Showing papers on "Pyroclastic rock published in 1986"
TL;DR: Hyperconcentrated flood-flow deposits are distinguished from normal, dilute streamflow deposits by lack of crossstratification in sand facies and by very poor sorting, poor imbrication, and numerous clasts with long axes oriented parallel to flow direction in gravel facies as discussed by the authors.
Abstract: Modern and ancient volcaniclastic sedimentary sequences contain depositional units whose features cannot be attributed to fully turbulent, dilute stream flow or viscous debris flow. The characteristics of these poorly sorted sediments suggest rapid deposition from high-concentration dispersions but not en masse . Sedimentation thus appears related to high-discharge flows intermediate in sediment/water ratio between stream flow and debris flow. The term “hyperconcentrated flood flow” is proposed for describing this intermediate condition. Hyperconcentrated flood-flow deposits are distinguished from debris-flow deposits by lack of matrix support or reverse grading and instead exhibit distribution normal grading and horizontal stratification. These deposits are distinguished from normal, dilute stream-flow deposits by lack of cross-stratification in sand facies and by very poor sorting, poor imbrication, and numerous clasts with long axes oriented parallel to flow direction in gravel facies. The horizontal bedding that dominates sandy hyperconcentrated flood-flow deposits consists of sediment too coarse grained and strata too thick to have been produced in the boundary layer of the upper-flow regime and should not be confused with the more familiar thin, graded laminae of fine- to medium-grained sand often associated with parting lineation. Hyperconcentrated flood-flow deposits are not unique to volcanic settings; they also occur in arid, alluvial-fan sequences. Debris-flow and hyperconcentrated flood-flow deposits, however, are much thicker and more extensive in volcanic regions than on alluvial fans because explosive volcanism leads to rapid mobilization of large volumes of sediment and water on a scale unparalleled in nonvolcanic settings. In volcanic regions, therefore, these deposits have greater preservation potential, show greater lateral variability, and are more voluminous. Transformation of channelized debris flow to hyperconcentrated flood flow by dilution with stream water, recently observed at Mount St. Helens, is recorded in ancient volcaniclastic sequences and may serve as the primary mechanism for generating hyperconcentrated flood flow.
469 citations
TL;DR: Pantelleria is a volcanic island with a subaerial eruptive history longer than 300 Ka as mentioned in this paper, which is dominated by the 6 km wide Cinque Denti caldera, which formed ca. 45 Ka during the Green Tuff eruption.
Abstract: Situated in a submerged continental rift, Pantelleria is a volcanic island with a subaerial eruptive history longer than 300 Ka. Its eruptive behavior, edifice morphologies, and complex, multiunit geologic history are representative of strongly peralkaline centers. It is dominated by the 6-km-wide Cinque Denti caldera, which formed ca. 45 Ka ago during eruption of the Green Tuff, a strongly rheomorphic unit zoned from pantellerite to trachyte and consisting of falls, surges, and pyroclastic flows. Soon after collapse, trachyte lava flows from an intracaldera central vent built a broad cone that compensated isostatically for the volume of the caldera and nearly filled it. Progressive chemical evolution of the chamber between 45 and 18 Ka ago is recorded in the increasing peralkalinity of the youngest lava of the intracaldera trachyte cone and the few lavas erupted northwest of the caldera. Beginning about 18 Ka ago, inflation of the chamber opened old ring fractures and new radial fractures, along which recently differentiated pantellerite constructed more than 25 pumice cones and shields. Continued uplift raised the northwest half of the intracaldera trachyte cone 275 m, creating the island's present summit, Montagna Grande, by trapdoor uplift. Pantellerite erupted along the trapdoor faults and their hingeline, forming numerous pumice cones and agglutinate sheets as well as five lava domes. Degassing and drawdown of the upper pantelleritic part of a compositionally and thermally stratified magma chamber during this 18-3-Ka episode led to entrainment of subjacent, crystal-rich, pantelleritic trachyte magma as crenulate inclusions. Progressive mixing between host and inclusions resulted in a secular decrease in the degree of evolution of the 0.82 km3 of magma erupted during the episode. The 45-Ka-old caldera is nested within the La Vecchia caldera, which is thought to have formed around 114 Ka ago. This older caldera was filled by three widespread welded units erupted 106, 94, and 79 Ka ago. Reactivation of the ring fracture ca. 67 Ka ago is indicated by venting of a large pantellerite centero and a chain of small shields along the ring fault. For each of the two nested calderas, the onset of postcaldera ring-fracture volcanism coincides with a low stand of sea level. Rates of chemical regeneration within the chamber are rapid, the 3% crystallization/Ka of the post-Green Tuff period being typical. Highly evolved pantellerites are rare, however, because intervals between major eruptions (averaging 13−6 Ka during the last 190 Ka) are short. Benmoreites and mugearites are entirely lacking. Fe-Ti-rich alkalic basalts have erupted peripherally along NW-trending lineaments parallel to the enclosing rift but not within the nested calderas, suggesting that felsic magma persists beneath them. The most recent basaltic eruption (in 1891) took place 4 km northwest of Pantelleria, manifesting the long-term northwestward migration of the volcanic focus. These strongly differentiated basalts reflect low-pressure fractional crystallization of partial melts of garnet peridotite that coalesce in small magma reservoirs replenished only infrequently in this continental rift environment.
