Period Of Intense Volcanic Activity Research Articles

Lessons Learnt from Monitoring the Etna Volcano Using an IoT Sensor Network through a Period of Intense Eruptive Activity.

This paper describes the successes and failures after 4 years of continuous operation of a network of sensors, communicating nodes, and gateways deployed on the Etna Volcano in Sicily since 2019, including a period of Etna intense volcanic activity that occurred in 2021 and resulted in over 60 paroxysms. It documents how the installation of gateways at medium altitude allowed for data collection from sensors up to the summit craters. Most of the sensors left on the volcanic edifice during winters and during this period of intense volcanic activity were destroyed, but the whole gateway infrastructure remained fully operational, allowing for a very fruitful new field campaign two years later, in August 2023. Our experience has shown that the best strategy for IoT deployment on very active and/or high-altitude volcanoes like Etna is to permanently install gateways in areas where they are protected both from meteorological and volcanic hazards, that is mainly at the foot of the volcanic edifice, and to deploy temporary sensors and communicating nodes in the more exposed areas during field trips or in the summer season.

Mercury chemostratigraphy as a proxy of volcanic-driven environmental changes in the Aptian-Albian transition, Araripe Basin, northeastern Brazil

The stratigraphy of mercury (Hg) in sections of the Araripe Basin straddling the Aptian-Albian transition, encompassing the Romualdo, Ipubi, Crato and Barbalha (formerly Rio da Batateira)Formations showed positive excursions of Hg concentrations without correlation with organic carbon (C-org) contents. These incursions occurred during the deposition of the Barbalha Formation, which occurred in the Aptian under warm climate and high bio-productivity; in the Crato Formation, in the Albian, and also during the deposition of the Romualdo Formation. The Hg enrichments suggest external sources of Hg to the basin. The positive Hg excursion in the Ipubi Formation, unlike the others, was shown to be correlated with an increase in the deposition of C-org, suggesting internal processes as major river of Hg accumulation in sediments. The positive Hg excursions independent of C-org accumulation are concomitant to periods of intense volcanic activity in the Kerguelen Large Igneous Provinces (LIP). The excursion observed in the Ipubi Formation, however, seems associated with an event of anoxia, resulting in sediments rich in C-org, scavenging Hg from the water column. The Hg chemostratigraphy helps to understand of the paleoenvironmental changes in the Araripe basin, allowing understanding some of the drivers of such changes as the influence of volcanic activity and events of anoxia, as well as major biological crises.

Evaluation of possible impact on human health of atmospheric mercury emanations from the Popocatépetl volcano.

The contribution of Hg from volcanic emanations is decisive for assessing global mercury emissions given the impact of this highly toxic contaminant on human health and ecosystems. Atmospheric Hg emissions from Popocatépetl volcano and their dispersion were evaluated carrying out two gaseous elemental mercury (GEM) surveys during a period of intense volcanic activity. Continuous GEM measurements were taken for 24h using a portable mercury vapor analyzer (Lumex RA-915M) at the Altzomoni Atmospheric Observatory (AAO), 11km from the crater. In addition, a long-distance survey to measure GEM was conducted during an automobile transect around the volcano, covering a distance of 129km. The evaluation of the GEM data registered in the fixed location showed that heightened volcanic activity clearly intensifies the concentration of atmospheric Hg, extreme values around 5ngm-3. Highest concentrations of GEM recorded during the mobile survey were about 10ngm-3. In both surveys, the recorded concentrations during most of the measurement time were below 2ngm-3, but measurements were taken at a considerable distance from the crater, and GEM is subject to dilution processes. During both surveys, recorded GEM did not exceed the 200ngm-3 concentration recommended by the WHO (Air quality guidelines for Europe, 2000) as the regulatory limits for Hg in the atmospheric environment for long-term inhalation. Because this study was carried out in inhabited areas around the volcano during a period of intense volcanic activity, it can be concluded that the Popocatépetl does not represent a risk to human health in terms of Hg.

