Andesitic Eruptions Research Articles

The last subduction-related volcanism in the northern tip of the Arabian-Nubian Shield: A Neoproterozoic arc preceding the terminal collision of East and West Gondwana

The medium- to high-K calc-alkaline subduction-related Dokhan volcanic rocks of the Wadi Hamad area record two flare-up events during the history of a Neoproterozoic continental island arc, an early pulse dominated by andesite and a later pulse dominated by dacite. The trace element abundances and (La/Yb)n ratios of the basalts, andesites and dacites all overlap, contrary to the expectations of a single fractional crystallization trend, suggesting the need for three distinct parental magmas for the three groups. The parental magmas of the basalts and andesites were generated by variable degrees of partial melting of subduction-metasomatized mantle, whereas that of the dacites requires mixing of mantle and crustal melts. The andesites and dacites both evolved mostly through fractionation of clinopyroxene and plagioclase, accompanied by apatite and Fe-Ti oxides in the more evolved dacitic rocks. Oscillatory and reverse zoning in clinopyroxene and plagioclase indicates that magma replenishment and mixing played a role in the genesis of the basalts and andesites. Depth-sensitive geochemical parameters show that the earlier andesites evolved at deeper levels in the arc crust compared with the later dacites, but not so deep as to stabilize garnet as a fractionating phase. Estimated values of the arc crust thickness indicate that the crust thickened from ∼35 km to ∼50 km from the time of basalt eruption to the time of andesite eruption, probably implying a high rate of crustal growth. The estimated arc crust thickness during the later subduction flare-up is slightly less than that of the earlier one despite ongoing magmatic addition, implying that arc crust delamination began before the production of the later dacites. The subduction-related geochemical characteristics of the Dokhan volcanic rocks suggest that the terminal collision between East and West Gondwana in the tip of the Nubian Shield occurred at ∼600 Ma.

Storage conditions of the mafic and silicic magmas at Cotopaxi, Ecuador

The 2015 reactivation of the Cotopaxi volcano urges us to understand the complex eruptive dynamics of Cotopaxi for better management of a potential major crisis in the near future. Cotopaxi has commonly transitioned from andesitic eruptions of strombolian style (lava flows and scoria ballistics) or nuées ardentes (pyroclastic flows and ash falls) to highly explosive rhyolitic ignimbrites (pumiceous pyroclastic flows), which entail drastically different risks. To better interpret geophysical and geochemical signals, Cotopaxi magma storage conditions were determined via existing phase-equilibrium experiments that used starting materials chemically close to the Cotopaxi andesites and rhyolites. The results suggest that Cotopaxi's most mafic andesites (last erupted products) can be stored over a large range of depth from ~7km to ≥16km below the summit (pressure from ~200 to ≥400MPa), 1000°C, NNO +2, and contain 4.5–6.0±0.7wt% H2O dissolved in the melt in equilibrium with ~30–40% phenocrysts of plagioclase, two pyroxenes, and Fe-Ti oxides. These mafic andesites sometimes evolve towards more silicic andesites by cooling to 950°C. Rhyolitic magmas are stored at 200–300MPa (i.e. ~7–11km below the summit), 750°C, NNO +2, and contain ~6–8wt% H2O dissolved in a nearly aphyric melt (<5% phenocrysts of plagioclase, biotite, and Fe-Ti oxides). Although the andesites produce the rhyolitic magmas by fractional crystallization, the Cotopaxi eruptive history suggests reactivation of either reservoirs at distinct times, likely reflecting flux or time fluctuations during deep magma recharge.

History of Red Crater volcano, Tongariro Volcanic Centre (New Zealand): Abrupt shift in magmatism following recharge and contrasting evolution between neighboring volcanoes

Red Crater volcano is one of several contemporaneously active vents on the Tongariro Volcanic Centre. Its history provides an opportunity to investigate the contrasting magmatic evolutionary paths of closely-spaced volcanoes. Rocks erupted at Red Crater over the last ~3.4ka display typical subduction-related trace element and isotopic signatures. Those erupted pre-1.8ka are medium-K andesites (SiO2 ~59–62wt%). They represent the most voluminous magmas and were emplaced in 5 lava flow events. An abrupt shift to the eruption of basaltic andesite (SiO2 ~53–54wt%) with less radiogenic Sr-Nd-Pb isotope ratios, occurred post-1.8ka. This period comprised 6 smaller volume, lava flow episodes and the contemporaneous development of a scoria cone.Plagioclase phenocrysts in post-1.8ka lava flows have resorbed cores with diverse textural and compositional growth patterns, as would be expected from the disruption of a crystal mush. They are similar to phenocrysts of the pre-1.8ka lava flows. The post-1.8ka plagioclase is distinguished from those in the older lavas by overgrowths with elevated An (~70–90), FeO and MgO contents, that mantle the resorbed cores (~An50–70). These rims are compositionally similar to groundmass plagioclase. This demonstrates that new mafic magma was intruded into the system, mixing with and entraining relic crystals from the older andesite system. Iron and Mg zoning patterns in the crystals are not consistent with significant re-equilibration via diffusion. Hence, the generation of eruptible magma during the last 1.8ka required repeated mafic intrusion events. The emptying of the older andesitic magma reservoir early in the volcano's history removed buoyancy barriers to the direct eruption of more mafic magmas.This pattern of magmatism is not recorded at the contemporaneously active Ngauruhoe volcano, just 3km to the SSW. Ngauruhoe rocks are compositionally distinct and are more heterogeneous in isotopic composition. Although mafic recharge is also evident, larger volumes of magma with more radiogenic compositions were erupted and the history of activity extends farther back in time than that of Red Crater. This is consistent with the development of a larger silicic reservoir beneath Ngauruhoe that could have acted as a buoyancy filter preventing direct eruption of mafic magma. The eruptive products of the two volcanoes reveal the diverging development of adjacent magmatic reservoirs that lack lateral connectivity at a scale in the order of 102–103m. There is limited literature on the comparative magmatic evolution of closely-spaced conduit/storage systems at arc volcanoes, reflecting the limitations of geochronological data at centennial and millennial timescales. However, such investigations provide insight into andesite assembly and the contrasting volcanism that could be expected in future activity.

