Satellite Vents Research Articles

Controlling influence of water and ice on eruptive style and edifice construction in the Mount Melbourne Volcanic Field (northern Victoria Land, Antarctica)

The Mount Melbourne Volcanic Field (MMVF) is part of the West Antarctic Rift System, one of Earth’s largest intra-continental rift zones. It contains numerous small, compositionally diverse (alkali basalt–benmoreite) flank and satellite vents of Late Miocene–Pliocene age (≤12.50 Ma; mainly less than 2.5 Ma). They demonstrate a wide range of morphologies and eruptive mechanisms despite overlapping compositions and elevations, and they occur in a relatively small area surrounding the active Mount Melbourne stratovolcano. The volcanic outcrops fall into several main categories based on eruptive style: scoria cones, tuff cones, megapillow complexes, and shield volcanoes. Using the analysis of lithofacies and appraisal of the internal architectures of the outcrops, we have interpreted the likely eruptive setting for each center and examined the links between the environmental conditions and the resulting volcanic edifice types. Previous investigations assumed a glacial setting for most of the centers but without giving supporting evidence. We demonstrate that the local contemporary environmental conditions exerted a dominant control on the resulting volcanic edifices (i.e., the presence or absence of water, including ice or snow). The scoria cones erupted under dry subaerial conditions. Products of highly explosive hydrovolcanic eruptions are represented by tuff cones. The water involved was mainly glacial (meltwater) but may have been marine in a few examples, based on a comparison of the contrasting internal architectures of tuff cones erupted in confined (glacial) and unconfined (marine, lacustrine) settings. One of the glaciovolcanic tuff cones ceased activity shortly after it began transitioning to a tuya. The megapillow complexes are highly distinctive and have not been previously recognized in glaciovolcanic successions. They are subglacial effusive sequences emplaced as interconnected megapillows, lobes, and thick simple sheet lavas. They are believed to have erupted at moderately high discharge and reduced cooling rates in partially drained englacial vaults under ice, probably several hundred meters in thickness. Finally, several overlapping small shield volcanoes crop out mainly in the Cape Washington peninsula area. They are constructed of previously unrecognized multiple ‘a‘ā lava-fed deltas, erupted in association with a thin draping ice cover c. 50–145 m thick. Our study highlights how effectively water in all its forms (e.g., snow, ice, and any meltwater) or its absence exerts a fundamental control on eruption dynamics and volcano construction. When linked to published ages and40Ar/39Ar dates produced by this study, the new environmental information indicates that the Late Pliocene–Pleistocene landscape was mainly an icefield rather than a persistent topography-drowning ice sheet. Ice thicknesses also generally increased toward the present.

Characteristics of Sub-Aerially Emplaced Pyroclasts in the Surtsey Eruption Deposits: Implications for Diverse Surtseyan Eruptive Styles

The 1963–1967 shallow-to-emergent eruption in Iceland’s Vestmannaeyjar earned a place in the history of volcanology by creating the island of Surtsey while under close observation of volcanologist Sigurdur Thorarinsson (Sigurður Þórarinsson in Icelandic). This is an example of what is now called Surtseyan volcanism, and it included explosive and effusive phases from multiple vents that formed the island of Surtsey itself, as well as one fully subaqueous pyroclastic edifice and two additional, but ephemeral, islands. Sigurdur Thorarinsson identified tephra jetting and continuous uprush as characteristic types of subaerial explosive activity of Surtseyan volcanism. Subaerial cone-forming deposits of Surtseyan volcanism are typically poorly sorted, with fine-grained beds rich in sideromelane ash fragments, punctuated by larger, ubiquitously composite bombs, whereas deposits sampled by coring deep into the submarine edifice include fines-poor horizons dominated by vesicular coarse sideromelane ash. Here, we present new textural data and highlight the diversity of pyroclasts and microtextures from Surtsey (Surtur I and Surtur II) and its satellite vents (Surtla, Syrtlingur and Jolnir), in the context of Surtseyan volcanism. We used several sample sets. Some were collected during the 3.5-year long eruption and were conserved in the Icelandic Natural History Museum, including one sample from the core drilled into Surtsey in 1979. Other samples were collected during more recent field campaigns on Surtsey Island. In closing, we discuss the implications of this diversity for the range of activity and products produced by Surtsey.

