low heat flux and large variations of lithospheric thickness in the canadian shield 1 ten new heat flux determinations have been made using measurements in 22 mining exploration boreholes located at latitudes higher than 51 n in the canadian shield they provide data in poorly sampled regions near the core of the north american craton where one expects the lithosphere to be thickest the new heat flux values are all smaller than 34 mw m 2 and are among the lowest recorded so far in the shield for all the new sites there is no relationship between heat flux and heat production in surface rocks in the canadian shield heat flux variations occur at wavelengths 100 km and are mostly of crustal origin local averages in two 250 250 km windows located on archean areas at high latitudes on either side of james bay are 29 mw m 2 and 31 mw m 2 the lowest values found so far at this scale in the canadian shield s wave traveltime delays derived from tomographic models provide the additional constraints needed to resolve differences of deep lithospheric thermal structure there is no significant correlation between average surface heat flux and traveltime delays within the canadian shield confirming that variations of the surface heat flux are mostly of crustal origin traveltime delays cannot be explained by variations in crustal heat production only and require variations of heat supply to the lithosphere and or radiogenic heat production in the lithospheric mantle these variations are associated with changes of lithospheric thickness that may be as large as 80 km the heat flux at the base of the superior lithosphere is constrained to be 11 2 mw m 2

gas evolution in eruptive conduits combining insights from high temperature and pressure decompression experiments with steady state flow modeling abstract in this paper we examine the consequences of bubble nucleation mechanism on eruptive degassing of rhyolite magma we use the results of published high temperature and pressure decompression experiments as input to a modified version of conflow the numerical model of mastin and ghiorso 2000 u s g s open file rep 00 209 53 pp and mastin 2002 geochem geophys geosyst 3 10 1029 2001gc000192 for steady two phase flow in vertical conduits synthesis of the available experimental data shows that heterogeneous nucleation is triggered at p 120 150 mpa and leads to disequilibrium degassing at extreme h2o supersaturation in this latter case nucleation is an ongoing process controlled by changing supersaturation conditions exponential bubble size distributions are often produced with number densities of 106 109 bubbles cm3 our numerical analysis adopts an end member approach that specifically compares equilibrium degassing with delayed disequilibrium degassing characteristic of homogeneously nucleating systems the disequilibrium simulations show that delaying nucleation until p 150 mpa restricts degassing to within 1500 m of the surface fragmentation occurs at similar porosity in both the disequilibrium and equilibrium modes 80 vol but at the distinct depths of 500 m and 2300 m respectively the vesiculation delay leads to higher pressures at equivalent depths in the conduit and the mass flux and exit pressure are each higher by a factor of 2 0 residual water contents in the melt reaching the vent are between 0 5 and 1 0 wt roughly twice that of the equilibrium model

shear wave splitting and subcontinental mantle deformation splitting observations are interpreted in terms of the strain induced lattice preferred orientation of mantle minerals especially olivine we consider three hypotheses concerning the origin of the continental anlsotropy 1 strain associated with absolute plate motion as in the oceanic upper mantle 2 crustal stress and 3 the past and present internal deformation of the subcontinental upper mantle by tectonic episodes it is found that the last hypothesis is the most successful namely that the most recent significant episode of internal deformation appears to be the best predictor of q for stable continental regions this is interpreted as fossil anisotropy whereas for presently active regions such as alaska the anisotropy reflects present day tectonic activity in the stable portion of north america there is a good correlation between delay time and lithospheric thickness this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant the acceptance of this hypothesis has several implications for subcontinental mantle deformation first it argues for coherent deformation of the continental lithosphere crust and mantle during orogenies this implies that the anisotropic portion of the lithosphere was present since the deformational episode and rules out the addition of undeformed material to this layer by subsequent underplating or conductive growth of the thermal boundary layer one of the most important issues in the study of orogenies is the need to reconcile the formation of thickened lithosphere with the paradoxically high mantle temperatures often associated with orogenic episodes most efforts to date have focussed on modes of deformation whereby the cold lithospheric mantle is removed by convective instability or delamination and replaced by warm asthenosphere these models however are incompatible with the evidence for preserved coherent lithospheric deformation rather the deformed mantle appears to have been heated in place we suggest that the elevated mantle temperatures may be due to the strain heating accompanying the deformation

a scaling growth model for bubbles in basaltic lava flows abstract pahoehoe an and massive lavas from mount etna show common statistical properties from one sample to another which are independent of scale size over certain ranges the gas vesicle distribution shows two scale invariant regimes with number density n v v b 1 where v is the volume and empirically b 0 for small bubbles and b 1 for medium to large bubbles we introduce a bubble growth model which explains the b 1 range by a strongly non linear cascading growth regime dominated by a quasi steady state coalescence process the small bubble region is dominated by diffusion its role is to supply small bubbles to the coalescence regime the presence of measured dissolved gas in the matrix glass is consistent with the notion that bubbles generally grow in quasi steady state conditions the basic model assumptions are quite robust with respect to the action of a wide variety of processes since we only require that the dynamics are scaled over the relevant range of scales and that during the coalescence process bubble volumes are approximately conserved the model also predicts a decaying coalescence regime with b 1 associated with a depletion of the gas source or alternatively a loss of large vesicles through the surface of the flow our model thus explains the empirical evidence pointing to the coexistence of two different growth mechanisms in subsurface lava flows but acting over distinct ranges of scale with non linear coalescence as the primary growth process the total vesicularity of each sample can then be well estimated from the partial vesicularity of each growth regime without any outlier problems

temperature and rheological properties of the mantle beneath the north american craton from an analysis of heat flux and seismic data 1 we combine heat flux data and seismic velocity models for the north american lithosphere to derive constraints on thermal conditions and deformation mechanisms in the underlying convecting mantle local heat flux averages that are not affected by shallow crustal heat production contrasts allow calculation of reliable lithospheric geotherms and uncertainty ranges for consistency with the seismic data the mantle potential temperature beneath north america must lie within a 1290 c 1450 c range close to that for the oceanic mantle sampled at mid ocean ridges the heat flux at the base of the lithosphere varies laterally from 11 3 mw m 2 beneath the 250 km thick archean core of the superior province to 15 3 mw m 2 beneath the thinner younger appalachians province it is shown that the most likely cause of such rates of heat supply into the north american continent is small scale convection in an unstable boundary layer beneath the rigid mechanical lithosphere this allows useful constraints on the mantle rheological properties we show that the most likely deformation mechanism is dislocation creep in wet mantle rocks ranges for the mantle temperature water content and rheological parameters could be tightened very significantly once strong constraints are obtained on radiogenic heat production in the lithospheric mantle

localized imaging of the uppermost mantle with usarray pn data 1 usarray has now provided several years of high quality seismic data and improved ray coverage for much of the western united states this allows increased resolution for regional studies of the lithosphere and deeper structure of the north american continent in this study we use pn phases in the usarray data set to solve for velocity structure in the uppermost mantle in the western united states this article focuses on localized imaging techniques that complement traditional pn tomography analysis we apply waveform cross correlation to obtain inter station travel times between the closely and uniformly spaced usarray stations this allows us to use traces without phase picks and reduces errors associated with the picking we obtain differential times that can directly be used to fit locally for slowness and depending on the approach for the direction and curvature of the incoming wavefront the various measurements of incoming wavefronts at different sub arrays provide constraints on azimuthal variations in velocity the traditional tomography approach and the local fitting method reveal similar large scale features no regularization is applied with the local method and the resulting velocity maps reveal smaller scale structures than the tomographic images

textural heterogeneities in pumices from the climactic eruption of mount pinatubo 15 june 1991 and implications for magma ascent dynamics the climactic event of mount pinatubo represents one of the most thoroughly studied eruptions of the century and has provided important insights into the dynamics of explosive volcanism we have performed detailed textural analyses of the white and gray pumices of the plinian and pyroclastic flow deposits and found that differences in color and clast density reflect different crystal and vesicle amounts and size distributions white pumice has higher vesicularity deformed and highly coalesced vesicles with thin walls euhedral phenocrysts and microlite free groundmass gray pumice shows lower vesicularity wider ranges in vesicle number density limited coalescence vesicles with thick walls that are less deformed phenocrysts and microphenocrysts with abundant solution pitting and groundmass containing ubiquitous microlites and crystal fragments the presence of white and gray pumice varieties and the broad range in vesicularity and vesicle number density that characterizes both of them appear to record the complexities of conduit processes such as magma vesiculation and fragmentation and the development of conduit regions marked by different rheological behaviors in particular the results of this study suggest the likely importance of intense shear and viscous dissipation at the conduit walls a mechanism that may be responsible for the creation and discharge of the gray pumice of this eruption along with the dominant white variety

