Exogenous Dome Research Articles

The Deterioration of Geoheritage in the Central Spanish Volcanic Region by Open-Pit Mining

The geoheritage of the Central Spanish Volcanic Region (Ciudad Real) has been severely affected by open pit mining since the beginning of the twentieth century until 2017. Field work, photointerpretation of aerial images, and consultation of the Mining Cadaster of the Spanish Geological Survey (IGME) have been used to characterize the impact that open-pit mining has on the geomorphology of this volcanic area. As a result of this intense mining activity, uninterrupted over the course of a century, 18.76 million tons of basalt and more than 16.11 million tons of scoriaceous deposits (considered as industrial minerals) have been extracted, which has caused irreversible damage to the region’s geoheritage. Some thirty volcanoes have been destroyed, with cinder cones and exogenous domes being particularly affected. Currently, the validity of this mining exploitation system represents a serious threat to the conservation of the rich volcanic heritage of the area. Therefore, urgent measures are needed to rationalize the management of natural resources, namely, to make their use and preservation compatible for the future generations.

Using a discrete element approach to model lava dome emplacement and collapse

Lava dome collapses can lead to explosive activity and pyroclastic flow generation which makes them one of the most deadly consequences of volcanic activity. The mechanisms linked to a collapse are however still poorly understood and very few numerical models exist that investigate the actual collapse of a lava dome after emplacement. We use a discrete element method implemented in the modelling software Particle Flow Code to investigate lava dome growth, but also go further to test the stability of the dome under the following conditions: increased internal pressure; switch in extrusion direction caused by partial cooling of the dome; and extrusion of lava onto variable underlying topography. We initially show the morphology development of a growing lava dome, and how the rheological boundary between core and talus evolves throughout the lifetime of a dome and with varied solidus pressures. Through visualisation of strain accumulation within the lava dome we show superficial rockfall development due to interaction with topography, whereas large deep-seated failures occur when the dome is exposed to internal overpressures. We find that a switch in extrusion direction promotes a transition from endogenous to exogenous dome growth and leads to lava lobe formation. We demonstrate that lava dome collapse exhibits many features similar to common landslides and by investigating strain patterns within the dome, we can use numerical modelling to understand features that elude field observations.

A22 磁気岩石学的解析による雲仙火山、平成溶岩ドームの噴出プロセス : 外成ドームと内成ドームの違い(火山の地質・噴火年代 (2), 日本火山学会2005年秋季大会)

A22 磁気岩石学的解析による雲仙火山、平成溶岩ドームの噴出プロセス : 外成ドームと内成ドームの違い(火山の地質・噴火年代 (2), 日本火山学会2005年秋季大会)

Airborne photogrammetry and geomorphological analysis of the 2001–2012 exogenous dome growth at Molodoy Shiveluch Volcano, Kamchatka

In 2001, after a six-year pause in extrusive activity, lava dome growth resumed at Molodoy Shiveluch Volcano. The new period of dome growth (2001–present) has morphological features that were uncommon during the previous periods of the dome formation (1980–1981, 1993–1995): numerous lava lobes and crease structures. Thus, the current dome growth is mostly of an exogenous type with short periods of endogenous growth that occurred in 2003, 2005, and 2010. Geomorphological interpretation of stereo photo images has revealed elements of the dome that are hardly distinguishable in single photographs. We have made detailed descriptions of the dome morphology covering all the dates of the available images. By using photogrammetric processing of aerial photographs, we created Digital Terrain Models and topographic maps of the lava dome and defined its volumes for 2001 (0.19km3), 2003 (0.47km3), 2005 (0.48km3), 2010 (0.54km3), and 2012 (0.63km3). We also defined other morphometric characteristics: absolute and relative heights, as well as the dimensions of the dome and its elements for the investigated period. Taking into account large partial failures of the dome in 2005 (>0.11km3) and 2010 (0.28km3), we suggest that the volume of the extruded material for the whole 1980–2012 period was no less than 1.02km3. The average extrusion rate over the 2001–2012 period exceeded 225,000 m3/day. The transition from endogenous to exogenous dome growth was possibly caused by change in extruded material physical properties due to an increase of SiO2. On the basis of geomorphological analysis of the current lava dome features, we suggest the possible process of the exogenous dome formation at Molodoy Shiveluch. The crease structures detected at Molodoy Shiveluch were classified into three groups according to their shapes: radial, bilaterally symmetrical, and irregular. These crease structures are morphologically similar to those formed at Unzen Volcano during the 1990–1995 eruption. Some revealed morphological features of the crease structures show that the role of gravity is insignificant in their formation. We assume that the crease structures at Molodoy Shiveluch were formed by inner stresses due to thermal and solidification gradients.

