Plume Top Research Articles

A hydrographic transient above the Salty Dawg hydrothermal field, Endeavour segment, Juan de Fuca Ridge

During systematic repeat hydrography cruises to the Endeavour segment of the Juan de Fuca Ridge in the summers of 2004, 2005, and 2006, we encountered a transient increase in the water column heat content above the Salty Dawg hydrothermal field. First observed in July 2005 and mapped in greater detail in August 2005, this feature was not a typical event or megaplume since potential temperature anomalies were continuously elevated from the plume top to the seafloor. During the summer of 2005, the heat content in the waters above Salty Dawg was elevated ∼30 TJ, and the plume top was over 150 m higher in the water column than the other years measured. Based on scaling analyses, an order of magnitude increase in the volume flux from Salty Dawg would be required to generate a neutrally buoyant plume of this size. This observation was unexpected because no substantial earthquakes were detected in the time frame of this increased heat flux. The duration of the transient suggests possible forcing mechanisms: advancement of a cracking front, a small‐scale dike intrusion, aseismic crustal movement, fracture of a flow constriction to a previously unaccessible reservoir, an increase of heat in an underlying magma chamber, or movement of melt within the axial magma chamber. The transient disappeared before returning in August 2006, likely due to thermal expansion of shallow host rock, decreasing the permeability. Should such increases in seafloor heat flux prove to be common, the rate of hydrothermal cooling could be faster than previously thought.

Geodynamics of the Early Precambrian, Part 2: Formation of continental crust and sedimentary basins, specificity of the lithosphere

This part of work is aimed at consideration of problems concerning evolution of geodynamic processes based on characteristic structures of the Early Precambrian and their rocks. Considered in the paper are structures characterizing existence of mature continental crust (sedimentary basins), development of the crust (metamorphic belts and granitoids) and underlying upper mantle (lithospheric keel), and amalgamation of crustal domains (continents, microcontinents, terrains) into supercontinents. Geodynamic phenomena of the Early Precambrian comparable with the Phanerozoic ones are distinguishable, on the one hand, and their changes with time of evolutionary character are detectable, on the other. The latter are satisfactorily explainable by secular lowering of the mantle temperature in general and above the top of mantle plumes.

Modelling of thermochemical plumes and implications for the origin of the Siberian traps

Thermochemical plumes form at the base of the lower mantle as a consequence of heat flow from the outer core and the presence of local chemical doping that decreases the melting temperature. Theoretical and experimental modelling of thermochemical plumes show that the diameter of a plume conduit remains practically constant during plume ascent. However, when the top of a plume reaches a refractory layer, whose melting temperature is higher than the melt temperature in the plume conduit, a mushroom-shaped plume head develops. Main parameters (melt viscosity, ascent time, ascent velocity, temperature differences in the plume conduit, and thermal power) are presented for a thermochemical plume ascending from the core–mantle boundary. In addition, the following relationships are developed: the pressure distribution in the plume conduit during the ascent of a plume, conditions for eruption-conduit formation, the effect of the P– T conditions and controls on the shape and size of a plume top, heat transfer between a thermochemical plume and the lithosphere (when the plume reaches the bottom of a refractory layer in the lithosphere), and eruption volume versus the time interval t 1 between plume formation and eruption. These relationships are used to determine thermal power and time t 1 for the Tunguska syneclise and the Siberian traps as a whole. The Siberian and other trap provinces are characterized by giant volumes of lavas and sills formed a very short time period. Data permit a model for superplumes with three stages of formation: early (variable picrites and alkali basalts), main (tholeiite plateau basalts), and final (ultrabasic and alkaline lavas and intrusions). These stages reflect the evolution of a superplume from the ascent of one or several independent plumes, through the formation of thick lenses of mantle melts underplating the lithosphere and, finally, intrusion and extrusion of differentiated mantle melts. Synchronous syenite–granite intrusions and bimodal volcanism abundant in the margins of the Siberian traps are the result of melting of the lower crust at depths of 65–70 km under the effect of plume melts.

