magnetostratigraphic constraints on the age of the lower beaufort group western karoo basin south africa and a critical analysis of existing u pb geochronological data we studied three partially overlapping sections with a composite thickness of 600 m in the upper permian fluvial siltstones and fine grained sandstones of the abrahamskraal formation the basal unit of the beaufort group in the karoo basin of western cape province south africa paleomagnetic analysis reveals three components of natural remanent magnetization nrm heating to 180 c removes a remanent magnetization parallel to the present day field which is interpreted as a viscous overprint an intermediate unblocking temperature component is removed by heating to 450 c this direction is always of normal polarity and is identical to a regional overprint imparted during the early jurassic emplacement of the karoo large igneous province a high temperature component isolated above 450 c is of dual polarity and is interpreted as primary on the basis of a positive reversals test the virtual geomagnetic pole position for the abrahamskraal formation computed from the average high temperature characteristic remanent magnetization direction is in agreement with the late permian directions for stable gondwana and with previous results from the lowermost abrahamskraal formation and waterford formation at the ouberg pass section the predominantly normal polarity of this magnetization is in agreement with either a middle late lopingian age ca 254 256 ma or a late guadalupian age ca 262 ma according to the global geomagnetic polarity time scale we integrate these new results with existing magnetostratigraphic biostratigraphic and geochronological results from the karoo basin with particular emphasis on the controversy over zircon age data reported from the underlying ecca group

variations in column height and magma discharge during the may 18 1980 eruption of mount st helens abstract peak eruption column heights for the b1 b2 b3 and b4 units of the may 18 1980 fall deposit from mount st helens have been determined from pumice and lithic clast sizes and models of tephra dispersal column heights determined from the fall deposit agree well with those determined by radar measurements b1 and b2 units were derived from plinian activity between 0900 and about 1215 hrs b3 was formed by fallout of tephra from plumes that rose off pyroclastic flows from about 1215 to 1630 hrs a brief return to plinian activity between 1630 and 1715 hrs was marked by a maximum in column height 19 km during deposition of b4 variations in magma discharge during the eruption have been reconstructed from modelling of column height during plinian discharge and mass balance calculations based on the volume of pyroclastic flows and coignimbrite ash peak magma discharge occurred during the period 1215 1630 hrs when pyroclastic flows were generated by collapse of low fountains through the crater breach pyroclastic flow deposits and the widely dispersed co ignimbrite ash account for 77 of the total erupted mass with only 23 derived from plinian discharge a shift in eruptive style at noon on may 18 may have been associated with increase in magma discharge and the eruption of silicic andesite mingled with the dominant mafic dacite increasing abundance of the silicic andesite during the period of highest magma discharge is consistent with the draw up and tapping of deeper levels in the magma reservoir as predicted by theoretical models of magma withdrawal return to plinian activity late in the afternoon when magma discharge decreased is consistent with theoretical predictions of eruption column behavior the dominant generation of pyroclastic flows during the may 18 eruption can be attributed to the low bulk volatile content of the magma and the increasing magma discharge that resulted in the transition from a stable convective eruption column to a collapsing one

hydrometeor enhanced tephra sedimentation constraints from the 18 may 1980 eruption of mount st helens 1 uncertainty remains on the origin of distal mass deposition maxima observed in many recent tephra fall deposits in this study the link between ash aggregation and the formation of distal mass deposition maxima is investigated through reanalysis of tephra fallout from the mount st helens 18 may 1980 msh80 eruption in addition we collate all the data needed to model distal ash sedimentation from the msh80 eruption cloud four particle size subpopulations were present in distal fallout with modes at 2 2 4 2 5 9 and 8 3 settling rates of the coarsest subpopulation closely matched predicted single particle terminal fall velocities sedimentation of particles 100 m was greatly enhanced predominantly through aggregation of a particle subpopulation with modal diameter 5 9 0 2 19 3 m mammatus on the msh80 cloud provided a mechanism to transport very fine ash particles with predicted atmospheric lifetimes of days to weeks from the upper troposphere to the surface in a matter of hours in this mechanism ash particles initiate ice hydrometeor formation high in the troposphere subsequently the volcanic cloud rapidly subsides as mammatus develop from increased particle loading and cloud base sublimation rapid fallout occurs as the cloud passes through the melting level in a process analogous to snowflake aggregation aggregates sediment en masse and form the distal mass deposition maxima observed in many recent volcanic ash fall deposits this work provides a data resource that will facilitate tephra sedimentation modeling and allow model intercomparisons

