Emplacement temperatures of pyroclastic and volcaniclastic deposits in kimberlite pipes in southern Africa

Palaeomagnetic techniques for estimating the emplacement temperatures of volcanic deposits have been applied to pyroclastic and volcaniclastic deposits in kimberlite pipes in southern Africa. Lithic clasts were sampled from a variety of lithofacies from three pipes for which the internal geology is well constrained (the Cretaceous A/K1 pipe, Orapa Mine, Botswana, and the Cambrian K1 and K2 pipes, Venetia Mine, South Africa). The sampled deposits included massive and layered vent-filling breccias with varying abundances of lithic inclusions, layered crater-filling pyroclastic deposits, talus breccias and volcaniclastic breccias. Basalt lithic clasts in the layered and massive vent-filling pyroclastic deposits in the A/K1 pipe at Orapa were emplaced at >570°C, in the pyroclastic crater-filling deposits at 200–440°C and in crater-filling talus breccias and volcaniclastic breccias at <180°C. The results from the K1 and K2 pipes at Venetia suggest emplacement temperatures for the vent-filling breccias of 260°C to >560°C, although the interpretation of these results is hampered by the presence of Mesozoic magnetic overprints. These temperatures are comparable to the estimated emplacement temperatures of other kimberlite deposits and fall within the proposed stability field for common interstitial matrix mineral assemblages within vent-filling volcaniclastic kimberlites. The temperatures are also comparable to those obtained for pyroclastic deposits in other, silicic, volcanic systems. Because the lithic content of the studied deposits is 10–30%, the initial bulk temperature of the pyroclastic mixture of cold lithic clasts and juvenile kimberlite magma could have been 300–400°C hotter than the palaeomagnetic estimates. Together with the discovery of welded and agglutinated juvenile pyroclasts in some pyroclastic kimberlites, the palaeomagnetic results indicate that there are examples of kimberlites where phreatomagmatism did not play a major role in the generation of the pyroclastic deposits. This study indicates that palaeomagnetic methods can successfully distinguish differences in the emplacement temperatures of different kimberlite facies.     

Pyroclastic deposits as a guide for reconstructing the multi-stage evolution of the Somma-Vesuvius Caldera

 The evolution of the Somma-Vesuvius caldera has been reconstructed based on geomorphic observations, detailed stratigraphic studies, and the distribution and facies variations of pyroclastic and epiclastic deposits produced by the past 20,000 years of volcanic activity. The present caldera is a multicyclic, nested structure related to the emptying of large, shallow reservoirs during Plinian eruptions. The caldera cuts a stratovolcano whose original summit was at 1600–1900 m elevation, approximately 500 m north of the present crater. Four caldera-forming events have been recognized, each occurring during major Plinian eruptions (18,300 BP "Pomici di Base", 8000 BP "Mercato Pumice", 3400 BP "Avellino Pumice" and AD 79 "Pompeii Pumice"). The timing of each caldera collapse is defined by peculiar "collapse-marking" deposits, characterized by large amounts of lithic clasts from the outer margins of the magma chamber and its apophysis as well as from the shallow volcanic and sedimentary units. In proximal sites the deposits consist of coarse breccias resulting from emplacement of either dense pyroclastic flows (Pomici di Base and Pompeii eruptions) or fall layers (Avellino eruption). During each caldera collapse, the destabilization of the shallow magmatic system induced decompression of hydrothermal–magmatic and hydrothermal fluids hosted in the wall rocks. This process, and the magma–ground water interaction triggered by the fracturing of the thick Mesozoic carbonate basement hosting the aquifer system, strongly enhanced the explosivity of the eruptions. Received: 24 November 1997 / Accepted: 23 March 1999     

Submarine basaltic volcanism: Morphometric parameters for discriminating hyaloclastites from hyalotuffs

Hyaloclastites are volcaniclastic rocks generated by non-explosive granulation of volcanic glass which takes place when basaltic magmas are quenched by contact with water; hyaloclastites are common products of deep submarine basaltic central volcanoes (seamounts and guyots). We suggested (Honnorlz, 1966) calling hyalotuffs the actual pyroclastic rocks which are generated by phreatomagmatic and phreatic explosions taking place when basaltic volcanoes crupt in shallow waters; hyalotuffs are restricted to shallow subaqueous conditions since no volcanic explosion can occur in deep seas. The distinction between hyaloclastites and hyalotuffs is therefore a useful tool when reconstructing the paleogeography of ancient submarine volcanic edifices and the mechanism by which their lavas were emplaced. We propose using two sets of morphometric parameters to discriminate the hyaloclastites from the hyalotuffs. The granules making up these two volcaniclastic rock types plot in different areas of either 1) a ternary diagram the apexes of which represent the grain planarity (P), convexity (V) and concavity (C) % («roundness» according to Szadeczky-Kardoss); or 2) a binary diagram relating the number of grain corners (N) to their planarity (P) %.     