193 citations
TL;DR: In this article, a particular K-Ar technique, the Cassignol technique, has been developed in order to date Upper Pleistocene and Holocene volcanic rocks, which is capable of achieving KAr dates as young as 2000 a with a few centuries accuracy.
Abstract: A particular KAr technique, the Cassignol technique, has been developed in order to date Upper Pleistocene and Holocene volcanic rocks. We describe here its principles and its technology. The limit of detectability of the radiogenic Ar portion corresponds to an error of less than 103 a for K-rich minerals and a few 103 a for basalts. The reliability of the results and the validity of the correction for atmospheric contamination have been checked by analysing historical lavas and by comparison with data obtained from radiocarbon and thermoluminescence dating methods. The results demonstrate that the technique is capable of achieving KAr dates as young as 2000 a with a few centuries accuracy. A precision of ± 1.5% is obtained for samples older than 105 a. Moreover, in rocks older than 107 a, the technique permits the accurate dating of minute amounts of pure separated mineral phases. A reconstruction of the recent volcano-tectonic evolution of the Naples area has been carried out. It allows us to establish a model for estimating volcanic hazards in the Phlegrean Fields. The dated terrestrial pyroclastic deposits have been recovered from cores collected in the central Tyrrhenian Sea. The land-sea correlations allow us to refine the chronology of the marine oxygen isotope records for the past 60,000 a.
174 citations
TL;DR: The Jemez volcanic field has a record of volcanism extending back to before 13 Ma as mentioned in this paper, with minor rhyolitic eruptions occurring along N-S rift-bounding faults.
Abstract: The Jemez volcanic field straddles the western margin of the Rio Grande rift where the rift is intersected by the Jemez lineament in north central New Mexico. The field has a record of volcanism extending back to before 13 Ma. Initial basaltic activity was related to active rifting, with minor rhyolitic eruptions occurring along N-S rift-bounding faults. Between 10 and 7 Ma, voluminous andesitic volcanism took place in the central Jemez Mountains area, overwhelming contemporaneous basaltic and rhyolitic magmatism. An apparent tectonic lull took place from 7 to 4 Ma, accompanied by lower eruption rates. During this interval, dacitic magmas were erupted to form the Tschicoma volcanic center, but mafic and rhyolitic volcanism virtually ceased. Since 4 Ma, accompanying resumption of rifting, a growing silicic magma system has been present under the central part of the Jemez Mountains, ultimately evolving to the magma body that produced the voluminous rhyolitic Bandelier tuffs. Explosive rhyolitic eruptions from this large magma body have occurred many times since 3 to 4 Ma. Early eruptions, 3.6–2.8 Ma, produced high-silica rhyolite ignimbrites restricted to the southwest part of the Jemez Mountains; formation of these units may have been accompanied by caldera collapse. These events were followed by the two caldera-forming Bandelier Tuff ignimbrite eruptions, 1.45 and 1.12 Ma. Post caldera explosive and effusive rhyolite eruptions have also tapped the magma body from vents generally located along ring fractures after both Bandelier events. Vent and caldera locations for the rhyolitic eruptions during 3–4 Ma have been inferred from grain size characteristics, dispersal patterns, and facies variations in the Plinian deposits and ignimbrites. Lithic breccia zones of the pre-Bandelier ignimbrites indicate possible caldera sources in the southwest part of the present Valles caldera. During the eruption of both Bandelier tuffs, initial plinian falls and early pyroclastic flows emanated from vents centrally located in the Jemez Mountains. In the lower Bandelier Tuff eruption a transition to ring fracture vents occurred before the emission of later pyroclastic flows, but there is no strong evidence to suggest such a transition occurred during the upper Bandelier eruption. Calderas associated with the lower and upper Bandelier tuffs (Toledo and Valles, respectively) are almost identical in location, as are the Plinian vent sites for these two large eruptions. The Toledo embayment, northeast of the Valles caldera, contains lava domes from up to 3.6 Ma and may be a caldera or crater associated with early explosive dacitic volcanism in the Tschicoma volcanic center. Post-1.4 Ma lava domes also fill this depression. The main volcanic features of the Jemez Mountains field, including the Valles caldera complex, eruption vents, and the apical graben of the post-Valles-Redondo resurgent block, appear to be aligned along the NE-SW trending Jemez fault zone. This zone, the local expression of a Precambrian basement feature (the Jemez lineament), has exerted strong control on the location and style of eruptions from the Bandelier rhyolitic magma system.
173 citations
TL;DR: The most recent eruption at the Mono Craters occurred in the fourteenth century A.D. Radiocarbon dates and dendrochronological considerations constrain the eruption to a period between 1325 and 1365 as discussed by the authors.
