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Miocene rhyolites (19.2 ± 0.9 Ma) at the Jabal Shama in western Saudi Arabia represent an example of rift-related silicic volcanism that took place during the formation of the Red Sea. They mostly consist of tuffaceous varieties with distinct flow banding, and pea-sized spherulites, obsidian and perlitized rhyolite tuffs. Although they have the geochemical signature of A 2 -type rhyolites, these silicic rocks are not typically alkaline but alkali-calcic to calc-alkaline. They developed in a within-plate regime and possibly derived from a recycled mafic subducted slab in depleted sub-continental mantle beneath the western Arabian plate. The Jabal Shama rhyolites are younger in age than their Miocene counterparts in Yemen and Ethiopia. The Jabal Shama spherulites consist of hydrous alkali feldspar-silica radial intergrowths with an occasional brown glass nucleus. Carbonate- and glass-free spherulites give up to 4.45 wt% L.O.I. The hydrous nature of these silicates and the absence of magnetite in the spherulites is a strong indication of oxidizing conditions. The spherulites contain hydrous feldspars with up to ∼6 wt% H 2 O, and they develop by diffusion and devitrification of glass in the rhyolite tuff at ∼800 °C. Owing to higher undercooling due to supersaturation, the radial hydrous phases within spherulites might grow faster and led to coagulation. The polygonal contacts between spherulites and the ∼120° dihedral angle suggest solid-state modification and recrystallization as the process of devitrification proceeds as low as ∼300 °C. The sum of FeO + MgO is positively correlated with total alkalies along with magnetite oxidation in the matrix to Fe-oxyhydroxides, and to the incorporation of OH − into silicates within the spehrulites themselves. Structural H 2 O in glass of the Jabal Shama perlite (obsidian) is considerable (∼9–12 wt%) with 3.72–5.6 wt% L.O.I. of the whole-rock. The presence of deleterious silica impurities would lower the ore grade due to poor expansion. Zeolitization of the rhyolite tuff was limited or incomplete in comparison with that of Pliocene-Pleistocene rift-related mafic volcanics in the western Arabian plate.

Felsic intrusions present ubiquitous structures. They result from the differential interactions between the magma components (crystal, melt, gas phase) while it flows or when the flow is perturbed by a new magma injection. The most obvious structure consists in fabrics caused by the interactions of rotating grains in a flowing viscous melt. New magma inputs through dikes affect the buk massif flow, considered as global within each mineral facies. A review of the deformation and flow types developing in a magma chamber identifis the patterns that could be expected. It determines their controlling parameters and summarizes the tools for their quantification. Similarly, a brief review of the rheology of a complex multi-phase magma identifies and suggests interactions between the different components. The specific responses each component presents lead to instability development. In particular, the change in vorticity orientation, associated with the switch between monoclinic to triclinic flow is a cause of many instabilities. Those are preferentially local. Illustrations include fabric development, shear zones and flow banding. They depend of the underlying rheology of interacting magmas. Dikes, enclaves, schlieren and ladder dikes result from the interactions between the magma components and changing boundary conditions. Orbicules, pegmatites, unidirectional solidification textures and miarolitic cavities result from the interaction of the melt with a gaseous phase. The illustrations examine what is relevant to the bulk flow, local structures or boundary conditions. In each case a field observation illustrates the instability. The discussion reformulates instability observations, suggesting new trails for ther description and interpretation in terms of local departure to a bulk flow. A brief look at larger structures and at their evolution tries to relate these instabilities on a broader scale. The helical structures of the Řicany pluton, Czech Republic and by the multiple granitic intrusions of Dolbel, Niger illustrate such events.

At Tucker Hill, in Central Otago, New Zealand, a series of pseudotachylyte veins are hosted in quartzofeldspathic schist. Chilled margins, microlites, flow banding, and the crystallisation of mineral phases absent from the host rock provide unequivocal evidence for melting during pseudotachylyte formation. Whole rock analyses of pseudotachylyte reveal c. 3× enrichment of K2O, Ba, and Rb, and similar depletion of Na2O, CaO, Sr, and Eu, as compared to host schist. Formation age of pseudotachylyte is 95.9±1.8 Ma as measured by total fusion 40Ar/39Ar analyses. Stepwise heating of pseudotachylyte matrix yields an excellently defined 40Ar/39Ar plateau age of 96.0±0.3 Ma. These well-defined ages are attributed to the presence of potassium feldspar, low abundance of inherited lithic material from the host rock, and few fluid inclusions containing extraneous Ar. We propose that formation of these pseudotachylyte veins was related to Cretaceous extensional uplift and exhumation of the Otago Schist.