Petrology, magnetostratigraphy and geochronology of the Miocene volcaniclastic Tepoztlán Formation: implications for the initiation of the Transmexican Volcanic Belt (Central Mexico)

The volcaniclastic Tepoztlán Formation (TF) represents an important rock record to unravel the early evolution of the Transmexican Volcanic Belt (TMVB). Here, a depositional model together with a chronostratigraphy of this Formation is presented, based on detailed field observations together with new geochronological, paleomagnetic, and petrological data. The TF consists predominantly of deposits from pyroclastic density currents and extensive epiclastic products such as tuffaceous sandstones, conglomerates and breccias, originating from fluvial and mass flow processes, respectively. Within these sediments fall deposits and lavas are sparsely intercalated. The clastic material is almost exclusively of volcanic origin, ranging in composition from andesite to rhyolite. Thick gravity-driven deposits and large-scale alluvial fan environments document the buildup of steep volcanic edifices. K-Ar and Ar-Ar dates, in addition to eight magnetostratigraphic sections and lithological correlations served to construct a chronostratigraphy for the entire Tepoztlán Formation. Correlation of the 577 m composite magnetostratigraphic section with the Cande and Kent (1995) Geomagnetic Polarity Time Scale (GPTS) suggests that this section represents the time intervall 22.8–18.8 Ma (6Bn.1n-5Er; Aquitanian-Burdigalian, Lower Miocene). This correlation implies a deposition of the TF predating the extensive effusive activity in the TMVB at 12 Ma and is therefore interpreted to represent its initial phase with predominantly explosive activity. Additionally, three subdivisions of the TF were established, according to the dominant mode of deposition: (1) the fluvial dominated Malinalco Member (22.8–22.2 Ma), (2) the volcanic dominated San Andrés Member (22.2–21.3 Ma) and (3) the mass flow dominated Tepozteco Member (21.3–18.8 Ma).     

Eocene volcanism during the incipient stage of Izu–Ogasawara Arc: Geology and petrology of the Mukojima Island Group,the Ogasawara Islands

The Ogasawara Islands mainly comprise Eocene volcanic strata formed when the Izu–Ogasawara–Mariana Arc began. We present the first detailed volcanic geology, petrography and geochemistry of the Mukojima Island Group, northernmost of the Ogasawara Islands, and show that the volcanic stratigraphy consists of arc tholeiitic rocks, ultra‐depleted boninite‐series rocks, and less‐depleted boninitic andesites, which are correlatable to the Maruberiwan, Asahiyama and Mikazukiyama Formations on the Chichijima Island Group to the south. On Chichijima, a short hiatus is identified between the Maruberiwan (boninite, bronzite andesite, and dacite) and Asahiyama Formation (quartz dacite and rhyolite). In contrast, these lithologies are interbedded on Nakodojima of the Mukojima Island Group. The stratigraphically lower portion of Mukojima is mainly composed of pillow lava, which is overlain by reworked volcaniclastic rocks in the middle, whereas the upper portion is dominated by pyroclastic rocks. This suggests that volcanic activity now preserved in the Mukojima Island Group records growth of one or more volcanoes, beginning with quiet extrusion of lava under relatively deep water followed by volcaniclastic deposition. These then changed into moderately explosive eruptions that took place in shallow water or above sea level. This is consistent with the uplift of the entire Ogasawara Ridge during the Eocene. Boninites from the Mukojima Island Group are divided into three types on the basis of geochemistry. Type 1 boninites have high SiO₂ (>57.0 wt.%) and Zr/Ti (>0.022) and are the most abundant type in both Mukojima and Chichijima Island Groups. Type 2 boninites have low SiO₂ (<57.1 wt.%) and Zr/Ti (<0.014). Type 3 boninites have 57.6–60.7 wt.% SiO₂ and are characterized by high CaO/Al₂O₃ (0.9–1.1). Both type 2 and 3 boninites are common on Mukojima but are rare in the Chichijima Island Group.     