Paleolakes in the Northwest Hellas Region, Mars: Implications for the Regional Geologic History and Paleoclimate

AbstractHellas basin is one of the largest and oldest impact basins on the Martian surface. Its surrounding highland regions have undergone complicated geologic processes after the formation of Hellas basin. However, the geologic and climatic histories of the highlands surrounding Hellas are still unclear. Paleolakes provide us clues to answer these questions. In this study, we made a detailed investigation of paleolakes in the northwest Hellas region with high‐resolution imaging, topographic, and spectral data. A total of 64 paleolakes were identified with diameters larger than 4 km, in which 49 are newly reported. We calculated basic hydrologic parameters of the paleolakes and analyzed the sedimentary landforms, resurfacing processes, and aqueous minerals in the lake basins. Comprehensive analyses of the geomorphology, mineralogy, and age of the paleolakes revealed the geologic and climatic histories of the northwest Hellas region: a climate transition from warm and wet to semi‐arid happened in Noachian, and then lakes drained in a cold and dry climate in the early Hesperian, contemporary with or followed by a period of intense volcanic activity peaked around 3.3 Ga, and finally in the Amazonian, an extensive glacial event around 0.9 Ga resurfaced most of the paleolakes in the region south of 25°S. Our study also supports the existence of a “Hellas Ocean” and indicates that the Martian climate could have variations on regional scales and further studies are still needed to clarify the details.

Genetic homogeneity of Anopheles maculatus in Indonesia and origin of a novel species present in Central Java

BackgroundAnopheles maculatus (s.s.) is an important vector of malaria in Indonesia. Previously it was considered the only member of the Maculatus Group present in Indonesia. A novel species was recently identified in the Kulon Progo District in Central Java. Until recently, few investigations have been conducted looking at An. maculatus genetic diversity in Indonesia, including allopatric island populations.MethodsIndonesian An. maculatus (s.l.) samples were collected in several locations in Java, Lesser Sunda Island group, Sumatra and in Kulon Progo (Yogyakarta, central Java) where a novel species has been identified. Samples from a 30-year-old colony of the Kulon Progo population were also included in the analysis. Maximum-likelihood analysis established the phylogenies of the ITS2 (nuclear) and cox1 (mitochondrial) markers. Putative times of separation were based on cox1 genetic distances.ResultsTwo species of the Maculatus Group are present in Indonesia. The novel sibling species is more closely related to Anopheles dispar than to An. maculatus (s.s.). Anopheles maculatus (s.s.) samples are homogeneous based on the ITS2 sequences. Indonesian samples and An. dispar belong to the same cox1 maternal lineage and differ from all other known members of the Maculatus Group. Divergence time between the different populations found in Java was estimated using an established cox1 mutation rate.ConclusionsA novel species within the Maculatus Group, most closely related to An. dispar, is confirmed present in the Kulon Progo area of Central Java. The divergence of this species from An. maculatus (s.s.) is explained by the stable refugia in the Kulon Progo area during the quaternary period of intense volcanic activity throughout most of Java. This novel species awaits detailed morphological description before applying a formal species name. For the interim, it is proposed that the Kulon Progo population be designated An. maculatus var. menoreh to distinguish it from An. maculatus (s.s.).

Ground-based observations of the [SII] 6731 Å emission lines of the Io plasma torus

AbstractThe Io Plasma Torus (IPT) is a doughnut-shaped structure of charged particles, composed mainly of sulfur and oxygen ions. The main source of the IPT is the moon Io, the most volcanically active object in the Solar System. Io is the innermost of the Galilean moons of Jupiter, the main source of the magnetospheric plasma and responsible for injecting nearly 1 ton/s of ions into Jupiter's magnetosphere. In this work ground-based observations of the [SII] 6731 Å emission lines are observed, obtained at the MacMath-Pierce Solar Telescope. The results shown here were obtained in late 1997 and occurred shortly after a period of important eruptions observed by the Galileo mission (1996-2003). Several outbursts were observed and periods of intense volcanic activity are important to correlate with periods of brightness enhancements observed at the IPT. The time of response between an eruption and enhancement at IPT is still not well understood.

