Silicic Volcanism Research Articles

Terrestrial events across the Permian–Triassic boundary along the Yunnan–Guizhou border, SW China

The border area between western Guizhou and eastern Yunnan Provinces in SW China is an ideal place to undertake research considering the terrestrial–ecological system evolution across the Permian–Triassic boundary (PTB). The study of plant and palynomorph fossils, clay minerals, inorganic geochemistry and sedimentary facies in this area enable us to interpret the events occurring at that time. The extinction pattern of the flora interpreted from megafloral and palynomorph data is demonstrated by a sudden decline of species numbers at the PTB after a long-term of gradual changes, followed by a delayed extinction in the basal Triassic. The two boundary claybeds (Beds 66 and 68 in the Chahe Section, beds 47 and 49 in the Zhejue Section) are considered to be volcanogenic. The inorganic geochemical anomalies occurred between Beds 63 and 69, Chahe Section and Beds 45 and 50, Zhejue Section. Sedimentary facies changed from channels of braided rivers, into flood plains of braided rivers, then to shallow lakes, reflecting a gradual transgression by lakes across the area. Our conclusions are that the mass extinction across the PTB in western Guizhou and eastern Yunnan was probably caused by the Siberian basaltic eruption episode and the siliceous volcanism in South China. These lithospheric events represented by volcanisms heralded a series of climatic and environmental events, giving rise to a catastrophe for the biosphere.

Silicic magmas of Protector Shoal, South Sandwich arc: indicators of generation of primitive continental crust in an island arc

Protector Shoal, the northernmost and most silicic volcano of the South Sandwich arc, erupted dacite–rhyolite pumice in 1962. We report geochemical data for a new suite of samples dredged from the volcano. Geochemically, the dredge and 1962 samples form four distinct magma groups that cannot have been related to each other, and are unlikely to have been related to a single basaltic parent, by fractional crystallization. Instead, the silicic rocks are more likely to have been generated by partial melting of basaltic lower crust within the arc. Trace element and Sr–Nd isotope data indicate that the silicic volcanics have compositions that are more similar to the volcanic arc than the oceanic basement formed at a back-arc spreading centre, and volcanic arc basalts are considered to be the likely source for the silicic magmas. The South Sandwich Islands are one of several intra-oceanic arcs (Tonga–Kermadec, Izu–Bonin) that have: (1) significant amounts of compositionally bimodal mafic–silicic volcanic products and (2) 6.0–6.5 km s−1P-wave velocity layers in their mid-crusts that have been imaged by wide-angle seismic surveys and interpreted as intermediate-silicic plutons. Geochemical and volume considerations indicate that both the silicic volcanics and plutonic layers were generated by partial melting of basaltic arc crust, representing an early stage in the fractionation of oceanic basalt to form continental crust.

Formation of Aromatum Chaos, Mars: Morphological development as a result of volcano‐ice interactions

Morphological examination of the Aromatum Chaos depression on Mars supports earlier suggestions that it is a site of cryosphere disruption and release of pressurized water trapped in an underlying aquifer. We infer that the cause of cryosphere disruption was intrusion of a volcanic sill, confined laterally by earlier intruded dikes, and consequent melting of ice by heat from the sill. The vertical extents and displacements of blocks of terrain on the floor of the depression, together with an estimate of the cryosphere thickness, constrain the vertical extent of ice melting and hence the thickness of the sill (∼100 m) and the depth at which it was intruded (∼2–5 km). At least ∼75% of the volume of material removed from Aromatum Chaos must have been crustal rock rather than melted ice. Water from melted cryosphere ice played a negligible role in creating the depression, the process being dominated by released aquifer water. For sediment loads of 30–40% by volume, 10,500–16,500 km3 of aquifer water must have passed through the depression to carry away rock as entrained sediment and erode the associated Ravi Vallis channel. These required water volumes are 2–3 times larger than the amount of water that could reasonably be contained in aquifers located beneath the area of incipiently collapsed ground to the west of Aromatum Chaos and suggest a much larger water source. Given that this source probably also fed the nearby Shalbatana Vallis outflow channel, Gangis Chasma is the most likely candidate.

