quartz recrystallization regimes c axis texture transitions and fluid inclusion reequilibration in a prograde greenschist to amphibolite facies mylonite zone ribeira shear zone se brazil abstract the crystal plastic behavior of quartz mylonites from the ribeira shear zone se brazil a major strike slip structure that was active during a prograde metamorphic phase related to the neoproterozoic brasiliano pan african orogeny was investigated using a multi method approach geothermobarometry results indicate deformational conditions ranging from 300 to 630 c and 500 700 mpa a strong correlation between mapped metamorphic zones and a dominance of different dynamic recrystallization mechanisms of quartz occurs within the mylonite zone bulging recrystallization blg dominates within the chlorite zone between 300 and 410 c subgrain rotation recrystallization sgr operates within the biotite zone from 410 to 520 c and grain boundary migration recrystallization gbm dominates in the garnet zone above 520 c the development of quartz c axis textures is mainly governed by temperature and dynamic recrystallization mechanisms textures from blg zone mylonites are characterized by maxima around z sgr zone mylonites display single girdles or asymmetric type i crossed girdles and gbm zone mylonites comprise maxima around y and intermediate between x and z the scarcity or absence of water bearing fluid inclusions in quartz mylonites from the sgr and gbm zones which are dominated by carbonic inclusions suggests water deficient conditions whereas blg zone mylonites are dominated by water bearing inclusions this evidence indicates that water was available in the protoliths but has been eliminated with increasing deformation and deformation temperature no effect of the water content variation on the quartz microstructural and recrystallized grain size evolution was detected and little influence on c axis texture development was observed most of the fluid inclusion densities were reequilibrated during the shear zone exhumation history recording a decompression in the range of 300 500 mpa while microstructural reequilibration effects related to the prograde metamorphism are largely preserved fluid inclusion microstructures and densities from two sgr zone samples preserved evidence for a near isothermal compression within the interior of the ribeira shear zone during the prograde metamorphism

petrofacies of eocene sand injectites of the tumey giant injection complex california usa abstract the forearc succession of great valley group in central california provides some of the best examples of giant sand injection complexes in the world and is therefore considered valuable analogues for injectite systems in the subsurface several sand injection complexes are well described in the outcrop and subsurface however the petrographic characteristics of injectites are still poorly documented in this paper we present the results of an integrated study of field observations quantitative and qualitative sandstone petrography provenance and petrofacies analysis of the tumey giant injection complex tgic in order to understand its lithostratigraphy and petrological evolution and its impacts on reservoir petrofacies characteristics and fluid migration the tgic intrudes into a 450 m thick deep water succession of slope mudrocks and sandy channel fills of the kreyenhagen shale eocene forming an interconnected network of sandstone sills dykes and injection breccias the complex generated a horizontal and vertical plumbing system for fluid migration connecting isolated sandy channel fills among low permeability mudrocks the primary detrital composition diagenetic products microtextures and provenance signatures allowed for the definition and discrimination of depositional and intrusive petrofacies and their genetic relations petrofacies associations confirm that the gypsum cemented feldspathic litharenites from the kreyenhagen shale channel fills are the only source for the injection complex eodiagenetic compaction and extensive gypsum cementation reduced the primary porosity of the complex while telodiagenetic dissolution of autigenic constituents formed pervasive secondary porosity the underlying calcite cemented arkosic sandstones of the lodo and domengine formations acted as barriers for fluid flow aiding lateral fluid migration and overpressure buildup within the overlying kreyenhagen channel fills intense grain microfracturing occurred during sand remobilization and injection along with erosion of the host mudrocks this study has significance for the generation of reliable stratigraphic and petrological models for sand injection complex genesis and evolution that consequently can help the understanding and exploration of injectite complexes elsewhere

shock effects in quartz compression versus shear deformation an example from the rochechouart impact structure france abstract different quartz microstructures from rochechouart impact breccias indicative of shock induced compression and shear deformation are compiled to obtain information on the mean stress and the deviatoric components of the shock wave associated stress annealed rhombohedral planar deformation features pdfs are widespread in quartz from rochechouart impact breccias as analysed by optical microscopy scanning electron and transmission electron microscopy shocked quartz can show mosaicism with domains of quartz microstructures in the rochechouart impact breccias that record shock induced shear deformation are mechanical brazil twins and planar fractures that are supposed to represent microfaults these shock effects indicate high differential stress on the order of a few gpa quartz that contains brazil twins and planar fractures shows no or few rhombohedral pdfs indicating relatively low shock pressures 20 gpa are apparently not accompanied by high differential stresses only at attenuated shock pressure

shock effects and pre shock microstructures in hydrothermal quartz veins from the rochechouart impact structure france abstract microfabrics in hydrothermal quartz veins in gneisses of the massif central france from the rochechouart impact structure and st paul la roche 40 km to the se of the centre of the structure have been investigated in the quartz veins from the rochechouart impact structure planar deformation features in the basal plane and cataclastic zones indicate low shock pressures

a tem investigation of shock metamorphism in quartz from the vredefort dome south africa the origin of the vredefort structure in south africa is still debated several causes have been discussed namely asteroid impact internal gas explosion or tectonic processes evidence of dynamic rock deformation is pervasive in the form of planar features in quartz grains shatter cones veins of pseudotachylite and occurrence of coesite and stishovite high pressure quartz polymorphs a number of these characteristics is widely believed to support an impact origin however the planar features in quartz which are generally considered as one of the strongest indicators of impact are in the vredefort case considered as anomalous when compared with those from accepted impact structures we have investigated by optical and transmission electron microscopy tem the defect microstructures in quartz grains from different lithologies sampled at various places at the vredefort structure whatever the locality only thin mechanical brazil twin lamellae in the basal plane are observed by tem so far such defects have only been found in quartz from impact sites but always associated with sets of thin glass lamellae in rhombohedral planes 10 1n with n 1 2 3 and 4 at the scale of the optical microscope brazil twins in 0001 are easily detected in vredefort quartz grains because of the numerous tiny fluid inclusions which decorate them similar alignments of tiny fluid inclusions parallel to other planes are also detected optically but at the tem scale no specific shock defects are detected along their traces if these inclusion alignments initially were shock features they are now so severely weathered that they can no longer be recognized as unambiguous shock lamellae fine grained coesite was detected in the vicinity of narrow pseudotachylite veinlets in a quartzite specimen but stishovite was not found even in areas where its occurrence was previously reported finally definite evidence of high temperature annealing was observed in all the samples these observations lead us to the conclusion that our findings regarding microdeformation in quartz are consistent with an impact origin for the vredefort structure most of the original shock defects are now overprinted by an intense post shock annealing episode only the thin mechanical twin lamellae in the basal plane have survived