the timing of the cape orogeny new 40ar 39ar age constraints on deformation and cooling of the cape fold belt south africa abstract we conducted a 40ar 39ar thermochronological study of samples from three transects across the cape fold belt of south africa metamorphic conditions along these transects decrease from the greenschist facies in the southern hinterland to the anchizonal foreland to the north nine new 40ar 39ar plateau ages and two mini plateau ages from handpicked muscovite are combined with one earlier published biotite mineral age and reveal two age modes the older population ranges from 261 3 to 276 5 ma n 4 and the younger from 248 2 to 254 6 2 1 ma n 4 the remaining three ages 400 ma are detrital and reflect unreset ages electron microprobe analyses and x ray diffraction studies indicate little mineralogical variation of micaceous material across the belt samples dominantly host muscovite clay minerals such as illite or smectite are absent microstructural characterization of rock fabrics across the belt reveals different mechanisms of strain partitioning and heterogenous deformation of different lithologies specifically pressure solution fracture and limited crystal plasticity are the dominant deformation mechanisms in the shale dominated bokkeveld group and witteberg groups in the foreland region resulting in recrystallization of phyllosilicate minerals strain partitioning into narrow locally deformed horizons of the table mountain group is accompanied by dynamic grain boundary recrystallization in quartz a weak trend of southward younging ages is interpreted as the preservation of deformation ages ca 275 260 ma and younger cooling ages ca 255 245 ma although it remains plausible that these populations represent two deformation events these new age constraints significantly narrow the temporal window of cape fold belt tectonic activity and provide a firm basis for evaluating the depositional patterns in the foreland karoo basin

macro and micro factors affecting coal seam quality and distribution in southern africa with particular reference to the no 2 seam witbank coalfield south africa abstract petrographical and palynological coal quality investigations relating to variations in one of the best known and well researched coalfields in africa the witbank coalfield and one of the most commercially important seams within this coalfield the no 2 seam are highlighted in this paper the investigations were based on 14 channel samples covering a 40 20 km 2 area in the witbank coalfield the results indicate that the coals in the no 2 seam were formed at the stage of major change over from dwyka to ecca floras as illustrated by the progressive dying out of dwyka pollens and spores and the introduction of ecca floral elements this also suggests a rapid amelioration of climate over the period of peat accumulation following a period of continental glaciation furthermore it is proposed that the in seam horizons reflect fluctuating of cyclic environments probably due to changing water level and plant communities due to changing growth environments saccate pollens appear to occur in association with vitrinite rich microlithotypes trilete spores with inertodetrite and carbominerite rich microlithotypes and plicates with inertite semifusite inertodetrite microlithotypes acritarchs nonspinose are present in parts of the seam where inertodetrite is common and in association with unusual suites of trace and major elements possibly indicating changing geochemical environments in high groundwater conditions petrographic results indicate that the coals are medium rank hypobituminous and show wide variations in maceral mineral and microlithotype compositions both vertically and laterally on the basis of microlithotype and mineral group analyses investigations were undertaken to establish vertical subdivisions by the study of historical patterns and possible lateral correlations of these subdivisions in the area under investigation these methods were then compared with distribution of selected chemical and geochemical parameters the evidence suggests that a common sequence of major events and some minor events occurred during the accumulation of the no 2 seam as manifested by petrographic and palynological facies three major periods or zones are recognized these tepresent periods of relatively stability quiescence separated by major in seam disruptive events namely flood deposits bearing high proportions of detrital organic material or minor clastic sedimentation these zones are subdivided into several subzones representing periods of reworking local flooding or rise in ground water levels and or changing geochemical environments these events can be recognized throughout the area under investigation but the manifestation of these events varies from area to area depending upon topographic situation proximity to river channels valley flanks or palaeoshorelines degree of subsidence and compaction geochemical environment and length of time under a specific set of conditions thus channels in close proximity to one another or in similar positions relative to valley flanks or axes appear to be remarkably similar to one another direct correlation of stratigraphic zones and sub zones from sample site of sample site was therefore possible although the methods to achieve this are complex and require the support of several supplementary complementary interpretative techniques coal quality variations and distribution within the original seam are therefore explicable in terms of the detailed factors listed above

global magsat anomaly maps at ground level abstract four maps of the vertical component of the magnetic anomaly field are derived at ground level from magsat data at satellite altitude measurements performed below and above an altitude of 425 km are treated as two independent datasets by two different methods 1 a local downward continuation inverse method and 2 a global spherical harmonic decomposition for each map one and two dimensional energy density spectra are obtained and correlation coefficients are computed it is shown that anomalies with wavelengths as short as 700 km can be significantly recovered at ground level corresponding to a ground resolution of 350 km the improved correlation between anomalies and geodynamic features achieved with downward continued maps is illustrated with the example of southeast asia

balloon borne observations of stratospheric aerosol and condensation nuclei during the year following the mt st helens eruption thirty six balloon flights carrying various particle counters are used to study the stratospheric aerosol layer during the year following the mt st helens volcanic eruption which occurred on may 18 1980 aerosol enhancements were observed to altitudes as high as 26 km for the first time stratospheric condensation nuclei r 0 01 m were observed in conjunction with a volcanic eruption time variation of these small particles suggests their continuous nucleation for at least 1 months after the eruption particles formed in the stratosphere following this eruption were generally smaller than those observed following the eruption of fuego in 1974 this may be related to a relative excess in condensation sites at the time of the mt st helens eruption thus resulting in smaller particles due to increased competition for the available sulfurous vapors involved in the nucleation process the excess mass concentration of volatile material which remained in the stratosphere about 6 months after the eruption was of the order 10 7g cm 2 column and if assumed to be uniform over the northern hemisphere would amount to about 0 25 tg this is about 3 times the background level but only one half that injected by the eruption of fuego in 1974

