Main Vent Research Articles

Compositional variability in the ascending fluxes from a hydrothermal plume

Sequentially sampling sediment traps set with 33 day sampling intervals were deployed with current meters on three moorings in the northeast Pacific Ocean between July 1994 and May 1995. One mooring was deployed near the Main Vent field on Endeavour Ridge (On‐Axis site, 47°57.0′N, 129°05.7′W), a second, 3 km west of the Main vent site (West site), and the third, 43 km northeast of the Main vent site (East or background site). Ascending and descending particles were collected near 1600 and 2000 m depth, well above and within the top of laterally spreading hydrothermal plumes. The elemental composition of particles was used to evaluate their origins: biogenic fluxes were indicated by elevated Ca or Si, hydrothermal fluxes by elevated Fe, Mn, and Cu, and lithogenic fluxes by elevated Ti. We link temporal variability in both the ascending and descending particle composition and flux to variations in lateral transport of hydrothermal constituents and to seasonal drawdown of hydrothermal plume particles by biogenic material from the upper ocean. The relatively low hydrothermal Fe content of ascending material late in the experiment is thought to be due to uptake by descending biogenic material. These results suggest that seasonal productivity and particle export from the ocean surface can modulate the hydrothermal flux of elements to the waters above and to the sediments below.

Volcanic Sequence and Alteration at the Parys Mountain Volcanic-Hosted Massive Sulfide Deposit, Wales, United Kingdom: Applications of Immobile Element Lithogeochemistry

Lenses of massive Zn-Pb-Cu sulfides at the Parys Mountain deposit in Anglesey, northwestern Wales, occur at and near the contact between Ordovician shales and overlying rhyolites. The rhyolites, which have been dated as Lower Silurian (Parrish, 1999), have a thickness of a few hundred meters and are overlain by Lower Silurian shales. The rhyolites and shales strike northeasterly for 2 to 3 km and dip to the north. Minor basalts are present but there are no intermediate rocks. Cu-bearing stockwork veins occur in the upper portion of the Ordovician shales, and Cu-bearing mineralization was mined from the Lower Silurian shales in the 18th century. Precambrian basement rocks are present to the south and north of Parys Mountain. Application of immobile element lithogeochemical techniques to >1,000 drill core and outcrop samples allows division of the rhyolites into five main chemical types (A, B, C, D1, and D2), the shales into three main types, and the mafic rocks into two types. The distribution of these units has been mapped across the property. Where individual volcanic chemical units have lateral continuity, they are considered to represent singular geologic events and thus are time markers. Correlations based on volcanic facies alone are of limited use, because a chemical unit can pass laterally through different volcanic facies, whereas in strongly altered zones the original volcanic facies cannot be recognized at all. The chemostratigraphic units, on the other hand, can be traced through even the most altered zones. Consequently they provide an improved means of correlating volcanic units on local to property scales. The first volcanic rocks to accumulate above the Ordovician shales were thin flows and volcaniclastic beds of rhyolite B and a laterally adjacent lens of largely massive rhyolite C. The main massive sulfide lenses in the western part of the property are associated with these volcanic events. These units were covered by thick flows of rhyolite A in the western part of the property. Rhyolites D and A are the main chemical types in the eastern part. Rhyolite D has two subtypes: D1 is generally massive and flow banded, whereas D2 is pyroclastic. It is suggested that rhyolite D1 was erupted first, followed by less fractionated rhyolite D2, which was the last volcanic event prior to the accumulation of the Silurian shales. The main vents for rhyolite A in the west and D in the east were separated by about 1 km. The alteration minerals at Parys Mountain are sericite, chlorite, quartz, pyrite, and Fe-bearing carbonates. Mass changes calculated for the rhyolites and the shales show a general property-scale increase in alteration in the downdip direction. FeO and MgO additions of up to 10 and 20 wt percent, respectively, are present in footwall shales up to 50 m below the massive sulfide lenses (in areas where stockwork veins are present). Similar FeO and MgO gains occur in rhyolites up to 50 m above the ore lenses. In rhyolites with Fe + Mg gains, Si and K have generally been lost. In areas lateral to the massive sulfides, shales and rhyolite B have gained large amounts of silica. Most Parys Mountain rhyolites have moderately high Nb contents and slightly inclined rare earth element (REE) patterns, whereas the basalts are enriched in the high field strength elements (HFSE) including the light REE. The primary geochemical features of the volcanic rocks, together with regional geologic relationships in northwestern Wales, suggest that volcanism occurred during intraplate rifting of a small marine basin, which was underlain by Precambrian continental crust. The small volumes of all rhyolite types (and basalt) at Parys Mountain suggest that they were derived from small, discrete pods of magma, which were tapped by basement faults. Most of the massive sulfides accumulated immediately before, and during, the first outpourings of rhyolitic lava.

