Bubble coalescence in basalt flows: comparison of a numerical model with natural examples

Gas accumulation in magma may be aided by coalescence of bubbles because large coalesced bubbles rise faster than small bubbles. The observed size distribution of gas bubbles (vesicles) in lava flows supports the concept of post-eruptive coalescence. A numerical model predicts the effects of rise and coalescence consistent with observed features. The model uses given values for flow thickness, viscosity, volume percentage of gas bubbles, and an initial size distribution of bubbles together with a gravitational collection kernel to numerically integrate the stochastic collection equation and thereby compute a new size spectrum of bubbles after each time increment of conductive cooling of the flow. Bubbles rise and coalesce within a fluid interior sandwiched between fronts of solidification that advance inward with time from top and bottom. Bubbles that are overtaken by the solidification fronts cease to migrate. The model predicts the formation of upper and lower vesicle-rich zones separated by a vesicle-poor interior. The upper zone is broader, more vesicular, and has larger bubbles than the lower zone. Basaltic lava flows in northern California exhibit the predicted zonation of vesicularity and size distribution of vesicles as determined by an impregnation technique. In particular, the size distribution at the tops and bottoms of flows is essentially the same as the initial distribution, reflecting the rapid initial solidification at the bases and tops of the flows. Many large vesicles are present in the upper vesicular zones, consistent with expected formation as a result of bubble coalescence during solidification of the lava flows. Both the rocks and model show a bimodal or trimodal size distribution for the upper vesicular zone. This polymodality is explained by preferential coalescence of larger bubbles with subequal sizes. Vesicularity and vesicle size distribution are sensitive to atmospheric pressure because bubbles expand as they decompress during rise through the flow. The ratio of vesicularity in the upper to that in the lower part of a flow therefore depends not only on bubble rise and coalescence, but also on flow thickness and atmospheric pressure. Application of simple theory to the natural basalts suggests solidification of the basalts at 1.0±0.2 atm, consistent with the present atmospheric pressure. Paleobathymetry and paleoaltimetry are possible in view of the sensitivity of vesicle size distributions to atmospheric pressure. Thus, vesicular lava flows can be used to crudely estimate ancient elevations and/or sea level air pressure.     

Some selected problems concerning the tectonics of granitoids of the Strzegom-Sobótka massif (SW Poland)

Directional structures in the SE part of the Strzegom-Sobótka massif were studied with mesostructural and petrotectonic methods. These structures are most pronounced in quarries at Strzeblów and Chwaków. They were studied there byCloos 1922a, b and taken by him as reference structures for the Q, S and L joints system. New results from this study defined more precisely the types of structures and directions of related stresses. Statistical measurements of joints and veins performed in some quarries scattered over the massif area led to the conclusion that the pluton was subjected to a complex field of tectonic stresses.

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Zusammenfassung Die Richtung der tektonischen Strukturen im Südostteil des Strzegom-Sobótka-Massives lassen sich in den Steinbrüchen von Strzeblów und Chwaków nach gefügekundlichen und petrographischen Methoden studieren. Hier hat auchHans Cloos 1922 seine Untersuchungen zum System der Q, S und L Klüfte durchgeführt. Neue Ergebnisse präzisieren diese Strukturtypen und ihre Beziehung zum tektonischen Stre\. Statistische Aufnahmen der Klüfte und Gänge über das gesamte Massiv führen zu dem Schlu\, da\ der Pluton während und nach seiner Intrusion komplexen Stre\feldern unterworfen war.

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Résumé Dans la partie sud du massif granitique de Strzegom-Sobótka les structures directionnelles ont été étudiées par les méthodes mésostructurale et pétrographique. Ces structures sont le mieux exprimées dans les carrières de Strzeblów et de Chwalkow, où d'ailleurs H.Cloos (1922 a, b) les a prises comme références pour ses systèmes de joints O, S et L. Les résultats de la présente étude permettent d'en définir avec plus de précision les divers types ainsi que les directions des contraintes qui en furent responsables. Le relevé systématique des joints et des veines, étendu à un certain nombre de carrières à travers tout le massif permet de conclure que le pluton a été le siège de champs de contraintes complexes pendant et après son intrusion.