dynamics of coupled diffusive and decompressive bubble growth in magmatic systems bubble growth in an ascending parcel of magma is controlled both by diffusion of oversaturated volatiles and decompression as the magma rises we have developed a numerical model which explores the processes involved in water exsolution from basaltic and rhyolitic melts rising at a constant rate from magma chamber depths of 4 and 1 km while the model does not attempt to simulate natural eruptions it sheds light on the processes which control eruptive behavior under various conditions ascent rates are defined such that a constant rate of decompression dp dt is maintained a variety of initial ascent rates are considered in the model from 1 m s to 100 m s for basalts and from a few centimeters per second to 10 m s for rhyolite at the base of the conduit the model results indicate that for any reasonable ascent rate basaltic melt degasses at a rate sufficient to keep the dissolved volatile concentration at equilibrium with the decreasing ambient pressure rhyolitic melt reaches the surface at equilibrium if its ascent rate is less than 1 m s but it can erupt with high oversaturation at greater ascent rates the latter may lead to explosive eruptions if the ascent rate of rhyolite is 10 m s or more then melt barely degasses at all in the conduit and erupts with the highest oversaturation possible for the case of slow magma rise bubble growth is limited by decompression for the case of rapid magma rise bubble growth is limited by diffusion the results of our simple model do not accurately simulate natural volcanic eruptions but suggest that subsequent more complex models may be able to simulate eruptions using the insights regarding diffusive and decompressive bubble growth processes explored in this study numerical modeling of volcanic degassing may eventually lead to better prediction of eruption timing energetics and hazards of active volcanoes

evolution of melt vapor surface tension in silicic volcanic systems experiments with hydrous melts 1 we evaluate the melt vapor surface tension of natural water saturated dacite melt at 200 mpa 950 1055 c and 4 8 5 7 wt h2o we experimentally determine the critical supersaturation pressure for bubble nucleation as a function of dissolved water and then solve for at those conditions using classical nucleation theory the solutions obtained give dacite melt vapor surface tensions that vary inversely with dissolved water from 0 042 0 003 j m 2 at 5 7 wt h2o to 0 060 0 007 j m 2 at 5 2 wt h2o to 0 073 0 003 j m 2 at 4 8 wt h2o combining our dacite results with data from published hydrous haplogranite and high silica rhyolite experiments reveals that melt vapor surface tension also varies inversely with the concentration of mafic melt components e g cao feototal mgo we develop a thermodynamic context for these observations in which melt vapor surface tension is represented by a balance of work terms controlled by melt structure overall our results suggest that cooling crystallization and vapor exsolution cause systematic changes in that should be considered in dynamic modeling of magmatic processes

the kapuskasing uplift a geological and geophysical synthesis over the past decade the kapuskasing uplift has been the subject of intense geological and geophysical investigation as lithoprobe s window on the deep crustal structure of the archean superior province enigmatic since its recognition as a positive gravity anomaly in 1950 the structure has been variably interpreted as a suture rift transcurrent shear zone or intracratonic thrust diverse studies including geochronology geothermobarometry and various geophysical probes provide a comprehensive three dimensional image of archean 2 75 2 50 ga crustal evolution and proterozoic 2 5 1 1 ga cooling and uplift the data favour an interpretation of the structure as an intracratonic uplift related to hudsonian collision eastward across the southern kapuskasing uplift erosion levels increase from 10 km in the michipicoten greenstone belt through the wawa gneiss domain 10 20 km into granulites 20 30 km of the kapuskasing structural zone juxtaposed against the low grade swayze greenstone belt al

what makes hydromagmatic eruptions violent some insights from the keanakāko i ash kilauea volcano hawai i abstract volcanic eruptions at the summit of kilauea volcano hawai i are of two dramatically contrasting types 1 benign lava flows and lava fountains and 2 violent mostly prehistoric eruptions that dispersed tephra over hundreds of square kilometers the violence of the latter eruptions has been attributed to mixing of water and magma within a wet summit caldera however magma injection into water at other volcanoes does not consistently produce widespread tephras to identify other factors that may have contributed to the violence of these eruptions we sampled tephra from the keanak ko i ash the most recent large hydromagmatic deposit and measured vesicularity bubble number density and dissolved volatile content of juvenile matrix glass to constrain magma ascent rate and degree of degassing at the time of quenching bubble number densities 9 10 4 1 10 7 cm 3 of tephra fragments exceed those of most historically erupted kilauean tephras 3 10 3 1 8 10 5 cm 3 and suggest exceptionally high magma effusion rates dissolved sulfur average 330 ppm and water 0 15 0 45 wt concentrations exceed equilibrium saturation values at 1 atm pressure 100 150 ppm and 0 09 respectively suggesting that clasts quenched before equilibrating to atmospheric pressure we interpret these results to suggest rapid magma injection into a wet crater perhaps similar to continuous uprush jets at surtsey estimates of reynolds number suggest that the erupting magma was turbulent and would have mixed with surrounding water in vortices ranging downward in size to centimeters such fine scale mixing would have ensured rapid heat exchange and extensive magma fragmentation maximizing the violence of these eruptions

heat flow heat generation and crustal temperatures in the northern canadian cordillera thermal control of tectonics 1 the heat flow in the northern canadian cordillera between 59 and 64 n is very high 105 22 mw m2 the average upper crust heat generation is 4 6 2 4 microw m3 giving a reduced deep heat flow of 59 mw m2 similar to the southern canadian cordillera 49 53 n estimated moho temperatures of 800 1000 c and lithosphere only 50 60 km thick are substantiated by slow upper mantle seismic velocities and by high elevations associated with crust only 32 35 km thick the new data support the conclusion that all of the north and south america cordilleras are hot the paleoproterozoic wopmay orogen to the east has a remarkably high heat flow of 90 15 mw m2 for such an old and stable region however basement samples indicate high heat generation and estimated deep temperatures are intermediate between the cordillera and the slave craton further to the east heat flow from the yellowknife area of the slave craton 53 mw m2 implies low lithosphere temperatures in agreement with kimberlite xenolith p t data the high temperatures in the cordillera result in a very thin and weak lithosphere and may allow a lower crust detachment the north and central cordillera have been inland of the coastal queen charlotte transform fault since subduction stopped in the eocene thus some process has maintained high temperatures in the former back arc for more than 40 ma

thermal properties of granulite facies rocks in the precambrian basement of finland and estonia abstract we present results of heat production thermal conductivity and p wave velocity measurements of 252 rock samples from five granulite facies areas in finland and estonia these compositionally mainly intermediate archaean and palaeoproterozoic metamorphic rocks have relatively high heat production values mean values averaged by areas range from 0 57 to 2 24 w m 3 the lowest values are in the varpaisjarvi area which is the oldest and most mafic and where the highest metamorphic pressure occurred 8 11 kbar whereas the highest heat production is found in the turku granulite belt where the metamorphic pressure was 4 6 kbar the heat production decreases with the increase of the metamorphic pressure however a general numerical relationship cannot be presented because of considerable variations in heat production data there is no significant difference in the metamorphic temperatures of the studied granulites and therefore no dependence on temperature is found further our data suggest no relationship between heat production and p wave velocity the mean thermal conductivity of granulites varies from 3 0 to 3 5 w m 1 k 1 slightly elevated thermal conductivity values in the varpaisjarvi and lapland granulite areas can be attributed to higher sillimanite and quartz contents respectively

heat flow gravity and structure of the abitibi belt superior province canada implications for mantle heat flow the results of new heat flow measurements are presented for eleven sites located in the abitibi greenstone belt of the archean superior province these new and previously published heat flow data are used with gravity and seismic data to constrain the crustal structure and composition of the abitibi subprovince and to determine the mantle heat flow beneath the canadian shield in order to analyze the long wavelength variations in heat flow and bouguer gravity anomaly the crust was divided into three layers with distinctive composition and physical properties average density heat production and thickness for each layer were varied within a range compatible with measurements and yielded crustal models consistent with gravity and heat flow data the dominant contribution to crustal heat production is that of a tonalitic type layer which has the lowest density the models require mantle heat flow to range between 10 and 14 mw m 2 the westward increase in heat flow requires the tonalitic layer to thicken from the grenville front toward the kapuskasing uplift at the expense of the greenstone supracrustals long wavelength gravity data are more affected by changes in crustal thickness than by composition and the low bouguer anomaly near the eastern grenville front is caused by 5 6 km of crustal thickening in greenstone terranes the long wavelength variations in heat flow are more useful for determining average crustal composition and the thickness of supracrustals than the gravity data

