Silicic Lava Flows Research Articles

The possible role of thermal feedback in the eruption of siliceous magmas

Using values of viscosity and its temperature dependence, density, heat capacity and thermal conductivity appropriate to rhyolitic liquids an analysis of the macroscopic heat balances of these liquids flowing under constant stress has been carried out. The analysis shows that for shear couples and shear stresses expected in the flow of silicic lava on the earth's surface thermal feedback may occur after time periods ranging from a few seconds to several days and may cause rapid increases in temperature. For silicic magmas flowing in a dike or circular conduit under a constant pressure gradient these thermal instabilities may occur after a few days to several weeks for dike half thicknesses or conduit radii ranging from a few centimeters to a few meters. For silicic lava flows this phenomenon may be important in producing commonly observed flow structures such as pumiceous banding and color banding. Local temperature increases would have the effect of reducing local water solubility, increasing diffusion rates, and increasing nucleation and growth rates of vapor bubbles, thus causing highly vesicular bands that parallel the shear planes. Temperature increases could also cause increases in f O 2 within a fluid layer resulting in oxidation and red color banding. For silicic lavas flowing in dikes or conduits, the possible thermal instabilities are accompanied by instabilities in volume rate of flow which may contribute to rapid vesiculation of upward-moving magma and result in explosive volcanism.

Geochemistry of the andesite flank lavas of three composite cones within the Atitlán Cauldron, Guatemala

Three composite cones have grown on the southern edge of the previously existing Atitlan Cauldron, along the active volcanic axis of Guatemala. Lavas exposed on the flanks of these cones are generally calc-alkaline andesites, but their chemical compositions vary widely. Atitlan, the largest and most southerly of the three cones, has recently erupted mainly pyroclastic basaltic andesites, while the flanks of San Pedro and Toliman are mantled by more silicic lava flows. On Toliman, 74 different lava units have been mapped, forming the basis for sequential sampling. Rocks of all three cones are consistently higher in K2O, Rb, Ba and REE than other Guatemalan andesites. Atitlan’s rocks and late lavas from Toliman have high Al2O3 content, compared to similar andesites from other nearby cones. All major and trace element data on the rocks are shown to be consistent with crystal fractionation involving phases observed in the rocks. If such models are correct, significant differences in the relative proportions of fractionation phases are necessary to explain the varied compositions, in particular higher Al2O3 rocks have fractionated less plagioclase. We speculate that inhibition of plagioclase fractionation could occur in chambers where PH2O is greater and when repose intervals are shorter. The distribution of volcanic vents throughout Guatemala which show this postulated «inhibition of plagioclase fractionation» is systematic with such vents lying just to the south of the main axis. The andesites of the three cones cannot be simply related to the late-Pleistocene rhyolites which are apparently associated with cauldron formation, because unlike the andesites, the rhyolites have markedly depleted heavy REE abundances. Recent dacitic lavas from vents south of San Pedro volcano and silicic pyroclastic rocks which mantle the slopes the San Pedro may reflect residual post-cauldron rhyolitic volcanism.

Paleozoic interarc basin in eastern Australia and a modern New Zealand analogue

Abstract An analogue approach to the geology of the middle Paleozoic Hill End Trough of New South Wales suggests a close parallel between its latest Silurian-Early Devonian (Pridoli-Siegenian) configuration and that of the southern apex of the Havre Trough where it impinges on the North Island of New Zealand, that is, at the junction of a largely intraoceanic multiple arc system and a continental block. This analogy, rather than one with the interarc basins of wholly intraoceanic multiple arc systems adopted by other authors, is favoured by: dimensional comparability, the basinaxial location of silicic lava flows, the silica-intermediate to silica-rich character of magmatic products within and around the basin apex, the broad continental magmatic province immediately beyond the basin apex, the probable substantial epiclastic component of the basin fill, the probable lack of a truly oceanic basement to the basin fill, and by metallogenetic parallels. If the analogy is valid, then the Hill End Trough was, a...

Hydrogeology of Ash Flow Tuff: A Preliminary Statement

Ash flow tuffs, silicic pyroclastic rocks commonly mapped as silicic lava flows (rhyolite, dacite, or quartz latite) on many pre‐1960 geologic maps, cover tens of thousands of square miles in the southwestern states and aggregate between 35,000 and 60,000 mi3 in the Great basin alone. In contrast to silicic lava flows but similar to basalts, ash flow tuffs are sheetlike bodies; individual flows have been traced for as much as 100 miles. Welding, the cohesion of molten glassy shards and pumice fragments during emplacement of these rocks, results in significant vertical variations in physical properties within individual flows. Interstitial porosity and permeability vary from 70% and 2 gpd/ft2 (gallons per day per square foot), respectively, in nonwelded portions or zones of ash flows to less than 5% porosity and virtually zero permeability in densely welded zones. Primary and secondary joints spaced several feet apart in nonwelded zones are as close as a fraction of an inch in densely welded zones. Fracture transmissibility of welded zones within ash flow tuffs varies from 100–100,000 gpd/ft. The nonwelded zones are aquitards. Spring groups with discharges ranging from 20–200 cfs (cubic feet per second) emerge from welded tuff in eastern Idaho and Sumatra, Indonesia. The magnitude of the spring discharge attests to the regional hydraulic continuity of these strata. Development of groundwater from welded tuff in the Southwest appears several decades away because of depth of burial, large variations in yield, and locally poor water quality. Nevertheless, further study of these rocks is warranted to test the assumption common to many water budget studies that interbasin movement through and ‘mountain front recharge’ from silicic volcanic rocks are negligible.