Geochronological evolution of the potentially active Iliniza Volcano (Ecuador) based on new K-Ar ages

The youngest volcanism of the Ecuadorian Volcanic Front (Western Cordillera) is mainly dominated by highly explosive events, including the growth and violent destruction of lava domes, and the formation of thick pyroclastic sequences. Deposits associated with such eruptive dynamics have been identified at Iliniza, a compound volcano located in the Western Cordillera with a poorly defined evolutionary history. We present the first K-Ar ages of Iliniza volcano combined with stratigraphic data, numerical reconstructions, and geochemical analyses, providing a new perspective on its evolution. Our results show that Iliniza volcano is much younger than previously proposed. The Iliniza twin-peaked shape is the result of the superposition of two andesitic to dacitic stratovolcanoes. (1) The North Iliniza (NI) edifice was constructed by two lava successions and an intermediate satellite vent, showing a short range of mainly effusive activity between 123 ± 6 and 116 ± 2 ka. (2) The South Iliniza (SI) edifice began its construction through the Lower SI stage when massive lavas dated at ~45 ka formed a basal cone. During the Upper SI stage, the uppermost part of this edifice was destroyed by the highly explosive Jatuncama phase (VEI 5) leaving a 30–40 m-thick ignimbrite sequence. The subsequent extrusion of several dacitic lava domes reconstructed the South Iliniza summit at around 35 ka. The Terminal SI stage corresponds to the emission of several andesitic lavas between 31 ± 4 and 25 ± 3 ka. The Iliniza eruptive activity extended into the Late Pleistocene and Holocene with the extrusion of the Tishigcuchi lava dome, and the emission of the Pongo lava flow dated at 6 ± 4 ka. Based on the proposed eruptive history, we suggest that a revised volcanic hazard assessment of the potentially active Iliniza volcano is required.

Transcrustal magmatic systems: evidence from andesites of the southern Taupo Volcanic Zone

Studies synthesizing field work, numerical simulations, petrology, geochemistry and geophysical observations indicate that the compositional diversity of arc lavas results from evolution of mantle-derived magmas by mixing, assimilation and fractional crystallization. This evolution occurs within complexes called transcrustal magmatic systems. The mafic lower parts of such zones, called hot zones, are difficult to probe. However, a satellite vent near the stratovolcano Ruapehu in the southern Taupo Volcanic Zone (New Zealand) comprises materials that may originate from a hot zone. Magnesian andesites (Mg# 60–65 ) from the Ohakune scoria cone contain primitive olivine (Fo 85–91 ), high-Mg# clinopyroxene (Mg# 81–88 ) and orthopyroxene (Mg# 76–83 ), but lack plagioclase. Disequilibrium of Ohakune crystals and groundmass suggests that the crystal cargo of Ohakune andesites was scavenged from deeper and more primitive levels of the magmatic system. Mineral constraints on temperature and pressure indicate that the hot zone initially formed at mid- to lower-crustal pressures (3.5–7.0 ± 2.8 kbar). We interpret the mafic mineralogy and presence of disequilibrium features as evidence that these andesites and their crystal cargo are products of a hot zone in the middle to lower crust. Products of the hot zone may appear before products of the systems that form the bases of mature stratovolcanoes such as Ruapehu. Supplementary material: Details of petrography, geochemical and arc olivine data, and MELTS modelling data are available at https://doi.org/10.6084/m9.figshare.c.5494984

Pre-eruptive conditions at satellite vent eruptions at Teide-Pico Viejo complex (Tenerife, Canary Islands)

The Teide-Pico Viejo (T-PV) stratovolcanoes constitute one of the most potentially active volcanic complexes in Europe. T-PV was traditionally considered as a non-explosive system, but recent studies have pointed out the explosive character of their phonolitic magmas, including plinian and subplinian eruptions and the generation of pyroclastic density currents, as is evidenced in the volcanological record of the last 30 kyr. This explosive activity is mostly associated with satellite dome-like vents, which are characterised by presenting progressive transitions between explosive and effusive activity. In order to improve our understanding of these types of eruptions and their controlling mechanisms, we conducted a petrological and mineral characterisation of the products from the different eruptive phases of Pico Cabras dome. This permitted us to constrain the pre-eruptive conditions of the magma and to infer the potential factors that control the changes in the volcanic activity of dome-forming eruptions at T-PV and, in particular, the transition between explosive and effusive phases.Feldspar (anorthoclase and sanidine), clinopyroxene (diopside and augite), biotite, amphibole (kaersutite), magnetite, ilmenite, sodalite, and glass samples were analysed using EPMA and Micro-XRF. Our results suggest the presence of a compositionally, thermally, and volatile stratified magma chamber at 100 ± 50 MPa prior to the Pico Cabras eruption (between 9210 and 5911 years BP), which was characterised by a progressive change in the style of activity, from an efficiently fragmented magma dispersed by a sustained plume, to a poorly-fragmented, low-height, non-convective fountain (forming the fountain-fed lava dome) to a glass-bearing lava flow activity. The transition in the eruptive style was controlled by changes in temperature and amount of volatiles dissolved in the phonolitic melt (i.e., differences in magma rheology). The initial explosive phase was related to the emission of highly evolved phonolitic magma ponding at 725–825 °C while containing 3.5–5 wt% H2O and located in the upper part of the magma reservoir. The magma that fed the phonolitic lava flow formed the main body of the reservoir and was stored below the preceding melt at 880 ± 30 °C and with 2.5–3 wt% H2O dissolved. Feldspar zonation suggests that the eruption was triggered by the intrusion of hotter mafic magma at the base of the phonolitic chamber, which induced the overturn of the interior of the reservoir. However, the interaction between the internal magmas was short enough to prevent the hybridization phenomena among them. The compositions of some feldspars from the most explosive phase are equivalent to those found in the El Abrigo eruption, the last caldera-forming episode (ca. 190 ka), demonstrating that T-PV volcanic system can already produce evolved and highly explosive magmas, a factor that should be considered in future hazard assessment for Tenerife.

