correlation of dielectric permittivity of volcanic units on venus with age 1 volcanic plains are the most common surface units on venus a survey of distinctive contrasts of the radiophysical properties associated with boundaries of volcanic units was carried out only those sites where the sign of the dielectric permittivity difference can be reliably inferred from radiophysical data were considered the inferred contrasts in the dielectric permittivity across the unit boundaries were correlated with the relative stratigraphic positions of the units it turned out that locally older units usually have locally lower dielectric permittivity this phenomenon can be a result of a set of processes of surface alteration and mantling formation which decrease the dielectric permittivity with time the observations are not consistent however with a gradual and globally uniform decrease of the permittivity thus it is impossible to instantly use the radiophysical properties to date the surface material the results show that further studies of the radiophysical data sets from the magellan data collection are promising for understanding the surface mantles and mechanisms of their formation and evolution as well as for some inferences about ages

submarine canyons and channels in the lower st lawrence estuary eastern canada morphology classification and recent sediment dynamics abstract series of submarine canyons and channels observed in the lower st lawrence estuary lsle provide an opportunity to analyze in great detail the morphology spatial distribution and modern activity of such systems in a relatively shallow 300 m semi enclosed basin based on their geomorphology and physical settings the canyons and channels were classified into four categories according to their feeding sources ancient or recent glacially fed river fed longshore drift fed and sediment starved systems their activity was interpreted based on geomorphological characteristics such as the presence of bedforms related to gravity flows backscatter intensity axial incision and the presence of rapidly deposited layers in surficial sediments river fed deltas were interpreted as inactive mainly because suspended sediment concentrations at river mouths are low preventing the generation of hyperpycnal currents or delta lip failures related to high sediment supply longshore drift fed canyons present where the coastal shelf narrows were found to be episodically active probably due to earthquakes or extreme storm events unlike other longshore drift fed canyons observed elsewhere in the world they are active infrequently because of the modern low rates of sediment supply to their heads the most active canyons are the sediment starved type and were observed near pointe des monts their activity is probably due to slope failures and to the presence of strong hydrodynamic processes therefore sediment supply is not the main mechanism responsible for modern canyon and channel activity in the lsle however sediment supply has been an important factor during the formation of the submarine channels and canyons their quasi exclusive occurrence on the quebec north shore is attributed to its larger watershed and important sedimentary delivery during deglaciation the quebec north shore watershed is 20 times greater than the quebec south shore watershed which favored the transport of greater volumes of sediment during the early holocene moreover the slope proximity to the shore led to the formation of longshore drift fed systems on the north shore when sediment supplied to rivers were transferred on a narrow shelf

a new analysis of the magnitude of the february 1663 earthquake at charlevoix quebec abstract this paper presents a new and comprehensive analysis of the magnitude of the 1663 charlevoix quebec earthquake based on a modified mercalli intensity scale mmi of about vi from reports of damage to chimneys and a masonry wall in roxbury and boston massachusetts the best estimate of the moment magnitude of this earthquake is m 7 3 to 7 9 from mmi attenuation relations using ground motion attenuation relations and the threshold for chimney and masonry damage from fragility curves for seventeenth and eighteenth century dwellings in boston the magnitude of the 1663 earthquake is at least m 6 9 if the damage occurred on very soft soil conditions and is m 7 3 to m 7 7 if the damage took place on firm soil or bedrock both the best estimate of a length of 73 km for the most active section of the charlevoix seismic zone and an estimated fault area of 73 km 25 km for the 1663 earthquake are consistent with an earthquake of m 7 3 to 7 6 based on scaling relationships also in the 1811 1812 new madrid earthquake sequence only the 7 february 1812 shock the largest of the sequence caused chimney damage beyond about 600 km the 7 february new madrid event is thought to have taken place on the reelfoot fault for which the length of the recent earthquake activity and of the suspected 1812 rupture is less than that of the active seismicity zone at charlevoix these observations suggest that the 1663 charlevoix earthquake was approximately comparable in magnitude to or even larger than the largest of the 1811 1812 new madrid earthquakes and therefore was at least m 6 8 when put together the several lines of analysis in this study indicate that the best estimate of the size of the 1663 earthquake is m 7 5 0 45

seismic activity and structural features in the charlevoix region quebec the charlevoix region is historically the most active earthquake zone in eastern canada understanding the links between its seismicity and the faults of the region is important for the assessment

reconstructing thermal properties of firn at summit greenland from a temperature profile time series we have constrained the value for thermal diffusivity of near surface snow and firn at summit station greenland using a fourier type analysis applied to hourly temperature measurements collected from eight thermistors in a closed off air filled borehole between may 2004 and july 2008 an implicit finite difference method suggests that a bulk diffusivity of 25 3m2 a 1 is the most reasonable for representing macroscale heat transport in the top 30 m of firn and snow this value represents an average diffusivity and in a conduction only model generates temperature series whose phase shifts with depth most closely match those of the summit borehole data rms difference between measurements and model output is 6 days this bulk value derived numerically and corroborated analytically is useful over large tracts of the greenland ice sheet where density and microstructure are unknown

crater associated dark diffuse features on venus properties of surficial deposits and their evolution 1 in order to assess the nature of crater associated radar dark diffuse features ddfs on venus and to understand their formation and evolution we analyzed magellan radar roughness emissivity and reflectivity data in the vicinity of craters accompanied by these features following others we assumed that ddfs are deposits mantles of ejected material emplaced during formation of the impact crater the majority of radar dark parabolas the youngest ddfs are characterized by a smooth mantle atmosphere interface having low root mean square rms slopes on scales of 1 100 m as derived from magellan altimeter data older ddfs also often have areas with low rms slopes suggesting that the mantle rms slopes can be preserved for geologically long periods of time some parabolas and older ddfs have asymmetric small scale decimeter scale relief that is interpreted to be dunes that formed as a result of eolian processes this implies that the mantle material is mobile and can saltate under the influence of wind action on average aging of these features is accompanied by a decrease of mantle material dielectric permittivity the most efficient mechanism for parabola degradation seems to be the removal of mantle material from the site of initial deposition by subsequent winds we found a few examples of features that could be interpreted to be the result of in situ modification of the primary mantle

merging magellan emissivity and sar data for analysis of venus surface dielectric properties abstract integration of magellan datasets for venus is an important part of characterizing the geology of the surface we analyze the global average behavior of radar backscatter and emissivity and find that for incidence angles greater than 30 the two parameters can be modeled by the behavior of a single dielectric surface e 4 15 with variable roughness based on this result we propose a model which relates the hh backscatter and h polarized emissivity to values for a relative roughness fraction and dielectric constant maps of the model solutions are presented for the region 34 s 54 n removal of the contribution of roughness to the emissivity permits rapid identification of regions with anomalous composition or density in smooth areas the model typically yields dielectric constant values in good agreement with those calculated from the altimeter derived reflectivities but it appears that the model is more reliable than the hagfors results for rough surfaces the permittivity in highland regions rises with altitude and the data for these areas are more consistent with a single high dielectric interface single and multiple scattering than with internal volume scattering higher dielectric constants are associated with both smooth and rough terrains but the general tendency is toward greater roughness with higher permittivity this may suggest that a mechanical weathering process occurs in the formation of high dielectric mineral phases there are variations in dielectric constant within lava flow complexes up to values of 9 which may reflect differences in the chemical content of the rook three major types of mantling deposits are identified 1 high permittivity e 7 8 units associated with parabolic crater features and interpreted to be fine grained material with a high proportion of metallic phases 2 low permittivity e 2 3 crater related splotches or haloes interpreted to be fine grained material with minimal or weathered metallic minerals 3 low permittivity deposits with no associated crater tentatively attributed to soil or pyroclastic ignimbrite eruptions within each type of deposit there is a range of surface roughness and radar backscatter strength which are related to the original surface roughness and the depth of the mantling material