The Taurewa Eruptive Episode: evidence for climactic eruptions at Ruapehu volcano, New Zealand

 The ca. 10,500 years B.P. eruptions at Ruapehu volcano deposited 0.2–0.3 km³ of tephra on the flanks of Ruapehu and the surrounding ring plain and generated the only known pyroclastic flows from this volcano in the late Quaternary. Evidence of the eruptions is recorded in the stratigraphy of the volcanic ring plain and cone, where pyroclastic flow deposits and several lithologically similar tephra deposits are identified. These deposits are grouped into the newly defined Taurewa Formation and two members, Okupata Member (tephra-fall deposits) and Pourahu Member (pyroclastic flow deposits). These eruptions identify a brief (<ca. 2000-year) but explosive period of volcanism at Ruapehu, which we define as the Taurewa Eruptive Episode. This Episode represents the largest event within Ruapehu's ca. 22,500-year eruptive history and also marks its culmination in activity ca. 10,000 years B.P. Following this episode, Ruapehu volcano entered a ca. 8000-year period of relative quiescence. We propose that the episode began with the eruption of small-volume pyroclastic flows triggered by a magma-mingling event. Flows from this event travelled down valleys east and west of Ruapehu onto the upper volcanic ring plain, where their distal remnants are preserved. The genesis of these deposits is inferred from the remanent magnetisation of pumice and lithic clasts. We envisage contemporaneous eruption and emplacement of distal pumice-rich tephras and proximal welded tuff deposits. The potential for generation of pyroclastic flows during plinian eruptions at Ruapehu has not been previously considered in hazard assessments at this volcano. Recognition of these events in the volcanological record is thus an important new factor in future risk assessments and mitigation of volcanic risk at Tongariro Volcanic Centre. Received: 5 July 1998 / Accepted: 12 March 1999     

Large phreatomagmatic vent complex at Coombs Hills, Antarctica: Wet, explosive initiation of flood basalt volcanism in the Ferrar-Karoo LIP

The Mawson Formation and correlatives in the Transantarctic Mountains and South Africa record an early eruption episode related to the onset of Ferrar-Karoo flood basalt volcanism. Mawson Formation rocks at Coombs Hills comprise mainly (≥80% vol) structureless tuff breccia and coarse lapilli tuff cut by irregular dikes and sills, within a large vent complex (>30 km²). Quenched juvenile fragments of generally low but variable vesicularity, accretionary lapilli and country rock clasts within vent-fill, and pyroclastic density current deposits point to explosive interaction of basalt with groundwater in porous country rock and wet vent filling debris. Metre-scale dikes and pods of coherent basalt in places merge imperceptibly into peperite and then into surrounding breccia. Steeply dipping to sub-vertical depositional contacts juxtapose volcaniclastic rocks of contrasting componentry and grainsize. These sub-vertical tuff breccia zones are inferred to have formed when jets of debris + steam + water passed through unconsolidated vent-filling deposits. These jets of debris may have sometimes breached the surface to form subaerial tephra jets which fed subaerial pyroclastic density currents and fall deposits. Others, however, probably died out within vent fill before reaching the surface, allowing mixing and recycling of clasts which never reached the atmosphere. Most of the ejecta that did escape the debris-filled vents was rapidly recycled as vents broadened via lateral quarrying of country rock and bedded pyroclastic vent-rim deposits, which collapsed along the margins into individual vents. The unstratified, poorly sorted deposits comprising most of the complex are capped by tuff, lapilli tuff and tuff breccia beds inferred to have been deposited on the floor of the vent complex by pyroclastic density currents. Development of the extensive Coombs Hills vent-complex involved interaction of large volumes of magma and water. We infer that recycling of water, as well as recycling of pyroclasts, was important in maintaining water supply for phreatomagmatic interactions even when aquifer rock in the vent walls lay far from eruption sites as a consequence of vent-complex widening. The proportion of recycled water increased with vent-complex size in the same way that the proportion of recycled tephra did. Though water recycling leaves no direct rock record, the volcaniclastic deposits within the vent complex show through their lithofacies/structural architecture, lithofacies characteristics, and particle properties clear evidence for extensive and varied recycling of material as the complex evolved. Editorial responsibility: J. Donnelly-Nolan     

