Search

This study presents a description of a rhyolite lava-forming eruption, including the conduit system, degassing history during the lava flow dynamics. We examined the Pleistocene Shiroyama rhyolite lava on Himeshima Island, Japan. The lava is mainly characterized by locally developed obsidian. Based on the structural variation, the obsidian lithofacies correspond to the shallow conduit. The geological investigation and FTIR analyses showed that gas removal from the conduit magma proceeded via vesiculation, fracturing, and brecciation, allowing formation of the dense obsidian. Since the lava originally maintained some extent of water, the lava effervesced just after the effusion. This vesiculation resulted in pervasive bubble coalescence and the formation of abundant permeable pathways. The volcanic gasses escaped via those pathways, allowing collapse of the bubbles and deflation of the lava. AMS (anisotropy of magnetic susceptibility) results indicate that the lava spread concentrically.

Cryptodomes are shallow-level intrusions that cause updoming of overlying sediments or other rocks. Understanding the formation of cryptodomes is important for hazard assessment, as cryptodome-forming eruptions are one of the major triggering factors in sector collapse. This paper describes internal structures of a Quaternary subaerial rhyodacite cryptodome at Ogariyama, Usu volcano, Japan (the Ogariyama dome), and examines the textural differences between subaerial and subaqueous cryptodomes to extend our knowledge of these phenomenon. The Ogariyama dome, which is one of the youngest subaerial cryptodomes in the world (

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.

The Sgurr of Eigg Pitchstone on the Isle of Eigg, NW Scotland, is a crystal-rich, trachydacitic, partially vitrophyric rock, which has previously been interpreted both as a lava and as a sill. We interpret this rock as a chemically zoned, rheomorphic, lava-like ignimbrite that formed during a sustained pyroclastic eruption. The Sgurr of Eigg Pitchstone can be subdivided into discrete emplacement units distinguished by the following features: (1) their present-day weathering characteristics; (2) the orientation, spacing and morphology of the columnar joints; (3) sharp, undulating boundaries with marked topographic breaks. The absence of weathered surfaces, palaeosols, pyroclastic fall deposits or sedimentary rocks at emplacement unit boundaries suggests deposition from a single eruption. The emplacement units, some of which display upper and basal vitrophyres, represent distinct depositional packages that record several rapidly emplaced ignimbrites, which welded, cooled and devitrified as a simple, essentially single, cooling unit, during eruption from a sustained, low pyroclastic column. The Sgurr of Eigg Pitchstone displays a pervasive base-parallel flow banding, which is folded into intrafolial recumbent isoclinal folds. The flow banding and folds indicate that rheomorphism occurred throughout deposition. The Sgurr of Eigg Pitchstone is interpreted as an erosional remnant of an extensive ignimbrite sheet, the first such unit recorded within the North Atlantic Igneous Province.

International audience; The ease at which exsolving volatiles can migrate though magma and outgas influences the explosivity of a volcanic eruption. Volcanic rocks often contain discrete discontinuities, providing snapshots of strain localisation processes that occur during magma ascent and extrusion. Whether these features comprise pathways for or barriers to fluid flow is thus of relevance for volcanic eruption and gas emission modelling. We report here on nine discontinuity-bearing andesite blocks collected from Volcán de Colima, Mexico. We present a systematic porosity and permeability study of fifty cores obtained from the blocks collected, and interpret the genetic processes of the discontinuities through detailed microstructural examination. Bands in pumiceous blocks were inferred to be relicts of inhomogeneous bubble expansion which, despite significantly increasing porosity, do not markedly affect permeability. Other discontinuities in our blocks are interpreted to be shear strain-induced flow banding, cavitation porosity, and/or variably healed fractures. In each of these cases, an increase in permeability (up to around three orders of magnitude) was measured relative to the host material. A final sample contained a band of lower porosity than the host rock, characterised by variably infilled pores. In this case, the band was an order of magnitude less permeable than the host rock, highlighting the complex interplay between dilatant and densifying processes in magma. We therefore present evidence for significant permeability anisotropy within the conduit and/or dome of a volcanic system. We suggest that the abundance and distribution of strain localisation features will influence the escape or entrapment of volatiles and therefore the evolution of pore pressure within active volcanic systems. Using a simple upscaling model, we illustrate the relative importance of permeable structures over different lengthscales. Strain localisation processes resulting in permeability anisotropy are likely to play an important role in the style, magnitude, and recurrence interval of volcanic eruptions.