the evolution of bubble size distributions in volcanic eruptions we review observations of bubble size distributions bsds generated during explosive volcanic eruptions and laboratory explosions as inferred from vesicle size distributions found in the end products unimodal polymodal exponential and power law bsds are common even in the absence of coalescence and both power law and exponential distributions have been generated in the same eruption to date theoretical models have proposed incompatible mechanisms for producing the various distributions we here present a unifying mechanism data from our laboratory analogue experiments suggest that power law distributions are associated with highly non equilibrium degassing a numerical model is developed in which bubbles nucleate repeatedly and grow in the spaces between those of previous generations where in a non equilibrium degassing scenario the volatile concentration remains high this process causes the bsd to evolve from unimodal through exponential into a power law the exponent of the power law is a measure of the number of nucleation events or the duration of the nucleation period compared with the timescale of bubble growth the mathematical inevitability of the evolution from unimodal poissonian to power law is discussed the findings may resolve the apparent contradiction between the equilibrium degassing conduit flow models and the non equilibrium degassing conditions derived from bubble growth models of explosive volcanic eruptions the process of ongoing nucleation is the mechanism whereby the volcanic system maintains near equilibrium in the case of rapid depressurisation and slow volatile diffusion

upper mantle velocity temperature conversion and composition determined from seismic refraction and heat flow 1 we compile upper mantle pn velocities from seismic refraction wide angle reflection surveys in the southern superior province of the canadian shield and compare them with temperatures at the moho deduced from heat flow data calculated moho temperatures and pn velocities correlate well showing that in this area pn depends primarily on temperature the obtained values of v pn t depend weakly on the assumed value of moho heat flow and are on the order of 6 0 10 4 10 km s 1 k 1 within the range of temperature derivatives obtained in laboratory studies of ultramafic rocks comparison between observed pn velocities and predicted values for several mineralogical models at moho temperatures allows constraints on both the moho heat flow and the shallow mantle composition for all moho heat flows undepleted clinopyroxene rich mantle compositions do not allow a good fit to the data for depleted mantle compositions temperatures consistent with the observed pn velocities correspond to values of moho heat flow larger than 12 mw m 2 for our preferred moho heat flow of 15 mw m 2 the best fit mantle composition is slightly less depleted than models for average archean subcontinental lithospheric mantle this may be due to rejuvenation by melt related metasomatism during the keweenawan rifting event the similarity in pn t conversion factors estimated from this empirical large scale geophysical study and those from laboratory data provides confidence in the absolute temperature values deduced from heat flow measurements and seismic studies

delayed disequilibrium degassing in rhyolite magma decompression experiments and implications for explosive volcanism recent numerical models and analog shock tube experiments show that disequilibrium degassing during magma ascent may lead to violent vesiculation very near the surface in this study a series of decompression experiments using crystal free rhyolite melt were conducted to examine the development of large supersaturations due to delayed homogenous spontaneous bubble nucleation melts were saturated at 900 c and 200 mpa with either 5 2 wt dissolved h2o or with 4 2 wt h2o and 640 ppm co2 and isothermally decompressed at linear rates of either 0 003 0 025 or 8 5 mpa s to final pressures between 25 and 175 mpa additional isobaric saturation experiments 900 c 200 25 mpa using pure h2o or mixed h2o co2 fluids establish reference equilibrium solubility curves values homogenous nucleation is triggered in both h2o only and h2o co2 experiments once the supersaturation pressure pss reaches 120 150 mpa and the melt contains two times its equilibrium water contents bubble number density and nucleation rate depend on the supersaturation pressure with values on the order of 102 cm3 and 1 cm3 s for pss 120 mpa 106 cm3 and 103 105 cm3 s for pss 130 150 mpa and 107 cm3 and 106 cm3 s for pss 160 175 mpa nucleation rates are consistent with classical nucleation theory and infer an activation energy for nucleation of 1 5 10 18 j nucleus a critical bubble radius of 2 10 9 m and an effective surface tension for rhyolite at 5 2 wt h2o and 900 c of 0 10 0 11 n m the long nucleation delay limits the time available for subsequent diffusion such that disequilibrium dissolved h2o and co2 contents persist to the end of our runs the disequilibrium degassing paths inferred from our experiments contrast markedly with the equilibrium or quasi equilibrium paths found in other studies where bubble nucleation occurs heterogenously on crystals or other discontinuities in the melt at low pss homogenous and heterogenous nucleation rates are comparable however as are bubble number densities so that at a given decompression rate it appears that nucleation mechanism rather than nucleation rate determines degassing efficiency by fixing the pressure depth at which vesiculation commences and hence the time available for equilibration prior to eruption although real systems are probably never truly crystal free our results show that rhyolitic magmas containing up to 104 crystals cm3 and perhaps as high as 106 crystals cm3 are controlled by homogenous rather than heterogenous nucleation during ascent

pn velocity and cooling of the continental lithosphere average upper mantle p sub n velocities and heat flow were computed within continental physiographic provinces in north america from published data p sub n velocity and heat flow data display an inverse relationship and were found to correlate with a statistically significant correlation coefficient temperatures at the crust mantle boundary were estimated from the heat flow values and these were used to demonstrate a correlation between p sub n velocity and temperature of upper mantle material the value of partialp sub n partialt p thus obtained 4 4 x 10 sup 4 to 8 1 x 10 sup 4 km s sup 1 sup 0 c sup 1 is within the range of temperature derivatives determined from laboratory studies of ultramafic rocks this dependence of p sub n velocity on temperature implies that one possible explanation for the observed geographical distribution in upper mantle seismic velocity is that the variation in p sub n velocity is primarily a temperature effect combined with the relationship between heat flow and crustal age for continents the p sub n versus heat flow correlation suggests a relationship between p sub n velocity and crustal age probably due to progressive cooling of the continental lithosphere after a thermotectonic event

anisotropy and vp vs in the uppermost mantle beneath the western united states from joint analysis of pn and sn phases pn and sn phases are valuable for resolving velocity structure in the mantle lid as they propagate horizontally right below the moho relatively few sn tomography attempts have been made compared to pn because sn is often highly attenuated or buried in p wave coda usarray has greatly increased data coverage for regional phases and both pn and sn are routinely picked by network analysts here we jointly invert pn and sn arrival time residuals with a modified time term analysis and a regularized tomography method and present new maps of crustal thickness uppermost mantle p velocity perturbations vp vs ratios and azimuthal anisotropy strength and orientation beneath the western united states the results indicate partially molten mantle below the snake river plain and the colorado plateau the seismic structure of the top 40 km of the mantle below the colorado plateau differs from that seen at greater depths in other studies such as surface wave or teleseismic body wave tomography whereas the snake river plain anomaly just below the moho is comparable to structures seen at about 200 km depth pn fast axes provide complementary information to sks shear wave splitting observations and our analysis indicates that in several regions in the western united states the orientation of azimuthal anisotropy changes with depth in the upper mantle however we have so far been unable to resolve shear wave splitting directly in sn waveforms which seem to be dominated by sn sv energy

bnd bubble number density decompression rate meter for explosive volcanic eruptions abstract this decade has been witness to substantial progress in the understanding of bubble dynamics and its role in volcanic eruptions a comparison between the results of experiments and a model for bubble nucleation shows that bubble number density bnd can be calculated as a function of decompression rate for given physical properties such as the diffusivity of water in a silicate melt and interfacial tension within an acceptable margin of error conversely in this paper we propose a method hereafter referred to as bnd decompression rate meter to estimate the decompression rate by using bnd data of natural pumice samples from volcanic eruptions as a result of the application of the bnd decompression rate meter to pumice and scoria from explosive eruptions it is found that the decompression rates increase in the range from 10 6 to 10 8 pa s with the eruption column heights these rates are proportional to the 4th power of the eruption column heights and are linear to the discharge rates the absolute values of the estimated decompression rates are very high and cannot be explained by the existing models of conduit flows in order to explain such high rates of decompression and the correlation with discharge rate we propose a possible model according to which the bubble nucleation propagates downward as a rarefaction shock wave in the conduit

homogeneous bubble nucleation in rhyolitic magmas an experimental study of the effect of h2o and co2 1 rapid decompression experiments were performed to study homogeneous bubble nucleation in a crystal free rhyolitic liquid at 800 c bubble nucleation was produced by lowering the pressure at 1 10 mpa s 1 from an initial value between 200 and 295 mpa to a final value below the volatile saturation pressure psat six volatile compositions with 4 1 7 7 wt h2o and 10 1200 ppm co2 were investigated for each composition we determined the critical pressure phon below which homogeneous nucleation can proceed the samples quenched below phon showed a nucleated core with a large number of uniformly spaced bubbles with decreasing pressure bubble number densities increased from 1015 m 3 the degree of supersaturation required for homogeneous nucleation phon psat phon increased with decreasing h2o content or increasing co2 content huge values of phon 135 to 310 mpa were measured in the h2o poor compositions 4 1 4 6 wt h2o 50 1100 ppm co2 much lower values from 60 to 160 mpa were obtained in the h2o rich compositions 7 0 7 7 wt h2o 10 630 ppm co2 the high phon in liquids with 4 5 wt h2o should result in the buildup of large degrees of supersaturation during magma ascent a very late nucleation event and a rapid explosive vesiculation by contrast rhyolitic liquids with much larger water contents have higher saturation pressures and much lower phon bubble nucleation may therefore occur at depth in the volcanic conduit favoring a subsequent near equilibrium degassing