Geochemistry and Sr-Nd-Pb isotopes of Monte Amiata Volcano, Central Italy: evidence for magma mixing between high-K calc-alkaline and leucititic mantle-derived magmas

Monte Amiata is a small volcano composed by trachytic to olivine latitic lava flows and domes emplaced in a very short time between 305 and 231 ka. The main petrographic features are represented by the occurrence of i) abundant rounded magmatic enclaves increasing in dimension and quantity passing from early to late erupted Monte Amiata volcanic rocks, ii) large sanidine megacrysts, mainly confined in the second stage of activity characterised by the emplacement of exogenous domes and massive lava flows, and iii) mafic olivine latitic lava flows, with intermediate compositions between the early silica-rich volcanic rocks and the most mafic rounded magmatic enclaves hosted by the Monte Amiata volcanic rocks. The occurrence of rounded magmatic enclaves testifies fresh magma injection and stirring within a differentiated magma reservoir. This triggered the pouring out of the viscous trachydacitic resident magma. A reverse differentiation pathway is observed with time of magma emplacement, which is accompanied by the decrease of silica contents and increase of MgO and compatible elements passing from early trachydacites to final olivine-latites. The same timely reverse differentiation pathway is observed among magmatic enclaves, with the most mafic terms hosted by final olivine-latitic lava flows. Fine-grained rounded magmatic enclaves, indeed, range in composition from potassic trachybasalt (absarokite) to olivine-latite. The overall geochemical and isotopic features agree with a mixing process between a highly differentiated (i.e., high silica), and partially crystallised, high-K calc-alkaline end- member and a mafic ultrapotassic magma possibly leucite-bearing. Absence of leucite in the Amiata rocks and enclaves is due to high-silica activity of derived magmas caused by the high-silica end-member of the mixing process.

Seismogenic frictional melting in the magmatic column

Abstract. Lava dome eruptions subjected to high extrusion rates commonly evolve from endogenous to exogenous growth and limits to their structural stability hold catastrophic potential as explosive eruption triggers. In the conduit, strain localisation in magma, accompanied by seismogenic failure, marks the onset of brittle magma ascent dynamics. The rock record of exogenous dome structures preserves vestiges of cataclastic processes and thermal anomalies, key to unravelling subsurface processes. Here, a combined structural, thermal and magnetic investigation of a shear band crosscutting a large block erupted in 2010 at Soufrière Hills volcano (SHV) reveals evidence of faulting and frictional melting within the magmatic column. The mineralogy of this pseudotachylyte vein offers confirmation of complete recrystallisation, altering the structure, porosity and permeability of the material, and the magnetic signature typifies local electric currents in faults. Such melting events may be linked to the step-wise extrusion of magma accompanied by repetitive long-period (LP) drumbeat seismicity at SHV. Frictional melting of Soufrière Hills andesite in a high velocity rotary shear apparatus highlights the small slip distances (< 15 cm) thought to be required to bring 800 °C magma to melting point at upper conduit stress conditions (10 MPa). We conclude that frictional melting is a common consequence of seismogenic magma fracture during dome building eruptions and that it may govern the ascent of magma in the upper conduit.

Enhanced crystal fabric analysis of a lava flow sample by neutron texture diffraction: A case study from the Castello d'Ischia dome

The crystal fabric of a lava has been analyzed for the first time by neutron texture diffraction. In this study we quantitatively investigate the crystallographic preferred orientation of feldspars in the Castello d'Ischia (Ischia Island, Italy) trachytic exogenous dome. The crystallographic preferred orientation was measured with the monochromatic neutron texture diffractometer SV7 at the Forschungszentrum Jülich in Germany and a Rietveld refinement was applied to the sum diffraction pattern. The complementary thin section analysis showed that the three‐dimensional crystal shape and the corresponding shape preferred orientation are in agreement with the quantitative orientation distributions of the neutron texture data. The (0k0) crystallographic planes of the feldspars are roughly parallel to the local flow bands, whereas the other corresponding pole figures show that a pivotal rotation of the anorthoclase and sanidine crystals was active during the emplacement of this lava dome. In combination with scanning electron microscopy investigations, electron probe microanalysis, XRF, and X‐ray diffraction, the Rietveld refinement of the neutron diffraction data indicates a slow cooling dynamic on the order of several months during their crystallization under subaerial conditions. Results attained here demonstrate that neutron texture diffraction is a powerful tool that can be applied to lava flows.