Fluorine and Chlorine Contents in the Products of the 1108 (Tennin) Eruption of Asama Volcano

Fluorine and chlorine have been determined for all kinds of the products of the 1108 activity of Asama volcano. The activity displayed various features that are characteristic of andesitic eruptions, i.e., scoria (pumice) fall (B-scoria L), intermediate-type pyroclastic flow (the Oiwake pyroclastic flow) and lava flow (the Kamino-butai lava flow). Their F-Cl variation patterns are widely varied according to the natures of the eruption in a similar way to those of the 1783 activity of Asama (Yoshida and Tsuchiya, 2004). The B-scoria L samples show uniform and the highest Cl contents in average. The Kamino-butai lava flow samples have low and uniform Cl contents. These results coincide with those obtained by the study on the 1783 activity and may be explained by the enrichment of volatiles in the uppermost part of the magma column before the eruption. The F and Cl contents of Oiwake pyroclastic flow samples show very large variances. Some Oiwake samples take almost the same halogen contents as the B-scoria L samples. Whereas, the other samples show the lowest F and Cl contents and the largest variances, and indicate strong release of gas at or after the effusion. The detailed examination of topographical distribution of the F and Cl contents shows that the Oiwake pyroclastic flow is composed of many flow units of high-halogen and of low-halogen contents and may suggest the transition of the mode of the formation of the pyroclastic flow with different degassing conditions. In the low-halogen flow units, volatiles might be mostly released in the early stage of the flow.

Global major events in Miocene and its significance: Revelation from data mining

The GEOROC (Geochemistry of Rocks of the Ocean-sand Continents) database is a comprehensive global petrological database and it is constantly updated and available online to users around the world. Based on this database, we analyze the age data of the global Cenozoic igneous rocks including convergent margin andesite and basalt, continental rift andesite and basalt, ocean island andesite and basalt, adakite, and so on. Statistical analysis reveals that the global magmatism either became active in the Miocene or peaked in the Miocene. Magmatic rocks formed in different tectonic settings represent different geological effects: convergent margin magmatic rock suggests the speed up of subduction; the huge eruption of rift andesite and basalt indicates the occurrence of plate extension; the sharp increase of ocean island andesite and basalt indicates an abnormally active lower mantle, which brings materials from the lower mantle into the crust; a large scale of medium-acid magmatic rock represents the lower crust at high temperature and the asthenosphere mantle strongly upwelling, which leads to the crust intensely reforming; adakite is usually related to strongly tectonic pressing and results in crustal thickening in these areas. In addition, there was a series of significant events in the Miocene. For instance, the Tibetan Plateau rapidly uplifted in the middle Miocene, leading to the formation of the Loess Plateau; the Andean Mountains rose abruptly probably during 8–3 Ma; Tethys dried up from 20–5 Ma, which may have been a cause of the global desertification; the development of the western Pacific marginal sea basins mainly occurred during the late Oligocene to early Miocene. We tried to explain the connection among all of these events as follows. Generally, magmatism usually goes with tectonic activities, and their interaction may change the paleogeography, paleoclimate, and paleoenvironment. The accelerated motion of the Pacific Plate in the Miocene could have thickened or raised the crust, resulting in the formation of the Andean mountains. The interaction between the Indian plate and Eurasian plate in the Miocene induced the speedy uplifting of the Tibetan Plateau. The new mountain and plateau reformed the original landscape and landform, which made these areas easily suffer weathering and erosion, and the resulting sediments were taken into global ocean basalts with ocean currents. As a result, the accumulation rate of oceanic sediments would have risen quickly. Furthermore, the upraising of the Andean and Tibetan Plateaus also influenced the general atmospheric circulation, which ultimately changed the environment; this might be an important reason why primitive man originated from the Miocene. In addition, as for China, the strike-slip fault in the Red River, the expansion of the South China Sea, and the formation of the Taklimakan Desert may be more or less related to the magmatism in the Miocene. There remain a number of problems to explore. Therefore, studying the interrelationship among magmatism, tectonic activity, climate, and environment in the Miocene should be a priority for geoscience workers in the present and future. Besides, scientific research is gradually entering the big data era, and geological science is no exception. The primary characteristic of big data research is not how big the data is but how creative the thought is. In this study, we took advantage of all available data to investigate the relationship among the data sets. The findings reveal that data mining with a global database can provide new insights and new lines of inquiry for scientific research in the future.