Volcanic seismicity beneath Chuginadak Island, Alaska (Cleveland and Tana volcanoes): Implications for magma dynamics and eruption forecasting

Cleveland and Tana are remote volcanoes located in the central Aleutian volcanic arc on the eastern end of the Islands of Four Mountains (IFM). The persistently active Mount Cleveland volcano, on the western side of Chuginadak Island, is surrounded by several closely spaced Quaternary volcanic centers including Carlisle, Herbert, Kagamil, Tana, and Uliaga, and numerous small satellite vents on Chiginadak between Cleveland and Tana. The Alaska Volcano Observatory (AVO) installed two permanent broadband seismometers on Chuginadak Island in 2014, and we operated a temporary broadband network focused on the western side of the island in 2015–2016. Collectively, these stations provided the first seismic observations of this frequently active volcano and the surrounding Holocene-aged volcanic vents. During the study period (July 2014–January 2019), eruptive activity at Cleveland was characterized by small explosions separated by periods of lava effusion that formed small domes in the volcano's summit crater. We characterize seismicity beneath Chuginadak Island through automated analysis of event waveform frequency content, development of a one-dimensional P-wave velocity model, calculation of earthquake hypocenters, magnitudes, focal mechanisms, and identification of earthquake families. This analysis reveals the full range of seismic event types expected in a highly active volcanic environment and includes Volcano-Tectonic (VT) earthquakes, Long-Period (LP) events, and explosion signals. LP events appear to cluster at shallow depth beneath the active crater of Mount Cleveland and almost all of the explosions occur without identifiable short-term (hours to days) seismic precursors. VT earthquakes beneath Mount Cleveland occur at depths of 2 to 8 km below sea level (BSL) and range in magnitude from −0.2 to 1.8. VT focal mechanisms have horizontal P-axes that align with the regional axis of maximum stress. These observations, and a relatively slow one-dimensional seismic velocity model, are consistent with a shallow body of magma that is fed through a deeper conduit system. The time-history of VT earthquakes and shallow LP events suggest their occurrence may track the transfer of magma and fluids from the mid-crust to the shallow portions of the conduit system and may provide a means to anticipate future explosions and periods of dome growth. VT hypocenters also extend ~7 km northeast of Cleveland's summit at depths of 5 to 10 km BSL, under a group of Holocene-aged vents between Mount Cleveland and Tana. These earthquakes have vertically-oriented P-axes and a greater percentage occur in families. These observations, combined with observations of vent orientation and morphology and gas flux, suggest the area between Cleveland and Tana represents a zone of complicated volcano-tectonic interaction, similar to calderas elsewhere in the Aleutian arc. The presence of a larger volcanic system in the eastern IFM could influence magmatism and account for the multiple closely spaced volcanic centers in this region.

Petrology and geochemistry of three Early Holocene eruptions from Makushin Volcano, Alaska

Makushin stratovolcano, Alaska, produced three, highly explosive, andesitic eruptions between ~ 9292 and 6215 yBP. Those eruptions are informally named the CFE (“crater-forming eruption”), Nateekin, and Driftwood Pumice, and they deposited significant tephra fallout in the present-day port of Dutch Harbor and City of Unalaska area. The focus of this study is to examine the geochemistry and petrology of those eruptions to better understand Makushin volcano hazards, andesite petrogenesis and eruption triggering by mafic recharge processes. The CFE, Nateekin, and Driftwood Pumice samples range from basaltic andesite to dacite but are predominantly andesitic (SiO2 = 55.6 to 63.5 wt%). The CFE deposits are slightly compositionally stratified, with the top CFE samples slightly more mafic (55 to 60 wt% SiO2) than the basal deposits (58 to 60 wt% SiO2). Disequilibrium mineral compositions and textures in the CFE, Nateekin, and Driftwood Pumice samples, combined with two pyroxene thermometry and An-rich plagioclase microlites (An80) found only in the top of the CFE deposits, provide evidence for repetitive mafic recharge triggering those eruptions, consistent with prior studies. We compare the Makushin geochemical data with data from select satellite vents and cones in the Makushin Volcanic Field (MVF) from prior studies, to examine possible genetic relationships. The geochemical data and Rhyolite-MELTS models run at crustal storage conditions (2 kbar, fO2 = Ni-NiO, and 1.5 and 3.5 wt% H2O) indicate that no single parental magma supplies the MVF satellite cones and Makushin volcano. Instead, two component mixing models better fit the MVF geochemical array. Our Makushin results compare well with models of predominantly andesitic volcanoes that require mafic recharge to mobilize the andesites and trigger eruptions.

