Characterization of juvenile pyroclasts from the Kos Plateau Tuff (Aegean Arc): insights into the eruptive dynamics of a large rhyolitic eruption

Silicic pumices formed during explosive volcanic eruptions are faithful recorders of the state of the magma in the conduit, close to or at the fragmentation level. We have characterized four types of pumices from the non-welded rhyolitic Kos Plateau Tuff, which erupted 161,000 years ago in the East Aegean Arc, Greece. The dominant type of pumice (>90 vol.%) shows highly elongated tubular vesicles. These tube pumices occur throughout the eruption. Less common pumice types include: (1) “frothy” pumice (highly porous with large, sub-rounded vesicles), which form 5–10 vol.% of the coarsest pyroclastic flow deposits, (2) dominantly “microvesicular” and systematically crystal-poor pumices, which are found in early erupted, fine-grained pyroclastic flow units, and are characterized by many small (<50 μm in diameter) vesicles and few mm-sized, irregular voids, (3) grey or banded pumices, indicating the interaction between the rhyolite and a more mafic magma, which are found throughout the eruption sequence and display highly irregular bubble shapes. Except for the grey-banded pumices, all three other types are compositionally identical and were generated synchronously as they are found in the same pyroclastic units. They, therefore, record different conditions in the volcanic conduit leading to variable bubble nucleation, growth and coalescence. A total of 74 pumice samples have been characterized using thin section observation, SEM imagery, porosimetry, and permeametry. We show that the four pumice types have distinct total and connected porosity, tortuosity and permeability. Grey-banded pumices show large variations in petrophysical characteristics as a response to mingling of two different magmas. The microvesicular, crystal-poor, pumices have a bimodal bubble size distribution, interpreted as reflecting an early heterogeneous bubble nucleation event followed by homogeneous bubble nucleation close to fragmentation. Finally, the significant differences in porosity, tortuosity and permeability in compositionally identical tube and frothy pumices are the result of variable shear rates in different parts of the conduit. Differential shear rates may be the result of either: (1) pure shear, inducing a vertical progression from frothy to tube and implying a relatively thick fragmentation zone to produce both types of pumices at the same time or (2) localized simple shear, inducing strongly tubular vesicles along the wall and near-spherical bubbles in the centre of the conduit and not necessarily requiring a thick fragmentation zone.     

Late Quaternary history of the Vakinankaratra volcanic field (central Madagascar): insights from luminescence dating of phreatomagmatic eruption deposits

The Quaternary Vakinankaratra volcanic field in the central Madagascar highlands consists of scoria cones, lava flows, tuff rings, and maars. These volcanic landforms are the result of processes triggered by intracontinental rifting and overlie Precambrian basement or Neogene volcanic rocks. Infrared-stimulated luminescence (IRSL) dating was applied to 13 samples taken from phreatomagmatic eruption deposits in the Antsirabe–Betafo region with the aim of constraining the chronology of the volcanic activity. Establishing such a chronology is important for evaluating volcanic hazards in this densely populated area. Stratigraphic correlations of eruption deposits and IRSL ages suggest at least five phreatomagmatic eruption events in Late Pleistocene times. In the Lake Andraikiba region, two such eruption layers can be clearly distinguished. The older one yields ages between 109?±?15 and 90?±?11 ka and is possibly related to an eruption at the Amboniloha volcanic complex to the north. The younger one gives ages between 58?±?4 and 47?±?7 ka and is clearly related to the phreatomagmatic eruption that formed Lake Andraikiba. IRSL ages of a similar eruption deposit directly overlying basement laterite in the vicinity of the Fizinana and Ampasamihaiky volcanic complexes yield coherent ages of 68?±?7 and 65?±?8 ka. These ages provide the upper age limit for the subsequently developed Iavoko, Antsifotra, and Fizinana scoria cones and their associated lava flows. Two phreatomagmatic deposits, identified near Lake Tritrivakely, yield the youngest IRSL ages in the region, with respective ages of 32?±?3 and 19?±?2 ka. The reported K-feldspar IRSL ages are the first recorded numerical ages of phreatomagmatic eruption deposits in Madagascar, and our results confirm the huge potential of this dating approach for reconstructing the volcanic activity of Late Pleistocene to Holocene volcanic provinces.     