Mass budget partitioning during explosive eruptions: insights from the 2006 paroxysm of Tungurahua volcano, Ecuador

AbstractHow and how much the mass of juvenile magma is split between vent‐derived tephra, PDC deposits and lavas (i.e., mass partition) is related to eruption dynamics and style. Estimating such mass partitioning budgets may reveal important for hazard evaluation purposes. We calculated the volume of each product emplaced during the August 2006 paroxysmal eruption of Tungurahua volcano (Ecuador) and converted it into masses using high‐resolution grainsize, componentry and density data. This data set is one of the first complete descriptions of mass partitioning associated with a VEI 3 andesitic event. The scoria fall deposit, near‐vent agglutinate and lava flow include 28, 16 and 12 wt. % of the erupted juvenile mass, respectively. Much (44 wt. %) of the juvenile material fed Pyroclastic Density Currents (i.e., dense flows, dilute surges and co‐PDC plumes), highlighting that tephra fall deposits do not depict adequately the size and fragmentation processes of moderate PDC‐forming event. The main parameters controlling the mass partitioning are the type of magmatic fragmentation, conditions of magma ascent, and crater area topography. Comparisons of our data set with other PDC‐forming eruptions of different style and magma composition suggest that moderate andesitic eruptions are more prone to produce PDCs, in proportions, than any other eruption type. This finding may be explained by the relatively low magmatic fragmentation efficiency of moderate andesitic eruptions. These mass partitioning data reveal important trends that may be critical for hazard assessment, notably at frequently active andesitic edifices.

A high-resolution 40Ar/39Ar lava chronology and edifice construction history for Ruapehu volcano, New Zealand

Ruapehu is an active ~150km3 andesite-dacite composite volcano located at the southern end of the Taupo Volcanic Zone, New Zealand. The growth of the present-day edifice has occurred throughout coeval eruptive and glacial histories since ~200ka. We present high-precision 40Ar/39Ar eruption ages for 46 samples and whole-rock major element geochemical data for 238 samples from lava flows. These new and existing data are interpreted in the context of geomorphologic and geologic mapping, volcano-ice interaction processes and glacier reconstructions to present an improved chronostratigraphic framework and new edifice evolution history for Ruapehu. Sub-glacial to ice-marginal effusive eruption of medium-K basaltic-andesites and andesites constructed the northern portion of the exposed edifice between ~200 and 150ka, and a wide southeast planèze as well as parts of the northern, eastern and western flanks between ~160 and 80ka. None of the dated lava flows have ages in the range of 80–50ka, which may reflect an eruptive hiatus. Alternatively the lack of ages may be the result of erosion and burial of lavas and syn-eruptive glacial conveyance of lava flows to the ring-plain during glacial advance at 70–60ka. From ~50–15ka edifice growth occurred via effusive eruptions onto the glaciated flanks of the volcano, resulting in construction of ice-bounded planèzes and ridges. The distribution of dated ice-marginal lavas indicates a general decrease in glacier thicknesses over this time, except for a short-lived period centred at ~31ka when peak ice cover was reached. The compositional ranges of medium- to high-K andesite and dacite lava flows within 50–35ka eruptive packages define broadly bimodal high- and low-MgO trends. Lavas erupted at ~35–22ka have compositions that fill a transitional geochemical field between older dacites and younger andesites. Large-scale retreat of flank glaciers since ~15ka has resulted in intra-valley lava flow emplacement at elevations below ~1500m on the edifice. Between 15 and 10ka unstable cones were constructed through effusive activity in the presence of remnant summit and upper flank glaciers and the emplacement of eruptive deposits on top of hydrothermally altered and fragmental sub-glacial and ice-marginal deposits. Debuttressing of two northern summit cones and a southern summit cone as ice underwent continued post-glacial retreat pre-conditioned the edifice for two major sector collapses. Ultimate triggering of the collapse events and deposition of debris avalanche deposits on the northern and south-eastern flanks may have been the result of tectonic or volcanic activity. The northern collapse scar is infilled by a new cone comprising <10ka lava flows that form the upper northern and eastern flanks of the modern edifice. Late Holocene-Recent eruptive activity has occurred through Crater Lake, which occupies the site of the collapsed southern cone. Deglaciation did not induce enhanced magma flux rates at Ruapehu as indicated by calculated eruptive volumes and their chronologies. However, volcano-ice interactions have had a fundamental influence on eruptive styles, sector collapse events, and the shape of the edifice by modifying the distribution, morphology and preservation of eruptive deposits for at least 200kyr. Our approach to integrating field and geochronological datasets from Ruapehu is directly relevant to the evolution of glaciated Quaternary composite cones worldwide, particularly those along the circum-Pacific continental volcanic arcs.

