‘Snake River (SR)-type’ volcanism at the Yellowstone hotspot track: distinctive products from unusual,high-temperature silicic super-eruptions

A new category of large-scale volcanism, here termed Snake River (SR)-type volcanism, is defined with reference to a distinctive volcanic facies association displayed by Miocene rocks in the central Snake River Plain area of southern Idaho and northern Nevada, USA. The facies association contrasts with those typical of silicic volcanism elsewhere and records unusual, voluminous and particularly environmentally devastating styles of eruption that remain poorly understood. It includes: (1) large-volume, lithic-poor rhyolitic ignimbrites with scarce pumice lapilli; (2) extensive, parallel-laminated, medium to coarse-grained ashfall deposits with large cuspate shards, crystals and a paucity of pumice lapilli; many are fused to black vitrophyre; (3) unusually extensive, large-volume rhyolite lavas; (4) unusually intense welding, rheomorphism, and widespread development of lava-like facies in the ignimbrites; (5) extensive, fines-rich ash deposits with abundant ash aggregates (pellets and accretionary lapilli); (6) the ashfall layers and ignimbrites contain abundant clasts of dense obsidian and vitrophyre; (7) a bimodal association between the rhyolitic rocks and numerous, coalescing low-profile basalt lava shields; and (8) widespread evidence of emplacement in lacustrine-alluvial environments, as revealed by intercalated lake sediments, ignimbrite peperites, rhyolitic and basaltic hyaloclastites, basalt pillow-lava deltas, rhyolitic and basaltic phreatomagmatic tuffs, alluvial sands and palaeosols. Many rhyolitic eruptions were high mass-flux, large volume and explosive (VEI 6–8), and involved H₂O-poor, low-δ¹⁸O, metaluminous rhyolite magmas with unusually low viscosities, partly due to high magmatic temperatures (900–1,050°C). SR-type volcanism contrasts with silicic volcanism at many other volcanic fields, where the fall deposits are typically Plinian with pumice lapilli, the ignimbrites are low to medium grade (non-welded to eutaxitic) with abundant pumice lapilli or fiamme, and the rhyolite extrusions are small volume silicic domes and coulées. SR-type volcanism seems to have occurred at numerous times in Earth history, because elements of the facies association occur within some other volcanic fields, including Trans-Pecos Texas, Etendeka-Paraná, Lebombo, the English Lake District, the Proterozoic Keewanawan volcanics of Minnesota and the Yardea Dacite of Australia. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. This paper constitutes part of a special issue dedicated to Bill Bonnichsen on the petrogenesis and volcanology of anorogenic rhyolites.     

Volcanological aspects of peralkaline silicic welded ash-flow tuffs

Peralkaline silicic welded ash-flow tuffs differ characteristically in a number of properties from most calc-alkaline welded tuffs, due to their generally lower viscosity and higher temperatures. For example, individual cooling units are relatively small (less than 30 m thick, less than 5 km³ in volume); rocks can be thoroughly welded and crystallized to feldspar, quartz, and mafic minerals; several primary deformational structures (e.g. lineations, stretching of pumice, folds, ramp structures) indicate late stage laminar creep, resulting from the low yield strength of the nearly homogeneous glass of very low viscosity. Theoretical considerations also suggest that peralkaline melts are of low viscosity and high temperature, as inferred from,e.g., their chemical composition (high iron- and alkali-, and low alumina-concentrations). The low viscosity may also be due to trapping of volatiles. Absence or paucity of OH-bearing phenocryst phases, paucity of pyroclastic rocks, other than ash flow tuffs, formed from highly explosive eruptions, and apparently high crystallization temperatures, indicate that peralkaline silicic magmas are comparatively dry. The common occurrence of peralkaline ash-flow tuffs may be due to an increased water content of the magmas, resulting also in amphibole phenocrysts in some welded tuffs, or to specific volcanotectonic conditions. Ash flows of peralkaline composition move as particularly dense particulate flows. This type of flowage and the very rapid welding of the low viscosity glass lead to a high degree of homogenization of the fine glass shards. This in turn inhibits complete degassing of the collapsing ash flow. Semiclosed systems result where gas overpressures can develop and where volatiles play an important role by fluxing crystallization and transporting dissolved matter. Several types of vesicles can form under these conditions: (a) Spherical vesicles within collapsed ash and pumice particles formed after deposition of the ash flow. (b) Round or irregular vesicles transsecting pyroclastic particles, vesicle sheets, and large cavities, several m in diameter, may form in a largely homogenized ash-flow tuff beneath tightly welded layers. (c) Lensoid cavities formed during granophyric crystallization of large pumice particles. Small ash particles of peralkaline composition may assume spherical shapes due to their low viscosity and in some cases, expansion of bubbles. They form during transport and are preserved under low load pressure in the top part of cooling units. Globule lavas and most froth flows are interpreted as welded ash-flow tuffs, some of their unusual features being due to their peralkaline composition.     