The physical volcanology of the 1600 eruption of Huaynaputina, southern Peru

Volcan Huaynaputina is a group of four vents located at 16°36'S, 70°51'W in southern Peru that produced one of the largest eruptions of historical times when ~11 km3 of magma was erupted during the period 19 February to 6 March 1600. The main eruptive vents are located at 4200 m within an erosion-modified amphitheater of a significantly older stratovolcano. The eruption proceeded in three stages. Stage I was an ~20-h sustained plinian eruption on 19–20 February that produced an extensive dacite pumice fall deposit (magma volume ~2.6 km3). Throughout medial–distal and distal parts of the dispersal area, a fine-grained plinian ashfall unit overlies the pumice fall deposit. This very widespread ash (magma volume ~6.2 km3) has been recognized in Antarctic ice cores. A short period of quiescence allowed local erosion of the uppermost stage-I deposits and was followed by renewed but intermittent explosive activity between 22 and 26 February (stage II). This activity resulted in intercalated pyroclastic flow and pumice fall deposits (~1 km3). The flow deposits are valley confined, whereas associated co-ignimbrite ash fall is found overlying the plinian ash deposit. Following another period of quiescence, vulcanian-type explosions of stage III commenced on 28 February and produced crudely bedded ash, lapilli, and bombs of dense dacite (~1 km3). Activity ceased on 6 March. Compositions erupted are predominantly high-K dacites with a phenocryst assemblage of plagioclase>hornblende>biotite>Fe–Ti oxides±apatite. Major elements are broadly similar in all three stages, but there are a few important differences. Stage-I pumice has less evolved glass compositions (~73% SiO2), lower crystal contents (17–20%), lower density (1.0–1.3 g/cm3), and phase equilibria suggest higher temperature and volatile contents. Stage-II and stage-III juvenile clasts have more evolved glass (~76% SiO2) compositions, higher crystal contents (25–35%), higher densities (up to 2.2 g/cm3), and lower temperature and volatile contents. All juvenile clasts show mineralogical evidence for thermal disequilibrium. Inflections on a plot of log thickness vs area1/2 for the fall deposits suggest that the pumice fall and the plinian ash fall were dispersed under different conditions and may have been derived from different parts of the eruption column system. The ash appears to have been dispersed mainly from the uppermost parts of the umbrella cloud by upper-level winds, whereas the pumice fall may have been derived from the lower parts of the umbrella cloud and vertical part of the eruption column and transported by a lower-altitude wind field. Thickness half distances and clast half distances for the pumice fall deposit suggests a column neutral buoyancy height of 24–32 km and a total column height of 34–46 km. The estimated mass discharge rate for the ~20-h-long stage-I eruption is 2.4×108 kg/s and the volumetric discharge rate is ~3.6×105 m3/s. The pumice fall deposit has a dispersal index (Hildreth and Drake 1992) of 4.4, and its index of fragmentation is at least 89%, reflecting the dominant volume of fines produced. Of the 11 km3 total volume of dacite magma erupted in 1600, approximately 85% was evacuated during stage 1. The three main vents range in size from ~70 to ~400 m. Alignment of these vents and a late-stage dyke parallel to the NNW–SSE trend defined by older volcanics suggest that the eruption initiated along a fissure that developed along pre-existing weaknesses. During stage I this fissure evolved into a large flared vent, vent 2, with a diameter of approximately 400 m. This vent was active throughout stage II, at the end of which a dome was emplaced within it. During stage III this dome was eviscerated forming the youngest vent in the group, vent 3. A minor extra-amphitheater vent was produced during the final event of the eruptive sequence. Recharge may have induced magma to rise away from a deep zone of magma generation and storage. Subsequently, vesiculation in the rising magma batch, possibly enhanced by interaction with an ancient hydrothermal system, triggered and fueled the sustained Plinian eruption of stage I. A lower volatile content in the stage-II and stage-III magma led to transitional column behavior and pyroclastic flow generation in stage II. Continued magma uprise led to emplacement of a dome which was subsequently destroyed during stage III. No caldera collapse occurred because no shallow magma chamber developed beneath this volcano.