The Chahnaly Low-Sulfidation Epithermal Gold Deposit, Western Makran Volcanic Arc, Southeast Iran

The Chahnaly low-sulfidation epithermal Au deposit and nearby Au prospects are located northwest of the intermittently active Bazman stratovolcano on the western end of the Makran volcanic arc, which formed as the result of subduction of the remnant Neo-Tethyan oceanic crust beneath the Lut block. The arc hosts the Siah Jangal epithermal and Kharestan porphyry prospects, near Taftan volcano, as well as the Saindak Cu-Au porphyry deposit and world-class Reko Diq Cu-Au porphyry deposit, near Koh-i-Sultan volcano to the east-northeast in Pakistan. The host rocks for the Chahnaly deposit include early Miocene andesite and andesitic volcaniclastic rocks that are intruded by younger dacitic domes. Unaltered late Miocene dacitic ignimbrites overlie these rocks. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb zircon geochronology data yield ages between 21.8 and 9.9 Ma for the acidic-intermediate regional volcanism. The most recent volcanic activity of the Bazman stratovolcano involved extrusion of an olivine basalt during Pliocene to Quaternary times. Interpretation of geochemical data indicate that the volcanic rocks are synsubduction and calc-alkaline to subalkaline. The lack of a significant negative Eu anomaly, a listric-shaped rare earth element pattern, and moderate La/Yb ratios of host suites indicate a high water content of the source magma. Gold and electrum are temporally and spatially related to a series of structurally controlled, 030°-trending, subvertical hydrothermal breccias with chalcedony-adularia that cut porphyritic andesite and andesitic volcaniclastic rocks. Gold is associated with pyrite, a siliceous matrix of hydrothermal breccia, and previously formed vein clasts, as well as with iron oxides and hydroxides in oxidized zones. Rare silver minerals include Ag-bearing electrum and naumannite, iodargyrite, an unnamed silver diiodide, and hessite. Hydrothermal alteration is generally well developed surrounding the ore-bearing hydrothermal breccia. The main types of alteration in the area include an inner ~0.5- to 20-m-thick gold-bearing hydrothermal breccia composed of quartz-chalcedonyadularia-illite-pyrite, a ~5- to 50-m-thick zone of quartz, chalcedony, pyrite, illitic phengite, phengite, illitic muscovite, illite, illitic paragonite, paragonite, muscovite, montmorillonite and, rarely, siderite, and a 30- to 70-m outer propylitic zone of Fe-Mg chlorite, calcite, ankerite, dolomite, epidote, palygorskite, and pyrite. The Chahnaly Au deposit formed during the early stages of magmatism. LA-ICP-MS zircon U-Pb geochronology of host andesite and 40Ar/39Ar dating of two samples of gold-associated adularia show that the ore-stage adularia (19.83 ± 0.10 and 19.2 ± 0.5 Ma) is younger, by as much as 1.5 million years, than the volcanic host rock (20.32 ± 0.4 Ma). Therefore, either hydrothermal activity continued well after volcanism or a second magmatic event rejuvenated hydrothermal activity. This second magmatic event may be related to eruption of porphyritic andesite at ~20.32 ± 0.40 Ma, which is within error of ~19.83 ± 0.10 Ma adularia. The new LA-ICP-MS zircon U-Pb host rock and vein adularia 40Ar/39Ar ages suggest that early Miocene magmatism and mineralization in the Bazman area is of a similar age to that of the Saindak porphyry and Tanjeel porphyry center of the giant Reko Diq deposit. This confirms the existence of early Miocene arc magmatism and mineralization along the Iranian part of the Makran volcanic arc. Ore, alteration mineralogy, and alteration patterns indicate that the Chahnaly deposit is a typical low-sulfidation epithermal Au deposit, located in a poorly explored part of the Makran volcanic arc in Iran.

The magmatic and eruptive response of arc volcanoes to deglaciation: Insights from southern Chile

In tectonic settings where decompression melting drives magmatism, there is compelling evidence that changes in ice loading or water loading across glacial-interglacial cycles modulate volcanic activity. In contrast, the response of subduction-related volcanoes remains unclear. A high-resolution postglacial eruption record from a large Chilean stratovolcano, Mocho-Choshuenco, provides new insight into the arc magmatic response to ice-load removal. Following deglaciation, we identify three distinct phases of activity characterized by different eruptive fluxes, sizes, and magma compositions. Phase 1 (13–8.2 ka) was dominated by large dacitic and rhyolitic explosive eruptions. During phase 2 (7.3–2.9 ka), eruptive fluxes were lower and dominated by moderate-scale basaltic andesite eruptions. Since 2.4 ka (phase 3), eruptive fluxes have been elevated and of more intermediate magmas. We suggest that this time-varying behavior reflects changes in magma storage time scales, modulated by the changing crustal stress field. During glaciation, magma stalls and differentiates to form large, evolved crustal reservoirs. Following glacial unloading, much of the stored magma erupts (phase 1). Subsequently, less-differentiated magma infiltrates the shallow crust (phase 2). As storage time scales increase, volcanism returns to more evolved compositions (phase 3). Data from other Chilean volcanoes show a similar tripartite pattern of evacuation, relaxation, and recovery, suggesting that this could be a general feature of previously glaciated arc volcanoes.