A statistical-based approach for determining the intensity of unrest phases at Stromboli volcano (Southern Italy) using one-step-ahead forecasts of displacement time series

The evaluation of the intensity of unrest phases at active volcanoes is a crucial topic in volcano hazard studies. This is particularly troublesome in the case of persistently active volcanoes like Stromboli (Southern Italy), where intense eruptive summit activity (overflows, strong spattering, powerful explosions) has in some cases anticipated a flank eruption. In this context, a new approach for the analysis of displacement data is introduced. Daily displacements of the Stromboli crater terrace measured between January 1, 2010, and August 7, 2014, by a ground-based interferometric synthetic aperture radar system were compared, in retrospect, to displacement predictions provided by an auto-regressive integrated moving average-based model. The methodology consisted in assessing when the actual displacements exceeded a fixed probability threshold for the forecasts (~95 %). Two sets of data were consequently produced: (1) series of residuals between actual displacements and model threshold (“anomalies”) and (2) series of normalized residuals between actual displacements and model threshold (“normalized anomalies”). This permitted to statistically identify and quantify the anomalous deformation at the crater terrace over the reference time interval of the analysis. Anomalies started to occur before each period of intense volcanic activity, highlighting the possibility to discern between background activity and unrest. Moreover, results indicated that the inflation of the crater terrace during the preparatory phase of the 2014 flank eruption was characterized by the greatest amount of anomalous deformation.

Phylogenetic position of the endemic Mount Oku rat,Lamottemys okuensis(Rodentia: Muridae), based on molecular and morphological data

Lamottemys okuensis Petter, 1986, is restricted to the Mount Oku montane forest in the central-northern part of the Cameroon Volcanic Line (CVL; Central West Africa). It is endangered and has a decreasing population trend. The genus is monotypic and little is known about its phylogeny and evolutionary history. Using both molecular and morphological evidence, we tested two competing systematic hypotheses involving Lamottemys: whether it is more closely related to Desmomys or to Oenomys. We also discuss Lamottemys’ biogeographical origin. Molecular phylogenies indicated a sister-group relationship with the Ethiopian montane forest endemic genus Desmomys. This reinforces the hypothesis of recurrent connections over time between West and East African mountains, probably facilitated by climate changes that led to expansions and contractions of montane environments. Our molecular dating, consistent with a period of intense volcanic activity in Central East Africa and the CVL, suggests that the geological history of East and Central Africa also contributed to promoting the isolation of Lamottemys in the CVL. Moreover, this study provides the most comprehensive phylogenetic analysis for the whole tribe Arvicanthini, sampling 17 of the 18 member genera. In this rodent group, stephanodonty probably appeared independently with grasses expansion. These habitat changes probably promoted the evolution of specialist rodents able to feed on abrasive vegetal matter by a dental adaptation. © 2015 The Linnean Society of London

New constraints on the subsurface geology of the Mexico City Basin: The San Lorenzo Tezonco deep well, on the basis of 40Ar/39Ar geochronology and whole-rock chemistry

