cosmic impact event in lake sediments from central europe postdates the laacher see eruption and marks onset of the younger dryas abstractthe younger dryas yd climate episode 12 850 11 650 calendar years before present cal bp is an event recorded widely across the northern hemisphere we conducted multiple analyses at

rabaul caldera papua new guinea volcanic hazards surveillance and eruption contingency planning abstract planning for volcanic contingencies at the rabaul caldera complex is a difficult task requiring urgent attention in view of the results of volcano surveillance and historical eruption frequency historical eruption periodicities have ranged from about 2 to 59 years the most recent eruptions were in 1937 1943 and caused more than 500 fatalities in may 1937 surveillance since 1971 has revealed uplift of part of the caldera floor and increasing seismicity in a ring structure believed to define the caldera bounding fault or faults these phenomena are interpreted as the prelude to a possible eruption potential eruption sites are all close to rabaul town and outlying residential areas and include the volcanoes of sulphur creek last active about 1850 vulcan and tavurvur 1937 1943 and rabalanakaia but new sites should also be considered in relation to the results of volcano surveillance a wide range of volcanic hazards accompanying different scales of eruption must be considered for the rabaul area the most likely types of eruption may be similar to the relatively small events that have been witnessed over the last century whose hazards included air fall tephra pumice rafts pyroclastic flows and surges volcanic earthquakes volcanic gas discharges tsunamis lightning strikes and the fall of mud rain torrential run off and mudflows as a result of accompanying rain storms other potential eruption types of much larger scale could involve cauldron subsidence which could cause destructive tsunamis and could be accompanied by explosive activity of major proportions air fall tephra is considered to be one of the main hazards from a future eruption and its effects in and around rabaul are primary influences in the choice of evacuation routes and points of refuge hence theoretical tephra dispersal patterns have been considered in some detail air fall tephra deposition from rabaul eruption clouds at heights of less than about 4 km is determined by a biannual change in wind direction from southeast in may to october november trade winds to northwest during the monsoon from december to march april in contrast clouds that rise higher than about 4 km are blown by east winds all year round another principal hazard is pyroclastic flows and surges which took place during the 1937 vulcan eruption but have until recently been overlooked although flows or surges or both are clearly distinguishable on eruption photographs the lack of suitable evacuation routes and problems in choosing safe and reasonably accessible refuge points compound the difficulties in preparing a practicable contingency plan however in small moderate eruptions the safest areas around rabaul are considered to be at the duke york islands about 30 km to the east and at cape gazelle about 25 km to the southeast seaborne evacuation to main ports in central new britain or on the neighbouring islands of new ireland and bougainville or overland evacuation using a southwards expanded road system would be necessary in the event of forewarning of a major eruption

magnetic remanence in the murchison meteorite the murchison meteorite is a carbonaceous chondrite containing a small amount of chondrules various inclusions and matrix with occasional porphyroblasts of olivine and or pyroxene it also contains amino acids that may have served as the necessary components for the origin of life magnetic analyses of murchison identify an ultrasoft magnetic component due to superparamagnetism as a significant part of the magnetic remanence the rest of the remanence may be due to electric discharge in the form of lightning bolts that may have formed the amino acids the level of magnetic remanence does not support this possibility and points to a minimum ambient field of the remanence acquisition we support our observation by showing that normalized mineral magnetic acquisition properties establish a calibration curve suitable for rough paleofield determination when using this approach 1 2 of the natural remanence left in terrestrial rocks with trm and or crm determines the geomagnetic field intensity irrespective of grain size or type of magnetic mineral with the exception of hematite the same method is applied to the murchison meteorite where the measured meteorite remanence determines the paleofield minimum intensity of 200 2000 nt during and or after the formation of the parent body

a millennium of geomagnetism 1 the history of geomagnetism began around the year 1000 with the discovery in china of the magnetic compass methodical studies of the earth s field started in 1600 with william gilbert s de magnete gilbert 1600 and continued with the work of among others edmond halley charles augustin de coulomb carl friedrich gauss and edward sabine the discovery of electromagnetism by hans christian oersted and andre marie ampere led michael faraday to the notion of fluid dynamos and the observation of sunspot magnetism by george ellery hale led sir joseph larmor in 1919 to the idea that such dynamos could sustain themselves naturally in convecting conducting fluids from that came modern dynamo theory of both the solar and terrestrial magnetic fields paleomagnetic studies revealed that the earth s dipole had undergone reversals in the distant past and these became the critical evidence in establishing plate tectonics finally the recent availability of scientific spacecraft has demonstrated the intricacy of the earth s distant magnetic field as well as the existence of magnetic fields associated with other planets and with satellites in our solar system

volcanic ash clusters tephra rafts and scavengers abstract much of the volcanic ash deposited in the vicinity of pullman washington from the may 18th eruption of mt st helens in 1980 fell in the form of composite ash clusters 0 25 to 0 5 mm in diameter particles in the clusters ranged from sub micron to more than 40 microns in size the porous clusters rafted large ash particles great distances and scavenged particles of all sizes as the wind blew them eastward the result is poorly sorted ash deposits as far as 644 km east missoula within individual clusters textural relationships suggest that both mechanical interlocking and electrostatic attraction played a role in cluster formation the clusters are very fragile and may never be preserved in the stratigraphic record but it is suggested that this kind of ash transport may well have occurred in the geologic past documentation of this ash cluster fall will promote a better understanding of the physical aspects of the downwind distribution of fine grained volcanic ash the poor size sorting in some ancient ash deposits may thus be explained

lodestone natures only permanent magnet what it is and how it gets charged magnetite and titanomagnetite exhibit magnetic properties which are attributable to the micro structures developed during oxidation and exsolution all magnetite iron ores which are lodestones contain maghemite these lodestones have hc between 10 and 30 mt sirm between 8 and 18 am kg and ri between 0 10 and 0 26 magnetite titanomagnetite and metals have rem values ratio of nrm to sirm 0 05 samples called fulgarites obtained from the smithsonian institution have rem values ranging from 0 45 to 0 92 the rem value serves as a witness parameter to the magnetic fields associated with the lightning bolt if a high rem value say 0 1 can be verified as not to be due to contamination by man and does not contain md hematite then the rock has lrm lightning remanent magnetization the magnetic field associated with lightning can be revealed from an isothermal remanent acquisition ra curve