Water-magma interaction and plume processes in the 2008 Okmok eruption, Alaska

Eruptions of similar explosivity can have divergent effects on the surroundings due to differences in the behavior of the tephra in the eruption column and atmosphere. Okmok volcano, located on Umnak Island in the eastern Aleutian Islands, erupted explosively between 12 July and 19 August 2008. The basaltic andesitic eruption ejected ∼0.24 km3 dense rock equivalent (DRE) of tephra, primarily directed to the northeast of the vent area. The first 4 h of the eruption produced dominantly coarse-grained tephra, but the following 5 wk of the eruption deposited almost exclusively ash, much of it very fine and deposited as ash pellets and ashy rain and mist. Meteorological storms combined with abundant plume water to efficiently scrub ash from the eruption column, with a rapid decrease in deposit thickness with distance from the vent. Grain-size analysis shows that the modes (although not their relative proportions) are very constant throughout the deposit, implying that the fragmentation mechanisms did not vary much. Grain-shape features consistent with molten fuel-coolant interaction are common. Surface and groundwater drainage into the vents provided the water for phreatomagmatic fragmentation. The available water (water that could reach the vent area during the eruption) was ∼2.8 × 1010 kg, and the erupted magma totaled ∼7 × 1011 kg, which yield an overall water:magma mass ratio of ∼0.04, but much of the water was not interactive. Although magma flux dropped from 1 × 107 kg/s during the initial 4 h to 1.8 × 105 kg/s for the remainder of the eruption, most of the erupted material was ejected during the lower-mass-flux period due to its much greater length, and this tephra was dominantly deposited within 10 km downwind of the vent. This highlights the importance of ash scrubbing in the evaluation of hazards from explosive eruptions.

A 5000-year record of multiple highly explosive mafic eruptions from Gunung Agung (Bali, Indonesia): implications for eruption frequency and volcanic hazards

The 1963 AD eruption of Agung volcano was one of the most significant twentieth century eruptions in Indonesia, both in terms of its explosivity (volcanic explosivity index (VEI) of 4+) and its short-term climatic impact as a result of around 6.5 Mt SO2 emitted during the eruption. Because Agung has a significant potential to generate more sulphur-rich explosive eruptions in the future and in the wake of reported geophysical unrest between 2007 and 2011, we investigated the Late Holocene tephrostratigraphic record of this volcano using stratigraphic logging, and geochemical and geochronological analyses. We show that Agung has an average eruptive frequency of one VEI ≥2–3 eruptions per century. The Late Holocene eruptive record is dominated by basaltic andesitic eruptions generating tephra fall and pyroclastic density currents. About 25 % of eruptions are of similar or larger magnitude than the 1963 AD event, and this includes the previous eruption of 1843 AD (estimated VEI 5, contrary to previous estimations of VEI 2). The latter represents one of the chemically most evolved products (andesite) erupted at Agung. In the Late Holocene, periods of more intense explosive activity alternated with periods of background eruptive rates similar to those at other subduction zone volcanoes. All eruptive products at Agung show a texturally complex mineral assemblage, dominated by plagioclase, clinopyroxene, orthopyroxene and olivine, suggesting recurring open-system processes of magmatic differentiation. We propose that erupted magmas are the result of repeated intrusions of basaltic magmas into basaltic andesitic to andesitic reservoirs producing a hybrid of bulk basaltic andesitic composition with limited compositional variations.

Palaeoproterozoic volcanism and granitic magmatism in the Ngualla area of the Ubendian Belt, SW Tanzania: Constraints from SHRIMP U–Pb zircon ages, and Sm–Nd isotope systematics

Secondary ion microprobe U–Pb zircon data and Sm–Nd isotopic compositions are presented for the volcanic and associated granitic rocks in the northern part of the Lupa terrane in the Ngualla area of the Ubendian Belt, southwestern Tanzania. The U–Pb data reveals two distinct episodes of volcanism in the area: a ∼1943Ma volcanism involving the eruption of basaltic andesites and a ∼1871Ma event during which porphyritic dacites (and minor rhyolites) erupted. The data further shows that the last episode of volcanism was coeval with the emplacement of potassic granites at ∼1878Ma. An older age of ∼2682Ma was obtained from zircons inherited from the pre-existing Archaean crust in a potassic granite. The basaltic andesites exhibit ɛNd (1943Ma) values of −3.33 to −6.24 and TDM ages of 2521–2883Ma whereas the porphyry dacites show ɛNd (1871) values of −5.72 to −5.94 and TDM ages are 2546–2573Ma. These TDM ages are 600–700Ma older than the emplacement ages of both the basaltic andesites and porphyry dacites. The potassic granites exhibit the most negative ɛNd (1878) values of all the rocks (−6.00 to −11.2) in the area and their TDM ages are 2568–3283Ma, which are 700–1400Ma older than their emplacement age. The ɛNd(t) values for both the Ngualla volcanic and granitic rocks are much lower than the corresponding mantle value of +6.13 at 1871Ma.The emplacement of ∼1.87Ga coeval felsic plutonic and volcanic rocks in the Ngualla area of the Ubendian Belt is also a feature of the nearby Usagaran Belt and this corroborates previous evidence that plutonism and volcanism in both the Usagaran and Ubendian belts took place at the same time. The presence of ∼1.87Ga granites in both the Usagaran and the Ubendian belt implies that granitic magmatism of this age was a regional thermal event that covered a significant portion of the Proterozoic terranes of southeastern and southwestern Tanzania. Inherited Archaean zircon crystals coupled with Negative ɛNd and Nd model ages 600–1400Ma older than their respective emplacement ages all suggest Palaeoproterozoic crust is derived, in part, by re-worked Archaean crust at the southwest corner of the Tanzanian Craton.