preferred orientation distribution of shock induced planar microstructures in quartz and feldspar shocked quartz and feldspar grains commonly exhibit planar microstructures such as planar fractures planar deformation features and possibly microtwins which are considered to have formed by shock metamorphism their orientation and frequency are typically reported to be randomly distributed across a sample the goal of this study is to investigate whether such microstructures are completely random within a given sample or whether their orientation might also retain information on the direction of the local shock wave propagation for this work we selected samples of shatter cones which were cut normal to the striated surface and the striation direction from three impact structures keurusselka finland and charlevoix and manicouagan canada these samples show different stages of pre impact tectonic deformation additionally we investigated several shocked granite samples selected at different depths along the drill core recovered during the joint iodp icdp chicxulub expedition 364 mexico in this case thin sections were cut along two orthogonal directions one parallel and one normal to the drill core axis all the results refer to optical microscopy and universal stage analyses performed on petrographic thin sections our results show that such shock related microstructures do have a preferred orientation but also that relating their orientation with the possible shock wave propagation is quite challenging and potentially impossible this is largely due to the lack of dedicated experiments to provide a key to interpret the observed preferred orientation and to the lack of information on postimpact orientation modifications especially in the case of the drill core samples

magnetic fabric of araguainha complex impact structure central brazil implications for deformation mechanisms and central uplift formation article i nfo the weakening mechanisms involved in the collapse of complex impact craters are controversial the ara guainha impact crater in brazil exposes a complex structure of 40 km in diameter and is an excellent object to address this issue its core is dominated by granite in addition to microstructural observations magnetic studies reveal its internal fabric acquired during the collapse phase all granite samples exhibit impact related planar deformation features pdfs and planar fractures pfs which were overprinted by cataclasis cataclastic deformation has evolved from incipient brittle fracturing to the development of discrete shear bands in the center of the structure fracture planes are systematically decorated by tiny grains b10 m of magnetite and hematite and the orientation of magnetic lineation and magnetic foliation obtained by the anisotropies of magnetic susceptibility ams and anhysteretic remanence aar are perfectly coaxial in all studied sites therefore we could track the orientation of deformation features which are decorated by iron oxides using the ams and aar the magnetic fabrics show a regular pattern at the borders of the cen tral peak with orientations consistent with the fabric of sediments at the crater s inner collar and complex in the center of the structure both the cataclastic flow revealed from microstructural observations and the structural pattern of the magnetic anisotropy match the predictions from numerical models of complex im pact structures the widespread occurrence of cataclasis in the central peak and its orientations revealed by magnetic studies indicate that acoustic fluidization likely operates at all scales including the mineral scales the cataclastic flow made possible by acoustic fluidization results in an apparent plastic deformation at the macroscopic scale in the core

shock metamorphic microfeatures in chert from the jebel waqf as suwwan impact structure jordan the petrographic investigation of a shocked chalcedony quartzine and quartz bearing allochthonous chert nodule probably upper cretaceous recovered from surficial wadi gravels in the inner parts of the central uplift of the approximately 6 km in diameter jebel waqf as suwwan impact structure jordan reveals new potential shock indicators in microfibrous spherulitic silica in addition to well established shock metamorphic effects in coarser crystalline quartz the microcrystalline chert groundmass exhibits a macroscopic dendritic and suborthogonal fracture pattern commonly associated with thin recrystallization bands that intersect the pre existing diagenetic chert fabric fibrous aggregates of quartzine spherulites in chalcedony quartzine quartz veinlets locally have a shattered appearance and show conspicuous curved fractures perpendicular to the quartzine fiber direction and parallel to 0001 that commonly trend subparallel to planar fractures pfs in neighboring shocked quartz quartz exhibits pfs feather features ffs and mainly single sets of planar deformation features pdfs parallel to the basal plane 0001 brazil twins and rarely additional pdfs parallel to 1013 shock petrography indicates shock pressures of 10 gpa and high shock induced differential stresses that affected the chert nodule the internal crosscutting relationships of primary diagenetic and impact related deformational features together with shockpressure estimates suggest that the curved fractures across quartzine spherulites might represent specific low to medium pressure shock metamorphic features possibly in structural analogy to basal plane pfs in quartz the dendritic suborthogonal fractures in the microcrystalline chert groundmass and recrystallization bands are likely related to impact induced shear deformation and recrystallization respectively and cannot be considered as definite shock indicators

an assessment of techniques for the deflocculation and removal of clays from sediments used in phytolith analysis this paper describes an experiment in which a comparison is made between two techniques of deflocculation and clay removal generally used for palynological extraction in the experiment methods used commonly for pollen extraction are applied to phytolith extraction the palynological products of the different preparation techniques are compared by reference to several attributes the time and efficiency in the preparation method the weight of residue resulting from each method the palynological composition clarity of the palynomorphs on the slides and measures of differential loss and selection by palynomorph shape and size the results are compared and discussed in relation to the still settling technique which is most commonly used for phytolith extraction it is concluded that centrifugation is a reliable and fast method that can be used to remove clays from fine grained sediments for the extraction of phytoliths and other silica microfossils high levels of diversity were recorded for morphotypes across all size classes it produced superior results to the sieving technique and there is no reason to assume that it cannot be used to produce results comparable with still settling if calculations of settling times consider the implications of sediment density on fluid viscosity adoption of the centrifugation method for clay removal from palynological sediment samples can greatly reduce extraction times and therefore it is a recommended procedure