The rio caliente ignimbrite: Analysis of a compound intraplinian ignimbrite from a major late quaternary Mexican eruption

The Rio Caliente ignimbrite is a multi-flow unit orcompound ignimbrite formed during a major late Quaternary explosive rhyolitic eruption of La Primavera volcano, Mexico. The eruption sequence of the ignimbrite is complex and it occurs between lower and upper plinian air-fall deposits. It is, therefore, anintraplinian ignimbrite. Air-fall layers, pyroclastic surge, mudflow and fluviatile reworked pumice deposits also occur interbedded between ignimbrite flow units. A chaotic near-vent facies of the ignimbrite includes co-ignimbrite lag breccias segregated from proximal pumice flows. The facies locates a central vent but one which could not have been associated with a well defined edifice. Many of the lithics in the exposed lag breccias and near-vent facies of the ignimbrite appear to be fragments of welded Rio Caliente ignimbrite, and indicate considerable vent widening, or migration, during the eruption. Nearer vent the ignimbrite is thickest and composed of the largest number of flow units. Here it is welded and is a simple cooling unit. Evidence suggests that it was only the larger thicker pumice flows that escaped to the outer parts of the sheet. Detailed analysis of four flow units indicates that the pumice flows were generally poorly expanded, less mobile flows which would be produced by collapse of low eruption columns. The analogy of a compound ignimbrite with a compound lava flow is, therefore, good — a compound lava flow forms instead of a simple one when the volumetric discharge rate (or intensity) is low, and in explosive eruptions this predicts lower eruption column heights. A corollary is that the ignimbrite has a high aspect ratio. The complex eruption sequence shows the reinstatement of plinian activity several times during the eruption after column collapse occurred. This, together with erosional breaks and evidence that solidified fragments of already welded ignimbrite were re-ejected, all suggest the eruption lasted a relatively significant time period. Nearly 90 km³ of tephra were erupted. The associated plinian pumice fall is one of the largest known having a volume of 50 km³ and the ignimbrite, plus a co-ignimbrite ash-fall, have a volume of nearly 40 km³. Published welding models applied to the reejected welded blocks indicate an eruption duration of 15-20d, and a maximum average magma-discharge rate of 1.4 × 10⁴ m³/s for the ignimbrite. This is low intensity when compared with available data from other ignimbrite-forming eruptions, and concurs with all the geological evidence presented. The total eruption duration was perhaps 15-31d, which is consistent with other estimates of the duration of large magnitude explosive silicic eruptions.     

Criteria for recognition of constructional silicic lava flow surfaces

Surface-exposure dating (SED) methods typically rely on the measurement of a geochemical parameter that systematically changes with time. A pivotal task in the calibration of many of these techniques is to demonstrate that lava flow surfaces sampled for dating have not experienced erosion. Although criteria for identification of constructional basaltic lava flow surfaces have been published, no such criteria presently exist for the recognition of constructional silicic flows. Here we present several criteria for identifying constructional silicic lava flow features in the field. First, crease structures are fractures with easily identified, curved, striated walls that are commonly observed on recent and active silicic lava flows. Crease structures form during extrusion, and are resistant to mechanical disintegration because they expose dense material from the flow interior. Second, some crease structures break apart during formation, leaving a deposit of striated blocks on the flow surface. Crease structure blocks are striated on only one side, whereas blocks from internal columnar joints exposed through erosion are striated on two or more sides. Only the striated side of the crease structure block is definitively constructional. Finally, many silicic flow surfaces exhibit expanded or breadcrusted textures. These features consist of a dense, fractured rind, 1 –2cm thick, enclosing highly vesicular material. Breadcrust flow textures appear similar to breadcrust bombs produced during volcanic explosions, so it is imperative to demonstrate that they are part of the lava flow surface. These criteria should enable investigators to positively identify constructional silicic lava flow surfaces when calibrating an SED method.     

Ignimbrites of the Roque Nublo group, Gran Canaria, Canary Islands

 Non-welded, lithic-rich ignimbrites, hereintermed the Roque Nublo ignimbrites, are the most distinctive deposits of the Pliocene Roque Nublo group, which forms the products of second magmatic cycle on Gran Canaria. They are very heterogeneous, with 35–55% volume lithic fragments, 15-30% mildly vesiculated pumice, 5–7% crystals and 20–30% ash matrix. The vitric components (pumice fragments and ash matrix) are largely altered and transformed into zeolites and subordinate smectites. The Roque Nublo ignimbrites originated from hydrovolcanic eruptions that caused rapid and significant erosion of vents thus incorporating a high proportion of lithic clasts into the eruption columns. These columns rapidly became too dense to be sustained as vertical eruption columns and were transformed into tephra fountains which fed high-density pyroclastic flows. The deposits from these flows were mainly confined to palaeovalleys and topographic depressions. In distal areas close to the coast line, where these palaeovalleys widened, most of the pyroclastic flows expanded laterally and formed numerous thin flow units. The combined effect of the magma–water interaction and the high content of lithic fragments is sufficient to explain the characteristic low emplacement temperature of the Roque Nublo ignimbrites. This fact also explains the transition from pyroclastic flows into lahar deposits observed in distal facies of the Roque Nublo ignimbrites. The existence of hydrovolcanic eruptions generating high-density pyroclastic flows, unable to efficiently separate the water vapour from the vitric components during transport, also accounts for the intense zeolitic alteration in these deposits. Received: 5 November 1996 / Accepted: 3 March 1997     