four seismogenic environments in eastern canada abstract although the seismogenic environments in eastern canada have some common features the respective seismotectonic environments are influenced by local features the seismotectonic environment in four seismogenic regions are described in order to illustrate how local features are reacting with the ambient plate tectonic stress a common feature to modify the nature of the seismic activity the charlevoix earthquakes are occurring along steeply dipping planes formed during the opening of lapetus ocean in the proterozoic perturbed by a meteor impact during the paleozoic and reactivated by the opening of the present day atlantic ocean in the mesozoic theoretical curves of maximum shear stress versus depth based on a wet upper crust and a wet quartz rheology in the lower crust for which the heat flow is 1 0 hfu 42 mw m2 can account for the depth distribution of earthquakes in this region potential trigger mechanisms are gravitationally induced stress water infiltration and vertical shear strain changes approximately 3 10 8 yr due to post glacial rebound the unusually deep focus 29 km of the m 5 9 saguenay earthquake of 1988 could be due in part to the lower heat flow in this region as compared with that in the charlevoix region which is about 70 km to the east the miramichi earthquake sequence is occurring on conjugate faults within an intrusion at depths shallower than 10 km theoretical curves of maximum shear stress versus depth based on a wet quartz rheology in the lower crust for which the heat flow is 1 5 hfu can account for this depth distribution the steep gradient in the vertical ground movement rate 5 10 8 yr in the epicentral area which is probably due to post glacial rebound is a potential trigger mechanism for favorably oriented faults the normal fault seismic regime along northeastern baffin island can be explained by a combination of topography and post glacial rebound which can generate horizontal extensional stresses capable of overcoming the ambient compressional plate tectonic stress post glacial rebound stress can act as a trigger mechanism for the ne dipping faults in this region in the grand banks region there is seismic evidence indicating that the slump that caused the destructive tsunami also generated the strong seismic m 7 2 signals but double couple solutions that satisfy certain criteria are not ruled out the absence of surface faulting especially in the epicentral regions of large earthquakes is not properly understood but several possibilities are near surface layering that could mask the underlying fault offsets and a spatiotemporal variation in the ambient plate tectonic stress field as knowledge of the causative factors of eastern canada earthquakes and also the data base for empirical earthquake scaling relations increase an overview of the seismotectonics of these mid plate earthquakes should be enhanced

temperature measurements and heat transfer in near surface snow at the south pole to study near surface heat flow on the antarctic ice sheet snow temperatures were measured at south pole station to a depth of 3 m at 15 min intervals during most of 1992 solar heating and water vapor transport were negligible during the 6 month winter as was inter grain net thermal radiation leaving conduction as the dominant heat transport mechanism the rate of temperature change at depth over 15 min intervals was smaller than that at the surface by one order of magnitude at 20 cm depth and two orders of magnitude at 1 m depth a finite difference model with conduction as the only heat transfer mechanism and measured temperatures as the upper and lower boundary conditions was applied to foursets of three thermistors each the thermal conductivity was estimated as that which minimized the difference between modeled and measured 15 min changes in temperatures at the center thermistor the thermal conductivity obtained at shallow depths above 40 cm was lower than that given by existing parameterizations based on density probably because the snow grains were freshly deposited cold and poorly bonded a model using only vertical conduction explains on average 87 ofthe observed 15 min temperature changes at less than 60 cm depth and 92 below 60 cm the difference between modeled andmeasured temperature changes decreased with depth the discrepancies between model and observation correlated more strongly with the air snow temperature difference than with the product of that difference with the square of the wind speed suggesting that the residual errors are due more to non vertical conduction and to sub grid scale variabilis of the conductivity than to windpumping the residual heating rate not explained by the model of vertical conduction exceeds 0 2 w m 3 only in the top 60 cm of the near surface snow

bell regio venus integration of remote sensing data and terrestrial analogs for geologic analysis the geology and surface morphology of bell regio 18 42 n 32 58 e are investigated using a combination of magellan venera and analogous terrestrial data the properties of surface units are compared to either direct terrestrial analog measurements or to the behaviors predicted by theoretical models five major volcanic sources are identified from geologic mapping tepev mons nefertiti corona a large shield volcano east of tepev and two small edifices southeast of tepev the volcano api mons lies northeast of the main bell uplift the oldest volcanic units are associated with an extensive low shield volcano east of tepev mons and a small edifice southeast of tepev the annular flow apron of tepev mons formed next with volcanism at a second small edifice on the southeast flank of tepev mons producing the youngest flow units comparisons between magellan data terrrestrial radar images and field topography profiles suggest that only three units resemble terrestrial a a flows the remainder are consistent with smoother pahoehoe type surfaces this suggests that most of the flow units were erupted at relatively low volume effusion rates 100 m3 s over long periods of time or had very low viscosities one flow unit has a knobby texture which resembles large tumuli or ridge structures much of the tectonic deformation in the area is in the form of fractures circumferential to the edifices and isolated blocks of tessera terrain there are no rift zones such as those which occur at beta regio atla regio or western eistla regio tepev mons is characterized by very steep slopes up to 40 along the east flank a relatively flat summit and two large 11 and 31 km calderas though covered in places by high dielectric material the tepev mons summit area is relatively smooth with the exception of annular rough deposits which ring the two calderas these calderas are quite shallow on the order of a few hundred meters at most with no discernible difference in radius between individual altimeter footprints inside and outside the larger eastern feature magellan and venera data suggest that the eastern caldera is covered by a layer of unconsolidated material ejecta from miriam crater has produced a halo with a dielectric constant of 6 7 surrounding nefertiti corona and potanina crater ejecta is the likely source material for radar dark wind streaks south of tepev mons a low dielectric e 2 3 triangular region on the south flank of tepev mons may be a pyroclastic or crater ejecta deposit which has been spread westward by the wind this region has undergone a wide variety of volcanic processes with a major shift in eruptive style from the older low relief eastern volcanic center to the steep slopes and large summit calderas characteristic of tepev mons and the two smaller edifices

reconstruction of snow firn thermal diffusivities from observed temperature variation application to iceberg c16 ross sea antarctica 2004 07 two inverse methods are proposed as a means of estimating the thermal diffusivity of snow and firn from continuous measurements of their temperature the first method is applicable to shallow depths where temperature experiences diurnal variations and is based on the fact that phase and amplitude of these diurnal variations are functions of the thermal diffusivity the second method is applicable to the deeper part of the firn layer and is based on a simple least squares estimation technique the methods applied here differ from various methods used for borehole paleothermometry in that observations are continuous in time and performance constraints on model data misfit can be applied over a finite temporal period both methods are tested on temperature records from thermistor strings operating in the upper 2 5m of firn on iceberg c16 ross sea antarctica from 2004 to 2007 results of the analysis show promise in identifying melting events and the movement and refreezing of meltwater within the snow firn layer