Abstract: The most recent eruption at the Mono Craters occurred in the fourteenth century A.D. Evidence for this event includes 0.2 km 3 of pyroclastic fall, flow, and surge deposits and 0.4 km 3 of lava domes and flows. These rhyolitic deposits emanated from aligned vents at the northern end of the volcanic chain. Hence we have named this volcanic episode the North Mono eruption. Initial explosions were Plinian to sub-Plinian events whose products form overlapping blankets of air fall tephra. Pyroclastic flow and surge deposits lie upon these undisturbed fall beds within several kilometers of the source vents. Extrusion of five domes and coulees, including Northern Coulee and Panurn Dome, completed the North Mono eruption. Radiocarbon dates and dendrochronological considerations constrain the eruption to a period between A.D. 1325 and 1365. The lack of lacustrine laminae or aeolian and fluvial beds between individual pyroclastic beds suggests that the explosive phases of the eruption took place over a period of not more than several months. Within the resolution of the available radiocarbon and dendrochronologic dates, the North Mono eruption is contemporaneous with the latest eruption of the Inyo volcanic chain, about 20 km to the south. However, the Inyo tephra blanket clearly overlies, and thus postdates, all North Mono tephra. Minor disturbance of the North Mono tephra prior to deposition of the Inyo tephra indicates that the period of time between the North Mono and Inyo eruptions was probably no more than a year or two. This near contemporaneity of the two eruptions suggests a genetic relationship. Liquefaction of North Mono sands on the floor of Mono Lake occurred twice during the waning stages of the North Mono eruption and 3 times immediately before and after pulses of the Inyo eruption. This is evidence that five earthquakes of M L 5.5 occurred during the North Mono and Inyo eruptions. The chemical and textural similarity of the erupted products and their nearly simultaneous evacuation from aligned vents indicates that the North Mono eruption resulted from intrusion of a dike beneath the northern 6 km of the volcanic chain. Several observations suggest that dike intrusion beneath the Mono Craters has replaced normal faulting as the mechanism for elastic rebound and permanent extension of the crust at this latitude. However, dike widths compatible with relief of purely tectonic strains (3 m) are probably too narrow to have allowed the North Mono magma to erupt. Overpressurtzation of the Mono Craters magma reservoir by another mechanism, perhaps magma mixing, appears necessary as well. The
158 citations
TL;DR: In this article, two-dimensional computer simulations of avalanches using a simplified physical model are made. And they are shown to be more mobile than landslides in nonvolcanic areas.
144 citations
TL;DR: A small Plinian eruption of the Nevado del Ruiz volcano in Colombia ejected 3.5 x 1010 kilograms of mixed dacite and andesite tephra on 13 November 1985, which caused surface melting of approximately 10% of the volcano's ice cap, leading to meltwater floods.
Abstract: A small Plinian eruption of the Nevado del Ruiz volcano in Colombia ejected 35 x 1010 kilograms of mixed dacite and andesite tephra on 13 November 1985, with a maximum column height of 31 kilometers above sea level Small pyroclastic flows and surges, generated during the initial stage of the eruption, caused surface melting of approximately 10% of the volcano9s ice cap, leading to meltwater floods The erosive floods incorporated soils and loose sediments from the volcano9s flanks and developed into lahars, which claimed at least 25,000 lives
141 citations
TL;DR: In this paper, the authors presented field and grain-size data for the ring-vent products of the ring vent of Mt. Mazama and interpreted them as coignimbrite lag breccia formed within the deflation zone of the collapsing ringvent eruption columns.
132 citations
TL;DR: In this article, geochronological and volcanological data from volcanics of the island group of Milos (South Aegean active volcanic arc), allow four main cycles of volcanic activity to be distinguished, ranging in age between 3.5 and 0.1 m.
126 citations
TL;DR: In this article, a model for the evolution of the late Archaean Abitibi greenstone belt of the Superior Province of Canada is proposed based on structural, geochemical, sedimentological and geochronological studies.
Abstract: Based on structural, geochemical, sedimentological and geochronological studies, we have formulated a model for the evolution of the late Archaean Abitibi greenstone belt of the Superior Province of Canada. The southern volcanic zone (SVZ) of the belt is dominated by komatiitic to tholeiitic volcanic plateaux and large, bimodal, mafic-felsic volcanic centres. These volcanic rocks were erupted between approximately 2710 Ma and 2700 Ma in a series of rift basins formed as a result of wrench-fault tectonics.The SVZ superimposes an older volcanic terrane which is characterized in the northern volcanic zone (NVZ) of the Abitibi belt and is approximately 2720 Ma or older. The NVZ comprises basaltic to andesitic and dacitic subaqueous massive volcanics which are cored by comagmatic sill complexes and layered mafic-anorthositic plutonic complexes. These volcanics are overlain by felsic pyroclastic rocks that were comagmatic with the emplacement of tonalitic plutons at 2717 ±2 Ma.The tectonic model envisages the SVZ to have formed in a series of rift basins which dissected an earlier formed volcanic arc (the NVZ). Analogous rift environments have been postulated for the Hokuroko basin of Japan, the Taupo volcanic zone of New Zealand and the Sumatra and Nicaragua arcs. The difference between rift related ‘submergent’ volcanism in the SVZ and ‘emergent’ volcanism in the NVZ resulted in the contrasting metallogenic styles, the former being characterized by syngenetic massive sulphide deposits, whilst the latter was dominated by epigenetic ‘porphyry-type’ Cu(Au) deposits.
124 citations
TL;DR: In this article, eight new K-Ar determinations are presented which, together with field relations and previous studies, are used to construct a chronology of the volcanic build-up of Fuerteventura.