Reconstruction of a major caldera-forming eruption from pyroclastic deposit characteristics: Kos Plateau Tuff, eastern Aegean Sea

The 161 ka explosive eruption of the Kos Plateau Tuff (KPT) ejected a minimum of 60 km³ of rhyolitic magma, a minor amount of andesitic magma and incorporated more than 3 km³ of vent- and conduit-derived lithic debris. The source formed a caldera south of Kos, in the Aegean Sea, Greece. Textural and lithofacies characteristics of the KPT units are used to infer eruption dynamics and magma chamber processes, including the timing for the onset of catastrophic caldera collapse.The KPT consists of six units: (A) phreatoplinian fallout at the base; (B, C) stratified pyroclastic-density-current deposits; (D, E) volumetrically dominant, massive, non-welded ignimbrites; and (F) stratified pyroclastic-density-current deposits and ash fallout at the top. The ignimbrite units show increases in mass, grain size, abundance of vent- and conduit-derived lithic clasts, and runout of the pyroclastic density currents from source. Ignimbrite formation also corresponds to a change from phreatomagmatic to dry explosive activity. Textural and lithofacies characteristics of the KPT imply that the mass flux (i.e. eruption intensity) increased to the climax when major caldera collapse was initiated and the most voluminous, widespread, lithic-rich and coarsest ignimbrite was produced, followed by a waning period. During the eruption climax, deep basement lithic clasts were ejected, along with andesitic pumice and variably melted and vesiculated co-magmatic granitoid clasts from the magma chamber. Stratigraphic variations in pumice vesicularity and crystal content, provide evidence for variations in the distribution of crystal components and a subsidiary andesitic magma within the KPT magma chamber. The eruption climax culminated in tapping more coarsely crystal-rich magma. Increases in mass flux during the waxing phase is consistent with theoretical models for moderate-volume explosive eruptions that lead to caldera collapse.     

A facies interpretation of the eruption and emplacement mechanisms of the upper part of the Neapolitan Yellow Tuff,Campi Flegrei,southern Italy

This study focuses on the upper part, Member B, of the Neapolitan Yellow Tuff (NYT). Detailed measurements of stratigraphic sections within the unlithified pozzolana facies show that Member B is composed of at least six distinct depositional units which each record a complex fluctuation between different styles of deposition from pyroclastic density flows. Six lithofacies have been identified: (1) massive valleyponded facies, the product of non-turbulent flows; (2) inverse-graded facies formed by flows that were turbulent for the majority of transport but were deposited through a non-tubulent basal regime; (3) regressive sand-wave facies, the product of high-concentration, turbulent flows; (4) stratified facies, the product of deposition from turbulent, low-particle-concentration, flows; (5) particle aggregate and (6) vesicular ash lithofacies, both of which are considered to have formed by deposition from turbulent, low-concentration flows. Although the whole eruption may have been phreatomagmatic, facies 1–4 are interpreted to be the product of dry eruptive activity, whereas facies 5 and 6 are considered to be of wet phreatomagmatic eruptive phases. Small-scale horizontal variations between facies include inverse-graded lithofacies that pass laterally into regressive sand-wave structures and stratified deposits. This indicates rapid transition from non-turbulent to turbulent deposition within the same flow. Thin vesicular ash and particle aggregate layers pass laterally into massive valley-ponded vesicular lithofacies, suggesting contemporaneous wet pyroclastic surges and cohesive mud flows. Three common vertical facies relations were recognised. (1) Massive valley-ponded and inverse-graded facies are overlain by stratified facies, suggesting decreasing particle concentration with time during passage of a flow. (2) Repeated vertical gradation from massive up into stratified facies and back into massive beds, is indicative of flow fluctuating between non-turbulent and turbulent depositional conditions. (3) Vertical alternation between particle aggregates and vesicular facies is interpreted as the product of many flow pulses, each of which involved deposition of a single particle aggregate and vesicular ash layer. It is possible that the different facies record stages in a continuum of flow processes. The deposits formed are dependent on the presence, thickness and behaviour of a high-concentration, non-turbulent boundary layer at the base of the flow. The end members of this process are (a) flows that transported and deposited material from a non-turbulent flow regime and (b) flows that transported and deposited material from a turbulent flow regime.     