Physical weathering and modification of a rhyolitic hyaloclastite in Iceland

Fragmental volcanic glass or ‘hyaloclastite’ is a common glaciovolcanic eruption product that is formed in large abundance during basaltic, andesitic and rhyolitic subglacial eruptions. The physical weathering of rhyolitic hyaloclastites differs notably from basaltic hyaloclastites due to differences in cementation and edifice consolidation. As rhyolitic glasses are also much rarer, comparatively little is known about their physical weathering and fracturing characteristics. In the presented study, we provide a process-oriented analysis of the physical modification of subglacially erupted rhyolitic hyaloclastites from the Blahnukur edifice in Torfajokull (Iceland). Frost weathering experiments were performed to determine how vesicular glass particles fragment to finer particle sizes. The surficial porosity of the glass drives such frost weathering through the process of pore pressurisation and was quantified using high-pressure mercury intrusion. Uniaxial compression experiments were carried out to understand how the glass structure responds to the application of external stress. The observed fracturing in both experimental treatments was found to adhere to fractal statistics, which allowed the compression experiments to be used in conjunction with the frost weathering experiments for inferring the fracturing characteristics of rhyolitic volcanic glasses. Transport processes by wind and gravity were simulated by long-duration abrasion experiments in rock tumblers (through granular avalanching), but these low-energy particle interactions were not found to significantly abrade particles. A notable result from our fragmentation experiments was the production of <10 μm particles. This size range is considered respirable and illustrates how physical weathering can continuously create potentially harmful ash textures; a process which is often overlooked in health hazard assessments after volcanic eruptions. Fragmentation by post-eruptive weathering can lead to overestimations of the fine ash fraction produced by syneruptive fragmentation and granulometric studies therefore need to be appreciative of the effects of such secondary fracturing processes.

Feeding andesitic eruptions with a high-speed connection from the mantle

Convergent margin volcanism is ultimately fed by magmas generated in the mantle, but the connection between the mantle and the eruption at the surface is typically obscured by cooling, crystallization and magma mixing within the crust. Geophysical techniques are also not very effective in the lower and middle crust, where seismic events are rare and resolution is generally poor. It has thus been unclear how fast mantle-derived magmas transit the crust and recharge crustal magma chambers. Here we use diffusion modelling of nickel zonation profiles in primitive olivines from diverse primary melts to show how mantle recharge may occur on timescales as short as eruptions themselves. In Irazú volcano in Costa Rica, magmas apparently ascend from their source region in the mantle through crust about 35 kilometres thick in just months to years, recharging hybrid basaltic andesites over the course of the eruption. These results show that large stratovolcanoes with shallow magma chambers may still preserve the deep record of their mantle origin in olivine crystals. This approach--documenting magma ascent timescales from the mantle beneath a convergent margin stratovolcano--can be applied to other eruptions that record magma mixing with recharge melts. Signs of volcanic unrest are typically monitored at the surface or upper crust; new efforts should look deeper, tracking magma movement from the base of the crust to the surface in the months to years before eruptions.

The effect of changing regional tectonics on an arc volcano: Methana, Greece

The peninsula of Methana has the longest recorded volcanic history of any volcanic centre in the South Aegean Arc. Regional fault patterns in the arc changed during the Pliocene–Quaternary, with E–W-striking listric faulting increasingly important through the Quaternary, as recorded in well-dated sedimentary basins. This study investigates how the geochemistry and eruptive style of volcanic products is influenced by regional tectonics. The volcanic stratigraphy of Methana was refined using radiometric dating, lithogeochemistry and field observations that included recording deformational structures and enclave abundance. Small N–S-striking Pliocene domes and a central volcano of uncertain type (phase A) were eroded to produce a widespread volcaniclastic apron (phase B). In the early Quaternary, an explosive central volcano with flank eruptions of andesite developed (phase C). Dacite domes and small andesitic stratovolcanoes formed throughout the mid and late Quaternary (phases D–H).Basaltic andesite and andesite of phase C are the least evolved rocks, characterised by high TiO2. Rocks that have experienced important assimilation and fractional crystallisation in the crust have a high abundance of enclaves, Th, U and alkalies (Na, K). Ni and Cr are abundant in phase A andesites, due to crystallisation and entrainment of olivine and pyroxene, whereas phase H andesites have the highest relative abundance of Ba, Rb and Sr from crystallisation and entrainment of hornblende and biotite.Pliocene domes of phase A were emplaced on N–S-striking listric faults during regional E–W extension. Onset of NE–SW faulting, arguably crustal scale and strike slip in character, led to the eruption of the least evolved rocks of phase C. Thereafter, E–W-striking faults controlled the location of volcanism. Volcanism in phases F and G was particularly voluminous and was synchronous with the onset of steep normal faulting in the Gulf of Corinth, 150km west of Methana. These steep faults cross-cut older, lower angle, listric faults. It is times of changes in regional fault patterns, as in phases C, F and G, that have the greatest influence on the eruptive style and abundance of volcanic products.