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

fragmentation of magma during plinian volcanic eruptions the ratio of the volume of vesicles gas to that of glass liquid in pumice clasts v g v l reflects the degassing and dynamic history experienced by a magma during an explosive eruption v g v l in pumices from a large number of plinian eruption deposits is shown here to vary by two orders of magnitude even between pumices at a given level in a deposit these variations in v g v l do not correlate with crystallinity or initial water content of the magmas or their eruptive intensities despite large ranges in these variables gas volume ratios of pumices do however vary systematically with magma viscosity estimated at the point of fragmentation and we infer that pumices do not quench at the level of fragmentation but undergo some post fragmentary evolution on the timescale of plinian eruptions pumices with viscosities 109 pa s can expand after fragmentation as long as their bubbles retain gas at a rate inversely proportional to their viscosity once the bubbles connect to form a permeable network and lose their gas expansion halts and pumices with viscosities 105 pa s can collapse under the action of surface tension textural evidence from bubble sizes and shapes in pumices indicates that both expansion and collapse have taken place the magnitudes of expansion and collapse therefore depend critically on the timing of bubble connectivity relative to the final moment of quenching we propose that bubbles in different pumices become connected at different times throughout the time span between fragmentation and quenching after accounting for these effects we derive new information on the fragmentation process from two characteristics of pumices the most important is a relatively constant minimum value of v g v l of 1 78 64 vol vesicularity in all samples with viscosities 105 pa s this value is independent of magma composition and thus reflects a property of the eruptive mechanism the other characteristic is that highly expanded pumices 85 vol vesicularities are common which argues against overpressure in bubbles as a mechanism for fragmenting magma we suggest that magma fragments when it reaches a vesicularity of 64 vol but only if sheared sufficiently strongly the intensity of shear varies as a function of velocity in the conduit which is related to overpressure in the chamber so that changes in overpressure with time are important in controlling the common progression from explosive to effusive activity at volcanoes

heat flow and structure of the lithosphere in the eastern canadian shield twenty two new determinations of heat flow and radiogenic heat production in the superior and grenville provinces of the canadian shield are presented the new data and previously published measurements strongly constrain the thermal structure of the eastern canadian shield in the abitibi greenstone belt heat flow gradually increases from 29 mw m 2 near the grenville front to 44 mw m 2 east of the kapuskasing uplift this heat flow variation is interpreted in terms of crustal thickening and increased thickness of a tonalitic layer with average heat production of about 1 1 w m 3 this interpretation based on estimated heat production of major rock types in the region is consistent with crustal models derived from recent seismic reflection and refraction studies it also leads to an estimate of about 12 mw m 2 for the mantle heat flow beneath the area the average heat flow in the grenville province 41 10 mw m 2 is the same as that of the superior province this similarity and the lack of significant variation of heat flow across the grenville front indicate that the crust on both sides of the front has similar heat production and thus composition in the western part of the grenville province heat flow reaches high values in the vicinity of the boundary between the allochtonous polycyclic and monocyclic belts where enriched granitic plutons are found in the crystalline terranes in the central part of the grenville province heat flow and heat production are related to each other the parameters of the linear heat flow heat production relationship qr 30 2 in mw m 2 and d 7 1 1 7 km are close to those of the much younger appalachian province implying that the higher appalachian heat flow is due solely to higher heat production in the upper crust the data provide no evidence for variation of mantle heat flow between the superior grenville and appalachian provinces whose tectonic ages range between 2700 and 400 ma the small value of the mantle heat flow about 12 mw m 2 implies that the depth to the 450 c isotherm which controls the effective elastic thickness of the lithosphere is very sensitive to crustal heat production

heat flow in the nipigon arm of the keweenawan rift northwestern ontario canada 1 in the archean superior province the nipigon embayment in the area of lake nipigon north of lake superior is covered by midproterozoic sediments intruded by keweenawan diabase sills it has been interpreted as a failed arm of the ca 1100 ma keweenawan rift six new heat flow values in this area show that the region of low heat flow associated with the keweenawan rift in lake superior extends northwards along the western margin of the nipigon embayment the average heat flow in the nipigon area 39 5 mwm 2 is only slightly lower than

archean crust as revealed in the kapuskasing uplift superior province canada in the central superior province of the canadian shield a 120 km wide transition from the low grade michipicoten greenstone belt to the high grade kapuskasing structural zone represents an oblique section through some 20 km of archean crust uplifted along a northwest dipping thrust fault the restored vertical section through the upper and middle crust consists of three megalayers with undulating boundaries 1 0 to 25 km a high grade heterogeneous gneissic assemblage in part older than the upper supracrustal succession

continental rupture and the creation of new crust in the salton trough rift southern california and northern mexico results from the salton seismic imaging project a refraction and wide angle reflection seismic profile along the axis of the salton trough california and mexico was analyzed to constrain crustal and upper mantle seismic velocity structure during active continental rifting from the northern salton sea to the southern imperial valley the crust is 17 18 km thick and approximately one dimensional the transition at depth from colorado river sediment to underlying crystalline rock is gradual and is not a depositional surface the crystalline rock from 3 to 8 km depth is interpreted as sediment metamorphosed by high heat flow deeper felsic crystalline rock could be stretched preexisting crust or higher grade metamorphosed sediment the lower crust below 12 km depth is interpreted to be gabbro emplaced by rift related magmatic intrusion by underplating low upper mantle velocity indicates high temperature and partial melting under the coachella valley sediment thins to the north and the underlying crystalline rock is interpreted as granitic basement mafic rock does not exist at 12 18 km depth as it does to the south and a weak reflection suggests moho at 28 km depth structure in adjacent mexico has slower midcrustal velocity and rocks with mantle velocity must be much deeper than in the imperial valley slower velocity and thicker crust in the coachella and mexicali valleys define the rift zone between them to be 100 km wide in the direction of plate motion north american lithosphere in the central salton trough has been rifted apart and is being replaced by new crust created by magmatism sedimentation and metamorphism

the structure of basaltic scoria and reticulite and inferences for vesiculation foam formation and fragmentation in lava fountains in this investigation pyroclast structures are used to constrain degassing in basaltic lava fountains vesicle size shape number density interconnectedness and packing character are quantified and related to 1 the kinetics of bubble nucleation and growth 2 the structural evolution of magmatic foams and 3 the influence of vesiculation rate on magma fragmentation measurements made on a diverse suite of pyroclasts from kilauea volcano indicate that basaltic foams evolve through an initially disordered closed celled spherical state to a well ordered open celled polyhedral state as the vesicularity rises from 75 to 98 the structural changes occur rapidly 10 s in the conduit and fountain in response to an intense vesiculation burst vesicle size distribution systematics indicate bubble nucleation rates 2 104 events cm 3s 1 that are approximately three orders of magnitude greater than those found for effusive eruptive activity bubble growth rates 9 10 4 cm s exceed effusive estimates by a factor of 3 the observed runaway rate of bubble production indicates strong supersaturations at the onset of nucleation we speculate that the rise speed of the magma as it reaches the level where significant volatile exsolution begins determines the intensity of the vesiculation burst and hence the vigor of the eruption rapid expansion and acceleration of the magma under these conditions may provide the impetus for fragmentation

measurement of bubble size distributions in vesiculated rocks with implications for quantitative estimation of eruption processes abstract this paper outlines methods for determining a bubble size distribution bsd and the moments of the bsd function in vesiculated clasts produced by volcanic eruptions it reports the results of applications of the methods to 11 natural samples and discusses the implications for quantitative estimates of eruption processes the analysis is based on a quantitative morphological stereological method for 2 dimensional imaging of cross sections of samples one method determines with some assumptions the complete shape of the bsd function from the chord lengths cut by bubbles the other determines the 1st 2nd and 3rd moments of distribution functions by measurement of the number of bubbles per unit area the surface area per unit volume and the volume fraction of bubbles comparison of procedures and results of these two distinct methods shows that the latter yields rather more reliable results than the former though the results coincide in absolute and relative magnitudes results of the analysis for vesiculated rocks from eleven subplinian to plinian eruptions show some interesting systematic correlations both between moments of the bsd and between a moment and the eruption column height or the sio2 content of magma these correlations are successfully interpreted in terms of the nucleation and growth processes of bubbles in ascending magmas this suggests that bubble coalescence does not predominate in sub plinian to plinian explosive eruptions the moment moment correlations put constraints on the style of the nucleation and growth process of bubbles the scaling argument suggests that a single nucleation event and subsequent growth with any kind of bubble interaction under continuous depressurization which leads to an intermediate growth law between the diffusional growth r m t 2 3 at a constant depressurization rate and the ostwald ripening r m t 1 3 under a constant pressure where rm and t are the mean radius of bubble and the effective time of diffusion respectively occurred in the eruptions it is emphasized that the bsd in vesiculated rocks from terrestrial volcanoes can be used to estimate quantitatively eruption processes such as the initial saturation pressure and magma ascent velocity in a volcanic conduit