GPR studies of pyroclastic deposits: Subsurface information where there are no outcrops

We have collected ground penetrating radar (GPR) data to evaluate its applications and limitations for obtaining subsurface information about pyroclastic deposits where there are no exposures. The surveys were conducted mainly in the Ubehebe hydro-volcanic field (Death Valley National Park, California, U.S.) that is composed of well preserved deposits left by base surges—turbulent flows generated by explosive interaction between magma and water. Base surges have high (i.e., magmatic) temperatures and move horizontally on the surface of the ground with hurricane velocity, causing extensive damage and loss of life. The main vent in this area, Ubehebe Crater, is a spectacular feature about 700–800 m in diameter and 235 m deep. Other, much smaller, tuff rings are south and west of the main crater.

Pressure suppression pool mixing in passive advanced BWR plants

In the SBWR passive boiling water reactor, the long-term post-accident containment pressure is determined by the combination of noncondensible gas pressure and steam pressure in the wetwell gas space. The suppression pool (SP) surface temperature, which determines the vapor partial pressure, is very important to overall containment performance. Therefore, the thermal stratification of the SP due to blowdown is of primary importance. This work looks at the various phases and phenomena present during the blowdown event and identifies those that are important to thermal stratification, and the scaling necessary to model them in reduced size tests. This is important in determining which of the large body of blowdown to SP data is adequate for application to the stratification problem. The mixing by jets from the main vents is identified as the key phenomena influencing the thermal response of the suppression pool and analytical models are developed to predict the jet influence on thermal stratification. The analytical models are implemented into a system simulation code, TRACG, and used to model thermal stratification behavior in a scaled test facility. The results show good general agreement with the test data.

The Lamongan volcanic field, East Java, Indonesia: physical volcanology, historic activity and hazards

The Lamongan volcanic field (LVF) in East Java, Indonesia, consists of up to 61 basaltic cinder or spatter cones, a minimum of 29 prehistoric maars, and a central compound complex comprising three main vents including the historically active Lamongan volcano. The field lies on the Sunda volcanic arc and abuts onto the currently active Tengger caldera–Semeru complex to the west. Between 1799 and 1898, Lamongan was persistently active and discharged up to 15 lava flows of basaltic to basaltic andesite composition, which is probably more than what was erupted by any other Indonesian volcano in that period. Several seismic crises have been recorded in the LVF since Lamongan's last eruption. There are no existing records of cinder cone or maar-forming eruptions in the area. Despite the possible hazards posed by future activity in the LVF, the history and structure of the field have been subject to little scrutiny. This contribution presents the results of a detailed morphometric and volumetric investigation of prehistoric vents and historic lava flows in the LVF, using field data, historic records, satellite imagery and modelling of erupted volumes. A typical LVF maar is characterised as a crater with a diameter of 450 m; generated by an eruption with a model ejecta volume of 0.016 km 3 dense rock equivalent (DRE) and a model juvenile component volume of 0.0096 km 3 DRE. Diameter/depth ratios of some maars display a morphological freshness comparable to recently formed phreatomagmatic eruption craters elsewhere. The median LVF cinder/spatter cone has a basal diameter of 600 m, a height of 75 m, a volume of 0.009 km 3 and a model total erupted volume of 0.014 km 3. Variation in cone basal diameter to height ratios indicates that the age of some prehistoric cones may not exceed a few centuries, and morphological classification identifies the youngest prehistoric vents on the northern flanks of the central edifice. Historic activity from vents on the western flanks of Lamongan produced at least ∼0.05 km 3 of basaltic to basaltic andesite lava. Resurfacing rates at Lamongan in the 19th Century imply an age of ∼900–2100 years for the surface of the volcano. The steady-state eruptive rate from 1843–1898 is evaluated as 0.025 m 3 s −1, which is low in comparison to estimates from many presently active systems. Using eruptive rate and volume data, the age of the LVF is estimated as between 13 and 40 ka, placing it among the youngest dated Indonesian volcanoes. Complex regional tectonics in conjunction with magmatic pressure have probably been the major influences on the distribution of volcanism in the LVF. The occurrence of maar-forming eruptions may have been controlled by a porous substrate, possibly a deposit from the Holocene Iyang–Argapura complex to the east. Volcanic hazards from future eruptions in the LVF include base surges, ash flows, ashfall and ejected blocks associated with phreatomagmatic activity, and ashfall and ejecta from magmatic eruptions.