Pyroclastic phases of a rhyolitic dome-building eruption: Puketarata tuff ring,Taupo Volcanic Zone,New Zealand

The 14 ka Puketarata eruption of Maroa caldera in Taupo Volcanic Zone was a dome-related event in which the bulk of the 0.25 km³ of eruption products were emplaced as phreatomagmatic fall and surge deposits. A rhyolitic dike encountered shallow groundwater during emplacement along a NE-trending normal fault, leading to shallow-seated explosions characterised by low to moderate water/magma ratios. The eruption products consist of two lava domes, a proximal tuff ring, three phreatic collapse craters, and a widespread fall deposit. The pyroclastic deposits contain dominantly dense juvenile clasts and few foreign lithics, and relate to very shallow-level disruption of the growing dome and its feeder dike with relatively little involvement of country rock. The distal fall deposit, representing 88% of the eruption products is, despite its uniform appearance and apparently subplinian dispersal, a composite feature equivalent to numerous discrete proximal phreatomagmatic lapilli fall layers, each deposited from a short-lived eruption column. The Puketarata products are subdivided into four units related to successive phases of:(A) shallow lava intrusion and initial dome growth; (B) rapid growth and destruction of dome lobes; (C) slower, sustained dome growth and restriction of explosive disruption to the dome margins; and (D) post-dome withdrawal of magma and crater-collapse. Phase D was phreatic, phases A and C had moderate water: magma ratios, and phase B a low water: magma ratio. Dome extrusion was most rapid during phase B, but so was destruction, and hence dome growth was largely accomplished during phase C. The Puketarata eruption illustrates how vent geometry and the presence of groundwater may control the style of silicic volcanism. Early activity was dominated by these external influences and sustained dome growth only followed after effective exclusion of external water from newly emplaced magma.     

The 1906 eruption of Vesuvius: from magmatic to phreatomagmatic activity through the flashing of a shallow depth hydrothermal system

The April 1906 eruption of Vesuvius is the type-example of the final eruptions that close the short cycles of semi-persistent activity that characterized the volcano in the 1631–1944 period. The eruption had a marked explosive character that accompanied the emission of lava from several vents on the southern slopes of the volcano. The observed sequence of events was characterized by repeated fluctuations of the magma level within the conduit, by large lava fountains, by conduit partial collapses, and by the final explosive decapitation of the summit cone. Contemporary chronicles, although frequently contradictory, allow reconstruction of the eruption, which can be divided into four main phases: (1) lateral lava effusions; (2) lava fountains; (3) gas-pyroclasts column; (4) low dense clouds. Pyroclastic deposits of the Monte Somma ridge and northeastern slope can be related to observed and described events and mainly refer to the 2nd and 3rd phases. The increase in the degree of fragmentation of the juvenile component together with the marked increase of the lithic component and morphologic evidence emphasize the repeated occurrence of magmawater interaction. This was most spectacular in the 3rd phase of the eruption in which, after the decapitation of the cone, a high gas-pyroclasts eruption column was formed. Because of the nature of the lithic fragments (mainly hydrothermally altered and metasomatic rocks), the huge amount of steam, and the high lithic/juvenile ratio, it is unlikely that the largest part of the energy in play was related to the contact between magma and cold phreatic water. We suggest that most of the steam involved in this phase of the eruption came from flashing of the hydrothermal system connected to the very shallow feeding system of the volcano and formed as a consequence of repeated subsurface intrusions between 1872 and 1906. Juvenile products were ejected through the eruption, and represent (at least) two different magma bodies: the first (older) was erupted during the initial phase of the eruption and was exhausted at the beginning of the lava fountains phase, when fresh magma was involved in the eruption.     

