amethyst bearing lava flows in the parana basin rio grande do sul brazil cooling vesiculation and formation of the geodic cavities size distribution data obtained from detailed field study of bubbles and amethyst geodes in the basaltic lava flows of the serra geral formation ametista do sul rio grande do sul brazil are used in cooling and vesiculation models to infer the origin and the formation processes of the geodic cavities coupled field observations and modelling results emphasize that 1 the formation of geodes in the studied lava flow can be explained qualitatively and quantitatively by the exsolution of magmatic gas from the supersaturated melt with no need for external surface water supply 2 the vertically elongated habits of the geodes result from higher cooling rate of the magma in contact with the accumulating bubbles and 3 the abnormal metre sized geodes with their branching habits result not only from the diffusive decompressive bubble growth but also from the coalescence of inwards progressing tubular cavities

late mesozoic and cenozoic palaeomagnetism of australia i a redetermined apparent polar wander path summary palaeomagnetic measurements have been carried out on one cretaceous and five cenozoic sedimentary sequences of australia of those one is in the carnarvon basin of north western australia and the others are in the otway basin of south eastern australia the new results are used together with those published previously to define a revised late mesozoic cenozoic apparent polar wander path for australia this path differs from earlier basalt derived paths by the absence of a large westward excursion and zig zag irregularities it is characterized instead by a substantially straight cenozoic trajectory a sharp bend in the late cretaceous and a non uniform rate of apparent polar wander the early tertiary segment of the new path lies north of the original paths thus eliminating a discrepancy that has been noted previously between the indian and australian palaeomagnetic data luyendyk rennick peirce and klootwyk peirce a re examination of the original data suggests that this discrepancy as well as two others in the australian results may have been caused principally by incomplete time averaging of remanence directions because the volcanics on which the results were based had been extruded episodically

effect of low temperature treatments on pseudo thellier paleointensity determination 1 in paleointensity determination on multidomain md magnetite demagnetization is always easier than restoring remanence in companion acquisition steps as a result paleointensity results follow a convex down curve on the arai plot in an attempt to overcome this problem the effect of systematic low temperature demagnetization ltd on pseudo thellier results was examined ltd was applied not only to the initial anhysteretic remanent magnetization arm simulating natural remanent magnetization or nrm but also after each partial anhysteretic remanent magnetization parm acquisition step three practical benefits of systematic ltd were discovered 1 ltd treated pseudo thellier results always yielded more accurate intensity values regardless of grain size and sample lithology 2 ltd improved the behavior of md magnetite in paleointensity determination although not as much as for single domain and pseudosingle domain grains 3 an unanticipated bonus was a more even distribution of data points on the arai plot resulting from the fact that ltd causes a shift of points toward the upper end of the arai curves constructing a three dimensional map of the distributions and ltd was successful in quantitatively explaining behavior on the arai plots

middle permian paleomagnetism of the sydney basin eastern gondwana testing pangea models and the timing of the end of the kiaman reverse superchron abstract paleomagnetic and geochronologic data from the eastern margin of gondwana have been obtained from the gerringong volcanics in the southern sydney basin australia the corresponding paleomagnetic pole at 56 9 s 154 8 e n 131 a95 9 1 has a 40ar 39ar plagioclase plateau age of 265 05 0 35 0 46 ma from the bumbo latite and overlaps with recent radio isotopic and paleomagnetic results published from western gondwana the long documented inconsistency between middle permian eastern and western gondwanan paleomagnetic datasets is most likely an artefact of a lack of reliable paleomagnetic data from eastern gondwana for this period a number of well dated and recently published ca 265 ma paleomagnetic results from gondwana and laurussia are shown to be consistent with the wegenerian pangea a configuration with a loose n s fit of the continents for the middle permian the lack of crustal overlap negates the need for a pangea b configuration which if valid must have been assembled to pangea a by ca 265 ma the reverse polarity bumbo latite was sampled from the kiaman type section located in the southern sydney basin three cases of normal polarity were detected in the overlying saddleback dapto and berkeley latites previously assigned to the kiaman reverse superchron krs we review krs aged magnetostratigraphic data and propose that an age assignment of 265 ma most likely represents the termination of the non reversing field with longer stable intervals of normal polarity recorded and able to be correlated globally

mineralisation of amethyst bearing geodes in ametista do sul brazil from low temperature sedimentary brines evidence from monophase liquid inclusions and stable isotopes fluid inclusion studies in combination with hydrogen oxygen and sulphur isotope data provide novel insights into the genesis of giant amethyst bearing geodes in early cretaceous parana continental flood basalts at amestita do sul brazil monophase liquid inclusions in colourless quartz amethyst calcite barite and gypsum were analysed by microthermometry after stimulating bubble nucleation using single femtosecond laser pulses the salinity of the fluid inclusions was determined from ice melting temperatures and a combination of prograde and retrograde homogenisation temperatures via the density maximum of the aqueous solutions four mineralisation stages are distinguished in stage i celadonite chalcedony and pyrite formed under reducing conditions in a thermally stable environment low 34sv cdt values of pyrite 25 to 32 suggest biogenic sulphate reduction by organotrophic bacteria during the subsequent stages ii amethyst goethite and anhydrite iii early subhedral calcite and iv barite late subhedral calcite and gypsum the oxidation state of the fluid changed towards more oxidising conditions and microbial sulphate reduction ceased three distinct modes of fluid salinities around 5 3 3 4 and 0 3 wt nacl equivalent characterise the mineralisation stages ii iii and iv respectively the salinity of the stage i fluid is unknown due to lack of fluid inclusions variation in homogenisation temperatures and in 18o values of amethyst show evidence of repeated pulses of ascending hydrothermal fluids of up to 80 90 c infiltrating a basaltic host rock of less than 45 c colourless quartz and amethyst formed at temperatures between 40 and 80 c while the different calcite generations and late gypsum precipitated at temperatures below 45 c calculated oxygen isotope composition of the amethyst precipitating fluid in combination with d values of amethyst hosted fluid inclusions 59 to 51 show a significant 18o shift from the meteoric water line this 18o shift high salinities of the fluid inclusions with chloride sulphate composition and high 34s values of anhydrite and barite 7 5 to 9 9 suggest that sedimentary brines from deeper parts of the guarani aquifer system must have been responsible for the amethyst mineralisation

multicomponent magnetizations from the mississippian mauch chunk formation of the central appalachians and their tectonic implications previous paleomagnetic study of mississippian red beds of the mauch chunk formation from the central appalachians reported shallow inclination directions a positive fold test was obtained after thermal demagnetization typically to only 550 c the near equatorial position for north america indicated by these results has been used to support the idea of a circa 15 latitudinal tectonic offset of the acadia displaced terrain in the northern appalachians the present study of 153 samples from 29 sites occupied in the mauch chunk formation of eastern and southern pennsylvania reveals multicomponent magnetizations a characteristic component declination d 161 0 inclination i 27 8 alpha95 a95 7 9 with unblocking temperatures usually concentrated above 650 c and with dual polarity 10 sites normal 13 sites reversed is isolated in 23 sites and passes a fold test at the 99 confidence level it is interpreted as an early acquired hematitic detrital or chemical magnetization of pre alleghanian orogeny and pre kiaman reverse polarity interval age a southern hemisphere paleolatitude of 15 for the sampling area is indicated another component has generally lower unblocking temperatures which however can extend up to 650 c or more and therefore is also likely to be carried by hematite this secondary magnetization d 170 6 i 5 3 a95 5 0 isolated in 25 sites is of uniform reverse polarity and application of fold tests suggests that it is of synfolding origin a near equatorial paleolatitude 3 n at the time of acquisition is obtained consistent with permo carboniferous kiaman directions from north america incomplete removal of the secondary component can explain the previous results from the mauch chunk comparison of the present paleomagnetic results with those from similar age rocks in the canadian maritimes does not support a latitudinal offset of acadia with respect to interior north america in the early carboniferous in fact according to these and other paleomagnetic data thought to be representative of this age the atlantic bordering continents already were near to a pangea configuration the alleghanian and particularly the hercynian orogenies therefore may not be associated with the closure of a large latitudinal ocean later in the carboniferous and permian alleghanian deformation however may have resulted in the partial bending of the pennsylvanian salient on the basis of apparently systematic deviations in paleomagnetic declinations

waves of bubbles in basaltic magmas and lavas initially homogeneous suspensions of bubbles in basaltic magmas and lava flows may become unstable and form rising waves or layers of bubbles we derive a set of model equations for the two phase bubble liquid system and present the results of a linear stability analysis and numerical simulations periodic vesicle layers are preserved in columbia liver flows with a spacing of 1 m the prediction of the stability analysis combined with a simple model for solidification of the lava suggests that vesicle layers may represent waves of bubbles formed within the flow the spacing of layers is determined by a balance between the growth of bubbles and hydrodynamic self diffusion vesicular layers preserved in columbia liver flows may thus form by hydrodynamic processes and do not require the repeated injection of fresh volatile rich magma or a periodic nucleation of bubbles waves of bubbles may also develop in magma chambers and conduits and could play a role in eruption dynamics