Thermal observation of the 1986 eruption of Izu-Oshima volcano (Japan) using Landsat TM data

Surface temperatures of the erupted materials and fumaroles in the 1986 eruption of Izu-Oshima volcano, Japan, have been measured using Landsat TM data. A negative correlation between the band 6 pixel-integrated temperature and the thickness of the lava flows was observed. This temperature variation is probably controlled by the surface structure - thickness of the upper clinker in conjunction with cracks. Fumaroles, arranged in a U-shape around the main vent was observed. Their pixel-integrated temperatures decrease in proportion to distance from the vent.

Petrology of lavas from the Puu Oo eruption of Kilauea Volcano: III. The Kupaianaha episode (1986-1992)

The Puu Oo eruption has been remarkable in the historical record of Kilauea Volcano for its duration (over 13 years), volume (>1 km3) and compositional variation (5.7–10 wt.% MgO). During the summer of 1986, the main vent for lava production moved 3 km down the east rift zone and the eruption style changed from episodic geyser-like fountaining at Puu Oo to virtually continuous, relatively quiescent effusion at the Kupaianaha vent. This paper examines this next chapter in the Puu Oo eruption, episodes 48 and 49, and presents new ICP-MS trace element and Pb-, Sr-, and Nd-isotope data for the entire eruption (1983–1994). Nearly aphyric to weakly olivine-phyric lavas were erupted during episodes 48 and 49. The variation in MgO content of Kupaianaha lavas erupted before 1990 correlates with changes in tilt at the summit of Kilauea, both of which probably were controlled by variations in Kilauea's magma supply rate. These lavas contain euhedral olivines which generally are in equilibrium with whole-rock compositions, although some of the more mafic lavas which erupted during 1990, a period of frequent pauses in the eruption, accumulated 2–4 vol.% olivine. The highest forsterite content of olivines (∼85%) in Kupaianaha lavas indicates that the parental magmas for these lavas had MgO contents of ∼10 wt.%, which equals the highest observed value for lavas during this eruption. The composition of the Puu Oo lavas has progressively changed during the eruption. Since early 1985 (episode 30), when mixing between an evolved rift zone magma and a more mafic summit reservoir-derived magma ended, the normalized (to 10 wt.% MgO) abundances of highly incompatible elements and CaO have systematically decreased with time, whereas ratios of these trace elements and Pb, Sr, and Nd isotopes, and the abundances of Y and Yb, have remained relatively unchanged. These results indicate that the Hawaiian plume source for Puu Oo magmas must be relatively homogeneous on a scale of 10–20 km3 (assuming 5–10% partial melting), and that localized melting within the plume has apparently progressively depleted its incompatible elements and clinopyroxene component as the eruption continued. The rate of variation of highly incompatible elements in Puu Oo lavas is much greater than that observed for Kilauea historical summit lavas (e.g., Ba/Y 0.09 a–1 vs ∼0.03 a–1). This rapid change indicates that Puu Oo magmas did not mix thoroughly with magma in the summit reservoir. Thus, except for variable amounts of olivine fractionation, the geochemical variation in these lavas is predominantly controlled by mantle processes.

