Tree-ring evidence for 1842–1843 eruptive activity at the Goat Rocks dome,Mount St. Helens,Washington

Until the 18 May 1980 eruption of Mount St. Helens, a debris fan and adjacent forest downslope from the dacitic Goat Rocks dome, on the north flank of the volcano, contained evidence that the dome was active in 1842 or 1843. The fan was destroyed by the debris avalanche of 18 May. Before 1980, the oldest tree cored on the debris fan showed that the fan predated 1855 by a few years. The young age of this tree suggests that the dome was active several decades after extrusion of the nearby andesitic floating island lava flow, dated to 1800. An anomalous series of narrow rings that starts with the 1843 ring is present in cores from two older trees adjacent to the fan. These ring-width patterns imply that these trees were damaged in late 1842 or early 1843 by flowage material from the dome; the trees were probably singed by an ash-cloud surge that originated on the dome as a hot-rock avalanche. Several lines of evidence suggest that the anomalous ring patterns record tree injury by surge, rather than by lahars or nonvolcanic causes (climate or insects). First, comparable ring patterns formed in all sampled trees that survived the 18 May surge, but formed in only a few sampled trees abraded or partially buried by 18 May lahars. Second, a 13-cm fine-ash layer, consistent with either tephra fall or surge emplacement, was present on the 1840s forest floor; yet the lack of similar tree-ring responses to 1980 tephra fall shows that such minor tephra fall could not have caused the ring patterns. Third, identical 1843 narrow-ring patterns are absent in control trees further from the volcano. The ring patterns of the trees adjacent to the Goat Rocks fan provide the first field evidence that the dome was active in late 1842 or early 1843. Thus, the new tree-ring dates confirm stratigraphic evidence for the youth of the activity of the Goat Rocks dome. They also link historical accounts of mid nineteenth century volcanism at Mount St. Helen with continuing dome extrusion. The dates additionally corroborate and revise the dacite-andesite-dacite petrologic cycle interpretation of Mount St. Helens' Goat Rocks eruptive period (1800–1857). They constrain the cycle to no more than 43 years. Lastly, the dates support the notion that the vent that erupted the 1800 dacitic T tephra was different from the one that produced the Goat Rocks dome. We infer that the magma that formed the floating-island lava flow plugged the T tephra vent. This forced residual magma from the compositionally zoned magma chamber into an alternate conduit. The second conduit produced the unnamed 1842 lithic tephra and the Goat Rocks dome.     

A new tree-ring date for the “floating island” lava flow,Mount St. Helens,Washington

Anomalously narrow and missing rings in trees 12 m from Mount St. Helens' floating island lava flow, and synchronous growth increases in trees farther from the flow margin, are evidence that this andesitic flow was extruded between late summer 1799 and spring 1800 a.d., within a few months after the eruption of Mount St. Helens' dacitic layer T tephra. For ease of reference, we assign here an 1800 a.d. date to this flow. The new date shows that the start of Mount St. Helens' Goat Rocks eruptive period (1800–1857 a.d.) resembled the recent (1980–1986) activity in both petrochemical trends and timing. In both cases, an initial explosive eruption of dacite was quickly succeeded by the eruption of more mafic lavas; dacite lavas then reappeared during an extended concluding phase of activity. This behavior is consistent with a recently proposed fluid-dynamic model of magma withdrawal from a compositionally zoned magma chamber.     

Multiphase petrography of volcanic rocks using element maps: a method applied to Mount St. Helens, 1980–2005