kinetics of bubble nucleation in a rhyolitic melt an experimental study of the effect of ascent rate in order to characterize the effect of ascent rate on the kinetics of bubble nucleation in a rhyolitic magma we performed three series of experiments decompressed at rates of either 1000 167 or 27 8 kpa s the experiments were carried out in an externally heated pressure vessel at 800 c and in the pressure range 260 59 mpa the starting material was a crystal free and bubble free rhyolitic glass containing 7 0 wt dissolved h2o in all the decompression experiments homogeneous bubble nucleation began at 90 2 mpa that is 150 mpa below the water saturation pressure of the silicate liquid 240 mpa the degree of supersaturation phon required to trigger homogeneous bubble nucleation was almost independent of decompression rate phon is the difference between the saturation pressure and the nucleation pressure nucleation pressure decreased by 3 mpa for a 36 fold increase in decompression rate these results are in good agreement with the classical theory of nucleation assuming a rhyolite h2o surface tension of 0 106 n m 1 our major experimental finding is that after a short nucleation event the nucleation rate dropped and the bubble number density n reached a stationary value that was strongly sensitive to decompression rate 6 8 mm 3 at 27 8 kpa s 470 mm 3 at 167 kpa s and 5800 mm 3 at 1000 kpa s the smaller value of n at low decompression rate was compensated by a larger mean bubble size so that at a given pressure vesicularity was almost independent of decompression rate the experimental values of n can be reproduced within a factor 0 3 1 4 using a relationship derived from published numerical simulations of vesiculation in ascending magmas the nucleation behavior in our experiments is dictated by a competition between bubble nucleation and diffusive bubble growth which depletes in water the surrounding liquid and therefore reduces the degree of volatile supersaturation once a critical value of n is attained diffusive bubble growth can keep pace with decompression and prevent the degree of volatile supersaturation in the liquid to increase with decreasing pressure the strong correlation between bubble number density and decompression rate has fundamental volcanological implications if we extrapolate the experimental data to the typical ascent rates of silicic magmas we obtain bubble number densities 10 3 101 mm 3 for homogeneous nucleation 10 1 103 mm 3 for heterogeneous nucleation that are orders of magnitude smaller than those measured in most natural pumices we therefore propose that the large values of n in silicic pumices may be due to two successive nucleation events 1 a first event which occurs relatively deep in the volcanic conduit and which yields a moderate number of bubbles and 2 a second nucleation event yielding a very large number of small bubbles and presumably related to the dramatic increase of decompression rate that precedes fragmentation the small bubble number densities associated with homogeneous nucleation suggest that a strong departure from equilibrium degassing should be the rule even at slow ascent rates

uppermost mantle pn velocity model for the afar region ethiopia an insight into rifting processes the afar depression ethiopia offers unique opportunities to study the transition from continental rifting to oceanic spreading because the process is occurring onland using traveltime tomography and data from a temporary seismic deployment we describe the first regional study of uppermost mantle p wave velocities vpn we find two separate low vpn zones as low as 7 2 km s 1 beneath regions of localized thinned crust in northern afar indicating the existence of high temperatures and potentially partial melt the zones are beneath and off axis from contemporary crustal magma intrusions in active magmatic segments the dabbahu manda hararo and erta ale segments this suggests that these intrusions can be fed by off axis delivery of melt in the uppermost mantle and that discrete areas of mantle upwelling and partial melting thought to characterize segmentation of the uppermost mantle at seafloor spreading centres are initiated during the final stages of break up

pn tomography of the western united states using usarray 1 usarray has now provided several years of high quality seismic data and improved ray coverage for much of the western united states which will enable increased resolution for studies of the lithospheric and deeper structure of the north american continent here we analyze pn arrival times from the transportable stations of usarray to resolve crustal thickness and uppermost mantle structure we use 123 008 pn picks from april 2004 to october 2009 as measured by the array network facility at epicentral distances from 180 to 1450 km these picks are derived from 778 stations at 70 km spacing and 7903 earthquakes and quarry blasts applying the classic time term method we use a regularized least squares inversion to estimate crustal thickness variations and image velocity perturbations in the uppermost mantle just below the moho we also consider upper mantle anisotropy and describe the velocity perturbations with a cos 2 azimuthal variation our crustal thickness map generally agrees with receiver function results from other researchers but differs in some details we obtain an average upper mantle velocity of 7 93 km s with higher velocities beneath eastern washington and northern idaho and lower velocities near the california mexico border the sierra nevada the northern coastal california region and the greater yellowstone area we observe large anisotropic anomalies in southern california as well as in the snake river plain area these results should complement other seismic studies e g body and surface wave tomography and shear wave splitting to provide information about composition temperature and tectonic processes in the western united states

numerical study of nucleation and growth of bubbles in viscous magmas the nucleation and growth processes of bubbles in viscous magmas with a constant decompression rate have been numerically investigated based on a formation which accounts for effects of viscosity as well as diffusivity interfacial tension and decompression rate the numerical solutions show two regimes in the nucleation and growth process a diffusion controlled regime and a viscosity controlled regime mainly depending on the decompression rate initial saturation pressure and viscosity the basic mechanism common to both regimes is that growth governs nucleation through depletion of degassing components in basaltic eruptions the vesiculation is essentially controlled by diffusion and the viscosity controlled regime is limited to very high decompression rate and very small water content when andesitic magma saturated by water at 10 mpa is decompressed through the propagation of rarefraction wave induced by a landslide as took place in the mount st helens 1980 eruption the vesiculation is controlled by the viscosity up to 100 m depth on the other hand in a rhyolitic magma for the same situation vesiculation is controlled by the viscosity over the whole depth of the magma column in the viscosity controlled regime the vesicularity may be 90 or less as seen in silicic pumice whereas in the diffusion controlledmore regime the vesicularity equals or exceeds 98 such as in reticulite in hawaiian basalt an observed variation of the number density of bubbles by several orders of magnitude in plinian eruptions and the correlation with the sio2 content can be attributed approximately to the dependence of diffusivity of viscosity on sio2 content and temperature assuming the apparent correlation between sio2 content and temperature of magma less

experimental study of water degassing from phonolite melts implications for volatile oversaturation during magmatic ascent abstract we have experimentally studied degassing of phonolite melts between 50 and 200 mpa and 825 and 850 c our results indicate that decompressed phonolite melts contain equilibrium water contents regardless of decompression rate or final pressure in addition the total porosities of our experiments agree with predicted equilibrium values regardless of final pressure or decompression rate combined observations of mean bubble diameters porosities and arrangement of bubbles in our experiments indicate that degassing occurs preferentially in the large bubble fraction of the population because of this retardation of growth in the small bubble population occurs this may lead to bimodal bubble size distributions in completely degassed phonolites our results indicate that water saturated phonolites at temperatures 800 c can degas in equilibrium while rhyolites follow a disequilibrium trend at decompression rates of 0 5 1 0 mpa s thus hotter water saturated phonolitic magmas are likely to maintain equilibrium magma water contents during ascent at rates up to 20 m s which are typical of highly explosive eruptions these results may apply to eruptions such as the 79 ad eruption of vesuvius which is the type location for plinian style explosive eruptions however caution must be used when applying these results to lower temperature more differentiated and water undersaturated phonolite magmas if fragmentation can occur at depths of 2 km or greater in the conduit our results indicate that highly explosive eruptions of phonolitic magma may occur in the absence of water oversaturation in the magma during ascent fast ascent rates and high shear stresses in the conduit may provide a better explanation for fragmentation in this case

laboratory simulations of sustained volcanic eruptions many violent eruptions are driven by rapid exsolution of dissolved volatiles within liquid magma the accelerating two phase mixture emerges at the vent as a sustained quasi steady discharge lasting for periods from hours to days refs 1 2 the initial growthin discharge rate commonly observed subsequent fluctuations and discrete pulses and shocks remain largely unexplained we have simulated volcanic conduit flows by producing sustained quasi steady explosions in a liquid undergoing rapid exsolution of a gas this was done by rapidly decompressing large volumes of co 2 saturated water the results reveal fluctuations in discharge rate that reflect heterogeneities in the two phase mixture that form spontaneously as a consequence of the size and geometry of the experimental system an initial transient with a growing discharge rate is observed in experiments in which material is erupted from a spherical flask up a narrow neck that mimics the magma chamber conduit assembly of volcanic systems the fragmentation region propagates down the neck during the initial transient until it reaches a stable position at the top of the flask at which point a quasi steady discharge ensues