Entrainment and bending in a major hydrothermal plume, Main Endeavour Field, Juan de Fuca Ridge

[1] We measure expansion rate and bending in a 23-hour time series of acoustic images of the lower 25 m section of a buoyant hydrothermal plume rising from Grotto vent in the Main Endeavour Field, Juan de Fuca Ridge. We then calculate entrainment coefficient, the constant of proportionality relating mean inflow velocity at the plume edge to maximum mean upward velocity within the plume. The plume section alternately bends southwest at relatively high inclinations (37°) and northeast at lower inclinations at irregular intervals twice during this time period, apparently driven by current reversals in the mixed semi-diurnal tidal cycle. The measured expansion rates (0.11–0.25 m/m) and calculated entrainment coefficients (0.07–0.18) are directly proportional to the degree of bending (R2 = 0.75). The loss of buoyancy flux related to enhanced mixing in a stratified environment during bending may contribute to reduction of potential rise height consistent with predicted (∼400 m) and measured (300–350 m) plume tops.

Thermo-chemical structure in the mantle arising from a three-component convective system and implications for geochemistry

Numerical simulations are used to characterize structures and observational signatures of thermo-chemical convection with three component of differing densities nominally corresponding to regular mantle (the depleted Mid Oceanic Ridge Basalt (MORB) source), primitive mantle and recycled crust. Strong heterogeneity, similar to that observed in seismic tomography models, is generated near the CMB by sweeping of the highest density ‘recycled’ component into piles by convective downwellings. Meanwhile, the medium density ‘primitive’ material is widely dispersed by the convection and forms ‘blobs’. Two types of upwelling plumes are observed: large plumes arising from the top of dense piles and small-scale plumes arising from the base of the transition zone resembling seismic tomography images of upwelling plumes [Zhao, Earth Planet. Sci. Lett. 192 (2001) 251], although the origin of these plumes is unclear. The horizontal spectra of the seismic velocity had anomalies at different depths are most similar to tomographic models when dense basal piles are present. Comparing to geochemically-required residence times, the source of HIMU (this type of rock has high ratio of helium isotope anomaly) Ocean Island Basalt (OIB) is more likely to be dense ‘piles’, while lower mantle blobs have been proposed as a location of primitive material. Thus the combination of blob structures and deep mantle piles may be plausible way of simultaneously satisfying geochemical and seismological constraints on mantle structure.

Magnetic fields over active tectonic zones in ocean

The aim of our work is to estimate the electromagnetic effects that can be detected in the submarine zones with hydrothermal activity. It is known that meso-scale flows appear in the regions over underwater volcanoes or hot rocks. Their origin is connected with heat flux and hot jets released from underwater volcanoes or faults in a sea bottom. Values of mean velocities and turbulent velocities in plumes were estimated. Quasiconstant magnetic fields induced by a hot jet and a vortex over a plume top are about 1–40 nT. Variable magnetic fields are about 0.1–1 nT. These magnetic disturbances in the sea medium create an additional natural electromagnetic background that must be considered when making detailed magnetic surveys.

Particle aggregation in volcanic eruption columns

We develop a model to describe the formation of aggregates in a volcanic eruption column. The model combines a description of the rate of collision and sticking of particles with a model of the vertical transport in the eruption column. We thereby determine the evolution of the grain size distribution as a function of height in the eruption column. We consider aggregates in which liquid water provides the binding agent. For sufficiently large eruption columns we find that this limits the vertical extent of the zone where aggregates may form since near the source, all water is in the vapor form, while in the upper part of the column the mixture becomes very cold and freezes. However, in many cases, we find that the particles spend sufficient time in the central region, where the water is in the liquid form, that a substantial amount of aggregation occurs. Furthermore, we predict that owing to the reduction in the binding efficiency of water as particle size increases [Gilbert and Lane, 1994], there is a relatively narrow size distribution of aggregates at the plume top. We also model the airfall deposits associated with this aggregate‐rich distribution of particles which is injected to the top of the eruption column. We show that the relatively narrow size distribution of particles at the top of the column, coupled with the gravitational settling and transport by ambient winds, may lead to enhanced deposition close to the source and in some cases secondary thickening of the deposit. The relatively near‐source deposition of fine ash in these deposits is associated with the fallout of aggregates. As a simple application, we show that the present dynamical aggregation model is consistent with the secondary thickening of the deposit from the May 18, 1980, eruption of Mount St. Helens.