An extremely large magnitude eruption close to the Plio-Pleistocene boundary: reconstruction of eruptive style and history of the Ebisutoge-Fukuda tephra, central Japan

An extremely large magnitude eruption of the Ebisutoge-Fukuda tephra, close to the Plio-Pleistocene boundary, central Japan, spread volcanic materials widely more than 290,000 km² reaching more than 300 km from the probable source. Characteristics of the distal air-fall ash (>150 km away from the vent) and proximal pyroclastic deposits are clarified to constrain the eruptive style, history, and magnitude of the Ebisutoge-Fukuda eruption.Eruptive history had five phases. Phase 1 is phreatoplinian eruption producing >105 km³ of volcanic materials. Phases 2 and 3 are plinian eruption and transition to pyroclastic flow. Plinian activity also occurred in phase 4, which ejected conspicuous obsidian fragments to the distal locations. In phase 5, collapse of eruption column triggered by phase 4, generated large pyroclastic flow in all directions and resulted in more than 250–350 km³ of deposits. Thus, the total volume of this tephra amounts over 380–490 km³. This indicates that the Volcanic Explosivity Index (VEI) of the Ebisutoge-Fukuda tephra is greater than 7. The huge thickness of reworked volcaniclastic deposits overlying the fall units also attests to the tremendous volume of eruptive materials of this tephra.Numerous ancient tephra layers with large volume have been reported worldwide, but sources and eruptive history are often unknown and difficult to determine. Comparison of distal air-fall ashes with proximal pyroclastic deposits revealed eruption style, history and magnitude of the Ebisutoge-Fukuda tephra. Hence, recognition of the Ebisutoge-Fukuda tephra, is useful for understanding the volcanic activity during the Pliocene to Pleistocene, is important as a boundary marker bed, and can be used to interpret the global environmental and climatic impact of large magnitude eruptions in the past.     

Characterization of volcanic deposits with ground-penetrating radar

 Field-based studies of surficial volcanic deposits are commonly complicated by a combination of poor exposure and rapid lateral variations controlled by unknown paleotopography. The potential of ground-penetrating radar (GPR) as an aid to volcanological studies is shown using data collected from traverses over four well-exposed, Recent volcanic deposits in western Canada. The deposits comprise a pumice airfall deposit (3–4 m thick), a basalt lava flow (3–6 m thick), a pyroclastic flow deposit (15 m thick), and an internally stratified pumice talus deposit (60 m thick). Results show that GPR is effective in delineating major stratigraphic contacts and hence can be used to map unexposed deposits. Different volcanic deposits also exhibit different radar stratigraphic character, suggesting that deposit type may be determined from radar images. In addition, large blocks within the pyroclastic deposits are detected as distinctive point diffractor patterns in the profiles, showing that the technique has potential for providing important grain-size information in coarse poorly sorted deposits. Laboratory measurements of dielectric constant (K') are reported for samples of the main rock types and are compared with values of K' for the bulk deposit as inferred from the field data. The laboratory values differ significantly from the "field" values of K'; these results suggest that the effectiveness of GPR at any site can be substantially improved by initial calibration of well-exposed locations. Received: 10 May 1996 / Accepted: 27 December 1996     

The role of regional-scale faults in controlling a trapdoor caldera,Coromandel Peninsula,New Zealand

Regional gravity data from an eroded Miocene to Pliocene volcanic arc exposed in the Coromandel Peninsula, New Zealand, delineate a circular − 26-mGal, 15-km-diameter gravity anomaly. This anomaly, which has steep gradients on its northern and western margins but shallow gradients elsewhere, correlates with relatively young volcanic and volcaniclastic rocks within a broad topographic depression. Gravity modelling, using an exponentially decreasing density contrast with depth profile, requires very low-density rocks (ca. 2280 kg m^− 3) in the near-surface to account for the observed anomaly, giving a total depth of ca. 2.8 km for these rocks. The northern and western margins of this body dip steeply inward at 70°, whereas the southern and eastern margins have shallow inward dips (20–30°). The western margin coincides with the regional-scale Mangakino Fault, but the northern margin, recognizable only in the geophysical data (and named here the Ohinemuri Fault), is partially buried under younger volcanic rocks. We interpret these deep and steeply bounded, low-density volcanics in terms of a trapdoor caldera, faulted on its northern and western margins, with its hinge on the southern and eastern margins. Epithermal deposits are spatially associated with the Mangakino and Ohinemuri Faults, suggesting that both structures may have influenced hydrothermal fluid flow. These deposits pre-date caldera fill, indicating that caldera development followed pre-existing regional faults. These results delineate the subsurface geometry of a trapdoor caldera and highlight the role of pre-existing, regional-scale faults in controlling such caldera location and collapse.     