The Tonga–Kermadec arc and Havre–Lau back-arc system: Their role in the development of tectonic and magmatic models for the western Pacific

The convergent plate boundary marked by the Tonga–Kermadec arc system is one of the clearest examples of an intra-oceanic arc on the modern Earth. Arising from ground breaking research in the 1970's and continuing through the present, the arc has become a testing ground for defining and explaining models for the tectonic and magmatic evolution of intra-oceanic subduction systems. Despite its simple oceanic setting the arc shows significant variations along its length in terms of rock types and structure and because of the lack of involvement of continental crust these allow refinement of the details of or models of how arcs develop and evolve. Of particular importance are the variations in convergence rates and vectors between the major plates and the effect these have on the tectonic evolution of the arc and back-arc. The erupted rocks of the arc have provided the means of testing the contribution of subducted sediment, slab, mantle wedge and overlying crust and most recently tests of P– T–time paths for recycled sediments. Although the arc is dominated by basalts and basaltic andesites, there is increasing evidence for significant proportions of silicic magmas erupted in recent times. Silicic volcanism is also evidenced in the presence of caldera structures revealed by detailed multibeam mapping that have been a feature of research in the last decade. As we enter the 21st century, the Tonga–Kermadec arc continues as an archetypical natural laboratory for the testing of new ideas in subduction zone tectonics and petrology.

Geochronology (SHRIMP II) of zircons from Archean stratotectonic associations of Karelian greenstone belts: Significance for stratigraphic and geodynamic reconstructions

Individual grains of zircon from the Archean Kostomuksha, North Karelian and Matkalakhta greenstone belts, which are situated respectively in western, northern and eastern Karelia, are studied using the ion microprobe SHRIMP II. As a result, the oldest 207Pb/206Pb ages of 3151 ± 4.6 and 3329 ± 16 Ma are first determined for detrital zircons from northern and eastern Karelia. The 207Pb/206Pb ages estimated for two subsequent metamorphic events of Archean Eon in eastern Karelia correspond to 3.25 and 3.17–3.10 Ga. The age value of 2711 ± 9.6 Ma is determined for silicic volcano-plutonic complex and quartz stockwork in northern Karelia and the date 2821 ± 15 Ma for magmatic rocks of eastern Karelia. Silicic volcanics from an oceanic plateau section in the Kostomuksha belt are dated at 2791.7 ± 6.1 Ma for the first time in the Archean of Fennoscandia. The oldest detrital zircons from siliciclastic metasediments determine the stabilization time of Archean continental nuclei in East Fennoscandia. The younger generation of greenstone belts is exemplified in the Karelian craton by the Matkalakhta and Kostomuksha structures comprising rock associations less than 2.82 Ga old, mafic rocks of the Kontokki Group included. Geological history of these belts corresponds to geodynamic mesocycle 90–110 Ma long and to the Archean global epoch of metallogeny, which was responsible for origin of most valuable deposits of base and precious metals.

The transition from explosive to effusive eruptive regime: The example of the 1912 Novarupta eruption, Alaska

The shift from explosive to effusive silicic volcanism seen in many historical eruptions reflects a change in the style of degassing of erupted magma. This paper focuses on such a transition during the largest eruption of the twentieth century, the 1912 eruption of Novarupta. The transition is recorded in a dacite block bed, which covers an elliptical area of 4 km 2 around the vent. Approximately 700 studied blocks fall into four main lithologic categories: (1) pumiceous, (2) dense, (3) flow-banded dacites, and (4) welded breccias. Textural analyses of the blocks indicate portions of the melt underwent highly variable degrees of outgassing. Vesicle populations show features characteristic of bubble coalescence and collapse. A decrease in measured vesicularity and increased evidence for bubble collapse compared with pumice from earlier Plinian episodes mark the transition from closed- to open-system degassing. Block morphology and textures strongly suggest the magma was first erupted as a relatively gas-rich lava dome/plug, but incomplete out-gassing led to explosive disruption. Heterogeneous degassing of ascending magma began in Plinian Episode III and resulted in instability during Episode IV dome growth and a (series of) Vulcanian explosion(s). Modeling of the dynamics of explosion initiation and ejecta dispersal indicates that a significant concentration in gas is required to produce the explosions responsible for the observed block field dispersal. The amount of gas available in the hot pumiceous dome material appears to have been inadequate to drive the explosion(s); therefore, external water most likely contributed to the destruction.