The San Lorenzo Tezonco deep well, drilled in 2012 by “Sistema de Aguas de la Ciudad de México” (Water Supply System of Mexico City) in the Mexico Basin to a depth of 2008m, offers an excellent opportunity to explore the subsurface stratigraphy and general geology of the region. Unfortunately, only chipped samples from this well were recovered, which were examined and analyzed for whole-rock chemistry, 40Ar/39Ar, and U–Pb zircon geochronology, in order to reconstruct the lithology of the well. Contrary to previous deep wells of the Mexico Basin, no basement sedimentary rocks were found in this one. While the upper 70m are composed of lacustrine sediments associated with Lake Texcoco, volcanic rocks make up the majority of the well and range in age from more than 18Ma to 0.25Ma. Andesitic lavas are the most abundant products of the stratigraphic column, followed by acidic products represented by dacitic and rhyolitic lavas and ignimbrite deposits. Less abundant are basaltic andesite lavas appearing in the upper and lower parts of the column. The thickest sequence of the well is represented not only by Miocene volcanic rocks ranging from 5 to 17Ma, suggesting a period of intense volcanic activity in this area producing mainly andesitic lavas, but also by thick rhyolitic ignimbrite deposits dated at 5Ma. These deposits suggest the presence of a caldera structure, probably buried by subsequent volcanic products and lacustrine sediments. Trace element concentrations suggest that volcanism is likely produced in a subduction environment with typical negative anomalies of Nb, Ta, Ti, and P and positive anomalies in Pb and Cs. We correlated the well units with units outcropping in mountain ranges in the surrounding area, with the recognition of the following units or formations: Eocene Andesite, San Nicolás Basaltic Andesite, Tepoztlán Formation, Miocene Volcanism, Sierra de las Cruces, and Chichinautzin Volcanic Field products. By correlating the two closest deep wells (Mixhuca and Tulyehualco) we conclude that the San Lorenzo Tezonco well is located in a graben-like structure hosting the Santa Catarina volcanic range.

Erratum: Geochronology of Late Cenozoic volcanism in the area of Lake Van, Turkey: An example of developmental dynamics for magmatic processes [Doklady earth sciences, 433(2), pp. 1031–1037

An isotope-geochronological study has been performed to examine the products of Late Cenozoic collision volcanism on the northern coast of Lake Van, Turkey. We obtained 45 new K—Ar dates, based on which the principal time characteristics of volcanic activity in the region have been determined. Volcanic activity in the northern coast of Lake Van has lasted ∼15 myr; it has had an expressed discrete nature, when periods of intense volcanic activity alternated with long-lasting pose periods. Four stages of Neogene—Quaternary volcanism have been identified: Middle Miocene (15.0—13.5 Ma), Late Miocene (10—9 Ma), Pliocene (5.8—3.7 Ma), and Quaternary (1.0—0.4 Ma). The average duration of the stages was 1—2 myr; the stages were separated from each other with periods of inactivity of approximately equal lengths (∼3 myr). For each of the Pliocene and Quaternary stages, three additional phases of volcanism have been identified, which were separated from each other with short time intervals (a few hundred thousand years). The last burst of volcanic activity in the study area occurred ∼400 ka; similar to Quaternary volcanism in general, it was not characterized by a high intensity. An important result of the studies performed was to confirm the existence of a separate Middle Miocene stage of collision volcanism for the Caucasian—Anatolian Segment of the Alpine Fold Belt. New geochronological data generated presented in this paper indicate that Neogene—Quaternary volcanism in this portion of the belt started much earlier (∼15 Ma) than assumed by the majority of the previous researchers.

Geochronology of Late Cenozoic volcanism in the area of Van Lake, Turkey: An example of development dynamics for magmatic processes