Quantitative textural analyses including crystal size distributions (CSDs) provide insights into crystallisation kinetics of magmatic systems. Investigations of volcanic crystal textures often rely on greyscale variations on backscattered electron images to identify crystal phases, which must then be thresholded and/or traced manually, a laborious task, and investigations are typically restricted to a single crystal phase. A method is presented that uses energy-dispersive X-ray element maps to generate textural data. Each pixel is identified as a crystal phase, glass or vesicle according to relative chemical composition enabling concurrent acquisition of multiphase CSD, crystallinity and mineral mode data. Data processing is less time intensive for the operator but considerable instrument time is required to generate element maps. The method is applied to 17 dacite samples from the 1980–1986 and 3 from the 2004–2005 eruptive periods of Mount St. Helens volcano (USA) to provide quantitative insights into multiphase textural evolution. All of the CSDs are curved and concave-up in the standard CSD plot with curvature increasing with plagioclase content. To facilitate comparisons with previous studies, CSDs for microlites (<50?μm length crystals) are approximated as straight lines. The line intercepts and slopes provide information on n ⁰ (nucleation density) and characteristic length or Gτ (the product of growth rate (G) and residence time (τ)), respectively. These parameters, as well as the total groundmass crystallinity, show distinct differences between explosive deposits from summer 1980 and post-summer 1980 domes. Post-summer 1980 microlite n ⁰ values are mostly at the lower end of the range of those measured for summer 1980 samples. Total groundmass crystallinities during summer 1980 are between 10 and 30?vol.%, whereas post-summer 1980 crystallinity increases to between 39 and 51?vol.%. The range of n ⁰ values is similar to those previously published for Mount St. Helens but Gτ is consistently higher. Gτ of a May 1985 sample analysed in this study is approximately 2?μm higher compared with previously published data for the same sample when processed using similar methodologies. Groundmass crystallinity data show similar trends to those previously published for the 1980 to 1986 eruption, increasing sharply after summer 1980 then increasing more gradually during the dome-building phase of the eruption. The effects of varying L, the apparent crystal size, and crystal aspect ratio on resultant CSDs are also investigated. Whilst relative temporal variations in CSDs for a given dataset can be investigated, absolute values from different studies cannot be compared unless methods of data acquisition and processing are exactly the same.     

Crystallization of microlites during magma ascent: the fluid mechanics of 1980–1986 eruptions at Mount St Helens

Eruptions of Mount St Helens (Washington, USA) decreased in intensity and explosivity after the main May 18, 1980 eruption. As the post-May 18 eruptions progressed, albitic plagioclase microlites began to appear in the matrix glass, although the bulk composition of erupted products, the phenocryst compositions and magmatic temperatures remained fairly constant. Equilibrium experiments on a Mount St Helens white pumice show that at 160 MPa water pressure and 900°C, conditions deduced for the 8 km deep magma storage zone, the stable plagioclase is An₄₇. The microlites in the natural samples, which are more albitic, had to grow at lower water pressures during ascent. Isothermal decompression experiments reported here demonstrate that a decrease in water pressure from 160 to 2 MPa over four to eight days is capable of producing the albitic groundmass plagioclase and evolved melt compositions observed in post-May 18 1980 dacites. Because groundmass crystallization occurs over a period of days during and after decreases in pressure, microlite crystallization in the Mount St Helens dacites must have occurred during the ascent of each magma batch from a deep reservoir rather than continuously in a shallow holding chamber. This is consistent with data on the kinetics of amphibole breakdown, which require that a significant portion of magma vented in each eruption ascended from a depth of at least 6.5 km (160 MPa water pressure) in a matter of days. The size and shape of the microlite population have not been studied because of the small size of the experimental samples; it is possible that the texture continues to mature long after chemical equilibrium is approached. As the temperature, composition, crystal content and water content of magma in the deep reservoir remained approximately constant from May 1980 to at least March 1982, the spectacular decrease in eruption intensity during this period cannot be attributed to changes in viscosity or density of the magma. Simple fluld mechanical considerations indicate, however, that the observed changes in mass flux of magma can be modelled by a five-fold decrease in conduit radius from 35 to 7 m, produced perhaps by plating of magma along the conduit walls. The decreased ascent rates which accompanied the decrease in conduit radius can explain the change from closed-system to open-system degassing and the shift from explosive to effusive eruptions during 1980.     

Petrology of lavas from the 2004–2005 flank eruption of Mt. Etna,Italy: inferences on the dynamics of magma in the shallow plumbing system

Following the 2001 and 2002–2003 flank eruptions, activity resumed at Mt. Etna on 7 September 2004 and lasted for about 6 months. This paper presents new petrographic, major and trace element, and Sr–Nd isotope data from sequential samples collected during the entire 2004–2005 eruption. The progressive change of lava composition allowed defining three phases that correspond to different processes controlling magma dynamics inside the central volcano conduits. The compositional variability of products erupted up to 24 September is well reproduced by a fractional crystallization model that involves magma already stored at shallow depth since the 2002–2003 eruption. The progressive mixing of this magma with a distinct new one rising within the central conduits is clearly revealed by the composition of the products erupted from 24 September to 15 October. After 15 October, the contribution from the new magma gradually becomes predominant, and the efficiency of the mixing process ensures the emission of homogeneous products up to the end of the eruption. Our results give insights into the complex conditions of magma storage and evolution in the shallow plumbing system of Mt. Etna during a flank eruption. Furthermore, they confirm that the 2004–2005 activity at Etna was triggered by regional movements of the eastern flank of the volcano. They caused the opening of a complex fracture zone extending ESE which drained a magma stored at shallow depth since the 2002–2003 eruption. This process favored the ascent of a different magma in the central conduits, which began to be erupted on 24 September without any significant change in eruptive style, deformation, and seismicity until the end of eruption.     