Sequence and eruptive style of the 1783 eruption of Asama Volcano, central Japan: a case study of an andesitic explosive eruption generating fountain-fed lava flow, pumice fall, scoria flow and forming a cone

The 3-month long eruption of Asama volcano in 1783 produced andesitic pumice falls, pyroclastic flows, lava flows, and constructed a cone. It is divided into six episodes on the basis of waxing and waning inferred from records made during the eruption. Episodes 1 to 4 were intermittent Vulcanian or Plinian eruptions, which generated several pumice fall deposits. The frequency and intensity of the eruption increased dramatically in episode 5, which started on 2 August, and culminated in a final phase that began on the night of 4 August, lasting for 15 h. This climactic phase is further divided into two subphases. The first subphase is characterized by generation of a pumice fall, whereas the second one is characterized by abundant pyroclastic flows. Stratigraphic relationships suggest that rapid growth of a cone and the generation of lava flows occurred simultaneously with the generation of both pumice falls and pyroclastic flows. The volumes of the ejecta during the first and second subphases are 0.21 km³ (DRE) and 0.27 km³ (DRE), respectively. The proportions of the different eruptive products are lava: cone: pumice fall=84:11:5 in the first subphase and lava: cone: pyroclastic flow=42:2:56 in the second subphase. The lava flows in this eruption consist of three flow units (L1, L2, and L3) and they characteristically possess abundant broken phenocrysts, and show extensive "welding" texture. These features, as well as ghost pyroclastic textures on the surface, indicate that the lava was a fountain-fed clastogenic lava. A high discharge rate for the lava flow (up to 10⁶ kg/s) may also suggest that the lava was initially explosively ejected from the conduit. The petrology of the juvenile materials indicates binary mixing of an andesitic magma and a crystal-rich dacitic magma. The mixing ratio changed with time; the dacitic component is dominant in the pyroclasts of the first subphase of the climactic phase, while the proportion of the andesitic component increases in the pyroclasts of the second subphase. The compositions of the lava flows vary from one flow unit to another; L1 and L3 have almost identical compositions to those of pyroclasts of the first and second subphases, respectively, while L2 has an intermediate composition, suggesting that the pyroclasts of the first and second subphases were the source of the lava flows, and were partly homogenized during flow. The complex features of this eruption can be explained by rapid deposition of coarse pyroclasts near the vent and the subsequent flowage of clastogenic lavas which were accompanied by a high eruption plume generating pumice falls and/or pyroclastic flows.Editorial responsibility: T. Druitt     

Estimating silicic lava vesicularity with thermal remote sensing: a new technique for volcanic mapping and monitoring

 Remote monitoring of active lava domes provides insights into the duration of continued lava extrusion and detection of potentially associated explosive activity. On inactive flows, variations in surface texture ranging from dense glass to highly vesicular pumice can be related to emplacement time, volatile content, and internal structure. Pumiceous surface textures also produce changes in thermal emission spectra that are clearly distinguishable using remote sensing. Spectrally, the textures describe a continuum consisting of two pure end members, obsidian and vesicles. The distinct spectral features of obsidian are commonly muted in pumice due to overprinting by the vesicles, which mimic spectrally neutral blackbody emitters. Assuming that this energy combines linearly in direct proportion to the percentage of vesicles, the surface vesicularity can be estimated by modeling the pumice spectrum as a linear combination of the glass and blackbody spectra. Based on this discovery, a linear retrieval model using a least-squares fitting approach was applied to airborne thermal infrared data of the Little Glass Mountain and Crater Glass rhyolite flows at Medicine Lake Volcano (California) as a case study. The model produced a vesicularity image of the flow with values from 0 to ∼70%, which can be grouped into three broad textural classes: dense obsidian, finely vesicular pumice, and coarsely vesicular pumice. Values extracted from the image compare well with those derived from SEM analysis of collected samples as well as with previously reported results. This technique provides the means to accurately map the areal distributions of these textures, resulting in significantly different values from those derived using aerial photographs. If applied to actively deforming domes, this technique will provide volcanologists with an opportunity to monitor dome-wide degassing and eruptive potential in near-real-time. In July 1999 such an effort will be possible for the first time when repetitive, global, multispectral thermal infrared data become available with the launch of the Advanced Spaceborne Thermal Emission and Reflectance Radiometer (ASTER) instrument aboard the Earth Observing System satellite. Received: 25 June 1998 / Accepted: 14 December 1998     