what was the paleogene latitude of the lhasa terrane a reassessment of the geochronology and paleomagnetism of linzizong volcanic rocks linzhou basin tibet the paleogene latitude of the lhasa terrane southern tibet can constrain the age of the onset of the india asia collision estimates for this latitude however vary from 5 n to 30 n and thus here we reassess the geochronology and paleomagnetism of paleogene volcanic rocks from the linzizong group in the linzhou basin the lower and upper parts of the section previously yielded particularly conflicting ages and paleolatitudes we report consistent 40ar 39ar and u pb zircon dates of 52 ma for the upper linzizong and 40ar 39ar dates 51 ma from the lower linzizong are significantly younger than u pb zircon dates 64 63 ma suggesting that the lower linzizong was thermally and or chemically reset paleomagnetic results from 24 sites in lower linzizong confirm a low apparent paleolatitude of 5 n compared to the upper part 20 n and to underlying cretaceous strata 20 n detailed rock magnetic analyses end member modeling of magnetic components and petrography from the lower and upper linzizong indicate widespread secondary hematite in the lower linzizong whereas hematite is rare in upper linzizong volcanic rocks of the lower linzizong have been hydrothermally chemically remagnetized whereas the upper linzizong retains a primary remanence we suggest that remagnetization was induced by acquisition of chemical and thermoviscous remanent magnetizations such that the shallow inclinations are an artifact of a tilt correction applied to a secondary remanence in lower linzizong we estimate that the paleogene latitude of lhasa terrane was 20 4 n consistent with previous results suggesting that india asia collision likely took place by 52 ma at 20 n

on the resolution of multivectorial remanences abstract low temperature demagnetization ltd promotes single domain sd like alternating field af and thermal demagnetization properties of anhysteretic remanent magnetization arm and thermoremanent magnetization trm by eliminating low coercivity or low unblocking temperature fractions of remanence we have investigated the effectiveness of ltd in improving the independence of partial arms parms independence of parms and of partial trms ptrms is crucial because multivectorial directional analyses and paleointensity determinations are otherwise compromised in normalized vector plots of af demagnetization of orthogonal parms ltd treated samples showed universal improvement in the degree of independence the greatest improvement was for synthetic sd and pseudo single domain magnetites and natural lake sediments on the other hand independence barely improved for synthetic multidomain magnetite granites and gabbros in general ltd provides a better resolution of superimposed vectors in multicomponent remanence the direction of the original arm corresponding to primary nrm in nature is perfectly recovered however the parm the overprint is not perfectly resolved especially for overprinting coercivities

a palaeomagnetic study of jurassic intrusives from southern new south wales further evidence for a pre cenozoic dipole low the results of rock magnetic thermal demagnetization and thellier palaeointensity studies are presented for two high level intrusions from southern new south wales australia the gingenbullen dolerite gb 34 5 s 150 3 e and the gibraltar microsyenite gs 34 5 s 150 4 e were emplaced during the second of three major phases of igneous activity that affected the southern sydney basin and are k ar dated at 172 and 178 ma respectively the magnetic mineralogy of the two intrusions is different the gb is dominated by single domain sd pseudo single domain grains of magnetite whereas the gs has both sd magnetite and haematite components although the haematite component does not record a stable remanence the gb records two opposed components of magnetization above 100 c occupying distinct parts of the blocking temperature spectrum the lower blocking temperature lbt component resides between 150 and 400 c and is of normal polarity having a mean direction of d i 021 79 95 8 k 63 with a corresponding vgp at lat long 54 s 137 e dp 12 dm 13 this component is believed to be a tcrm or tvrm related to the initial opening of the tasman sea at 90 ma the higher blocking temperature hbt chrm component is recognized between 450 and 580 c and is of reverse polarity with a mean direction of d i 174 81 95 11 k 33 and a corresponding vgp at lat long 52 s 153 e dp 17 dm 18 the chrm for the gs resides in the 100 450 c region and although the scatter is high has a similar mean direction d i 167 79 95 24 k 26 to the hbt component of the gb with a pole position at lat long 54 s 158 e dp 30 dm 31 there is no evidence of the gb lbt component in samples from the gs suggesting that this intrusion was magnetically unaffected by the later event which is also seen for other intrusions in the sydney basin the vgps calculated from the chrm directions both plot on the jurassic segment of the australian apwp dunlop et al 1997 providing further palaeomagnetic confirmation for the k ar ages thellier palaeointensity results were of variable quality as expected for rocks of this age the mean vdm values calculated for the chrms of the gb and gs were 1 3 1022 and 4 1 1022 am2 respectively approximately 16 and 51 per cent of the present value on application of a cooling rate correction estimate the gb result falls to a minimum of 0 87 1022 am2 approximately 11 per cent of the present day value representing an overestimate of up to 56 per cent these values arising from a time window previously lacking in thellier estimates provide further confirmation of the existence of a pre cenozoic dipole low the vdm values derived from the overprint in the gb samples yield an average of 4 8 1022 am2 from a range between 44 and 81 per cent of the present day mean vdm and are tentatively attributed to the cretaceous normal superchron cns these results may document evidence for a previously unrecognized low dipole moment during the bulk of the cns a possibility that requires further investigation by performing palaeointensity work on igneous rocks with radiometric ages between 110 and 90 myr that carry unequivocal primary trms

an example of multicomponent magnetization in welded tuffs a case study of upper cretaceous welded tuffs of eastern russia abstract four distinct components of natural remanent magnetization were isolated from a single site in welded tuffs in the upper cretaceous kisin group of the sikhote alin mountain range russia in order to contribute toward a basis for an interpretation of multicomponent magnetization rock magnetic experiments were performed on the welded tuffs all four magnetization components essentially reside in magnetite the lowest temperature component up to 300 c component a d 349 3 i 60 9 95 7 3 n 7 is a present day viscous magnetization the third removed component component c d 41 4 i 51 8 95 3 5 n 8 isolated over the temperature range of 450 560 c is a primary remanence the second and fourth demagnetized components component b d 174 7 i 53 1 95 21 2 n 3 and component d d 188 1 i 64 5 95 4 0 n 8 respectively are secondary magnetizations related to a thermal event in sikhote alin between 66 and 51 ma components b and d were acquired through different remagnetization processes component b is ascribed to a thermoviscous remanent magnetization carried by single domain magnetite and component d is a chemical remanent magnetization

from alkali basalt to phonolite in hand size samples vapor differentiation effects in the bouzentes lava flow cantal france abstract the bouzentes lava flow is a 20 m thick alkali basalt flow emplaced during the last stage of formation of the cantal stratovolcano at 4 2 ma its upper part has 1 to 20 cm thick vesicle rich segregation sheets which recur every 0 1 2 m these horizontal veins are hawaiitic in composition they are characterized by hypertrophic development of their minerals pegmatoids and by glassy phonolitic segregation vesicles internal differentiation within the bouzentes lava flow was triggered by an unusually high water content as suggested by pre emptive iddingsite alteration of olivine phenocrysts the proposed model of formation of the segregation sheets includes the upward motion of diapirs of residual melt plus addition of vapor from the bottom of the central liquid lens to the base of the upper solidified crust of the cooling lava flow olivine settling appears to have been inhibited or at least retarded by upward migration of melt plus vesicles most of the features observed in bouzentes recall the internal differentiation processes usually described within thick hawaiian lava lakes the segregation vesicles are believed to result from an increase of gas solubility in residual melt during the crystallization process

complex layering of the orange mountain basalt new jersey usa the orange mountain basalt of new jersey is a mesozoic formation consisting of three units a single lower inflated sheet lobe about 70 m thick omb1 a middle pillow basalt about 10 to 20 m thick omb2 and an upper compound pahoehoe flow about 20 to 40 m thick omb3 the orange mountain basalt is part of the central atlantic magmatic province quarry and road cut exposures of omb1 near paterson new jersey display some unusual layering that is the focus of this study omb1 exposures displays the typical upper crust core and basal crust layers of sheet lobes but throughout the patterson area also display distinct light gray layers of microvesicular basalt mineralized with albite directly over the basal crust and under the upper crust the lower microvesicular layer is associated with mega vesicular diapirs we propose that the upper and lower microvesicular layers were composed of viscous crust that was suddenly quenched before it could devolatilize immediately before the solidification of the core during initial cooling the bottom of the basal layer was mineralized with high concentrations of calcite and albite during a high temperature hydrothermal event subsequent albitization as well as zeolite prehnite and calcite precipitation events occurred during burial and circulation of basin brine heated by recurring palisades magmatism below the orange mountain basalt some of the events experienced by the orange mountain basalt are unusual and place constraints on the fluid dynamics of thick flood basalt flows in general the late penetration of vesicular diapirs through the entire thickness of the flow interior constrains its viscosity and solidification history

thermoviscous remagnetization in some appalachian limestones experimental evidence shows that blocking temperature relaxation time theory for single domain magnetite can grossly underestimate laboratory thermal demagnetization temperatures of a present day viscous remanence in devonian limestones of new york state thermoviscous remagnetization at moderately elevated burial temperatures during the late paleozoic cannot therefore be readily excluded for the origin of the stable secondary component in these rocks more generally these results emphasize that single domain thermal activation theory cannot be assumed a priori to correctly predict temperature of thermoviscous remanence acquisition in all magnetite bearing rocks the prob leto