Thermoremanence in red sandstone clasts and emplacement temperature of a quaternary pyroclastic deposit (Catalan Volcanic Zone,ne Spain)

Summary The application of the progressive thermal demagnetization procedure of volcanic rock debris has been frequently used to determine the emplacement temperatures of pyroclastic deposits and thus to characterize the nature of these volcanic deposits. This debris consists of a mixture of juvenile fragments derived from the explosive fragmentation of erupting magma and an assortment of lithic clasts derived mainly from the walls of a volcanic conduit, as well as from the ground. The temperature at which the clasts were deposited can be estimated by analyzing its remanent magnetization. To do this, oriented samples of clasts are subjected to progressive thermal demagnetization and the directions of the resulting remanent vectors provide the necessary information. Clasts of basalt, andesite, limestone, pumice and homebricks have previously been used to estimate the emplacement temperatures of pyroclastic deposits. According to our data, clasts of red sandstones also seem to be good carriers of thermoremanent magnetization. We have carried out a paleomagnetic study on a Quaternary, lithic-rich, massive, pyroclastic deposit from the Puig d'Adri volcano (Catalan Volcanic Zone), which contains a large number of red sandstone clasts. It is concluded that the studied deposit cannot be considered as a lahar or as a pyroclastic surge deposit, considering both the emplacement temperature and the morphological features.Presented at 3rd Biennial Meeting on New Trends in Geomagnetism, Smolenice Castle, West Slovakia, June 22–29, 1992     

Geology of a complex kimberlite pipe (K2 pipe,Venetia Mine,South Africa): insights into conduit processes during explosive ultrabasic eruptions

K2 is a steep-sided kimberlite pipe with a complex internal geology. Geological mapping, logging of drillcore and petrographic studies indicate that it comprises layered breccias and pyroclastic rocks of various grain sizes, lithic contents and internal structures. The pipe comprises two geologically distinct parts: K2 West is a layered sequence of juvenile- and lithic-rich breccias, which dip 20–45° inwards, and K2 East consists of a steep-sided pipe-like body filled with massive volcaniclastic kimberlite nested within the K2 pipe. The layered sequence in K2 West is present to > 900 m below present surface and is interpreted as a sequence of pyroclastic rocks generated by explosive eruptions and mass-wasting breccias generated by rock fall and sector collapse of the pipe walls: both processes occurred in tandem during the infill of the pipe. Several breccia lobes extend across the pipe and are truncated by the steep contact with K2 East. Dense pyroclastic rocks within the layered sequence are interpreted as welded deposits. K2 East represents a conduit that was blasted through the layered breccia sequence at a late stage in the eruption. This phase may have involved fluidisation of trapped pyroclasts, with loss of fine particles and comminution of coarse clasts. We conclude that the K2 kimberlite pipe was emplaced in several distinct stages that consisted of an initial explosive enlargement, followed by alternating phases of accumulation and ejection.     

Geology, ore deposits and hydrothermal venting in Bahía Concepción, Baja California Sur, Mexico