Detailed field observations of structures within the flow front of a Holocene trachyandesite lava from Snaefellsnes, Iceland, are presented. The lava provides exceptional three-dimensional exposure of complex brittle and ductile deformation textures that record processes of lava fracture and quenching driven by external water. The flow front interior is characterised by structures consisting of a large (metre-scale) curviplanar master fracture with many smaller (centimetre-scale) subsidiary fractures perpendicular to the master fracture. Such structures have previously been recognised in a range of lava compositions from basalt to dacite and called pseudopillows or pseudopillow fractures. We propose the term pseudopillow fracture systems to emphasise the consistent package of different fracture types occurring together. All documented occurrences of pseudopillow fracture systems are in lavas that have been inferred to interact with an aqueous coolant (i.e. liquid water, ice or snow). We use fracture surface textures and their orientation in relation to flow banding to identify three distinct types of master fracture and two types of subsidiary fractures. Master fracture surface textures used to identify fracture mechanisms include chisel marks (striae), cavitation dimples, river lines and rough/smooth fracture surface textures. These indicate both brittle and ductile fracture happening on different types of master fracture. Chisel marks on subsidiary fractures indicate comparative cooling rates, cooling directions and isotherm orientations at the time of fracture. We propose a model for pseudopillow fracture system formation taking into account all the various fracture types, textures and fracture propagation mechanisms and discuss their implications for interaction mechanisms between lava flows and external coolants.

Some large metabasaltic pillows in the uppermost part of the Palaeoarchaean Hooggenoeg Complex in the Barberton Greenstone Belt, South Africa, exhibit flow-banded margins and homogeneous cores that have a different texture and compositional variation. The margins consist of millimetre to several centimetre thick alternating bands of pale green spherulitic and darker, conspicuously variolitic varieties of non-vesicular and aphyric metabasalt, previously inferred to be due to mingling of two different types of lava. The dark cores have sharp, aphanitic contacts with the flow-banded carapaces. They lack flow banding, have coarse-grained interiors and exhibit well-preserved primary textures with pseudomorphs after prismatic pyroxene set in a groundmass containing skeletal plagioclase. The compositional range of samples from these cores is unlike that of the flow-banded metabasalt but is similar to a 19 m thick lobate metabasalt flow ~150 m stratigraphically further up, at the local top of the Hooggenoeg volcanic sequence. The pillow cores are inferred to result from the later refilling of drained hollow pillows.

The Miocene Grey9s Landing ignimbrite reaches 70 m thick and covers at least 400 km 2 in the central region of the Snake River Plain. It shows particularly intense welding and rheomorphic deformation, and although parts are eutaxitic, most is lava-like with flow-banding and no fiamme. A near-ubiquitous penetrative flow lamination, associated with a well-developed elongation lineation, is folded into small intrafolial tight to isoclinal oblique and sheath folds, which are refolded by larger folds in the upper parts. Structural and kinematic analysis reveals that welding and early deformation occurred rapidly during deposition from a very hot (≤1000 °C), high-mass-flux pyroclastic density current that flowed westward across a graben-faulted landscape. As hot particles were deposited, they rapidly agglutinated and coalesced, and underwent noncoaxial shear in a subhorizontal ductile shear zone close to the current-deposit interface. The shear zone is interpreted to have been less than 2 m thick. It produced and deformed the rheomorphic fabric, and it migrated upward with the rising current-deposit interface during aggradation, so that it transiently affected all levels of the resultant thick ignimbrite. Deformation was progressive, and after the density current had dissipated, viscous spreading and downslope flow continued and involved an increasingly thick portion of the sheet. This folded the flow banding and F 1 intrafolial isoclines into larger sheath folds, and into more upright periclines near the top of the ignimbrite. We demonstrate that structural and kinematic analysis can elucidate the emplacement history of rheomorphic ignimbrites.

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.

The Eucarro Rhyolite is remarkable in being far more voluminous (>675 km3) and extensive than most felsic lavas (

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.