Retrieval of microphysical and morphological properties of volcanic ash plumes from satellite data: Application to Mt Ruapehu, New Zealand

AbstractA quantitative analysis of the properties of several Mt Ruapehu, New Zealand, ash plumes has been performed using multispectral satellite data from the AVHRR‐2 and ATSR‐2 instruments. The analysis includes: identification of the plume from background clouds using the ‘reverse’ absorption effect in the thermal channels: modelling and retrieval of particle sizes; determination of the plume height from cloud shadows, stereoscopy and meteorological data; and estimates of the mass of fine particles (radii less than 10 μm). A new spectral technique for identifying opaque, silica‐rich ash clouds is demonstrated by utilizing the near‐infrared (1.6 μm) and visible (0.67 μm) channels of the ATSR‐2, and the optical properties of a simple volcanic cloud are presented for use in radiative transfer studies. It is found that the Ruapehu eruption cloud contained silica‐rich ash particles with radii generally less than a few micrometres. The distribution of fine particles is monomodal with a dominant mode peak of about 3 μm radius. Mass loadings of fine particles are found to be in the range ≈︁1 to ≈︁7 mg m−3, and are consistent with estimates of mass loadings of volcanic clouds from eruptions of other volcanoes. The height of the plume top, derived from radiosonde data and plume‐top temperatures in the opaque regions, was found to be between 7.5 and 8.5 km, while the plume thickness was estimated to be between 1.5 and 3 km. Cloud height derived from ATSR‐2 stereoscopy on a different plume gave heights in the range 5 to 8 km.The results of this study provide important information on the optical properties of nascent volcanic eruption plumes. This information may prove useful in determining the potential effects of volcanic clouds on local climate, and in assessing any hazard to aviation.

Tomographic evidence for a narrow whole mantle plume below Iceland

Iceland is known to be a centre of both hotspot and Mid-Atlantic Ridge activity. A plume origin for the hotspot has been suggested by observational, laboratory and numerical investigations as well as by several seismological studies. The latter have, however, not yielded a detailed image of the entire mantle volume below Iceland. We present tomographic images of a narrow low velocity anomaly below Iceland, extending from the core-mantle boundary to the surface. From these images we infer a bent plume with a diameter ≤500 km, rising from a broad root zone (>1000 km) and culminating in a plume top with a diameter of ∼1200 km. The anomaly amplitude can, if only controlled by temperature, be converted into excess temperatures in the upper and lower mantle of the order of 200–300 K and ∼200 K, respectively. The wavy shape of the anomaly together with its lateral branches, may indicate that the position of the Iceland hotspot is not stationary.

Dependence of heat island phenomena on stable stratification and heat quantity in a calm environment

Dependence of heat island atmosphere on stable stratification and the heat quantity is elucidated by (i) visualizing and measuring the thermal flow over a heated source in a calm and stratified conditions in a laboratory experiment, (ii) finding new type of flow patterns, (iii) determining flow states by the PSD gradients, (iv) comparing with the existing results, and (v) discussing physically to extend the laboratory experiment to a real urban atmosphere. The heat quantity transferred from urban areas makes flow patterns tall. The stable stratification suppresses the plume height, increases the swaying amplitude and turbulence, and produces a vortex pair near the plume top. With increasing the heat quantity and/or decreasing the stable stratification, the plume changes from the type of the steady fumigation with local circulations accumulating heat, which agrees well with Oke's model, to the types of fumigation with the swaying motion, the mushroom, the maximum amplitude of the swaying motion, having a glasses-like vortex pair, and shedding vortices upwards, wherein the last two types of plumes the urban heat easily dissipates upwards. The results agree well with heat island phenomena which occur during night in winter and gigantic columns of clouds which occur during daytime in summer.