The Upper Pliocene Kalogeros cryptodome is exposed on the northeastern part of Milos, Greece. The dacite cryptodome is 800-1300 m across and at least 120 m high. It is inferred to have intruded wet, unconsolidated pumiceous sediments in a shallow marine environment. The Kalogeros cryptodome includes five facies. The coherent dacite facies (80 vol%) consists of massive, non-vesicular dacite and is characterised by radial columnar joints 20-250 cm across. The banded dacite facies (15 vol%) encircles the coherent core and is up to 40 m thick. It comprises alternating bands (0.5-4 m thick) of pale grey and black dacite. The fractured dacite facies (less than 1 vol%) forms an irregular zone (1-3 m thick) at the outer margin of the cryptodome. The outermost massive dacite breccia facies ( less than 2 vol%) consists of blocky to polyhedral dacite clasts (1-40 cm in diameter) and is characterised by domains of jigsawfit and clast-rotated breccia. The stratified dacite breccia (2vol%) is clast-to-matrix supported, monomictic, poorly sorted and composed of dacite clasts up to several metres in diameter. The facies association collectively records endogenous growth of a cryptodome that involved a continuous magma supply during a single intrusive phase and simple expansion (inflation). During emplacement, the margins of the Kalogeros cryptodome were quench fragmented, forming an outer domain of intrusive hyaloclastite and intensely fractured dacite. The near-solid outer carapace insulated the hotter, less viscous interior. Laminar shear accompanied inflation, generating large-scale flow banding around the outer part of the core. Once stagnant, concentric isotherms were established within the cryptodome and controlled the orientation of columnar joints. Cryptodomes are characterised by a well-developed internal concentric distribution of distinctive textural domains, modest autoclastic breccia, and the absence of redeposited autoclastic facies.

Mesoscopic structures in four large cavities on the Big Obsidian Flow, OR, USA, provide evidence of links between large-scale surface folding, cavity formation, and hazardous explosive activity originating from within the flow. Stereographic analysis of flow banding, stretched bubble lineation, striations, and mesoscopic fold axes indicates that three of the cavities are near-cylindrical folds. Cavity fold axes are oriented parallel to large-scale compressional flow ridge axes, suggesting that cavities form in response to shortening during flow advance. Large surface folds develop as the upper 10 m of the cooler flow surface buckles when the flow front cools, slows, and stalls against topographic barriers. Void space is created in fold hinges as obsidian layers shear past each other and pull apart. The resultant cavities may serve as reservoirs for exsolved volatiles or surface water which may later vent explosively to form explosion craters on the flow surface.

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 nearvent 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 (V1) and for horizontal spreading of a fluid whose volume is equal to that of Obsidian Dome (V1.6). 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. fl 2002 Elsevier Science B.V. All rights reserved.

Hazardous explosive activity may sporadically accompany the extrusion of silicic lava domes. Model- ling of the emplacement of silicic domes is therefore an important task for volcanic hazard assessment. Such modelling has been hampered by a lack of a sufficiently accurate rheological database for silicic lavas with crys- tals and vesicles. In the present study, the parallel-plate viscometry method was applied to determine the shear viscosity of five natural rhyolitic samples from a vertical section through the Ben Lomond lava dome, Taupo Volcanic Centre, New Zealand. Rheological measure- ments were performed at volcanologically relevant tem- peratures (780-950°C) and strain rates (10 -5 -10 -7 s -1 ). Although these samples are in the metastable state, vis- cosity determinations, melt composition, as well as water and crystal contents of samples were demonstrably sta- ble during experiments. For samples containing up to 5 vol.% microlites, the composition of the melt, rather than the physical effect of suspended crystals, had great- er influence on the effective viscosity of the silicic mag- ma. Samples with 10 vol.% microlites and containing a flow banding defined by microlites show no significant orientational effects on apparent viscosity. The rheologi- cal measurements were used together with a simple cool- ing model to construct thermal and viscosity profiles re- vealing conditions during the emplacement of the Ben Lomond lava dome.

The Islands of Ponza and Gavi (western Pontine Archipelago, Italy) preserve parts of a subaqueously emplaced, high-silica (75–77% SiO2) rhyolitic lava flow that overlies the margins of three older domes. Exposures of the lava alone cover 24 km2, and with a thickness of 150 m exposed in the cliffs, the preserved volume of the lava flow is 3.6 km3, and the original volume is likely to have exceeded 7 km3. The lava flow consists of coherent obsidian and several hyaloclastite facies, including in situ and clast-rotated breccias and sandstones, which all exhibit gradational relationships. The coherent obsidian occurs as separate domains and is often flow banded. Although flow folds occur, the banding is generally subhorizontal, dipping gently to the NNE. The hyaloclastite, representing 90% of the lava by volume, has a diffuse layering varying from 0.5 to 2 m in thickness. This layering also dips gently to the NNE and is marked in places by alternation of some coherent horizons, coarse and finely fragmented in situ hyaloclastite, pumiceous hyaloclastite, and clast-rotated hyaloclastite. The layering is laterally continuous and consistent in orientation over most of the 13 km length of Ponza: the subhorizontal orientation of the flow banding and presence of resedimented deposits in the upper parts of the sequence indicate that the unit is a lava flow. The resedimented deposits comprise both proximal and distal deposits of debris flows and turbidity currents initiated by gravitational instability of the upper and marginal parts of the lava flow and include stratified monomictic obsidian breccias and bedded vitric siltstones and sandstones. The lava flow has been intruded and crosscut by rhyolitic bodies of various ages, including discordant, coherent to internally fractured contemporaneous rhyolite bodies, and late, penecontemporaneous dikes with glassy margins and alteration haloes.