OFHC copper pins with 10 ppm oxygen were slid against alumina at a load of 50 N and sliding speeds of 0.1 ms−1 to 4.0 ms−1. The wear characteristics of copper were related to the strain rate response of copper under uniaxial compression between strain rates of 0.1 s−1 and 100 s−1 and temperatures in the range of 298 K to 673 K. It is seen that copper undergoes flow banding at strain rates of 1 s−1 up to a temperature of 523 K, which is the major instability in the region tested. These flow bands are regions of crack nucleation. The strain rates and temperatures existing in the subsurface of copper slid against alumina are estimated and superimposed on the strain rate response map of copper. The superposition shows that the subsurface of copper slid at low velocities is likely to exhibit flow band instability induced cracking. It is suggested that this is the reason for the observed high wear rate at low velocities. The subsurface deformation with increasing velocity becomes more homogeneous. This reduces the wear rate. At velocities >2 ms−1 there is homogeneous flow and extrusion of thin (10 μm) bands of material out of the trailing edge. This results in the gradual increase of wear rate with increasing velocity above 2.0 ms−1.

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.

Momo-iwa, Rebun Island, Hokkaido, Japan, is a dacite cryptodome 200-300 m across and 190 m high. The dome is inferred to have intruded wet, poorly consolidated sediment in a shallow marine environment. The internal structure of the dome is concentric, with a massive core, banded rim, and narrow brecciated border, all of which are composed of compositionally uniform feldspar-phyric dacite. Boundaries between each of the zones are distinct but gradational. The massive core consists of homogeneous coherent (unfractured) dacite and is characterized by radial columnar joints 60-200 cm across. The banded rim encircles the massive core and is 40 m wide. It is characterized by large-scale flow banding parallel to the dome surface. The flow banding comprises alternating partly crystalline and more glassy bands 80-150 cm thick. The outermost brecciated border is up to 80 cm thick, and consists of in situ breccia and blocky peperite. The in situ breccia comprises polyhedral dacite clasts 5-20 cm across and a cogenetic granular matrix. The blocky peperite consists of polyhedral dacite clasts 0.5-2 cm across separated by the host sediment (mudstone). The internal structures of the dome suggest endogenous growth involving a continuous magma supply during a single intrusive phase and simple expansion from the interior. Although much larger, the internal structures of Momo-iwa closely resemble those of lobes in subaqueous felsic lobe-hyaloclastite lavas.

Flow banded rocks of the middle Tertiary Aliso lava dome, in the Tumacacori Mountains of southern Arizona, contain light-colored dacitic flow bands, dark-colored rhyolitic flow bands, and dark-colored ellipsoidal enclaves of rhyolite. Groundmass glass of the light- and dark-colored flow bands is trachyte and high-silica rhyolite, respectively. Enclaves are fine grained, holocrystalline, and phenocryst-poor. A two-stage sequence of mixing and mingling of discrete magmas can account for the compositional and textural characteristics of the Aliso rocks: (1) enclave magma mixed with dark band magma and (2) the enclave/dark band magma composite mingled with alkalic trachyte magma, now represented by the light bands. Both mingling events may have happened in the magma chamber, or partially in the magma chamber and partially in the volcanic conduit. Deformation of the magmas and the development of flow bands are likely to have occurred in the conduit. During flow, the highly crystalline enclave magma was slightly stretched and deformed, contaminating the dark band magma. The dark band magma deformed into thin bands, decoupling in some instances from the rhyolitic enclaves which it hosted. Enclaves and phenocrysts from the dark band magma were locally captured by the light band magma. Calculations of viscosities of the three magmas across fanges of equilibration temperature, strain rate, water concentration, and crystallinity indicate that magma viscosities are more strongly affected by water concentration and crystal abundance than by temperature or strain rate.