crystal mush compaction in the cohassett flood basalt flow hanford washington abstract the 40 m thick central part of the cohassett flow which was probably emplaced by inflation of an earlier more ti rich flow exhibits textural evidence of crystal mush compaction during solidification although most of the crystallization started in the roof zone as water circulated through fractures in the crust plumes of dense crystal mush must have dripped from the roof to the floor of the sheet thus displacing upward the colonnade entablature boundary the mush initially consisted of loosely stacked plagioclase crystals and liquid surrounding rapidly grown pyroxene oikocrysts containing radial chadacrysts of plagioclase upon compaction in the floor zone the liquid between the loosely stacked plagioclase crystals was expelled as the crystals packed tightly together to form sub parallel grains wrapped around the pyroxene oikocrysts which recrystallized into granular aggregates of augite and pigeonite stacks of sinking plagioclase crystals and rising liquid accumulated on the upper and lower sides respectively of horizontal plagioclase phenocrysts this asymmetric texture clearly indicates the relative motion of solids and liquid the degree of compaction as determined from variations in the amount of interstitial liquid and quantitative measures of textural anisotropy accounts for the chemical variation through the central part of the flow with compatible elements being concentrated in the lower part of the flow and incompatible elements concentrated in the upper part modeling of the compaction process using the iridium melt infiltration and reaction program of boudreau a 2003 iridium a program to model reaction of silicate liquid infiltrating a porous solid assemblage computers and geosciences 29 423 429 shows that the observed chemical and compaction dilation profiles are to be expected in a 40 m thick sheet of basaltic magma solidifying on the earth s surface compaction of crystal mush should be even more likely in slowly cooled intrusive bodies

spongy pahoehoe in hawaii a study of vesicle distribution patterns in basalt and their significance a well defined pahoehoe lava type that is very common medially and distally in hawaii is characterized by a high concentration and fairly uniform distribution of spherical or near spherical vesicles measurements of vesicle sizes and concentrations have been made on ten of these spongy pahoehoe lava flow units the vesicles increase in size toward the middle accompanied by a moderate increase in lava porosity the close approach to bilateral symmetry on either side of the horizontal median plane and the common occurrence of a median gas blister shows that no significant upward movement of vesicles occurred suggesting that the lava possessed a yield strength and was more or less static olivine phenocrysts when present are however concentrated in the lower half of the same flow units showing that the lava previously lacked a significant yield strength the vesicles are regarded as early inherited from the vent but the size characteristics of the vesicle population are a late formed feature vesicles grew in static lava mostly by coalescence and it is postulated that coalescence was promoted by the presence of abundant diktytaxitic voids which punctured the walls of contiguous vesicles zones in which the vesicle concentration is lower and the vesicles are larger and strongly deformed interrupt the symmetry of some spongy pahoehoe units and gas blisters higher than the median plane occur in many examples these zones are interpreted to result from late stage shearing and point to a mechanism by which vesicles may be eliminated from a lava

comment on philpotts and philpotts 2005 crystal mush compaction in the cohassett flood basalt flow hanford washington flood basalt lava flows have generally been assumed to be homogeneous with little variation in flow composition this concept was reinforced by the ability to map many flows over large distances and thus develop a workable regional stratigraphy as in the columbia river basalt group swanson et al 1979 as detailed studies were carried out on columbia river basalt group flows however it became clear that some lava flows did not fit this model and that considerable variation occurred in some flows e g reidel and fecht 1987 reidel 1998 2000 2005 one of the most complex compositional patterns occurs in the cohassett flow of the sentinel bluffs member grande ronde basalt reidel 2000 2005 philpotts and philpotts 2005 propose that the compositional variation observed in the central portion of the cohassett flow from corehole dc 6 is the result of a process that they have called compaction their conclusions are based on an analysis of textures that were observed in vertically oriented samples complimented by quantitative modeling of the compositional profile the purpose of my comment is to provide an alternative explanation for the compositional pattern ob

paleointensity determination on the late precambrian tudor gabbro ontario paleointensity determination by the coe modified thellier method was carried out on the 1100 ma tudor gabbro of southern ontario the tudor has a bivectorial natural remanent magnetization nrm the c nrm is a grenvillian uplift thermoviscous remagnetization which overprints the a nrm the surviving primary thermoremanent magnetization trm of the tudor intrusion a sharp junction at 530 c between the a and c vectors in laboratory thermal demagnetization matches the peak metamorphic reheating temperature of 480 500 c in nature when the difference in time scales is taken into account the unblocking temperature distributions of the a nrm and of laboratory trm also match a is carried by single domain or small pseudo single domain grains probably magnetite rods exsolved in plagioclase or pyroxene at temperatures above 580 c arai plots of nrm versus partial trm are linear from 520 c or 530 c to 580 c for 45 specimens of 19 samples from 9 sites the 45 reliable paleointensity values from these specimens give a virtual axial dipole moment vadm of 4 6 0 8 1022 a m2 around 1100 ma this is about one half of the modern field intensity but is similar to the mean cretaceous cenozoic intensity and to most archean and proterozoic vadms keweenawan rocks whose ages are within 20 myr of the tudor gabbro have a vadm of 11 4 1022 a m2 the difference between the tudor and keweenawan vadms is similar to differences recorded over similar time intervals in the cretaceous and seems to represent normal variation in geomagnetic field strength throughout earth s history

magnetic overprinting in southeastern australia and the thermal history of its rifted margin the magnetizations of many rock units in southeastern australia have been severely overprinted interpretation of early work was hindered by this and one of the aims of the current research is to ascertain the nature of the overprint mechanism its geological significance and its regional extent results from three igneous bodies from the sydney basin which have been studied previously in a reconnaissance fashion have provided a new insight into the reason for such widespread magnetic overprinting the hornsby breccia hb and milton monzonite mm yield similar magnetic signatures both containing discrete high and low temperature magnetizations with directions of dec 72 inc 83 95 4 and dec 13 inc 76 95 2 respectively for hb and dec 60 inc 78 95 4 and dec 348 inc 79 95 6 respectively for mm the corresponding pole positions are 29 s 166 e a95 7 and 59 s 139 e a95 4 for hb and 22 s 171 e a95 7 and 55 s 158 e a95 11 for mm the paleomagnetic direction and pole from the third body studied the mogo hill basalt mh are dec 241 inc 81 95 5 and 41 s 130 e a95 9 this pole position and the two low temperature overprint pole positions from hb and mm are consistent with ages ranging from 100 m y to approximately 70 m y when compared to the late mesozoic apparent polar wander path we relate the overprinting to uplift erosion and rapid supracrustal cooling of the southeastern margin of australia associated with initial phases of marginal rift development prior to seafloor spreading in the tasman sea the temperatures of rocks now exposed may have reached 200 c or higher before the rapid cooling blocked and stabilized the overprint magnetizations

changes in remanence coercivity and domain state at low temperature in magnetite abstract submicron magnetite crystals with mean sizes of 0 037 0 10 and 0 22 m undergo major changes in hysteresis properties and domain states in crossing the verwey transition t v 120 k the 0 037 m crystals are single domain sd both in the cubic phase at room temperature t 0 and in the monoclinic phase below t v the 0 10 and 0 22 m crystals have a mixture of sd and two domain 2d states at room temperature t 0 but mainly sd structures below t v in agreement with micromagnetic calculations coercive force h c increases on cooling through t v by a factor 3 5 in the submicron magnetites and 40 in a 1 3 mm single crystal because of the high crystalline anisotropy and magnetostriction of monoclinic magnetite as a result domain walls and sd moments are so effectively pinned below t v that all remanence variations in warming or cooling are reversible however between 100 k and t 0 remanence behavior is variable saturation remanence sirm produced in monoclinic magnetite at 5 k drops by 70 100 in warming across t v with minor recovery in cooling back through t v ultimate levels at 5 k of 23 37 for the submicron crystals and 3 for the 1 3 mm crystal in contrast sirm produced in the cubic phase at 300 k decreases 5 35 submicron or 95 1 3 mm during cooling from 300 to 120 k due to continuous re equilibration of domain walls but there is little further change in cooling through t v itself however the submicron magnetites lose a further 5 15 of their remanence when reheated through t v these irreversible changes in cycling across t v and the amounts of the changes have potential value in determining submicron magnetite grain sizes the irreversibility is mainly caused by 2d sd transformations on cooling through t v which preserve or enhance remanence while sd 2d transformations on warming through t v cause remanence to demagnetize

paleomagnetism and paleothermometry of the sydney basin 1 thermoviscous and chemical overprinting of the milton monzonite the early triassic 245 ma milton monzonite of the sydney basin australia has four distinct components of natural remanent magnetization nrm with only slightly overlapping ranges of unblocking temperatures the low temperature lt component the first to be thermally demagnetized is thought to be a late cretaceous 100 ma thermoviscous overprint acquired in slow cooling during uplift the high temperature ht component the second to be demagnetized is probably the primary thermoremanent magnetization trm of the milton intrusion but could possibly be a jurassic overprint lt and ht are usually carried by magnetite and occasionally by pyrrhotite samples from nine sites have a further nrm component which unblocks at higher temperatures than ht but below the magnetite curie temperature of 580 c this component is argued to be a chemical remanent magnetization crm because of its discrete range of high unblocking temperatures above those of the thermal components ht and lt and is called crm1 crm1 has almost the same direction as lt and is likely carried by authigenic magnetite produced during uplift 100 ma samples from five sites have a fourth nrm component with a direction resembling that of ht but carried by hematite this fourth component could be a primary trm but is more likely a crm and is therefore called crm2 the ht crm2 mean direction is d 50 i 75 5 defining a paleopole at 16 s 172 e the ht crm2 paleopole falls near 150 ma on the australian apparent polar wander path but is a considerable distance from paleopoles of permian and early triassic age there is no known tectonic or other remagnetizing event in the sydney basin around 150 ma for this reason we propose that the ht crm2 paleopole defines a new triassic segment of the australian polar wander path the lt crm1 mean direction is d 348 i 79 with a paleopole falling at 56 s 1580 e near 100 ma on the polar wander path this age is consistent with uplift and cooling related to initial rifting of the tasman sea