The Middle Scoria sequence: A Holocene violent strombolian, subplinian and phreatomagmatic eruption of Okmok volcano, Alaska

The Middle Scoria deposit represents an explosive eruption of basaltic andesite magma (54 wt. % SiO₂) from Okmok volcano during mid-Holocene time. The pattern of dispersal and characteristics of the ejecta indicate that the eruption opened explosively, with ash textural evidence for a limited degree of phreatomagmatism. The second phase of the eruption produced thick vesicular scoria deposits with grain texture, size and dispersal characteristics that indicate it was violent strombolian to subplinian in style. The third eruptive phase produced deposits with a shift towards grain shapes that are dense, blocky, and poorly vesicular, and intermittent surge layers, indicating later transitions between magmatic (violent strombolian) to phreatomagmatic (vulcanian) eruptive styles. Isopach maps yield bulk volume estimates that range from 0.06 to 0.43 km³, with ~ 0.04 to 0.25 km³ total DRE. The associated column heights and mass discharge values calculated from isopleth maps of individual Middle Scoria layers are 8.5 – 14 km and 0.4 to 45 × 10⁶ kg/s. The Middle Scoria tephras are enriched in plagioclase microlites that have the textural characteristics of rapid magma ascent and relatively high degrees of effective undercooling. Those textures probably reflect the rapid magma ascent accompanying the violent strombolian and subplinian phases of the eruption. In the later stages of the eruption, the plagioclase microlite number densities decrease and textures include more tabular plagioclase, indicating a slowing of the ascent rate. The findings on the Middle Scoria are consistent with other explosive mafic eruptions, and show that outside of the two large caldera-forming eruptions, Okmok is also capable of producing violent mafic eruptions, marked by varying degrees of phreatomagmatism.     

Micro-erosion meter measurements of travertine deposition rates: a case study from Louie Creek,Northwest Queensland,Australia

Although originally designed to measure surface denudation, the micro-erosion meter (MEM) can be adapted easily to measure deposition rates of chemical sediments such as travertines and speleothems. At Louie Creek, northwest Queensland, Australia, travertine deposition rates measured using the MEM average 4·15 mm a⁻¹. However, this figure masks considerable rate variability. Both purely hydraulic and hydraulically related variables appear to be the major mechanisms controlling deposition rates. The most rapid rates occur within relatively high-energy hydraulic regimes (impact and flow zones), whilst deposition rates in pools separating individual travertine barrages (standing water zones) are relatively slow. Dsposition rate variations within spray and impact zones are related directly to discharge. The highest rates in flow zones correlate with the incorporation into the travertine of in situ and allochthonous biogenic material, such as caddis fly larvae, green algal mats and phytoclasts, which proliferate or are entrapped easily under such hydraulic conditions. Considerable spatial variability in deposition rates also prevails. The highest rates for a given set of hydraulic conditions occur at two sites, the Upper Everglades and the Lower Everglades. The MEM also measures net erosion of travertines. At Louie Creek, most of the travertine erosion occurs in the wet season and is confined primarily to standing water zones. © 1997 John Wiley & Sons, Ltd.     

Base surge deposits,eruption history,and depositional processes of a wet phreatomagmatic volcano in Central Anatolia (Cora Maar)