Contemporaneously emplaced submarine volcaniclastic deposits and pillow lavas from multiple sources in the island arc Brook Street Terrane, Southland, New Zealand

ABSTRACT Volcaniclastic and coherent volcanic rocks of the Permian island arc Brook Street Terrane crop out along the Southland coast, New Zealand, near Riverton. Extensive along-strike exposures reveal lateral variations in these submarine deposits and provide a rare opportunity to examine deposits of an arc environment within a thin slice of time. We describe pillow lavas, dikes, hydroclastic breccias, tuffs, lapilli tuffs and argillites, all hydrothermally altered and metamorphosed to prehnite-pumpellyite facies. Trace element geochemical data indicate multiple primitive island arc to MORB-like magma sources. Two separate volcanic centres apparently tapped different mantle domains in a source region transitional between the arc and back-arc. The centres’ proximity and inferred small sizes suggest they were satellite vents of a larger arc volcano. Tuff and lapilli tuff were formed as hyaloclastite during eruption of pillow lava, and redeposited by subaqueous mass flows. Peperitic contacts indicate that the lavas intruded unconsolidated volcaniclastic deposits. We infer that hydroclastic fragmentation associated with pillow emplacement produced debris that was then intruded by later pillows, with local peperite formation, as eruption persisted. The hydroclastic debris from these satellite vents was resedimented in turbidite fans with channels, and periodic turbidity currents delivered fine-grained material from the volcano’s main vent.

Holocene volcanism at the Quetrupillán Volcanic Complex (39°30’ S, 71°43’ W), southern Chile

This paper provides the first detailed description of Holocene volcanism at the Quetrupillán Volcanic Complex. This volcanic complex consists of a truncated and eroded stratocone plus sixteen well-preserved satellite vents on its lower flanks. Intense scouring of the stratocone’s flanks (presumably by ice) has removed much evidence of its Holocene eruptions, and thus the Holocene construction of the stratocone (i.e. number and volume of eruptions) cannot be determined. The sixteen satellite vents are the products of an uncertain number of eruptions, with trachyte comprising ~97% of the lava erupted. Geochemical analysis of tephra layers from three logged sections in nearby valleys provides evidence of three explosive eruptions from Quetrupillán. In these sections, no evidence of pyroclastic density current deposits was identified, which may suggest that explosive volcanic hazards from Quetrupillán are less than indicated on current hazard maps.

An overview of the Muara Laboh geothermal system, Sumatra

The Muara Laboh geothermal system is a liquid-dominated, fracture-controlled reservoir showing some characteristics of both intrusion-related and fault circulation systems. The developed reservoir has moderate to high-temperature (230–310 °C) with a long NNW outflow (160–230 °C) extending to the Sapan Malulong boiling chloride springs. Reservoir fluids generally have low salinity (∼400-1600 ppm Cl) with benign chemistry and low non-condensable gas (NCG) content (∼0.5 to 2 wt% in steam). The proven reservoir is divided into distinct SW and NE sectors. The principal deep upflow zone (270 to 310 °C) is located in the SW and is associated with Patah Sembilan volcano and satellite vents (Anak Patah Sembilan). The SW upflow is characterized by relatively low salinity (∼400-600 ppm Cl), whereas fluids in the NE emerge from a second upflow along faults and fractures at 240–250 °C (1200–1600 ppm Cl) and ascend to an initial-state steam cap near the Idung Mancung fumarole. The shallow NE part of the geothermal system is hosted mainly by Quaternary to Miocene Age andesitic to rhyolitic rocks, whereas the deeper SW part of the system occurs in a Mesozoic to Cenozoic Age plutonic complex and its host rocks that are cut by younger dikes. Rock porosity and permeability trends can be related to reservoir age, volcanic deposit type, and alteration history. Fracture permeability is high, whereas matrix porosity (<1-7%) and permeability are moderate to low and decrease with depth. The volcanic sequence is capped by tuffs dated at ∼34,000 to 41,000 years BP, incapsulated in debris flows. This sequence may have formed during and shortly after likely sector collapse episodes of the Patah Sembilan volcano. The shallow clay cap of the system wraps around the crater, suggesting that it was excavated during crater formation.Permeable fracture patterns and their origins differ somewhat between the deep SW and the shallow NE reservoir. In the deep SW reservoir, fluids ascend along fractured margins of stock and dike intrusions. Steeply dipping intrusions and open fractures strike from WNW to NNW, subparallel to the main Great Sumatra Fault trend, with a secondary set of N to NE fractures and intrusions parallel with the inferred maximum horizontal stress direction (SHMax). In the shallower NE steam cap and outflow, N to NE steeply dipping fractures dominate, with bounding N to NNW faults. Bulk rock alteration includes shallow argillic and transitional clay zones overlying phyllic and propylitic zones. High-T propylitic alteration (secondary amphibole) and rare potassic alteration (secondary biotite) are associated with intrusion. Vein paragenesis in the SW reservoir indicates that portions of the initially permeable propylitic zone have been sealed by calcite and quartz ± prehnite. Late calcite veins suggest ingress of steam-heated bicarbonate waters contributed to resealing of the system. These relationships suggest that the SW system underwent extensive boiling, fluid loss, and inflow of dilute bicarbonate waters, possibly related to sector collapse and partial excavation of the topseal of the system (now Patah Sembilan crater).