interactions between human activity volcanic eruptions and vegetation during the holocene at garua and numundo west new britain png this paper reviews recent fossil phytolith analysis from wet tropical west new britain papua new guinea the holocene vegetation has been influenced by spatially and temporally diverse patterns of both prehistoric human settlement and catastrophic volcanic events we have hypothesized different landscape responses and recovery pathways to events during the last six millennia phytolith sequences on the coastal lowlands the willaumez peninsula and nearby island of garua provide details of vegetational change and human interactions at different landscape scales since c 5900 cal yr b p during this period four major volcanic eruptions c 5900 3600 1700 and 1400 cal yr b p have disrupted the landscape the evidence provides detailed descriptions of temporal and spatial patterning in the impacts and changes in the vegetation in particular vegetation responded differently from one event to another reflecting both forest recovery from seed bank and shooting and the influence of prehistoric people on recovering vegetation furthermore after some events landscape recovery was moderately uniform while after others there was considerable landscape partitioning although these differences largely relate to airfall tephra type distribution and magnitude the partitioning is more strongly influenced by human activity

feather features a low shock pressure indicator in quartz 1 feather features are a recently discovered type of microstructure in shocked quartz they occur as short parallel to subparallel lamellae with spacing similar to that of planar deformation features pdfs these lamellae are always found in combination with a planar fracture pf feather features are crystallographically controlled 0001 101 and 112 orientations are common while 103 and 102 orientations are lacking samples from the nordlinger ries and the matt wilson impact structures are compared with plane wave shock recovery experiments 16 gpa microscopic analysis of quartz grains with feather features show that their formation is linked to shearing along the associated pfs during shock deformation the generation of shear fractures with feather features in plane wave shock recovery experiments suggests a pressure range of 7 10 gpa although further constraints are needed for an upper and lower pressure limit the orientation of feather features in rock samples is suggested to be controlled by the direction of the principal axis of stress with sheared pfs at 45 angles to the axis and most feather feature lamellae aligned parallel to the axis 20 the majority of feather feature lamellae emanate from the pfs in the same direction and in experiments point in the direction from which the shock wave came on the basis of their appearance in shocked quartz grains from 26 impact craters and the current lack of reports of these features in endogenically deformed crustal rocks their uniqueness as a new type of impact induced planar microstructure alongside pdfs and pfs is proposed

mechanical twinning in quartz shock experiments impact pseudotachylites and fault breccias abstract increasing use of diffraction methods to study preferred orientation of minerals has established that quartz in deformed rocks not only displays characteristic c axis orientation patterns but that there is also generally a distinct difference in the orientation of positive and negative rhombs in the trigonal quartz crystal structure positive and negative rhombs are structurally different and particularly negative rhombs e g 01 1 1 are much stiffer than positive rhombs e g 10 1 1 here we focus on the role of mechanical dauphine twinning under stress as a cause of this difference and illustrate with ebsd measurements ubiquitous twinning in quartz bearing rocks subjected to high stresses characteristic twinning is observed in experimentally shocked sandstones and stishovite bearing quartzites from the vredefort meteorite impact site in south africa similar twinning is documented for quartz associated with pseudotachylites from the santa rosa mylonite zone in southern california whereas quartz in underlying ductile mylonites are more or less twin free it suggests that twinning was produced by local seismic stresses that caused fracture and frictional melting on fault surfaces quartz bearing breccias from the safod san andreas fault observatory at depth drilling project also show evidence of twinning and suggest high seismic stresses in the currently creeping segment of the san andreas fault at parkfield from these observations it appears that dauphine twin microstructures can be diagnostic of high local and transient stresses

pseudotachylyte controversy fact or friction high speed slip experiments performed on westerly granite using friction welding apparatus reveal that comminution is an essential precursor to melting by friction observations of slip surfaces via analytical scanning electron microscopy sem document the following sequence of events occurring in 2 s with increasing velocity up to 2 m s fracture progressive comminution surface melting of mineral fragments fragment to fragment adhesion and finally production of a fragment laden melt supported suspension explosive dehydration and melting of the epidote group mineral allanite indicates that temperatures of at least 1000 c were realized at the interface this is corroborated by calculation of the temperature rise for the known operating conditions contrary to earlier proposals these results show that comminution and frictional melting are complementary and not mutually exclusive processes depending on the velocity shear stress displacement relations prevailing during frictional slip rocks produced in seismogenic zones can be predominantly comminuted wall rock or fragment melt mixes pseudotachylytes

a late triassic age for the rochechouart impact structure france argon 40 argon 39 laser spot fusion dating of pseudotachylyte from the 25 km diameter roche chouart impact structure of western central france yields a matrix age of 214 c 8 ma 20 field evidence indicates that the pseudotachylyte was generated during the modification stage of the impact process prob ably during transient cavity collapse this new age is considerably older than the previously accepted age of 186 8 ma for this structure which was obtained from hydrothermally altered melt sheet samples the new age is in accordance with earlier paleomagnetic and fission track data which indicated that rochechouart was formed during the late triassic moreover the new determination is in agreement with the regional geolog ical setting and field relations of the structure the new age of 214 2 8 ma falls within the norian stage of the triassic system