pyroclastic flow deposits on venus as indicators of renewed magmatic activity radar bright deposits on venus that have diffuse margins suggest eruptions that distribute debris over large areas due to ground hugging flows from plume collapse we examine deposits in eastern eistla western eistla phoebe and dione regiones using magellan data and earth based radar maps the radar bright units have no marginal lobes or other features consistent with viscous flow their morphology radar echo strength polarization properties and microwave emissivity are consistent with mantling deposits comprised of few cm or larger clasts this debris traveled downhill up to 100 km on modest slopes and blanketed lava flows and tectonic features to depths of tens of cm to a few meters over areas up to 40x103 km2 there is evidence for ongoing removal and exhumation of previously buried terrain a newly identified occurrence is associated with a ridge belt south of ushas mons we also note radar bright streaks of coarse material west of rona chasma that reflect the last traces of a deposit mobilized by winds from the formation of mirabeau crater if the radar bright units originate by collapse of eruption columns with coarse fragmental material entrained and fluidized by hot gases then their extent suggests large erupted volatile co2 or h2o amounts we propose that these deposits reflect the early stage of renewed magmatic activity with volatile rich disrupted magma escaping through vents in fractured regions of the upper crust rapidly eroding under venus surface conditions or buried by subsequent eruptions these markers of recently renewed activity have disappeared from older regions

rapid techniques for determining annual accumulation applied at summit greenland we have determined accumulation histories by identifying annual layer horizons in records obtained by three independent methods 1 glaciochemical analysis on a core 2 density profiling in the borehole from which the core was taken using the neutron probe np technique and 3 borehole optical stratigraphy bos again in the same borehole we also used three different techniques for determining density to convert annual layer thickness to accumulation 1 gravimetric measurements on core samples 2 measurement of density using np and 3 a simple empirical model based on regional climatology the result is nine different accumulation time series three of which are completely independent the chemical analysis and np derived accumulation time series are correlated and the 70year means are in agreement the bos derived accumulation 70 year mean is slightly lower probably due to a combination of the empirical density model s underestimate of the density profile and the misidentification of sub annual events in the shallow part of the borehole as annual horizons

deglacial and postglacial paleoseismological archives in mass movement deposits of lakes of south central quebec investigation of seismic activity in eastern canada is important for natural hazard management as two major active seismic zones with many historical records are located in the region the western

geologic evolution of the akna montes atropos tessera region venus the investigated area comprises an arcuate mountain belt akna montes in western ishtar terra associated with an outboard plateau atropos tessera to the west and a volcanic plateau lakshmi planum to the east eight geologic units have been recognized on the basis of their geomorphic and structural characteristics as they appear on magellan radar images our stratigraphic analysis shows that the geological evolution of the study area can be explained by four main steps 1 formation of the older substrata of atropos tessera and lakshmi 2 extensive plains emplacement 3 an orogenic phase including the formation of akna montes and 4 local emplacement of younger plains the tectonic evolution shows a deformational sequence characterized by contraction shear and topographic relaxation this sequence is interpreted to be a consequence of the variation of crustal stresses and crustal thickening during orogenic events as observed for terrestrial high plateaus associated with a mountain belt i e himalaya and tibet andes and altiplano in order to estimate the amount of crustal shortening associated with the akna montes we considered two end members for structural style of the mountain belt a symmetric fold model and a fault bend fold model the models are theoretical because terrestrial orogenic belts are often formed by a combination of different compressional structures however symmetric and fault bend faults represent the minimum and maximum crustal shortening respectively and thus they do place bounds on the amount of strain recorded by akna montes the first model yields a shortening value less than 1 whereas a range of 17 34 is derived for the second model the large difference between these values underscores the importance of fold geometries for estimating strain and to place constraints on geodynamic models for mountain belt formation on the basis of our study we think that a combination of mantle downwelling and horizontal convergence may provide a good explanation of the geology and tectonics we observed in the akna montes atropos tessera region

firn model intercomparison experiment firnmice evolution of cold dry snow and firn plays important roles in glaciology however the physical formulation of a densification law is still an active research topic we forced eight firn densification models and one seasonal snow model in six different experiments by imposing step changes in temperature and accumulation rate boundary conditions all of the boundary conditions were chosen to simulate firn densification in cold dry environments while the intended application of the participating models varies they are describing the same physical system and should in principle yield the same solutions the firn models all produce plausible depth density profiles but the model outputs in both steady state and transient modes differ for quantities that are of interest in ice core and altimetry research these differences demonstrate that firn densification models are incorrectly or incompletely representing physical processes we quantitatively characterize the differences among the results from the various models for example we find depth integrated porosity is unlikely to be inferred with confidence from a firn model to better than 2 m in steady state at a specific site with known accumulation rate and temperature firn model intercomparison experiment can provide a benchmark of results for future models provide a basis to quantify model uncertainties and guide future directions of firn densification modeling

snow crystal growth with varying surface temperatures and radiation penetration the temperature field is derived for a sinusoidally varying surface temperature with varying solar radiation penetration the growth rates of snow crystals are calculated to explain the rapidly growing layers of faceted crystals i e depth hoar that form just below the surface at high altitudes and in polar snow the solutions also show that a layer of wet snow can exist just below the surface even on days when the surface temperature remains sub freezing

earthquake source parameters and state of stress for the northeastern united states and southeastern canada from analysis of regional seismograms we examine 16 moderate size earthquakes that have occurred since 1990 in the northeastern united states and southeastern canada source parameters such as focal mechanism focal depth and seismic moment are determined using a regional waveform inversion technique in which the best fitting double couple mechanism is obtained through a grid search over strike dip and rake angles most of the 16 events and 10 previous ones with well determined focal mechanisms show horizontal compression with near horizontal p axes striking northeasterly which is consistent with the maximum horizontal stress orientation for midplate north america four shocks along the st lawrence river however have p axes trending southeast 100 130 which is not compatible with the regional stress field and requires a local stress perturbation close to the st lawrence river we examine several generating mechanisms for this localized stress variation including that a rift pillow structure may exist in the lower crust below the st lawrence river all of the 11 shocks in the united states occurred at shallow depths from 2 to 8 km the 15 canadian events are systematically deeper with foci between 5 and 28 km this geographic variation in focal depth is well correlated with the difference in surface heat flow values between the older grenville and younger appalachian provinces in the study area we also examine apparent discrepancies between m w and m b lg magnitude for 17 events five earthquakes of very similar m w show variations in m b lg as large as 0 6 which implies that for seismic events with similar seismic moment in the study area high frequency seismic radiation behavior differs considerably manuscript received 23 october 2002