Lava channel roofing,overflows, breaches and switching: insights from the 2008–2009 eruption of Mt. Etna

During long-lived basaltic eruptions, overflows from lava channels and breaching of channel levées are important processes in the development of extensive 'a'ā lava flow-fields. Short-lived breaches result in inundation of areas adjacent to the main channel. However, if a breach remains open, lava supply to the original flow front is significantly reduced, and flow-field widening is favoured over lengthening. The development of channel breaches and overflows can therefore exert strong control over the overall flow-field development, but the processes that determine their location and frequency are currently poorly understood. During the final month of the 2008–2009 eruption of Mt. Etna, Sicily, a remote time-lapse camera was deployed to monitor events in a proximal region of a small ephemeral lava flow. For over a period of ~10 h, the flow underwent changes in surface elevation and velocity, repeated overflows of varying vigour and the construction of a channel roof (a required prelude to lava tube formation). Quantitative interpretation of the image sequence was facilitated by a 3D model of the scene constructed using structure-from-motion computer vision techniques. As surface activity waned during the roofing process, overflow sites retreated up the flow towards the vent, and eventually, a new flow was initiated. Our observations and measurements indicate that flow surface stagnation and flow inflation propagated up-flow at an effective rate of ~6 m h⁻¹, and that these processes, rather than effusion rate variations, were ultimately responsible for the most vigorous overflow events. We discuss evidence for similar controls during levée breaching and channel switching events on much larger flows on Etna, such as during the 2001 eruption.     

Insights into eruption dynamics from textural analysis: the case of the May, 2008, Chaitén eruption

The May, 2008, Chaitén (southern Chile) eruption was characterized by several explosive events, each associated with plumes which reached up to about 19?km above sea level on May 6. A study of the textural and physical features of the juvenile clasts erupted during the climactic phase of the 2008 eruption of Chaitén is presented. Pumice clasts show unimodal density distribution (main mode at 600?kg/m³), average vesicularity of about 69?%, a glassy groundmass with no microcrystals, and vesicles with dimension between ～1?μm and ～2?mm. They also show a unimodal vesicle size distribution with most frequent vesicle size in the range 0.05–0.08?mm and an estimated vesicle number density of 1.3?±?0.5?×?10⁵?mm^?3 related to a rapid nucleation event produced during the late phases of magma rise. This is confirmed by the absence of microcrystals that could otherwise have delayed vesicle formation and allowed the magma to maintain a low viscosity and a supersaturation in volatiles. Vesiculation and fragmentation were triggered by a sudden decompression of the melt associated with the opening of the volcanic conduit (～10?MPa?s^?1).     

Twelve years of ground-tilt measurements on the Soufriere of St. Vincent, 1977–1989

Five ground-tilt stations were established on the flanks of the Soufriere of St. Vincent; two in 1977, two in 1980, and one in 1983. Four of these stations have survived; two consist of lines oriented radially to the volcano, and the other two consist of cross-shaped arrays. Collectively, this network showed that the volcano inflated gradually before the eruption of 1979 and then deflated rapidly during the eruption and for about a year after it had ended. The volcano then reinflated during much of 1981 and inflated only slightly, if at all, during the 7-year period 1982–1989. The measured amount of ground tilt from 1977 to 1989 was relatively small; the maximum recorded change of radial tilt at a station 6.5 km from the summit of the volcano totaled about 20 rad. Local seismicity correlates well with measured ground deformation: numerous earthquakes accompanied the 1979–1981 deflation/inflation cycle; relatively few earthquakes occurred during the 1982–1989 period of little or no ground deformation. In the hope that the experience we have gained might be of use to others who are considering a program of ground-tilt monitoring on volcanoes elsewhere in the humid tropics, we describe many practical aspects of our program that have evolved over the years. For example: (1) stainless steel bench marks cemented into solid bedrock appear to be stable for at least 12 years, (2) bench marks can be located in concrete-filled drums where bedrock is absent, (3) bench marks should be concealed to protect them from vandalism, (4) care must be exercised where sight lines graze the ground (<0.5 m), and (5) automatic levels are preferable because of their efficiency of operation, especially with inexperienced personnel.     