new late permian paleomagnetic data from argentina refinement of the apparent polar wander path of gondwana the late paleozoic early mesozoic apparent polar wander path of gondwana is largely constructed from relatively old paleomagnetic results many of which are considered unreliable by modern standards paleomagnetic results derived from sedimentary sequences which are generally poorly dated and prone to inclination shallowing are especially common here we report the results of a joint paleomagnetic geochronologic study of a volcanic complex in central argentina u pb dating of zircons has yielded a robust age estimate of 263 0 1 6 2 0 ma for the complex paleomagnetic analysis has revealed a pretilting primary permian magnetization with dual polarities rock magnetic experiments have identified pseudo single domain titano magnetite and hematite as the mineralogic carriers of the magnetization lightning induced isothermal remagnetizations are widespread in the low coercivity magnetic carriers the resulting paleomagnetic pole is 80 1 s 349 0 e a95 3 3 n 35 and it improves a late permian mean pole calculated from a filtered south american paleomagnetic data set more broadly this new high quality igneous based paleomagnetic pole falls between the previously distinct late permian segments of the laurussian and gondwanan apparent polar wander paths suggesting that the long recognized disparity between these large paleomagnetic data sets may be primarily due to the inclusion of low quality or systemically biased data

particle sedimentation and diffusive convection in volcanic ash clouds 1 understanding the longevity of volcanic ash clouds generated by powerful explosive eruptions is a long standing problem for assessing volcanic hazards and the nature and time scale of volcanic forcings on climate change it is well known that the lateral spreading and longevity of these clouds is influenced by stratospheric winds particle settling and turbulent diffusion observations of the recent 2010 eyjafjallajokull and 2011 grimsvotn umbrella clouds as well as the structure of atmospheric aerosol clouds from the 1991 mt pinatubo event suggest that an additional key process governing the cloud dynamics is the production of internal layering here we use analog experiments on turbulent particle laden umbrella clouds to show that this layering occurs where natural convection driven by particle sedimentation and the differential diffusion of primarily heat and fine particles give rise to a large scale instability where umbrella clouds are particularly enriched in fine ash this particle diffusive convection strongly influences the cloud longevity more generally cloud residence time will depend on fluxes due to both individual settling and diffusive convection we develop a new sedimentation model that includes both sedimentation processes and which is found to capture real time measurements of the rate of change of particle concentration in the 1982 el chichon 1991 mt pinatubo and 1992 mt spurr ash clouds a key result is that these combined sedimentation processes enhance the fallout of fine particles relative to expectations from individual settling suggesting that particle aggregation is not the only mechanism required to explain volcanic umbrella longevity

long wavelength magnetic anomalies in the caribbean plate boundaries and allochthonous continental blocks vector magnetic anomaly maps are derived at satellite altitude from magsat data over the caribbean and middle america to minimize smoothing of the anomaly field caused by averaging the data over the whole altitude range of magsat measurements two maps are derived in two nonoverlapping altitude ranges 350 420 and 420 550 km a composite map of the vertical component of the anomaly field is then computed at ground level this map displays large positive anomalies associated with fast subduction zones these anomalies define discontinuities in the downgoing slab associated with seismic gaps positive anomalies are also consistently observed above allochthonous blocks of continental origin in the caribbean area and suggest that the average magnetization is larger in the continental lithosphere elongated magnetic lows are observed above transform boundaries of the caribbean plate the cayman trough anomaly is consistent with magnetization of the order of 1 a m in the upper mantle demagnetization of the lithosphere along the bocono el pilar fault system which bounds the caribbean plate to the south may be accounted for by frictional heating along shear zones the pattern of anomalies along the southern margin of the united states is related to its accretionary tectonic history various estimates of the vertically integrated magnetization of both oceanic and continental lithospheres are computed using these anomalies and provide constraints on the vertical distribution of lithospheric sources of magsat anomalies

geophysical interpretation of the magnetic anomalies of china derived from magsat data the ambiguities about geophysical implications based on the correlation of scalar magnetic anomalies and geological features were investigated a method was developed to convert scalar magnetic anomalies into a map of the lateral variations of magnetic susceptibility of the lithosphere this map is directly correlated with the causative sources the method is based on spherical harmonic analysis of lateral variations seen on the scalar magnetic anomaly map and those of the lithospheric magnetic susceptibility the harmonic coefficients are related through the fundamental causality relationship governing a magnetized body and its associated scalar magnetic anomaly the main features of the resulting magnetic susceptibility anomalies are outlined

can the thermal thickness of the continental lithosphere be estimated from magsat data abstract the long wavelength magnetic anomaly field derived from magsat data is modelled by large scale lateral magnetization variations in the continental lithosphere of europe central and southeast asia a simple model with six domains with uniform magnetization is shown to be consistent with this long wavelength anomaly field in this model all major alpine mountain belts are associated with magnetization lows and proterozoic cratons with magnetization highs in particular a significant contrast of magnetization is associated with the tornquist tesseyre line the structural boundary between the paleozoic western part of europe and the proterozoic eastern domain it is thus proposed that the magnetization contrasts and their associated long wavelength magnetic anomalies may be attributed to differences of tectono thermal age of the lithosphere the main magnetization contrasts displayed by the model are then shown to be associated with seismic velocity contrasts this observation supports the hypothesis that magnetization variations are generated by magnetic sources distributed over the whole upper lithosphere down to the depth of the 600 c isotherm to support this conclusion it is shown that the magnetic thickness predicted by the magnetization model obtained is in good agreement with the depth of the 600 c deduced from heat flow measurements