late quaternary morphostratigraphy of lake st joseph southeastern canadian shield evolution from a semi enclosed glacimarine basin to a postglacial lake abstract this paper reports and describes multibeam echosounder subbottom profiler and sediment sampling data collected in lake st joseph southern quebec a formerly glacimarine sedimentary basin that has been glacio isostatically uplifted to form a small lake its study provides a unique opportunity for investigating changes in sedimentary environments from a glacial sea to a postglacial lake multibeam bathymetry allows the observation of two basins separated by a bathymetric ridge and of mass movement deposits in both basins high resolution seismic stratigraphy reveals the presence of seven seismic units these seven units relate to the advance and retreat of the laurentide ice sheet over the region following ice retreat transparent u3 and high amplitude reflection u4 units were deposited during the champlain sea invasion the transparent unit likely represents a rapidly retreating glacier which allowed the deposition of massive silt and clay whereas the high amplitude reflectors which are internally composed of transparent to chaotic lenses are interpreted as the result of a deceleration of ice retreat or a stabilization of the glacier and of glacio isostatic rebound overlying the champlain sea sediments a unit composed of smaller laminations u5 suggests that it was deposited during the marine to lacustrine transition in a paraglacial environment this is supported by its presence in the northern basin where the riviere aux pins mouth is located and its absence in the southern basin where no river is observed the bathymetric ridge between the two basins thus limited sediment transport following the complete retreat of the champlain sea organic rich sediments u7 were deposited in the lake in the northern basin the presence of the riviere aux pins allowed their deposition in the form of rhythmites whereas the absence of rivers in the southern basin resulted in the deposition of an acoustically transparent unit a mass movement u6 identified during the deposition of unit 7 occurred around 1250 ad and is interpreted as earthquake triggered

solar heating rates and temperature profiles in antarctic snow and ice observations of temperature maxima at about 10 cm depth in cold antarctic snow during summer have previously been explained by proposing that solar heating is distributed with depth whereas thermal infrared cooling is localized at the surface the solid state greenhouse an increase in temperature from the surface to 10 cm depth t 4 k found by rusin 1961 on the antarctic plateau was successfully reproduced by schlatter 1972 in a combined radiative transfer and heat transfer model

seismicity along st lawrence paleorift faults overprinted by a meteorite impact structure in charlevoix quebec eastern canada abstract the charlevoix seismic zone csz in quebec has the highest number of earthquakes in eastern canada with cataloged events diffusely distributed between the st lawrence paleorift faults in this study we relocate 1242 out of 1639 earthquakes in the csz from 1988 to 2010 using double difference relocations with phase arrivals from manual picks and waveform cross correlations and requiring a strict set of quality control criteria our results show a diffuse distribution of earthquakes beneath the st lawrence river within a meteorite impact structure and clear lineations of event hypocenters outside the inner boundary of the impact structure the diffuse seismicity distribution coupled with a lower cumulative seismic moment within the impact structure despite a higher number of events suggests the presence of highly fractured crust and weakened pre existing faults a lack of m n 4 0 earthquakes and a wide variation of focal mechanism p axis orientations also provide evidence for weakened pre existing faults relocated earthquakes beneath the north shore of the st lawrence river illuminate a broad southeast trending linear structure suggesting the geometry of the gouffre northwest river fault a paucity of seismicity between the north shore and the river may correspond to fault controlled basins of unconsolidated sediments at the surface and volumes of high seismic velocity material at midcrustal depths online material earthquake catalogs

geomorphology and volcanology of maat mons venus abstract full resolution fmidr magellan radar backscatter images have been used to characterize the geology and volcanology of the volcano maat mons on venus this volcano has often been identified by remote sensing techniques as one of the volcanoes on the planet that could have been recently active and is the highest volcano on venus with a relief of 9 km the summit of maat mons is characterized by a caldera complex 26 30 km in diameter with at least six remnant pit craters 10 km in diameter preserved in the walls of the caldera suggesting that multiple small volume 1 km in diameter on the flanks these pits appear to have formed by collapse with no effusive activity associated with their formation no evidence for explosive volcanism can be identified despite the relatively low atmospheric pressure 55 bar near the summit there is also a lack of evidence for lava channels deformation features within the caldera and thrust faults on the flanks indicating that the physical volcanology of maat mons is simpler than that of typical martian and terrestrial shield volcanoes preservation of fine scale 3 4 pixels structures within the pit craters and summit pits is consistent with geologically very recent activity but no evidence for current activity can be identified

some seismological and geometric features of intraplate earthquakes to understand the nature and cause of intraplate earthquakes we carried out a systematic search for common features associated with them various geological geophysical and seismic data were analyzed and geometric and seismic parameters were compared for 29 intraplate events m 4 5 occurring worldwide the results suggest that intraplate earthquakes occur by the reactivation of pre existing zones of weakness in a compressional stress regime which is generally oriented parallel to the absolute direction of plate motion two styles of faulting were observed for type a earthquakes observed in the eastern united states eastern china western europe and west africa deformation occurs by strike slip motion on steeply dipping faults type b events are usually associated with thrust or normal faulting and were observed in eastern canada fennoscandia australia and peninsular india focal depths did not explain the differences between type a and type b earthquakes rather the differences are due to the perturbation of a regional stress field by a local stress field the regional stress field for type a events is primarily due to ridge push forces the method of stress perturbation varied from region to region the earthquakes in the type b category were further divided into three subcategories type b1 occurring in eastern canada and fennoscandia type b2 occurring in australia peninsular india and central brazil and type b3 occurring in the gulf coast of the u s the inferred causes of the perturbing stresses are deglaciation intracontinental resistive forces resulting from the collision of plates and the deposition of a large thickness of sediments respectively type a events were found to be associated with intersecting faults and generally supported the intersection model of talwani 1988 and had analogs in plate boundary events

stress induced by topography and crustal density heterogeneities implication for the seismicity of southeastern canada abstract the stress induced by the topography and density heterogeneities in the lithosphere has been calculated for a large region covering most of southeastern canada the calculations assume that the lithosphere consists of a three dimensional layered elastic slab overlaying an inviscid fluid the calculations show that the deviatoric stress difference induced by density heterogeneities and the topographic load is of the same order as the regional tectonic stress tens of mpa stresses appear largest along the saint lawrence valley where most of the seismicity is located e g the charlevoix and the lower saint lawrence areas the charlevoix seismic zone was analyzed in greater detail and theoretical focal mechanisms were computed this analysis shows that the stress induced by surface and internal loading is compressional in this area and the principal stress orientation is consistent with earthquake focal mechanisms and in situ measurements

weighted euler deconvolution of gravity data euler deconvolution is used for rapid interpretation of magnetic and gravity data it is particularly good at delineating contacts and rapid depth estimation the quality of the depth estimation depends mostly on the choice of the proper structural index and adequate sampling of the data the structural index is a function of the geometry of the causative bodies for gravity surveys station distribution is in general irregular and the gravity field is aliased this results in erroneous depth estimates by weighting the euler equations by an error function proportional to station accuracies and the interstation distance it is possible to reject solutions resulting from aliasing of the field and less accurate measurements the technique is demonstrated on bouguer anomaly data from the charlevoix region in eastern canada