Rheology of arc dacite lavas: experimental determination at low strain rates

Andesitic–dacitic volcanoes exhibit a large variety of eruption styles, including explosive eruptions, endogenous and exogenous dome growth, and kilometer-long lava flows. The rheology of these lavas can be investigated through field observations of flow and dome morphology, but this approach integrates the properties of lava over a wide range of temperatures. Another approach is through laboratory experiments; however, previous studies have used higher shear stresses and strain rates than are appropriate to lava flows. We measured the apparent viscosity of several lavas from Santiaguito and Bezymianny volcanoes by uniaxial compression, between 1,109 and 1,315 K, at low shear stress (0.085 to 0.42 MPa), low strain rate (between 1.1 × 10−8 and 1.9 × 10−5 s−1), and up to 43.7 % total deformation. The results show a strong variability of the apparent viscosity between different samples, which can be ascribed to differences in initial porosity and crystallinity. Deformation occurs primarily by compaction, with some cracking and/or vesicle coalescence. Our experiments yield apparent viscosities more than 1 order of magnitude lower than predicted by models based on experiments at higher strain rates. At lava flow conditions, no evidence of a yield strength is observed, and the apparent viscosity is best approached by a strain rate- and temperature-dependent power law equation. The best fit for Santiaguito lava, for temperatures between 1,164 and 1,226 K and strain rates lower than 1.8 × 10−4 s−1, is $$ \log {\eta_{\text{app}}} = - 0.738 + 9.24 \times {10^3}{/}T(K) - 0.654 \cdot \log \dot{\varepsilon } $$ where η app is apparent viscosity and $$ \dot{\varepsilon } $$ is strain rate. This equation also reproduced 45 data for a sample from Bezymianny with a root mean square deviation of 0.19 log unit Pa s. Applying the rheological model to lava flow conditions at Santiaguito yields calculated apparent viscosities that are in reasonable agreement with field observations and suggests that internal shear heating may be significant ongoing heat source within these flows, enabling highly viscous lava to travel long distances.

Surface textures and dynamics of the 2005 lava dome at Shiveluch volcano, Kamchatka

Shiveluch is one of the largest and most active andesitic volcanoes of the Kuril-Kamchatka arc. It commonly alternates between Vulcanian explosive eruptions and periods of dome growth and subsequent dome collapse–driven block-and-ash flows. The volcano was in an extended period of heightened activity for most of the period 2004–2010. We examined this activity in detail using thermal infrared (TIR) remote sensing as part of the urgent request protocol (URP) program of the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instrument and confirmed the results with ground-based photography and airborne TIR camera data. High-spatial-resolution TIR images were collected during both daytime and nighttime satellite overpasses prior to and following the large explosive event/eruption of 27 February 2005 and the dome growth that followed. During a field campaign in August 2005, a helicopter overflight designed to acquire visible and TIR data of the active dome was performed. This was a nadir-looking, low-altitude overflight and the first ever of Shiveluch volcano involving non-Russian scientists. The image data revealed an active crease structure in the center of the dome with a distinctly different, crescent-shaped, high-temperature (>380 °C) zone roughly perpendicular to the crease. In order to provide a time context and estimate extrusion rates, the airborne data were compared to the spaceborne ASTER data and long-distance ground-based photography of the dome acquired by our Russian colleagues. The presence of a crease structure and the complex thermal pattern on the surface were both unexpected discoveries that reveal the way in which exogenous dome growth was occurring at the time. This highly active period at Shiveluch provides a unique example to better understand silicic lava dome growth using TIR data. The results also demonstrate a straightforward approach for fusing ground, air, and spaceborne image data, which could be applied to other active domes around the world.