Diverse dynamics of Holocene mafic-intermediate Plinian eruptions at Mt. Taranaki (Egmont), New Zealand

Over the last 5000 years, at least 53 eruptive episodes have occurred at Mt. Taranaki (western North Island, New Zealand), from either its summit crater (~ 2500 m) or a satellite vent on Fanthams Peak (~ 1900 m). The magmas erupted have a wide range of compositions from basaltic to trachy-andesitic (~ 48–60 wt% SiO2). Five large-magnitude episodes from this sequence were studied so as to characterize a typical range of explosive eruption styles at andesitic stratovolcanoes, including three eruptions from the summit crater and two from Fanthams Peak. Sustained eruption columns characterized the climactic phase of all five eruptions, but these were interspersed with pulsating, collapsing, or oscillating conditions. Eruption columns reached between 14 to 29 km in height and ejected minimum volumes of 0.1–1.1 km3 at mass discharge rates of 1 × 107–2 × 108 kg/s, indicating magnitudes of 4.1 to 5.1. The simplest eruptions occurred from Fanthams Peak with basaltic magmas producing high-climactic eruption columns rapidly after vent opening, followed by gentle waning phases or a passage into a lava-fountaining phase. Eruptions of higher-silica magmas at the summit vent, by contrast, showed longer pre-climactic eruptive phases with either dome growth or complex phases of vent clearance and blockage producing unsteady eruption columns. The latter eruption types produced block-and-ash flows, lateral-blast surges, and column-collapse pumice-and-ash flows, with run-out distances of 3–19 km, covering 5–70 km2 with volumes of up to 0.022 km3. Our results demonstrate that very different eruption scenarios may occur at different vent locations, or with subtly different compositions erupted, on the same stratovolcano so that emergency management planning must take such a range of possibilities into account.

Degassing and magma mixing during the eruption of Surtsey Volcano (Iceland, 1963–1967): the signatures of a dynamic and discrete rift propagation event

The eruption of Surtsey (Iceland, 1963–1967) is a rare example of observed volcanism at a propagating rift tip, where Iceland’s Eastern Volcanic Zone is advancing southwesterly toward the Reykjanes Ridge. We use olivine-hosted melt inclusions, embayments, and matrix glasses to investigate major element, trace element, and volatile characteristics of Surtsey’s magmatic system, and to parameterize decompression models that describe degassing of C-H-S-O species during this well-studied eruption. Major elements show that the inclusions represent heterogeneous melts with a range of compositions similar to those of Icelandic lavas. Trace elements discriminate between two groups of inclusions: the majority are compositionally similar to Eastern Volcanic Zone alkalic basalts, and the remainder are notably more depleted in incompatible trace elements and trend toward fields defined by tholeiites on and around Iceland. Strongly correlated Cl and Nb for all inclusions indicates that seawater did not affect Surtsey’s volatile budget, despite having clearly affected its eruption style. Both types of inclusions are volatile rich (≤1.47 wt.% H2O; ≤2534 ppm CO2 as measured or ≤5286 ppm once corrected for vapor bubbles and post-entrapment crystallization). They are more hydrous than primary melts in the onshore Eastern Volcanic Zone and have parental CO2/Nb (≤590) at the upper end of regional estimates. Saturation pressures calculated from corrected CO2 values indicate that melts in both inclusion types initially crystallized at similar depths in the upper mantle (17–21 km), and then partially crystallized during ascent through the lower crust (7–13 km). Historical data show that gases emitted on Surtsey shifted from being relatively reduced in 1964/1965 to more oxidized in 1967, after a protracted period of effusive activity and resurgence of pyroclastic activity from satellite vents. Closed-system degassing models predict the compositions and redox states of the 1964/1965 gases extremely well, but cannot account for the oxidized gases emitted in 1967, which may have been contaminated by ambient air in a system that was opening as the eruption waned. Surtsey’s pyroclastic resurgence can be explained by recharge from ephemeral and compositionally heterogeneous magma bodies, tapped from possibly as deep as the mantle-crust boundary, in a process consistent with the progressively increasing interconnection between magma bodies that is typical at propagating rift tips. The eruption of Surtsey therefore shows that magma system evolution at rift tips can occur in dynamic and discrete events, with influx of new magma having explosive consequences.