Dynamics of explosive paroxysms at open-vent andesitic systems: High-resolution mass distribution analyses of the 2006 Tungurahua fall deposit (Ecuador)

Long-lasting andesitic eruptions sometimes include strong short-lived explosive events, which can pose significant hazards in populated regions. The origin and dynamics of such violent eruptions remain poorly known and may involve a combination of different factors. Tungurahua volcano, Ecuador, reawakens in 1999 and is an example of such an open-vent system that experienced a strong and deadly andesitic pyroclastic flow-forming event in August 2006. Inspection of the deposits suggested that the event could have been triggered by magma mixing (coexistence of both silicic pumices and andesitic scoria in the tephra), magma–water interaction (presence of xenolithic clasts) or deep andesitic magma reinjection (based on mineral chemistry). Here we investigate these options by performing a high-resolution mass budget analysis of the scoria fall deposit. This is achieved by analysing componentry compositions and their mass distribution pattern in the layer, which allow us to document and integrate exponential and power laws mass decay rates over wide areas. The results yield a total mass for the tephra layer of ∼2×1010kg. The pumice mass fraction is far too small (<0.4%) to account for the high explosivity of the 2006 event. Similarly, the xenoclastic mass fraction is unexpectedly small (0.2%) and suggests limited magma–water interaction. Instead, we interpret these xenoclasts as a result of upper conduit erosion at a rate of ∼30cm/h during the paroxysm. Altogether our results support an explosive event fed by a deep gas-rich andesitic reinjection, which would have incorporated a pocket of older differentiated magma and eroded the upper conduit during the sub-plinian event. The high-resolution mass-based approach reveals useful to decipher the origin of the violent 2006 paroxysm and has potential to improve magnitude determinations of ancient eruption by considering componentry mass instead of volume. It is also applicable for monitoring purposes in the context of ongoing crises at andesitic volcanoes worldwide

Airborne filter pack measurements of S and Cl in the plume of Redoubt Volcano, Alaska February–May 2009

Filter pack data from six airborne campaigns at Redoubt Volcano, Alaska are reported here. These measurements provide a rare constraint on Cl output from an andesitic eruption at high emission rate (>104t d−1 SO2). Four S/Cl ratios measured during a period of lava dome growth indicate a depth of last magma equilibration of 2–5km. The S/Cl ratios in combination with COSPEC SO2 emission rate measurements indicate HCl emission rates of 1500–3600t d−1 during dome growth. SO2 and HCl emission rates at Redoubt Volcano correlate with each other and were low prior to the eruption, high during the eruption, and low after the eruption. S/Cl ratios measured by filter pack at andesitic volcanoes have a small range of variance, with no clear trends seen for eruptive versus passive activity. The very few S/Cl ratio measurements by filter pack at andesitic volcanoes are not as predictive of future volcanic activity as has been demonstrated for basaltic volcanoes. This may be because there are so few of these measurements. We have demonstrated it is possible to collect these samples by air between explosions during lava dome-building eruptions. We recommend more filter pack sampling be performed at andesitic volcanoes to determine the technique's utility for volcano monitoring. Filter pack data has been demonstrated to be useful for calculating the depth of magma equilibration at volcanoes including Redoubt Volcano.

Geochemistry and Petrology of the Most Recent Deposits from Cotopaxi Volcano, Northern Volcanic Zone, Ecuador

Cotopaxi volcano is located in the Northern Volcanic Zone of the South American Andes. Pyroclastic deposits and lava flows from Cotopaxi comprise basaltic andesites, andesites and rhyolites that have erupted since 13 200 years BP. Nine rhyolite eruptions were produced in at least five separate events, punctuated by intermittent andesite eruptions. High La/Yb (>5) and 230Th excesses in the andesites are consistent with equilibration of magma with garnet-bearing lower crust or mantle, and numerical models show that lower crustal assimilation–fractional crystallization involving crystallization of amphibole and plagioclase is sufficient to create the observed variations in trace elements. The Cotopaxi andesites contain intergrowths of plagioclase + pyroxene, and at least four populations of plagioclase crystals indicate pervasive magma mixing. Sr, Nd and Pb isotopes are consistent with 5–20% assimilation of radiogenic crust, but higher levels of mafic crust (cryptic assimilation) and/or source contamination by volcanogenic sediments are also likely. The Cotopaxi rhyolites formed from compaction and extraction of high-SiO2 melt from an andesitic or dacitic crystal mush. On the basis of U-series data, the residence time of the rhyolitic melts is of the order of 74 kyr. Temporal variations in MgO and Sr of the Cotopaxi andesites reflect frequent recharge of the Cotopaxi system, variable recharge composition and a system that has not become more evolved over time.