palaeomagnetism of the dundas fossey trough tasmania oroclinal rotation and late cretaceous overprinting abstract the dundas fossey trough a curved belt of early palaeozoic rocks in western and northern tasmania is hypothesised to have developed as an orocline driven by asthenospheric flow behind a rapidly retreating subduction zone on the east gondwana margin rocks of the wurawina supergroup from the trough were sampled for a palaeomagnetic test of this hypothesis rock magnetic analysis identifies both magnetite and hematite remanence carriers hematite carries a normal polarity overprint which was acquired at relatively low temperature thermal demagnetisation over the magnetite unblocking range reveals a linear trend that defines component a which displays both normal and reversed polarity component a passes a modified version of the fold test indicating that it is pre deformational the declination of component a correlates linearly with regional strike thus passing the orocline test a palaeomagnetic pole for the cambrian owen group after correction for oroclinal rotation is displaced from the gondwana polar wander path suggesting that the western limb of the trough may have been rotated anticlockwise by about 40 prior to the development of the orocline presumably during known early to middle ordovician deformation adjustment of the palaeomagnetic declination data to account for this earlier phase of rotation improves the fit parameters of the orocline test the palaeomagnetic results are consistent with the orocline model for the dundas fossey trough and support the inference that the trough extends beneath later cover in southeastern tasmania overprint component b yields a mean pole in broad agreement with other overprints previously observed both in tasmania and the eastern seaboard of the australian mainland and like them appears to have been acquired during the late cretaceous normal polarity superchron we speculate that magnetic overprinting in tasmania is related to enhanced oxidation by meteoric water driven by regional uplift

chemostratigraphy of the neoproterozoic alona bay lavas ontario a basal sequence of flood basalt lavas associated with the neoproterozoic midcontinent rift system crops out in alona bay along the southeastern shore of lake superior in ontario the alona bay lava succession is about 1200 m thick and lies just north of the well studied contemporaneous mamainse point formation detailed chemostratigraphy of the alona bay lavas suggests they are grossly correlative with the basal portion of the mamainse point formation for instance like the basal part of mamainse point formation the alona bay section contains numerous high mgo lavas and can be subdivided into 4 5 groups with distinct chemical characteristics chemical variations within the alona bay groups are largely the result of fractional crystallization likely at moderate pressures one small group of alona bay lavas also carries the compositional imprint of crustal contamination the remaining inter group chemical distinctions at alona bay are the consequence of temporal changes in partial melting and source ch

a lava flow without a source the cohassett flow and its compositional components sentinel bluffs member columbia river basalt group abstract the eruption of sentinel bluffs member lava flows at approximately 15 5 ma marked the end of grande ronde basalt volcanism the most voluminous period of the columbia river basalt group more than 10 000 km3 of lava erupted from northerly trending feeder dikes in eastern washington and northern oregon and flowed westward down an ancestral paleoslope covering more than 169 700 km2 of the flood basalt province the sentinel bluffs member consists of six phases or types defined by their compositions lava flows having the first compositional type were the most voluminous and reached the pacific ocean the volume of later lava flows having the other compositional types declined with time until the final eruption produced the second largest volume of basalt at the source area compositional variation in sentinel bluffs lava flows is relatively small this homogeneity allows the individual compositional types and thus phases of the eruption to be easily recognized farther west however composition

vesicle layering in solidified intrusive magma bodies a newly recognized type of igneous structure we report a novel type of layering structure in igneous rocks the layering structure in the ogi picrite sill in sado island japan is spatially periodic and appears to be caused by the variation in vesicle volume fraction the gas phase forming the vesicles apparently exsolved from the interstitial melt at the final stage of solidification of the magma body we call this type of layering caused by periodic vesiculation in the solidifying magma body vesicle layering the presence of vesicle layering in other basic igneous bodies pillow lava at ogi and dolerite sill at atsumi japan implies that it may be a fairly common igneous feature the width of individual layers slightly but regularly increases with distance from the upper contact the layering plane is perpendicular to the long axes of columnar joints regardless of gravitational direction suggesting that the formation of vesicles is mainly controlled by the temperature distribution in the cooling magma body we propose a model of formation of vesicle layering which is basically the same as that for liesegang rings the interplay between the diffusion of heat and magmatic volatiles in melt and the sudden vesiculation upon supersaturation both play important roles

segregation vesicles cylinders and sheets in vapor differentiated pillow lavas examples from tore madeira rise and chile triple junction abstract we conducted a detailed field and laboratory study of internal segregation structures of two hand size pillow lavas samples they were dredged respectively on the josephine seamount tore madeira rise tmr and on a small quaternary volcanic edifice located on the continental edge of the trench close to the chile triple junction ctj both pillows display a combination of four types of segregation structures spherical vesicles pipe vesicles vesicle cylinders and vesicle sheets observed so far only within subaerial basalt flows typically 2 10 m thick in particular the samples offer a remarkable exposure of the transition between pipe vesicles and cylinders we show that the vesicle sheets are not generated by the same mechanism in both occurrences they do not seem to be connected to cylinders in the ctj pillow as they are in the tmr pillow the two pillows are geochemically distinct the tmr being alkaline and the ctj calc alkaline two types of internal differentiation are proposed the first one implies the extraction of the residual liquid from the host lava and transport towards the segregation structures whereas the other one results from in situ crystallization within one given structure in the latter case glass composition is highly dependant on the nature of the neighbouring crystallizing minerals the degree of crystallization required to produce a crystal framework strong enough for generating the segregation structures seems to be lower in pillows ca 25 crystallization than in vapor differentiated basaltic lava flows 35 crystallization

interactions between bubbles in magmas and lavas effects of bubble deformation the interactions between two deformable bubbles are studied in order to determine the effects of deformation on bubble dynamics and to determine the limits in which the effects of deformation can be ignored deformation due to hydrodynamic interactions with other bubbles leads to alignment of horizontally offset bubbles and thus an enhanced rate of coalescence bubble alignment may produce spatial inhomogeneities in bubble concentrations in magmas which implies temporal variations in the concentration and size of bubbles in erupting magmas and spatial variations in solidified lavas an approximate quantitative model for the rate of coalescence of deformable bubbles is developed based on a series of experiments the effects of deformation can be ignored if the largest bubbles have radii less than about 5 mm in silicate magmas

p type and s type pahoehoe a study of vesicle distribution patterns in hawaiian lava flows abstract two main types of hawaiian pahoehoe lava p type pipe vesicle bearing and s type spongy are characterized their origin is inferred from features of the vesicle sizes and abundances particularly in their chilled margins which record frozen in the vesicle population at the time and place of solidification s type margins are distinctly more vesicular than p type porosity and vesicle size and range in s type margins are highest at the vent vesicle size reaches a minimum in medial areas and then increases while porosity decreases onto the flatter ground of the coastal terrace the vesicles are inferred to be part of the complement of bubbles that was present at the vent modified by loss and coalescence during travel in lava tubes the minimum porosity occurs on the coastal terrace and is the result of degassing during a longer residence time in lava tubes there the systematically lower porosity of p type margins than s type is attributed to a still longer residence allowing more time for bubble coalescence and loss p type pahoehoe is common only on flatter ground such as the coastal terrace it is found where tumuli and lava rises are widespread and where extensive networks of lava expansion clefts occur through which lava moving below the surface crust can lose gas without significant cooling additionally p type pahoehoe shows clear evidence that a strong inward movement and growth of bubbles took place from the outermost selvage into an inner selvage concentration zone a mechanism that might cause this inward movement is surface tension operating against deforming forces in the steep viscosity gradient of flow margins causing down gradient bubble translation inward movement does not usually occur in the upper crust of thick 2 m flow units because of the absence of strong deforming forces in the shallow velocity gradient there many of the larger bubbles that entered the inner selvage rose buoyantly to form pipe vesicles p type pahoehoe is characterized by sampled profiles across 20 flow units continued gas loss through surface clefts caused a general overall reduction in vesicle content within each flow unit to less than that in the upper crust and the pronounced depletion of vesicles from its lower half is attributed to scavenging by ascending pipe bubbles in a small proportion of flow units overall gas loss was small and a median gas blister fed by pipe bubbles developed instead

characterizing the superparamagnetic grain distribution f v hk by thermal fluctuation tomography 1 in 1965 d j dunlop showed that the joint distribution of particle volumes and microcoercivities f v hk0 can be determined for magnetically monomineralic thermally stable single domain ssd ensembles by taking advantage of the joint temperature and field dependence of relaxation time we have developed a procedure that follows dunlop s strategy to obtain f v hk0 for ensembles containing both superparamagnetic and ssd grains based on backfield remanence curves measured over a range of temperatures each point on the derivative curves represents the integrated contribution from grains that lie along a corresponding blocking contour on the neel plot a suitable set of such line integral samples can be used to reconstruct the f v hk0 distribution using the methods of tomographic imaging samples of the basal tiva canyon tuff have narrow size distributions of elongate ti poor titanomagnetite tomographic inversion of the low temperature backfield spectra yield sharply peaked f v hk0 distributions from which we calculate modal grain dimensions in good agreement with those observed by transmission electron microscopy analysis of synthetic samples containing bimodal populations clearly distinguishes the two modes because our simplified forward calculations incompletely account for the effects of orientation distribution the width of the coercivity distribution at each temperature is underestimated and consequently the inverse calculations yield grain distributions that are overly broad frequency and temperature dependent susceptibilities calculated for the inverted f v hk0 distributions accord fairly well with measured susceptibilities for the weakly interacting tiva canyon samples less well for a moderately interacting paleosol specimen and poorly for a strongly interacting ferrofluid