Geochemical evolution of magmatism in Archean granite-greenstone terrains

Evolution of Archean magmatism is one of the key problems concerning the early formation stages of the Earth crust and biosphere, because that evolution exactly controlled variable concentrations of chemical elements in the World Ocean, which are important for metabolism. Geochemical evolution of magmatism between 3.5 and 2.7 Ga is considered based on database characterizing volcanic and intrusive rock complexes of granite-greenstone terrains (GGT) studied most comprehensively in the Karelian (2.9–2.7 Ga) and Kaapvaal (3.5–2.9 Ga) cratons and in the Pilbara block (3.5–2.9 Ga). Trends of magmatic geochemical evolution in the mentioned GGTs were similar in general. At the early stage of their development, tholeiitic magmas were considerably enriched in chalcophile and siderophile elements Fe2O3, MgO, Cr, Ni, Co, V, Cu, and Zn. At the next stage, calc-alkaline volcanics of greenstone belts and syntectonic TTG granitoids were enriched in lithophile elements Rb, Cs, Ba, Th, U, Pb, Nb, La, Sr, Be and others. Elevated concentrations of both the “crustal” and “mantle-derived” elements represented a distinctive feature of predominantly intrusive rocks of granitoid composition, which were characteristic of the terminal stage of continental crust formation in the GGTs, because older silicic rocks and lithospheric mantle were jointly involved into processes of magma generation. On the other hand, the GGTs different in age reveal specific trends in geochemical evolution of rock associations close in composition and geological position. First, the geochemical cycle of GGT evolution was of a longer duration in the Paleoarchean than in the Meso-and Neoarchean. Second, the Paleoarche an tholeiitic associations had higher concentrations of LREE and HFSE (Zr, Ti, Th, Nb, Ta, Hf) than their Meso-and Neoarchean counterparts. Third, the Y and Yb concentrations in Paleoarchean calc-alkaline rock associations are systematically higher than in Neoarchean rocks of the same type, while their La/Yb ratios are in contrast lower than in the latter. These distinctions are likely caused by evolution of mantle magmatic reservoirs and by changes in formation mechanisms of silicic volcanics and TTG granitoids. The first of these factors was likely responsible for appearance of sanukitoid magmatic rocks in the Late Mesoarchean. Representative database considered in the work includes ca. 500 precision analyses of Archean magmatic rocks.

Subsurface Structure of the "Petit-spot" Intra-plate Volcanism, in the Northwestern Pacific

Single-channel seismic reflection surveys were conducted in the northwestern Pacific to investigate subsurface structure and nature of volcanic eruption of small volcanic knolls considered to be formed by petit-spot intra-plate volcan- ism, and also morphology of a sedimentary basin and oceanic basement around the knolls in the area. Seismic profiles reveal the sedimentary layer is acoustically transparent, having some horizontal reflectors within the layer. Oceanic crust of the Pacific Plate lies flatly beneath the sedimentary layer with nearly constant thickness of ~200-300 m. The layer beneath the small knolls is acoustically opaque, possibly caused by inhomogeneous structure due to feeder dykes or cryptodome and vol- canic deposits. Strong reflectors at base of the sedimentary layer, identified in the vicinity of small knolls, are probably re flec- tions from volcanic sills. The oceanic crust beneath the small knolls is depressed by weight of the volcanic deposits.