Abstract Bahía Concepción is located in the eastern coast of the Baja California peninsula and it is shaped by northwestern–southeastern normal faults. These are associated with a 12–6 Ma rifting episode, although some have been reactivated since the Pliocene. The most abundant rocks correspond to the arc related Comondú Group, Oligocene to Miocene, which forms a mainly calc‐alkaline volcanic and volcaniclastic sequence. There are less extensive outcrops of sedimentary rocks, lava flows, domes and pyroclastic rocks of Pliocene to Quaternary ages. The Neogene volcanism in the area indicates a shift from a subduction regime to an intraplate volcanism related to continental extension and the opening of an oceanic basin. The Bahía Concepción area contains numerous Mn ore deposits, being the biggest at El Gavilán and Guadalupe. The Mn deposits occur as veins, breccias and stockworks, and are composed by Mn oxides (pyrolusite, coronadite, romanechite), dolomite, quartz and barite. The deposits are hosted in volcanic rocks of the Comondú Group and, locally, in Pliocene sedimentary rocks. Thus, the Mn deposits formed between the Middle Miocene and the Pliocene. The mineralized structures are associated with Miocene northwestern–southeastern fault systems, which are analogous to those associated with the Cu‐Co‐Zn‐Mn deposits of El Boleo. The Bahía Concepción area also bears subaerial and submarine hot springs, which are associated with the same fault systems and host rocks. The submarine and subaerial geothermal manifestations south of the bay are possibly related with recent volcanism. The geothermal manifestations within the bay are intertidal hot springs and shallow submarine diffuse venting areas. Around the submarine vents (5–15 m deep, 87°C), Fe‐oxyhydroxide crusts with pyrite and cinnabar precipitate. In the intertidal vents (62°C), aggregates of opal, calcite, barite and Ba‐rich Mn oxides occur covered by silica‐carbonate stromatolitic sinters. Some 10–30 cm thick crustiform veins formed by chalcedony, calcite and barite are also found close to the vents. The hydrothermal fluids exhibit mixed isotopic compositions between δ¹⁸O‐enriched meteoric and local marine water. The precipitation of Ba‐rich Mn oxides around the vent sites could be an active analog for the processes that produced Miocene to Pliocene hydrothermal Mn‐deposits.     

Volcano-sedimentary stratigraphy in the Valsequillo Basin,Central Mexico inferred from electrical resistivity soundings

Initial results of an electrical resistivity survey of the volcano-sedimentary sequence of the Valsequillo basin in central Mexico are presented. The volcano-sedimentary deposits preserve rich paleontological, paleoclimatic and paleoenvironmental records, which include extinct megafauna remains associated with human artifacts. The report of possible 38 ka old human footprint tracks in the Xalnene tuff attracted renewed interest in the basin stratigraphy. We examine the shallow stratigraphic sequence in the Xalnene tuff outcrop plain northwest of Cerro Toluquilla volcano using vertical resistivity soundings (VES). Inversion models of VES soundings show a layered structure of high and low resistivity units, which characterize the Xalnene tuff, lacustrine and fluvial sediments and volcanic rocks. 2-D resistivity cross sections document three major units corresponding to the Xalnene tuff and sediments filling a <30 m deep basin lying on volcanic rocks. Resistivity models provide further support for the association of Xalnene tuff with the Toluquilla volcano and emplacement of the pyroclastic deposits on a shallow lacustrine environment. The resistivity cross sections constrain the thickness of the tuff layers and underlying lacustrine sediments. Observations during the data acquisition field work provide insight on the possible origin of the apparent tracks, which seem to develop from erosion processes acting on quarrying marks. Further analysis and experimental evidence is required to understand the morphology and weathered patterns. The tuff layers are being removed by quarrying operations and the outcrops significantly altered. Adequate conservation measurements should be implemented to preserve the deposits for scientific research.     

Volcanic dry avalanche deposits — Identification and comparison with nonvolcanic debris stream deposits

A “volcanic dry avalanche deposit” is defined as a volcaniclastic deposit formed as a result of a large-scale sector collapse of a volcanic cone associated with some form of volcanic activity. Avalanche transport occurred in response to the gravitational field, in a manner similar to the transport of nonvolcanic debris streams (e.g. Hsü, 1975). Such deposits are characterized by megablock structure — deformed and fractured large blocks up to several hundreds meters in diameter. A megablock preserves original layering, intrusive contacts or weathered surfaces of the source volcanic edifice. Surface topography of the deposit is characterised with hummocky relief. Ratios of fell height to travel distance for volcanic dry avalanche deposits are between 0.18 and 0.06. This range is similar but smaller than the value of 0.58 to 0.08 for nonvolcanic debris stream deposit. This similarity suggests similar transportation mechanisms. Excessive travel distances as defined by Hsü (1975), calculated for volcanic dry avalanche deposits, give values larger than for debris stream deposits of the same volume. The difference is explained by lower rigidity of the collapsing mass due to the existence of soft pyroclastic layers, alteration around the vent, development of fractures owing to new cryptodome intrusion, and boiling of supercritical fluid contained within the collapsed mass.     