the downward continuation of magsat crustal anomaly field over southeast asia a general method for the local downward continuation at ground level of geopotentials measured by low orbiting satellites at different altitudes is proposed such a continuation is the inverse solution of the external dirichlet problem the stability of the solution with respect to the altitude and the distribution of data points is discussed principal component analysis is shown to perform well and leads to a stable solution the method proposed has the advantage of being directly applicable to measured data points which are not evenly spaced and at different altitude as an illustration it is applied to magsat data over southeast asia two independent maps of the vertical component of the anomaly field at a constant altitude near ground level are derived for dawn descending orbits and dusk ascending orbits data separately the two maps reveal similar anomalies and confirm the correlation of the magsat derived anomaly field with the mosaic structure of the continental crust in southeast asia stable continental blocks are associated with positive anomalies and intervening collision belts correspond to well defined negative anomalies a very prominent low is observed above the himalaya and extends over the tibet plateau confirming an anomalous crustal and thermal structure of the plateau

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

magnetization of the pacific ocean lithosphere deduced from magsat data magsat anomalies over the pacific ocean are related to volcanic plateaus and the edge effect of the pacific cretaceous quiet zone pcqz four prominent plateaus shatsky rise the mid pacific mountains hess rise and manihiki plateau are considered in this study the major lateral magnetization contrasts associated with these plateaus are considered to arise from the topographic swells and the crustal roots with root thicknesses estimated from topography using an airy isostatic model shatsky rise and the mid pacific mountains are older than the cretaceous quiet period and their natural remanent magnetization nrm is not a significant component of the magnetization contrasts that cause the magsat anomalies associated with these plateaus their magnetic properties are characterized by effective magnetic susceptibility contrasts of 0 07 0 01 s i per unit volume averaged over their topographic swells and crustal roots equivalent to 0 04 if averaged over the entire crustal thicknesses swell plus root plus intervening layer this value agrees well with the estimates of previous investigators for these plateaus and for similar plateaus in other oceans two plateaus of cretaceous quiet period age within the pcqz hess rise and manihiki plateau have origins similar to those of shatsky rise and the mid pacific mountains and they probably have similar effective susceptibility contrasts but they may also carry cretaceous quiet period nrm magnetic anomalies due to hess rise and manihiki plateau calculated from the effective susceptibility contrast deduced for shatsky rise and the mid pacific mountains are substantially larger than the observed magsat anomalies of these plateaus there are also magsat anomalies over parts of the pacific where no plateaus exist some of these anomalies are demonstrated as probably due to a contrast in nrm between the pcqz and its surroundings the total remanent magnetization contrast magnetization contrast in amperes per meter times thickness in meters equivalent to the dipole moment per unit area of the pcqz is 14 000 20 000 amperes 30 which cannot be simply explained by the absolute values of remanent magnetization predicted by conventional models of oceanic crustal magnetic properties modelling of the cretaceous quiet zones in the northern atlantic indicates a similar total remanent magnetization contrast these results may be explained by including uppermost mantle remanent magnetization with an intensity of about 1 2 a m 1 if magnetization persists to a depth of 7 below the seismic moho i e down to the depth of the crustal roots of the plateaus including nrm in the uppermost mantle of the pcqz also serves to reduce the calculated amplitude of the hess rise and manihiki plateau magsat anomalies if the nrm of the crustal roots is less than the nrm of the surrounding uppermost mantle on this basis the estimated intensity of nrm in the uppermost 7 km of the pcqz mantle is 1 3 a m 1 in very good agreement with the intensity estimated from the pcqz edge effect anomalies these estimates of uppermost mantle nrm are probably a maximum limit recognizing that other sources may also contribute to the total remanent magnetization contrasts of the cretaceous quiet zones

the global continent ocean magnetization contrast abstract a schematic global model for the distribution of magnetization in the lithosphere is devised this model is characterized by a uniform stepwise change of radially integrated susceptibility at continent ocean boundaries and an axial dipolar inducing field a spherical harmonic expansion of the magnetic field produced by this source distribution is computed then the standard method of truncation at degree n 13 is used to determine the respective contribution of this field to the main field and to the lithospheric anomaly field the latter is shown to display significant anomalies near continental margins as well as within continents and oceanic basins the energy density spectrum of this field is shown to be very similar to the spectrum of the magsat derived lithospheric anomaly field furthermore several features of this field are shown to account for unexplained anomalies of the magsat field this supports the hypothesis that such an abrupt magnetization variation may occur at several continental margins and contribute to the signal in the magsat derived long wavelength anomaly maps this hypothesis provides an order of magnitude estimate for the magnetization difference between oceanic and continental lithospheres finally it is argued that the synthetic anomaly field produced by this model provides an upper bound to the energy density spectrum of the lithospheric field at low harmonic degrees and concluded that the lithospheric field does not contribute significantly to the earth s field below degree 13