The microstructural flow behavior of titanium and copper under uniaxial compression has been qualitatively related to the surface morphology and subsurface flow during plane strain abrasion. Uniaxial compression tests were carried out on titanium in the temperature range 298-673 K and copper at 298 K at strain rates in the range 0.1-100 $s^{-1}$. Titanium undergoes adiabatic shear banding at strain rates $>1s^{-1}$, the intensity of which decrease with decreasing strain rate and increasing temperature. This adiabatic shear banding is associated with a peak in the flow stress. Flow bands were observed in copper at low strain rates $( \leq 1 s^{-1})$. There is no peak flow stress associated with the flow banding in copper. The adiabatic shear bands in titanium and flow bands in copper are sites of crack formation. It is found that the strain rate response of both these materials plays an important role in plane strain abrasion of these metals by tungsten carbide wedges. The intense adiabatic shear banding in titanium at 298 K results in intense cracking of the abraded track and the bow wave and inhibits subsurface flow. The subsurface flow was found to be high at 673 K where no intense adiabatic shear banding is observed. In copper subsurface cracking was observed. This is associated with the flow bands observed at low strain rates.

While continuing our investigation of the geology of copper occurrences in central Anatolia, it was noted that there were a few places in the highlands south of Kayseri, such as Bakirdagi, with the Turkish word for copper (bakir) in their name. However, the search among place names with bakir (dotless i) is complicated by the presence of place names using the similar sounding word bakir (doted i ) meaning virgin or untouched. Enquires with local residents brought to our attention a number of slag sites which we hoped would be for copper. Inspection of these sites revealed that these slags showed no signs of copper alteration and that, when broken open with a rock hammer, showed prills of iron. These slags closely resembled the iron slags that we had previously investigated at Yaprakli in the Cankiri Province north-east of Ankara. As with the slags at Yaprakli, these slags are fully exposed with no sediment cover which would suggest a rather young age. To obtain some idea of their age, charcoal embedded in a selected piece of slag from one of the sites was submitted for AMS radiocarbon dating. The slag which was from Farasaolugu Mevkii, near Yahyali, gave an age of 393 years BP with an expected standard deviation of 39 years. A graph of 14C age versus calibrated calendar age gives an expected primary calendar age of 1470 CE and a secondary calendar age of 1610 CE. The primary age is similar to the average date for the slags at Yaprakli of 1412 CE, and places the slag in early Ottoman times. Most of the sites were similar in size to many of those at Yaprakli, but none were as small nor as large as some of those sites. The individual pieces of slag have the appearance of scoriaceous lava and are 8–10 cm in diameter. The slags, in roughly ‘hand’ size pieces, generally have the texture of a compact ceramic usually with abundant vesicles. While some pieces are glass and some pieces have ropey surfaces, none of the pieces showed any flow banding. Compositionally the iron slags consist mainly of kirschsteinite, calcic fayalite, fayalite, wustite, plagioclase, metallic iron and interstitial glass. Slag samples with high lime contents contain modal wollastonite and calcic-augite. The interstitial glass between the crystalline phases is normative in kalsilite-anorthite-wollastonite-fayalite-quartz. Most of the glasses lie adjacent to but on the silica-rich side of the anorthite-fayalite join, and on both sides of the anorthite-fayalite cotectic. This along with the compositions of the calcic-fayalites suggests that the interstitial glass formed at temperatures in the 1,100–1,200°C range. The glasses of the Develi-Yahyali slags are distinctly richer in iron and lime and lower in silica than those from Yaprakli.

Exceptionally well-preserved pillowed and massive phenocryst-free metabasaltic lava flows in the uppermost part of the Palaeoarchaean Hooggenoeg Complex of the Barberton Greenstone Belt exhibit both flow banding and large leucocratic varioles. The flow banding is defined by blebs and bands of pale and dark green metabasalt and was the result of mingling of two types of basalt (Robins et al. in Bull Volcanol 72:579–592, 2010a). Varioles occur exclusively in the dark chlorite-, MgO- and FeO-rich metabasalt. Varioles are absent in the outermost rinds of pillows and increase in both abundance and size towards the centres of pillows. In the central parts of some pillows, they impinge to form homogeneous pale patches, bands or almost homogenous cores. Individual varioles consist essentially of radially orientated or outwardly branching dendritic crystals of albite. Many varioles exhibit concentric zones and finer-grained rims. Some varioles seem to have grown around tiny vesicles and vesicles appear to have been trapped in others between a core and a finer-grained rim. The matrix surrounding the ocelli contains acicular pseudomorphs of actinolite and chlorite after chain-like, skeletal Ca-rich pyroxenes that are partly overgrown by the margins of varioles. Varioles are enriched in the chemical constituents of feldspar but contain concentrations of immobile TiO2, Cr, Zr and REE that are similar to the host metabasalts. The shape, distribution, texture and composition of the varioles exclude liquid immiscibility and support an origin by spherulitic crystallisation of plagioclase from severely undercooled basalt melt and glass. Nucleation of plagioclase was strongly inhibited and took place on vesicles, on the bases of drainage cavities and along early fractures. Eruption in deep water and retention of relatively high concentrations of volatiles in the melt may be the principal cause of spherulitic crystallisation in the interiors of pillows rather than only in their margins as in younger submarine flows. publishedVersion