An isotope-geochronological study has been performed to examine the products of Late Cenozoic collision volcanism on the northern coast of Van Lake, Turkey. We obtained 45 new K-Ar dates, based on which the principal time characteristics of volcanic activity in the region have been determined. The total duration of magmatic activity in the area of the northern coast of Van Lake has lasted ∼15 myr; it has had an expressed discrete nature, when periods of intense volcanic activity alternated with lasting breaks in eruptions. Four stages of Neogene-Quaternary volcanism have been identified: Middle Miocene (15.0–13.5 myr), Late Miocene (10–9 myr), Pliocene (5.8–3.7 myr), and Quaternary (1.0–0.4 Ma). The average duration of the stages has been 1–2 myr; the stages were separated from each other with periods of inactivity of approximately equal lengths (∼3 myr). For each of the Pliocene and Quaternary stages, three additional phases of volcanism have been identified, which were separated from each other with short time intervals (a few hundred thousand years). The last burst of volcanic activity in the area in question took place ∼400 ka; similar to Quaternary volcanism in general, it was not characterized by a high intensity. An important result of the studies performed was to confirm the existence of a separate Middle Miocene stage of collision volcanism for the Caucasian-Anatolian Segment of the Alpine Fold Belt. The data generated allow concluding that Neogene-Quaternary volcanism in this portion of the belt started much earlier (∼15 Ma) than assumed by the majority of the previous researchers.

Nitrogen geochemistry as a tracer of fluid flow in a hydrothermal vent complex in the Karoo Basin, South Africa

We have investigated the N geochemistry of minerals and rocks from contact metamorphic aureoles and hydrothermal vent complexes (HVC) in the Karoo Basin in South Africa. The HVC formed during phreatic eruptions associated with rapid devolatilization and pressure build-up in contact aureoles around early Jurassic sill intrusions. By combining outcrop data from a HVC and core data from contact aureoles, we investigate the relationship between light element release during metamorphism and vertical fluid migration. Sandstone and breccia from the HVC contain early-diagenetic ammonium NH 4 + -bearing feldspar (buddingtonite) and illite. Ammonium occupies up to 95% of the A site in feldspar, corresponding to concentrations up to 5.2 wt% N. Bulk-rock N isotope data for rocks from inside and outside the hydrothermal vent complex fall into two distinct groups. Background samples have δ 15N air between +1.5‰ and +4.9‰, whereas minerals from the vent complex have δ 15N in the range +7.5 to +10.6‰. The N geochemistry of contact metamorphic shale from the lower stratigraphic units of the Karoo Basin shows that the vitrinite reflectance and δ 15N values are positively correlated. Shale with reflectivity values >4%Ro are enriched in 15N, with δ 15N values between +6‰ and +14‰, implying the release of isotopically light N into metamorphic fluids (probably as N 2). We suggest that the relatively high δ 15N values of the early-diagenetic buddingtonite in the HVC reflect exchange of buddingtonite with N-bearing fluids ascending from greater depth after their release during contact metamorphism and dehydration. We present a qualitative model whereby hydrothermal vent complexes represent fluid flow structures after their formation, focusing N-bearing metamorphic fluids sourced in deeper levels of the basin. The release of organic N from sediments at depth in volcanic basins could play a role in the geochemical cycle of N, becoming particularly important during periods of intense volcanic activity.

Cyclic magma storages and transfers at Piton de La Fournaise volcano (La Réunion hotspot) inferred from deformation and geochemical data

Piton de La Fournaise is in a period of intense volcanic activity since 1998. To constrain the magma dynamics responsible for this activity, we combined GPS ground deformation monitoring interpreted through numerical modelling and geochemistry. Two cycles of continuous volcano inflation are evidenced for the May 2004–December 2005 period, with a rest from March to October 2005. These inflations are consistent with two cycles of compatible major element enrichment in the emitted lavas. Numerical models indicate that the pressurization of a single magma reservoir may be responsible for the observed pre-eruptive inflations of the volcano. The reservoir, located at 2300 m depth, has a radius of ∼ 500 m. At the beginning of each cycle, dykes propagate from the roof of the reservoir and yield eruptions of differentiated basalt near the summit. At the end of the cycle, dykes propagate from the eastern sidewall of the reservoir and yield distal eruptions of primitive magmas away from the summit. The volumes of magma emitted during the primitive eruptions seem too large to explain the surface deformations and therefore suggest some refill of the reservoir by deeper magmas. Our results may be used to predict the location and lava volume of future eruptions at Piton de La Fournaise volcano, depending on the timing of these eruptions within a cycle of volcanic activity.