Eruptive history of South Sister, Oregon Cascades

South Sister is southernmost and highest of the Three Sisters, three geologically dissimilar stratovolcanoes that together form a spectacular 20 km reach along the Cascade crest in Oregon. North Sister is a monotonously mafic edifice as old as middle Pleistocene, Middle Sister a basalt–andesite–dacite cone built between 48 and 14 ka, and South Sister is a basalt-free edifice that alternated rhyolitic and intermediate modes from 50 ka to 2 ka (largely contemporaneous with Middle Sister). Detailed mapping, 330 chemical analyses, and 42 radioisotopic ages show that the oldest exposed South Sister lavas were initially rhyolitic ~ 50 ka. By ~ 37 ka, rhyolitic lava flows and domes (72–74% SiO 2) began alternating with radially emplaced dacite (63–68% SiO 2) and andesite (59–63% SiO 2) lava flows. Construction of a broad cone of silicic andesite–dacite (61–64% SiO 2) culminated ~ 30 ka in a dominantly explosive sequence that began with crater-forming andesitic eruptions that left fragmental deposits at least 200 m thick. This was followed at ~ 27 ka by growth of a steeply dipping summit cone of agglutinate-dominated andesite (56–60.5% SiO 2) and formation of a summit crater ~ 800 m wide. This crater was soon filled and overtopped by a thick dacite lava flow and then by > 150 m of dacitic pyroclastic ejecta. Small-volume dacite lavas (63–67% SiO 2) locally cap the pyroclastic pile. A final sheet of mafic agglutinate (54–56% SiO 2) – the most mafic product of South Sister – erupted from and drapes the small (300-m-wide) present-day summit crater, ending a summit-building sequence that lasted until ~ 22 ka. A 20 kyr-long-hiatus was broken by rhyolite eruptions that produced (1) the Rock Mesa coulee, tephra, and satellite domelets (73.5% SiO 2) and (2) the Devils Chain of ~ 20 domes and short coulees (72.3–72.8% SiO 2) from N–S vent alignments on South Sister's flanks. The compositional reversal from mafic summit agglutinate to recent rhyolites epitomizes the frequently changing compositional modes of the South Sister locus throughout its lifetime. South Sister is part of a reach of the Cascades unusually active in the last 50 kyr, characterized by high vent density, N–S vent alignments, and numerous eruptive units of true rhyolite (≥ 72% SiO 2) that distinguishes it from much of the Quaternary Cascade arc; these are eruptive expressions of the complex confluence of arc and intraplate magmatic–tectonic regimes.

Hydrothermal alteration and K-Ar ages of Neogene-Quaternary magmatic-hydrothermal systems at Toyoha-Muine area in southwest Hokkaido, Japan

This paper presents a review of hydrothermal alteration and K–Ar age data from the Toyoha-Muine area (TMA), where the Toyoha polymetallic (Ag–Pb–Zn–Cu–In) deposit is located near the Pliocene andesitic volcano that formed Mt Muine. Systematic prospect-scale mapping, sampling, X-ray analysis and microscopic observation show that hydrothermal alteration is divided into two groups: acid-pH and neutral-pH alteration types. The former is further divided into mineral assemblages I, II and III, while the latter into mineral assemblages IV and V. Different mineralogical features in five mineral assemblages are summarized as follows: (I) Quartz (silicified rock); (II) Pyrophyllite or dickite; (III) Kaolinite or halloysite ± alunite; (IV) Sericite or K-feldspar; and (V) Interstratified minerals (illite/smectite and chlorite/smectite) and/or smectite. K–Ar radiometric ages determined on twenty-eight K-bearing samples (whole volcanic rocks and separated hydrothermal minerals) mainly fall into one of three periods: Early Miocene (24.6–21.4 Ma), Middle–Late Miocene (12.5–8.4 Ma) and Pliocene–Pleistocene (3.2–0 Ma). These three periods are characterized as follows. Early Miocene: A minor hydrothermal activity, which might be genetically related to the intermediate or felsic magmatic activities, formed mineral assemblage IV at 24.6 Ma in the northern part of the TMA. Middle to Late Miocene: The basaltic intrusion, andesitic eruption, and granodiorite intrusions induced hydrothermal activities between 12.5 and 8.4 Ma, resulting in the formation of a mineral assemblage IV with some base metal mineralization. Pliocene–Pleistocene: An andesitic eruption formed Mt Muine between 3.2 and 2.9 Ma. The andesitic activity was associated with acid-pH mineral assemblages I, II and III locally around the volcano. Latent magmatic intrusions subsequent to the andesitic eruption generated hydrothermal activities that formed mineral assemblages IV and V between 1.9 and 0 Ma in the southern and southeastern parts of Toyoha deposit at depth, overprinting the Middle to Late Miocene alteration. The hydrothermal activities also formed mineral assemblages I, II and III along the Yunosawa fault (east of the Toyoha deposit) and assemblage III in the south and southeast of the Toyoha deposit near the surface.