Endogenous and exogenous growth of the monogenetic Lemptégy volcano, Chaîne des Puys, France

The monogenetic Lemptegy volcano in the Chaine des Puys (Auvergne, France) was quarried from 1946 to 2007 and offers the possibility to study scoria cone architecture and evolution. This volcano was originally 50–80 m high, but scoria excavation has resulted in a 50-m-deep hole. Beginning in the 1980s, extraction was carried out with the advice of volcanologists so that Lemptegy’s shallow plumbing system and three-dimensional stratigraphy have been preserved. Detailed mapping enabled key stratigraphic units to be distinguished and the constructional phases to be reconstructed. The emplacement and evolution of the shallow plumbing system have also been unraveled. The growth of this monogenetic scoria cone included two temporally well-separated eruptions from closely spaced vents. The activity included Hawaiian, Strombolian and Vulcanian explosions, lava effusion, cryptodome and dome formation, partial collapse, satellite vent formation, eruptive pauses, and intrusion emplacement with consequent uplift. The cone shape, structure, and hence the local stress field, plumbing system, and thermal state were continuously changing, which in turn influenced the eruptive style and location. The plumbing system morphology and microtectonic structures both record local stress field and magmatic flow direction changes. Lemptegy volcano’s internal architecture, stratigraphy, and evolution show how complex a monogenetic volcano can be.

Growth after collapse: the volcanic and magmatic history of the Neostromboli lava cone (island of Stromboli, Italy)

This study focuses on the Holocene Neostromboli lava cone which is part of the Stromboli volcano (Aeolian Islands, Italy). It represents an important example of the structure and behaviour of lava cone re-growth shortly after a large-scale volcano sector collapse. Neostromboli’s eruptive style is dominated by extrusion of sheet-like a’a lava flows and associated autoclastic breccias. The lava cone developed during five phases of activity (phases A to E) representing discrete episodes of lava effusion characterised by rapid changes in magma composition, spatial development of the feeder system and physical properties of erupted lavas through time. Although they all belong to the potassic series (KS) with SiO2 49–55 wt% and MgO 2.50–6.25 wt%, the Neostromboli lavas have distinct petrogenetic characteristics ranging from leucite-free and low-Sr shoshonitic basalts (phases A and B) to leucite-bearing shoshonitic basalts with high (phase C) and intermediate (phase D) Sr contents, and to leucite-free, biotite-bearing shoshonites (phase E). Three different batches of mantle-derived, relatively primitive basic magmas fed the activity of phases A, C and D. The evolution phases A to B is inferred to have occurred in a continuously replenished, tapped and crystallizing (RTF) magma chamber whereas phase E products were dominantly generated by fractional crystallization from phase D magmas. The Neostromboli cone shows summit, flank and satellite vents and sheet intrusions with different geometry and location through time, reflecting the interplay of regional tectonics, precursory instability processes and volcano load.

Magma emplacement into the Lemptégy scoria cone (Chaîne Des Puys, France) explored with structural, anisotropy of magnetic susceptibility, and Paleomagnetic data

The Lemptegy volcano is a small monogenetic scoria cone located in the Chaine des Puys, Auvergne, France, which erupted about 32,000 years ago. A first edifice (Lemptegy 1) formed during a trachybasalt eruption as a group of satellite vents of the Puy de Gouttes scoria cone. A second trachyandesitic edifice (Lemptegy 2) formed soon after and completely covered Lemptegy 1 with an 80-m-high breached cone. Since 1946, the Lemptegy volcano has been quarried for scoria and today offers unprecedented three-dimensional exposure of the subvolcanic plumbing system. To map the internal flow architecture of the plumbing system and to study the subvolcanic deformation of Lemptegy 2, structural mapping, petrographic observations, anisotropy of magnetic susceptibility (AMS), rock magnetic, and paleomagnetic data were collected. Field structural mapping and thin section study of tension gashes, Riedel shears, striations as well as ductile shear zones and bubbles allow the direction and sense of the magma flow to be determined. Twenty AMS sites were established in ten dikes (one to four sites in each dike) with 504 specimens analyzed and 479 specimens used to infer magma flow patterns. Structural data, the maximum susceptibility axis (K 1), and the imbrication of the magnetic foliation (K 1–K 2) planes indicate both upward and downward sense of flow, as well as flow toward and away from the central vent. Rock magnetic experiments reveal that a cubic Fe–Ti oxide phase, likely low-Ti titanomagnetite, is the principal magnetic phase carrying both the remanence and anisotropy. Paleomagnetic data from some sites yield statistically distinct, at the 95 % confidence level, remanence directions while at other sites the data are indistinguishable at the 95 % confidence level. The paleomagnetic results, observed steeply tilted scoria layers, internal unconformities, and faults show that as each dike was emplaced, it displaced earlier dikes evidencing subvolcanic deformation. The Lemptegy 2 volcano shares similarities in terms of inferred eruption style and structures with other scoria cones, such as Cerro Negro (Nicaragua), and thus provides an excellent field laboratory to investigate active scoria cones world-wide.