terrestrial heat flow in lake superior using oceanographic heat flow techniques 162 measurements of heat flow were made in lake superior during the summers of 1966 and 1967 these data are of high quality with precisions with respect to intercomparisons typically in the 3 5 range the data define two very clear features one is a trough of low heat flow values which runs continuously for 650 km along the northern edge of the lake with values ranging between 0 46 and 0 98 heat flow units hfu 19 2 41 0 mw m2 this feature correlates with surface exposure of keweenawan mafic volcanics it is believed to delineate a major crustal separation associated with the midcontinent rift and is filled to crustal thicknesses with mafic intrusives and extrusives this feature has not been imaged with the seismic reflection profiling of glimpce the other heat flow feature is an arcuate ridge of high heat flow values 1 0 1 45 hfu 41 8 60 7 mw m2 parallel to and south of the heat flow trough the highest areas of this ridge correspond to areas of t

vesiculation of may 18 1980 mount st helens magma the may 18 1980 eruption of mount st helens washington produced both white and gray pumice similar in composition but varying in phenocryst microlite and vesicle content the white pumice has fewer phenocrysts no microlites and higher vesicularity and is thus less dense than the gray in addition vesicles in the white pumice are larger and more interconnected than those in the gray both white and gray have effective crystal free vesicularities 85 7 and 72 2 respectively close to the traditionally accepted fragmentation vesicularity of 75 77 given that the two pumice types were erupted concurrently for several phases of the eruption and therefore probably had similar eruption paths we suggest that differences in vesicularity and vesicle size distributions result from the presence or absence of microlites the presence of microlites in the gray pumice not only increased the magma viscosity and effective vesicularity but appears to have aided bubble nucleation and hindered bubble expansion and coalescence thus magmas with microlites may fragment at a lower bulk vesicularity than those without microlites fragmented microlite bearing clasts are also likely to expand less after fragmentation and therefore more closely preserve the bubble distribution and structure at the time of magma fragmentation

crustal geochemistry in the wawa foleyet region ontario fifty three rock samples from the kapuskasing structural zone ksz and 56 from the wawa domal gneiss terrane wgt ontario have been analysed for major elements 32 trace elements and 18o d was measured in a few samples average chemical compositions for the ksz and wgt regions have been calculated from map unit averages weighted by regional abundance compared with estimates of the composition of the upper continental crust ucc the ksz is enriched in al fe mg ca p transition elements sc and sr depleted in si b most rare earth elements zr hf nb ta li na k u th ba and rb but similar in composition to estimates of the lower continental crust the wgt is closer in composition to the ucc these data support the interpretation on geophysical and petrological grounds that the crust here is layered and has been uplifted the wgt and ksz regions representing progressively deeper levels igneous rock and orthogneiss 18o values in the ksz and wgt show good correlation with the weigh

the effect of magma flow on nucleation of gas bubbles in a volcanic conduit abstract we solve the dynamics of magma ascent and the kinetics of bubble nucleation and growth simultaneously which allow us to predict bubble sizes and number densities under ascent conditions as magma rises toward the surface the pressure decreases and eventually becomes less than the solubility pressure when the degree of supersaturation becomes great enough bubbles nucleate nucleation will stop as the concentration of volatiles in the melt decreases due to growth of existing bubbles and hence the degree of supersaturation decreases we show that a second nucleation event may occur just below the fragmentation level near that level the degree of supersaturation continuously increases as the magma is rapidly decompressed as a result nucleation will not stop until fragmentation occurs this second nucleation event should be taken into account when interpreting bubble size distribution measurements made on natural pumices the bubbles of the second nucleation event have high internal gas pressures up to 2 mpa greater than the liquid pressure suggesting that the second nucleation event may enhance fragmentation of magma we apply the model to the calculation protocol defined at the volcanic eruption mechanism modeling workshop durham 2002 we found that as a result of disequilibrium degassing fragmentation occurs higher in the conduit than under equilibrium degassing

satellite observations and interpretation of the 1991 mount pinatubo eruption plumes we demonstrate the use of hourly geostationary meteorological satellite gms and noaa polar orbiting advanced very high resolution radiometer avhrr satellite images of the june 1991 mount pinatubo volcanic eruption plumes in providing details of the timing changing eruptive style and umbrella plume movement and dynamics a chronology of explosive events which culminated in a 9 hour long climactic phase on june 15 has been determined from satellite data and favorably compared to ground based and other ancillary information maximum eruption column altitudes of 40 km and column top undercooling in excess of 55 c below the ambient atmosphere occurred during the climactic phase phases dominated by plinian versus coignimbrite eruptive plumes can be distinguished on the images analysis of the spreading umbrella plume during the climactic phase suggests that over the first 4 5 hours the plume spread laterally as a gravitational intrusion before becoming advected in the ambient winds we compare the rate of plume growth to a model of a radially symmetric continuously fed intrusion to describe the motion the rate of plume growth shows good agreement with this simple model for the first 4 5 hours from column top heights we determine eruption rates and obtain an independent estimate of the total volume of magma erupted from june 12 16 of 5 5 km3 dense rock equivalent dre in good agreement with an independent estimate of the total eruptive volume made from deposits on land and submarine ash fall of up to 5 3 km3 dre we also show that secondary explosions in the pinatubo ignimbrite deposits produced ash plumes reaching altitudes as high as 19 km in september 1991 this study indicates that realistic interpretation and monitoring of major eruption plumes can be accomplished by analysis of high temporal resolution satellite data

thermal regime of the lithosphere in the canadian shieldthis article is one of a series of papers published in this special issue on the theme lithoprobe parameters processes and the evolution of a continent geothermal studies were conducted within the framework of lithoprobe to systematically document variations of heat flow and surface heat production in the major geological provinces of the canadian shield one of the main conclusions is that in the shield the variations in surface heat flow are dominated by the crustal heat generation horizontal variations in mantle heat flow are too small to be resolved by heat flow measurements different methods constrain the mantle heat flow to be in the range of 12 18 mw m 2 most of the heat flow anomalies high and low are due to variations in crustal composition and structure the vertical distribution of radioelements is characterized by a differentiation index di that measures the ratio of the surface to the average crustal heat generation in a province determination of mantle temperatures requires the knowledge of both the surface heat flow and di mantle temperatures increase with an increase in surface heat flow but decrease with an increase in di stabi

bubble nucleation in rhyolitic melts experiments at high pressure temperature and water content abstract we report the first measurement of bubble nucleation in hydrated rhyolitic melts in response to pressure release two rhyolitic obsidians one containing less than 1 of microlites of fe ti oxides and the other about 20 of various crystals were hydrated at 150 mpa and 780 850 c after saturation was reached 5 3 5 5 wt water pressure was lowered and the samples were allowed to nucleate and grow bubbles for various amounts of time before the final rapid quenching of the experiments the results demonstrate the importance of heterogeneous nucleation microlites of fe ti oxides are very efficient as sites for bubble nucleation in their presence modest nucleation was observed even after decompression by 6 10 8 bubbles cm 3 in the absence of microlites no nucleation occurred at p mpa at p 10 mpa bubbles also nucleated on crystals of biotite zircon and apatite modest nucleation 10 3 10 5 cm 3 took place even in crystal free samples but it was still heterogeneous when p exceeded 80 mpa nucleation in crystal free samples became extensive 10 5 10 7 cm 3 the lack of correlation of bubble density with either time or decompression suggests that nucleation was still heterogeneous nucleation rates were controlled mainly by the availability of sites rates were faster than 10 6 cm 3 s 1 when microlites were present and faster than 10 5 cm 3 s 1 in the absence of microlites at p 70 mpa narrow size distributions in most samples suggests that nucleation took place immediately after the pressure drop the experimental data we present here indicate that the presence or absence of efficient nucleation sites can lead to two distinct modes of bubble formation when a large number of efficient sites e g fe ti oxide are present bubble nucleation requires very little supersaturation and to a good approximation gas and magma are in equilibrium in magmas that are crystal free or contain crystals that are inefficient at nucleating bubbles very high degrees of supersaturation are required in order to initiate nucleation these two modes of exsolution may lead to contrasting styles of convection pressure build up and eruption