trajectories of the mount st helens eruption plume the plume of the major eruption of mount st helens on 18 may 1980 penetrated 10 to 11 kilometers into the stratosphere attaining heights of 22 to 23 kilometers wind shears rapidly converted the plume from an expanding vertical cone to a thin slightly inclined lamina the lamina was extruded zonally in the stratosphere as the lower part moved eastward at jet stream velocities while the upper part slowly moved westward in the region of nonsteady transition from the westerlies to the summer stratospheric easterlies trajectories computed to position the nasa u 2 aircraft for sampling in the plume are described plume volume after 8 hours of strong volcanic emission is estimated at 2 x 106cubic kilometers only about 1 percent of this volume is attributed to the volcano the rest was entrained from the environment

small scale features in the earth s magnetic field observed by magsat a spherical harmonic expansion to degree and order 29 is derived using a selected magnetically quiet sample of magsat data global maps representing the contribution due to terms of the expansion above n 13 at 400 km altitude are compared with previously published residual anomaly maps and shown to be similar even in polar regions an expansion with such a high degree and order displays all but the sharpest features seen by the satellite and gives a more consistent picture of the high order field structure at a constant altitude than do component maps derived independently the appendix is available with the entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b83 008 2 50 payment must accompany order

new global vector magnetic anomaly maps derived from magsat data new global maps of the vector magnetic anomaly field of earth are derived from magsat data improvement on earlier published vector and scalar maps is achieved with the following procedure 1 the noise level of vector measurements associated with the discontinuities of the in flight sensor attitude determination is reduced by selecting the data 2 a monthly model of the main field is computed to account for secular variation using only quiet day data 3 the time dependence of the ring current effect is taken into account using hourly values of the dst index based on ground obseivations 4 sectorial indices based on ground observations are introduced to select profiles corresponding to low magnetic activity these indices provide a better discrimination of quiet data than the global kp index usually used for that purpose 5 a time and longitude dependent dynamic model of the equatorial electrojet is derived and used to correct magnetic measurements at low latitudes anomaly maps of x and z components are obtained using selected measurements within the reduced altitude range 350 425 km these maps are compared with earlier magsat anomaly maps and their respective resolutions are discussed these maps evidence large anomalies of lithospheric origin within the oceanic domain in particular the z map displays a characteristic pattern of long wavelength anomalies in these areas it is first shown that some of these anomalies particularly in the north atlantic and indian oceans may be correlated with the age of the lithosphere then a significant similarity between magnetic and geoid anomalies above ocean basins is evidenced this suggests that long wavelength oceanic magnetic anomalies may also be attributed to variations of the thickness of the magnetized layer associated with ocean floor topography

simulation of the 1980 eruption of mount st helens using the ash tracking model puff abstract the dispersal of volcanic ash from the may 18 1980 eruption of mount st helens msh has been simulated using the lagrangian ash tracking model puff previous applications of the model were limited to smaller short lived eruptions with ash dispersal occurring mainly within the troposphere two high resolution atmospheric reanalysis datasets era 40 and ncep ncar 40 allowed msh ash cloud dispersal to be simulated up to 30 km elevation the 1980 eruption was divided into two distinct eruptive phases 1 an initial relatively short lived blast surge phase that injected ash up to 30 km and 2 a subsequent nine hour plinian phase that maintained an average eruption column height of 16 km using puff the two phases of the msh eruption were modeled separately based on a range of individual input parameters and then combined to produce an integrated simulation of the entire eruption the trajectory and areal extent of the modeled atmospheric ash cloud best match the actual distribution of msh ash when input parameters are set to values inferred from satellite and radar data collected on may 18 1980 the prevailing wind field exerts the strongest control on the advection and ultimate position of the modeled ash cloud making the maximum column height and the vertical distribution of ash the most sensitive of the puff input parameters for this event the results indicate that the puff model works well at simulating the dispersal of ash injected well into the lower stratosphere from a moderate relatively long lived eruption such as msh however attempts to use puff to recreate some granulometric aspects of the msh fallout deposit such as the maximum particle size as a function of distance from source were not successful puff consistently predicts much greater fallout distances for small ash particles

magnetic anomaly modeling at the indo eurasian collision zone abstract one of the consistent features of satellite crustal magnetic anomaly maps is the large negative anomaly in the indo eurasian collision zone this region is modeled by the introduction of magnetic blocks representing the tibetan asian and himalayan mountain regions the crustal thickness of these blocks is based on the principle of isostasy it has been shown that the removal of the core field from the magsat data removes most of the long wavelength anomalies associated with the ocean continent susceptibility contrast and that some of the observed anomalies after processing of the satellite data are a manifestation of this contrast to account for this the final model is a superposition of the above block model on a global model of typical continent and ocean susceptibility called a standard earth magnetization model semm a combination of the three blocks superposed on the semm reproduces the major features of the observed magsat anomalies block model susceptibilities of the asian and himalayan regions are the same as for the continental portion of the semm albeit with a greater crustal thickness thus the crust in these regions is typical by contrast the satellite magnetic data require an integrated susceptibility in the tibetan region which is considerably lower than typical two possible explanations are suggested one that the origin and tectonic history of the tibetan region differs from the neighboring regions in such a way as to result in lower susceptibility this is consistent with current thinking that the tibetan plateau is made up of several fragments or micro continents which were attached to asia prior to the collision of india with asia the other possible explanation is that the curie isotherm in the tibetan region is elevated compared to the asian and himalayan regions