use of joint growth directions and rock textures to infer thermal regimes during solidification of basaltic lava flows abstract thermal contraction joints form in the upper and lower solidifying crusts of basaltic lava flows and grow toward the interior as the crusts thicken lava flows are thus divided by vertical joints that by changes in joint spacing and form define horizontal intraflow layers known as tiers entablatures are tiers with joint spacings less than about 40 cm whereas colonnades have larger joint spacings we use structural and petrographic methods to infer heat transfer processes and to constrain environmental conditions that produce these contrasting tiers joint surface morphology indicates overall joint growth direction and thus identifies the level in a flow where the upper and lower crusts met rock texture provides information on relative cooling rates in the tiers of a flow lava flows without entablature have textures that develop by relatively slow cooling and two joint sets that usually meet near their middles which indicate mostly conductive cooling entablature bearing flows have two main joint sets that meet well below their middles and textures that indicate fast cooling of entablatures and slow cooling of colonnades entablatures always occur in the upper joint sets and sometimes alternate several times with colonnades solidification times of entablature bearing flows constrained by lower joint set thicknesses are much less than those predicted by a purely conductive cooling model these results are best explained by a cooling model based on conductive heat transfer near a flow base and water steam convection in the upper part of an entablature bearing flow calculated solidification rates in the upper parts of such flows exceed that of the upper crust of kilauea iki lava lake where water steam convection is documented use of the solidification rates in an available model of water steam convection yields permeability values that agree with measured values for fractured crystalline rock we conclude therefore that an entablature forms when part of a flow cools very rapidly by water steam convection flooding of the flow top by surface drainage most likely induces the convection colonnades form under conditions of slower cooling by conductive heat transfer in the absence of water

stratified ejecta boulders as indicators of layered plutons on the moon abstract high resolution images of stratified ejecta boulders on the lunar nearside reveal layers of alternating low and high albedo material we measured the thickness and albedo of each alternating light and dark layer from 29 stratified boulders located in aristarchus crater and mare undarum the results were used to test hypotheses to explain the origin of the observed strata in these impact ejected boulders morphologically these boulders demonstrate cross bedding trough shaped layering tapered layering and cumulate enclaves we interpret these characteristics to be evidence that these layers result from periodic disruption by convection or density currents within a cooling layered igneous intrusion we demonstrate that the layering observed in these boulders cannot be the result of known processes occurring on the surface but instead suggests a history of complex intrusive igneous processes within the lunar crust

curie temperatures of titanomagnetite in ignimbrites effects of emplacement temperatures cooling rates exsolution and cation ordering pumices ashes and tuffs from mt st helens and from novarupta contain two principal forms of titanomagnetite homogeneous grains with curie temperatures in the range 350 500 c and oxyexsolved grains with similar bulk composition containing ilmenite lamellae and having curie temperatures above 500 c thermomagnetic analyses and isothermal annealing experiments in combination with stratigraphic settings and thermal models show that emplacement temperatures and cooling history may have affected the relative proportions of homogeneous and exsolved grains and have clearly had a strong influence on the curie temperature of the homogeneous phase the exsolved grains are most common where emplacement temperatures exceeded 600 c and in laboratory experiments heating to over 600 c in air causes the homogeneous titanomagnetites to oxyexsolve rapidly where emplacement temperatures were lower curie temperatures of the homogeneous grains are systematically related to overburden thickness and cooling timescales and thermomagnetic curves are generally irreversible with lower curie temperatures measured during cooling but little or no change is observed in room temperature susceptibility we interpret this irreversible behavior as reflecting variations in the degree of cation ordering in the titanomagnetites although we cannot conclusively rule out an alternative interpretation involving fine scale subsolvus unmixing short range ordering within the octahedral sites may play a key role in the observed phenomena changes in the curie temperature have important implications for the acquisition stabilization and retention of natural remanence and may in some cases enable quantification of the emplacement temperatures or cooling rates of volcanic units containing homogeneous titanomagnetites

palaeomagnetism of the early permian mount leyshon intrusive complex and tuckers igneous complex north queensland australia summary this study provides reliable precisely defined and well dated early permian 286 6 ma palaeomagnetic poles for australia from the mount leyshon intrusive complex mlic and the tuckers igneous complex tic both complexes are associated with prominent negative magnetic anomalies indicating the presence of rocks carrying stable remanence of reverse polarity with a koenigsberger ratio greater than unity the characteristic remanence carried by the intrusive phases and by locally remagnetized contact metamorphosed host rocks is always of reverse polarity consistent with acquisition during the permo carboniferous kiaman reverse superchron the corresponding palaeopoles confirm that australia occupied high latitudes in the early permian the pole positions are mlic lat 43 2 s long 137 3 e dp 6 0 dm 6 4 q 6 tic lat 47 5 s long 143 0 e dp 6 0 dm 6 6 q 6 permian palaeomagnetic overprinting is detectable at considerable distances from the mlic 2 3 km well beyond the zone of visible alteration the primary nature of the early permian palaeomagnetic signature is established by full baked contact aureole tests at both localities other new data from australia are consistent with the poles reported here comparison of the australian african and south american apparent polar wander paths apwp suggests that mean permian and triassic poles from west gondwana particularly from south america are biased by remagnetization in the jurassic cretaceous and that the late palaeozoic mesozoic apwp for gondwana is best defined by australian data the australian apwp exhibits substantial movement through the mesozoic provided only that the time averaged palaeofield was zonal the early triassic palaeomagnetic data from australia provide an important palaeogeographic constraint that the south geographic pole was within or very close to se australia around 240 ma the new early permian poles are apparently more consistent with pangaea b type reconstructions of gondwana and laurussia than with the pangaea a2 configuration this may be partly an artefact of reconstruction problems within gondwana as systematic differences between approximately coeval apparently reliable permo carboniferous poles from africa south america and australia are evident in standard gondwana reconstructions these discordances require a tighter fit of the southern continents suggesting that some attenuation of continental margins not accounted for in the reconstructions has occurred during breakup of gondwana or that the fit between east and west gondwana needs to be substantially modified if stretching of continental margins during breakup of supercontinents is a general phenomenon it may help to ameliorate but not solve the long standing controversy regarding pangaea reconstructions although alternative pangaea reconstructions such as pangaea b may reconcile poles from laurussia with australian poles in the late carboniferous early permian no plausible reconstruction can bring the early triassic poles into agreement this suggests that persistent departures from a pure dipole field may have been present in the early triassic lesser but still significant non dipole effects may also have been present during the late carboniferous and permian and may help resolve the pangaea a versus b controversy without requiring substantial attenuation of continental margins or intracontinental deformation we suggest that the most parsimonious interpretation of the palaeomagnetic and geological information is that laurussia and gondwana remained in a pangaea a2 type configuration through the permian and triassic discordance between the apwps for these two supercontinents is attributable mainly to persistent non dipole components of the geomagnetic field which were most important in the early triassic

hawaiian magma reservoir processes as inferred from the petrology of gabbro xenoliths in basalt kahoolawe island gabbro xenoliths in a tholeiitic lava of kahoolawe island hawaii a 1 3 1 4 ma shield volcano are 1 3 cm in size and comprised of plagioclase clinopyroxene and orthopyroxene gabbro textures while intergranular and in part subophitic are open due to 28 48 vol of vesicular basalt occupying xenolith space vesicles in and around the xenoliths are lined or filled with rhyolitic glass segregation vesicles the host is evolved tholeiite mgo 6 1 wt with phenocrysts microphenocrysts and glomerocrysts of olivine clinopyroxene orthopyroxene and plagioclase and megacrysts 1 cm of plagioclase the sr isotope ratio of one xenolith is 0 70489 the host basalt ratio is 0 70460 xenolith isotope composition grain resorption and clinopyroxene fs12 5 15wo38 35 5 orthopyroxene fs19 5 24wo4 1 and plagioclase an68 65or0 8 1 2 compositions suggest that these gabbros crystallized from kahoolawe tholeiitic magma of essentially the same composition as the host basalt but pre dating the magma represented by the host based on the absence of intergranular fe ti oxide phases from the pl cpx opx assemblages and the open vuggy textures we envision crystallization on a reservoir roof at temperatures 1100 c entrainment of gabbro assemblages and plagioclase megacrysts from a roof mush suspension zone occurred during convection associated with replenishment of the magma reservoir these open textured gabbro xenoliths are therefore not fragments of preexisting coarse grained bodies such as sills or segregation veins rhyolitic glass in vesicles represents a gas effervescence filtration process that forced fractionated residual liquids from the groundmass into voids associated with the xenoliths

the occurrence and origin of late paleozoic remagnetization in the sedimentary rocks of north america although it has been known for 25 years that some paleozoic sedimentary rock units in europe and north america were remagnetized during pennsylvanian or permian time it is only very recently that the complex and widespread nature of the late paleozoic remagnetization phenomenon has been generally acknowledged it is now recognized that many paleozoic paleomagnetic poles for north america that were once considered reliable are in fact the result of remagnetization and as a consequence the paleomagnetic data base for the paleozoic is undergoing rapid and drastic revision the causes of late paleozoic remagnetization in north america are currently the focus of much interest and active research the remagnetization can reside in either hematite or magnetite and different remagnetization mechanisms have been important in different settings chemical remagnetization processes some related to specific diagenetic events are dominant in hematite bearing sandstones and carbonates in magnetite bearing carbonates both chemical and thermoviscous remagnetization processes appear to have been important but it is difficult to determine which process is the dominant one in some settings some of the observed remagnetizations can be linked to the migration of chemically active and perhaps hot fluids during the mountain building events that affected much of the continent during the late paleozoic paleomagnetic studies promise to be important in assessing the role of orogeny in driving fluid migrations within sedimentary basins and in constraining the age of the migrations and the nature of the fluids