Cora Maar is a Quaternary volcano located to the 20 km northwest of Mount Erciyes, the largest of the 19 polygenetic volcanic complexes of the Cappadocian Volcanic Province in central Anatolia. Cora Maar is a typical example of a maar-diatreme volcano with a nearly circular crater with a mean diameter of c.1.2 km, and a well-bedded base surge-dominated maar rim tephra sequence up to 40 m in thickness. Having a diameter/depth ratio (D/d) of 12, Cora is a relatively “mature” maar compared to recent maar craters in the world.Cora crater is excavated within the andesitic lava flows of Quaternary age. The tephra sequence is not indurated, and consists of juvenile clasts up to 70 cm, non-juvenile clasts up to 130 cm, accretionary lapilli up to 1.2 cm in diameter, and ash to lapilli-sized tephra. Base surge layers display well-developed antidune structures indicating the direction of the transport. Both progressive and regressive dune structures are present within the tephra sequence. Wavelength values increase with increasing wave height, and with large wavelength and height values. Cora tephra display similarities to Taal and Laacher See base surge deposits. Impact sags and small channel structures are also common. Lateral and vertical facies changes are observed for the dune bedded and planar bedsets.According to granulometric analyses, Cora Maar tephra samples display a bimodal distribution with a wide range of Md_φ values, characteristic for the surge deposits. Very poorly sorted, bimodal ash deposits generally vary from coarse tail to fine tail grading depending on the grain size distribution while very poorly sorted lapilli and block-rich deposits display a positive skewness due to fine tail grading.     

Subaqueous,rhyolitic dome-top tuff cones: a model based on the Devonian Bunga Beds,southeastern Australia and a modern analogue

The relics of two Late Devonian subaqueous rhyolitic dome-top tuff and pumice cone successions are preserved in the Bunga Beds outlier of the Boyd Volcanic Complex, southeastern Australia. These cone successions and other rhyolitic volcanics of the Bunga Beds are associated with turbidite and other deep-water massflow sedimentary rocks. The two cone successions have a generally similar stratigraphy. At the base, flow-banded, variably autobrecciated and quench-fragmented rhyolite, representing an intrusive to extrusive dome, is overlain by rhyolitesediment breccia, representing extrusion of the dome through the deep-water sediment pile and resedimentation down its flanks. In the northern cone succession an overlying, succession of bedded pumiceous crystal-rich to crystal-poor tuffs represents the onset of pyroclastic activity and growth of a tuff cone. An overyling debris flow deposit represents degradation of part of the cone. The topmost unit, a stratified pumice succession, is thought to represent another cone-building eruptive phase, and is separated from the underlying strata by a major slide surface. The southern cone succession contains less tuff and abundant pumice, and is also terminated by a debris-flow deposit, indicating cone degradation. A modern analogue for the inferred eruptive style and sequence is the 1953–1957 rhyolite eruption that formed the Tuluman Island lava-tuff cone complex in the Bismarck Sea. The eruptions were often cyclical consisting of an initial inferred submarine-lava-forming stage, passing into a pumicecone-forming stage, in some cases a subaeriallava-forming stage, and a final stage, following the cessation of volcanism, during which the cones collapsed gravitationally or were destroyed by wave erosion. Using observations from both the Tuluman Island eruptions and the preserved stratigraphies of the Devonian tuff cones, a dynamic model is proposed for the formation of subaqueous rhyolitic dome-top tuff and pumice cones.     

Rothenberg scoria cone,East Eifel: a complex Strombolian and phreatomagmatic volcano

Rothenberg scoria cone Eifel formed by an alternation of three Strombolian and three phreatomagmatic eruptive phases. Eruptions took place from up to six vents on a 600 m-long fissure, building an early tuff ring and then two coalescing scoria cones. Strombolian volcanism dominated volumetrically, as the supply of external water was severely limited. Magma/water interaction only occurred during the opening stages of eruption at any vent, when discharge rates were low and the fragmentation surface was below the water table. The phreatomagmatic deposits consist of relatively well-sorted fall beds and only minor surge deposits. They contain juvenile clasts with a wide range of vesicularity and grain size, implying considerable heterogeneity in the assemblage of material ejected by the phreatomagmatic explosions. the transition from phreatomagmatic to Strombolian eruption at any vent was rapid and irreversible, and Strombolian volcanism persisted even when eruption rates are inferred to have waned at the close of each eruptive phase as, by then, the fragmentation surfaces were high in the growing cones and water was denied access to the magma. The Strombolian deposits are relatively homogenous, consisting of alternating coarser- and finer-grained, well-sorted fall beds erupted during periods of open-vent eruption and partial blockage of the vent respectively. The intervals of Strombolian eruption were always a delicate balance between discharge of freely degassing magma and processes such as ponding of degassed magma in the vent, collapse of the growing cones, and repeated recycling of clasts through the vent. Clear evidence of the instability of the Rothenberg cones is preserved in numerous unconformities within deposits of the inner walls of the cones. The close of Strombolian phases was probably marked by a decreasing supply of magma to the vents accompanied by ponding and stagnation of lava in the craters.     