Triassic volcanism in the Eastern Fore-Caucasus: evolution and geodynamic interpretation

The aim of this paper is to describe the Triassic regional geodynamic framework of the Eastern Fore-Caucasus area from data on volcanics and related sedimentary deposits. These deposits, only known by drilling, represent the lowermost horizons of a slightly deformed Mesozoic and Cenozoic cover and rest unconformably on folded, mainly Devonian to Carboniferous, series. They are thought to be, for the most part, postcollisional. The sedimentary record of Early to Middle Triassic evolution allows visualisation of uplift events associated with continental basin formation, preceding and following the opening of a deep passively subsiding marine basin. Tectonic instability, indicated by tilted strata, prevailed throughout the whole period. The subsidence maximum, corresponding with a total immersion below sea level, occurred during the Olenekian (Early Triassic). During Middle Triassic times, the area was continuously uplifted and progressively emerged to become strictly continental with a fluvio-lacustrine sedimentation during the Late Triassic. Two periods of intense volcanic activity occurred during Early-Middle and Late Triassic times. The former comprises: (1) subaerial explosive eruptions of rhyolitic magma, the vents of which are presumably located about 50–100 km southwest of the study area; and (2) submarine effusive eruptions of basalts and rhyolites within an actively subsiding trough. The numerous unconformities within the volcano-sedimentary sequence testify to a tectonically active setting. During the Late Triassic, eruptions were subaerial, mainly explosive ( nuées ardentes), and rhyo-dacitic. Basaltic and andesitic lava flows and domes were minor. In both eruptive periods, mafic magmas had a calc-alkaline affinity and their diversity would have been the result of partial melting of a heterogeneous metasomatized subcontinental mantle. Several compositional gaps within the acidic group reflect melting of distinct components of the continental crust. Fractional crystallisation or even partial melting trends can be deciphered within mafic and rhyolitic groups. The geodynamic background of Early-Middle Triassic volcanism was one of extension. The precise nature of the extension is unclear but could include active rifting, back-arc extension, or even transtension in a strike-slip fault zone. With their great contribution of felsic rocks (about 90%), the Late Triassic subaerial volcanics can trace an important crustal partial melting event. They could belong to an overheated zone of an Andean-type volcanic belt.

Phylogeography of skinks (Chalcides) in the Canary Islands inferred from mitochondrial DNA sequences.

Mitochondrial DNA (mtDNA) evolution was investigated in skinks of the genus Chalcides found in the Canary Islands (Ch. sexlineatus, Ch. viridanus and Ch. simonyi), together with some North African congenerics (Ch. polylepis and Ch. mionecton). Several sites were included within islands to cover areas of known within-island geographical variation in morphology. Skinks from the islands of El Hierro and La Gomera appear to be sister taxa. The relationships between this clade and the Tenerife and Gran Canarian skinks were not fully resolved, although the best working hypothesis indicated monophyly with the former, with the latter forming a closely related outgroup. Ch. simonyi from Fuerteventura was more distantly related to the Western Canary Island skinks and did not show close relationships with the North African species Ch. mionecton and Ch. polylepis. Possible colonization sequences for the four most Western Canary Islands were considered. El Hierro appears to have been colonized relatively recently from La Gomera, commensurate with the recent origin of this island, while dispersal between La Gomera and Tenerife and between Gran Canaria and Tenerife or La Gomera appears to have taken place considerably earlier. Substantial within-island haplotype divergence was found in Gran Canaria and Tenerife. This may be a result of recent periods of intense volcanic activity found within these two islands. Lower levels of within-island differentiation are found in La Gomera and El Hierro and may be explained by lower levels of volcanic activity during recent geological history and a more recent colonization, respectively.