timing and controls on the delivery of coarse sediment to deltas and submarine fans on a formerly glaciated coast and shelf the evolution of deltas and submarine fans is often envisioned as largely controlled by relative sea level variations however in some cases relative sea level can have less effect on delta and submarine fan activity than sediment supply and shelf geomorphology in order to document the relative importance of these three factors on deltaic and submarine fan evolution in a formerly glaciated environment this paper documents the delivery of coarse sediment to the laurentian channel eastern canada the well constrained stratigraphic and geomorphologic framework of both the glacio isostatically uplifted deltas and the modern laurentian channel fans allows us to document and contrast the evolution of river fed deltas river fed canyon fan systems and longshore drift fed fans during deglacial and postglacial times the evolution of these different types of fans can be divided into three phases the first phase was characterized by delta progradation on the shelf while relative sea level was at its maximum although already falling and the ice margin gradually retreated inland the second phase was characterized by the delivery of deltaic sediment in the deep realm of the laurentian channel permitted by the supply of large amounts of glaciogenic sediments derived from the retreating ice margin and the lowering of the relative sea level at the same time sediment instability along the steep laurentian channel formed small incisions that evolved into submarine canyons where the narrow shelf allowed the trapping of longshore sediment the third phase was characterized by the withdrawal of the ice margin from the watershed of the main rivers and the drastic decrease in sediment supply to the deltas consequently the delta fronts experienced strong coastal erosion even though relative sea level was still lowering in some cases and the eroded sediments were transferred onto the shelf and to adjacent bays this transfer of coastal sediments allowed the continued activity of longshore drift fed canyons the retreat of the ice margin from the watersheds thus controlled the supply of sediment and induced a change in delta type passing from river dominated to wave dominated deltas this paper highlights the role of the type of sediment supply ice contact glaciofluvial and longshore drift in the timing and activity of submarine fans in high latitude environments a conceptual model is proposed for high latitude shelves where sediment delivery to submarine fans is mostly controlled by structural inheritance watershed area and shelf geomorphology rather than relative sea level fluctuations therefore although relative sea level fell during delta progradation this study demonstrates that it was not the main contributor to delta and submarine fan growth this has wider implications for the extraction of sea level information from stratigraphic successions

a pyroclastic flow deposit on venus explosive volcanism on venus is severely inhibited by its high atmospheric pressure and lack of water this paper shows that a deposit located near 16 s 144 e here referred to as scathach fluctus displays a number of morphological characteristics consistent with a pyroclastic flow deposit these characteristics particularly its lack of channelisation and evidence for momentum rather than cooling limited flow length contrast with fissure fed lava flow deposits the total erupted volume is estimated to have been between 225 km3 and 875 km3 but this may have been emplaced in more than one event interaction between scathach fluctus and a small volcanic cone constrain the flow velocity to 48 m s 1 and plausible volatile concentrations to at least 1 8 wt h2o 4 3 wt co2 or 6 1 wt so2 the latter two implying magma sourced directly from the mantle the deposit has radar characteristics particularly an exponential backscatter function that are similar to those of nearly half the planetary surface implying that pyroclastic deposits may be much more common on venus than has been recognised to date and suggesting both a relatively volatile rich mantle and a volcanic source for atmospheric so2

physically based modeling of atmosphere to snow to firn transfer of h2o2 at south pole quantitative interpretation of ice core chemical records requires a detailed understanding of the transfer processes that relate atmospheric concentrations to those in the snow firn and ice a unique 2 year set of year round surface snow samples at south pole and snow pits with associated accumulation histories were used to test a physically based model for atmosphere to firn transfer of h2o2 the model which extends our previous transfer modeling at south pole into the snowpack is based on the advection dispersion equation and spherical diffusion within representative snow grains required physical characteristics of the snowpack such as snow temperature and ventilation were estimated independently using established physical models the surface snow samples and related model simulations show that there is a repeatable annual cycle in h2o2 in the surface snow at south pole it peaks in early spring and surface snow concentration is primarily determined by atmospheric concentration and temperature with some dependence on grain size the snow pits and associated model simulations point out the importance of accumulation timing and annual accumulation rate in understanding the deposition and preservation of h2o2 and 18o at south pole long term snowpack simulations suggest that the firn continues to lose h2o2 to the atmosphere for at least 10 12 years 3 m after burial at current south pole temperatures and accumulation rates

low radar emissivity signatures on venus volcanoes and coronae new insights on relative composition and age abstract multiple studies reveal that most of venus highlands exhibit anomalously high radar reflectivity and low radar emissivity relative to the lowlands this phenomenon is thought to be the result of atmosphere surface interactions in the highlands due to lower temperatures these reactions are a function of rock composition atmospheric composition and degree of weathering we examine the magellan radar emissivity altimetry and sar data for all major volcanoes and coronae on venus we characterize and classify edifices according to the pattern of the variation of radar emissivity with altitude the volcanic highlands can be classified into 7 distinct patterns of emissivity that correspond to at least 3 discrete types of mineralogy based on the altitude temperature of the emissivity anomalies the majority of emissivity anomalies support the hypothesis of a weathering phenomenon at high altitude 6053 km but we also find strong emissivity anomalies at lower altitudes that correspond spatially to individual lava flows indicating variations in mineralogy within an evolving volcanic system the emissivity signature of tallest volcanoes on venus are consistent with the presence of ferroelectric minerals in their rocks while volcanic edifices in western ishtar terra and eastern aphrodite terra are consistent with the presence of semiconductor minerals sapas mons and pavlova corona are also consistent with ferroelectrics but at a different curie temperature than the other volcanoes in atla regio the spatial distribution of radar emissivity classes correlates to different geologic settings indicating that different mantle source regions deep shallow plumes and possible convergence zones may contribute to differences in mineralogy for the studied edifices finally we show that the emissivity signatures of idunn maat and other volcanic edifices are consistent with relatively fresh and unweathered rocks indicating recent or possibly current volcanism on venus

source characterization and fault plane determination for mblg 1 2 to 4 4 earthquakes in the charlevoix seismic zone quebec canada two earthquake doublets and two multiplets recorded by the charlevoix telemetered network cltn in the charlevoix seismic zone csz of southern quebec canada have been analyzed using an empirical green s function egf method to derive the relative source time functions rstf s of seven master events with mblg 1 2 to 4 4 we identified the doublets and multiplets using a waveform cross correlation and relative event location technique to verify that each earthquake pair had similar focal mechanisms and hypocentral locations three component s waveforms recorded by the high dynamic range 126 db instrumentation of the cltn were used to extract the rstf s the rstf s reveal that six of the seven events are simple with single source pulses having durations of 0 05 to 0 2 sec another earthquake 920310 0545 m 3 3 appears to be a double event with two episodes of rupturing azimuthal variations of the rstf pulse amplitudes and widths provide strong evidence for the rupture directivities of five of the earthquakes m 1 2 to 4 4 the azimuthal variations in the rstf pulse amplitudes were used to estimate the rupture directions and rupture velocities lower bound estimates of the rupture velocity range from 0 5 to 0 7 vs estimates of the rupture direction were combined with p wave focal mechanisms for the four largest events m 3 3 to 4 4 to identify the fault plane for these earthquakes source parameters were measured for the rstf s of the master events including seismic moments of 3 5 1018 to 5 3 1021 dyne cm fault radii of 100 to 330 m and static stress drops of 2 to 90 bars the fault radii and stress drop estimates for m 3 events agree well with estimates obtained by other researchers for m 3 to 4 5 earthquakes in the csz we also observed apparent scaling between the stress drop and the earthquake size which has been reported in other studies of stress drop in northeastern north america