the role of hematite ilmenite solid solution in the production of magnetic anomalies in ground and satellite based data abstract remanent magnetization rm of rocks with hematite ilmenite solid solution hiss minerals at all crustal levels may be an important contribution to magnetic anomalies measured by ground and satellite altitude surveys the possibility that lower thermal gradient relatively deep in the crust can result in exsolution of hiss compositions with strong remanent magnetizations rm was studied for two bulk compositions within the hiss system samples from granulite terrane around wilson lake labrador canada contains titanohematite with exsolved ferrian ilmenite lamellae other samples from the anorthosite terrane of allard lake quebec canada contain ferrian ilmenite with exsolved titanohematite lamellae in both cases the final exsolved titanohematite has similar ti content and carries dominant magnetic remanence with rem nrm sirm where nrm is the natural remanent magnetization and sirm is the saturation isothermal remanent magnetization that is comparable to the ti free end member the rm was acquired prior to exsolution and the ilmeno hematite rich rock possesses thermal remanent magnetization trm whereas rocks with hemo ilmenite possess chemical remanent magnetization crm in both cases we found fairly large homogeneous grains with low demagnetizing energy that acquired intense rm the magnetism of the ilmeno hematite solid solution phases is not significantly perturbed by the continuous reaction ilmeno hematite titanohematite solid solution hence the occurrence of hiss in rocks that cooled slowly in a low intensity magnetic field will have an intense magnetic signature characterized by a large rem

magnetic petrology of arc xenoliths from japan and aleutian islands xenoliths from japan and the aleutian islands were studied for the purpose of identifying source rocks with sufficient magnetization to account for positive subduction zone magsat anomalies two types of magnetic lithologies are identified 1 amphibolites and homblende gabbros containing up to 8 6 titanomagnetite mostly as large unoxidized multidomain crystals and 2 pyroxene gabbros and metagabbros with up to 20 3 titanomagnetite usually in association with primary ilmenite the first type is represented by xenoliths from ichinomegata japan and adak island in the aleutians the second restricted to iki island japan and kanaga island aleutians has abundant coarse grained titanomagnetite which in most samples has been oxidized producing secondary trellis type ilmenite lamellae a few iki island xenoliths show more advanced oxidation arc xenoliths show 3 orders of magnitude range in natural remanent magnetization and saturation remanence and about 2 orders of magnitude range in magnetic susceptibility nearly all iki island rocks and many from ichinomegata and the aleutians possess induced magnetization 2 a m 1 and greater with the expectation of additional magnetization due to viscous remanence at elevated temperature the tio 2 content of titanomagnetite together with the presence ot appreciable al 2 o3 and mgo ensure that most xenoliths have curie points between 400 and 550 c remanence acquisition curves exhibit a range reflecting varying degrees of magnetic hardening due to oxidation correspondingly each xenolith site groups distinctly on a plot of magnetic hysteresis ratios this paper indicates that lithologies with total magnetizations of many a m 1 exist in lower crust arc settings their volumetric distribution is uncertain based on the limited xenolith population reported herein

two archaeomagnetic intensity maxima and rapid directional variation rates during the early iron age observed at iberian coordinates implications on the evolution of the levantine iron age anomaly abstract variations of geomagnetic field in the iberian peninsula prior to late iron age times are poorly constrained here we report 14 directional and 10 palaeointensity results from an archaeomagnetic study carried out on 17 combustion structures recovered from six archaeological sites in eastern spain the studied materials have been dated by archaeological evidences and supported by radiocarbon dates 8th 5th centuries bc rock magnetic experiments indicate that the characteristic remanent magnetization is carried by a low coercivity magnetic phase with curie temperatures of 500 575 c most likely titanomagnetite maghemite with low titanium content archaeointensity determinations were carried out by using the classical thellier thellier experiment including ptrm checks and magnetic anisotropy corrections a new full vector iberian paleosecular variation curve for the iron age is presented high fluctuation rates on both directions and intensities are observed during the early iron times that seems to be related with the levantine iron age anomaly liaa the most prominent anomaly of the geomagnetic field of the last three millennia two intensity maxima were observed at iberian coordinates the oldest around 750 bc associated with easterly declinations of around 23 and the second 275 yrs later 475 bc with northerly directions the related virtual axial dipole moment was up to 14 10 22 am2 for the oldest materials 750 bc and reaching 16 10 22 am2 for the materials corresponding to the end of the early iron age in order to investigate the origin of the unusually high fluctuations of the palaeofield we have developed a new global geomagnetic field reconstruction the shawq ironage model which is based on a critical revision of the global archeomagnetic and volcanic dataset the new model provides an improved description of the evolution of the liaa which is related to a normal flux patch at the core mantle boundary cmb below arabian peninsula clearly observed at around 950 bc this flux patch expanded towards the north west while decreasing in intensity reaching iberia at around 750 bc around 600 500 bc it underwent a revival below the european continent after that it seems to vanish in situ

trm arm and isr of two natural magnetites of md and psd grain size summary trm arm and isr stabilities with respect to alternating fields and high and low temperature treatment are compared for a set of artificial rock specimens the specimens contain grain size fractions of a homogeneous natural magnetite and a natural magnetite showing exsolution lamellae the grain size fractions are in the psd md range and vary between 5 and 250 m for large md grains the coercivity spectra of arm are similar to those of isr but differ from those of trm particularly in the lower part of the spectrum for small 5 m grains the coercivity spectrum of arm is similar to that of trm and both spectra are different from the coercivity spectrum of isr the use of arm instead of trm to determine palaeo intensities is discussed in relation to the results obtained during high and low temperature treatment arm behaves similarly to isr but unlike trm for all grain sizes studied the results of this study are discussed in terms of a remanence composed of psd surface moments and md and psd bulk moments

paleointensity determination using anhysteretic remanence and saturation isothermal remanence 1 determining the strength of ancient planetary magnetic field is pivotal to understanding the evolution of planets and asteroids in the solar system while the thellier type double heating technique provides the most faithful field strength information for rocks carrying a thermoremanent magnetization trm many extraterrestrial rock samples respond unfavorably to heat treatment the present study systematically examined the two well known normalization techniques that avoid any heating by comparing the ratios of trm anhysteretic remanent magnetization arm and trm saturation isothermal remanent magnetization sirm both the ratios of trm arm and trm sirm are dependent on the grain size as well as the volume concentration of magnetite the remanence ratios also were found to increase as the alternating field increased during demagnetization for fine grained magnetite a new calibration relation of 2 60 1 32 for the trm arm and of 3 62 1 28 10 2 for the trm sirm was defined for an external field of 50 t the trm sirm is superior to trm arm because the former showed less dispersion in the grain size dependence the volume concentration dependence and the stability against af demagnetization in addition the standard error of the mean for the trm sirm ratio was smaller than that for the trm arm ratio thus whenever heating is inapplicable the sirm method seems to be a better choice than the arm method however it should be emphasized that the uncertainty of trm arm and trm sirm is still nearly an order of magnitude larger than that of the high fidelity thellier estimation and thus must be limited in use only when samples are irreversibly altered during heating in practice the best approach is to carry out both arm and sirm methods and check whether the two estimations agree within the uncertainties