Abstract: Eight new K-Ar determinations are presented which, together with field relations and previous studies, are used to construct a chronology of the volcanic build-up of Fuerteventura. The earliest island-building volcanics are late Cretaceous to early Tertiary alkaline basaltic volcaniclastic sediments, and these may be correlated with the early gabbro/pyroxenite intrusions which were probably subvolcanic to the early edifice. This was followed by a carbonatitic subvolcanic complex then by two later high-level gabbro/pyroxenite plutons, the latter being early Miocene or older. The emplacement of a subsequent middle Miocene gabbro/syenite ring complex caused widespread resetting of the Ar contents of the earlier rocks, but nevertheless consideration of the mineralogy of the rocks in conjunction with the field relations enables the long volcanic history ( c . 80 Ma) to be reconstructed.
TL;DR: In this paper, the authors modeled the time history of magma withdrawal through a central vent from a flat-roofed chamber strongly stratified in density and viscosity and quantitatively predicted a mechanism for the generation of compositional gaps in ignimbrites following either a short eruption hiatus or an abrupt increase or decrease of the discharge during an otherwise quasi-steady eruption.
Abstract: The time history of magma withdrawal through a central vent from a flat-roofed chamber strongly stratified in density and viscosity has been numerically modeled. Important parameters include the geometry of the reservoir; the initial vertical compositional profile; the ratio of viscous, inertial, and gravitational forces; and the basal normal stress driving the eruption. Finite critical stresses in the range 102 to 106 Pa are required to initiate and maintain an eruption. The composition-time history of erupted magma depends strongly on the reservoir/conduit width ( A ) such that large A increases (1) the time interval during which mixed magma is erupted, (2) the steady-state time ( ts ), defined as the time at which the composition of erupted magma is within 1% of the initial basal composition, and (3) the fraction of silicic magma trapped within the chamber. Steady-state times increase by a factor of two as the viscosity contrast increases from 1 to 102, and they become independent of viscosity variations for contrasts > 104. It is possible to distinguish continuous from discontinuous (i.e., layered) pre-eruptive gradients within chambers by comparing synthesized and measured geochemical stratigraphic sections for particular pyroclastic flow deposits. A mechanism for the generation of compositional gaps in ignimbrites following either a short eruption hiatus or an abrupt increase or decrease of the discharge during an otherwise quasi-steady eruption is quantitatively predicted. Most important, a compositional gap or a series of gaps within a pyroclastic deposit does not necessarily mean that one existed within the chamber before the eruption. It is impossible to invert stratigraphically controlled geochemical data to obtain in situ chamber compositional structure if one does not have detailed information regarding the location of vents and the variation of magma discharge with time during a pyroclastic eruption.
TL;DR: A review of gravity data reflecting shallow concealed structures on volcanoes demonstrates that valuable information may be derived on the geological development of such structures and on the dynamic evolution of active volcanoes as mentioned in this paper.
TL;DR: The authors distinguish three eruptive units of pyroclastic flows (T1, T2, and T3; T for trass) within the late Quaternary Laacher See tephra sequence.
Abstract: We distinguish three eruptive units of pyroclastic flows (T1, T2, and T3; T for trass) within the late Quaternary Laacher See tephra sequence. These units differ in the chemical/mineralogical composition of the essential pyroclasts ranging from highly differentiated phonolite in T1 to mafic phonolite in T3. T1 and T2 flows were generated during Plinian phases, and T3 flows during a late Vulcanian phase. The volume of the pyroclastic flow deposits is about 0.6 km3. The lateral extent of the flows from the source vent decreases from > 10 km (T1) to 1000 N/m2 is consistent with the divergence of lithic size/distance curves from purely Newtonian models; the transport of lithics must be treated as in a Bingham fluid. The flow temperature probably decreased from T1 (300°–500°C) to T3 (<200°C). A large-scale longitudinal variation in the flow units from proximal through medial to distal facies dominantly reflects temporal changes during the progressive collapse of an eruption column. Only a small amount of fallout tephra was generated in the T1 phase of eruption. The pyroclastic flows probably formed from relatively low ash fountains rather than from high Plinian eruption columns.
TL;DR: A large diameter borehole core from an epiclastic kimberlite remnant on the farm Stompoor in the Prieska district, Cape Province, contains a continuous 76 m section of fossiliferous sediments interpreted as having accumulated within a crater-lake during the Late Cretaceous as mentioned in this paper.
Abstract: A large diameter borehole core from an epiclastic kimberlite remnant on the farm Stompoor in the Prieska district, Cape Province, contains a continuous 76 m section of fossiliferous sediments interpreted as having accumulated within a crater-lake during the Late Cretaceous. Three distinct facies associations reflect depositional processes that prevailed in offshore areas of the original lake. Facies Association A: matrix-supported pebble conglomerates comprising a chaotic assemblage of pyroclastic, basement and country rocks set in a fine-grained matrix. Flat, non-erosional basal surfaces with ‘frozen’ rip-up clasts, the protrusion of matrix-supported clasts above the upper surfaces and a direct relationship between maximum clast size and bed thickness suggest deposition from debris flows that originated subaerially on pyroclastic talus cones surrounding the crater. Facies Association B: alternating thin beds of matrix-supported granule conglomerate, structureless fine-grained sandstone and parallel laminated mudrock. Small fining-upward sequences within these beds are comparable to turbidite Bouma Tade, Tde. Numerous partings display petrified fish and frog skeletons, as well as bivalve, gastropod and ostracode shells, leaf impressions, insect wings and a possible bird bone. These beds were deposited by thin debris-flows and turbidity underflows interspersed with periods of ‘pelagic’ sedimentation. Facies Association C: microlaminated mudstone beds containing scattered ‘dropstone lapilli’. The lamination is imparted by alternating Ca-rich/Ca-poor layers which may reflect climatic seasonality. They are interpreted as the result of seasonally influenced suspension settling through a thermally stratified water column.