nuclear blast induced nanotextures in quartz and zircon within trinitite the intense heat and pressure resulting from the detonation of the world s first nuclear device in the new mexico desert july 16 1945 severely altered the arkosic sand producing the fused glassy material referred to as trinitite the study of trinitite is key to the development of nuclear forensic techniques that can provide crucial information about a nuclear event such as device composition and radionuclide distribution moreover nuclear blasts are often considered analogs to catastrophic natural events such as meteorite impacts and it is well documented that with increasing impact severity zircon and quartz grains deform systematically in trinitite a sufficient number of primary quartz and zircon grains remain identifiable here a multi technique approach focused ion beam scanning electron microscopy transmission electron microscopy and micro raman spectroscopy is employed to study the micrometer to nanometer scale deformation features in altered grains of zircon and quartz to constrain blast pressure and temperature conditions trinitite zircon grains consistently show an outer halo of fibrous baddeleyite radiating from a relatively unaltered core hrtem images show complex twinning likely originating from an intermediate tetragonal zirconia precursor trinitite quartz grains show various states of melting that appear to vary predictably with depth below the surface of the desert sand grains occurring deeper than 1 5 cm are crystalline with occasional planar fractures at the optical scale at shallower depths a systematic increase in quartz vitrification is observed considered together these data suggest maximal temperatures in excess of 1500 c and pressures of 10 gpa the latter being considerably less than for any natural impact event taken in a broader context the implications of this work extend toward exploiting the use of advanced imaging techniques to improve our understanding of mineral processes in extreme non equilibrium environments at the near atomic scale

the pseudotachylite from champagnac in the rochechouart meteorite crater france as part of a comprehensive study on pseudotachylites from established impact structures cryptoexplosion structures and diverse tectonic settings a vein system containing melt rock and hydrothermal crystallizations from the champagnac quarry in the basement of the rochechouart impact structure northwest haut limousin france was characterised by petrographic and geochemical means the lack of characteristic shock phenomena in melt clasts deformation is restricted to kink bands in mica and local planar fracturing in quartz and feldspar the angular shapes of clasts and the structural field relations lead to the conclusion that the melt fraction of these vein fillings from the champagnac quarry represents pseudotachylite produced by friction during lateral shear movements these veins differ from the impact breccia dyke fillings al breccias reported by lambert 1981 bulk chemical analysis of vein fillings and defocussed beam electron probe results obtained on glassy pseudotachylite favor in situ formation of melt and rule out the possibility that it represents injected impact melt compositional variations are observed in this vein system and are interpreted to result from varied contributions of mafic and felsic components mica plus opaque minerals versus quartz and feldspar glass analyses indicate that total melting as well as partial melting of parent rock occurred according to petrographic evidence the temperature at melt formation is estimated at 700 750 c and the pressure is estimated to have been less than 1 gpa the hydrothermal component of vein fillings was generated simultaneously with the pseudotachylite melt and produced distinctive petrographic and geochemical effects within the vein system 1 sericitization in clasts and adjacent host rock which altered andesitic feldspar to pure albite and 2 selective removal of sr from vein fillings a rb sr isotope study on vein fillings yielded consistently low sr and 87sr contents but highly varied rb sr ratios and an apparently well defined isotopic equilibration among seven vein filling samples an apparent 217 m y isochron results it is shown that the rb sr data cannot be explained by mixing processes and are not in agreement with normal radiogenic evolution of the sr isotope system isotopic equilibration by diffusion seems improbable therefore the following genetic process for the champagnac pseudotachylite vein system is suggested 1 clastic breccias were formed by impact induced lateral slip along preexisting planes of weakness locally these breccias were transformed into pseudotachylite by friction melting 2 circulation of hydrothermal fluids 700 c high ph2o led to sericitization of plagioclase of the breccia clasts and immediate host rocks and modified the sr isotopic system of pseudotachylite 3 during the subsequent cooling period hydrothermal phases crystallized and the various textures of pseudotachylite matrix and clasts developed before 4 long term tectonic activity crosscutting fractures and additional hydrothermal modifications took place

planar deformation features in shocked quartz a transmission electron microscopy investigation abstract we present a detailed investigation by transmission electron microscopy tem of the fine structure of planar deformation features pdf in quartz grains within which shock metamorphism has been detected optically and which originate from various sites slate islands la malbaie and manicouagan lake canada ries crater germany the toba caldera sumatra the vredefort complex in south africa and the k t boundary at raton basin colorado pdfs appear on tem micrographs as straight and very narrow tem reveals many more pdfs than are optically detected probably because only concentrations of such defects in narrow areas can induce an optically detectable contrast caution should therefore be exercised when applying criteria based on the optical observation of single or multiple sets of pdfs pdfs in samples from slate islands and raton basin exhibit a peculiar morphology a high density of very tiny bubbles precipitated on the dislocation lines comparison with the behavior of wet quartz strongly suggests that these materials experienced lengthy annealing at moderate pressure and temperature

microraman spectroscopy of anomalous planar microstructures in quartz from mt oikeyama discovery of a probable impact crater in japan abstract planar deformation features pdfs and planar fractures pfs have been found and confirmed by optical microscope observations and microraman spectroscopy in quartz grains from mt oikeyama akaishi mountains central japan for which the semicircular topographic feature of the ridge suggests a crater formed by an impact event according to the optical microscope observations a low shock pressure 8 10 gpa is estimated by the occurrence of basal or pdfs leading to lack of multiple sets of pdfs in addition a new type of planar microstructure was found in several quartz grains the microraman characteristics of pdfs in quartz from mt oikeyama show the amorphous state indicating the presence of weak broad bands at 400 and 800 cm 1 peak positions these characteristics are indicative of pdfs that are limited to shocked quartz this indicates an impact origin for distinct planar microstructures in quartz from mt oikeyama

rapid phytolith extraction for analysis of phytolith concentrations and assemblages during an excavation an application at tell es safi gath israel abstract a rapid phytolith extraction procedure is described that allows phytolith concentrations and morphotype assemblages to be analyzed within hours this procedure enables the results of these analyses to be used during an archaeological excavation in order to better understand how plants were used the new procedure was tested using a standard phytolith extract and two experimental phytolith sediment blends and found to be both accurate and precise the reliability of partial slide counting was evaluated and found to be as accurate and precise as existing phytolith quantification methods the new extraction and counting procedures were applied to an archaeological site tell es safi gath israel the results demonstrate how information on phytolith concentrations in sediments that are available from one day to the next can be used during an excavation to more effectively document the local features of interest and obtain better information