interpretation of domainal groundmass textures in basalt lavas of the southern lamington volcanics eastern australia the arrangement of crystals vesicles and glass in the matrix of a volcanic rock records information about the kinematics accompanying its formation simple rock textures involve homogeneous distributions of features such as aligned crystals whereas more complex rock textures involve inhomogeneous distributions comprising subregions or domains of homogeneous textural types in the groundmass of volcanic rocks two types of domainal textures are defined by 1 differences in the degree of alignment or direction of alignment of crystals and 2 different concentrations of crystals a petrographic survey of basalt lavas of the tertiary lamington volcanics in northeast new south wales australia reveals the presence of both crystal alignment and crystal concentration domainal textures alignment domains are interpreted as shear zones overprinting homogenous aligned textures during flow the formation of crystal concentration domains appears to involve multiple processes including 1 crystal clustering during flow 2 complex overprinting of dilatant groundmass shear zones 3 melt segregations due to stress or 4 postvesiculation processes

thermal evolution of rifted continental margins new evidence from fission tracks in basement apatites from southeastern australia the palaeozoic granitic basement of southeastern australia shows a regular pattern of apatite fission track ages which is dominated by a rapid decrease in apparent age towards the rifted continental margin apparent ages away from the margin vary between about 230 and 360 myr but drop sharply to minimum values of around 80 myr near the coast just south of the sydney basin and 150 175 myr elsewhere along the coast fission track length information allows the age pattern to be interpreted as the result of partial to complete resetting of the older ages by a widespread thermal event at about 80 100 myr this event accompanied the rift phase of passive margin development and peaked just before the onset of sea floor spreading in the adjacent tasman sea the zone of reduced apatite ages is deeply embayed around the gippsland basin supporting the suggestion that it originated as a cretaceous failed arm of the tasman rift system the thermal event has affected apatites up to about 130 km from the margin and has produced a characteristic set of bimodal track length distributions in apatites with partially reset ages the bimodality gradually disappears towards the oldest apatite ages which have negatively skewed unimodal distributions the youngest apatites also tend towards unimodal length distributions indicating that thermal annealing of pre existing tracks in these samples went almost to completion maximum temperatures reached in samples presently exposed in the coastal area were thus probably about 110 120 c thermal activity was accompanied by considerable uplift across the region followed by gradual erosion to expose the present land surface post breakup erosion was generally greater towards the new coastline with near coastal samples being previously buried a probable 1 5 2 5 km

the demagnetization temperature necessary to remove viscous remanent magnetization recent experimental studies have shown that the temperature necessary to demagnetize viscous remanent magnetization vrm in magnetite bearing rocks is significantly higher than predicted by standard neel single domain theory the walton time temperature equation which has been invoked by some to explain the enhanced thermal stability actually relates intensities of vrms acquired at different temperatures rather than describing the unblocking of a previously acquired vrm we show that the low field approximation lfa used in all previous formulations which predicts equal vrm acquisition and decay rates is not valid for a magnetite assemblage in the earth s field a medium field approximation mfa predicts significantly faster vrm acquisition and therefore greater thermal stability in decay for the larger particles in the assemblage conventional theory does apply to red bed acquisition decay experiments since the lfa is valid for hematite

the south range breccia belt of the sudbury impact structure a possible terrace collapse feature the south range breccia belt srbb is an arcuate 45 km long zone of sudbury breccia in the south range of the 1 85 ga sudbury impact structure the belt varies in thickness between tens of meters to hundreds of meters and is composed of a polymict assemblage of huronian supergroup 2 49 2 20 ga nipissing diabase 2 2 ga and proterozoic granitoid breccia fragments ranging in size from a few millimeters to tens of meters the srbb matrix is composed of a fine grained 100 m assemblage of biotite quartz and ilmenite with trace amounts of plagioclase zircon titanite epidote pyrite chalcopyrite pyrrhotite and occasionally chlorite the srbb hosts the frood stobie vermilion and kirkwood quartz diorite offset dykes the former being associated with one of the largest ni cu pge sulphide deposits in the world optical petrography and whole rock geochemistry concur with previous studies that have suggested that the matrix of the srbb is derived from comminution and at least partial frictional melting of the wall rock huronian supergroup lithologies rare earth element ree data from all sampled lithologies associated with the srbb exhibit crustal signatures when normalized to c1 chondrite values additionally ree data from the quartz diorites disseminated sulphides in sudbury breccia and a sample of an aphanitic biotite hornblende tonalite dyke exhibit flat slopes when compared to the mafic and felsic norites quartz gabbro and granophyre units of the sudbury igneous complex sic which suggests that these lithologies are representative of bulk sic melt we suggest that the srbb was formed by high strain rate 1 m s gravity driven seismogenic slip of the inner ring of the sudbury impact structure during postimpact crustal readjustment crater modification stage failure of the hanging wall may have facilitated the injection of bulk sic melt into the srbb along with the ni cu pge sulphides of the frood stobie deposit postimpact penokean 1 9 1 7 ga tectonism particularly northwest directed shearing along the south range shear zone and associated thrust faulting could account for the present subvertical orientation of the srbb and the apparent lack of a connection at depth with the sic

two stage vesiculation in the cohassett flow of the grande ronde basalt south central washington the cohassett flow in south central washington is considered a possible host for a nuclear waste repository at the hanford site and is being investigated as part of the basalt waste isolation project the cohassett flow of the grande ronde basalt contains two vesicular zones separated by 24 m of dense sparsely vesicular basalt the vesicular zones represent accumulations of upward migrating bubbles sequentially frozen in by the downward passage of the upper solidification front the separation of the two zones suggests a hiatus in the exsolution of aqueous vapor from the lava the flow top vesicular zone formed during and immediately following emplacement of the flow from vapor bubbles nucleated during ascent and eruption whereas the internal vesicular zone formed after emplacement from bubbles apparently nucleated at the lower solidification front homogeneous nucleation of vapor was probably initially driven by large oversaturation pressures in the erupting lava after emplacement heterogeneous nucleation of vapor was probably initiated by crystallization in the margins of the flow

origin of vesicle layering and double imbrication by endogenous growth in the birkett basalt flow columbia river plateau abstract the 40 m thick birkett basalt pahoehoe flow at sentinel gap in the columbia river plateau has an unusually thick 15 m upper vesicular zone this zone includes a striking layering in which the layers have contrasted vesicle abundances and sizes most layers show a reverse grading of vesicle size and abundance the layering is interpreted to have grown endogenously by the cyclic injection of vesicular lava layers under the growing top crust accommodated by uplift of that crust grading of the layers resulted from vesicle growth and ascent each injection occurred at or near the boundary between vesicular and non vesicular lava of the preceding layer and split that layer into an upper vesicular part and a lower non vesicular part critical to this interpretation are 1 a pervasive foliation and lineation defined by the parallelism of strongly flattened and elongate vesicles transects the vesicle layers obliquely and 2 the magnetic fabric the anisotropy of magnetic susceptibility is oriented similarly to the vesicle foliation and also defines a cryptic foliation in the non vesicular zone having a dip opposed to that in the layered zone these foliations are interpreted to be opposed imbrications and indicate the flow azimuth of the lava they strongly support the concept of lava growth by successive thin sill like insertions of fresh vesicular lava between hot but static and effectively solid floor and roof

surface to tunnel seismic tomography studies at yucca mountain nevada 1 a surface to tunnel seismic survey was conducted to estimate fracture intensity and distribution in the proposed nuclear waste repository area at yucca mountain nevada a 5 km long source line and a 3 km long receiver line were located on top of yucca mountain ridge and inside the exploratory study facility esf tunnel respectively numerical modeling showed that the first arrival p waves are mainly propagated in the repository horizon at depth therefore two dimensional travel time inversions were performed after the actual source locations on the ridge were projected onto the layer of interest at depth static corrections were applied to absorb the scatter in the new source locations and to correct for variations in source and receiver coupling and travel time shifts zero offset amplitude variations of the first arrivals across the receiver line inside the tunnel showed strong correlation with the fracture density determined by measurements along the tunnel wall wave guidance by subhorizontal fractures is suggested as a possible source for this phenomenon tomographic inversion of the travel time data revealed a low velocity zone in the south central area of the repository horizon conversion of the velocity tomography results to fracture density tomograms showed good correlation with fracture intensity mapped along tunnel walls the fractured zone extends over a large area in the southern region of the potential repository in particular southwest of the intensely fractured zone encountered in the esf tunnel the rock in the northern section of the repository appears more competent compared to the highly fractured areas in the south

the degassing of hawaiian tholeiite accepting the gerlach and graeber 1985 estimates of the initial co2 and h2o concentrations we have calculated the variation of the concentrations of these gases dissolved and or exsolved in hawaiian tholeiite from the moment it is generated until it solidifies at the earth s surface these computations are extensions of our previous work bottinga and javoy 1989 1990a 1990b on the nucleation growth and ascent of bubbles in mid oceanic ridge basalt the present study is different in that we consider low pressure p 1 bar bubble nucleation and the presence of h2o our results indicate that 1 hawaiian liquid tholeiite is supersaturated with respect to co2 when it erupts 2 degassing of the basaltic liquid takes place during different stages each of which gives rise to a compositionally distinct bubble population which to a large extent is lost before the formation of a new population 3 fumarolic gas compositions are affected by the pressure and temperature at which gas bubbles are released from the lava and the velocity with which lava is ejected from the magma chamber