the 1980 eruptions of mount st helens physical and chemical processes in the stratospheric clouds we investigate the physical and chemical properties of the stratospheric clouds produced by the recent eruptions of the mount st helens volcano the large and diverse set of observational data collected in the high altitude plumes of the may 18 may 25 and june 13 1980 eruptions is organized and analyzed for evidence of the processes at work the data are used to guide and constrain detailed model simulations of the volcanic clouds for this purpose a comprehensive one dimensional model of stratospheric sulfate aerosols sulfur precursor gases and volcanic ash and dust is utilized the model accounts for gas phase and condensed phase heterogeneous chemistry in the clouds aerosol nucleation and growth and cloud expansion computational results are given for the time histories of the gaseous species concentrations aerosol size distributions and ash burdens of the eruption clouds the long term buildup of stratospheric aerosols in the northern hemisphere and the persistent effects of injected chlorine and water vapor on stratospheric ozone are also investigated we conclude that so2 water vapor and ash were probably the most important substances injected into the stratosphere by the mount st helens volcano both with respect to their widespread effects on composition and their impact on climate despite the seemingly large magnitude of the mt st helens eruption we also find that the volcano had little influence on the climate 0 05 k surface cooling in the northern hemisphere or on stratospheric ozone 0 2 maximum hemispherical depletion the remaining problems in the field of volcanic cloud physics and chemistry and the requirements for future experimental work are also discussed

magsat scalar anomalies and major tectonic boundaries in asia the distribution of magsat scalar anomalies is compared with locations of major tectonic boundaries in asia foldbelts separate large scale positive and negative anomalies inside and outside the siberian platform and sediment filled aulacogens within the platform separate the anomalies into two parts which overlie the anabar and aldan shields tectonic boundaries appear to exercise some control over the extent and distribution of the satellite observed crustal anomalies

magnetic measurements aboard a stratospheric balloon abstract we have measured the total intensity of the earth s magnetic field above the western mediterranean along a 2000 km profile flown at an altitude of 40 km a proton magnetometer was aboard a cnes centre national d etudes spatiales stratospheric balloon the core field and transient external field contributions have been carefully determined and removed from the data the resulting profile reveals significant anomalies of 10 20 nt amplitudes with 100 1200 km wavelengths these anomalies have a clear crustal origin and can be related to heterogeneities in magnetisation in the oceanic algero provencal basin and to major faults of the spanish continental crust comparison of our new data with previous data obtained over the same area at about 1 km altitude aeromagnetic surveys and about 400 km altitude magsat demonstrates the potential of stratospheric balloon measurements in bridging the 200 1500 km gap that exists in the knowledge of crustal anomalies a gap that is unavoidable if only aeromagnetic and satellite like magsat data are used

influence of particle aggregation on deposition of distal tephra from the may 18 1980 eruption of mount st helens volcano the may 18 1980 eruption of mount st helens msh produced an extensive ashfall deposit in washington idaho and montana with a minumum volume of 0 55 km3 tephra an unusual feature of the deposit is the occurrence of a second thickness maximum 325 km ene of msh near ritzville washington grain size and component abundance analysis of samples along the main dispersal axis indicates that ash in this region is very fine grained mean size 22 m poorly sorted polymodal and rich in glass shards and pumice fragments a computer simulation of ash fallout from an atmospherically dispersed eruption plume was developed to evaluate various hypotheses for the origin of the distal ash characteristics particularly the thickness versus distance relationship the model was constrained by observations of the eruption column height elevation of major ash transport lateral spreading of the eruption plume and atmospheric wind structure in the vicinity of msh results of different simulations indicate that the second thickness maximum cannot be attributed to either decreased wind velocities over central washington or injection of fine ash above the horizontal wind velocity maximum near the tropopause for the model to fit the observed characteristics of the deposit significant particle aggregation of ash finer than 63 m must be invoked the best fit occurs when ash less than 63 m is aggregated into particles several hundred microns in diameter with a settling velocity of 0 35 m s support for this process comes from the observation and collection of fragile ash clusters of similar size which fell at pullman washington during the may 18 eruption sorem 1982 the premature fallout of fine ash as particle aggregates is a fundamental process in the origin of the grain size characteristics variations in component abundances and thickness versus distance relationship of the may 18 msh ash fall deposit