shock experiments on pre heated α and β quartz i optical and density data abstract discs of single crystal quartz unheated and pre heated to 275 c and 540 c i e quartz and 630 c i e quartz were experimentally shocked to pressures ranging from 20 to 40 gpa with the shock front propagating parallel to either 10 1 0 or 0001 refractive indices density and the orientation of planar deformation features pdfs were determined on the recovered quartz samples refractive indices of pre heated quartz are unaffected up to 25 gpa but density starts to decrease slightly up to this pressure above 25 gpa pre heating causes drastic variations refractive indices and birefringence of quartz shocked at ambient temperature decrease continuously until complete isotropization is reached at 35 gpa in quartz shocked at 630 c refractivity drops discontinuously in the interval from 25 to 26 gpa and complete transformation to diaplectic glass is reached at 26 gpa density follows the trends demonstrated by the optical parameters with higher pre shock temperatures yielding lower density at a given shock pressure these results indicate that the threshold pressure for the onset of transformation to diaplectic quartz glass is largely temperature invariant lying at 25 gpa whereas the pressure limit for complete transformation decreases with increasing pre shock temperature from 35 to 26 gpa quartz shocked parallel to 0001 always has a higher density and refractivity than that shocked parallel to 10 1 0 indicating a significant influence of the structural anisotropy this is also evident from the distribution of pdf orientations pressures 25 gpa cause in quartz shocked parallel to 10 1 0 pdfs that are predominantly oriented parallel to 10 1 2 while quartz shocked to the same pressures but parallel to 0001 contains almost exclusively pdfs parallel to 10 1 3 pdf orientations in quartz shocked at ambient temperature parallel to 10 1 0 show the following characteristics i the frequency of 10 1 3 is high at 20 gpa declines with increasing pressure and finally is totally absent at 32 gpa ii 10 1 3 begins to occur at 25 gpa increases and finally is the only remaining orientation at 32 gpa and iii 10 1 2 and 11 2 2 are only present at 20 and 25 gpa in contrast quartz shocked at 630 c parallel to 10 1 0 displays a broad pdf distribution with indistinct maxima and 10 1 3 is totally absent the small structural difference between and quartz which is reflected in the pdf distribution should allow in principle evaluation of the pre shock temperature in naturally impacted metamorphic rocks the results substantiate that formation of shock effects in quartz is dependent on i shock pressure ii pre shock temperature and iii shock wave direction so far the latter two parameters have not been considered in the application of experimental results to impact sites

shock metamorphism of quartz in nature and experiment ii significance in geoscience the occurrence of shock metamorphosed quartz is the most common petrographic criterion for the identification of terrestrial impact structures and lithologies its utility is due to its almost ubiquitous occurrence in terrestrial rocks its overall stability and the fact that a variety of shock metamorphic effects occurring over a range of shock pressures have been well documented these shock effects have been generally duplicated in shock recovery experiments and thus serve as shock pressure barometers after reviewing the general character of shock effects in quartz the differences between experimental and natural shock events and their potential effects on the shock metamorphism of quartz are explored the short pulse lengths in experiments may account for the difficulty in synthesizing the high pressure polymorphs coesite and stishovite compared to natural occurrences in addition post shock thermal effects are possible in natural events which can affect shock altered physical properties such as refractive index and cause annealing of shock damage and recrystallization the orientations of planar microstructures however are unaffected by post impact thermal events except if quartz is recrystallized and provide the best natural shock barometer in terms of utility and occurrence the nature of planar microstructures particularly planar deformation features pdfs is discussed in some detail and a scheme of variations in orientations with shock pressure is provided the effect of post impact events on pdfs is generally limited to annealing of the original glass lamellae to produce decorated pdfs resulting from the exsolution of dissolved water during recrystallization basal 0001 pdfs differ from other pdf orientations in that they are multiple mechanical brazil twins which are difficult to detect if not partially annealed and decorated the occurrence and significance of shock metamorphosed quartz and its other phases namely coesite stishovite diaplectic glass and lechatelierite are discussed for terrestrial impact structures in both crystalline non porous and sedimentary porous targets the bulk of past studies have dealt with crystalline targets where variations in recorded shock pressure in quartz have been used to constrain aspects of the cratering process and to estimate crater dimensions at eroded structures in sedimentary targets the effect of pore space results in an inhomogeneous distribution in recorded shock pressure and temperature which requires a different classification scheme for the variation of recorded shock compared to that in crystalline targets this is discussed along with examples of variations in the relative abundances of planar microstructures and their orientations which are attributed to textural variations in sedimentary target rocks examples of the shock metamorphism of quartz in distal ejecta such as at the k t boundary and from nuclear explosions are illustrated and are equivalent to that of known impact structures except with respect to characteristics that are due to long term post shock thermal effects finally the differences between the deformation and phase transformation of quartz by shock and by endogenic tectonic and volcanic processes are discussed we confirm previous conclusions that they are completely dissimilar in character due to the vastly different physical conditions and time scales typical for shock events compared to tectonic and volcanic events well characterized and documented shock effects in quartz are unequivocal indicators of impact in the natural environment

rochechouart impact crater melt breccias record no geomagnetic field reversal abstract impact melt breccias from the rochechouart france meteorite crater possess paleomagnetic directions with both normal and reverse polarities raising the question whether shock from the collision initiated a geomagnetic field reversal stepwise thermal demagnetization together with a suite of rock magnetic experiments optical microscopy raman spectroscopy and electron microprobe analyses identify adjacent multiphase titanohematite in the samples containing normal and mixed polarities typical of lithologies bearing self reversal behavior stemming from magnetic exchange interaction melt breccias possessing mostly titanium free hematite as the magnetic remanence carrier have solely reverse polarity directions leading to the conclusion that the rochechouart meteorite impact did not spawn a geomagnetic event samples displaying paleomagnetic directions with normal or mixed polarity yield unreliable paleointensity data 30 samples with reverse polarity that pass stringent data selection criteria from thellier thellier paleointensity experiments with alteration tail and additivity checks define an average field value of 12 8 3 7 t this translates into a virtual dipole moment of 2 7 0 8 10 22 a m 2 which is relatively low but within uncertainty of other mesozoic paleointensity data