Tahiti: Geochemical evolution of a French Polynesian Volcano

The island of Tahiti, the largest in French Polynesia, comprises two major volcanoes aligned NW‐SE, parallel with the general trend of the Society Islands hotspot track. Rocks from this volcanic system are basalts transitional to tholeiites, alkali basalts, basanites, picrites, and evolved lavas. Through K‐Ar radiometric dating we have established the age of volcanic activity. The oldest lavas ( ∼1.7 Ma) crop out in deeply eroded valleys in the center of the NW volcano (Tahiti Nui), while the main exposed shield phase erupted between 1.3 and 0.6 Ma, and a late‐stage, valley‐filling phase occurred between 0.7 and 0.3 Ma. The SW volcano (Tahiti Iti) was active between 0.9 and 0.3 Ma. There is a clear change in the composition of lavas through time. The earliest lavas are moderately high SiO2, evolved basalts (Mg number (Mg# = Mg/Mg+Fe2+) 42–49), probably derived from parental liquids of composition transitional between those of tholeiites and alkali basalts. The main shield lavas are predominantly more primitive olivine and clinopyroxene‐phyric alkali basalts (Mg# 60–64), while the later valley‐filling lavas are basanitic (Mg# 64–68) and commonly contain peridotitic xenoliths (olivine+orthopyroxene+clinopyroxene+spinel). Isotopic compositions also change systematically with time to more depleted signatures. Rare earth element patterns and incompatible element ratios, however, show no systematic variation with time. We focused on a particularly well exposed sequence of shield‐building lavas in the Punaruu Valley, on the western side of Tahiti Nui. Combined K‐Ar ages and magnetostratigraphic boundaries allow high‐resolution age assignments to this ∼0.7‐km‐thick flow section. We identified an early period of intense volcanic activity, from 1.3 to 0.9 Ma, followed by a period of more intermittent activity, from 0.9 to 0.6 Ma. Flow accumulation rates dropped by a factor of 4 at about 0.9 Ma. This change in rate of magma supply corresponds to a shift in activity to Tahiti Iti. We calculated the composition of the parent magma for the shield‐building stage of volcanism, assuming that it was in equilibrium with Fo89 olivine and that the most primitive aphyric lavas were derived from this parent by the crystallization of olivine alone. The majority of the shield lavas represent 25 to 50% crystallization of this parent magma, but the most evolved lavas represent about 70% crystallization. From over 50 analyzed flow units we recognize a quasi‐periodic evolution of lava compositions within the early, robust period of volcanic activity, which we interpret as regular recharge of the magma chamber (approximately every 25±10 kyr). Volcanic evolution on Tahiti is similar to the classic Hawaiian pattern. As the shield‐building stage waned, the lavas became more silica undersaturated and isotopic ratios of the lavas became more MORB‐like. We propose that the Society plume is radially zoned due to entrainment of a sheath of viscously coupled, depleted mantle surrounding a central core of deeper mantle material. All parts of the rising plume melt, but the thermal and compositional radial gradient ensures that greater proportions of melting occur over the plume center than its margins. The changing composition of Tahitian magmas results from lithospheric motion over this zoned plume. Magmas erupted during the main shield‐building stage are derived mainly from the hot, incompatible element‐enriched central zone of the plume; late‐stage magmas are derived from the cooler, incompatible element‐depleted, viscously coupled sheath. A correlation between Pb/Ce and isotope ratios suggests that the Society plume contains deeply recycled continental material.

High-resolution [formula omitted] chronology of Oligocene volcanic rocks, San Juan Mountains, Colorado

The central San Juan caldera complex consists of seven calderas from which eight major ash-flow tuffs were erupted during a period of intense volcanic activity that lasted for approximately 2 m.y. about 26–28 Ma. The analytical precision of conventional K-Ar dating in this time interval is not sufficient to unambiguously resolve this complex history. However, 40Ar 39Ar incremental-heating experiments provide data for a high-resolution chronology that is consistent with stratigraphie relations. Weighted-mean age-spectrum plateau ages of biotite and sanidine are the most precise with standard deviations ranging from 0.08 to 0.21 m.y. The pooled estimate of standard deviation for the plateau ages of 12 minerals is about 0.5 percent or about 125,000 to 135,000 years. Age measurements on coexisting minerals from one tuff and on two samples of each of two other tuffs indicate that a precision in the age of a tuff of better than 100,000 years can be achieved at 27 Ma. New data indicate that the San Luis caldera is the youngest caldera in the central complex, not the Creede caldera as previously thought.