Young pumice deposits on Nisyros,Greece

The island of Nisyros (Aegean Sea) consists of a silicic volcanic sequence upon a base of mafic-andesitic hyaloclastites, lava flows, and breccias. We distinguish two young silicic eruptive cycles each consisting of an explosive phase followed by effusions, and an older silicic complex with major pyroclastic deposits. The caldera that formed after the last plinian eruption is partially filled with dacitic domes. Each of the two youngest plinian pumice falls has an approximate DRE volume of 2–3 km³ and calculated eruption column heights of about 15–20 km. The youngest pumice unit is a fall-surge-flow-surge sequence. Laterally transitional fall and surge facies, as well as distinct polymodal grainsize distributions in the basal fall layer, indicate coeval deposition from a maintained plume and surges. Planar-bedded pumice units on top of the fall layer were deposited from high-energy, dry-steam propelled surges and grade laterally into cross-bedded, finegrained surge deposits. The change from a fall-to a surge/flow-dominated depositional regime coincided with a trend from low-temperature argillitic lithics to high-temperature, epidote-and diopside-bearing lithic clasts, indicating the break-up of a high-temperature geothermal reservoir after the plinian phase. The transition from a maintained plume to a surge/ash flow depositional regime occurred most likely during break-up of the high-temperature geothermal reservoir during chaotic caldera collapse. The upper surge units were possibly erupted through the newly formed ringfracture.     

Volcanic stratigraphy of large-volume silicic pyroclastic eruptions during Oligocene Afro-Arabian flood volcanism in Yemen

A new stratigraphy for bimodal Oligocene flood volcanism that forms the volcanic plateau of northern Yemen is presented based on detailed field observations, petrography and geochemical correlations. The >1 km thick volcanic pile is divided into three phases of volcanism: a main basaltic stage (31 to 29.7 Ma), a main silicic stage (29.7 to 29.5 Ma), and a stage of upper bimodal volcanism (29.5 to 27.7 Ma). Eight large-volume silicic pyroclastic eruptive units are traceable throughout northern Yemen, and some units can be correlated with silicic eruptive units in the Ethiopian Traps and to tephra layers in the Indian Ocean. The silicic units comprise pyroclastic density current and fall deposits and a caldera-collapse breccia, and they display textures that unequivocally identify them as primary pyroclastic deposits: basal vitrophyres, eutaxitic fabrics, glass shards, vitroclastic ash matrices and accretionary lapilli. Individual pyroclastic eruptions have preserved on-land volumes of up to ∼850 km³. The largest units have associated co-ignimbrite plume ash fall deposits with dispersal areas >1×10⁷ km² and estimated maximum total volumes of up to 5,000 km³, which provide accurate and precisely dated marker horizons that can be used to link litho-, bio- and magnetostratigraphy studies. There is a marked change in eruption style of silicic units with time, from initial large-volume explosive pyroclastic eruptions producing ignimbrites and near-globally distributed tuffs, to smaller volume (<50 km³) mixed effusive-explosive eruptions emplacing silicic lavas intercalated with tuffs and ignimbrites. Although eruption volumes decrease by an order of magnitude from the first stage to the last, eruption intervals within each phase remain broadly similar. These changes may reflect the initiation of continental rifting and the transition from pre-break-up thick, stable crust supporting large-volume magma chambers, to syn-rift actively thinning crust hosting small-volume magma chambers.Electronic Supplementary Material Supplementary material is available for this article at     