Eruption processes and deposit characteristics at the monogenetic Mt. Gambier Volcanic Complex, SE Australia: implications for alternating magmatic and phreatomagmatic activity

The ～5 ka Mt. Gambier Volcanic Complex in the Newer Volcanics Province, Australia is an extremely complex monogenetic, volcanic system that preserves at least 14 eruption points aligned along a fissure system. The complex stratigraphy can be subdivided into six main facies that record alternations between magmatic and phreatomagmatic eruption styles in a random manner. The facies are (1) coherent to vesicular fragmental alkali basalt (effusive/Hawaiian spatter and lava flows); (2) massive scoriaceous fine lapilli with coarse ash (Strombolian fallout); (3) bedded scoriaceous fine lapilli tuff (violent Strombolian fallout); (4) thin–medium bedded, undulating very fine lapilli in coarse ash (dry phreatomagmatic surge-modified fallout); (5) palagonite-altered, cross-bedded, medium lapilli to fine ash (wet phreatomagmatic base surges); and (6) massive, palagonite-altered, very poorly sorted tuff breccia and lapilli tuff (phreato-Vulcanian pyroclastic flows). Since most deposits are lithified, to quantify the grain size distributions (GSDs), image analysis was performed. The facies are distinct based on their GSDs and the fine ash to coarse+fine ash ratios. These provide insights into the fragmentation intensities and water–magma interaction efficiencies for each facies. The eruption chronology indicates a random spatial and temporal sequence of occurrence of eruption styles, except for a “magmatic horizon” of effusive activity occurring at both ends of the volcanic complex simultaneously. The eruption foci are located along NW–SE trending lineaments, indicating that the complex was fed by multiple dykes following the subsurface structures related to the Tartwaup Fault System. Possible factors causing vent migration along these dykes and changes in eruption styles include differences in magma ascent rates, viscosity, crystallinity, degassing and magma discharge rate, as well as hydrological parameters.     

Mixed deposits of complex magmatic and phreatomagmatic volcanism: an example from Crater Hill,Auckland, New Zealand

 A series of alternating phreatomagmatic ("wet") and magmatic ("dry") basaltic pyroclastic deposits forming the Crater Hill tuff ring in New Zealand contains one unit (M1) which can only be interpreted as the products of mixing of ejecta from simultaneous wet and dry explosions at different portions of a multiple vent system. The principal characteristics of M1 are (a) rapid lateral changes in the thicknesses of, and proportions in juvenile components in individual beds, and (b) wide ranges of juvenile clast densities in every sample. M1 appears to have been associated with an elongate source of highly variable and fluctuating magma : water ratios and magma discharge rates. This contrasts with the only other documented mixed (wet and dry) basaltic pyroclastic deposits where mixing from two point sources of quite different but stable character has been inferred. Received: July 11, 1995 / Accepted: February 13, 1996     

Structures and facies associated with the flow of subaerial basaltic lava into a deep freshwater lake: The Sulphur Creek lava flow, North Cascades, Washington