Tectonic history of the Shikoku marginal basin

Studies of marine magnetic anomaly data from the Shikoku basin reveal magnetic lineations which strike northwest almost parallel to the trend of the Palau-Kyushu ridge. The lineation pattern is best developed in the western part of the basin and we can confidently identify a sequence of anomalies 7 through 5E between the base of the Palau-Kyushu ridge and the center of the basin. In the eastern part of the basin the basement morphology is rough and complex and magnetic anomalies can not be identified unequivocally. We infer that the Palau-Kyushu ridge and the Izu-Bonin island arc began separating about 27 m.y. B.P. An interval of rapid separation (4.2 cm/yr) occurred between 26 and 22.5 m.y. B.P. which approximately coincides with a period of intense volcanic activity in Japan. The observed magnetic lineation pattern and basement morphology can be best explained if the Shikoku basin formed at a two-limb spreading system during the Late Oligocene to Middle Miocene. Subsequently the eastern half of the basin was disrupted by fractures as the Iwo-Jima ridge collided with the Japanese islands. The accretionary process which formed the crust of the Shikoku marginal basin appears similar to that operating at mid-ocean ridges of the world.

A model for planetary igneous differentiation

Numerical models for igneous activity on any planet must be based on thermal properties and melting relationships for realistic multicomponent systems. A consistent model for the composition of the earth's mantle may be obtained by considering a mixture of basalt and dunite, as postulated by Ringwood and others, if the uranium, thorium, and potassium concentrations of the basalt are similar to that of basalts dredged from the deep ocean floors. Present evidence does not indicate that the bulk composition of the moon is appreciably different from that of the earth's mantle.On the basis of a review of existing phase equilibria and high pressure studies, we predict melting behavior as a function of pressure and temperature for a composition equivalent to 1 part basalt and 3 parts dunite. The specific heat and fusion properties of the important minerals in this system are reviewed. The implications of these data for an understanding of magma generation are discussed.A review of the factors influencing the thermal conductivity of rocks indicates that when considering the details of heat conduction under conditions likely to be encountered within the earth's upper mantle and the moon, a model invoking constant phonon conductivity and temperature independent photon mean free path may be unrealistic. A more likely, but still oversimplified, model has a phonon conductivity inversely proportional to temperature, and, unless scattering dominates, a photon mean free path directly proportional to temperature over the range of interest. Pressure should not significantly affect the thermal conductivity within the upper mantle, except by changing the stable mineral assemblage.When the more important aspects of the melting relationships and thermal properties of the model composition are selected and used to extend thermal history calculations of the moon to include the effects of differentiation through igneous activity, the calculations indicate that a brief period of intense volcanic activity is likely to have occurred at a time which is primarily a function of the moon's temperature at the time of its formation and the concentration of radioactive heat sources. If the temperature of the moon at the time of its formation were 0°C and the uranium content were 0.05 ppm, with Th/U and K/U in the ratio 3.7 and 104, respectively, melting would have begun about 1.5 × 109 years after formation, the intensity being largely controlled by the thermal conductivity. At lower U, Th, and K contents, the onset of volcanism would have been delayed or suppressed completely, depending on the conductivity. Properly selected samples of the moon's surface rocks should thus make it possible to determine its initial temperature. The total thickness of extrusive and intrusive igneous rocks is shown to be critically dependent on the efficiency of radiative heat transfer in removing heat from the interior.