tephra from the 1979 soufriere explosive eruption the explosive phase of the 1979 soufriere eruption produced 37 5 x 10 6 cubic meters dense rock equivalent of tephra consisting of about 40 percent juvenile basaltic andesite and 60 percent of a nonjuvenile component derived from the fragmentation of the 1971 1972 lava island during phreatomagmatic explosions the unusually fine grain size poor sorting and bimodality of the land deposit are attributed to particle aggregation and the formation of accretionary lapilli in a wet eruption column

stratified flow in pyroclastic surges stratified flow theory is applied to pyroclatic surges in an effort to gain insight into transport dynamics during explosive eruptions particle transport is assumed to be by turbulent suspension and calculations contained herein show that this is likely for many cases including the 18 may 1980 blast at mount st helens the discussion centers on the rouse number pn which represents a ratio of particle settling velocity to scale of turbulence the brunt vaisala frequency n which is the maximum possible frequency of internal waves the froude number fr representing the ratio of inertial forces to gravitational forces and the richardson number ri a ratio of buoyant restoring forces to turbulent mixing forces the velocity or flow power dependence of bed form wavelength in surge deposits is related to a velocity dependence of wavelength of internal waves in the turbulent surge this produces a decrease in dune wavelength with increasing distance from vent migration direction of bed forms is related tofr as it is defined for a continuously stratified flow proximal to distal facies variations in surge deposits reflect increasingpn andri as the flows move away from their sources this produces the progression from sandwave to massive to planar facies with increasing distance from vent where the long axis of topography is at low angles to the flow direction massive facies in topographic lows may from concurrently with sandwave facies on highs due to the higher particle concentration in the lows where long axis of topography is at high angles to flow direction denser lower parts of the surge may be dammed or blocked blocked material tends to form massive flows that may move down slope independent of the overriding surge a model incorporating turbulent transport stratified flow and time evolution of pyroclastic surges is proposed for deposits which have been attributed to both pyroclastic flow and pyroclastic surge transport by various workers during the initial high energy waxing phase of the eruptive event pn is sufficiently low that only coarse but poorly sorted material is deposited to form relatively coarse bottom layers as the event wanes remaining finer material is deposited through a thin bed load to produce overlying bedded and cross bedded veneer deposits throughout most of the event blocking occurs to produce relatively thick and massive deposits in valley bottoms

mechanism of the 1979 explosive eruption of soufriere volcano st vincent abstract the vulcanian explosive phase of the soufriere of st vincent in 1979 occurred when magma penetrated the water logged 1971 1972 lava island in the center of the crater lake the 13 day period of explosive and volcanoseismic activity ceased when the crater lake had dried up and subsequent activity was restricted to quiet lava extrusion the explosive phase produced 15 5 105m3 dense rock equivalent d r e of new magma and 22 106m3 d r e of lithic material from the 1971 1972 island whereas the following extrusive phase produced a dome of volume 47 106m6 considerations of the energetics of the explosive eruption indicate that the available heat content of the new magma 5 8 1016j was not sufficient to account for the observed phenomena including evaporation of the 25 106m3 crater lake and that additional 8 1016 j of heat remaining in the 1971 1972 lava mass are required to drive the eruption plumes field relations the high proportion of lithic material and the energy relations of the eruption are consistent with a phreato magmatic eruption mechanism involving a three way interaction of the 1971 1972 lava mass crater lake water and magma as a consequence of net veining and thermally induced hydraulic fracturing of the water logged 1971 1972 lava island by new magma in 1979

sedimentology and geomorphology of the deposits from the august 2006 pyroclastic density currents at tungurahua volcano ecuador the deposits of the pyroclastic density currents from the august 2006 eruption of tungurahua show three facies associations depending on the topographic setting the massive proximal cross stratified and distal cross stratified facies 1 the massive facies is confined to valleys on the slopes of the volcano it contains clasts of 1 m diameter to fine ash material is massive and interpreted as deposited from dense pyroclastic flows its surface can exhibit lobes and levees covered with disk shaped and vesicular large clasts these fragile large clasts must have rafted at the surface of the flows all along the path in order to be preserved and thus imply a sharp density boundary near the surface of these flows 2 the proximal cross stratified facies is exposed on valley overbanks on the upper part of the volcano and contains both massive coarse grained layers and cross stratified ash and lapilli bedsets it is interpreted as deposited from a dense pyroclastic flows that overflowed the gentle ridges of valleys of the upper part of the volcano and b dilute pyroclastic density currents created from the dense flows by the entrainment of air on the steep upper flanks 3 the distal cross stratified facies outcrops as spatially limited isolated and wedge shaped bodies of cross stratified ash deposits located downstream of cliffs on valleys overbanks it contains numerous aggrading dune bedforms whose crest orientations reveal parental flow directions a downstream decrease in the size of the dune bedforms together with a downstream fining trend in the grain size distribution are observed on a 100 m scale this facies is interpreted to have been deposited from dilute pyroclastic density currents with basal tractional boundary layers we suggest that the parental flows were produced from the dense flows by entrainment of air at cliffs and that these diluted currents might rapidly deposit through pneumatic jumps three modes are present in the grain size distribution of all samples independently of the facies which further supports the interpretation that all three facies derive from the same initial flows this study emphasizes the influence of topography on small volume pyroclastic density currents and the importance of flow transformation and flow stripping processes