radiogenic heat production in the continental crust abstract the thermal structure and evolution of continents depend strongly on the amount and distribution of radioactive heat sources in the crust determining the contribution of crustal rocks beneath a superficial layer is a major challenge because heat production depends weakly on major element composition and physical properties such as seismic wavespeed and density enriched granitic intrusives that lie at the current erosion level have a large impact on the surface heat flux but little influence on temperatures in the deep crust many lower crustal rocks that are poor in heat producing elements are restites from ancient orogenic events implying that enrichment of the upper crust was achieved at the expense of deeper crustal levels for the same total heat production concentrating heat sources in an upper layer acts to reduce temperatures in the lower crust thereby allowing stabilization of the crust the present day structure of the crust is a consequence of orogeny and should not be adopted for thermal models of the orogenic event itself this review summarizes information extracted from large data sets on heat flow and heat production and provides estimates of crustal stratification and heat production in several geological provinces analysis of global and regional data sets reveals the absence of a positive correlation between surface heat flow and crustal thickness showing that the average crustal heat production is not constant differences of heat flow between geological provinces are due in large part to changes of crustal structure and bulk composition collating values of the bulk crustal heat production in a few age intervals reveals a clear trend of decrease with increasing age this trend can be accounted for by radioactive decay indicating that thermal conditions at the time of crustal stabilization have not changed significantly for the average crustal thickness of 40 km moho temperatures are near solidus values at the time of stabilization suggesting an intrinsic thermal control on crustal thickness and heat production distribution crustal thickening by more than about 10 km above this mean value induces changes of gravitational potential energy that exceed the strength of the lithosphere for several provinces where strong constraints on heat production are available it is shown that prior to intracrustal fractionation only modest amounts of thickening were needed to generate the conditions of ultra high temperature metamorphism the tell tale signature of crustal heat production is anatectic and metamorphic events that lag the cessation of orogenic activity by several tens of million years crustal heat production is decreasing with time because it is due to the radioactive decay of uranium thorium and potassium its rundown is responsible for the secular cooling of lithospheric roots at a typical rate of about 100 k gy 1 implying complex thermal interactions with a convecting mantle that is not cooling at the same rate

global patterns of azimuthal anisotropy and deformations in the continental mantle summary we present a summary of measurements of azimuthal anisotropy in the continental mantle based on the sks technique and performed mostly with the active participation of the authors the directions of polarization of the fast quasi shear wave and the time delays between the quasi shear waves are obtained at nearly 70 locations in all continents except antarctica these data are interpreted in terms of lattice preferred orientation of olivine which is caused by deformations in the mantle the depth interval responsible for anisotropy is unknown but the data suggest that it may reach at least 300 km the fast directions in sks do not show clear correlation with the fast directions of the teleseismic p at the same seismograph stations in the regions of present day convergence the fast direction of anisotropy usually aligns with the plate boundary this correlation implies that the direction of shortening is the same in the crust and the upper mantle in the regions of rifting the inferred direction of mantle flow usually aligns with the direction of extension in the crust outside the regions of recent tectonic activity we most likely observe a combined effect of frozen anisotropy in the subcrustal lithosphere and of recently formed anisotropy in the asthenosphere on a global scale in these regions there is a positive correlation between the absolute plate velocity directions and the fast directions of anisotropy the correlation is especially strong in central and eastern parts of north america a clear absence of any evidence of large scale azimuthal anisotropy in the data of long range refraction profiling for the upper 100 km of the mantle of that region implies that the effect in sks is generated mainly at greater depths in the asthenosphere orientation of olivine at these depths reflects recent and present day flow in the mantle rather than processes of a distant geologic past

decompression experiments identify kinetic controls on explosive silicic eruptions 1 eruption intensity is largely controlled by decompression induced release of water rich gas dissolved in magma it is not simply the amount of gas that dictates how forcefully magma is propelled upwards during an eruption but also the rate of degassing which is partly a function of the supersaturation pressure pcritical triggering gas bubble nucleation high temperature and pressure decompression experiments using rhyolite and dacite melt reveal compositionally dependent differences in the pcritical of degassing that may explain why rhyolites have fueled some of the most explosive eruptions on record

experimental constraints on degassing of magma isothermal bubble growth during continuous decompression from high pressure numerical models predict that rapid ascent of hydrous magma can lead to supersaturation of dissolved volatile constituents possibly leading to explosive eruption we have performed controlled decompression experiments to investigate the ascent rates required to maintain bubble melt equilibrium high silica rhyolitic melts were saturated with water at 200 mpa and 825 c decompressed to lower pressures at constant rates of 0 025 0 25 0 5 and 1 0 mpa s 1 and then rapidly quenched isobarically other samples were saturated with water over the pressure range investigated to determine equilibrium water solubility in order to quantify degassing efficiency during decompression at a decompression rate of 0 025 mpa s 1 melt vapor equilibrium was maintained over the entire pressure range examined 200 to 17 5 mpa a single bubble nucleation event occurred in response to decompression and quenched bubble sizes can be modeled by a equilibrium bubble growth model that takes into account the number density of bubbles at decompression rates of 0 25 0 5 and 1 0 mpa s 1 rhyolitic melts could not degas in equilibrium when pressure decreased from 200 mpa to 140 mpa and water supersaturation p in the melt reached up to 60 mpa with higher values at faster decompression rates further pressure release resulted in near equilibrium degassing and p dropped significantly in each case p decreased when bubbles exceeded 10 vol a single heterogeneous bubble nucleation event occurred in each experiment when p 20 mpa no other bubbles nucleated despite p reaching 60 mpa which is probably too low to trigger homogeneous nucleation compared to estimates for magma decompression rates during lava dome eruptions our results indicate that magmas can degas efficiently throughout their ascent to the surface in explosive eruptions decompression rates may exceed those of this study and hence melts may become supersaturated with water such fast decompressions are expected however only when magma is highly vesicular which would aid approach to equilibrium degassing

experimental determination of ra mineral melt partitioning for feldspars and 226ra disequilibrium crystallization ages of plagioclase and alkali feldspar radium and other trace element partition coefficients have been experimentally determined for plagioclase and alkali feldspar in equilibrium with silicate melts in anorthitic plagioclase alkalis e g na k rb cs and the heavy earth alkalis e g ba ra are incompatibles in albitic plagioclase na is compatible whereas ca and sr are always compatible in plagioclase draplagioclase melt was determined to be 0 017 0 006 an91 0 025 0 009 an81 and 0 47 0 08 an34 for three different an contents the ratio dra dba increasing with decreasing an component in alkali feldspar k rb ba ra and sr are compatible whereas na cs and ca are incompatible draalkali feldspar melt was determined to 2 54 0 04 for a or75ab21an4 composition the ratio dra dba is 0 55 0 01 for plagioclase dra and xan are related as rt lndra kj mol 52 54 5 75 xan 4 25 1 51 and dra dba exp 9 47 6 21 xan 13 66 1 52 rt kj mol allowing calculation of dra from the an content of magmatic plagioclase for alkali feldspar we propose the equation dra dba exp 16 06 0 58 15 52 0 67 xor rt kj mol the experimentally determined partition coefficients are then used to recalculate th ra ba model ages leading to generally shorter feldspar residence times the variations encompass 20 to 90 shorter crystallization ages with respect to the originally calculated ones in two cases feldspar crystallization ages increase slightly for several cases the measured plagioclase and melt have either never been in equilibrium or the th ra ba system has been disturbed after crystallization

the structure of the mokai geothermal field based on geophysical observations abstract the mokai geothermal area about 25 km northwest of taupo has only minor thermal manifestations with a natural heat output of about 6 mw in late 1976 a roving dipole resistivity survey was made in the area surrounding mokai springs since then an extensive schlumberger resistivity survey has been made using two spacings nominally 600 m and 1200 m covering an area of about 4 b v n 2 around the mokai area interpretation of all the resistivity data suggests the area of mokai geothermal field below which high temperatures may be expected at depth is about 12 16 km 2 low resistivities from soundings within this area indicate highly conductive saline geothermal fluids at a depth of about 60 m although below this the resistivity increases suggesting a lowering of porosity the interpretation of resistivity measurements suggests a considerable subterranean flow of geothermal fluids northward from the mokai geothermal field and a further 76 mw of heat is estimated to be carried by springs and seeps into the deeply incised waipapa stream a second low resistivity region occurs at the waikato river about 12 km north of the geothermal field with an area of 3 4 km 2 there are indications that this anomaly is caused by hot geothermal fluids and it is suggested here that these geothermal fluids have their origin at the mokai geothermal field on this basis an estimate of the total heat output of the mokai geothermal area is 400 160 mw temperature in a drill hole within the geothermal field as outlined by this study reach a maximum of 290 c

shear wave splitting observations in the archean craton of western superior shear wave splitting observations in the archean superior province of the canadian shield show moderate to large delay times 1 1 2 1 s with azimuths suggesting upper mantle anisotropy is subparallel to the structural grain of the craton two regions with uniform anisotropy azimuth 62 7 and 87 3 are separated by a transitional zone showing strong dependence of observed azimuth on source direction suggesting that lateral structure is being observed no splitting was observed at stations near the coast of hudson bay continuing a pattern of weak splitting in surrounding proterozoic orogenic belts current absolute plate motion is consistent with the direction of anisotropy but would not explain the regional contrasts instead the anisotropy appears to be related to archean structure and tectonic history