Short-term periodicities in conglomerate bed thicknesses are interpreted as the result of successive block caving of a slump scar giving rise to several debris flows from the same source area. Seismic shock from nearby volcanism may have simultaneously triggered slumps on both subaerial and subaqueous slopes. Dropstone lapilli in Type C beds and the preponderance of load casting in Type B beds support this interpretation. An estimate of the time span involved in accumulating 76 m of crater lake sediments based on the possible seasonal imprint of Type C beds gives a figure of some 220,000 yr.
TL;DR: The Toledo-Valles caldera was formed at 1.47±0.06 Ma during the catastrophic eruption of the lower member of the Bandelier Tuff and with several other field and drilling-related observations as discussed by the authors.
Abstract: The Toledo caldera was formed at 1.47±0.06 Ma during the catastrophic eruption of the lower member, Bandelier Tuff. The caldera was obscured at 1.12±0.03 Ma during eruption of the equally voluminous upper member of the Bandelier Tuff that led to formation of the Valles caldera. Earlier workers interpreted a 9-km-diameter embayment, located NE of the Valles caldera (Toledo embayment), to be a remnant of the Toledo caldera. Drill hole data and new K-Ar dates of Toledo intracaldera domes redefine the position of Toledo caldera, nearly coincident with and of the same dimensions as the younger Valles caldera. The Toledo embayment may be of tectonic origin or a small Tschicoma volcanic center caldera. This interpretation is consistent with distribution of the lower member of the Bandelier Tuff and with several other field and drilling-related observations. Explosive activity associated with Cerro Toledo Rhyolite domes is recorded in tuff deposits located between the lower and upper members of the Bandelier Tuff on the northeast flank of the Jemez Mountains. Recorded in the tuff deposits are seven cycles of explosive activity. Most cycles consist of phreatomagmatic tuffs that grade upward into Plinian pumice beds. A separate deposit, of the same age and consisting of pyroclastic surges and flows, is associated with Rabbit Mountain, located on the southeast rim of the Valles-Toledo caldera complex. These are the surface expression of what may be a thicker, more voluminous intracaldera tuff sequence. The combined deposits of the lower and upper members of the Bandelier Tuff, Toledo and Valles intracaldera sediments, tuffs, and dome lavas form what we interpret to be a wedge-shaped caldera fill. This sequence is confirmed by deep drill holes and gravity surveys. This fill accumulated in depressions formed during precaldera rifting and episodes of caldera collapse. We interpret the Toledo-Valles caldera complex to be a pair of nearly coincident trapdoor calderas, with the hinge on the west side and thick caldera fill in the east.
TL;DR: In this article, micro-spherules intercepted in the plume of Mount Etna, Sicily, during moderate recurring volcanic activity were analyzed using analytical transmission and scanning electron microscopy (ATEM and ASEM).
Abstract: A possible volcanic origin has been suggested for the micrometre-sized spherules that have been discovered in polar snows or ice-cores1–5, and in the stratosphere6,7. However, although such particles, especially when black and magnetic, have been identified among the tephra deposits of some volcanoes3,8–12, their volcanic emission has never been directly observed. Here we describe micro-spherules intercepted in the plume of Mount Etna, Sicily, during moderate recurring volcanic activity. Their study, using analytical transmission and scanning electron microscopy (ATEM and ASEM) demonstrates the simultaneous presence in the plume of glassy silicate microspherules of various chemical compositions (47–98% SiO2).
TL;DR: In this article, the effect on the buoyancy due to mixing of air and ash has been modelled in the laboratory using mixtures of methanol and ethylene glycol (MEG), which have a nonlinear density behavior when mixed with water.
TL;DR: In this article, the pre-1982 paratropical forest litter from El Chichon volcano in Mexico has been used to estimate the species composition and structure of the surrounding forest.
Abstract: Volcaniclastic sediments proximal to El Chichon volcano (Chiapas, Mexico) preserve the pre-1982 paratropical forest litter and provide an accurate estimate of the species composition and structure of the surroundingforest. Deposition offine ash, when wet, encrusted leaves, providing an immediate compression-impression flora of the arboreal vegetation. Deposits as close as 3.5 km from the crater were sufficiently fine grained to preserve impressions of high quality. Plant remains, mostly leaves, are horizontally layered and rarely contorted, conditions usually attributed to slow sediment deposition rather than rapid, "event" deposition. The deposits consist of a basal, forest-litter layer preserved in coarse volcaniclastic sediments, and an overlying layer of arboreal leaves, abscissed (separated from the twig) in response to heavy ash fall. Organic deposits are capped by up to 2.5 m of normally graded pumice and airfall lapilli. Plant remains are well preserved 10-20 km from the crater in airfall ash and ash-pumice, 10-15 cm thick. Woody vegetation is accurately reflected in the proximal volcanic deposits. Herbaceous vegetation is more variable in preservation; it is normally buried by coarse volcaniclastics, along with the arboreal litter, and thus has a low preservation potential. Only the autochthonous forest-floor litter preserves an accurate representation of the localflora: both herbaceous and woody. Recognition of such organic layers, possibly by superpositional relationships with soil horizons, ts critical to interpretation of the succession of plants on volcanic substrates. The stratified nature of terrestrial organic deposits resulting from a single eruptive sequence suggests caution be exercised in interpreting fossiliferous volcaniclastics; multiple fossil horizons may reflect a single depositional event rather than a succession of assemblages in an area through time.