ice sheet elevation changes caused by variations of the firn compaction rate induced by satellite observed temperature variations 1982 2003 changes in the surface elevation of the greenland and antarctic ice sheets and ice shelves caused by variations in the rate of firn compaction are calculated with a time dependent firn densification model driven by two decades 1982 2003 of satellite observed monthly surface temperatures the model includes the effects of melting and refreezing both the direct changes in density and the subsequent effects on the densification rate as previously shown the temperature dependent rate of densification is largest in summer but changes in winter temperatures also have a significant effect over the last decade climate warming has enhanced the rate of compaction and lowered the average surface elevation of greenland by 1 8 cma 1 and most of west antarctica by 1 9 cma 1 in east antarctica a small cooling raised the average surface elevation by 0 14 cma 1

stress channelling and partitioning of seismicity in the charlevoix seismic zone quebec canada summary the charlevoix seismic zone csz in the st lawrence valley of quebec is historically the most active in eastern canada the structurally complex region comprises rift faults formed during the opening of the iapetus ocean superimposed by a 350 ma meteorite impact structure resulting in a circular highly fractured zone although seismicity is localized along two steeply dipping planar rift parallel zones previous work indicates that most of the large scale rift faults bound the low magnitude background seismicity rather than generate earthquakes themselves in order to gain insight into the mechanics of the partitioning of this seismicity a 2 d model of the csz was built using the stress analysis code flac the rift faults are represented by frictional discontinuities the heavily fractured impact structure is represented by an elastic continuum of reduced modulus boundary displacements are used to generate a regional stress field with the major horizontal component in the direction of tectonic loading given a high strength the rift faults have little effect on the stress patterns stress trajectories naturally flow around the crater of reduced elastic modulus leaving the fractured area with lower stresses than the background level however when the rift faults have low strength they are unable to support stress trajectories inclined to them due to the resolved shear stress exceeding their strength this prevents trajectories from flowing out of the rift effectively channelling higher magnitude stresses into the region of the impact structure between the faults low strength bounding faults can thus explain the localization of seismicity into linear bands rather than distributed seismicity throughout the impact structure it also explains how the rift faults act as boundaries to regions of low magnitude seismicity these results indicate that the interplay between faults of varying strength and zones of differing elastic modulus can give rise to complicated stress patterns and can explain many of the seismicity patterns observed in the csz this has implications for other intraplate seismic zones as it shows an example of how regional weak faults can modify stress conditions around local structures and drive seismicity the results are particularly relevant for other regions located within rifted crust such as the new madrid seismic zone which possibly display evidence of stress channelling

thermal effects due to air flow and vapor transport in dry snow the thermal effects of air flow forced through a snow sample are investigated numerically a new method for calculating vapor transport in snow is presented which allows for the determination of the effects of sublimation in this method the snow is not assumed to be saturated with water vapor results of the model show very good agreement with analytical and experimental results

the thermal conductivity of seasonal snow twenty seven studies on the thermal conductivity of snow k eff have been published since 1886 combined they comprise 354 values of k eff and have been used to derive over 13 regression equation and predicting k eff vs density due to large and largely undocumented differences in measurement methods and accuracy sample temperature and snow type it is not possible to know what part of the variability in this data set is the result of snow microstructure we present a new data set containing 488 measurements for which the temperature type and measurement accuracy are known a quadratic equation where is in g cm 3 and k eff is in w m 1 k 1 can be fit to the new data r 2 0 79 a logarithmic expression can also be used the first regression is better when estimating values beyond the limits of the data the second when estimating values for low density snow within the data set snow types resulting from kinetic growth show density independent behavior rounded grain and wind blown snow show strong density dependence the new data set has a higher mean value of density but a lower mean value of thermal conductivity than the old set this shift is attributed to differences in snow types and sample temperatures in the sets using both data sets we show that there are well defined limits to the geometric configurations that natural seasonal snow can take

natural convection in the subarctic snow cover the purpose of this study was to determine if air convects in a natural snow cover to detect convection the temperature field in the subarctic snow cover in fairbanks alaska was measured hourly during three winters 1984 1987 using an array of thermistors which were suspended on threads and allowed to be buried by snowfall the results indicate that convection occurred sporadically in 1984 1985 and almost continuously in 1985 1986 and 1986 1987 the evidence was 1 simultaneous warming and cooling at different locations in a horizontal plane in the snow and 2 horizontal temperature gradients of up to 16 c m 1 during the winter warm and cold zones developed in the snow and remained relatively fixed in space we interpret these zones to be the result of a diffuse plumelike convection pattern linked to spatial variations in the temperature of the snow soil interface air flow was inferred to have been primarily horizontal near the base of the snow and vertical elsewhere calculated flow speeds were of the order of 0 2 mm s 1 with a maximum value of 2 mm s 1 the convective circulation was time dependent with perturbations such as high wind or rapid changes in air temperature triggering periods when horizontal temperature gradients were strongest suggesting that these were also periods when the air flow was fastest the coincidence of depth hoar crystals with horizontal c axes and the horizontal flow lines at the base of the snow suggests that convection may have affected crystal growth directions

impact crater related surficial deposits on venus multipolarization radar observations with arecibo 1 the distribution of surficial deposits in the vicinity of impact craters on venus was studied using measurements of the polarization properties of the reflected radar wave subsurface scattering of an incident circularly polarized radar signal results in a linearly polarized component in the radar echo due to the differing transmission coefficients at a smooth at wavelength scales atmosphere surface boundary for the horizontal h and vertical v linearly polarized components of the incident wave arecibo 12 6 cm wavelength radar observations in 1999 and 2001 provided images of the surface of venus in the full stokes polarization parameters of the reflected echo from which images in the degree of linear polarization were derived these images show that substantial areas of sedna guinevere and lavinia planitias return a radar echo with a significant degree of linear polarization indicating that mantling deposits may be relatively widespread on the plains of venus the areas showing linear polarization enhancements are strongly correlated with topographic features primarily impact craters dome fields and windblown deposits including dune fields a strong linearly polarized echo component 10 40 linear polarization is found from regions near 45 impact craters including 5 parabolas these linear polarization features typically correspond to diffuse higher backscatter cross section bright regions in magellan images the linearly polarized component in these regions is attributed to subsurface echoes from a mantled substrate or from buried rocks

microstructural change around a needle probe to measure thermal conductivity of snow the thermal conductivity of snow determines to a large extent ground heat flux and snow metamorphism one common method of measuring the thermal conductivity of snow is with a needle probe we measured the microstructural changes in the snow around a typical needle using micro computed tomography the insertion of the needle probe caused structural changes up to a radial distance of 0 5 1 mm using a commercial needle probe with short 30 s heating time we measured thermal conductivity values in snow that were 50 lower than the conductivity measured using a calibrated guarded hot plate apparatus numerically simulated time temperature curves with an introduced air gap that simulated the needle damage reproduced the observed needle probe results well confirming that poor contact resistance between the needle and the snow will lead to measurement discrepancies and a large bias while most measurements in snow have been done using a long heating time and are not subject to this error the thermal conductivity of snow measured with needle probes with short measurement times should be avoided this same effect is probably observed in other brittle and highly porous materials a condition that may not be widely appreciated