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.

In order to understand the movement of large rock masses or allochthons on low-angle surfaces, we have studied the 3400-km2 Heart Mountain slide block of northwestern Wyoming and southwestern Montana. The Heart Mountain slide block was initiated on a 2° gradient, with its toe thrust a minimum of 45 km across an early Eocene landscape. The slide block moved on a basal layer that ranges in thickness from a few tens of centimeters to several meters. This basal layer commonly has a concrete-like appearance of rounded, mixed-lithology grains in a fine-grained carbonate matrix, and in some locations it has features similar to sedimentary deposits, including both normal and inverse grading, flow banding, turbidite-like structures, and clastic dikes containing pieces of carbonized wood. Nowhere did we observe crosscutting relationships in the basal layer or overlying clastic dikes, as would be expected from incremental or noncatastrophic emplacement. Results from cathodoluminescence and δ18O, δ13C, and 8...

Sheath-fold-like structures in pseudotachylyte samples from a gold mine in the Witwatersrand Basin (South Africa) are explained by injections of frictionally produced silicate melt within the shear zone due to the formation of high and low pressure zones. If the shear zone geometry is known, the orientation of the fold closures can be used to determine the direction and the movement sense. However, if it is not possible to establish the shear zone geometry, only the azimuth of movement can be estimated from the orientation of the sheath-fold-like folds. The flow banding in these fault rocks is indicative of laminar flow, which suggests a low Reynold's number.

Sandstone intrusions are widespread west of Santa Cruz, California and were emplaced during late Cenozoic tectonic deformation of this region. Among these is a very large and complex intrusion which is well exposed along the coastline at Yellow Bank Creek. Here, fluidized sands from the Miocene Santa Margarita Sandstone were injected upward into fractured biosiliceous rocks of the Santa Cruz Mudstone, probably due to faulting and seismic shaking. The complicated internal structure of this intrusion includes sedimentary xenoliths, fluidization structures, and secondary limonite staining. The latter likely occurred during oxidation by groundwater and produced conspicuous, complicated layering which serves to mask and confuse interpretations of the earlier-formed features. Among the earlier formed features are fluidization structures, comprising (1) flow banding which records injection of sands horizontally in silllike areas of the intrusion, and (2) heave structures which reflect mainly vertical injection of hydrocarbons and sands partially saturated with hydrocarbons into water-saturated sands. This latter type of injection appears to have occurred at a hydrocarbon front that was derived from either a localized petroleum accumulation or else from remnants of hydrocarbons that had mostly migrated updip prior to the clastic intrusion event. Dolomitic cementation occurred preferentially in the hydrocarbon-saturated sands due to degradation of the hydrocarbons. Paleotemperature estimates of the intrusive sandstones (by apatite fission track analysis) and of the host Santa Cruz Mudstone (by vitrinite reflectance) indicate maximum temperatures of about 60°C for the former, 50°C for the latter. Our data suggests that initial fluidization began in water-saturated sands of the bioturbated facies in the Santa Margarita Sandstone; following upward intrusion of these sands, fluidization and injection expanded into hydrocarbon-bearing sands within the cross-bedded facies of the same unit.