TL;DR: Valles Caldera 1 (VC-1) as mentioned in this paper is the first continuous cored well in the Valles caldera region, which was designed to obtain structural and stratigraphie information near the intersection of the ring fracture zone and the precaldera Jemez fault zone.
Abstract: Valles Caldera 1 (VC-1) is the first Continental Scientific Drilling Program (CSDP) core hole drilled in the Valles caldera and the first continuously cored well in the caldera region. The objectives of VC-1 were to penetrate a hydrothermal outflow plume near its source, to obtain structural and stratigraphie information near the intersection of the ring fracture zone and the precaldera Jemez fault zone, arid to core the youngest volcanic unit inside the caldera (Banco Bonito obsidian). Coring of the 856-m well took only 35 days to finish, during which all objectives were attained and core recovery exceeded 95%. VC-1 penetrates 298 m of moat volcanics and caldera fill ignimbrites, 35 m of precaldera volcaniclastic breccia, and 523 m of Paleozoic carbonates, sandstones, and shales. A previously unknown obsidian flow was encountered at 160 m depth underlying the Battleship Rock Tuff in the caldera moat zone. Hydrothermal alteration is concentrated in sheared, brecciated, and fractured zones from the volcaniclastic breccia to total depth with both the intensity and rank of alterations increasing with depth. Alteration assemblages consist primarily of clays, calcite, pyrite, quartz, and chlorite, but chalcopyrite and sphalerite have been identified as high as 450 m and molybdenite has been identified in a fractured zone at 847 m. Carbon 13 and oxygen 18 analyses of core show that the most intense zones of hydrothermal alteration occur in the Madera Limestone above 550 m and in the Madera and Sandia formations below 700 m. This corresponds with zones of most intense calcite and quartz veining. Thermal aquifers were penetrated at the 480-, 540-, and 845-m intervals. Although these intervals are associated with alteration, brecciation, and veining, they are also intervals where clastic layers occur in the Paleozoic sedimentary rocks.
TL;DR: In this paper, various parameters of the internal structure of a debris-avalanche deposit from ancestral Mount Shasta (size and percentage of block facies in each exposure, number and width of jigsaw cracks, and number of rounded clasts in matrix facies) were measured in order to study flow and emplacement mechanisms.
Abstract: Various parameters of the internal structure of a debris-avalanche deposit from ancestral Mount Shasta (size and percentage of block facies in each exposure, number and width of jigsaw cracks, and number of rounded clasts in matrix facies) were measured in order to study flow and emplacement mechanisms. Three types of coherent blocks were identified: blocks of massive or brecciated lava flows or domes, blocks of layered volcaniclastic deposits, and blocks of accidental material, typically from sedimentary units underlying Shasta Valley. The mean maximum dimension of the three largest blocks of layered volcaniclastic material is 220 m, and that of the lava blocks, 110 m. This difference may reflect plastic deformation of blocks of layered volcaniclastic material; blocks of massive or brecciated volcanic rock deformated brittly and may have split into several smaller blocks. The blocks in the deposit are one order of magnitude larger, and the height of collapse 1100 m higher, than the Pungarehu debris-avalanche deposit at Mount Egmont, New Zealand, although the degree of fracturing is about the same.This suggests either that the Shasta source material was less broken, or that the intensity of any accompanying explosion was smaller at ancestral Mount Shasta. The Shasta debris-avalanche deposit covered the floor of a closed basin; the flanks of the basin may have retarded the opening of jigsaw cracks and the formation of stretched and deformed blocks such as those of the Pungarehu debris-avalanche deposit.
TL;DR: The 6163m high volcano San Pedro in the Western Cordillera of the central Andes of North Chile, is a composite of two cones, each representing a different structural and chemical stage in the evolution of the volcano.
Abstract: The 6163 m high volcano San Pedro in the Western Cordillera of the central Andes of North Chile, is a composite of two cones, each representing a different structural and chemical stage in the evolution of the volcano. Eruption of thin lavas and scoria, dominantly of basaltic andesite composition, built up an older cone, which was subsequently modified by both glacial erosion and a major collapse event, akin to that of Mt. St. Helens in May 1980. A younger cone has subsequently grown within the crater, obscuring much of the detailed structure of the older cone. The younger cone was built up in four distinct phases of eruption which produced andesite and dacite lavas and pyroclastic flow deposits. The latter include both pumice flows and hot avalanche deposits. All of these products from the younger cone are chemically distinct from those of the older cone. Whilst much of the whole rock geochemistry can be explained by simple crystal-liquid fractionation involving the observed phenocryst phases, this process cannot adequately account for the petrographic textures and mineralogical variations, which show evidence for considerable thermal and chemical disequilibrium. Textural, mineralogical and chemical variations are consistent with models of late stage mixing between magmas of the same fractionation series, with chemical and thermal inputs from hot dense basalt or basaltic andesite injected into the base of the magma chamber.