Volcanic Setting of the Bajo de la Alumbrera Porphyry Cu-Au Deposit, Farallon Negro Volcanics, Northwest Argentina

The late Miocene Farallon Negro volcanics, comprising basaltic to rhyodacitic volcano-sedimentary rocks, host the Bajo de la Alumbrera porphyry copper-gold deposit in northwest Argentina. Early studies of the geology of the district have underpinned the general model for porphyry ore deposits where hydrothermal alteration and mineralization develop in and around porphyritic intrusions emplaced at shallow depths (2.5-3.5 km) into stratovolcanic assemblages. The Farallon Negro succession is dominated by thick sequences of volcanosedimentary breccias, with lavas forming a minor component volumetrically. These volcaniclastic rocks conformably overlie crystalline basement-derived sedimentary rocks deposited in a developing foreland basin southeast of the Puna-Altiplano plateau. Within the FarallAƒÂ³n Negro volcanics, volcanogenic accumulations evolved from early mafic to intermediate and silicic compositions. The younger and more silicic rocks are demonstrably coeval and comagmatic with the earliest group of mineralized porphyritic intrusions at Bajo de la Alumbrera. Our analysis of the volcanic stratigraphy and facies architecture of the Farallon Negro volcanics indicates that volcanic eruptions evolved from effusive to mixed effusive and explosive styles, as magma compositions changed to more intermediate and silicic compositions. An early phase of mafic to intermediate volcanism was characterized by small synsedimentary intrusions with peperitic contacts, and lesser lava units scattered widely throughout the district, and interbedded with thick and extensive successions of coarse-grained sedimentary breccias. These sedimentary breccias formed from numerous debris- and hyperconcentrated flow events. A later phase of silicic volcanism included both effusive eruptions, forming several areally restricted lavas, and explosive eruptions, producing more widely dispersed (up to 5 km) tuff units, some up to 30-m thickness in proximal sections. Four key features of the volcanic stratigraphy suggest that the Farallon Negro volcanics need not simply record the construction of a large steep-sided polygenetic stratovolcano: (1) sheetlike, laterally continuous debris-flow and other coarse-grained sedimentary deposits are dominant, particularly in the lower sections; (2) mafic-intermediate composition lavas are volumetrically minor; (3) peperites are present throughout the sequence; and (4) fine-grained lacustrine sandstone-siltstone sequences occur in areas previously thought to be proximal to the summit region of the stratovolcano. Instead, the nature, distribution, and geometry of volcanic and volcaniclastic facies suggest that volcanism occurred as a relatively low relief, multiple-vent volcanic complex at the eastern edge of a broad, >200-km-wide late Miocene volcanic belt and on an active foreland sedimentary basin to the Puna-Altiplano. Volcanism that occurred synchronously with the earliest stages of porphyry- related mineralization at Bajo de la Alumbrera apparently developed in an alluvial to ring plain setting that was distal to larger volcanic edifices.

40Ar/39Ar ages of silicic volcanic rocks in the Tauranga‐Kaimai area, New Zealand: Dating the transition between volcanism in the Coromandel Arc and the Taupo Volcanic Zone

Subduction‐related volcanism in the northern part of the North Island of New Zealand shifted abruptly during the late Pliocene. This study focuses on the transition, in time and space, from the NNW‐oriented Miocene‐Pliocene Coromandel Volcanic Zone to the northeast‐oriented active Taupo Volcanic Zone. The volcanic rocks marking this transition are exposed in the Tauranga Basin and adjacent Kaimai Range, and associated here with the recently defined Tauranga and Kaimai Volcanic Centres, respectively. New 40Ar/39Ar age determinations indicate that the transition occurred between 1.90 and 1.55 Ma, that is between the youngest age of silicic volcanism in the Tauranga‐Kaimai area, and the age of the oldest silicic volcanism in the Taupo Volcanic Zone. This interpretation is generally consistent with recent plate models and with the initiation of the Kermadec Arc within the last 2 m.y.