Textures in an exceptionally preserved effusive rhyolite conduit at Torfajokull, Iceland, indicate that rising magma repeatedly fractured and healed at shallow levels in the conduit (RFH process). Anastomosing tuffisite veins filled by fine-grained juvenile clasts were generated by shear fracture of highly viscous magma in the glass transition interval. Welding of the particulate material during subsequent deformation led to thorough healing of veins, allowing repeated fracture of the same body of magma. We propose that the RFH process is a rechargeable trigger mechanism for hybrid seismicity and show that the time scale of the process and the fractures formed by it are consistent with the repeat time and magnitude of hybrid earthquakes during silicic eruptions. The RFH process may also form the flow banding that is nearly ubiquitous in obsidian.

Magma mixing in silicic volcanic rocks of the Inyo chain is manifest by bimodal mineral chemistry and distinctive banding, defined by either variations in (1) glass color and chemistry or (2) microlite abundance within colorless glass. Bands composed of brown or microlite-rich, colorless glass are characterized by more Mg-rich mafic silicates and more calcic plagioclase than microlite-poor, colorless glass domains. A rhyolitic and a dacitic mixing end-member can be defined on the basis of this mineral distribution. Thermobarometry using the mineral assemblages unique to each end-member indicates that prior to mixing, the dacite was more oxidized and water-rich than the rhyolitic magma. Determined mineral crystallization conditions compare favorably with experimentally derived phase equilibria for melts of broadly similar composition. The spatial association of contrasting mineral assemblages with the flow banding in these rocks implies that the banding is a direct product of magma mixing. It formed as either (1) volumes of the comingled magmas were immediately vented to the surface and quenched, preserving domains of two distinct glass compositions, or (2) bands of dacitic magma precipitated microlites when comingled with the rhyolitic magma in an attempt for the two magmas to approach equilibrium.

SUMMARY: An appreciation of the positions within a magma chamber from which magma will be erupted can be gained from an understanding of the pattern of magma removal. Upwardly converging streamlines followed by a fluid escaping from a flat-topped reservoir into a cylindrical conduit cause magma from widely separated horizontal and vertical locations in a pre-eruption chamber to be simultaneously erupted. The volume erupted ( V ) is related to the maximum depth tapped ( R ) by the equation V = (2π/9) R 3 . Large ash-flow sheets (100 km 3 ) may therefore sample magma up to 5 km below the chamber roof. Analysis of the flow field suggests that about one half of a given sample of erupted magma represents material from the deepest part of the chamber being tapped at the time of eruption. Many petrographic features of chemically zoned ignimbrites can best be explained by the operation of such a flow-field during eruption, where a series of sub-spherical sampling shells intersect the horizontal layers in the magma chamber to produce heterogeneous ejecta. Mixed magmas erupted late in some zoned ash-flow sequences can originate by tapping across the boundary between silicic and underplating basaltic magmas which coexist in the source chamber rather than by convective mixing prior to eruption. Flow banding in rhyolites can result from the same fluid dynamic processes.

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.

A method is presented for using the change in angle between a plane and an initially perpendicular line together with the orientation of the finite-strain ellipsoid to calculate the axial ratio of the ellipsoid. Cleavage and stretching direction are used to give the orientation of the finite-strain ellipsoid. The method is applied to trails of amygdules which are presumed to have been initially perpendicular to the flow banding in the Pitts Head Ignimbrite of Snowdonia, North Wales. Results are in good agreement with those calculated using other strain markers.

The basalt of Jebel Et Toriya near Khartoum/Sudan has been studied in order to discover the mode of emplacement. For this purpose the chemical composition of an altered horizon in the bassalthas been analysed. Losses of Na, Mg, Ca,and the related gains of Al, Fe, Ti, V and Zr show element depletion and enrichment, which are common for rock weathering. This evidence permits a re-interpretation of the basalt mass comprising of two surface flows, separated from each other, by a period of chemical weathering. Some geological models of emplacement have been suggested and discussed to explain the occurrence of centripetally dipping flow banding and surface weathering.