TL;DR: Paleomagnetic study of Middle Jurassic volcanic and volcaniclastic rocks in southeastern Arizona yields a paleomagnetic pole located at 61.8°N, 116.0°E (k = 49.6, α95 = 6.2°).
Abstract: Paleomagnetic study of Middle Jurassic volcanic and volcaniclastic rocks in southeastern Arizona yields a paleomagnetic pole located at 61.8°N, 116.0°E (k = 49.6, α95 = 6.2°). These rocks from Corral Canyon represent an autochthonous segment of the Jurassic Cordilleran magmatic arc and were deposited directly on Upper Paleozoic carbonates of the cratonic miogeocline. Rock magnetic experiments demonstrate that the natural remanent magnetization of these rocks is dominantly a single component magnetization carried in both magnetite and hematite phases and is interpreted as a primary thermoremanent magnetism. Rb/Sr isotope geochemistry on whole-rock samples from eight welded ash flow tuffs yields an isochron age of 172.2 ± 5.8(2σ) Ma. The Corral Canyon pole falls between poles from Newark Trend Group II (179 ± 3 Ma) and Glance Conglomerate (151 ± 2 Ma).
TL;DR: Two drill-holes were carried out during 1983-84 by the “Joint Venture AGIP-EMS-ENEL on the island of Vulcano southwest of the Cratere della Fossa.
Abstract: Two drill-holes were carried out during 1983–84 by the “Joint Venture” AGIP-EMS-ENEL on the island of Vulcano southwest of the Cratere della Fossa. After passing through pyroclastics and lavas of the young volcanic centres of Vulcano the drill-holes penetrated an intrusion of monzogabbro to leuco-monzogabbro composition. In one of the holes the top of the intrusion occurs at 1360 m and the intrusive rocks are found to the bottom of the well at 2050 m. At this depth the temperature exceeds 419 °C and the temperature gradients are sufficiently steep that magma could well be reached only a few hundred metres deeper. Lava of the South Vulcano centre is metamorphosed by the intrusion.
TL;DR: The Ambler district is located along the southern margin of the Brooks Range, Alaska as mentioned in this paper, which is composed of two geologic terranes: the Ambler terrane consists of Devonian and older schistose metasedimentary rocks overlain by Middle Devonian to Lower Mississippian phyllitic shales and siltstones.
Abstract: The Ambler district is located along the southern margin of the Brooks Range, Alaska. Mineral exploration since the late 1950s in this remote area has led to the discovery of a number of volcanogenic massive sulfide deposits and several carbonate-hosted copper deposits. The district is composed of two geologic terranes. The Ambler terrane consists of Devonian and older schistose metasedimentary rocks overlain by Middle Devonian to Lower Mississippian phyllitic shales and siltstones. Late Devonian carbonate sediments (Bornite sequence) and mixed carbonate and volcanic rocks (Ambler sequence) are interbedded within this thick metasedimentary succession. A low-angle fault separates the Ambler terrane from the overlying, southerly derived Angayucham terrane which is composed of Devonian to middle -Mesozoic mafic and ultramafic rocks. Both terranes were deformed and metamorphosed during the Jurassic (?) to Cretaceous Brooks Range orogeny. Significant mineralization is restricted to the Ambler and Bornite lithologic successions. The Ambler sequence, which contains a number of volcanogenic massive sulfide deposits, averages 1.5 km in thickness and is composed of approximately 60 percent volcanic and volcaniclastic rocks, 25 percent carbonate sediments, and 15 percent pelitic sediments. The volcanic rocks are bimodal with rhyolite predominating over basalt. Much of the rhyolite was apparently deposited as submarine ash-flow tuff. Two major types of mineralization are recognized: massive sulfide adjacent to major hydrothermal vent areas, and laterally extensive massive sulfide lacking well-defined vent areas. The Bornite carbonate sequence contains significant copper mineralization within large, structurally controlled zones of hydrothermal dolomitization.The bimodal volcanic sequence is suggestive of a rifting-related tectonic environment. This interpretation is consistent with the distribution of carbonate and volcanic sequences on what may be high structural blocks and with possible coeval ocean-floor spreading to the south suggested by the Angayucham mafic volcanic rocks.
TL;DR: Miyakejima volcano erupted at 1515 hours on October 3, 1985 as discussed by the authors, and a 4.5km-long flank fissure produced the curtain of fire at higher vents from 500 m to 100 m in altitude.
TL;DR: In this paper, the authors show that the ultramafic pyroclastic rocks are chaotic to stratified mixtures of angular to subrounded glassy picritic blocks and a fine grained volcaniclastic matrix that consists primarily of plastically-deformed, glassy globules.
Abstract: Tertiary ultramafic volcanism on Gorgona Island, Colombia, is manifested not only by komatiite flows, but also by a more voluminous sequence of tuff breccias, which is cut by comagmatic picrite dikes The ultramafic pyroclastic rocks are chaotic to stratified mixtures of angular to subrounded glassy picritic blocks and a fine grained volcaniclastic matrix that consists primarily of plastically-deformed, glassy globules The entire deposit is interpreted to have formed by an explosive submarine eruption of phenocryst-laden picritic magma MgO contents of tuff breccias and picrite dikes range from 21 to 27 wt% Relative to nearby komatiite flows, these rocks are MgO-rich, and FeO-, TiO2- and Ni-poor HREE concentrations are very low (
TL;DR: The orientation of dykes from many of the islands of the Lesser Antilles island arc has been mapped as discussed by the authors, and the results show that the dominant NE-SW azimuth mode corresponds closely to the direction of faulting in the sedimentary cover of the backarc and the inferred tectonic fabric of the oceanic crust on which the arc is founded.