Photogrammetric monitoring of lava dome growth during the 2009 eruption of Redoubt Volcano

The 2009 eruption of Redoubt Volcano, Alaska, began with a phreatic explosion on 15 March followed by a series of at least 19 explosive events and growth and destruction of at least two, and likely three, lava domes between 22 March and 4 April. On 4 April explosive activity gave way to continuous lava effusion within the summit crater. We present an analysis of post-4 April lava dome growth using an oblique photogrammetry approach that provides a safe, rapid, and accurate means of measuring dome growth. Photogrammetric analyses of oblique digital images acquired during helicopter observation flights and fixed-wing volcanic gas surveys produced a series of digital elevation models (DEMs) of the lava dome from 16 April to 23 September. The DEMs were used to calculate estimates of volume and time-averaged extrusion rates and to quantify morphological changes during dome growth.Effusion rates ranged from a maximum of 35m3s−1 during the initial two weeks to a low of 2.2m3s−1 in early summer 2009. The average effusion rate from April to July was 9.5m3s−1. Early, rapid dome growth was characterized by extrusion of blocky lava that spread laterally within the summit crater. In mid-to-late April the volume of the dome had reached 36×106m3, roughly half of the total volume, and dome growth within the summit crater began to be limited by confining crater walls to the south, east, and west. Once the dome reached the steep, north-sloping gorge that breaches the crater, growth decreased to the south, but the dome continued to inflate and extend northward down the gorge. Effusion slowed during 16 April–1 May, but in early May the rate increased again. This rate increase was accompanied by a transition to exogenous dome growth. From mid-May to July the effusion rate consistently declined. The decrease is consistent with observations of reduced seismicity, gas emission, and thermal anomalies, as well as declining rates of geodetic deflation or inflation. These trends suggest dome growth ceased by July 2009. The volume of the dome at the end of the 2009 eruption was about 72×106m3, more than twice the estimated volume of the largest dome extruded during the 1989–1990 eruption. In total, the 2009 dome extends over 400m down the glacial gorge on the north end of the crater, with a total length of 1km, width of 500m and an average thickness of 200m.

Metalogenia de oro y cobre en América central

Los depósitos de oro y cobre en América Central se encuentran dentro de rocas del Cretácico Tardío y más jóvenes del arco insular volcánico. Basaltos toleíticos bajos en potasio forman la base de la agrupa-ción volcánica y fueron depositados en un arco insular intraoceánico. El vulcanismo se tornó mas potásico conel tiempo, y ya para el Mioceno tardío, las andesitas calco-alcalinas dominan la secuencia volcánica. Rocas volcánicas alcalinas aparecieron hace unos cinco millones de años pero han sido subreportadas en areas más anti-guas del arco.Depósitos porfídicos de cobre se centran sobre plutones calco-alcalinos de composición intermedia, diorita cuar-cífera a diorita monzonítica y que típicamente son anómalas en oro. Aunque algunos de los depósitos porfídi-cos contienen más del billón de toneladas, ninguno está en producción. Vetas auríferas tipo bonanza se agrupanalrededor de intrusiones subvolcánicas en distritos que asemejan la distribución espacial de volcanes activos.Bonanzas de metales preciosos, algunos de los cuales produjeron por encima del millón de onzas de oro, sonresponsables por la mayor parte de la producción histórica en América Central. Se han venido descubriendo depósitos auríferos a lo largo de una inconformidad regional que marca el nacimiento del arco insular actual delocéano. A pesar de su reciente descubrimiento, algunos de estos depósitos ya exceden el millón de onzas. Lasrocas caja incluyen domos exógenos y los mantos piroclásticos que los encierran.Los esfuerzos exploratórios de hoy se concentran sobre depósitos de alto volumen: stockworks relacionados conlos pórfidos, depósitos de skarn, stockworks epitermales y depósitos diseminados. Fallas longitudinales regionales paralelas a la Depresión de Nicaragua controlan la distribución, tanto del oro como del cobre a través deCentroamérica. Dataciones proveen documentación para mineralización a lo largo de la historia del arco, un período de cerca de 100 millones de años.

Magnetic petrology of the 1991–1995 dacite lava of Unzen volcano, Japan: Degree of oxidation and implications for the growth of lava domes