Distinguishing strongly rheomorphic tuffs from extensive silicic lavas

High-temperature silicic volcanic rocks, including strongly rheomorphic tuffs and extensive silicic lavas, have recently been recognized to be abundant in the geologic record. However, their mechanisms of eruption and emplacement are still controversial, and traditional criteria used to distinguish conventional ash-flow tuffs from silicic lavas largely fail to distinguish the high-temperature versions. We suggest the following criteria, ordered in decreasing ease of identification, to distinguish strongly rheomorphic tuffs from extensive silicic lavas: (1) the character of basal deposits; (2) the nature of distal parts of flows; (3) the relationship of units to pre-existing topography; and (4) the type of source. As a result of quenching against the ground, basal deposits best preserve primary features, can be observed in single outcrops, and do not require knowing the full extent of a unit. Lavas commonly develop basal breccias composed of a variety of textural types of the flow in a finer clastic matrix; such deposits are unique to lavas. Because the chilled base of an ashflow tuff generally does not participate in secondary flow, primary pyroclastic features are best preserved there. Massive, flow-banded bases are more consistent with a lava than a pyroclastic origin. Lavas are thick to their margins and have steep, abrupt flow fronts. Ashflow tuffs thin to no more than a few meters at their distal ends, where they generally do not show any secondary flow features. Lavas are stopped by topographic barriers unless the flow is much thicker than the barrier. Ash-flow tuffs moving at even relatively slow velocities can climb over barriers much higher than the resulting deposit. Lavas dominantly erupt from fissures and maintain fairly uniform thicknesses throughout their extents. Tuffs commonly erupt from calderas where they can pond to thicknesses many times those of their outflow deposits. These criteria may also prove effective in distinguishing extensive silicic lavas from a postulated rock type termed lava-like ignimbrite. The latter have characteristics of lavas except for great areal extents, up to many tens of kilometers. These rocks have been interpreted as ash-flow tuffs that formed from low, boiling-over eruption columns, based almost entirely on their great extents and the belief that silicic lavas could not flow such distances. However, we interpret the best known examples of lava-like ignimbrites to be lavas. This interpretation should be tested through additional documentation of their characteristics and research on the boiling-over eruption mechanism and the kinds of deposits it can produce. Flow bands, flow folds, ramps, elongate vesicles, and probably upper breccias occur in both lavas and strongly rheomorphic tuffs and are therefore not diagnostic. Pumice and shards also occur in both tuffs and lavas, although they occur throughout ash-flow tuffs and generally only in marginal breccias of lavas. Dense welding, secondary flow, and intense alteration accompanying crystallization at high temperature commonly obliterate primary textures in both thick, rheomorphic tuffs and thick lavas. High-temperature silicic volcanic rocks are dominantly associated with tholeiitic flood basalts. Extensive silicic lavas could be appropriately termed flood rhyolites.     

The emplacement of intermediate volume ignimbrites: A case study from Roccamonfina Volcano,Southern Italy

A model is presented for the emplacement of intermediate volume ignimbrites based on a study of two 6 km³ volume ignimbrites on Roccamonfina Volcano, Italy. The model considers that the flows were slow moving, and quickly deflated from turbulent to non-turbulent conditions. Yield strength and density increased whereas fluidisation decreased with time and runout of the pyroclastic flows. In proximal locations, on the caldera rim, heterogeneous exposures including discontinuous lithic breccias, stratified and cross-stratified units interbedded with massive ignimbrite suggest deposition from turbulent flows. In medial locations thick, massive ignimbrite occurs associated with three types of co-ignimbrite lithic breccia which we interpret as being emplaced by non-turbulent flows. Multiple grading of different breccia/lithic concentration types within single flow units indicates that internal shear occurred producing overriding or overlapping of the rear of the flow onto the slower-moving front part. This overriding of different parts of non-turbulent pyroclastic flows could be caused by at least two different mechanisms: (1) changes in flow regime, such as hydraulic jumps that may occur at breaks in slope; and (2) periods of increased discharge rate, possibly associated with caldera collapse, producing fresh pulses of lithic-rich material that sheared onto the slower-moving part of the flow in front.We propose that ground surge deposits enriched in pumice compared with their associated ignimbrite probably formed by a flow separation mechanism from the top and front of the pyroclastic flow. These turbulent clouds moved ahead of the non-turbulent lower part of the flow to form stratified pumice-rich deposits. In distal regions well-developed coarse, often clast-supported, pumice concentrations zones and coarse intra-flow-unit lithic concentrations occur within the massive ignimbrite. We suggest that the flows were non-turbulent, possessed a relatively high yield strength and may have moved by plug flow prior to emplacement.     

Volcanoes of the Cape Hoskins area,New Britain,territory of Papua and New Guinea

One active and ten extinct Quaternary volcanoes are described from the Cape Hoskins area, on the north coast of New Britain. They are mostly strato volcanoes built up of lava flows, lava domes, pyroclastic flows, lahars, tephra, and derived alluvial sediments. The volcanic products range in composition from basalt to rhyolite, but basaltic andesite and andesite predominate. Much of the area is covered by tephra, several metres thick, consisting mainly of rhyolitic pumice. The active volcano, Pago, is built up of several glacier-like lava flows, the last of which was formed during an eruption in 1914–18. Pago lies within a well-preserved caldera forming the central part of a broad low-angle cone, named Witori, which consists largely of welded and unwelded pyroclastic flow deposits. C-14 dates obtained on charcoal indicate that the caldera eruption occurred about 2500 years B. P. Another caldera of similar age lies south of Witori. Of the other eight volcanoes described four are relatively well-preserved steep-sided cones formed mainly of lava flows, one is a remnant of a low-angle cone with a caldera, and three are deeply eroded cones which have none of their constructional surfaces preserved.     