thermal conductivity of snow firn and porous ice from 3 d image based computations estimating thermal conductivity of snow firn and porous ice is key for modeling the thermal regime of alpine and polar glaciers whereas thermal conductivity of snow was widely investigated studies on firn and porous ice are very scarce this study presents the effective thermal conductivity tensor computed from 64 3 d images of microstructures of snow antarctic firn and porous ice at 3 20 and 60 c we show that in contrast with snow conductivity of firn and porous ice correlates linearly with density is approximately isotropic and is largely impacted by temperature we report that performances of commonly used estimates of thermal conductivity vary largely with density in particular formulas designed for snow lead to significant underestimations when applied to denser ice structures we present a new formulation to accurately estimate the thermal conductivity throughout the whole density range from fresh snow to bubbly ice and for any temperature conditions encountered in glaciers

a re examination of the 1925 charlevoix quebec earthquake the 1 march 1925 charlevoix earthquake is one of the largest earthquakes known to have occurred in southeastern canada because it occurred in one of the most active seismic zones of southeastern canada and near now heavily populated regions understanding this earthquake may have implications for seismic hazard assessments of the region previous attempts have been made to study this earthquake using modern waveform analysis techniques but the source parameters could be only partially constrained due to a lack of good data recently a number of seismograms that had not been previously studied were rediscovered warranting a second look at the earthquake the best overall solution is strike 42 7 dip 53 7 rake 105 10 depth 10 km seismic moment 3 1 2 5 1025 dyne cm mw 6 2 ms 6 2 0 3 mb 6 5 0 4 source duration 5 sec and stress drop 35 bars the focal mechanism is consistent with many recent earthquakes in the charlevoix seismic zone but inconsistent with the regional stress field

surface processes in the venus highlands results from analysis of magellan and arecibo data the highlands of venus are characterized by an altitude dependent change in radar backscatter and microwave emissivity likely produced by surface atmosphere weathering reactions we analyzed magellan and arecibo data for these regions to study the roughness of the surface lower radar backscatter areas at the highest elevations and possible causes for areas of anomalous behavior in maxwell montes arecibo data show that circular and linear radar polarization ratios rise with decreasing emissivity and increasing fresnel reflectivity supporting the hypothesis that surface scattering dominates the return from the highlands the maximum values of these polarization ratios are consistent with a significant component of multiple bounce scattering we calibrated the arecibo backscatter values using areas of overlap with magellan coverage and found that the echo at high incidence angles up to 70 from the highlands is lower than expected for a predominantly diffuse scattering regime this behavior may be due to geometric effects in multiple scattering from surface rocks but further modeling is required areas of lower radar backscatter above an upper critical elevation are found to be generally consistent across the equatorial highlands with the shift in microwave properties occurring over as little as 500 m of elevation these surfaces are not simply characterized however by the absence of a highly reflective component surface morphology and radar scattering properties suggest that a mantling deposit forms at the highest elevations most likely by in situ erosion of the original rock in ovda regio this process mantles or has removed surface festoon structure at the 1 to 10 m scale implying a significant depth for the weathered layer similar radar dark areas occur in maxwell montes but are apparently unrelated to the current topography of the region possible reasons for these observations include mass wasting from areas of steep slopes compositional or age differences within the montes vertical tectonic shifts of relict contacts local topographic effects on surface temperature or errors in the magellan topography data in the rugged terrain while there is evidence for some of these effects in the existing data no single model at present appears to satisfy all occurrences of high altitude radar dark terrain new measurements of the surface and lower atmosphere chemistry of venus are needed to further refine these conclusions

late quaternary glacial to postglacial sedimentation in three adjacent fjord lakes of the quebec north shore eastern canadian shield abstract high resolution swath bathymetry imagery allowed mapping in great detail the sublacustrine geomorphology of lakes pentecote walker and pasteur three deep adjacent fjord lakes of the quebec north shore eastern canada these sedimentary basins have been glacio isostatically uplifted to form deep steep sided elongated lakes their key geographical position and limnogeological characteristics typical of fjords suggest exceptional potential for long term high resolution paleoenvironmental reconstitutions acoustic subbottom profiles acquired using a bi frequency chirp echosounder 3 5 12 khz together with cm and m long sediment core data reveal the presence of four acoustic stratigraphic units the acoustic basement unit 1 represents the structural bedrock and or the ice contact sediments of the laurentide ice sheet and reveals v shaped bedrock valleys at the bottom of the lakes occupied by ice loaded sediments in a basin fill geometry unit 2 moraines observed at the bottom of lakes and in their structural valleys indicate a deglaciation punctuated by short term ice margin stabilizations following ice retreat and their isolation the fjord lakes were filled by a thick draping sequence of rhythmically laminated silts and clays unit 3 deposited during glaciomarine and or glaciolacustrine settings these sediments were episodically disturbed by mass movements during deglaciation due to glacial isostatic rebound ams 14c dating reveal that the transition between deglaciation of the lakes pentecote and walker watersheds and the development of para and post glacial conditions occurred around 8000 cal bp the development of the lake head river delta plain during the holocene provided a constant source of fluvial sediment supply to the lakes and the formation of turbidity current bedforms on the sublacustrine delta slopes the upper sediment succession i e 4 6 5 m consists of a continuous para to post glacial sediment drape unit 4 that contains laminated and massive sediment and series of rapidly deposited layers these results allow establishing a conceptual model of how a glaciated coastal fjord evolves during and after deglaciation in a context of rapid glacio isostatically induced forced regression

radar dark impact crater related parabolas on venus characterization of deposits with magellan emissivity data 1 we have analyzed the radar dark parabola deposits associated with impact craters on the surface of venus using the results of the magellan radio experiments in both passive and active modes the extent of mantle deposits is found to be much wider than the dark features observed in synthetic aperture radar sar images using the emissivity data we have classified features coinciding with the radar dark parabolas into four types related to both differences in ejecta deposition styles for craters of different diameters and stages in the degradation of crater related deposits a model for the microwave properties of the mantled surfaces is proposed comparison between the distribution of mantle thickness predicted by the model and the previously published independent results based on modeling of the cratering process show good agreement when the loss tangent of the mantle material is assumed to be within the 0 001 0 005 range this value is similar to that of dry powders of terrestrial and lunar rocks on the basis of this work we conclude that radar dark diffuse features due to these mantles can be up to several meters thick if the loss tangent is assumed to be 0 003 thin deposits as little as 7 cm thick are found to be responsible for the deposits seen in the emissivity parabolas but not observed in the sar images these results provide a basis for the interpretation of the character and thickness of mantle deposits their states of degradation processes associated with their formation and degradation and guides to their use in establishing stratigraphic relationships and ages of underlying geologic units on venus the results will also be useful in interpreting the nature of surface deposits at previous landing sites and predicting the nature of deposits likely to be encountered at future landing sites

interannual variations of shallow firn temperature at greenland summit firn temperature profiles are calculated in a thermal model using continuous surface temperatures derived from automatic weather station aws data and passive microwave data in the greenland summit region during the period 1987 1999 the results show that significant interannual variations of mean summer june to august and annual temperatures occur in the top 15 m in addition to the normal seasonal cycle of firn temperature at 5 m depth the seasonal cycle is damped to 13 of the surface seasonal amplitude but even at 15 m about 1 or 0 6 c of the seasonal cycle persists both summer and mean annual temperatures decrease from 1987 to 1992 followed by a general increasing trend interannual variability is 5 c at the surface but only is only dampened to 3 2 c at 10 m depth and 0 7 c at 15 m depth dampening of the interannual variability with depth is slower than dampening of the seasonal cycle because of the longer time constant of the interannual variation the warmer spring and summer temperatures experienced in the top 5 m due to both the seasonal cycle and interannual variations affect the rate of firn densification which is non linearly dependent on temperature during the 12 year period 1987 1999 the mean annual surface temperature is 29 2 c and the mean annual 15 m temperature is 30 1 c which is more than 1 c warmer than a 15 m borehole temperature representing the period of about 1959 and warmer than the best fit temperature history by alley and koci back to 1500 a d