To understand the oxidation state and process of oxidation of lava domes, we carried out magnetic petrological analyses of lava samples obtained from domes and block-and-ash-flow deposits associated with the 1991–1995 eruption of Unzen volcano, Japan. As a result, we recognize three different types of magnetic petrology, each related to deuteric high-temperature oxidation during initial cooling. Type A oxides are characterized by homogenous titanomagnetite and titanohematite, indicating a low oxidation state and high titanomagnetite concentrations. Type B oxides are weakly exsolved and contain titanohematite laths and rutile lenses, indicating a higher oxidation state. Type C oxides, which represent the highest oxidation state, are completely exsolved and composed of Ti-poor titanomagnetite, titanohematite, rutile, and pseudobrookite, indicating high hematite concentrations. Some grains in Types A and B show indications of reduction, which was related to interaction with volcanic gases subsequent to high-temperature oxidation. In terms of geological occurrence, the oxidation processes probably differed for endogenous and exogenous domes. Endogenous dome lavas are oxidized concentrically and are classified into the three types according to their location within the dome: samples from the surface are strongly oxidized and classified as Type C, while the inner part is unoxidized and classified as Type A. Exogenous dome lavas are unoxidized and assigned to Type A. Some samples show signs of reduction, which may have occurred around fumaroles. We propose that location within the dome and the process of dome growth are the factors that control oxidation.

ENDOGENOUS AND EXOGENOUS PLUTONS: THE INFLUENCE OF EMPLACEMENT STYLE ON CONTAMINATION OF GRANITIC MAGMA

Whether granitic magmas can detach and digest fragments of wallrock or mingle with other magmas depends in large part on the style of magma emplacement. Many composite plutons grow by relatively small pulses of magma, some of which may reach their solidus before injection of the next pulse. Composite “nested” plutons may grow as new pulses of magma invade and displace their predecessors, or “sheeted” plutons may grow as new magma follows contacts between wallrock and older intrusions. Contrary to a common assumption, the innermost parts of a composite intrusion are not necessarily the last to be emplaced or the most fractionated. Emplacement kinematics of some plutons and lava domes are similar. Lava domes grow in two ways. Endogenous domes inflate as new lava ascends and ponds within a cooler lava crust, and exogenous domes accrete as new lava ascends, breaches the lava crust, and spreads over the exterior. Exogenous growth of sheeted plutons offers less opportunity for magma mingling and mixing, but allows more wallrock assimilation (contamination) than endogenous growth of nested plutons.

Age constraints and characteristics of subaqueous volcanic construction, the Archean Hunter Mine Group, Abitibi greenstone belt

The Archean Hunter Mine Group (HMG) is a 6–7 km-thick south-facing volcanic sequence at the southern limit of the Northern Volcanic Zone, Abitibi greenstone belt. The HMG is composed of (1) felsic lava flows and endogenous and exogenous domes with autoclastic breccia deposits; (2) pyroclastic fire-fountain products and high- to low-concentration volcaniclastic sediment gravity flows of autoclastic and pyroclastic origin interstratified with iron-formation; (3) mafic pillowed and brecciated flows; (4) a prominent felsic dyke swarm; and (5) abundant mafic intrusions. The stratigraphy is complex, as inferred by detailed volcano-sedimentary facies mapping. A 5–7 km-wide, N-trending felsic dyke swarm, coupled with abundant synvolcanic and synsedimentary faults, chaotic breccias and mineralization suggest a central volcanic depression, which is consistent with a caldera structure. Voluminous effusive activity and explosive fire-fountaining eruptions contributed to caldera development. Rifting of this arc edifice is supported by the presence of felsic dykes and mafic tholeiitic sills. The lithofacies and overlying komatiites flows are consistent with a deep-water marine setting. U-Pb age determinations help constrain the HMG caldera formation to ≈6 million years. The sampled quartz–feldspar–phyric dykes yielded U-Pb zircon ages of 2728.3+4/−3.4 and 2728.9±0.8 Ma and together with a previously published dyke age of 2729.6±1.4 Ma from the same dyke swarm, a case is made that the swarm evolved over 1 million years (2728.6–2729.6 Ma). A lobate flow and flow breccia revealed similar ages of 2724.6+4.6/−1.7 and 2727.6+4.2/−2.0 Ma, whereas the up to 1 km-thick, gabbro-diorite Roquemaure sill precedes both felsic dykes and flows, as indicated by the 2731.8+2.2/−2.0 Ma age. The 2706.4+5.7/−4.3 Ma feldspar–quartz–phyric dykes occurred late in the history of the HMG and were not associated with subaqueous volcanic construction. The 207Pb/ 206Pb ages of analyzed zircon fractions have an inherited component within error of the oldest crystallization ages obtained for the HMG complex. Collectively, these age determinations and inheritance, as well as stratigraphy, support a tripartite division into a 2734–2730 Ma lower formational stage, a 2732 Ma middle formational stage and a 2730–2728 Ma upper formational stage. The study shows that age constraints without detailed facies mapping may lead to erroneous conclusions on volcanic construction and that felsic volcanic dykes and flows may inherit an earlier component of their own volcanic edifice.