Hydrothermal venting at endeavour ridge: effect on zooplankton biomass throughout the water column

Bio-acoustical data reveal that the hydrothermal plume emanating from the main vent field near 2100 m depth on Endeavour Ridge (47°57′N, 129°06′W) affects the distribution and migration of zooplankton throughout the entire water column. Net samples taken in July of 1991 and 1992 show that the standing stock of macrozooplankton integrated over the water column was considerably higher within several kilometers of the main vent site than at locations tens of kilometers from the vent site. Community analysis reveals that there were distinct shallow (<800 m depth) and deep (>800 m depth) faunal assemblages in the vent region. Shallow fauna infiltrated the deep zooplankton acoustic scattering layers in the immediate vicinity of Endeavour Ridge, producing a mixed assemblage of animals, including large numbers of juvenile filter-feeding copepods and their predators that normally inhabit the shallow layer. In contrast, the deep acoustic scattering layers found 11 km to the southeast and 15 km to the north of the central vent field in 1991, and 50 km to the west of the central vent field in 1992, were composed of distinctly deep-sea fauna. The enhanced, vertically-integrated biomass over the vent region appears to result from vertical migration of zooplankton. A simple circulation model indicates that fauna can make the round-trip journey between the top of the plume and the upper ocean without being advected beyond the range of the detectable hydrothermal effluent.

A petrologic re-investigation of the Adak volcanic center, central Aleutian arc, Alaska

The Adak volcanic center is located in the central part of the Aleutian arc and consists of three main volcanic vents. Andrew Bay Volcano, the oldest center, has been mostly removed by erosion. The next youngest vent, Mount Adagdak, was built in three major volcanic stages whereas Mount Moffett, the largest volcanic edifice, consists of a main cone and a parasitic cone each with several magmatic phases. Adak is unique compared to other modern Aleutian volcanic centers in that it contains two xenolith suites (Conrad and Kay, 1984; Debari et al., 1987). One suite consisting predominantly of mafic xenoliths occurs on Mount Moffett whereas an assemblage of ultramafic and mafic xenoliths is found on Mount Adagdak. Lavas erupted at Adak span the compositional range from 48.4 to 65.0 wt.% SiO2 and are characterized by significant variations in Al2O3, MgO, Sr, Ni and Cr. On Harker diagrams, this variability produces compositional trends with significant scatter. The Adak suite has total REE contents that vary from 32 to 154 ppm but do not correlate systematically with silica. (LaYb)n ratios range from 2.41 to 21.72 with the majority of lavas between 2.41 and 6.06. On process identification diagrams, the Adak suite plots as steeply sloping trends that contrast with the horizontal patterns of most other Aleutian centers. Measured isotopic ranges are large and nearly equal to those for the entire arc. Although they span similar silica ranges, subtle geochemical and isotopic differences distinguish the different volcanic vents of Adak. On Mount Moffett, a geochemically and isotopically distinct group of andesites (55.5–57.9 SiO2), the mafic andesites, occur on its NE flank. These lavas have elevated MgO, Ni and Cr but are depleted in Al2O3 relative to other Mount Moffett andesites with similar silica. They also have more heterogeneous REE abundances and isotopic ratios than most of the other andesites. Significant compositional differences exist between Adak and the other volcanic centers of the central Aleutian arc. Although these differences are characteristic of all geochemical systems, they are greatest for major and rare-earth elements and isotopic ratios. The lack of coherent relationships on major- and trace-element Harker diagrams, the isotopic variability, as well as the steeply sloping trends on REE process identification diagrams suggest that the Adak volcanic suite was not formed predominantly by closed-system crystal fractionation, but must be the product of a complex open-system process(es). The significant isotopic variability displayed by the suite suggests that contamination by an isotopically distinct contaminant must also have been an important petrologic component in the evolution of the suite. REE data are also suggestive of a role for magma mixing. Such a complex petrologic evolution is consistent with an immature lithospheric plumbing system. Based on REE systematics, the xenolith suites of Adak cannot, as previously proposed, be related to the host lavas or the rest of the Adak suite through crystal fractionation schemes. Rather they are probably accidental fragments derived from various depths along lithospheric conduits. In light of their relation to xenolith-bearing units, the mafic andesites of Adak presumably represent hybrid magmas formed during the interaction of ascending magmas with lithospheric wall rock. They are, therefore, characteristic of immature volcanic centers and unlikely to be related directly to the magmatic processes responsible for the generation of primary arc magmas. Because of the close proximity of the vents and the subtle compositional differences between their lavas, the Adak volcanic center was probably supplied by a single, deep lithospheric plumbing system that fed separate crustal magma chambers. The absence of historic volcanic activity on Adak suggests this plumbing system was abandoned before complete conduit development. This decline in magmatism may reflect a re-adjustment of volcano spacing within this part of the Aleutian arc.