anomalous unblocking temperatures viscosity and frequency dependent susceptibility in the chemically remagnetized trenton limestone abstract chemical remagnetization of many carbonate rock units occurred by diagenetic alteration of iron sulfide precursors and by authigenesis of fine grained magnetite which can commonly be recognized by a distinctive set of rock magnetic attributes these carbonates also typically exhibit anomalously high unblocking temperatures for recent thermoviscous components that have overprinted the characteristic chemical remanence recent work has shown that departures from neel theory as applied in the model of pullaiah et al can generally be attributed to vrm carried by multidomain particles to which the theory does not apply in the remagnetized carbonates however rock magnetic data all point to an overwhelming dominance of fine superparamagnetic and stable single domain particles therefore we explore possible alternative causes for the anomalous unblocking temperatures including maghemitization and magnetocrystalline rather than shape anisotropy new blocking unblocking models for these cases however fail to match the observed behavior leading to the conclusion that the anomalous unblocking temperatures are indeed probably attributable to multidomain carriers a possible alternative explanation is an increase in coercivity due to maghemitization of single domain particles after acquisition of the viscous overprint causing a significant increase in their unblocking temperatures new frequency dependent susceptibility measurements at low temperatures allow us to focus on the time temperature behavior of the particles at the superparamagnetic stable single domain threshold and we find that these do closely follow the predictions of neel theory as formulated in the models of pullaiah et al and walton middleton schmidt

paleothermometry of the sydney basin evidence from overprinting of magnetizations of late permian and mesozoic rocks and from the rank of permian coals and mesozoic phytoclasts coal particles suggests that surface rocks in the sydney basin eastern australia have been raised to temperatures of the order of 200 sup 0 c or higher as vitrinite reflectance an index of coal rank or coalification is postulated to vary predictably with temperature and time estimates of the paleotemperatures in the sydney basin based on observed vitrinite reflectance measurements can be made in conjunction with reasonable assumptions about the tectonic and thermal histories of the basin these estimates give maximum paleotemperatures of present day surface rocks in the range 60 249 sup 0 c depending on factors such as location in the basin the thickness of the sediment eroded and the maximum paleogeothermal gradient higher coal rank and consequently larger eroded thicknesses and paleogeothermal gradients occur along the eastern edge of the basin and may be related to seafloor spreading in the tasman sea on the basin s eastern margin a theory of thermal activation of magnetization entailing the dependence of magnetic viscosity on the size distribution of the magnetic grains is used to obtain an independent estimate of the maximum paleotemperaturesmore in the sydney basin this estimate places the maximum paleotemperature in the range 250 300 sup 0 c along the coastal region both coalification and thermal activation of magnetization models provide strong evidence of elevated paleotemperatures which in places exceed 200 sup 0 c and the loss of sediment thicknesses in excess of 1 km due to erosion less

recognizing distinct portions of seamounts using volcanic facies analysis examples from the archean slave province nwt canada abstract archean volcanic rocks in the mafic dominated ca 2 66 2 69 ga point lake and beaulieu river belts slave province northwest territories are significant in demonstrating the facies that characterize specific portions of pillow volcanoes or seamounts irrespective of tectonic setting three distinct localities mapped in detail display facies consistent with 1 proximal deep water 2 medial to distal deep water and 3 medial shallow water seamount settings the proximal facies in the point lake belt include a 55 m thick non vesicular pillowed sequence cut by numerous mafic dykes and sills dykes contain multiple chilled margins indicating successive magma pulses which contributed to edifice construction abundant feeder conduits in addition to the absence of fragmental facies and vesicles are typical of the central deep water portion of seamounts where growth is initiated the medial to distal deep water facies in the point lake belt are represented by a 30 80 m thick assemblage of disorganized pillow breccia and pillowed and massive flows with 5 27 vesicularity massive non vesicular hyaloclastite intermingled with sedimentary material fluidal peperite in addition to thin shale units interstratified with pillow breccia and hyaloclastite indicate that sedimentation and volcanism were contemporaneous an increase in fragmental units and vesicularity relative to the proximal deep water facies is suggestive of the medial to distal part of a seamount in shallower water bedded tuffs laterally along strike with massive flows are the results of turbidity current deposition immediately following localized subaqueous eruptions a medial shallow water seamount setting is represented in the beaulieu river belt by a 5 85 m thick sequence of vesicular lobate pillowed and massive flows stratified pillow breccia and hyaloclastite and mafic dykes vesicularity ranges from 21 49 in pillowed flows 5 40 in massive flows and 20 35 in pillow breccia and hyaloclastite stratified pillow breccia developed along steep flow fronts in shallow water whereas bedded hyaloclastite formed during reworking and redeposition of autoclastic hyaloclastite on seamount flanks in shallow water the volcanic facies associations in the study areas are analogous to those of modern seamounts associated with the mid atlantic ridge and east pacific rise as well as mesozoic cenozoic seamounts in the canary islands fiji southwest japan the sea of japan and cyprus volcanological studies in the point lake and beaulieu river volcanic belts and subsequent comparisons with phanerozoic analogues demonstrate the manner in which distinct portions of ancient seamounts can be recognized in similar archean terranes

time temperature relations for the remagnetization of pyrrhotite fe7s8 and their use in estimating paleotemperatures abstract paleotemperature controls the maturation of coal and hydrocarbons in sedimentary basins and is also important in determining paleogeothermal gradient and hence tectonic style in exhumed metamorphic terrains one method of estimating paleotemperature analyses the partial remagnetization of a rock due to heating in thick volcanic or sedimentary sequences over subcrustal heat sources such as plumes or at convergent plate margins the overprinted natural remanent magnetization nrm of a rock records both the age and the paleotemperature of remagnetization but a temperature correction from laboratory to geological time scales is required using theoretical time temperature relations time temperature relations are well known for magnetite fe 3 o 4 but are reported here for the first time for pyrrhotite fe 7 s 8 another common nrm carrier data for each mineral separately yield independent estimates of paleotemperature if geologically reasonable estimates of heating time can be made paleotemperature can be estimated without geological input if data for both minerals are combined together with the age of remagnetization determined from the paleomagnetic pole of the nrm overprint these paleotemperature estimates can be used to infer the history of heating and uplift following burial as a test case we examine thermally acquired nrm overprints carried by pyrrhotite fe 7 s 8 and magnetite fe 3 o 4 in the milton monzonite of southeastern australia these overprints record a heating event about 100 ma ago probably thermal doming prior to rifting of the tasman sea that upgraded coal rank in the sydney basin extrapolating from laboratory to geological times using the new time temperature contours for pyrrhotite we estimate that the presently exposed sydney basin in the vicinity of the milton monzonite was remagnetized by heating to 165 30 c for 100 ka assuming a paleogeothermal gradient of 70 c km appropriate for young or incipient rifts the depth of burial at the time of remagnetization is estimated to have been 2 3 0 4 km this figure is in excellent agreement with independent estimates based on reflectance data for the coal accessory mineral vitrinite

stability of foams in silicate melts bubble coalescence and the spontaneous disruption of high porosity foams in silicate melts are the result of physical expulsion of interpore melt syneresis leading to bubble coalescence and diffusive gas exchange between bubbles melt expulsion can be achieved either along films between pairs of bubbles or along plateau borders which represent the contacts between 3 or more bubbles theoretical evaluation of these mechanisms is confirmed by experimental results enabling us to quantify the relevant parameters and determine stable bubble size and critical film thickness in a foam as a function of melt viscosity surface tension and time foam stability is controlled primarily by melt viscosity and time melt transport leading to coalescence of bubbles proceeds along inter bubble films for smaller bubbles and along plateau borders for larger bubbles thus the average bubble size accelerates with time in silicate melts the diffusive gas expulsion out of a region of foam is effective only for water and even then only at small length scales as the diffusion of co2 is negligible the results of our analyses are applicable to studies of vesicularity of lavas melt degassing and eruption mechanisms

volcanic diapirs in the orange mountain flood basalt new jersey usa abstract diapir shaped structures 4 30 m high consisting of vesicular basalt have intruded into the interior of a 50 70 m thick subaerial orange mountain basalt flow exposed at several rock quarries in northern new jersey the basalt flowed onto a travertine encrusted mudflat saturated with alkali salts we propose that pressurized alkali vapors trapped under the lava created a vesicular and viscous flow bottom layer about 10 m thick vesicle coalescence within this layer increased its buoyancy where it locally accumulated into diapirs and displaced overlying lava large bubbles within the diapirs expanded upon intrusion into hot flow interiors where they explosively escaped leaving lenses of breccia some early diapirs reached the base of the upper lava crust these diapirs document vapor driven convection of large blobs of contaminated lava into the lava core of the orange mountain flow

effects of titanomagnetite reordering processes on thermal demagnetization and paleointensity experiments titanomagnetite fe3 xtixo4 0 x 1 is a common naturally occurring magnetic mineral critical to many paleomagnetic studies underlying most interpretations is the assumption that lacking chemical alteration curie temperature tc remains constant however recent work has demonstrated that tc of many natural titanomagnetites varies strongly as a function of thermal history independent of chemical alteration this is inferred to arise from reordering of cations and or vacancies in the crystal structure and changes occur at temperatures and times relevant to standard paleomagnetic thermal treatments because changes take place at t tclose will have their original tb spectrum truncated at t tclose predicted behavior during thellier type paleointensity experiments results in only modest deviations in nrm lost or ptrm gained from the non reordering case much larger deviations are predicted for ptrm checks compared to paleointensity results from titanomagnetite bearing pyroclastic deposits modeled non ideal behavior occurs in the same temperature intervals but is much more systematic reordering is likely one contributing factor to failure of paleointensity experiments this article is protected by copyright all rights reserved