minimal crustal magnetizations from satellite data abstract the minimum amplitude crustal magnetization needed to explain magsat vector anomaly data is found using harmonic splines as basis functions shure et al 1982 phys earth planet inter 28 215 229 and choosing a subset of the original basis parker and shure 1982 geophys res lett 9 812 815 to represent magnetization we suggest the optimum model of this type is obtained by associating representers only with north and vertically downwards data points given the larger uncertainties associated with east component magsat data following parker et al 1987 rev geophys 25 17 40 we find a model minimizing the r m s volume integral of magnetization over the magnetized lithosphere the numerical effort involved in producing a regional or continental scale model with given horizontal spatial detail with our method is less than that for an equivalent source dipole model when the magnetization is not assumed induced approximately equal to that for an induced magnetization equivalent dipole model and considerably less than that for a spherical harmonic apparent susceptibility model the method is demonstrated by producing a solution for two regions the contiguous usa for comparison with previously published models and africa and its continental margins magnetization models fit the data slightly worse than those of the downward continued anomaly field at the earth s surface comparison of our component of magnetization for the usa in the direction of the core field with the induced magnetization equivalent dipole model of mayhew 1984 earth planet sci lett 71 290 296 is very good as expected for magnetization either induced by or frozen remanently in an approximately dipole field either parallel or antiparallel to the current rotation axis the east component of magnetization is much smaller than the north and vertically downwards components we make no assumptions about the relative contributions of remanent and induced magnetization and the resulting models have a substantial remanent component however our models suffer from the severe non uniqueness common to all magnetization models whereby magnetization in a uniform susceptibility and thickness shell concentric with the earth s centre owing to any internal magnetic field that is then switched off generates no external magnetic field

magnetoclimatology a test of the wind vigour model using the 1980 mount st helens ash abstract magnetic mineralogy is now commonly employed in the study of past climate changes but the underlying magnetoclimatological mechanisms are not fully understood many important instances have been reported where pedogenesis leads to magnetic enhancement warmer more humid conditions produce higher magnetic signals however significant exceptions occur wherein colder more arid conditions lead to higher magnetic content one suggested explanation for the latter is the so called wind vigour model in which glacial intervals are associated with stronger more frequent winds which are more efficient at entraining and transporting dense iron oxide particles this notion is tested here by determining the relevant magnetic properties of the ash produced by the 1980 eruption of mount st helens a clear exponential decrease of magnetic susceptibility with increasing distance from the vent is observed this supports the wind vigour concept at least in some environments as a viable alternative to the pedogenetic model of magnetoclimatology

abrupt and gradual extinction among late permian land vertebrates in the karoo basin south africa the karoo basin of south africa exposes a succession of upper permian to lower triassic terrestrial strata containing abundant terrestrial vertebrate fossils paleomagnetic magnetostratigraphic and carbon isotope data allow sections to be correlated across the basin with this stratigraphy the vertebrate fossil data show a gradual extinction in the upper permian punctuated by an enhanced extinction pulse at the permian triassic boundary interval particularly among the dicynodont therapsids coinciding with negative carbon isotope anomalies

support for an a type pangea reconstruction from high fidelity late permian and early to middle triassic paleomagnetic data from argentina 1 a major disparity is observed between the late paleozoic early mesozoic apparent polar wander paths apwps of laurussia and gondwana when the landmasses are re assembled in a conventional a type pangea this discrepancy has endured from the earliest paleomagnetic reconstructions of the supercontinent and has prompted discussions of non dipole paleomagnetic fields and alternative paleogeographic models here we report on a joint paleomagnetic geochronologic study of late permian and early to middle triassic volcanic and volcaniclastic rocks from argentina which demonstrates support for an a type model without requiring modification to the geocentric axial dipole hypothesis new shrimp u pb and 40ar 39ar isotopic dating has reinforced the inferred age of the sequences which we estimate at 264 ma upper choiyoi group and 245 ma puesto viejo group field stability tests demonstrate that the volcanic rocks are carrying early primary magnetizations which yield paleopoles 73 7 s 315 6 e a95 4 1 n 40 upper choiyoi and 76 7 s 312 4 e a95 7 3 n 14 puesto viejo a comprehensive magnetic fabric analysis is used to evaluate structural restorations and to correct for magnetization anisotropy the paleomagnetic results derived from volcaniclastic rocks are interpreted to be affected by inclination shallowing and corrections are discussed a comparison of these new results with the existing permian triassic paleomagnetic data from gondwana suggests the presence of widespread bias in the latter we contend that such bias can explain the observed apwp disparity at least for late permian middle triassic time and that alternative paleogeographic reconstructions or non dipole paleomagnetic fields do not need to be invoked to resolve the discrepancy

mean ionospheric field correction for magsat data to obtain a meaningful anomaly map showing magnetic fields of crustal origin from satellite observations it is necessary to account adequately for and subtract the fields generated by the currents flowing in the ionosphere conventionally corrections are made separately for each path by empirical methods of fitting and subtracting linear quadratic or harmonic functions from the original data this procedure however lacks a sound physical basis we examined the field components after the main field and the magnetospheric field were subtracted from magsat data and found certain structures in the residual field the residual fields at 0600 and 1800 local times were separately averaged over longitude to obtain the effective contribution of ionospheric current system at these local times which is called the mean ionospheric field mif to obtain the crustal anomalies the mif was subtracted from the residual fields to account for the day to day variation in the ionospheric current system and for the difference in satellite altitude anomaly maps are constructed on 5 5 grid averages the dawn and dusk anomaly maps after the correction agree well with each other showing that the effect of the ionospheric field is well taken care of by the mif correction the amplitude of anomalies is large even at comparatively long wavelength and the maps successfully represent these crustal anomalies which have been greatly reduced in the previous maps