Io's volcanic and sublimation atmospheres

Computer simulations of SO 2 and H 2S frost sublimation and SO 2 day and nightside volcanic atmospheres on Io are presented. The models include a surface temperature which is a function of distance from the volcanic vent or subsolar point, sublimation and condensation with a sticking coefficient of unity, and a crude radiative transfer approximation. The full three-dimensional, axisymmetric gas dynamic equations are solved numerically using a time explicit, finite volume formulation. Neither a SO 2 frost sublimation atmosphere nor a SO 2 Pele-type eruption centered on the dayside extends into the nightside. However, if the volcanic emissions contain a gas with a lower condensation rate than SO 2, for example H 2S, then a Pele-type volcanic eruption centered on the dayside can reach the terminator with the pressure required by the Pioneer 10 radio occultation measurements. Also, the atmosphere generated by a SO 2 Pele-type volcano located at a distance of ∼400 km from the terminator can extend to the terminator with the required Pioneer 10 radio occultation pressure. An extended nightside atmosphere could be formed by horizontal flow from multiple volcanoes located on the nightside. Both sublimation and volcanic atmospheres produce horizontal supersonic winds away from the subsolar point or the volcanic vent. The sublimation atmosphere is driven mainly by horizontal pressure gradients determined by surface temperatures. The volcanic atmosphere is driven by pressure gradients determined by the source rate. The near surface momentum flux for a dayside Pele-type volcanic atmosphere is consistent with observations of winds blowing vent emissions at 700 km from the main vent. The volcanic atmosphere features a shock that forms as the gas falls supersonically toward Io's surface. The volcanic nightside shock comes closer to Io's surface than the dayside shock. No shock develops in the sublimation atmosphere. Sublimation and condensation produce patterns of surface deposits which are characteristic of the two types of atmospheres. A sublimation atmosphere develops a resurfacing band pattern. Sublimation removes mass from an equatorial band of ∼±27° of Iographic latitude, while condensation deposits mass poleward of ±27° latitude. The maximum condensation rate per Io rotation period occurs at ∼50° Iographic latitude and the rate decays to zero at ∼80° latitude. Volcanic atmospheres have condensation deposits in the form of rings that fall within the particulate plume and are caused by plume shock conditions. The volcanic model is quantitatively consistent with Voyager observations of ring deposits. Volcanic eruptions expose SO 2 and other possible gaseous constituents of the plume such as sodium to sputtering by corotating torus ions in times very short (∼ 1 hr) compared to diffusion and dissociation times. Thus, the source of neutrals to the Jovian magnetosphere reflects the volcanic source composition and meets the constraints imposed by multiple torus observations, particularly the ratio of oxygen to sulfur. Furthermore, the elevation of the exobase above the surface in the vicinity of volcanic vents may locally increase the neutral sputtering rate if the diversion of magnetospheric plasma flow around Io is considered.