Experimental Constraints on Parameters Controlling the Difference in the Eruptive Dynamics of Phonolitic Magmas: the Case of Tenerife (Canary Islands)

We have performed phase equilibrium experiments to determine the pre-eruptive conditions of the explosive eruption of Montana Blanca (2020 BP) that occurred from a satellite vent located in the east flank of Teide volcano (Tenerife Island). Crystallization experiments were performed using a phonolitic obsidian from the fall out deposit that contains 5 wt% anorthoclase, diopside and magnetite with minor amounts of biotite and ilmenite, set in a glassy matrix that contains microlites of Ca-rich alkali feldspars. Temperature was varied between 850oC and 800oC, and pressure between 200MPa to 50 MPa. The oxygen fugacity (fO2) was varied between NNO+0.2 (0.2 log units above to the Ni-NiO solid buffer) to NNO-2, whilst dissolved water contents varied from 7 to 1.5 wt%. Comparison between natural and experimental phase proportions and compositions indicates that the main body of phonolite magma was stored at 850±15oC, 50±20MPa, 2.5±0.5wt% H2O at an fO2 around NNO-0.5 prior to eruption, equivalent to depths between 1 to 2 km below the surface. Some clinopyroxene hosting H2O-rich melt inclusions possibly originates from intermittent supply of phonolitic magma stored at somewhat deeper levels (100 MPa). The Ca and Fe-rich composition of alkali feldspar phenocrysts rims and microlites attests to the intrusion of a mafic magma in the reservoir just prior to eruption, as borne out by banded pumices appearing in the later products of the eruptive sequence. The comparison with other phonolitic magmas from Tenerife and elsewhere (e.g., Vesuvius, Laacher See) shows that differences in the eruptive dynamic of phonolitic magmas can be correlated to differences in storage depths, along with variation in pre-eruptive volatile contents.

Eruption episodes and magma recharge events in andesitic systems: Mt Taranaki, New Zealand

Eruption episodes, where a series of eruption events are generically related, can include the eruption of a wide spectrum of volcanic activity over decadal periods. This paper concentrates on the opening phases of an eruption episode which occurred approximately 1800 yrs BP from Mt Taranaki, New Zealand. These events spanned the eruption of differing bulk compositions and styles from two distinct vent locations; an andesitic sub-plinian eruption from the summit vent and a scoria cone-building eruption of basaltic magma from a satellite vent. Compositional profiles and zoning textures of plagioclase, amphibole and clinopyroxene phenocrysts from the opening andesitic event show evidence of magma mixing and subsequent crystallisation just prior to the initiation of the eruption episode. Titanomagnetite grain morphology and Ti variation suggest that the magma mixing event occurred within a few days to weeks before the eruption acting as a trigger for it. We present a magmatic model which is constrained by the petrological observations and eruptions of the episode. In this model magma differentiation at depth causes its rise and recharging of a mid-crustal magma storage area at 5–7 km. Although the recharging magma differed slightly in oxygen fugacity and temperature, it was compositionally and physically similar enough to the residing andesitic magma to allow efficient mixing. The petrological characteristics described here can be readily observed and enable identification of mixing events in other recent eruption episodes.

Evidence for dike emplacement beneath Iliamna Volcano, Alaska in 1996

Two earthquake swarms, comprising 88 and 2833 locatable events, occurred beneath Iliamna Volcano, Alaska, in May and August of 1996. Swarm earthquakes ranged in magnitude from −0.9 to 3.3. Increases in SO 2 and CO 2 emissions detected during the fall of 1996 were coincident with the second swarm. No other physical changes were observed in or around the volcano during this time period. No eruption occurred, and seismicity and measured gas emissions have remained at background levels since mid-1997. Earthquake hypocenters recorded during the swarms form a cluster in a previously aseismic volume of crust located to the south of Iliamna’s summit at a depth of −1 to 4 km below sea level. This cluster is elongated to the NNW–SSE, parallel to the trend of the summit and southern vents at Iliamna and to the regional axis of maximum compressive stress determined through inversion of fault-plane solutions for regional earthquakes. Fault-plane solutions calculated for 24 swarm earthquakes located at the top of the new cluster suggest a heterogeneous stress field acting during the second swarm, characterized by normal faulting and strike-slip faulting with p-axes parallel to the axis of regional maximum compressive stress. The increase in earthquake rates, the appearance of a new seismic volume, and the elevated gas emissions at Iliamna Volcano indicate that new magma intruded beneath the volcano in 1996. The elongation of the 1996–1997 earthquake cluster parallel to the direction of regional maximum compressive stress and the accelerated occurrence of both normal and strike-slip faulting in a small volume of crust at the top of the new seismic volume may be explained by the emplacement and inflation of a subvertical planar dike beneath the summit of Iliamna and its southern satellite vents.