Mid-Jurassic age for the Botany Bay Group: implications for Weddell Sea Basin creation and southern hemisphere biostratigraphy

New U–Pb zircon ion-microprobe ages from the alluvial conglomerates and flood plain sediments of the Botany Bay Group demonstrate that sedimentation occurred at c. 167 Ma, coeval with rift-related silicic volcanism in the northern Antarctic Peninsula. In contrast, rift-related volcanism and sedimentation in the southern Antarctic Peninsula (Latady Basin) occurred at c. 183 Ma. The new data indicate that syn-rift sedimentation and volcanism was diachronous from south to north, consistent with early opening of the Weddell Sea embayment by anti-clockwise rotation of the Antarctic Peninsula in the Mid-Jurassic. A definitive date for the Botany Bay Group floras has important implications for Southern Hemisphere biostratigraphic correlations.

Evidences for Incipient Hydrothermal Event(s) in the Central Indian Basin: A Review

Abstract The Central Indian Basin (CIB, 10°–19°S) encompasses morphotectonic features such as seamounts, abyssal hills, faults, fracture zones and lineations and basic to silicic volcanics that were derived from different sources. Instances of incipient hydrothermal event (s) occurred as evident from: (1) spilites that probably formed due to low‐temperature hydrothermal action on the precursory basalts, (2) slabs and fragments of zeolitites which are the consequent products of palagonitization of the basaltic glasses, (3) chemical variations in the ferromanganese crusts, and (4) abundance of volcanogenic‐hydrothermal material (vhm) that includes ochrous sediments and magnetite spherules. The vhm suggests its origin by interaction of Fe‐rich hydrothermal solutions with surrounding sediments. The CIB spherules occur in sediments of ∼10 ka and ∼425 to 650 ka age and are orders of magnitude younger than the 50 to 60 Ma age of the basin. Incidentally, to the north of the basin at ODP Site 717–719 (1°S/81°E) hydrothermal precipitates of inorganic calcite and deposition of sulphide minerals (∼7.5 to 9 Ma and 0.5 Ma) occur as a result of the on‐going intraplate deformation. This paper reviews the above evidences that possibly indicate episodes of localized hydrothermal events in the CIB.

Possible distinguishing characteristics of very deepwater explosive and effusive silicic volcanism

Recent seafloor exploration has shown that volcanic-hosted massive sulfides (VHMS) occur in modern silicic calderas formed by highly explosive eruptions at substantial water depths. Sampling of these has so far been restricted to surficial deposits. Ancient analogs provide a time-integrated view of the structure and fill of deepwater calderas, but constraints on paleo-water depths have previously been lacking. The Ordovician Bald Mountain VHMS (northern Maine, USA) is unique in its degree of preservation, not only permitting detailed textural and structural analysis of the enclosing volcanic rocks, but also allowing fluid inclusion analysis to determine a paleo-water depth of eruption of greater than 1.45 km. The deposits that result from very deepwater explosive eruptions have distinctive textural characteristics recognized in other ancient examples where paleo-water depths are less well constrained. I propose that hydrostatic pressure suppresses the explosivity of the eruption sufficiently to retard the formation of fine ash, producing ignimbrite composed largely of pumice lapilli. It also inhibits exsolution of dissolved water in the magma, producing fluidal rhyolite eruptions and intrusions. These include silicic fire fountaining to produce proximal cones of hyalotuff or spatter accumulations, and intrusions of globular peperite.