luogufengite a new nano mineral of fe2o3 polymorph with giant coercive field luogufengite al bearing e fe 2 o 3 is a new member of fe 2 o 3 polymorphs discovered in late pleistocene basaltic scoria from the menan volcanic complex nearby rexburg idaho it is an oxidation product of fe bearing basaltic glass at high temperature and is associated with maghemite and hematite luogufengite is an euhedral or semi euhedral nano mineral with its crystal size ranging from 20 to 120 nm the mineral has a space group of pna 2 1 its unit cell parameters refined from synchrotron x ray powder diffraction pattern are a 5 0647 3 b 8 7131 6 c 9 3842 5 a and z 4 calculated density 4 905 g cm 3 with the doubled hexagonal abac packing of oxygen atoms the eight strongest lines of the measured x ray diffraction pattern d a i hkl are 3 197 27 3 022 2 945 29 1 013 2 708 100 122 2 437 35 8 131 1 716 24 4 204 1 507 40 7 135 and 1 458 37 2 330 the empirical formula is fe 1 71 al 0 24 mg 0 02 ti 0 03 o 3 the crystals display 110 twins with twin boundaries of 110 100 and 130 due to their pseudo hexagonal symmetry luogufengite is an important mineral that records paleomagnetism of volcanic rocks because of its large magnetic coercivity this unique magnetic property of the mineral may explain the observed unusually high remanent magnetization in some igneous and metamorphic rocks and even martian rocks with high remanent magnetization some intergrowths of magnetite with ilmenite exsolution lamellae or hematite with magnetite lamellar precipitates have luogufengite like 2d crystalline characteristics with the doubled hexagonal packing at the interface between cubic and rhombohedral structures luogufengite like nano domains at the magnetite hematite interfaces might be responsible for the large coercive field of lodestones that are partially oxidized magnetite with hematite micro precipitates

dilute gravity current and rain flushed ash deposits in the 1 8 ka hatepe plinian deposit taupo new zealand two groups of poorly sorted ash rich beds previously interpreted as rain flushed ashes occur in the ca ad 180 hatepe plinian pumice fall deposit at taupo volcano new zealand two ash beds with similar dispersal patterns and an aggregate thickness of up to 13 cm make up the lowermost group a group a beds extend 45 km north east of the vent and cover 290 km2 in the southern part of the group a distribution area a coarse ash to lapilli size plinian pumice bed deposit b separates the two group a beds the scarcity of lapilli material seen elsewhere from the still depositing pumice fall in group a beds indicates that they were rapidly transported and deposited however this rapid transportation and deposition did not produce cross bedding nor did it erode the underlying deposits it is proposed that thick 600 m but dilute gravity currents generated from the collapsing outer margin of the otherwise buoyant hatepe plinian eruption column deposited the group a beds the upper ash beds group c consist of one to seven layers attain an aggregate thickness of 35 cm and vary considerably in thickness and number of beds with respect to distance from vent group c beds contain variable amounts of ash mixed with angular plinian pumices and are genuine rain flushed ashes several recent eruptions at other volcanoes ukinrek maars vulcan rabaul la soufrere de guadeloupe and soufriere st vincent have produced gravity currents similar in style but much smaller than those envisaged for group a deposits the overloaded margins of otherwise buoyant eruption plumes generated these gravity currents laboratory studies have produced experimental gravity current analogues hazards from dilute gravity currents are considerable but often overlooked thus the recognition of gravity current deposits will contribute to more thorough volcanic hazard assessment of prehistoric eruption sequences

the influence of palaeotopography on facies architecture and pyroclastic flow processes of a lithic rich ignimbrite in a high gradient setting the abrigo ignimbrite tenerife canary islands abstract the late pleistocene 188 19 ka abrigo ignimbrite represents the last major explosive caldera forming eruption on tenerife it consists almost entirely of a moderate volume 1 8 km 3 light grey phonolitic lithic rich ignimbrite deposit although a possible abrigo member thin fall deposit is locally preserved beneath the ignimbrite suggesting an initial short lived stable plinian eruption column the ignimbrite deposit is dispersed radially around the lower flanks of the las canadas edifice and correlates to a clast supported lithic breccia on the eastern and northern caldera wall much of the ignimbrite was probably deposited offshore or within the caldera now buried by more recent lavas two major depositional units separated by a co ignimbrite ash cloud deposit and which display dramatic variations in thickness pyroclastic flows were channelised along and deposited within the same valleys that exist today and were commonly continuous and sheet like across the interfluves the basal zones of pyroclastic flows are envisaged as a bed load of coarse lithic lapilli and blocks transported under their own momentum and with the support of the surrounding fine ash gas granular flow

comment on a paper the origin of high magnetic remanence in fault pseudotachylites theoretical considerations and implication for coseismic electrical currents by e c ferre m s zechmeister j w geissman n mathana sekaran and k kocak

aspects of the validation of magnetic remanence in meteorites meteorite magnetic records constitute physical evidence of processes acting during early solar system evolution consequently the validation of these records is important in meteorite research the first step in the validation process should be the rem value the rem value is the ratio of natural remanence nrm to saturation remanent magnetization imparted by a 1 t magnetic field sirm the rem values range over 3 to 4 orders of magnitude for stony meteorites and for chondrules from allende c3v s1 bjurbole l4 s1 and chainpur ll3 s1 meteorites the rem values computed from published nrm and sirm data identify many orders of magnitude range in the rem values including rem values 100 10 3 these data suggest a dependence for the nrm intensity on the curatorial location from which the sample was obtained any earth rock acquiring thermoremanent magnetization trm in the geomagnetic field has a restricted range in rem mostly between 5 and 50 10 3 the exception being the mineral hematite in the multidomain size range the only terrestrial samples with rem much greater than 100 10 3 are those struck by lightning the rem value provides a physical basis for recognition between valid records and those that might be contaminated the isothermal remanence acquisition ra curve is presented as a contamination curve that allows an indication of the level of magnetic field contamination required to give the computed rem rm sirm value in the case of the bjurbole and chainpur chondrules with rem values 100 10 3 the ra curve indicates that unrealistically large contamination magnetic fields would be required to give rem values greater than 100 10 3 this would suggest contamination other than by a hand magnet that is normally available to an experimenter this would require an explanation that would involve large magnetic fields during chondrule formation or some extraordinary remanence acquisition mechanism that remains to be described magnetic contamination experiments using 80 and 40 mt magnets demonstrate that the rem values and extent of modification of the magnetic vector record are mineralogy dependent and this is mostly related to the amount and characteristics of the mineral tetrataenite the complexity of the meteorite records suggest validation of the record as a first step the rem value is the first physical statement that can be made in this validation