Deep towed 30 kHz sidescan sonar data from the Saharan Debris Flow deposit, west of the Canary Islands, show spectacular backscatter patterns which are interpreted in terms of flow banding, longitudinal shears, lateral ridges (levees) and transported blocks. Identification of these features is based on high resolution seismic profiles and on a comparison with similar structures seen in better known environments including other marine debris flows and slides, subaerial sediment failures (particularly rock fall avalanches), glaciers and lava flows. Flow banding in the debris flow, picked out by bands of differing backscatter intensity, is on a scale of tens to hundreds of metres. It is considered to result from flow streaming of clasts, with variation in clast size between bands. This primary fabric is cut by a series of distinct flow-parallel longitudinal shears. Broad, high backscatter longitudinal bands along the edge of and within the debris flow are interpreted as lateral ridges associated with multiple flow pulses; the high backscatter possibly reflects either a concentration of coarse grained material or chaotic sediments deposited from a turbulent flow. Coherent, low backscatter patches are interpreted as rafted blocks, although streamlined haloes of high backscatter material around some blocks indicates differential movement between block and flow, possibly during the waning stages of the flow. A non-turbulent debris flow model is preferred, in which a raft of more or less coherent material is carried along by a base undergoing laminar flow. Speculatively, the lack of turbulent mixing preserves original sedimentological heterogeneity from the debris flow source area, possibly in the form of clast size distributions. These heterogeneous sediments are drawn out into a flow-parallel banding which is imaged as the flow-parallel backscatter intensity banding. The upper raft of material responds to cross-flow velocity differences, and perhaps to variations in the timing of flow movement, primarily by longitudinal shearing. More complex deformation of the flow banding occurs at the flow margins and around obstacles in the flow, where lateral velocity shear would be expected to be highest.

The Reydarfjordur drill hole penetrated 1919 m of section with a recovery of 99.7%. Subaerial lava flows comprise 54% of the core, dikes 41%, and clastic rocks about 5%. In addition, a 1.3-km-thick section of lava flows and clastic rocks exposed above the drill site was measured and sampled. The lava flows are chiefly low-magnesia basalts and ferrobasalts with lesser amounts of icelandite, basaltic andesite, and olivine tholeiite. In the drill core, complete flows range from 1.2 to 19.7 m in vertical thickness and average 7.1 m; in the exposed section, flow thicknesses range from 1 to 30 m and average 5.2 m. Most of the flows are characterized by an upper scoriaceous zone 1–3 meters thick, grading downward through a zone of fractured basalt into a massive flow interior. Some flows also have a thin basal breccia. Olivine tholeiites and some low-magnesia basalts are commonly mottled, whereas ferrobasalts and icelandites have well developed flow banding. Most of the flows are fine- to medium-grained, aphyric to sparsely phyric, and moderately vesicular. A few are moderately plagioclase phyric; others have scattered phenocrysts of clinopyroxene and olivine. Over 100 intrusive units are recognized in the core, and they span approximately the same compositional range as the flows. They are characterized by having steeply inclined chilled contacts, a uniform massive aspect, and an absence of vesicles or brecciation. Most are less altered than the intruded lava flows and contain secondary minerals only along sparse high angle fractures. Many of the dikes are also aphyric, but some contain notable plagioclase or olivine phenocrysts. Groundmass textures range from fine-grained, intersertal to coarse-grained, ophitic. Clastic units occur typically as thin, varicolored layers between lava flows, rarely exceeding 0.5 m in thickness. These are chiefly fine- to coarse-grained, poorly- to well-bedded lithic tuffs showing varying degrees of reworking. Most are silicic to intermediate in composition, but basaltic units are also present. Two rhyolitic ash flow tuffs occur in the section, one of which is 30 m thick. We interpret the lavas to be the flanking portions of broad low-angled central volcanoes centered on silicic complexes. Some of the flows may have erupted from central vents but most probably represent flank eruptions from rift zones represented by the dike swarms. Most of the dikes were probably intruded laterally into the lava pile; thus they do not represent direct vertical feeders from deep magma bodies.

VoL. 11 The extinct multiple volcano, Tauhara, of late Pleistocene age, is north-east of Lake Taupo, and consists of five coalescing dacite cumulo-domes. Another dome, Maunganamu N, 3 miles south of Tauhara, is included in the study. The morphology and structure of these domes are described. The internal fan-shaped structure of the flow banding as seen in cross section suggests that each dome was formed from lava continuously extruded from separate vents. The superficial ash deposits of the Taupo ash sequence, which mantle the volcano, are described briefly.