geodynamic evidence for a chemically depleted continental tectosphere the tectosphere namely the portions of earth s mantle lying below cratons has a thermochemical structure that differs from average suboceanic mantle the tectosphere is thought to be depleted in its basaltic components and to have an intrinsic buoyancy that balances the mass increase associated with its colder temperature relative to suboceanic mantle inversions of a large set of geodynamic data related to mantle convection using tomography based mantle flow models indicate that the tectosphere is chemically depleted and relatively cold to 250 kilometers depth below earth s surface the approximate equilibrium between thermal and chemical buoyancy contributes to cratonic stability over geological time

heterogeneous bubble nucleation on fe ti oxide crystals in high silica rhyolitic melts the nucleation of h2o bubbles in magmas has been proposed as a trigger for volcanic eruptions to determine how bubbles nucleate heterogeneously in silicate melts experiments were carried out in which high silica rhyolitic melts were hydrated at 740 800 c and 50 175 mpa decompressed by 20 70 mpa and held at the lower pressures for 10 s before being quenched the hydration conditions were subliquidus and all samples contain blocky magnetite needle shaped hematite plagioclase magnetite is abundant at 800 c and high pressures whereas hematite becomes more abundant at lower temperatures and pressures bubbles nucleated in a single event in all samples with the number density nt of bubbles varying between 2 107 and 1 109 cm 3 at low degrees of supersaturation one to a few bubbles nucleate on faces of magnetite but at medium to high degrees of supersaturation multiple bubbles nucleate on single magnetite grains on hematite one to a few bubbles nucleated at the ends of the needle shaped crystals at medium supersaturations but formed along their entire lengths at high supersaturations nt increases as water diffusivity decreases indicating that the number of bubbles nucleated is influenced by their growth which depletes the melt with respect to h2o and lowers supersaturation if volcanic eruptions are triggered by bubble formation in magmas stored in shallow level magma chambers then the supersaturations needed for heterogeneous nucleation suggest that only small amounts of crystallization are needed whereas homogeneous nucleation is unlikely to trigger eruptions

thermal diffusivity of igneous rocks at elevated pressure and temperature thermal diffusivity measurements of seven igneous rocks were made to temperatures of 400 sup 0 c and pressures of 200 mpa the measuring method was based on the concept of cylindrical symmetry and periodic heat pulses the seven rocks measured were westerly rhode island granite climax stock nevada quartz monzonite pomona washington basalt atikokan ontario canada granite creighton ontario canada gabbro east bull lake ontario canada gabbro and stripa sweden granite the diffusivity of all the rocks showed a positive linear dependence on inverse temperature and excluding the east bull lake gabbro showed a linear dependence on quartz content quartz content varied from 0 to 31 by volume diffusivity in all cases rose or remained steady with increasing confining pressure the pressure effect was strongest at lowest pressures and vanished by levels between 10 and 100 mpa depending on rock type the pressure effect measured as a percentage change in diffusivity is stronger in the four rocks of granite composition than in the three of basaltic composition our results agree well with existing thermal diffusivity measurements at atmospheric pressure

geophysical evidence for chemical variations in the australian continental mantle 1 the relative density to shear velocity scaling provides a diagnostic for the presence of compositional variations in the mantle we invert shear wave velocity from a recent 3 d model and gravity anomalies for radial profiles of of the uppermost mantle beneath australia we performed calculations for the three major tectonic provinces that constitute the continent and found significant differences between them the profile for the phanerozoic region can be explained by thermal variations alone in contrast negative values of suggest that variations in composition are important between 75 and 150 km depth in the proterozoic continental lithosphere central australia it is likely that chemical variations are also required to explain the inferences for the archean craton west australia but poor tomographic resolution precludes a definitive conclusion the scaling factors found are consistent with chemical depletion of deep precambrian lithosphere which supports a tectosphere model for the australian continental roots

seismic anisotropy beneath the continents probing the depths of geology seismic anisotropy beneath continents is analyzed from shear wave splitting recorded at more than 300 continental seismic stations anisotropy is found to be a ubiquitous property that is due to mantle deformation from past and present orogenic activity the observed coherence with crustal deformation implies that the mantle plays a major if not dominant role in orogenies no evidence is found for a continental asthenospheric decoupling zone suggesting that continents are coupled to general mantle circulation

heat production in an archean crustal profile and implications for heat flow and mobilization of heat producing elements we have measured concentrations of heat producing elements th u and k in 58 samples representative of the main lithologies in a 100 km transect of the superior province of the canadian shield from the michipicoten wawa greenstone belt near wawa ontario through a domal gneiss terrane of amphibolite grade to the granulite belt of the kapuskasing structural zone near foleyet this transect has been interpreted as an oblique cross section through some 25 km of crust uplifted along a major thrust fault and thus provides an opportunity to examine in detail a continuous profile into deep continental crust of archean age mean heat production values for these terranes based on aereal distribution of major rock types and calculated from their th u and k concentrations are michipicoten greenstone belt 0 72 w m 3 wawa domal gneiss terrane amphibolite grade 1 37 w m 3 kapuskasing granulites 0 44 w m 3 among the silicic plutonic rocks tonalites granites and their derivative gneisses the relatively large variation in heat production correlates with modal abundances of accessory minerals including allanite sphene zircon and apatite we interpret these variations as primary pre metamorphic the relatively high weighted mean heat production of the domal gneiss terrane can be accounted for by the larger proportion there of late stage th u and k rich granitoid plutons these may have been derived from the underlying kapuskasing granulite terrane leaving it slightly depleted in heat producing elements transport of th u and k therefore could have taken place in silicate melts rather than in aqueous or carbonic metamorphic fluids this conclusion is supported by the lack of a statistically significant difference in heat production between tonalites tonalite gneisses and mafic rocks of amphibolite versus granulite grade the pre metamorphic radioactivity profile for this crustal section is likely to have been uniformly low with a mean heat production value less than 1 w m 3 this result is distinctly different from measured profiles in more silicic terranes which show decreasing heat production with depth this implies fundamental differences in crustal radioactivity distributions between granitic and more mafic terranes and may be an important factor in selective reactivation of lithologically different terranes possibly resulting in preferential stabilization of basic terranes in the geological record our results indicate that a previously determined apparently linear heat flow heat production relationship for the kapuskasing area does not relate to the distribution of heat production with depth low but significant heat production 0 4 0 5 w m 3 continues to lower crustal depths with no correlation to the depth parameter from the linear relationship this low heat production may be a minimum average granulite heat production and suggests that in general heat flow through the moho is 8 10 mw m 2 lower than the reduced heat flow calculated from the heat flow heat production regression

sustained blasts during large volcanic eruptions we carried out numerical simulations to investigate magma ascent in wide conduits during large explosive eruptions wide conduits allow high discharge rates low frictional pressure losses and shallow levels of explosive fragmentation of the magma within the conduit in contrast with the commonly modeled lower rate eruptions during which magma fragments inside the conduit at depth and feeds a vertical eruptive jet we find that for sufficiently high discharge rates 10 10 kg s 1 the fragmentation level may rise up to the surface gas rich unfragmented magma reaches the surface at high pressure and feeds a sustained volcanic blast geologic evidence for very high discharge rate eruptions wide conduits and shock waves in large pyroclastic flows supports the occurrence of this type of explosive eruption

lith5 0 a revised crustal model for canada based on lithoprobe results summary during the last 15 yr numerous seismic refraction experiments have been conducted overthe canadian landmass most as part of the lithoprobe initiative the new results providethe basis for revising existing global models of crustal structure e g crust5 1 mooney et al 1998 which do not incorporate the new lithoprobe data the crustal model discussedhere lith5 0 combines the lithoprobe and crust5 1 data to provide better constraints oncrustal structure in canada the objectives of the study are to 1 compile available new datainto a single consistent database of crustal thickness and velocity including the uppermostmantle pn velocity and 2 to construct a revised 5 5 crustal model of the canadianlandmass in a format that is generally compatible with existing global models the lithoproberesults suggest a 5 5 km average root mean square rms crustal thickness difference betweenlith5 0andcrust5 1 evenwiththeincorporationofthenewdata thegeographiccoveragein north and central canada remains sparse the new model does not explicitly incorporategeological constraints but instead relies on spherical spline interpolation for extrapolatingvalues into regions that lack control to illustrate the implications of this new crustal model we use lith5 0 to correct for the isostatically compensated crustal heterogeneity in order toobtain improved estimates of the dynamic surface topography the latter provides importantconstraints on the density structure of the uppermost mantle