unique thermoremanent magnetization of multidomain sized hematite implications for magnetic anomalies intense magnetic remanence 100 1000 a m associated with md hematite and or titanohematite and associated with high koenigsberger ratios 40 1000 indicate that magnetic remanence may dominate the total magnetization if these minerals are volumetrically significant titanohematite behaves similarly to hematite and thus the grain size dependence of trm acquisition in hematite is considered as a generalization the transition between truly md behavior and tendency towards sd behavior in hematite has been established to be between grain sizes of 0 1 and 0 05 mm in contrast to magnetite and titanomagnetite hematite exhibits inverse grain size dependence with md hematite acquiring a relatively intense trm in the geomagnetic field comparable to sub micrometer sized magnetite and only an order of magnitude less than sd magnetite consequently md hematite and by analogy titanohematite remanence may be of significance as a source of magnetic anomalies at all scales md hematite exhibits trm weak field acquisition behavior that is different from all other magnetic minerals being the only magnetic mineral having an rem trm sirm value e0 1 for trm acquisition in the geomagnetic field the very different trm behavior of md hematite in contrast to magnetite is due to two factors the first is the lesser influence of demagnetizing energy with respect to wall pinning energy at temperatures almost up to the curie temperature for hematite the second is the greater importance of the magnetostatic energy in the applied field which for hematite dominates the total energy at high temperatures fl 2000 published by elsevier science b v all rights reserved

the study of earth s magnetism 1269 1950 a foundation by peregrinus and subsequent development of geomagnetism and paleomagnetism 1 this paper summarizes the histories of geomagnetism and paleomagnetism 1269 1950 the role of peregrinus is emphasized in the sixteenth century a debate on local versus global departures of the field from that of an axial dipole pitted gilbert against le nautonier regular measurements were undertaken in the seventeenth century at the turn of the nineteenth century de lamanon de rossel and von humboldt discovered the decrease of intensity as one approaches the equator around 1850 three figures of rock magnetism were fournet remanent and induced magnetizations delesse remagnetization in a direction opposite to the original and melloni direction of lava magnetization acquired at time of cooling around 1900 brunhes discovered magnetic reversals in the 1920s chevallier produced the first magnetostratigraphy and hypothesized that poles had undergone enormous displacements matuyama showed that the earth s field had reversed before the pleistocene our review ends in the 1940s when exponential development of geomagnetism and paleomagnetism starts

fall out and deposition of volcanic ash during the 1979 explosive eruption of the soufriere of st vincent abstract the 1979 explosive activity of soufriere volcano st vincent commenced on april 13th and continued for two weeks producing eleven eruption columns the activity produced air fall ash with abundant accretionary lapilli plus minor pyroclastic flows and base surge deposits the dispersal of the columns was observed by an sms 1 geostationary satellite all but two of the plumes were dispersed entirely to the east the last event on april 26th formed a minor lobe to the east and a major lobe to the south southeast the april 26th column was observed to rise to 8 km within five minutes and ascended to a maximum of 14 km in the next hour the magma discharge rate was greater than 3000 m3 s the plume expanded at a rate of 4000 km2 hr and was dispersed at a mean velocity of 15 m s the ash deposits show bimodal grain size distribution with 95 of the ash collected being sub 1 mm diameter modelling of the ash fall out has allowed interpretation of grainsize parameters in a quantitative manner the fall velocities of ash particles appear to have been reduced by turbulence and downwind ascent of the plume which is attributed to fallout of ash reducing the plume density the coarse population of the april 26th deposit displayed a systematic change in md and away from source the fine mode displays uniform md and everywhere and is interpreted as having been deposited in the form of accretionary lapilli

ferric iron content in mg fe sio3 perovskite and post perovskite at deep lower mantle conditions we have determined the fe 3 fe ratio of al free mg fe sio 3 perovskite post perovskite and mg fe o ferropericlase synthesized at 99 to 187 gpa and 1830 to 3500 k based on the electron energy loss near edge structure elnes spectroscopy the results demonstrate that post perovskite includes minor amounts of ferric iron with fe 3 fe ratios of 0 11 to 0 21 these values are substantially lower than those of al rich post perovskite fe 3 fe 0 59 to 0 69 reported in a previous study suggesting that the fe 3 al 3 coupled substitution is important in post perovskite as in the case of perovskite the al bearing post perovskite in a pyrolitic mantle composition likely contains a considerable amount of ferric iron which affects various physical properties in the lowermost mantle

evidence for deposition of 10 million tonnes of impact spherules across four continents 12 800 y ago airbursts impacts by a fragmented comet or asteroid have been proposed at the younger dryas onset 12 80 0 15 ka based on identification of an assemblage of impact related proxies including microspherules nanodiamonds and iridium distributed across four continents at the younger dryas boundary ydb spherule peaks have been independently confirmed in eight studies but unconfirmed in two others resulting in continued dispute about their occurrence distribution and origin to further address this dispute and better identify ydb spherules we present results from one of the largest spherule investigations ever undertaken regarding spherule geochemistry morphologies origins and processes of formation we investigated 18 sites across north america europe and the middle east performing nearly 700 analyses on spherules using energy dispersive x ray spectroscopy for geochemical analyses and scanning electron microscopy for surface microstructural characterization twelve locations rank among the world s premier end pleistocene archaeological sites where the ydb marks a hiatus in human occupation or major changes in site use our results are consistent with melting of sediments to temperatures 2 200 c by the thermal radiation and air shocks produced by passage of an extraterrestrial object through the atmosphere they are inconsistent with volcanic cosmic anthropogenic lightning or authigenic sources we also produced spherules from wood in the laboratory at 1 730 c indicating that impact related incineration of biomass may have contributed to spherule production at 12 8 ka an estimated 10 million tonnes of spherules were distributed across 50 million square kilometers similar to well known impact strewnfields and consistent with a major cosmic impact event

the basal ash deposit of the sovana eruption vulsini volcanoes central italy the product of a dilute pyroclastic density current a low aspect ratio decimeter thick ash deposit axisymmetrically distributed around the latera caldera western vulsini volcanoes central italy has been studied by means of field and laboratory investigations field studies comprise facies analysis at centimeter scale and maximum clast size and deposit thickness measurements grain size and component distribution chemical composition and particle morphoscopic features have been determined on selected samples we discuss the co ignimbrite ash fall vs pyroclastic surge origin of the deposit and the hydrovolcanic vs magmatic eruption nature complex facies association textural features and grain size data rule out an ash fall origin for the whole deposit the hydrovolcanic nature of the eruption has been discarded on the grounds of componentry and morphoscopic features of vitric fragments we propose that the main body of the ash deposit formed from a radially expanding dilute turbulent pyroclastic density current originated by a continuous collapse of a low altitude a few kilometers eruptive column with a possible radial jet component