GOES weather satellite observations and measurements of the May 18, 1980, Mount St. Helens eruption

We demonstrate the use of Geostationary Operational Environmental Satellite (GOES) images of the May 18, 1980, Mount St. Helens volcanic plume in providing details of the dynamics and changing character of this major explosive eruption. Visible and thermal infrared (IR) data from a sequence of images at 30‐min intervals from 0850 to 1720 Local Time (LT) give information on dispersal and plume top temperature. Initial visible and IR images at 0850 show the top of a spreading co‐ignimbrite‐like umbrella plume and an overshooting column emerging from it, both rising off the ground‐hugging pyroclastic gravity flow generated by the opening directed blast. The overshooting column had a minimum temperature significantly colder than local ambient atmosphere, indicating substantial undercooling, and a maximum altitude of 31±2 km at 0920. This large plume system then formed a high‐velocity, radially spreading, gravitationally driven current before becoming advected in the wind field at an average downwind velocity of 29 m/s. Within 30 to 60 min the undercooled plume adjusted to ambient temperatures. After this initial event, the eruption column fluctuated over the next 8 hours between stable Plinian column rise and unstable conditions favoring column collapse, leading to pyroclastic flows and further development of co‐ignimbrite ash columns. Over this period of observation, profiles of plume top temperatures derived from IR data versus distance along the downwind axis have distinctive patterns, with steep slopes associated with co‐ignimbrite plumes and shallower slopes associated with Plinian plumes. Moreover, reflectance values from visible GOES data change from lower to higher during periods of transition from darker toned Plinian to lighter toned coignimbrite plumes indicating that in this case satellite data resolved changes in eruptive style from plumes with a coarser to a finer dominant particle size.

On the gas dynamics of the volcanic plumes on Io

Features of the volcanic plumes on Io observed by Voyager 1 such as the umbrella shape, the filamentary structure and the abrupt bends of the filaments, are explained based on a gas dynamic analysis. An analytic solution of the flow field between the shock and the plume top is given.

Ejection of atoms and molecules from Io by plasma-ion impact

The directions and energy distributions of neutral atoms and molecules ejected from Io are modeled analytically. The particles are emitted from the Io exosphere by collisions of incident plasma ions. Some of the Io atmosphere is attributed to particles sputtered off the surface by incoming plasma ions, while other departing particles originate at the top of volcanic plumes. Account is taken of the thermal motion of incident plasma and Io's gravitational field when calculating the distribution of the ejected particles. The distributions associated with each type of ejection force are projected to be different and highly dependent on the inclusion of realistic cross-sections. Attention is focused on Na-rich species and defining ejection processes which can be verified with ground-based observations. It is, however, concluded that no particles are ejected directly from the surface by Jean's escape.

FORCED PLUMES IN A STABLY STRATIFIED FLUID

Predictions are given of the final height to which a fluid emitted from a finite actual source with buoyant force will rise in a stably stratified fluid. The effects of velocity and temperature of effluent are discussed. The results show that the increase in the discharging velocity does not necessarily contribute to the increase in the height of the plume top, reducing it in some cases of small mass flux and relatively slow velocity, although increasing the temperature is always effective. Also, virtual sources are determined which correspond to the actual sources of mass, momentum and buoyancy.One of the practical problems to apply the proposed solution is an estimate of the necessary amount of temperature or velocity increase as required for stack gases of a thermal plant which is sufficient to penetrate the top of inversion layer in smog seasons and reach an upper atmosphere where they may be rapidly diffused by strong turbulence ; another such problem will be encountered in sewage disposal in the marine environments.