Abstract: The orientations of dykes from many of the islands of the Lesser Antilles island arc have been mapped Most of these dykes can be interpreted in terms of local or regional swarms derived from specific volcanoes of known age, with distinct preferred orientations Dykes are known from all Cenozoic epochs except the Palaeocene, but are most common in Pliocene, Miocene and Oligocene rocks A majority of the sampled dykes are basaltic, intrude volcaniclastic host rocks and show a preference for widths of 1–125 m Locally, dyke swarms dilate their hosts by up to 9% over hundreds of metres and up to 2% over distances of kilometres The azimuths of dykes of all ages show a general NE-SW preferred orientation with a second NW-SE mode particularly in the Miocene rocks of Martinique The regional setting for these minor intrusions is a volcanic front above a subduction zone composed of three segments: Saba-Montserrat, Guadeloupe-Martinique, St Lucia-Grenada The spacing of volcanic centres along this front is interpreted in terms of rising plumes of basaltic magma spaced about 30 km apart This magma is normally intercepted at crustal depths by dioritic plutons and andesitic/dacitic magma generated there Plumes which intersect transverse fracture systems or which migrate along the front can avoid these crustal traps Throughout its history the volcanic front as a whole has migrated, episodically, towards the backarc at an average velocity of about 1 km/Ma The local direction of plate convergence is negatively correlated with the local preferred orientation of dykes The dominant NE-SW azimuth mode corresponds closely to the direction of faulting in the sedimentary cover of the backarc and the inferred tectonic fabric of the oceanic crust on which the arc is founded A generalised model of the regional stress field that controls dyke intrusion outside of the immediate vicinity of central volcanic vents is proposed, in which the maximum horizontal stress parallels the volcanic front except in the northern segment where subduction of the Barracuda Rise perturbs the stress field There is also evidence of specific temporal changes in the stress field that are probably due to large scale plate kinematics
TL;DR: The southern volcanic zone (SVZ) of the Late Archean Abitibi belt of Canada is dominated by komatiitic to tholeiitic volcanic plateaus and large, bimodal, mafic-felsic volcanic centers as discussed by the authors.
Abstract: The southern volcanic zone (SVZ) of the Late Archean Abitibi belt of the Superior province of Canada is dominated by komatiitic to tholeiitic volcanic plateaus and large, bimodal, mafic-felsic volcanic centers. These volcanic rocks were erupted between about 2717 and 2700 Ma in a series of rift basins that formed as a result of wrench-fault tectonics. They overlie and juxtapose a volcano-plutonic assemblage characterized in the northern Abitibi belt. The age of the assemblage is about 2720 Ma or older, and it comprises basaltic to andesitic and dacitic subaqueous massive volcanics, cored by comagmatic sills and layered anorthositic complexes and overlain by felsic pyroclastic rocks that were comagmatic with the emplacement of tonalitic plutons at 2717 ± 2 Ma. A tectonic model is proposed in which the SVZ formed in a series of rift basins that dissected an earlier formed volcanic arc. Comparisons are made with rift environments that have been postulated for Phanerozoic areas such as the Hokuroko basin of Japan, the Taupo volcanic zone of New Zealand, and the Sumatra and Nicaragua arcs.
TL;DR: In this article, the effect of sedimentary processes is never strong enough to obscure the fundamental relationship of sandstone composition to provenance, but it accounts for most short-term variability in quartz/feldspar and feldspar/ volcanic rock fragment ratios.
Abstract: The Carnian sandstones of the Southern Alps are fluviodeltaic and lagoonal clastic wedges derived from volcanic source rocks changing with time from mesosilicic to persilicic lavas and pyroclastics. Two-thirds of the variance in detrital modes reflect this petrologic evolution of the source. The effect of sedimentary processes is never strong enough to obscure the fundamental relationship of sandstone composition to provenance, but it accounts for most short-term variability in quartz/feldspar and feldspar/ volcanic rock fragment ratios. Plagioclase and quartz grains are concentrated, respectively, in the very fine to fine fractions and in medium-grained sandstones. The quartz/feldspar ratio (Q/F) thus decreases through delta-plain upward-fining sequences, from channel-lag sediments ( /F = 1/3) to overbank sands (Q/F = 1/8). The sedimentary control on the composition of fluviodeltaic sands results from the coupling of mechanical effects in high-gradient headwater streams and hydraulic sorting in low-gradient depositional environments. Assuming that the effectiveness of mineralogical segregation in different grain-size fractions during transport depends primarily on paleorelief, a decrease in altitude during Carnian times is inferred within the Triassic volcanic source, which lay south of the sedimentary depocenter. Reworking by weak traction currents in lagoonal settings has further increased the textural maturity and the mineralogical stability of the sands. Volcanic rock fragments were reduced by selective destruction in marine environments, but the Q/F ratio was no affected, and a common value of 1/6 characterizes all lagoonal strata.