gas bubble dynamics model for shallow volcanic tremor at stromboli volcanic tremor at stromboli aeolian islands italy is correlated to small infrasonic transients ripepe et al 1996 which repeat almost rhythmically in time in a range between 0 8 and 1 2 s we demonstrate that infrasonic transients are associated to small gas bubble 0 5 m burstings which produces no transients in the seismic signal tremor ground displacement attenuates with the inverse of the distance from the craters indicating that the source is shallow short term energy release shows that infrasonic and seismic signals are linked to the same dynamical process while at the long term scale it is evident that the two signals are controlled by two distinctive mechanisms we suggest that the possible physical model acts in two steps first gas coalescence and then gas bursting in our model the seismic signal is related to the coalescence of a gas bubble from a layer of small bubbles while the infrasonic signal is linked to the bursting of the bubble when it reaches the magma surface gas bubbles could form by free coalescence in magma or could be forced to coalesce by a structural barrier we calculate that forced coalescence induces in magma a pressure change 104 pa 2 orders of magnitude higher than free coalescence and it explains best the tremor ground displacement 10 5 m recorded at stromboli moreover forced coalescence evidences the role of a structural barrier such as a dike in volcanic tremor source dynamics in this gas dynamic process the delay time of 1 2 s between infrasonic pulses could reflect the gas nucleation interval of basaltic magma thomas et al 1993 manga 1996 we propose that the source function for the shallow volcanic tremor at stromboli could be the viscoelastic reaction of the magma to the pressure decrease induced by gas bubble growth rate under constant depressurization the spectrum of our source function is controlled by the time duration of the pressure pulse which represents the viscoelastic relaxation time of the magma and gas bubble growth rate the predicted asymptotic decay of the frequency contents fits the spectral behavior of the volcanic tremor ground displacement recorded at stromboli we show that the same spectral behavior can be found in ground displacement spectra of volcanic tremor recorded on different volcanoes

paleomagnetism and paleothermometry of the sydney basin 2 origin of anomalously high unblocking temperatures the milton monzonite of southeastern australia was thermoviscously remagnetized as a result of cretaceous burial and uplift thermal demagnetization separates the low unblocking temperature lt overprint from the high unblocking temperature ht primary remanence with a relatively sharp junction between lt and ht components in vector projections for single domain grains the junction temperature tl between two such vectors corresponds to the maximum blocking temperature tr reactivated in nature apart from a correction for the difference between natural and laboratory timescales however measured tl values are distributed over an implausibly wide range 250 c for burial remagnetization of an untilted intrusion like the milton monzonite furthermore many tl values are anomalously high compared to the predictions of single domain theory multidomain grains are the cause of these anomalies samples pretreated before thermal demagnetization by zero field cycling to liquid nitrogen temperature so as to erase multidomain remanence and isolate single domain remanence do have the theoretically expected tl values in these samples realistic remagnetization time and temperature tr tr conditions in nature are predicted using the t t contours of pullaiah et al 1975 the anomalously high tl values before low temperature treatment are due to multidomain grains which carry 50 of the lt overprint the lt thermal demagnetization curve in samples dominated by multidomain grains is quasi exponential in shape with a high temperature tail extending almost to the curie point as predicted by multidomain theory these high lt unblocking temperatures which are much greater than plausible remagnetization temperatures reached in nature overlap and mask the lower part of the ht unblocking temperature spectrum driving up tl values and leading to inflated estimates of tr although multidomain remanence is a sufficient explanation of anomalously high unblocking temperatures of thermoviscous overprints in the milton monzonite chemical overprinting may be a factor in other lithologies and tectonic settings

an integrated magnetic and geological study of cataclasite dominated pseudotachylytes in the chiapas massif mexico a snapshot of stress orientation following slip summary the permian age chiapas massif in southeast mexico is locally host to well exposed pseudotachylyte vein networks the veins are black to dark grey and aphanitic in appearance and consist mostly of microbreccia of angular fragments of plagioclase k feldspar biotite and quartz in a cryptocrystalline microscopically irresolvable matrix evidence of melting is present in the form of glass seams dikelets glass clasts included in cataclasite and a distinct chemistry in the pseudotachylite veins pristine glass represents a relatively small volume of the pseudotachylite veins at an exposure along the tablon river valley where the host rock is a medium to coarse grained equigranular quartz diorite individual veins are consistently oriented about 280 are up to 16 mm wide tens of cm apart display a consistent left lateral offset and can be traced for several metres individual pseudotachylyte veins rarely cross each other and they cannot be directly linked to a regional scale fault pseudotachylytes are apparently formed by a combination of crushing comminution and frictional melting but they are cataclasite dominated textures indicate that cataclasis continued after frictional melting had ceased a 40ar 39ar age determination from whole rock chips of one vein shows a climbing ar release spectrum with a date of 114 ma as the most reliable age estimate for ar retention this result is interpreted in the context of pseudotachylyte formation recrystallization and resetting of k bearing minerals for the k ar system in the late early cretaceous ten veins were sampled for palaeomagnetic and magnetic fabric studies with samples collected from both the veins and their host rock remanence data give moderate natural remanent magnetization nrm intensities for both the veins e g nrm mean 6 6 10 3 a m 1 5 5 and host rock mean 7 7 10 3 a m 1 10 8 many samples of host rock yield an ill defined east west directed and shallow magnetization which we interpret as a late permian magnetization based on previous studies of the chiapas massif this magnetization resides in haematite veins as well as immediately adjacent host rock typically have well defined single polarity magnetizations of north northwest declination and moderate positive inclination and these resemble the cretaceous expected field direction the overall mean of the veins is of dec 348 7 and inc 33 6 k 30 5 and 95 12 3 n 6 site means rock magnetic parameters suggest that the remanence in the veins principally resides in low ti pseudo single domain magnetite and maghemite grains but haematite coarse grained magnetite rutile fe cr ni oxides and ilmenite are also present in the veins bulk magnetic susceptibility values range between 0 3 and 1 1 10 3 si volume units and host rock values do not differ significantly from vein values the orientations of the principal susceptibility axes in the host rock and the veins are indistinguishable in both magnetic foliations are near vertical and are essentially parallel to the nearly east west orientation of the veins nonetheless host rock fabrics are predominantly prolate whereas vein fabrics are oblate the mean susceptibility tensors of host rocks and pseudotachylyte are characterized by p t values of 1 041 0 327 and 1 033 0 302 respectively this result suggests that the fabric reflected by magnetic susceptibility anisotropy in the veins was formed under pure shear stress during cooling of the veins the lack of evidence of fabric rotation supports models that associate partial melt with viscous break during fault slip

surface movements of emplaced lava flows measured by synthetic aperture radar interferometry lava flows continue to move after they have been emplaced by flow mechanisms this movement is largely vertical and can be detected using differential synthetic aperture radar sar interferometry there are three main components to this motion 1 movement of surface scatterers resulting in radar phase decorrelation 2 measurable subsidence of the flow surface due to thermal contraction and clast repacking and 3 time dependent depression of the flow substrate these effects act in proportion to the thickness of the lava flow and decay with time although there is a time lag before the third component becomes significant we explore these effects using sar data from the ers satellites over the etna volcano sicily phase decorrelation on young thick a a lava flows persists for a few years and probably results from surface block rotations during flow contraction maximum measured subsidence rates of the 1991 1993 lava flow over a period of 70 days are about 0 7 mm day 1 but are potentially greater in areas of data decorrelation these rates fall to 2 7 10 2 mm day 1 after about 20 years in flows about 50 m thick sooner for thinner flows comparison with measured subsidence rates on kilauean lava lakes suggests that thermal contraction only accounts for about one third of the observed subsidence the remaining motion is thought to come from surface clast repacking during cooling and from creep mechanisms in the flow substrate measurements of postemplacement surface movement provide new constraints on the thermomechanical properties of lava flows and have cautionary implications for the interpretation of interferometric sar data of volcanoes 2001 american geophysical union

the evolution of lava flows from ephemeral vents at mount etna insights from vesicle distribution and morphological studies abstract lava flow units from ephemeral vents at mount etna are characterized by a cross flow subdivision into zones with different surface morphology symmetrically distributed with respect to the centerline of the flow these zones are 1 a central zone cp zone with a relatively smooth surface near the vent 2 a lateral zone lp zone covered by a carpet of clinkers overlying the massive lava body and 3 a lateral levee zone ll zone in many cases the cp and lp zones are separated by an inward dipping groove and the cp zone is from some decimeters up to more than 1 m thicker than the adjacent lp zone in sections perpendicular to the flow direction the vesicles corresponding with the cp zone are systematically distributed in a ring like region and are characterized by a sub elliptical shape with their minor axes along a direction radial to the center of such region vesicles corresponding with the lp zone are still elliptical and their major axes tend to be parallel to the nearest cooling surface vesicularity is the lowest at the center of the ring like region where vesicles approach a spherical shape in the upper part of the ring like region vesicle accumulation and coalescence below the crust produces one or more gas rich layers which act as levels of preferential detachment for the formation of lava tubes the observed morphological characteristics and reconstructed vesicle distribution patterns are consistent with a radial distribution of velocity within the lava flow unit and suggest the existence of an inner plug flow region where the velocity gradient is zero a simple evolution scheme is proposed in which a lava flow unit from ephemeral vent invariably evolves to a lava tube through the downward migration of the plug flow region the progressive reduction of the cross flow section due to cooling produces a pressure increase within the flowing lava body and the thickening of the active central portion of the lava flow unit in order to satisfy mass conservation