a late mesoproterozoic 40ar 39ar age for a melt breccia from the keurusselka impact structure finland field investigations in the eroded central uplift of the 30 km keurusselka impact structure finland revealed a thin dark melt vein that intersects the autochthonous shatter cone bearing target rocks near the homestead of kirkkoranta close to the center of the impact structure the petrographic analysis of quartz in this melt breccia and the wall rock granite indicate weak shock metamorphic overprint not exceeding 8 10 gpa the mode of occurrence and composition of the melt breccia suggest its formation as some kind of pseudotachylitic breccia 40ar 39ar dating of dark and clast poor whole rock chips yielded five concordant late mesoproterozoic miniplateau ages and one plateau age of 1151 10 ma 11 ma 2 mswd 0 11 p 0 98 considered here as the statistically most robust age for the rock the new 40ar 39ar age is incompatible with 1 88 ga svecofennian tectonism and magmatism in south central finland and probably reflects the keurusselka impact followed by impact induced hydrothermal chloritization of the crater basement in keeping with the crosscutting relationships in the outcrop and the possible influence of postimpact alteration the late mesoproterozoic 40ar 39ar age of 1150 ma should be treated as a minimum age for the impact the new 40ar 39ar results are consistent with paleomagnetic results that suggested a similar age for keurusselka which is shown to be one of the oldest impact structures currently known in europe and worldwide

estimating average shock pressures recorded by impactite samples based on universal stage investigations of planar deformation features in quartz sources of error and recommendations planar deformation features pdfs in quartz are the most widely used indicator of shock metamorphism in terrestrial rocks they can also be used for estimating average shock pressures that quartz bearing rocks have been subjected to here we report on a number of observations and problems that we have encountered when performing universal stage measurements and crystallographically indexing of pdf orientations in quartz these include a comparison between manual and automated methods of indexing pdfs an evaluation of the new stereographic projection template and observations regarding the pdf statistics related to the c axis position and rhombohedral plane symmetry we further discuss the implications that our findings have for shock barometry studies our study shows that the currently used stereographic projection template for indexing pdfs in quartz might induce an overestimation of rhombohedral planes with low miller bravais indices we suggest based on a comparison of different shock barometry methods that a unified method of assigning shock pressures to samples based on pdfs in quartz is necessary to allow comparison of data sets this method needs to take into account not only the average number of pdf sets grain but also the number of high miller bravais index planes both of which are important factors according to our study finally we present a suggestion for such a method which is valid for nonporous quartz bearing rock types which consists of assigning quartz grains into types a e based on the pdf orientation pattern and then calculation of a mean shock pressure for each sample less

the nature distribution and genesis of the coesite and stishovite associated with the pseudotachylite of the vredefort dome south africa abstract the vredefort dome represents the uplift centre of a well known 2 00 ga old impact structure of unusually large magnitude shock features like shatter cones planar features and high pressure silica polymorphs are common the present study deals with the description mode of occurrence field distribution and post shock metamorphic alteration of coesite and stishovite which were poorly documented up to now these minerals occur as unusually large crystals in the quartzite of the wall rock in contact with very thin pseudotachylite veins in the pseudotachylite itself which can be interpreted as a friction and or a shock recovery melt fine needles of kyanite are ubiquitous it is proposed that these thin pseudotachylites a type formed during the transit of the shock wave they preceded the development of thick pseudotachylite and microbreccia veins b type which formed during the tensional period which immediately followed from comparison with the model of formation of high pressure polymorphs in porous sandstone it is suggested that higher pressure and stress was concentrated along the a type veins at the arrival of the shock front at this time the quartz was transformed into a high pressure phase which was probably poorly crystalline behind the shock front that is during the rarefaction the pressure was progressively released and the polymorphs crystallized from this initial high pressure phase stishovite crystallized first followed by coesite the high pressure conditions may have lasted an unusually long time due to the magnitude of the vredefort impact this long time probably about one second may account for the large size of the coesite and stishovite crystals most of the high pressure silica polymorphs are corroded to a variable degree by secondary quartz and preserved only in a restricted area of the impact structure this alteration is attributed to post shock metamorphism due to the temperature of the rock before impact plus the heat left after shock recovery some of the evidence put forward by authors against the impact model is discussed according to the facts arising from this investigation it is concluded that an origin by impact remains the simplest and most realistic model to explain the origin of the vredefort dome

shock experiments on pre heated α and β quartz ii x ray and tem investigations quartz single crystals pre heated to different temperatures 20 275 540 and 630 c were experimentally shocked to pressures ranging from 20 to 40 gpa with the shock front propagating parallel to 1010 or 0001 shocked quartz pre heated to 630 c is nearly x ray amorphous at 26 gpa while quartz shocked at room temperature still displays several x ray diffraction lines up to 30 gpa all samples that were still crystalline when shocked above 25 gpa exhibit lattice expansion internal strain and reduced crystallite sizes lattice expansion was up to 1 for the constants a o and co and up to 3 for the cell volume v o quartz shocked parallel to 1010 always shows larger lattice constants and is more strained than quartz shocked parallel to 0001 indicating that shock effects are controlled by the structural anisotropy internal strain is anisotropic with minimum strain in the planes belonging to 1012 the dominant shock defect revealed by tem are sets of parallel lamellae consisting of pure sio 2 glass so called planar deformation features pdfs below 25 gpa pdfs are extremely rare and thin 30 nm at pressures above 25 gpa however pdfs occur frequently and are about ten times thicker this qualitative observation of increasing pdf frequency and thickness with increasing pressure is quantified by calculations of the amount of glass using x ray and density data the results of this study substantiate the crucial role of pdfs for the lattice expansion of quartz and the formation of diaplectic glass and lechatelierite lattice expansion is ascribed to the enormous expansion of pdfs during decompression leading to elastic stretching in adjacent crystalline areas the observation that pdfs occur predominantly parallel to 1012 where minimum strain is present supports this hypothesis the increasing amount of pdfs above 25 gpa is interpreted as the cause for the formation of diaplectic glass shock temperature calculations support the assumption that these pdfs consist under compression of superheated melt the heat is possibly sufficient to melt crystalline areas adjacent to the superheated pdfs at enhanced pressures such molten bands coalesce to a total melt which is quenched upon decompression thus diaplectic glass represents a quenched high pressure melt while lechatelierite sio 2 glass with a fluidal texture is interpreted as quenched low pressure melt