Emplacement and inflation of natrocarbonatitic lava flows during the March–April 2006 eruption of Oldoinyo Lengai,Tanzania

The most voluminous eruption of natrocarbonatite lava hitherto recorded on Earth occurred at Oldoinyo Lengai in March–April 2006. The lava flows produced in this eruption range from blocky 'a'a type to smooth-surfaced inflated pahoehoe. We measured lava inflation features (i.e. one tumulus and three pressure ridges) that formed in the various pahoehoe flows emplaced in this event. The inflation features within the main crater of Oldoinyo Lengai are relatively small-scale, measuring 1-5 m in width, 2.5–24.4 m in length and with inflation clefts less than 0.4 m deep. Their small sizes are in contrast to a tumulus that formed on the northwestern slope of the volcano (situated ~1140 m below the crater floor). The tumulus is roughly circular, measures 17.5 × 16.0 m, and is cut by a 4.4 m deep axial inflation cleft exposing two separate flow units. We measured the elastic properties (i.e. shear- and bulk moduli) of natrocarbonatitic crust and find that these are similar to those reported for basaltic crust, and that there is no direct correlation between magmastatic head and pressure required to form tumuli. All inflated flows in the 2006 event were confined by lateral barriers (main crater, erosional channel or erosional gully) suggesting that the two most important factors for endogenous growth in natrocarbonatitic lava flows are (1) lateral barriers that prevent widening of the flow, and (2) influx of new material beneath the viscoelastic and brittle crust.     

Sedimentological analysis of the tephra from the 12–15 August 1991 eruption of Hudson volcano

The tephra fallout from the 12–15 August 1991 explosive eruption of Hudson volcano (Cordillera de los Andes, 45°54 S-72°58 W; Chile) was dispersed on a narrow, elongated ESE sector of Patagonia, covering an area (on land) of more than 100 000 km². The elongated shape of the deposit, together with the relatively coarse mean and median values of the particles at a considerable distance from the vent, were the result of strong winds blowing to the southeast during the eruption. The thickness of the fall deposit decreases up to 250 km ESE from Hudson volcano, where it begins to thicken again. Secondary maxima are well developed at approximately 500 km from the vent. Secondary maxima, together with grainsize bimodality in individual layers and in the bulk deposit suggest that particle aggregation played an important role in tephra sedimentation. The fallout deposit is well stratified, with alternating fine-grained and coarsegrained layers, which is probably a result of strong eruptive pulses followed by relatively calm periods and/or changes in the eruptive style from plinian to phreatoplinian. The tephra is mostly composed of juvenile material: the coarse mode (mostly pumice) shifts to finer sizes with distance from the volcano; the fine mode (mostly glass shards) is always about 5/6 phi. Glass shards and pumice are mostly light gray to colorless. However, considerable amounts of dark, poorly vesiculated, blocky shards, suggest a hydromagmatic component in the eruption. A land-based tephra volume of 4.35 km³ was estimated, and a total volume of 7.6 km³ arose from an extrapolation, which took into account the probable volume sedimented in the sea. Bulk density ranges from 0.9 to 1.10 gr/cm³ (beyond 110 km from the vent). Rather uniform density values measured in crushed samples (2.45–2.50 gr/cm³ at all distances from the vent) reveal a relatively homogeneous composition. Mean and median sizes decrease rapidly up to 270 km from the vent; beyond that point they are more or less constant, whereas the maximum size (1 phi) shows a steady decrease up to 550 km. A concomitant improvement in sorting is observed. This is attributed to sorting due to wind transport combined with particle aggregation at different times and distances from the vent. The Hudson tephra fallout shares some strikingly similar features with the Mount St. Helens (18 May 1980) and Quizapu (1932) eruptions.     