Discrimination of lava dome activity styles using satellite-derived thermal structures

Thermal imaging from satellites is one methodology used for the monitoring and scientific investigation of volcanoes, including those characterised by active lava domes. To be most effective, the remote sensing techniques employed must allow the cause of any observed thermal anomaly to be identified, ideally using information contained within the remote sensing data itself, whilst using any ancillary field data to guide analysis and the necessary assumptions. This study investigates a method by which such discrimination maybe accomplished for activity at lava domes. For this purpose we use three Landsat Thematic Mapper (TM) scenes of Unzen Volcano (Japan) to determine the temperature structure of the active lava dome existing during 1991–1993. These data are of particularly high quality since the TM scenes were obtained during night-time overpasses and a large amount of supplementary data are available to parameterise the model used to retrieve the subpixel temperature structure of the dome surface. The results are matched to near-contemporaneous geological sketch maps of the then current situation in order to identify the TM pixel groupings corresponding to the different styles of dome activity, namely fumarolic degassing, exogenous and endogenous dome growth, and collapse deposits resulting from block and ash flows, small scale pyroclastic flows and rockfalls. The spatial and statistical characteristics of the resultant TM-derived temperature distributions are then investigated to determine rule-based criteria that may be used to differentiate the activity styles of lava domes based on their thermal structure within the remote sensing data. Results indicate that fumarolically active locations, regions of active dome growth, and areas of collapse deposition on the Unzen dome can be differentiated using only the statistical distribution of the hotspot temperatures and fractional areas identified via analysis of TM imagery. The statistics derived for hotspots at the fumarolically active areas show them to be, in general, significantly hotter (and smaller) than those found at the areas of active dome growth, whereas hotspots retrieved on the areas of collapse deposition are cooler (and larger). We find that the frequency distributions of hotspot temperatures extracted for the areas of endogenous and exogenous dome growth exhibit statistically inseparable means, but that discrimination between these two dome growth styles is possible using analysis of the spatial arrangement of the retrieved hotspots. Specifically the exogenous lobes are characterised by structures interpreted to be the effusing vent of new magma and the collapsing lobe front. These features are absent at locations showing only endogenous growth. It is hoped that the criteria developed here will prove useful during future quantitative analysis of the extended TM time-series available for Unzen, and for identification of thermal anomalies of uncertain origin at other active domes observed via satellite remote sensing.

Generation of block and ash flows during the 1990–1995 eruption of Unzen Volcano, Japan

Processes generating block and ash flows by gravitational dome collapse (Merapi-type pyroclastic flow) were observed in detail during the 1990–1995 eruption of Unzen volcano, Japan. Two different types were identified by analysis of video records and observations during helicopter flights. Most of the block and ash flows erupted during the 1991–1993 exogenous dome growth stage initially involved crack propagation due to cooling and flowage of the dome lava lobes. The mass around the crack became unstable, locally decreasing in tensile strength. Finally, a slab separated from the lobe front, fragmented progressively from the base to the top within a few seconds, and became a block and ash flow. Rock falls immediately followed, in response to local instability of the lobe front. Clasts in these rock falls fragmented and merged with the preceding flow. In contrast, block and ash flows during the endogenous dome growth stage in 1994 were generated due to local bulge of the dome. Unstable lava blocks collapsed and subsequently fragmented to produce block and ash flows.

Cerro Quemado, Guatemala: the volcanic history and hazards of an exogenous volcanic dome complex