U–Pb zircon geochronology of Late Devonian to Early Carboniferous extension‐related silicic volcanism in the northern New England Fold Belt

Laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) analysis of zircons confirm a Late Devonian to Early Carboniferous age (ca 360–350 Ma) for silicic volcanic rocks of the Campwyn Volcanics and Yarrol terrane of the northern New England Fold Belt (Queensland). These rocks are coeval with silicic volcanism recorded elsewhere in the fold belt at this time (Connors Arch, Drummond Basin). The new U–Pb zircon ages, in combination with those from previous studies, show that silicic magmatism was both widespread across the northern New England Fold Belt (>250 000 km2 and ≥500 km inboard of plate margin) and protracted, occurring over a period of ∼15 million years. Zircon inheritance is commonplace in the Late Devonian — Early Carboniferous volcanics, reflecting anatectic melting and considerable reworking of continental crust. Inherited zircon components range from ca 370 to ca 2050 Ma, with Middle Devonian (385–370 Ma) zircons being common to almost all dated units. Precambrian zircon components record either Precambrian crystalline crust or sedimentary accumulations that were present above or within the zone of magma formation. This contrasts with a lack of significant zircon inheritance in younger Permo‐Carboniferous igneous rocks intruded through, and emplaced on top of, the Devonian‐Carboniferous successions. The inheritance data and location of these volcanic rocks at the eastern margins of the northern New England Fold Belt, coupled with Sr–Nd, Pb isotopic data and depleted mantle model ages for Late Palaeozoic and Mesozoic magmatism, imply that Precambrian mafic and felsic crustal materials (potentially as old as 2050 Ma), or at the very least Lower Palaeozoic rocks derived from the reworking of Precambrian rocks, comprise basement to the eastern parts of the fold belt. This crustal basement architecture may be a relict from the Late Proterozoic breakup of the Rodinian supercontinent.

Geochronology of age‐progressive volcanism of the Oregon High Lava Plains: Implications for the plume interpretation of Yellowstone

The High Lava Plains province (HLP) is a late Cenozoic bimodal volcanic field at the northern margin of the Basin and Range province in southeastern Oregon that hosts a westward younging trend of silicic volcanism that crudely mirrors northeastward migration of silicic volcanism along the Yellowstone–Snake River Plain (YSRP) trend. We present 40Ar/39Ar ages for 19 rhyolite domes, 5 rhyolite ash flow tuffs, and 34 basaltic lavas from the HLP. The previously identified trend of westward migration of HLP rhyolites is confirmed. The rate of propagation is ∼33 km/m.y. from 10 to 5 Ma, slowing to ∼13 km/m.y. after 5 Ma. The duration of silicic volcanism at any locus is ∼2 m.y. Three older HLP dacite domes yielded ages of ∼15.5 Ma. Basalts are not age progressive. We identify several episodes of increased basaltic activity at 7.5–7.8, 5.3–5.9, and 2–3 Ma, with the younger episode likely continuing into the Recent. The HLP and YSRP trends emerged from the axis of middle Miocene basaltic volcanism of the Columbia River and Steens basalts. We propose a model in which (1) Miocene flood basalts and widespread silicic rocks are the result of emplacement of a plume head near the craton margin, enhanced by flow up a topographic gradient along the base of the lithosphere at the craton margin; (2) the HLP trend is the result of westward flow originating at the craton margin; and (3) the YSRP trend is the trace of the motion of the North American plate over the tail of the plume.

Magma Generation at a Large, Hyperactive Silicic Volcano (Taupo, New Zealand) Revealed by U–Th and U–Pb Systematics in Zircons

Young (<65 ka) explosive silicic volcanism at Taupo volcano, New Zealand, has involved the development and evacuation of several crustal magmatic systems. Up to and including the 26·5 ka 530 km3 Oruanui eruption, magmatic systems were contemporaneous but geographically separated. Subsequently they have been separated in time and have vented from geographically overlapping areas. Single-crystal (secondary ionization mass spectrometry) and multiple-crystal (thermal ionization mass spectrometry) zircon model-age data are presented from nine representative eruption deposits from 45 to 3·5 ka. Zircon yields vary by three orders of magnitude, correlating with the degrees of zircon saturation in the magmas, and influencing the spectra of model ages. Two adjacent magma systems active up to 26·5 ka show wholly contrasting model-age spectra. The smaller system shows a simple unimodal distribution. The larger system, using data from three eruptions, shows bimodal model-age spectra. An older 100 ka peak is interpreted to represent zircons (antecrysts) derived from older silicic mush or plutonic rocks, and a younger peak to represent zircons (phenocrysts) that grew in the magma body immediately prior to eruption. Post-26·5 ka magma batches show contrasting age spectra, consistent with a mixture of antecrysts, phenocrysts and, in two examples, xenocrysts from Quaternary plutonic and Mesozoic–Palaeozoic metasedimentary rocks. The model-age spectra, coupled with zircon-dissolution modelling, highlight contrasts between short-term silicic magma generation at Taupo, by bulk remobilization of crystal mush and assimilation of metasediment and/or silicic plutonic basement rocks, and the longer-term processes of fractionation from crustally contaminated mafic melts. Contrasts between adjacent or successive magma systems are attributed to differences in positions of the source and root zones within contrasting domains in the quartzo-feldspathic (<15 km deep) crust below the volcano.