Pyroclastic deposits of the November 13, 1985 eruption of Nevado del Ruiz volcano, Colombia

The November 13, 1985 eruption of Nevado del Ruiz produced a series of pyroclastic flows and surges that eroded channels on the surface of the summit glacier and generated lahars which descended down most of the rivers that drain the volcano. The stratigraphy of the proximal pyroclastic deposits indicates that there were at least four episodes to the eruption. Episode I, deposited an unusual surge consisting of small pieces of ice mixed with ash and exhibiting planar stratification. Ballistically emplaced fragments are also intercalated with this unit. During Episode II, at least two pyroclastic flows were erupted. Their deposits contain the most evolved pumice of the entire eruption; SiO₂ content of matrix glass ranges between 74.5 and 74.9%. Episode III is marked by the emplacement of a welded tuff with an average SiO₂ content of about 66% in the matrix glass. The final Episode IV was characterized by the development of a high-altitude eruption column and the emplacement of several nonwelded pyroclastic flows. Banded pumice are common in the pyroclastic flow as well as in the pumice fall deposits. Co-existing dark and light pumice bands differ in SiO₂ content by 3.5% and in general are similar to the composition of the welded pumice from Episode III.The compositional zonation of the pyroclastic deposits from Episode I to IV suggests that a nearsurface compositionally-stratified portion of the magma body was tapped during Episode II. During Episodes III and IV the main body of magma was involved although the coexistence of the compositionally distinct pumice clasts at similar stratigraphic levels argues for mixing of magma from different levels in the chamber during the eruptive process.     

Imbrication, flow direction and possible source areas of the pumice-flow tuffs near Bend, Oregon, U.S.A.

Imbrication, indicating flow and source direction, occurs in three Pleistocene or upper Pliocene pumice-flow tuffs exposed in a 700-km² area on the east flank of the Cascade Range near Bend, Oregon, and shows the location of previously unknown source vents of these tuffs. The imbrication is formed by inclined elongate and/or flat pumice or lithic fragments and locally by elongate plagioclase crystals. Imbrication is best developed within the lower zones of individual flow units; the pumiceous top zones also locally show imbrication directions parallel to that in the lower zones. Moreover, the areal pattern of size distribution of lithic and pumice fragments in the flows is concordant with the flow direction pattern indicated by imbrication.The upper pumice flow shows a fan-shaped pattern of flow directions indicated by imbrication which points to a western source. A possible vent, about 20 km west of Bend in the highland near Broken Top Volcano, is marked by many silicic domes and basaltic cinder cones where there is a 6–8 mgal negative Bouguer gravity anomaly. In contrast, imbrication in the middle and lower pumice flows indicates flow from a source southwest of Bend. Vents in this direction are not obvious. Possible buried vents are located about 30 km and 45 km southwest of Bend near Sitkum Butte and Lookout Mountain, respectively.     

Stratigraphy, chronology, and sedimentology of ignimbrites from the white trachytic tuff, Roccamonfina Volcano, Italy

We describe the stratigraphy, chronology, and grain size characteristics of the white trachytic tuff (WTT) of Roccamonfina Volcano (Italy). The pyroclastic rock was emplaced between 317 and 230 Ma BP during seven major eruptive events (units A to G) and three minor events (units BC, CD, and DE). These units are separated by paleosol layers and compositionally well-differentiated pyroclastic successions. Stratigraphic control is favored by the occurrence at the base of major units of marker layers. Four WTT units (1 to 4) occur within the central caldera. These are not positively correlated with specific extracaldera units.The source of most of the WTT units was the central caldera. Units B and C were controlled by the western wall of the caldera, whereas units D and E were able to overcome this barrier, spreading symmetrically along the flanks of MC. The maximum pumice size (MP) of units increases with distance from the caldera, whereas the maximum lithic size (ML) decreases. MP and ML of the marker layer of unit D (MKDa–MKDp) do not show any systematic variations with respect to the central caldera. In contrast, the thickness of surge MKDa decreases with distance from the source, and MKDp accumulates to the north of MC probably controlled, respectively, by mobility-transport power and by wind blowing northwards.The grain size characteristics of the WTT deposits are used for classifying the units. There is no systematic variation of the grain size as a function of stratigraphic height either among units or within single units. Large variation of components in subunit E1, with repetitive alternation of pyroclastic flow to surge through fallout vs. surge deposits, suggests that the process of eruption took place in a complex or piecemeal fashion.Pumice concentration zones (PCZ) occur at all WTT levels on the volcano, but they are much thicker and pumice clasts are much larger within the central caldera. These were probably originated by the disruption of lava (flow or dome) to pumice fragments and fine ash due to sudden depressurization and interaction with lake waters of the molten lava. Local basal PCZ are, in some cases, similar to the lapilli-rich “layer 1P” that has been described elsewhere, and may have been deposited from currents transitional between pyroclastic surge and flow. Other basal PCZ formed in response to small undulations in the substrate, or can be originated by fallout. Lenticular PCZ within ignimbrite interiors and tops are interpreted to record marginal pumice levees and pumice rafts, some of which were buried by subsequant pyroclastic flows.Lithic concentration zones (LCZ) also occur at various stratigraphic height within the extracaldera ignimbrites, whereas intracaldera LCZ are absent, probably due to the fact that ignimbrite currents are strongly energetic and erosive near vent. LCZ at the top of basal inversely graded layers are formed by mechanical sieving or dispersive pressure in response to variable velocity gradients and particle concentration gradients (a segregation process). Coarse LCZ and coarse lithic breccias (LB), that reside in the interior or tops of pyroclastic flows and that occur in medial to distal areas, are interpreted to be the result of slugs of lithic-rich debris introduced by vent collapse or rockslides into the moving pyroclastic flows along their flow paths. These LCZ become mixed to varying degrees due to differential densities and velocities relative to the pyroclastic flows (desegregation processes).     