Cerro Quemado is a Holocene, andesitic to dacitic, exogenous dome complex located in a back-arc volcanic field 5 km west of Quezaltenango, Guatemala. Activity in the back-arc Almolonga volcanic field began prior to 84,000 yr B.P. with growth and collapse of an andesitic stratovolcano and has continued to the present with sporadic eruption of silicic domes and flows. The Cerro Quemado dome complex covers about 12 km 2 , has a volume of nearly 2 km 3 , and comprises at least 8 vents. Historic activity is limited to extrusion of a blocky lava flow during a six-month period beginning in January 1818. Cerro Quemado lavas are biotite-bearing hornblende andesite and dacite. The calc-alkaline lavas have a disequilibrium mineral assemblage that includes: three populations of zoned plagioclase phenocrysts and microphenocrysts, hornblende, biotite, magnetite, two pyroxenes, olivine, and quartz. In addition, spherical inclusions (2 mm to 30 cm) of basaltic andesite (SiO 2 56 wt. %) are ubiquitous in Cerro Quemado lavas. this petrographic evidence points to an origin by mixing of felsic and mafic magma for Cerro Quemado. In the past, Cerro Quemado typically erupted viscous blocky lava flows (up to 120 m thick and 2.5 km long) and plug domes. The presence of tephra and pyroclastic flow deposits, however, indicates that explosive volcanism has also occurred at the dome. About 1150 yr B.P., the west flank of the dome gravitationally collapsed, producing a debris avalanche that underlies 13 km 2 of the Llano del Pinal valley, and avalanche caldera, 1 km × 1.5 km × 90 m deep. The caldera and avalanche deposit have identical volumes of approximately 0.1 km 3 . The collapse apparently precipitated a hot, laterally directed blast that swept about 40 km 2 of rugged terrain that includes Llano del Pinal valley and the east slopes of nearby Siete Orejas to an elevation of 3100 m. A pyroclastic flow overlying the fines-poor lateral blast deposit apparently resulted from remobilization and flow age of hot blast deposits off the steep slopes of Siete Orejas. Subsequently, a small Peleéan dome was emplaced in the upper part of the avalanche caldera. Following the catastrophic events of 1150 yr B.P., Cerro Quemado was dormant until 1818 A.D. Geologic mapping at Cerro Quemado volcano indicates that the principal volcanic hazards includes lava flow and exogenous dome growth, pyroclastic flow, debris avalanche, lateral blast, tephra fall, and mudflow. If Cerro Quemado erupts in the future Quezaltenango and environs (population more then 125,000) will be threatened.

Basement Structure and Implications for Hydrothermal Circulation Patterns in the Western Moat of Long Valley Caldera, California

Detailed surface mapping, subsurface drill hole data, and geophysical modeling are the basis of a structural and hydrothermal model for the western part of Long Valley caldera. Six fault zones are recognized in the western caldera with dominant orientations of north, northwest, and northeast, sub‐parallel to regional fault trends in the surrounding Sierran basement. The internal structural geometry of the cores of 12 exogenous domes inside and outside the caldera suggests that the domes erupted at the intersections of these principal fault trends rather than along the axis of a single dike. Gravity modeling and subsurface data from deep geothermal wells indicate that the floor of the caldera is segmented into a number of discrete fault blocks with varying offsets. One of the northeast trending fault zones, designated Discovery fault in this paper, appears to be part of the original Sierran embayment that existed before caldera collapse. Recent hydrothermal alteration occurs along Discovery fault strands and composite vertical offset of intracaldera volcanic units across the entire fault zone may be as much as 400 m. Field relationships, geophysical interpretations, and interpretaions of drill hole data suggest that this fault is a fundamental flaw in the western caldera. The preexisting tectonic framework of the basement rocks controlled the configuration of the western caldera floor and, through it, the location of postcollapse eruptive centers. These deep basement structures may also provide the high fracture density which controls circulation in the present geothermal system of Long Valley.

History and mechanism of eruption of soda-rhyolite and alkali basalt, Socorro Island, Mexico

Socorro Island is the summit of a large volcanic mountain located on the Clarion Fracture Zone in the east Pacific. Two major periods of volcanic activity can be recognized on the island. The first (pre-caldera) period was characterized by eruptions of olivine-poor alkali basalt, followed by quiet effusion of soda rhyolite including varieties transitional to pantellerite. This period of activity terminated with the formation of a caldera by collapse. A relatively prolonged period of quiescence ended with rifting and down-faulting of the western side of the island along a north-south fracture system, accompanied by violently explosive eruptions of soda rhyolite which built a large tephra cone over the position of the old caldera. The locus of eruptive activity moved outward and downward along tension fractures and old tectonic rifts as the central vents became blocked by domes of dense obsidian. Low level eruptions of viscous soda rhyolite including pantellerite commenced without preliminary explosive eruptions and built numerous endogenous and exogenous domes. Basaltic eruptions were rare and confined to low-level vents. During the growth of the volcano the direction of active rifting appears to have changed from east-west to northwest-southeast to north-south. Little is known of the submarine portion of the volcano, but the topography seems to reflect the three directions of rifting. The oldest submarine lavas are assumed to be basaltic and are probably of late Tertiary age. The eruptive history of Socorro suggests that the underlying magma column became stratified toward the end of the active period.