A boulder of mineralized breccia (300 mm across) was recently recovered from the Crom Allt river 3 km downstream [NN 336 293] of the abandoned Tyndrum Pb + Zn vein mineralization. The breccia style of mineralization of the boulder with base-metal sulphides is similar to the Tyndrum mineralization. However detailed examination of the boulder revealed it to have a complex mineralogy and fundamental differences from the mineralized breccias of the Tyndrum Pb + Zn veins (Pattrick, 1981). The boulder mineralization includes breccia fragments of a highly siliceous rock which in hand specimen appears rhyolitic with possible flow banding and ignim-britic textures. However thin section examination reveals the rock to comprise quartz clasts up to 10 mm across set in a fine grained matrix of quartz, muscovite and illite, a mineralogy more consistent with a low-grade, poorly-sorted metapsammite. In contrast the rock fragments in the Tyndrum vein breccias are mainly Grampian group (Eilde) quartzites which are much coarser-grained, better-sorted, highly feldspathic and of higher metamorphic grade. Polished and thin sections reveal a complex sequence of mineralization and formulating a definitive paragenetic sequence on a single boulder is difficult. However three stages of mineral precipitation can be recognised: (1) The rock fragments are usually rimmed by a growth of acicular quartz. This quartz is often overgrown by siderite (sometimes rhombic grains up to 5 mm) and sphalerite displaying ‘colour’ growth banding. Minor galena is present, often replacing the sphalerite. (2) Minor rebrecciation associated with precipitation of fine-grained quartz and anhedral galena caused . . .

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.

Certain microtonalite dykes cut across the flow banding of the Main Donegal Granite in the direction of an important cross-joint, yet they and their granite host are shown to have crystallized together and in the same manner as both the darker flow bands in the granite and the pelitic xenoliths within it. It appears that an embryonic joint system existed in the Donegal Granite before consolidation and crystallization were complete. It is suggested that the early dykes represent metasomatized pelitic enclave material and are not differentation products of granitic magma. Comparison is made with the basic analogue provided by the beerbachite enclaves and dykes of the hypersthene-gabbro of Ardnamurchan.

Many rhyolitic obsidian flows show consistent stratigraphic relations among textural units exposed in the flow fronts of undissected flows and in cross sections of older flows. The stratigraphy of the Holocene Little Glass Mountain rhyolitic obsidian flow consists of (from bottom to top): air-fall tephra deposits, basal breccia, coarsely vesicular pumice, obsidian, finely vesicular pumice, and surface breccia. Slightly crystalline rhyolite occurs near the vent areas. A model of obsidian-flow emplacement based on these textural relations is presented. This stratigraphy may reflect the distribution of volatiles within the magma source region, with the interlayered contact between coarsely vesicular pumice and overlying obsidian indicating stratification of volatiles in the magma body. The distribution of pumiceous and glassy zones along with the orientations of flow banding can be used to map the surface structure of rhyolitic obsidian flows. This complex structure, as mapped on the Little Glass Mountain flow, reflects both the initial flow stratigraphy and subsequent deformational processes. During emplacement of the lava, three processes disrupt the initial flow configuration: the rise of coarse pumice diapirs from the base of the flow, the inward propagation of fractures in areas of extension, and surface folding in sites of flow-parallel compression. Subvertical flow banding in vent areas indicates that fracturing accompanies the emergence of lava; most of the observed upper surface of a dome originates as a fracture plane. The structure of domes that form over vent areas may reflect the orientation of dike-like conduits as well as the local state of stress during extrusion.

Several silicic units of the Trans-Pecos volcanic field have outcrop and thin-section scale features of lava flows but areal extents and aspect ratios of ignimbrites. These voluminous rocks (up to hundreds of cubic kilometres per unit) are quartz trachytes to low-silica rhyolites (68% to 72%SiO 2 ). Lava flow features include flow banding and folding, elongated vesicles, and autobreccias and vitrophyres at the base and top of units. Pyroclastic flow features include sheetlike geometry, lateral extents up to 70 km, aspect ratios as low as 1:700, and areal extents up to 3000 km 2 . A few of these units are clearly rheomorphic ignimbrites, but others show no unambiguous evidence of a primary pyroclastic origin. Although no adequate explanation currently exists for the origin of the latter, we evaluate two end-member hypotheses: (1) they are ignimbrites in which extreme rheomorphism has obliterated primary internal features, and (2) they are highly viscous lavas with unusually high heat retention or effusion rates that allowed them to spread over great areas. Either origin requires a rock type and eruptive mechanism not commonly recognized.