Stratigraphy, age and correlation of middle Pleistocene silicic tephras in the Auckland region, New Zealand: A prolific distal record of Taupo Volcanic Zone volcanism

Coastal sections in the Auckland region reveal highly carbonaceous and/or highly weathered clay‐dominated cover‐bed successions with numerous discrete distal volcanic ash (tephra) layers, fluvially reworked siliciclastic (tephric) deposits, and two widely distributed pyroclastic density current (PDC) deposits generated from explosive silicic volcanism within the Taupo Volcanic Zone (TVZ). The younger of the two PDC deposits (informally named Waiuku tephra) is glass‐isothermal plateau fission‐track (ITPFT) dated at 1.00 ± 0.03 Ma and occurs in a normal polarity interval interpreted as the Jaramillo Subchron. Waiuku tephra is correlated with Unit E sourced from the Mangakino Volcanic Centre of the TVZ. Waiuku tephra can be subdivided into two distinctive units enabling unequivocal field correlation: a lower stratified unit (dominantly pyroclastic surge with fall component) and an upper massive to weakly stratified unit (pyroclastic flow). At many sites in south Auckland, Waiuku tephra retains basal “surge‐like” beds (<1.4 m thickness). This provides clear evidence for primary emplacement and is an exceptional feature considering the c. 200 km this PDC has travelled from its TVZ source area. However, at many other Auckland sites, Waiuku tephra displays transitional sedimentary characteristics indicating lateral transformation from hot, gas‐supported flow/surge into water‐supported mass flow and hyperconcentrated flow (HCF) deposits. The older PDC deposit is dated at 1.21 ± 0.09 Ma, is enveloped by tephras that are ITPFT‐dated at 1.14 ± 0.06 Ma (above) and 1.21 ± 0.06 Ma (below), respectively, and occurs below a short normal polarity interval (Cobb Mountain Subchron) at c. 1.19 Ma. This PDC deposit, correlated with Ongatiti Ignimbrite sourced from the Mangakino Volcanic Centre of TVZ, has laterally transformed from a gas‐supported, fine‐grained pyroclastic flow deposit at Oruarangi, Port Waikato, into a water‐supported volcaniclastic mass flow deposit farther north at Glenbrook Beach. The occurrence of Ongatiti Ignimbrite in Auckland significantly extends its northward distribution. Large numbers of post‐ and pre‐Ongatiti rhyolitic tephra layers, ranging in age from c. 1.31 to 0.53 Ma, are also recognised in the region, with some up to 0.5 m in compacted fallout thickness. Although some tephras can be attributed to known TVZ eruptions (e.g., Ahuroa/Unit D), many have yet to be identified in proximal source areas and remain uncorrelated. However, some can be reliably correlated to tephra layers occurring in marine to nearshore sequences of Wanganui Basin and deep‐sea cores retrieved east of North Island. The identification of previously unrecognised mid‐Pleistocene TVZ‐sourced tephra deposits in the Auckland region, and their correlation to the offshore marine record, represent an advance in the construction of a higher resolution history for the TVZ where, close to eruptive source, the record is fragmentary and obscured by deep burial, or erosion, or both.