New insights into Late Pleistocene explosive volcanic activity and caldera formation on Ischia (southern Italy)

A new pyroclastic stratigraphy is presented for the island of Ischia, Italy, for the period ∼75–50 ka BP. The data indicate that this period bore witness to the largest eruptions recorded on the island and that it was considerably more volcanically active than previously thought. Numerous vents were probably active during this period. The deposits of at least 10 explosive phonolite to basaltic-trachyandesite eruptions are described and interpreted. They record a diverse range of explosive volcanic activity including voluminous fountain-fed ignimbrite eruptions, fallout from sustained eruption columns, block-and-ash flows, and phreatomagmatic eruptions. Previously unknown eruptions have been recognised for the first time on the island. Several of the eruptions produced pyroclastic density currents that covered the whole island as well as the neighbouring island of Procida and parts of the mainland. The morphology of Ischia was significantly different to that seen today, with edifices to the south and west and a submerged depression in the centre. The largest volcanic event, the Monte Epomeo Green Tuff (MEGT) resulted in caldera collapse across all or part of the island. It is shown to comprise at least two thick intracaldera ignimbrite flow-units, separated by volcaniclastic sediments that were deposited during a pause in the eruption. Extracaldera deposits of the MEGT include a pumice fall deposit emplaced during the opening phases of the eruption, a widespread lithic lag breccia outcropping across much of Ischia and Procida, and a distal ignimbrite in south-west Campi Flegrei. During this period the style and magnitude of volcanism was dictated by the dynamics of a large differentiated magma chamber, which was partially destroyed during the MEGT eruption. This contrasts with the small-volume Holocene and historical effusive and explosive activity on Ischia, the timing and distribution of which has been controlled by the resurgence of the Monte Epomeo block. The new data contribute to a clearer understanding of the long-term volcanic and magmatic evolution of Ischia.     

Volcanology of the 2350 B.P. Eruption of Mount Meager Volcanic Complex, British Columbia, Canada: implications for Hazards from Eruptions in Topographically Complex Terrain

 The Pebble Creek Formation (previously known as the Bridge River Assemblage) comprises the eruptive products of a 2350 calendar year B.P. eruption of the Mount Meager volcanic complex and two rock avalanche deposits. Volcanic rocks of the Pebble Creek Formation are the youngest known volcanic rocks of this complex. They are dacitic in composition and contain phenocrysts of plagioclase, orthopyroxene, amphibole, biotite and minor oxides in a glassy groundmass. The eruption was episodic, and the formation comprises fallout pumice (Bridge River tephra), pyroclastic flows, lahars and a lava flow. It also includes a unique form of welded block and ash breccia derived from collapsing fronts of the lava flow. This Merapi-type breccia dammed the Lillooet River. Collapse of the dam triggered a flood that flowed down the Lillooet Valley. The flood had an estimated total volume of 10⁹ m³ and inundated the Lillooet Valley to a depth of at least 30 m above the paleo-valley floor 5.5 km downstream of the blockage. Rock avalanches comprising mainly blocks of Plinth Assemblage volcanic rocks (an older formation making up part of the Mount Meager volcanic complex) underlie and overlie the primary volcanic units of the Formation. Both rock avalanches are unrelated to the 2350 B.P. eruption, although the post-eruption avalanche may have its origins in the over-steepened slopes created by the explosive phase of the eruption. Much of the stratigraphic complexity evident in the Pebble Creek Formation results from deposition in a narrow, steep-sided mountain valley containing a major river. Received: 20 January 1998 / Accepted: 29 September 1998     

Volcanology and petrology of Mathews Tuya,northern British Columbia,Canada: glaciovolcanic constraints on interpretations of the 0.730 Ma Cordilleran paleoclimate

Petrological, volcanological and geochronological data collected at Mathews Tuya together provide constraints on paleoclimate conditions during formation of the edifice. The basaltic tuya was produced via Pleistocene glaciovolcanism in northern British Columbia, Canada, and is located within the Tuya volcanic field (59.195°N/130.434°W), which is part of the northern Cordilleran volcanic province (NCVP). The edifice comprises a variety of lithofacies, including columnar-jointed lava, pillow lava, massive dikes, and volcaniclastic rocks. Collectively these deposits record the transition from an explosive subaqueous to an effusive subaerial eruption environment dominated by Pleistocene ice. As is typical for tuyas, the volcaniclastic facies record multiple fragmentation processes including explosive, quench and mechanical fragmentation. All samples from Mathews Tuya are olivine-plagioclase porphyritic alkali olivine basalts. They are mineralogically and geochemically similar to nearby glaciovolcanic centers from the southeastern part of the Tuya volcanic field (e.g., Ash Mountain, South Tuya, Tuya Butte) as well as the dominant NCVP rock type. Crystallization scenarios calculated with MELTS account for variations between whole rock and glass compositions via low pressure fractionation. The presence of olivine microphenocrysts and the absence of pyroxene phenocrysts constrain initial crystallization pressures to less than 0.6 GPa. The eruption of Mathews Tuya occurred between 0.718 ± 0.054 Ma and 0.742 ± 0.081 Ma based on ⁴⁰Ar/³⁹Ar geochronology (weighted mean age of 0.730 Ma). The age determinations provide the first firm documentation for large (>700 m thick), pre-Fraser/Wisconsin glaciers in north-central British Columbia ~0.730 Ma, and correlate in age with glaciovolcanic deposits in Russia (e.g., Komatsu et al. Geomorph 88: 352-366, 2007) and with marine isotopic evidence for large global ice volumes ~0.730 Ma.     