crystallographic preferred orientations may develop in nanocrystalline materials on fault planes due to surface energy interactions a layer of substantially noncrystalline material composed of partially annealed nanopowder with local melt was experimentally generated by comminution during 1 5 mm total slip at 2 5 10 6 m s 1 pconf 0 5 gpa and 450 c or 600 c on saw cut surfaces in novaculite the partially annealed nanopowder comprises angular grains mostly 5 200 nm diameter in a variably dense packing arrangement a sharp transition from wall rock to partially annealed nanopowder illustrates that the nanopowder effectively localizes shear consistent with generation of nanoparticles during initial fragmentation not by progressive grain size reduction dislocation densities in nanopowder grains or immediate wall rock are not significantly high but there are planar plastic defects spaced at 5 200 nm parallel to the host quartz grain s basal plane we propose these plastic defects developed into through going fractures to generate nanocrystals the partially annealed nanopowder has a crystallographic preferred orientation cpo that we hypothesize developed due to surface energy interactions to maximize coincident site lattices csl during annealing this mechanism may also have generated cpos recently described in micro nanocrystalline calcite fault gouges

evidence for coeval late triassic terrestrial impacts from the rochechouart france meteorite crater 1 high temperature impact melt breccias from the rochechouart france meteorite crater record a magnetization component with antipodal normal and reverse polarities the corresponding paleomagnetic pole for this component lies between the 220 ma and 210 ma reference poles on the eurasian apparent polar wander path consistent with the 214 8 ma 40ar 39ar age of the crater late triassic tectonic reconstructions of the eurasian and north american plates place this pole within 95 confidence limits of the paleomagnetic pole from the manicouagan canada meteorite impact crater which is dated at 214 1 ma together these observations reinforce the hypothesis of a late triassic multiple meteorite impact event on earth

measuring effusion rates of obsidian lava flows by means of satellite thermal data abstract space based thermal data are increasingly used for monitoring effusive eruptions especially for calculating lava discharge rates and forecasting hazards related to basaltic lava flows the application of this methodology to silicic more viscous lava bodies such as obsidian lava flows is much less frequent with only few examples documented in the last decades the 2011 2012 eruption of cordon caulle volcano chile produced a voluminous obsidian lava flow 0 6 km3 and offers an exceptional opportunity to analyze the relationship between heat and volumetric flux for such type of viscous lava bodies based on a retrospective analysis of modis infrared data mirova system we found that the energy radiated by the active lava flow is robustly correlated with the erupted lava volume measured independently we found that after a transient time of about 15 days the coefficient of proportionality between radiant and volumetric flux becomes almost steady and stabilizes around a value of 5 106 j m 3 this coefficient i e radiant density is much lower than those found for basalts 1 108 j m 3 and likely reflects the appropriate spreading and cooling properties of the highly insulated viscous flows the effusion rates trend inferred from modis data correlates well with the tremor amplitude and with the plume elevation recorded throughout the eruption thus suggesting a link between the effusive and the coeval explosive activity modelling of the eruptive trend indicates that the cordon caulle eruption occurred in two stages either incompletely draining a single magma reservoir or more probably tapping multiple interconnected magmatic compartments

melt bearing impactites suevite and impact melt rock within the gardnos structure norway abstract melt bearing impactites dominated by suevite and with a minor content of clast rich impact melt rock are found within the central part of the gardnos structure they are preserved as the eroded remnants in the relatively small complex impact structure with a present diameter of 5 km these rocks have been mapped in the field and in the branden drill core and described according to mineralogy petrology including matrix litho clast and melt content as well as geochemistry based on our extensive field mapping a simple 3 d model of the original crater was constructed to estimate tentative volumes for the melt bearing impactites the variations in lithic and melt fragment content and chemistry of suevite matrix can mostly be explained by incorporation of mafic rocks into a dominant mixture of granitic gneissic and quartzitic target rocks reflecting mixing of material from different parts of the crater melt fragments within suevite occur with a variety of shapes and textures probably related to different original target rock composition to the various temperatures the individual fragments were subjected to during the impact event and deposition processes this study discusses the impact related deposits based on a sedimentological approach their overall composition and structures indicate dominating gravity flow processes in the final transportation and deposition of the suevite

thirty years of satellite derived lava discharge rates at etna implications for steady volumetric output 1 we present a 30 year long data set of satellite derived time averaged lava discharge rates tadr for mount etna volcano sicily italy spanning 1980 2010 and comprising 1792 measurements during 23 eruptions we use this to classify eruptions on the basis of magnitude and intensity as well as the shape of the tadr time series which characterizes each effusive event we find that while 1983 1993 was characterized by less frequent but longer duration effusive eruptions at lower tadrs 2000 2010 was characterized by more frequent eruptions of shorter duration and higher tadrs however roughly the same lava volume was erupted during both of these 11 year long periods so that the volumetric output was linear over the entire 30 year period increasing at a rate of 0 8 m3 s 1 between 1980 and 2010 the cumulative volume record can be extended back in time using data available in the literature this allows us to assess etna s output history over 5 centuries and to place the current trend in historical context we find that output has been stable at this rate since 1971 at this time the output rate changed from a low discharge rate phase which had characterized the period 1759 to 1970 to a high discharge rate phase this new phase had the same output rate as the high discharge rate phase that characterized the period 1610 1669 the 1610 1669 phase ended with the most voluminous eruption of historic times

