mobility, and if the available magma supply is large. To evaluate this possibility, a finite-difference numerical model was used to determine the rate of heat loss from units interpreted to be large rhyolite lavas. Cooling of such 100–300 m-thick units is very slow and is further retarded by a vesicular carapace and by evolution of latent heat. Thermal history alone suggests that large, thick lava flows could remain active for several decades. To constrain the duration of flow advance of these units, their inferred apparent viscosities are compared with those of active lava flows. Mean flow velocity of some large rhyolites in southwestern Idaho, USA, was estimated based on the observation that the units must still have been advancing by viscous flow at the start of spherulite crystallization, indicating perhaps a 10- to 18-year flow duration, based on studies of Obsidian Dome, California. Mean flow velocities would have been roughly 0.59 to 2.5 km/yr, and calculated average apparent viscosities of the flows ranged from 1.8 to 3.6 orders of magnitude greater than the viscosity of the lava itself. This relationship also holds for active andesite and basaltic andesite block lava flows, which have apparent viscosities 2.7 to 4 orders of magnitude greater than lava viscosities. This implies that large rhyolites may indeed have been emplaced in the same manner as less-silicic block lavas, and that long distance viscous flow is not unrealistic for rhyolite lavas. A good example of a large-volume lava is the 15-km 3 Rhyolite of the Badlands (SW Idaho), where vent relations show that lava effused from a fissure and advanced up to 9 km. More extensive rhyolites could also be emplaced solely by effusion. Extended flow of these lavas may lead to the remelting and removal of basal crumble breccias, and to the formation of welded shard textures in their margins, both processes tending to obscure the lava flow origin of the units. The great dimensions of lava-like rhyolitic units with volumes of 10 to at least 200 km 3 are not a sufficient reason to assume they erupted as ignimbrites.

Effects of compressibility on the flow of lava

Lava contains gas bubbles and hence is a compressible liquid whose density increases as a function of pressure. After eruption at the Earth's surface, it spreads at a rate which is a function of its thickness and it is compressed under its own weight. Therefore, both thickness and spreading rate are determined by a balance between viscous and compressible effects. Theoretical equations are derived for the shape and velocity of a compressible liquid spreading on a horizontal surface. Solutions are obtained for a fixed eruption rate Q. The radial extent of the flow increases proportional to t1/2. A dimensionless number C is defined which characterizes the importance of flow compression: \(C = \rho _0 \beta g\left( {\frac{{\mu Q}}{{\rho _0^2 g}}} \right)^{{1 \mathord{\left/ {\vphantom {1 4}} \right. \kern-\nulldelimiterspace} 4}}\), where ρ0 is bubbly lava density at atmospheric pressure, β compressibility, μ viscosity and g the acceleration of gravity. C can be thought of as the ratio of two characteristic length-scales, one for compression effects and one for viscous effects. The larger C is, the more important compressibility effects are. As C is increased, the flow becomes thinner because the liquid is compressed more and more efficiently. Compressibility acts to smooth out variations of flow thickness, which provide the driving force. Thus, all else being equal, a compressible liquid flows less rapidly than an incompressible one. When trying to infer the effective viscosity of a flow from its spreading rate, the neglect of compressibility leads to an overestimate. The various factors which act to determine the distribution of gas bubbles in lava flows are reviewed and discussed quantitatively. Comparison with data from Obsidian Dome (Eastern California) shows that disequilibrium effects are important and that bubble resorption during burial in a thick flow is not a pervasive phenomenon. The analysis is applied to the 1979 dome of Soufriere de Saint Vincent (W.I.). An effective value of compressibility for this 100-m-thick dome is 1.5x10−6 Pa−1. This implies that, all else being equal, the viscosity of this lava may be overestimated by a factor of 5 if no account is taken of the compressible nature of the flow.

Degassing of argon from microclines within the thermal aureole of the Obsidian Dome Conduit, Long Valley Caldera, California: Constraints on emplacement history

Age spectra for microclines separated from samples obtained within the thermal aureole of the conduit to Obsidian Dome are compared with those from a reference sample collected in an area removed from the thermal effects of intrusion. The age spectra are characterized by a zero‐age plateau at low fractional release and by an ∼80 Ma plateau at higher fractional releases. These plateaus correspond to subparallel arrays in the Arrhenius data. These arrays are attributed to two discrete diffusion domain size fractions characterized by identical activation energies but by discrete and different frequency grain size fractions (Lovera et al., 1989). The relative losses of 40Ar* do not show a systematic pattern when compared on the basis of total 40Ar* loss. The 40Ar* losses first increase and then fall with distance from the contact. However, when the data composing the high‐temperature array of the Arrhenius data are considered (the large diffusion domain size fraction), the pattern is consistent with that predicted by conductive heating of the aureole. This agreement requires the aureole to have been uniformly preheated to a temperature of ∼250°C prior to intrusion of the rhyolite dike and surrounding breccia funnel that make up the vent structure. Further, the data require that the material forming the breccia funnel must have been heated to a temperature approaching that of the magma prior to emplacement.