The ca. 8800 ¹⁴C yrs BP Sulphur Creek lava flowed eastward 12 km from the Schriebers Meadow cinder cone into the Baker River valley, on the southeast flank of Mount Baker volcano. The compositionally-zoned basaltic to basaltic andesite lava entered, crossed and partially filled the 2-km-wide and > 100-m-deep early Holocene remnant of Glacial Lake Baker. The valley is now submerged beneath a reservoir, but seasonal drawdown permits study of the distal entrant lava. As a lava volume that may have been as much as 180 × 10⁶ m³ entered the lake, the flow invaded the lacustrine sequence and extended to the opposite (east) side of the drowned Baker River valley. The volume and mobility of the lava can be attributed to a high flux rate, a prolonged eruption, or both. Basalt exposed below the former level of the remnant glacial lake is glassy or microcrystalline and sparsely vesicular, with pervasive hackly or blocky fractures. Together with pseudopillow fractures, these features reflect fracturing normal to penetrative thermal fronts and quenching by water. A fine-grained hyaloclastite facies was probably formed during quench fragmentation or isolated magma-water explosions. Although the structures closely resemble those developed in lava-ice contact environments, establishing the depositional environment for lava exhibiting similar intense fracturing should be confirmed by geologic evidence rather than by internal structure alone. The lava also invaded the lacustrine sequence, forming varieties of peperite, including sills that are conformable within the invaded strata and resemble volcaniclastic breccias. The peperite is generally fragmental and clast- or matrix-supported; fine-grained and rounded fluidal margins occur locally. The lava formed a thickened subaqueous plug that, as the lake drained in the mid-Holocene, was exposed to erosion. The Baker River then cut a 52-m-deep gorge through the shattered, highly erodible basalt.     

Cooling,degassing and compaction of rhyolitic ash flow tuffs: a computational model

Previous models of degassing, cooling and compaction of rhyolitic ash flow deposits are combined in a single computational model that runs on a personal computer. The model applies to a broader range of initial and boundary conditions than Riehle's earlier model, which did not integrate heat and mass flux with compaction and which for compound units was limited to two deposits. Model temperatures and gas pressures compare well with simple measured examples. The results indicate that degassing of volatiles present at deposition occurs within days to a few weeks. Compaction occurs for weeks to two to three years unless halted by devitrification; near-emplacement temperatures can persist for tens of years in the interiors of thick deposits. Even modest rainfall significantly chills the upper parts of ash deposits, but compaction in simple cooling units ends before chilling by rainwater influences cooling of the interior of the sheet. Rainfall does, however, affect compaction at the boundaries of deposits in compound cooling units, because the influx of heat from the overlying unit is inadequate to overcome heat previously lost to vaporization of water. Three density profiles from the Matahina Ignimbrite, a compound cooling unit, are fairly well reproduced by the model despite complexities arising from numerous cooling breaks. Uncertainties in attempts to correlate in detail among the profiles may be the result of the non-uniform distribution of individual deposits. Regardless, it is inferred that model compaction is approximately valid. Thus the model should be of use in reconstructing the emplacement history of compound ash deposits, for inferring the depositional environments of ancient deposits and for assessing how long deposits of modern ash flows are capable of generating phreatic eruptions or secondary ash flows.     

The Holocene Secche di Lazzaro phreatomagmatic succession (Stromboli,Italy): evidence of pyroclastic density current origin deduced by facies analysis and AMS flow directions

The edifice of Stromboli volcano gravitationally collapsed several times during its volcanic history (>100 ka–present). The largest Holocene event occurred during the final stage of the Neostromboli activity (∼13–5 ka), and was accompanied by the emplacement of phreatomagmatic and lahar deposits, known as the Secche di Lazzaro succession. A stratigraphic and paleomagnetic study of the Secche di Lazzaro deposits allows the interpretation of the emplacement and the eruptive processes. We identify three main units within the succession that correspond to changing eruption conditions. The lower unit (UA) consists of accretionary lapilli-rich, thinly bedded, parallel- to cross-stratified ash deposits, interpreted to indicate the early stages of the eruption and emplacement of dilute pyroclastic density currents. Upward, the second unit (UB) of the deposit is more massive and the beds thicker, indicating an increase in the sedimentation rate from pyroclastic density currents. The upper unit (UC) caps the succession with thick, immediately post-eruptive lahars, which reworked ash deposited on the volcano’s slope. Flow directions obtained by Anisotropy of Magnetic Susceptibility (AMS) analysis of the basal bed of UA at the type locality suggest a provenance of pyroclastic currents from the sea. This is interpreted to be related to the initial base-surges associated with water–magma interaction that occurred immediately after the lateral collapse, which wrapped around the shoulder of the sector collapse scar. Upward in the stratigraphy (upper beds of UA and UB) paleoflow directions change and show a provenance from the summit vent, probably related to the multiple collapses of a vertical, pulsatory eruptive column.     