magnetic study of meteorites recovered in the atacama desert chile implications for meteorite paleomagnetism and the stability of hot desert surfaces we conducted a detailed paleomagnetic study of 18 ordinary chondrites recovered in a hot desert the atacama desert chile to clarify the significance of the natural remanent magnetizations of weathered meteorites we show that the more weathered meteorites weathering grade w3 possess a stable magnetization unblocked over two temperature intervals 140 320 c and 500 550 c this magnetization was acquired on earth as indicated by the paleomagnetic directions of large meteorites 150 g that are in the direction of the terrestrial field rock magnetic data suggest that this stable magnetization is a chemical remanent magnetization acquired by maghemite and magnetite upon their formation as weathering products on the other hand the natural remanent magnetization of ordinary chondrites showing little or no weathering weathering grades w0 and w1 are much weaker and very unstable upon demagnetization a similar behavior to the magnetization of meteorite falls therefore up to the weathering grade w1 weathering does not largely overprint the original extraterrestrial magnetization of these meteorites this is confirmed at microscopic scale by magnetic imaging tetrataenite and kamacite have randomly oriented magnetization at mineral scale that is canceled out at larger scale 1 mm 3 whereas a small fraction of the weathering products has a weak but unidirectional magnetization that masks the weak extraterrestrial magnetization this suggests that extraterrestrial magnetization can survive mild weathering below weathering grade w2 and that such meteorites remain worthwhile targets for study of extraterrestrial paleomagnetism for meteorites with weathering grade w3 or more the extraterrestrial signal can only be studied at microscopic scale these data also show that over a time scale of several kyr to several tens of kyr large stones 150 g have remained still at the surface of the atacama desert whereas smaller ones have moved only by rotation around vertical axes evidencing the fine scale stability of this desertic surface

an example of enhanced tephra deposition driven by topographically induced atmospheric turbulence spatial variations in the thickness and grain size characteristics of tephra fall deposits imply that tephra depositional processes cannot be fully captured by models of single particle sedimentation from the base of the eruption plume here we document a secondary thickness maximum in a 9 75 ka tephra fall deposit from chaiten volcano chile cha1 eruption this secondary thickness maximum is notably coarser grained than documented historical examples being dominated by medium grained ash and an origin via particle aggregation is therefore unlikely in the region of secondary thickening we propose that high levels of atmospheric turbulence accelerated particles held within the mid to lower troposphere 0 to 6 km towards the ground surface we suggest that this enhancement in vertical atmospheric mixing was driven by the breaking of lee waves generated by winds passing over elevated topography beneath the eruption plume lower atmospheric circulation patterns may exert a significant control on the dispersal and deposition of tephra from eruption plumes across all spatial scales particularly in areas of complex topography

bimodal grain size distribution and secondary thickening in air fall ash layers investigation of the ashfall deposit from the 1980 mount st helens eruption reveals that the deposit is bimodal the finer population maintaining a uniform grain size distribution away from source and that the deposit exhibits secondary thickening beyond the 1 cm isopach grain size distribution shows that at least 64 of the deposit is finer than 63 m far more than is generally supposed for plinian eruptions an investigation of other ashfall deposits reveals similar grain size bimodality features which are interpreted as evidence of aggregation of fine particles in the eruption cloud our data suggest that aggregation may be a common process during ash fallout from explosive eruptions and that secondary thickening may often occur providing that other conditions are met

stratigraphy petrography and dispersion of the lower permian syn eruptive deposits in the viar basin spain abstract this paper presents a stratigraphic petrographic and sedimentologic analysis of felsic syn eruptive deposits of the southernmost permian basin in europe viar basin an inner orogenic transtensional half graben located between two major geotectonic zones of the iberian variscan belt sw spain the syn eruptive deposits of the viar basin represent a mappable lithostratigraphic unit that is herein named the los canchales formation this unit forms a wedge shaped body bounded by alluvial fluvial deposits red layers that are in gradational contact the stratigraphic architecture of the los canchales formation is complex with frequent facies changes and interfingering among pyroclastic rocks resedimented volcaniclastic rocks and sedimentary rocks abundant fossil flora is also recorded a wax wane cycle of volcanic activity is recognized during the syn eruptive period two short lived volcanic episodes were recorded including ignimbrites and a possible phreatomagmatic event explosive volcanism generated pyroclastic flow surge and ash fall deposits in cases collapsing the drainage network they also modified the vegetation and created a new depositional setting with four successive sedimentary environments vegetated floodplain ephemeral sandy braidplain palustrine environment and again vegetated floodplain input of pyroclasts followed a nw to se longitudinal axial dispersal pattern

deposition and generation of multiple widespread fall units from the c ad 1314 kaharoa rhyolitic eruption tarawera new zealand the c ad 1314 kaharoa eruption from tarawera volcano north island new zealand was a complex long duration multi episode rhyolitic event up to 13 discrete sub plinian to plinian style phases occurred in two stages from at least two different vents along an 8 km long fissure the early explosive stage produced seven pumice lapilli fall units dispersed solely to the southeast of the volcano while a later stage produced six fall units dispersed almost entirely to the north and northwest the duration of fallout for these 13 lapilli units is estimated to have totaled about 1 5 days the presence of massive textured ash layers between many of the lapilli fall units suggests that longer duration pyroclastic density current producing phases and more quiescent fallout intervals occurred between the intense explosive phases of the eruption total duration of ash fallout 7 5 days the ash deposition extended the minimum duration of the explosive episodes to at least 9 days based on settling rates of the ash possibly the explosive phase of eruptive activity lasted for 2 weeks including some brief hiatuses and pyroclastic density current deposition and was followed by 4 years of mainly lava dome forming activity at least 15 km3 7 3 km3 dense rock equivalent dre of tephra was erupted during the main explosive episodes and at least 9 1 km3 dre during the whole eruption more than doubling the previous estimate this may be an underestimate by up to 50 of the fallout volume eruption column heights for the two most widely dispersed fall units are estimated to have ranged from 25 28 km above the vents and durations of individual lapilli fall unit forming phases ranged from 1 5 to 4 h but the ash producing phases lasted much longer individually up to 30 h and represent a longer lived shut down to each eruptive episode time averaged mass discharge rates are estimated to have varied by more than one order of magnitude during the main plinian style phases from 107 to 108 kg s however the explosive phases were unsteady as indicated by fluctuating grain size with stratigraphic height in the lapilli fall units and common capping ash beds the north northwesterly dispersal of the later stage fall units and the possible duration of 4 days for these episodes indicate an unusual period of southerly to southeasterly winds therefore the main explosive part of the kaharoa eruption is most likely to have occurred during the austral winter to spring months the kaharoa eruption style is also shared by the three previous rhyolite dominated eruptions that have built tarawera volcano over the past 22 000 years knowledge of the widespread pumice and ash fallout from the kaharoa eruption is important in quantifying the hazard from this type of extended duration explosive activity and has implications for future similar rhyolitic activity in new zealand