Petrologic constraints on the spatial distribution of crustal magma chambers, Atka Volcanic Center, central Aleutian arc

The Atka volcanic center, the largest modern, central Aleutian magmatic complex, covers 360 km2 with an estimated eruptive volume of ~200 km3. The oldest exposed rocks, the Old Harbor Series, are a suite of basaltic andesite to dacite flows, pyroclastic rocks and lahars of presumed Tertiary age. The youngest phase of activity on Atka probably began 1–2 Ma ago near the center of the present volcanic field, and consists of basaltic and basaltic andesite flows which produced a large shield volcano, Atka Volcano. Subsequent to this activity, satellite vents formed along the margin of this structure. Collapse of the ancestral Atka Volcano produced a caldera 5 km in diameter and was accompanied by the eruption of a large dacitic flow (Big Pink). After caldera formation, four major volcanic cones (Korovin, Mount Kliuchef, Sarichef and Konia) grew around the margins of Atka caldera. This study focuses on the Atka Volcano, Korovin and Mount Kliuchef volcanic centers. Atka lavas range from basalt through dacite, with all major volcanic vents erupting lavas spanning similar compositional ranges. They are porphyritic with phenocrysts of plagioclase (25–36%), olivine (0.4–3.6%) or orthopyroxene (2.8%), clinopyroxene (1–12%) and a single Fe–Ti oxide (0–2%). Hydrous phases are lacking even in the most evolved lavas. Two generations of plagioclase occur in the Korovin suite but not in lavas from Mount Kliuchef. Korovin plagioclase also displays resorption surfaces and inclusion zones which are either absent in the phenocrysts of Mount Kliuchef lavas or very rare. Similarly, clinopyroxene in Korovin has reaction rims and abundant glass inclusions which are absent in the Mount Kliuchef lavas. Lavas erupted in the Atka volcanic field vary from 47 to 66% SiO2, although the complex is dominated volumetrically by basaltic lavas. On major-element Harker diagrams, mafic and intermediate Atka Volcano lavas form linear trends but the most siliceous lavas do not fall along these trends. By contrast, Mount Kliuchef lavas are compositionally bimodal whereas Korovin samples range from 51 to 62 wt% silica with intermediate lavas clustering around 55% SiO2. Incompatible trace-element abundances of Korovin lavas do not increase with silica content but those of Atka Volcano and Mount Kliuchef do. On incompatible-incompatible element diagrams, the three suites behave very differently. Differences also exist between the REE characteristics of the three Atka suites. The petrographic and geochemical differences between the three suites suggest formation by markedly different crustal magmatic processes. The characteristics of the Atka Volcano suite suggest formation of the basalt to dacite suite by crystal fractionation of an anhydrous phenocryst assemblage. Incompatible-incompatible element trends suggest the mafic Mount Kliuchef lavas were also produced by crystal fractionation, a conclusion supported by quantitative mass balance calculations. The difference in slope between the Atka Volcano and Mount Kliuchef suites indicates, however, that there were significant differences in these fractionation schemes. Modeling as well as geochemical characteristics suggest that the Mount Kliuchef dacites are not the product of crystal fractionation. They may represent (1) crustal melts, (2) complete melts of siliceous lava produced during earlier magmatic stages, or (3) residual liquids from the Atka Volcano magmatic system. In contrast to these suites, the Korovin lavas could not have been formed by crystal fractionation. Rather the andesites are probably the result of mixing basaltic and dacitic magmas. The Atka data suggest: (1) large volcanic centers may be supplied by a series of non-communicating, crustal magma chambers; (2) local factors may be important in determining liquid-lines-of-descent; and (3) very different magmatic processes may occur in adjacent magma chambers. If the complexity of the Atka suite is a common feature of arc magmatic plumbing systems, reconstructing differentiation processes requires analytical sample sets which are well constrained spatially and temporally.

Stratigraphy, structure, and volcanic evolution of the Pico Teide–Pico Viejo formation, Tenerife, Canary Islands

The structure and volcanic stratigraphy of the Pico Teide–Pico Viejo (PT–PV) formation, deriving from the basanite–phonolite stratovolcanoes PT and PV, and numerous flank vent systems, are documented in detail based on new field and photogeologic mapping, geomorphologic analysis, borehole data, and petrological and geochemical findings. Results provide insight into the structure and evolution of the PT–PV magma system, and the long-term, cyclic evolution of Tenerife's post-shield volcanic complex. The PT–PV formation comprises products of central volcanism, mainly emplaced into the Las Cañadas caldera (LCC), and contemporaneous products from adjacent rifts. PT–PV central volcanic products become more differentiated up-section with felsic lavas dominating the recent output of the system. This is attributed to the evolution of a shallow magma reservoir beneath PT that was emplaced early in the PT–PV cycle on the intra-caldera segment of Tenerife's post-shield rift system. The rift axis has been the focus of PT–PV intrusive and eruptive activity, and has controlled the location of the stratocones. The current geometry of the rifts reflects a major structural reorganisation defining the start of the PT–PV cycle at ∼0.18Ma, namely the truncation of the north side of the LCC/LCE by the giant Icod landslide. The internal stratigraphy of the PT–PV formation suggests that PT developed early, with PV developing as a satellite vent. Activity has since alternated between PT and PV due to episodes of vent blockage or chamber sealing. These processes have allowed significant volumes of phonolitic magmas to develop and accumulate within the PT chamber, which have vented through radial dike systems during tumescence episodes and from the rift system, which has permitted lateral magma transport. The PT–PV magma system is a potentially hazardous source of future, felsic eruptive activity on Tenerife.