explosive volcanism on venus transient volcanic explosions as a mechanism for localized pyroclast dispersal it is proposed that transient volcanic explosions of the vulcanian type may provide a mechanism for the generation and dispersal of pyroclastic material on venus the influence of the venusian high atmospheric pressure environment implies that continuous discharge plinian eruptive activity is relatively uncommon the tendency for suppression of exsolution and expansion of magmatic gases favors effusive eruptions however it may be possible for explosive activity to occur in a fashion analogous to vulcanian eruptions on earth as a result of the accumulation of hot pressurized gas under a coherent rock lid the explosion may be initiated by the failure of this retaining caprock causing the catastrophic release of the high pressure gas which expands out of the vent driving the fragmented caprock material ahead of it and displacing the surrounding atmosphere on earth the driving gas may originate either from vaporization of groundwater or from degassing of a stalled magma body in the near surface crust whereas on venus where the presence of crustal stores of volatile compounds is uncertain the latter option only is favored prolonged degassing may lead to an accumulation of gas sufficient to initiate an explosion this paper presents the results of a numerical model describing the explosion process under boundary conditions representing the venusian physical environment this involves treatments of the acceleration of the driving gas caprock and displaced atmospheric gas out of the vent and the subsequent motions and aerodynamic interactions between the atmosphere and the ejected blocks of fragmented caprock in this way predictions of the eruption velocities and of the resulting distribution of large solid ejecta can be obtained for likely conditions on venus deposits of large blocky debris are predicted to range up to a maximum distance of the order of 1 km from the vent on venus compared with distances of several kilometers commonly attained by ejecta from transient explosions on earth more typical blocky deposits may extend for only a few hundred meters which implies that they would not be detected in the magellan radar data however the possible presence of associated pyroclastic flow and fine grained ashfall deposits may constitute aids to the identification of sites of vulcanian eruptions on venus

transport of so2 by explosive volcanism on venus observations by the pioneer venus orbiter and many different types of analyses have suggested the possibility of contemporary explosive volcanism on venus the rise of volcanic eruption plumes on venus is reexamined using recent improvements in buoyant plume modeling the first order model applied to venus by previous authors features nonphysical discontinuous solutions for all of the model parameters and lacks internal consistency in the formulation of the governing equations this makes it difficult to assess the validity of the venus applications and conclusions derived from these models the model used here contains several improvements including two corrections to the formulation and a change in the criterion for the transition of the plume from the jet region to the buoyancy driven region the model used in earlier works assumed a discontinuous transition between these two regions resulting in an overestimate of the transition height as well as the maximum plume height the effect of the transition criterion is magnified on venus where the continuous solution appears to have very little dependence on initial vent size in contrast the discontinuous solution shows a very strong dependence on initial vent size the continuous solution used here indicates that plumes on venus become dominated by buoyancy effects almost immediately above the vent use of the discontinuous solution however suggests that jets up to 10 km above the vent are possible for the boundary conditions considered the combined effect of using the older model for conditions on venus is a 5 8 overestimate of the maximum plume height for vent radii ranging from 20 to 250 m the influence of latitude and elevation are also explored for large eruptions on venus plumes rising in the northern highlands would rise much higher than plumes with identical boundary conditions erupted in the equatorial lowlands this is due to the greater stability of the upper atmosphere at higher latitudes and the sharp decrease in atmospheric pressure as a function of altitude to examine the net effect of all the model assumptions and ambient influences eruptions are simulated for a range of conditions at maat mons and compared with results in the literature these simulations indicate that for small mass fluxes the new model predicts smaller plumes than the older model for larger mass fluxes however the new model predicts larger plumes than the older model because the maat mons summit elevation is already more than 9 km above the mean planetary radius the reduced atmospheric pressure results in a plume with enough buoyancy to more than compensate for all of the model effects these results continue to support the possibility that explosive eruptions on venus may be capable of producing plumes that rise buoyantly to heights detected by the pioneer venus orbiter

coldspots and hotspots global tectonics and mantle dynamics of venus magellan radar images and altimetry data and pioneer venus gravity data of major highlands and lowlands on venus are examined with the objective of relating these tectonic and volcanic landforms to convection within venus mantle two roughly circular lowlands atalanta and lavinia planitiae are bowl shaped depressions which contain compressional tectonic features and lack hotspot related features such as coronae and shield volcanoes they are identified as surface expressions of young mantle coldspots or regions of mantle downwelling highlands can be divided into two distinct groups volcanic rises and plateau shaped highlands volcanic rises are domical circular to elongate regions characterized by volcanic construction extensional tectonism large positive gravity and geoid anomalies centered on the highlands and large apparent depths of compensation volcanic rises include beta bell atla and western eistla regiones and are identified as surface expressions of large mantle hotspots or plumes plateau shaped highlands are steep sided elevated regions that range from circular alpha regio to elongate or irregular ovda regio tellus regio in shape their surfaces are dominated by complex ridged terrain rather than the shield volcanoes and flows that dominate volcanic rises most plateau shaped highlands contain compressional structures some of which commonly lie along their margins and have small gravity and geoid anomalies and small apparent depths of compensation compared to volcanic rises plateau shaped highlands which include those mentioned above and western ishtar terra thetis regio and phoebe regio are identified as surface expressions of mantle coldspots or regions of downwelling the highland coldspot features are elevated as a consequence of crustal thickening crustal thickening is absent or minor at lowland coldspots the importance of mantle downwelling in the tectonic deformation of venus suggests that its mantle like the mantle of earth is strongly heated from within the major venusian hotspots attest to the existence of large mantle plumes carrying heat upward from the planet s core

volcanic deposits in shield fields and highland regions on venus surface properties from radar polarimetry 1 we compare arecibo dual polarization radar image data for venus to magellan images and emissivity data to investigate the physical properties of volcanic deposits radar waves can easily penetrate smooth mantling layers such as ash aeolian and crater derived deposits if a circularly polarized radar wave refracts into a surface that is smooth at wavelength scales the vertical component of the wave will be preferentially transmitted resulting in a net linear polarized echo component arecibo polarimetry data were used to create maps of the degree of linear polarization in the radar echo we find that some volcanic fields in plains regions on venus are associated with enhanced linear polarization these fields sometimes have nearby windstreaks which suggest fine grained surface material and we infer that the radar wave is penetrating into mantling deposits that are a few centimeters to 1 m thick enhanced linear polarization values are also correlated with specific lava flows these lava flows have emissivity values of 0 80 to 0 84 similar to many other flows on venus the enhanced linear polarization may be produced by penetration of the radar wave into very smooth lava flows with internal air gaps high reflectivity low emissivity areas near the summits of theia and tepev montes also have a linearly polarized echo component consistent with surface penetration by the radar wave the cause of the high linear polarization in summit regions remains uncertain but perhaps the radar wave is able to penetrate into high dielectric material in limited cases of very smooth surface texture