how accurately can nrm sirm determine the ancient planetary magnetic field intensity abstract the present study tested grain size dependence of thermoremanent magnetization trm saturation isothermal remanent magnetization sirm as well as its stability against demagnetization the trm sirm ratio is dependent on the grain size of magnetite suggesting that a strong constraint on grain size characterization is necessary to be used as a paleointensity proxy in planetary magnetism in addition the trm sirm ratio increases as the alternating field increases for fine grained magnetite accuracy of trm sirm was investigated using historic showa lava erupted in 1946 it was observed that the natural remanent magnetization nrm sirm ratio of 0 032 is comparable to the field intensity of 46 80 t however the uncertainty of nrm sirm was an order of magnitude larger than that of the companion thellier estimation therefore the nrm sirm ratio can only provide crude estimations on the absolute planetary magnetic field intensity in practice anisotropy correction is advisable to reduce the scatter of nrm sirm

co ignimbrite ash fall deposits of the 1991 eruptions of fugen dake unzen volcano japan abstract fugen dake the main peak of unzen volcano began a new eruption sequence on november 17 1990 on may 20 1991 a new lava dome appeared near the eastern edge of the fugen dake summit small scale 10 4 10 6 m 3 in volume merapi type block and ash flows were frequently generated from the growing lava dome during may june 1991 these pyroclastic flows were accompanied by co ignimbrite ash plumes that deposited ash fall deposits downwind of the volcano three examples of co ignimbrite ash fall deposits from unzen pyroclastic flows are described the volume of fall deposits was estimated to be about 30 by volume of the collapsed portions of the dome that formed pyroclastic flows this proportion is smaller than that described for other larger co ignimbrite ash fall deposits from other volcanoes grain size distributions of the unzen co ignimbrite ash fall deposits are bi modal or tri modal most ashes are finer than 4 phi and two modes were observed at around 4 7 phi and 9 phi they are composed mainly of groundmass fragments fractions of another mode at around 2 phi are rich in crystals derived from dome lava some of the fine ash component fell as accretionary lapilli from the co ignimbrite ash cloud indicating either moisture or electrostatic aggregation we believe that the co ignimbrite ash of unzen block and ash flows were formed by the mechanical fracturing of the cooling lava blocks as they collapsed and moved down the slope these ashes were entrained into the convective plumes generated off the tops of the moving flows

high temperature characteristics of a natural titanomagnetite summary continuous recordings were made of thermal demagnetizations of isr arm and trm induced in artificial rock specimens the artificial rock specimens contained well defined grain size fractions of a natural titanomagnetite which under an optical microscope was homogeneous using a transmission electron microscope it was shown that submicroscopic inclusions exist within the titanomagnetite screening of the remanence of these inclusions by the surrounding titanomagnetite is thought to be responsible for the occurrence of peaks in the continuous thermal decay curves of isr and arm higher peaks observed in the case of trm are thought to be caused by the combined effects of screening and an anti parallel remanence in the titanomagnetite further submicroscopic exsolution processes which occur in the titanomagnetite as a result of repeated heating can be deduced from the development of new peaks from changes in the temperature at which the peak s occur their shape and their relative height the use of stepwise demagnetization techniques on the titanomagnetite yielded erroneous directional results due to the screening in the titanomagnetite the biasing effects due to screening could be avoided by using continuous thermal demagnetization techniques

long runout pyroclastic surge on a cretaceous alluvial plain republic of korea pyroclastic surge is a dilute and turbulent flow of volcanic gas and tephra that is commonly generated during explosive volcanic eruptions and can threaten lives along its flow paths assessing its travel distance and delineating future volcanic hazards have therefore been major concerns of volcanologists historical eruptions show that most pyroclastic surges travel a few tens of kilometres or less from their sources aeolian or aquagene processes have therefore been evoked for the emplacement of supposed surge deposits much beyond this distance here we show that a cretaceous tuff bed in korea was emplaced by an exceptionally powerful pyroclastic surge that flowed as far as the most powerful pyroclastic flows that formed the low aspect ratio ignimbrites lari this has significant implications for interpreting ancient volcanic eruptions and delineating volcanic hazards by pyroclastic surges and casts intriguing questions on the eruption dynamics and physics of long runout pyroclastic surges and their distinction from lari forming pyroclastic flows

a linear field dependence of thermoremanence in low magnetic fields abstract we tested a linear field dependence of thermoremanent magnetization trm to saturation isothermal remanent magnetization sirm ratio for magnetite containing natural samples the trm sirm shows a linear field dependence to very low field ranges

the 26 5 ka oruanui eruption new zealand an introduction and overview abstract the 26 5 ka oruanui eruption from taupo volcano in the central north island of new zealand is the largest known wet eruption generating 430 km3 of fall deposits 320 km3 of pyroclastic density current pdc deposits mostly ignimbrite and 420 km3 of primary intracaldera material equivalent to 530 km3 of magma erupted magma is 99 rhyolite and 90 pdc deposits range from mm to cm thick ultra thin veneers enclosed within fall material to 200 m thick ignimbrite in proximal areas the farthest travelled 90 km most energetic pdcs velocities 100 m s 1 occurred during phase 8 but the most voluminous pdc deposits were emplaced during phase 10 grain size variations in the pdc deposits are complex with changes seen vertically in thick proximal accumulations being greater than those seen laterally from near source to most distal deposits modern lake taupo partly infills the caldera generated during this eruption a 140 km2 structural collapse area is concealed beneath the lake while the lake outline reflects coeval peripheral and volcano tectonic collapse early eruption phases saw shifting vent positions development of the caldera to its maximum extent indicated by lithic lag breccias occurred during phase 10 the oruanui eruption shows many unusual features its episodic nature wide range of depositional conditions in fall deposits of very wide dispersal and complex interplay of fall and pdc activity