The contribution of sulfate to rainfall pH around Kilauea Volcano, Hawaii

Between December 1986 and June 1987, the mean pH of rainfall downwind of the Kilauea main vent was found to be 4.5 (range 4.0 to 5.6), 1.2 units higher than the year before (1985-86), although 84% of the 12 sequential samples fell below pH 5.0. The SO4 content, however, was 34% higher, averaging 18.5 mg L−1. Upwind, in open forest the mean pH 4.7 was little changed from that measured before. Mean SO4, however, has fallen to a low of 2.5 mg L−1, but, more significantly, in 9 out of 12 sequential samples S04 was not detectable at all (i.e. < 0.5 mg L−1). The calculated pH, assuming 100% H2SO4 would be 5.3 yet 58% of these samples fell below pH 5.0, the lowest being 4.0. Disparities between pH measured and calculated on the basis of SO4 content indicated that other acid species were present in the precipitation. Oxidation of rain samples with H2O2 greatly increased SO4 content and lowered pH downwind, but failed in most samples to alter either parameter in the upwind collections. These observations, together with the elimination of HCl and N03 by others, suggested that SO2 contributes significantly to acidity downwind, but that in most upwind samples a source of H+ other than mineral acids, presumably organic compounds, must be of major importance.

Changes in the geomagnetic total intensity observed before the eruption of Oshima Volcano in 1986.

Before the eruption of Oshima Volcano on November 15, 1986, 5 proton magnetometers were operating on the volcano. One of them, installed about 500m south from the center of the central crater, recorded anomalous changes in the total intensity preceding the eruption. The anomalous variation began around 1981. The total intensity decreased gradually at a rate of -5.3nT/year (-0.44nT/month), amounting to a 27nT decrease in 5 years. The decrease of the total intensity accelerated about six months before the eruption. The rate of decrease was estimated to be about -2.2nT/month. At the time of the eruption, a very fast and significant variation was observed. The total intensity decreased 4nT in one minute. Following the eruption from the central cone, fissure eruptions took place on November 21. A decrease in the total intensity of about 7nT was also observed at this time at a site 2km southeast of the central cone.The observed magnetic changes preceding the eruption are supposed to have been caused by the volcanic body being heated during the intrusion of magma, at the initial stage through the main vent, and at the later stage through a new path toward the newly formed crater, called A crater. The variation at the time of eruption is considered due to the disorientation of magnetic materials by the catastrophic outflow of gases and liquids through pores and cracks inside the volcano.

Late Miocene volcanic stratigraphy of central Banks Peninsula, Canterbury, New Zealand

Abstract Banks Peninsula consists of Late Miocene alkalic to transitional volcanic rocks erupted from two major composite volcanoes (Lyttelton and Akaroa) and a host of minor monogenetic vents. Four volcanic groups are defined in this paper: Lyttelton (11-9.7 Ma), Mt Herbert (9.7-8.0 Ma), Akaroa (9.0-8.0 Ma), and Diamond Harbour (7.0-5.8 Ma). Three further formations, which are petrographically distinct and volumetrically minor, were erupted between 8.1 and 7.3 Ma, marking the transition from Akaroa to Diamond Harbour volcanism. Eruption of 350 km3 of mildly alkalic to transitional hawaiite to trachyte lavas of the Lyttelton Volcanic Group constructed a major composite volcano which forms the northwestern half of the peninsula. Late in the history of activity, the Lyttelton crater was breached in the southeast. The Mt Herbert Volcanic Group, comprising 100 km3 of mildly alkalic lavas and pyroclastic rocks, was then erupted from two main vents on the southeastern segment of Lyttelton Volcano. It consists o...

First estimate of annual mercury flux at the Kilauea main vent

Mercury is the principal metal emitted in vapour form by volcanoes and other natural geothermal sources. Direct evidence for the release of mercury at volcanic, fumarolic and hydrothermal sites has been obtained in Hawaii, Iceland, Antarctica and the Pacific north-west1–6, and the relationship of mercury to volcanic activity elsewhere has been inferred from the analysis of indicator plants7,8. The relation of volcanogenic mercury to total atmospheric content has been a matter of some uncertainty. From 1971 to 1980, mercury and sulphur dioxide analyses were carried out on air samples collected immediately downwind of Halemaumau, the Kilauea main vent, in Hawaii. From these measurements, a Hg/SO2 ratio of 0.51 × 10−3 was derived. As reported here, applying this ratio to the recently determined sulphur dioxide mass output of Halemaumau, yields a calculated mercury flux of 2.6 × 108g annually. This value is consistent with recent evidence6 suggesting that volcanogenic mercury contributes significantly to the atmospheric total.