Rapid dome growth at Montagne Pelée during the early stages of the 1902–1905 eruption: a reconstruction from Lacroix’s data

Information obtained from various parts of the two books on Montagne Pelée by Lacroix enables an estimate to be made of the size of l’Etang Sec summit crater, the volume of the 1902–1905 lava dome and its growth rate at various stages of development. During the week preceding the 8 May nuée ardente, dome growth was between 28 and 38 m³ s^–1, leading to a volume of 17–23×10⁶ m³ on the morning of the catastrophe. Considering that significant parts of the dome (~1/3?) were removed by the 8 and 20 May climactic eruptions, a high magmatic flux could have continued until at least 27 May, when the total remaining volume was estimated to 53×10⁶ m³. After moderate activity in June–July (of order 10 m³ s^–1), vigorous dome growth resumed dramatically, leading to the third climactic eruption of 30 August (a true calculation for this period being not feasible because of poor quality of the data). From November 1902 to July 1903 most of the effusive activity was concentrated in the great spine (erupted volume ~15×10⁶ m³, magma flux 1.2 m³ s^–1), which was eventually destroyed by collapse and minor nuées ardentes. The end of the eruption was characterized by a very low effusion rate, <1 m³ s^–1 in average from August 1903 to October 1905.     

Effect of deposit alterations on the dating of herbivorous teeth from Arago cave by the ESR–U-series method

Geochemical alteration processes in archaeological sites may affect the environmental dose rate during the burial history and cause inaccuracy in age determination by palaeodosimetric dating methods, such as luminescence or ESR. In Arago Cave (Tautavel, France), the original composition of the sandy aeolian sediments of palaeoanthropological level G, where the Homo heidelbergensis Arago XXI skull was found, has been modified by both carbonation and phosphatization processes. Eight fossil teeth were collected from different geochemically affected parts of level G and analyzed by the ESR–U-series method. All the teeth are presumably contemporaneous. The results show that the samples from the phosphated area agree within error with those from the carbonated area. Surprisingly, the samples from the non-altered area show ages 100–150 ka younger. This difference is mainly due to the remarkable changes in the gamma dose rate over time. The measured in situ dose rate accounts for more than 50% of the total dose rate for all the samples. We observed that the samples' equivalent dose (D_E) were generally 20% lower in the non-altered area than in the carbonated and phosphated ones. These results show the crucial effect of the geochemical processes affecting the age calculation by ESR–U-series method in comparison with independent chronological data.     

Effects of shock pressure and temperature on titanomagnetite from ICDP cores and target rocks of the El’gygytgyn impact structure,Russia

The aim of this study was to investigate the effect of meteorite impacts on magnetic properties including magnetic susceptibility and the Verwey transition of Ti-poor titanomagnetite of volcanic rocks from the 3.6 Ma old El’gygytgyn impact structure located in the Okhotsk-Chukotka volcanic belt in north-eastern Russia. The target rocks consist mainly of rhyolite with some andesites, and is a rare example of impact structures within volcanic target rocks on Earth. 27 samples from outside the crater, the crater rim and from the depth interval 316 to 517 m below lake bottom (mblb) of the El’gygytgyn ICDP drilling were studied. A significant decrease of the average specific magnetic susceptibility by around 90% was observed between felsic volcanic rocks from the surface (18.1 × 10^-6 m³/kg) and the drill cores from near the crater central uplift (1.9 × 10^-6 m³/kg). Ferrimagnetic Fe-Ti oxide assemblages (Verwey transition temperature, T_V: -161 to -150°C, Curie temperature, T_C: 451 to 581°C), occurring in all studied samples, differ significantly. At the surface titanomaghemite is ubiquitously associated with titanomagnetite. The drill cores lack titanomaghemite, but either show a transformation into titanomagnetite and ilmenite or a strong fragmentation associated with a second T_V between -172 and -188°C. Reversible curves of temperature dependence of magnetic susceptibility in the suevite indicate high depositional temperatures of at least 500°C. In the polymict and monomict impact breccia mechanical deformation of titanomagnetite and temperatures of at least 200-350°C related to the shock are suggested from temperature dependent magnetic susceptibility cycling. Lowtemperature oxidation along strongly brecciated grain surfaces in titanomagnetite is suggested to cause the lower TV and we suggest that this phenomenon is related to postimpact hydrothermal activity. The strong magnetic susceptibility decrease at El’gygytgyn is mainly influenced by shock, and post-impact hydrothermalism causes a significant additional depletion. These observations explain why magnetic lows are a ubiquitous phenomenon over impact structures.