Geologic evolution of the Donguinyó-Huichapan caldera complex,central Mexican Volcanic Belt,Mexico

The Donguinyó-Huichapan caldera complex is located 110 km to the NNW of Mexico City, in the central sector of the Mexican Volcanic Belt. It is a 10 km in diameter complex apparently with two overlapping calderas, each one related to an ignimbrite sequence that contrasts in composition, mineralogy, welding, distribution, and physical aspect. The geologic evolution of this complex includes the following phases, 1) A first caldera formed at 5.0 ± 0.3 Ma, with the eruption of several discrete pulses of andesitic to trachydacitic pyroclastic flows that produced a series of densely welded ignimbrites; 2) At 4.6 ± 0.3 Ma, several small shield volcanoes and cinder cones built the rim of this caldera and erupted basaltic-andesite and andesitic lava flows; 3) At 4.2 ± 0.2 Ma, a second caldera was formed associated to the eruption of the Huichapan Tuff, which is a rhyolitic pyroclastic sequence consisting of minor unwelded ignimbrites, pumice fall and surge deposits, and a voluminous welded ignimbrite; 4) Also yielding an age of 4.2 ± 0.2 Ma, several trachydacitic lava domes were extruded along the new ring fracture and formed the rim of the Huichapan caldera, as well as five intra-caldera domes of dacitic and trachydacitic composition. Peripheral volcanism includes a large 2.5 ± 0.1 Ma shield volcano that was emplaced on the Huichapan caldera rim.The two calderas that form the Donguinyó-Huichapan complex have contrasting differences in volcanic styles that were apparently due to their differences in composition. Products erupted by the Donguinyó caldera are basaltic-andesite to trachydacitic in composition, whereas Huichapan caldera products are all high-silica rhyolites.     

Silicic lava dome growth in the 1934–1935 Showa Iwo-jima eruption, Kikai caldera, south of Kyushu, Japan

The 1934–1935 Showa Iwo-jima eruption started with a silicic lava extrusion onto the floor of the submarine Kikai caldera and ceased with the emergence of a lava dome. The central part of the emergent dome consists of lower microcrystalline rhyolite, grading upward into finely vesicular lava, overlain by coarsely vesicular lava with pumice breccia at the top. The lava surface is folded, and folds become tighter toward the marginal part of the dome. The dome margin is characterized by two zones: a fracture zone and a breccia zone. The fracture zone is composed of alternating layers of massive lava and welded oxidized breccia. The breccia zone is the outermost part of the dome, and consists of glassy breccia interpreted to be hyaloclastite. The lava dome contains lava with two slightly different chemical compositions; the marginal part being more dacitic and the central part more rhyolitic. The fold geometry and chemical compositions indicate that the marginal dacite had a slightly higher temperature, lower viscosity, and lower yield stress than the central rhyolite. The high-temperature dacite lava began to effuse in the earlier stage from the central crater. The front of the dome came in contact with seawater and formed hyaloclastite. During the later stage, low-temperature rhyolite lava effused subaerially. As lava was injected into the growing dome, the fracture zone was produced by successive fracturing, ramping, and brecciation of the moving dome front. In the marginal part, hyaloclastite was ramped above the sea surface by progressive increments of the new lava. The central part was folded, forming pumice breccia and wrinkles. Subaerial emplacement of lava was the dominant process during the growth of the Showa Iwo-jima dome.Editorial Responsibility J. McPhie     

Systematics of xenocrystic contamination: preservation of discrete feldspar populations at McCullough Pass Caldera revealed by Ar/Ar dating