Origin and evolution of Lord Howe Island, Southwest Pacific Ocean

Abstract Lord Howe Island is the eroded remnant of a large shield volcano. Tholeiitic lavas of the North Ridge Basalt comprise the main shield building phase and were erupted about 6.9 Ma ago. The Boat Harbour Breccia probably formed within the throat of the volcano and, together with the North Ridge Basalt, is intruded by numerous basaltic dykes, which grade into a cone sheet complex near the main vent. Large scale collapse of the summit area of the volcano produced a caldera which was filled rapidly by lavas of the Mount Lidgbird Basalt some 6.4 Ma ago, bringing to a close the volcanic history of Lord Howe Island. The shield volcano thus was built during a short interval in the late Miocene. Palaeomagnetic data show that the North Ridge Basalt and the Mount Lidgbird Basalt were erupted during periods when the geomagnetic field had normal polarity, and that their formation was separated by at least one interval of reversed polarity when the dykes and cone sheets were emplaced. The directions of magnetisa...

Role of lava tubes in flows from the Observatory Vent, 1971 Eruption on Mount Etna

SummaryThe role of tubes in aa flows may be more important than previously recognized. On Etna they are commonly found in the near-vent areas of aa flows. A study of tubes associated with the 1971 Observatory Vent on Etna suggests that ephemeral boccas on the flow surface downslope of the main vent were fed by tubes.

The 1974 eruption of mount etna

The 1974 Etna eruption that occurred on the lower, west part of the volcano is rather exceptional as it appeared independent of the central main vent activity. The products of this eruption also differ from those commonly emitted throughout historic times. They are almost aphyric and display an unusual order of crystallization, clinopyroxene being a near liquidus phase instead of plagioclase that is typical in all other recent lavas. The chemical composition is also slightly more basic than that of pyroclastics contemporaneously erupted by the summit Central Crater.

Mount Etna and the 1971 eruption - The summit of Mount Etna prior to the 1971 eruptions

The summit cone of Mt Etna is cut by an approximately NNE-SSW trending, narrow fissure zone that controls much of the terminal activity of this volcano. Within the central crater, which lies at the top of the summit cone, there are two main vents alined on this fissure zone: the chasm, a deep pit; and the 1964 crater. Both of these vents were blocked before 1971. The NE crater first opened in 1911 on the lower slopes of the summit cone on the same fissure zone, and has had an open conduit from which lava has been erupted at a constant rate from 1966 to the beginning of the 1971 eruption. In 1968 a small gas bocca known as the Bocca Nuova opened near a fissure that had previously erupted lavas in 1956. The Bocca Nuova collapsed in early 1970 to give a 100 m wide crater. It is suggested that collapse resulted from movement of magma at depth into the 1956 fissure causing sudden lowering in the magma column below the Bocca, and that continued migration of magma into the fissure eventually lead to the eruption of April 1971, the site of this eruption being in line with the fissure. A rise in temperature of fumaroles at the edge of the central crater after the Bocca Nuova collapse is attributed to this incursion of magma across the crater.

Acid volcanoes of San Miguel, Azores

The trachytic volcanoes of San Miguel Island are emplaced on either side of a young basaltic area, which could correspond to some active branch of the mid-Atlantic rift. Geological and geomagnetic surveys suggest crustal drift of about 1 cm/year on either direction. From the seismic anomaly data, shallow magma chambers have been inferred for those acid volcanoes, where owing to the crustal drift the main vents appear to have been shifted relatively to the fractures feeding the chambers from the deep upper mantle supply. The primitive magma is certainly basaltic; when the surface vents are directly above the deep feeding fractures, only basaltic lavas of some primitive composition can be extruded; but, when the upper vents are laterally displaced, the magma is delayed in the chambers and will differentiate, eventually into trachytic material capable of producing huge pumice explosions.