Paleomagnetic results from Late Carboniferous to Early Permian rocks in the northern Qiangtang terrane,Tibet, China,and their tectonic implications

Results of a systematic paleomagnetic study are reported based on Late Carboniferous to Early Permian sedimentary rocks on the north slope of the Tanggula Mountains,in the northern Qiangtang terrane(NQT),Tibet,China.Data revealed that magnetic minerals in limestone samples from the Zarigen Formation(CP^z)are primarily composed of magnetite,while those in sandstone samples from the Nuoribagaribao Formation(Pnr)are dominated by hematite alone,or hematite and magnetite in combination.Progressive thermal,or alternating field,demagnetization allowed us to isolate a stable high temperature component(HTC)in 127 specimens from 16 sites which successfully passed the conglomerate test,consistent with primary remnance.The tilt-corrected mean direction for Late Carboniferous to Early Permian rocks in the northern Qiangtang terrane is D_s=30.2°,I_s=-40.9°,k_s=269.0,a_(95)=2.3°,N=16,which yields a corresponding paleomagnetic pole at 25.7°N,241.5°E(dp/dm=2.8°/1.7°),and a paleolatitude of 23.4°S.Our results,together with previously reported paleomagnetic data,indicate that:(1)the NQT in Tibet,China,was located at a low latitude in the southern hemisphere,and may have belonged to the northern margin of Gondwana during the Late Carboniferous to Early Permian;(2)the Paleo-Tethys Ocean was large during the Late Carboniferous to Early Permian,and(3)the NQT subsequently moved rapidly northwards,perhaps related to the fact that the Paleo-Tethys Ocean was rapidly contracting from the Late Permian to Late Triassic while the Bangong Lake-Nujiang Ocean,the northern branch of the Neo-Tethys Ocean,expanded rapidly during this time.     

Partially melted lithic megablocks in the Yardea Dacite, Gawler Range Volcanics, Australia: implications for eruption and emplacement mechanisms

 Lithic megablocks ranging from <1 to 50 m in diameter occur in the Yardea Dacite, a widespread (12,000 km²), thick (>200 m) felsic volcanic unit in the Mesoproterozoic Gawler Range Volcanic Province (GRV) of South Australia. Throughout its vast extent, the Yardea Dacite shows typical lava-like features, in that it is massive, columnar jointed and evenly porphyritic with 30–40% crystals in a spherulitic and granophyric groundmass. In addition, flow banding is present at many locations. The megablocks are abundant at two sites 50 km apart, but isolated megablocks and smaller (<6 cm) lithic clasts are also scattered throughout the unit. At both sites the megablocks are matrix supported, non-graded, randomly oriented and show no evidence of being confined to a particular stratigraphic level in the dacite. The most abundant and largest megablocks are granitoids derived from older basement and from early-crystallised plutons of the Hiltaba Suite, which is broadly coeval and comagmatic with the GRV. The granitoid megablocks have been partially melted, most likely prior to eruption when resident in the thermal aureole of the Yardea Dacite magma chamber. The lithic megablock occurrences are unlike coarse pyroclastic breccias but are similar in distribution and abundance to xenoliths in lavas, consistent with the lava-like character of the host dacite. Using reasonable estimates of megablock density, magma density and magma viscosity, we show that the rise rate of the dacitic magma exceeded the settling velocity of the megablocks, implying that they could have been entrained and erupted effusively. All but the largest and least-melted megablocks would have remained suspended or else settled very slowly in the dacitic lava during outflow. The rapid rate of magma withdrawal required to produce such an extensive felsic sheet could have also triggered disintegration of the thermally stressed wallrock surrounding the magma chamber, dislodging megablocks that were later entrained and effusively erupted. Received: 11 November 1998 / Accepted: 18 April 1999