Single crystal ⁴⁰Ar/³⁹Ar dating of K-feldspars from silicic volcanic rocks containing xenocrysts often yields a spectrum of ages slightly older than those of juvenile sanidine phenocrysts. In contrast, feldspars from thin, low-volume units of the Tertiary (14 Ma) McCullough Pass Tuff define discrete age populations at 14 Ma, 15 Ma, and 1.3 Ga, reflecting the time of eruption, xenocrysts from an older ignimbrite exposed in the caldera wall, and Proterozoic basement K-feldspars, respectively. Conductive cooling and diffusion modelling suggests preservation of such discrete populations is likely only when xenocrystic material is incorporated into the magma very near or at the surface, or is engulfed in thin, rapidly cooled pyroclastic flows during emplacement. Incorporation of xenocrysts into the subvolcanic magma chamber, into thick rhyolite domes or lava flows, or into large, welded ignimbrite sheets will result in partial or total resetting of the K/Ar isotopic system. Similarly, petrographic evidence such as exsolution lamellae may be homogenized under these conditions but not in thin ignimbrites. Extremely low diffusion rates for disordering of the Al–Si tetrahedral siting of basement feldspars suggests that they will retain their ordered structural state given rhyolitic magma temperatures. Thus, even when petrographic and K/Ar isotopic evidence for xenocrystic contamination is obscured, it may be preserved in the form of Al–Si ordering.     

Snow-contact volcanic facies and their use in determining past eruptive environments at Nevados de Chillán volcano, Chile

Studies of the eruptive products from volcanoes with variable ice and snow cover and a long history of activity enable reconstruction of erupted palaeoenvironments, as well as highlighting the hazards associated with meltwater production, such as jökulhlaups and magma-water interaction. Existing difficulties include estimation of ice/snow thicknesses and discrimination between ice- and snow-contact lithofacies. We present field evidence from the Cerro Blanco subcomplex of Nevados de Chillán stratovolcano, central Chile, which has erupted numerous times in glacial and non-glacial periods and most recently produced andesitic lava flows in the 1861–1865 eruption from the Santa Gertrudis cone on the northwest flank of the volcano. The main period of lava effusion occurred during the winter of 1861 when the upper flanks of the volcano were reportedly covered in snow and ice. The bases and margins of the first lava flows produced are cut by arcuate fractures, which are interpreted as snow-contact features formed when steam generated from the melting of snow entered tensional fractures at the flow base. In contrast, the interiors and upper parts of these flows, as well as the overlying flow units, have autobrecciated and blocky textures typical of subaerial conditions, due to insulation by the underlying lava. Similar textures found in a lava flow dated at 90.0±0.6 ka that was emplaced on the northwest flank of Cerro Blanco, are also inferred to be ice and snow-contact features. These textures have been used to infer that a small valley glacier, overlain by snow, existed in the Santa Gertrudis Valley at the time of the eruption. Such reconstructions are important for determining the long-term evolution of the volcano as well as assessing future hazards at seasonally snow-covered volcanoes.     

Correlation and petrogenesis of silicic pyroclastic rocks in the Northern Pannonian Basin,Eastern-Central Europe: In situ trace element data of glass shards and mineral chemical constraints

The Neogene volcanism of the Carpathian–Pannonian Region, Eastern-Central Europe started with repeated explosive eruption of silicic magmas. Volcanic products consist mostly of non-welded and partially to densely welded ignimbrites, which cover large areas in the Pannonian Basin. Since this volcanism occurred during a long time interval, from 21 Ma to 13.5 Ma ago, these pyroclastic deposits have great stratigraphic importance, as well as providing valuable information about petrogenetic processes during the formation of the back-arc basin area. In this study, we used in situ trace element data of glasses (glass shards, pumice and fiamme glass) determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), combined with microprobe geochemical data of the main mineral phases (plagioclase, biotite), to correlate the scattered outcrops of the ignimbrite units. Based on these data, we distinguished four ignimbrite units in the Bükkalja Volcanic Field, Northern Pannonian Basin. Each of these units is characterized by specific geochemical fingerprints. Thorium, Nb, Y and the rare earth elements are the most effective discriminator elements in glasses. The modal composition of mineral phases (occurrence or lack of certain minerals) and chemistry of plagioclases and biotites are also good correlation tools, especially the Fe, Mg and Ti contents of biotites. We suggest that these correlation criteria, particularly the grain-specific LA-ICP-MS data, could be effectively used also in other ignimbrite fields and in wider tephrochronological studies. The in situ trace element composition of glasses, representing the liquid part of the erupted magma, can be also used to constrain the petrogenesis of the rhyolitic magmas. Trace element ratios such as La/Nb, La/Y and Th/Nb suggest the importance of minor (e.g., hornblende and ilmenite) and accessory (e.g., zircon, allanite) minerals controlling the composition of the erupted melt. Rhyolitic magmas probably evolved from metaluminous andesitic parental melts via fractional crystallization. Syn-eruptive magma mingling was detected in the genesis of the Middle Ignimbrite Unit, based on the strong intra-sample geochemical variation both in the glasses and in the phenocrysts.