The Holocene Pucón eruption of Volcán Villarrica, Chile: deposit architecture and eruption chronology

The Pucón eruption was the largest Holocene explosive outburst of Volcán Villarrica, Chile. It discharged >1.0 km³ of basaltic-andesite magma and >0.8 km³ of pre-existing rock, forming a thin scoria-fall deposit overlain by voluminous ignimbrite intercalated with pyroclastic surge beds. The deposits are up to 70 m thick and are preserved up to 21 km from the present-day summit, post-eruptive lahar deposits extending farther. Two ignimbrite units are distinguished: a lower one (P1) in which all accidental lithic clasts are of volcanic origin and an upper unit (P2) in which basement granitoids also occur, both as free clasts and as xenoliths in scoria. P2 accounts for ∼80% of the erupted products. Following the initial scoria fallout phase, P1 pyroclastic flows swept down the northern and western flanks of the volcano, magma fragmentation during this phase being confined to within the volcanic edifice. Following a pause of at least a couple of days sufficient for wood devolatilization, eruption recommenced, the fragmentation level dropped to within the granitoid basement, and the pyroclastic flows of P2 were erupted. The first P2 flow had a highly turbulent front, laid down ignimbrite with large-scale cross-stratification and regressive bedforms, and sheared the ground; flow then waned and became confined to the southeastern flank. Following emplacement of pyroclastic surge deposits all across the volcano, the eruption terminated with pyroclastic flows down the northern flank. Multiple lahars were generated prior to the onset of a new eruptive cycle. Charcoal samples yield a probable eruption age of 3,510 ± 60 ¹⁴C years BP.     

Geology of a submarine volcanic caldera in the Tonga Arc: Dive results

A submersible dive conducted on Volcano #1 located near 21° 09′S–175° 45′W on the Tonga Arc showed that the volcanic edifice with a caldera floor area of 30 km² located at and 450 m deep (b.s.l.=below sea level) was constructed recently during episodic volcanism. The sequential volcanic events are recorded along a faulted terrain formed in response to the collapse of the caldera wall. The post-caldera events are marked by occasional eruptions that have built scoriaceous cones associated with low-temperature hydrothermal venting and localized small-scale collapse features. The stratigraphy of the caldera wall indicates that the volcano was built by explosive volcanism alternating with quieter eruptive events. The repeated, violent explosive events formed ≤ 20 m thick sequences composed of alternating fine-grained ash beds and sand- to boulder-sized pyroclastic layers. During quieter volcanic events, dykes and massive flows intruded and/or accompanied the eruption of the volcaniclastic deposits throughout the sections of the wall explored. Massive columnar-jointed flows consist of viscous, silica-rich lavas forming tabular and giant radial-jointed (GRJ) flows formed in large (> 8 m in diameter) conduits and extruded onto the sea floor. In addition, massive lava flows forming sill-like complexes were observed underneath and near the giant radial-jointed columnar flows. Also, an intermittent quiet type of eruption produced vesicular lava flows, which are interbedded within the pyroclastic layered deposits. The massive and vesicular lavas consist of andesites and dacites with Ca-depleted (pigeonite) and Ca-enriched (salite) pyroxene, and intermediate (andesine-labradorite) to calcic (bytownite) plagioclase. They are depleted in total alkalis (Na₂O + K₂O < 3%), K₂O (< 1%), Zr/Y (< 1.8), Nb/Zr (< 0.01) and light Rare Earth Elements. We interpret that these andesite–dacite series were erupted after undergoing crystal-liquid fractionation in a magma chamber located underneath the caldera floor.