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

contribution of induced and remanent magnetization to long wavelength oceanic magnetic anomalies the magsat mission has provided very precise measurements of earth s magnetic field which have been used to derive accurate maps of the long wavelength magnetic anomaly field analyses of this field over localized areas have shown that induced magnetization is mainly responsible for the anomalies and that remanent magnetization is rarely detectable at magsat altitude a careful analysis of the data taking into account the various effects due to non lithospheric sources allows this field to be better described and consequently suggests two observations 1 long wavelength oceanic anomalies display many similarities with the distribution of seafloor topography suggesting a relationship between crustal thickening and magnetization enhancement 2 age related pattern i e ridge parallel of anomalies is observed in two of the larger oceanic basins indian and north atlantic oceans several forward models are constructed to investigate these correlations in the first model the effect of variations in crustal thickness was modeled assuming uniform susceptibility and that thickness varies proportionally to the seafloor topography in the second model the field due to remanent magnetization acquired during the cretaceous long normal period was modeled and shown to contribute significantly to the magsat field in order to make the modeled and observed maps fully comparable several effects must be taken into account a global magnetization contrast exists between oceans and continents and induces widely distributed anomalies because of the truncated spherical harmonic analysis the spectral content of the observed map in the north south direction is reduced because of the along track filtering due to the polar orbit of magsat when these effects are taken into account the model combining the contribution of induced and remanent magnetization display remarkable similarity to observed data this comparison constrains both induced and remanent magnetization of oceanic lithospheric rocks

seafloor spreading anomalies in the magsat field of the north atlantic we show that a simple magnetization model for the crust of the north atlantic basin will explain the major anomalies of the magsat intermediate wavelength magnetic field over the basin our seafloor spreading model incorporates anomalous skewness and enhanced magnetization near the spreading axis and at the cretaceous quiet zones the anomalies over the young oceanic crust indicate a decrease in susceptibility away from the spreading axis the enhanced magnetization over the cretaceous quiet zone could be due to an increased main field intensity or a change in the spreading center processes during the cretaceous though similar observations are not readily apparent for other basins the north atlantic correlation indicates that an investigation of the remanent magnetization of the ocean crust could provide a standard for the evaluation of the intermediate wavelength magnetic field the models presented in this paper demonstrate that a sizable percentage of the intermediate wavelength energy in the magsat field is derived from sources within the earth s crust it also demonstrates that remanent magnetization is an important source of the magsat field over oceanic areas we seek to demonstrate in this paper that although components of the main field are likely to contribute to the intermediate wavelength scalar field we can correlate the anomaly field over the north atlantic to the well studied thermal remanent magnetization of the oceanic crust with a minor component of induced magnetization associated with younger oceanic crust

mercury content of equisetum plants around mount st helens one year after the major eruption the mercury content of young equisetum plants collected around mount st helens was higher in the direction of yakima and toppenish washington northeast to east northeast than at any other compass heading and was about 20 times that measured around portland oregon the increase in substratum mercury was not as pronounced as that in plants but was also higher toward the northeast the direction taken by the may 1980 volcanic plume

band limited global scalar magnetic anomaly map of the earth derived from magsat data the magnetometer satellite magsat data are separated into two distinct sets the dawn and the dusk sets the two sets are analyzed independently because of the dawn dusk asymmetry of the external magnetic field components of the data two scalar magnetic anomaly maps the dawn and the dusk maps are derived using the spherical harmonic expressions of the respective data set the significant correlation of the two maps in the harmonic range of 18 41 indicates the repeatability of the magsat data in these harmonics the shortest repeatable wavelength is about 920 km a band limited global scalar magnetic anomaly map is derived at about 410 km altitude by arithmetic averaging of the dawn and the dusk maps the map is then downward continued to about 10 km altitude the downward continued magnetic anomalies are relatively more localized the main features of the anomalies are as follows 1 continents have relatively stronger magnetic anomalies than oceans especially the younger oceans 2 many circum pacific subduction zones have magnetic signatures and 3 there is no overall correlation between anomalies and major shields

wind driven dispersal of volcanic ash plumes and its control on the thermal structure of the plume top the dispersal of an ash plume as it propagates downwind with the ash settling under gravity is presented it is shown that wind shear the initial plume height and the plume grain size distribution have an important role in the dispersal of the ash in particular in determining the evolution of the upper surface and the leading edge of the ash cloud once the ash has thermally equilibrated with the atmosphere the temperature of the upper surface of the ash plume is directly related to its altitude as a result we can use the model to interpret satellite images of the temperature of the upper surface of ash plumes these calculations are compared with new analyses of satellite data from the 18 may 1980 eruptions of mount st helens in which both thermal infra red and visible goes satellite data were examined in accord with the data the model is able to predict the rate at which the mount st helens ash plume propagated downwind through a combination of the wind shear and gravitational settling of different size particles the model is also able to explain the observed thermal structure of the upper surface of the ash plume in which the temperature initially decreases but then increases with downwind distance as the ash falls through the tropopause