experimental studies of particle laden plumes experiments are described in which buoyant plumes of fresh water and solid particles are generated in a tank of salt water dependence of the behavior of the plumes on particle concentration and grain size was studied at low concentrations behavior was similar to single component plumes and the principal effect of the particles was to reduce the buoyancy flux measurements of particle concentration across the plumes demonstrated that there were substantial amounts of particle reentrainment from an outer sedimenting veil reentrainment caused the plume margins to become denser than the ambient fluid total particle flux in the plumes increased with height and can be several times greater than the flux at the plume source plumes with high particle concentration exhibit several kinds of convective instability which generated gravity currents and rapid sedimentation gravity currents were generated at the margins of concentrated plumes these flows are attributed to reentrainment causing the flow margins to have excess density when the plume density was close to but less than the density of the ambient fluid a fountain forms high in the plumes which collapsed asymmetrically to generate a gravity current on one side of the plume when the density of the particle laden flow was greater than the ambient fluid a symmetrical fountain formed and gravity currents moved across the tank floor depositing sediment at a well defined distance from the source buoyant fluid ascended from the flow fronts to form a ring of plumes around the central fountain these plumes coalesced to form a broad buoyant cloud the plumes are generated by the sedimentation from the gravity currents which resulted in the fresh water and some entrained particles being released all the experimental plumes ascended to the top of the tank and spread out as gravity currents beneath the upper surface particles often fall from this layer in discrete parcels of particle rich fluid with shapes similar to thermals the fall velocities were much greater than the terminal velocities of individual particles and the sedimenting region is inhomogeneous these observations indicate that sedimentation is enhanced by convective instabilities when particle rich fluid is emplaced above particle free fluid the experiments indicate that there may be several different mechanisms of generating pyroclastic flows and surges from the collapse of volcanic eruptions columns the experiments also demonstrate that the sedimentation of particles from a plume has an important influence on dynamical behavior

estimating peak currents at ground lightning impacts using remanent magnetization 1 the impact of lightning is permanently recorded in struck material by a secondary magnetization called lightning induced remanent magnetization lirm we introduce an original rock magnetic method based on alternating field demagnetization to define both the geometry and the intensity of the peak lightning magnetic field it also provides positive evidence than a material has effectively been struck on a schoolyard tree struck by lightning we were able to locate the impact to within a few centimeters to prove that it was a vertical negative current and to estimate a peak current intensity of 99 7 ka this value is 14 ka higher than the one provided by meteorage lightning detection network and offers an independent calibration

laboratory verification of submicron magnetite production in pseudotachylytes relevance for paleointensity studies abstract pseudotachylytes generally possess stable remanent magnetizations but the processes by which pseudotachylytes are magnetized remain poorly understood magnetic hysteresis and scanning electron microscope studies reveal that experimental frictional melting of granites produces dispersed submicron inclusions of weakly interacting pseudo single domain psd magnetite in artificial pseudotachylyte the magnetite inclusions are absent in the undeformed granite protolith and result from oxidation of fe in melt susceptible mafic minerals during the melt quenched event the pseudotachylytes acquired a stable thermal remanence in fine grained psd magnetites during the rapid cooling of the melt implying that fine grained magnetite has the potential for paleointensity determinations of contemporaneous magnetic fields with co seismic faulting in granitoids

the taupo eruption new zealand i general aspects the ca a d 186 taupo eruption was the latest eruption at the taupo volcanic centre occurring from a vent at horomatangi reefs now submerged beneath lake taupo in the central north island of new zealand minor initial phreatomagmatic activity was followed by the dry vent 6 km 3 hatepe plinian outburst large amounts of water then entered the vent during the 2 5 km 3 hatepe phreatoplinian ash phase eventually stopping the eruption though large amounts of water continued to be ejected from the vent area causing gullying of the ash deposits after a break of several hours to weeks phreatoplinian activity resumed generating the 1 3 km 3 rotongaio ash notable for its fine grainsize and for containing significant quantities of non or poorly vesicular juvenile material the vent area then became dry again and eruption rates and power markedly increased into the 23 km 3 taupo ultraplinian phase which is the most powerful plinian outburst yet documented synchronous with this ultraplinian activity lesser volumes of non to partially welded ignimbrite were generated by diversion of ejecta from or partial collapse of the eruption column the rapid rate of magma withdraw al during this phase removed support from the vent area to trigger local vent collapse and initiate the catastrophic eruption of the ca 30 km 3 taupo ignimbrite finally after some years lava was extruded on to the floor of the reformed lake taupo and floating fragments derived from the lava carapace were driven ashore the known eruption volume is more than 65 km 3 while additional volumes are represented by primary material now beneath lake taupo and layer 3 to the ignim brite phases a total volume of more than 105 km 3 is likely equivalent to more than 35 km 3 of magma plus more than 3 km 3 of lithic debris airfall deposits more than 10 cm thick blanketed 30000 km 2 of land east of the vent while ignimbrite covers a near circular area of 20000 km 2 widespread and locally severe ground shaking occurred during but mostly after the eruption associated with subsidence in the lake taupo basin secondary deposits are abundant above and extending beyond the taupo ignimbrite consisting of the products of surface water interacting with the still hot ignimbrite and subsequent water reworking of the light pumiceous materials the complexity and size of this eruption preclude accurate forecasting of the size nature and return period of the inevitable next eruption from the taupo volcanic centre

mineralogy of the sources for magnetic anomalies on mars abstract recent discovery of intense magnetic anomalies on mars which are due to remanent magnetization requires some explanation for the possible minerals responsible for the anomalous signature thermoremanent magnetization trm in single domain sd and multidomain md sized magnetite hematite and pyrrhotite all potential minerals are considered the intensity of trm in 0 05 mt is in descending order sd sized magnetite sd sized pyrrhotite md sized hematite md sized pyrrhotite md sized magnetite sd sized hematite the trm intensity is 50 of the sirm each of these minerals and estimated concentrations of magnetic remanence carriers assumed to be titanomagnetite in the shergotty nakhlaxhassigny martian meteorites are used in a thin sheet approximation model to reveal the concentration of each mineral required for the generation of an observed magnetic anomaly 1 500 nt at 100 km altitude assuming trm acquisition in a 0 05 mt magnetic field