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一个秃冻胀丘的热及水文动力学--关于其形成的暗示

Mud boils, classified as non-sorted circles, are a common patterned ground phenomenon of permafrost areas. They typically consist of a bare circle center of cryoturbated soil and are surrounded by vegetation on more stable soil. The objectives are to examine differences in seasonal hydrologic and thermal dynamic across this gradient of cryoturbation, i.e. the region below the organic border and the mud center. We installed instruments in a mud boil at a site close to Ny-@lesund, Spitsbergen, in September 1998. The bare soil circle center ranges about 1 m in diameter and is surrounded by a vegetated border consisting of a mixture of low vascular plants, mosses and lichens. Fine soil (>95% clay and silt) in the lower part of the profile is overlain by coarser grained material (silt and sand). The grain size distribution also suggests an upwelling of fine material in the center of the circle. Temperature and moisture sensors were installed over a vertical 1 m×1 m profile and hourly data recording started in September 1998. Surface irregularities, as well as variations of grain size and moisture, create a non-uniform thermal and hydrologic dynamic. We qualitatively analyze this dynamic with respect to the mud boils physical stability.     

Effects of eruption and lava drainback on the H2O contents of basaltic magmas at Kilauea Volcano

 Lava drainback has been observed during many eruptions at Kilauea Volcano: magma erupts, degasses in lava fountains, collects in surface ponds, and then drains back beneath the surface. Time series data for melt inclusions from the 1959 Kilauea Iki picrite provide important evidence concerning the effects of drainback on the H₂O contents of basaltic magmas at Kilauea. Melt inclusions in olivine from the first eruptive episode, before any drainback occurred, have an average H₂O content of 0.7±0.2 wt.%. In contrast, many inclusions from the later episodes, erupted after substantial amounts of surface degassed lava had drained back down the vent, have H₂O contents that are much lower (≥0.24 wt.% H₂O). Water contents in melt inclusions from magmas erupted at Pu'u 'O'o on the east rift zone vary from 0.39–0.51 wt.% H₂O in tephra from high fountains to 0.10–0.28 wt.% H₂O in spatter from low fountains. The low H₂O contents of many melt inclusions from Pu'u 'O'o and post-drainback episodes of Kilauea Iki reveal that prior to crystallization of the enclosing olivine host, the melts must have exsolved H₂O at pressures substantially less than those in Kilauea's summit magma reservoir. Such low-pressure H₂O exsolution probably occurred as surface degassed magma was recycled by drainback and mixing with less degassed magma at depth. Recognition of the effects of low-pressure degassing and drainback leads to an estimate of 0.7 wt.% H₂O for differentiated tholeiitic magma in Kilauea's summit magma storage reservoir. Data for MgO-rich submarine glasses (Clague et al. 1995) and melt inclusions from Kilauea Iki demonstrate that primary Kilauean tholeiitic magma has an H₂O/K₂O mass ratio of ∼1.3. At transition zone and upper mantle depths in the Hawaiian plume source, H₂O probably resides partly in a small amount of hydrous silicate melt. Received: 31 March 1997 / Accepted: 17 November 1997     

Low-O18 basalts from Iceland

The volcanic rocks of Iceland are anomalous in their oxygen isotope content. Recent tholeiitic and transitional alkali basalts from Iceland range in (δO¹⁸ from 1·8 to 5δ7%. Most of the tholeiitic basalts and their phenocrysts are at least 1% lower in δO¹⁸ than unaltered basalts from other oceanic islands or oceanic ridges. The Icelandic basalts that resemble ridge basalts in δO¹⁸ also resemble them in major element chemistry. δO¹⁸ values of alkali olivine basalts are closest to those of other oceanic islands. Secondary alteration processes have lowered as well as raised the δO¹⁸ values of some Icelandic rocks, but such surface mechanisms cannot account for the distribution of oxygen isotopes in the Recent basalts of Iceland. Three mechanisms that could give rise to the low-O¹⁸ magmas are (1) exchange of oxygen between magma and low-O¹⁸ hydrothermally altered rock, (2) exchange with low-O¹⁸ meteoric water, or (3) an exceptional mantle under Iceland. None of the above models can satisfactorily account for all the observations.     

Large lakes of the world: A global science opportunity

Recent commentaries have expressed growing concern about the present state of limnology in the United States. Two basic problems are perceived: l) there are no separate, dedicated funding programs in the US for basic limnologic research; and 2) limnology has failed to enter the global big science era of the 90's. While both the oceans and atmosphere are subjects of large, multinational global-scale scientific programs, similar large studies of lakes have been limited, at best, to regional studies. There are over 250 large lakes worldwide, spanning the globe from 80° N to 60° S, and ranging from hypersaline to fresh water. Altogether they contain over 68% of the earth's fresh liquid surface water. However, these large ecosystems are increasingly threatened by global anthropogenic change such as illconceived diversions, uncontrolled consumption, and progressive degradation of water and overall ecosystem quality. Global climate change is but another anthropogenic global-scale problem with potential future effects on large lakes. An organized, multinational framework for the study of large lakes on a global scale is outlined.GLERL Contribution No. 793