Io's volcanic control of Jupiter's extended neutral clouds

Dramatic changes in the brightness and shape of Jupiter's extended sodium nebula are found to be correlated with the infrared emission brightness of Io. Previous imaging and modeling studies have shown that varying appearances of the nebula correspond to changes in the rate and the type of loss mechanism for atmospheric escape from Io. Similarly, previous IR observational studies have assumed that enhancements in infrared emissions from Io correspond to increased levels of volcanic (lava flow) activity. In linking these processes observationally and statistically, we conclude that silicate volcanism on Io controls both the rate and the means by which sodium escapes from Io's atmosphere. During active periods, molecules containing sodium become an important transient in Io's upper atmosphere, and subsequent photochemistry and molecular-ion driven dynamics enhance the high speed sodium population, leading to the brightest nebulas observed. This is not the case during volcanically quiet times when omni-present atmospheric sputtering ejects sodium to form a modest, base-level nebula. Sodium's role as a “trace gas” of the more abundant species of sulfur (S) and oxygen (O) is less certain during volcanic episodes. While we suggest that volcanism must also affect the escape rates of S and O, and consequently their extended neutral clouds, the different roles played by lava and plume sources for non-sodium species are far too uncertain to make definitive comparisons at this time.

Middle Jurassic air fall tuff in the sedimentary Latady Formation, eastern Ellsworth Land

Rhyolitic volcanism along the proto-Pacific margin of Gondwana occurred at intervals throughout the Jurassic. Silicic melt generation has been interpreted as a result of interaction between mantle plumes and subduction modified lower crust. The rhyolitic Mount Poster Formation of the southern Antarctic Peninsula is c. 184 Ma in age (V1), whereas silicic volcanism of the northern Antarctic Peninsula is c. 168 Ma (V2). A thin, (13.5 cm) reworked air fall tuff, interbedded with sandstone and mudstone of the Latady Formation in the southern Antarctic Peninsula has a REE pattern similar to V2 volcanic material but is isotopically similar to the extracaldera, low-Ti rhyolites of the V1 Mount Poster Formation. The tuff is interbedded with lithofacies that have been assigned a Callovian age (164–159 Ma) in the west of the area. Simple mixing between a MASH source and reworked Early Jurassic (184 Ma) V1 volcanic material during V2 volcanism in the area explains the apparent discrepancy between the faunal age and the isotopic characteristics of the ash fall. This supports a Middle Jurassic (168 Ma) age that also corresponds to a 167 ± 3 Ma age from Mt Rex on the periphery of the Mount Poster Formation, which was previously thought to be anomalous.

Large magnitude silicic volcanism in north Afar: the Nabro Volcanic Range and Ma?alalta volcano

Much of the volcanological work carried out in north Afar (Ethiopia and Eritrea) has focused on the nature of Quaternary basaltic volcanic ranges, which have been interpreted by some as incipient oceanic ridges. However, we show here that comparable volumes of silicic magmas have been erupted in the region. In particular, the virtually undocumented Nabro Volcanic Range, which runs NNE for more than 100 km from the margin of the Danakil Depression to the Red Sea coast, has a subaerial volume of the order of 550 km3, comparable to the volume of the much better known Erta’Ale axial volcanic range. Nabro volcano itself forms part of an enigmatic double caldera structure with a neighbouring volcano, Mallahle. The twin caldera may have formed simultaneously with the eruption of between 20 and 100 km3 of ignimbrite, which is readily identified in Landsat Thematic Mapper imagery. This may have been the largest explosive eruption in north Afar, and is certain to have deposited a regionally distributed tephra layer which could in the future be located in distal sections as a stratigraphic marker. An integrated analysis of optical and synthetic aperture radar imagery, digital topographic data, field observations and limited geochemical measurements, permits here descriptions and first order inferences about the structure, stratigraphy and compositions of several major volcanoes of the Afar Triangle, and a reappraisal of their regional significance.