an investigation by transmission electron microscopy of planar deformation features in naturally shocked quartz we have investigated by transmission electron microscopy tem the planar deformation features pdf in quartz from various sites where shock metamorphism has been detected optically impact sites the vredefort complex south africa and the k t boundary at raton basin colorado pdfs are straight narrow bands of glass or microtwins or huge densities of dislocations such contrasting microstructures must reflect different shock scenarios in the vredefort complex the post shock thermal history has strongly overprinted the shock induced defects in the raton basin samples very tiny bubbles have precipitated on the dislocations strongly suggesting a lengthy annealing at moderate pressure and temperature this new information should be taken into account in any model of the origin of the k t transition in any case observation of pdfs by optical microscopy alone is not sufficient to unambiguously conclude on their origin

the gardnos impact structure norway petrology and geochemistry of target rocks and impactites abstract the gardnos structure norway is an approximately circular area of anomalously fractured and brecciated rock about 4 5 km in diameter emplaced in a metamorphic terrane composed chiefly of granitic gneisses with minor amphibolite and quartzite the original recognition of gardnos as a deeply eroded impact structure between 900 and 400 ma old has been followed up by detailed petrographic and chemical studies of approximately thirty samples of target rocks and various types of shocked rocks impactites deep erosion of the structure has erased the original rim removed much of the crater fill deposits and exposed large areas at or near the original crater floor however a wide variety of distinctive impactites fractured and blackened quartzites in the sub crater basement rocks lithic breccias and melt bearing breccias are still preserved these impactites show petrographic and chemical characteristics that confirm an impact origin distinctive planar deformation features pdfs in quartz and feldspar incipient melting of feldspar clasts in the melt bearing breccias close matches between the chemical composition of the breccias and mixtures of the target lithologies and the detection of an extraterrestrial component a minor extraterrestrial component 0 15 was detected in the melt bearing breccias based on significantly elevated ir and os contents and lower 187os 188os ratios compared to those in the target rocks the gardnos impactites are significantly enriched in c 5 10x over the exposed target rocks this may reflect the presence of a c rich shale overlying the metamorphic basement at the time of impact this idea is supported by 13c values of 28 1 to 31 5 measured in the impactites mixing calculations show that the chemical compositions of the impactites can be reproduced by mixtures of target rocks ranging from approximately 60 90 wt granite gneiss 0 30 wt amphibolite 0 12 wt quartzite and 3 19 wt of a c rich shale component the deeply eroded state of the structure and the preliminary state of detailed geologic mapping make crater reconstruction difficult one possible scenario involves the impact of a 300 m diameter stony meteorite that released 1019 j of energy and formed a transient cavity 3 km in diameter that evolved to a complex crater 5 km in diameter with a central uplift of about 350 m the original crater was filled with at least 0 3 km3 of allochthonous melt matrix breccias containing about 0 06 km3 of impact melt the structure underwent low grade greenschist metamorphism in caledonian time about 400 ma ago and was subsequently eroded to its present appearance the value of the gardnos structure for further cratering studies lies in its easy access to large areas of the original crater floor zone in the preservation of a possibly complete sequence of crater fill breccias beneath a cap of elastic sediments and in the unusual carbon enrichment of its impactites

formation mechanisms of planar deformation features in naturally shocked quartz abstract shock waves induce peculiar defects in quartz shock mosaics high pressure polymorphs coesite and stishovite diaplectic glass and planar deformation features pdfs together these features are the indices of shock metamorphism pdfs appear under the optical microscope as straight and narrow defects parallel to the 10 1n rhombohedral planes with n 3 and 2 as most frequent values although n 1 4 and i e basal plane are also found we report in the first part of this paper a detailed investigation by transmission electron microscopy of the fine structure of pdfs in shocked quartz grains from a variety of sites slate islands la malbaie and manson in north america the vredefort complex in south africa the ries crater in germany and the cretaceous tertiary boundary layer at raton basin colorado we distinguish four pdf fine structures 1 bands of dislocations 2 lamellae of mixtures in various proportions of amorphous silica and small crystallites 3 thin brazil twin lamellae 4 short parallel lamellae forming serrated ladder structures the possible formation mechanisms of these pdfs are discussed in the second part of the paper it is suggested that the dislocation bands are probably not original pdf structures but stem from a later recrystallization stage the brazil twins have been produced by relatively large deviatoric stresses which accompanied the shock wave the other pdf structures which consist of partial amorphization in the 10 1 n planes must result from elastic instabilities of the shear modulus of the quartz structure at high pressure in these planes theoretical estimates of the elastic stiffness coefficients indicate that amorphization should start at approximately 10 gpa the growth of the straight and narrow amorphous lamellae would be driven by the front of the shock wave

shock metamorphic features in quartz grains from the saarijarvi and soderfjarden impact structures finland shock metamorphic features at the saarijarvi d 2 km and soderfjarden d 6 5 km structures in finland have so far only been studied tentatively although both are considered to be proven impact structures this work presents the first detailed universal stage study of planar deformation features pdfs feather feature lamellae ffl and planar fractures pfs in quartz grains from a polymict impact breccia dike from soderfjarden and from sedimentary crater fill rocks from saarijarvi planar microstructures particularly pdfs are very rare and poorly developed or preserved in saarijarvi whereas in soderfjarden they are much more common and well defined miller bravais indices of the planar microstructures in both saarijarvi and soderfjarden are indicative of relatively low shock pressure but high shear conditions only compatible with an impact origin for these structures although a proterozoic age for saarijarvi cannot be ruled out the observations of shock features throughout the sedimentary crater fill sequence and a brecciated sedimentary dike below the crater floor are more consistent with a lower cambrian or younger impact age

examining rhyolite lava flow dynamics through photo based 3d reconstructions of the 2011 2012 lava flowfield at cordon caulle chile during the 2011 2012 eruption at cordon caulle chile an extensive rhyolitic flowfield was created in excess of 0 5 km3 in volume affording a unique opportunity to characterise rhyolitic lava advance in 2012 and 2013 we acquired approximately 2500 digital photographs of active flowfronts on the north and east of the flowfield these images were processed into three dimensional point clouds using structure from motion and multi view stereo sfm mvs freeware from which digital elevation models were derived sequential elevation models separated by intervals of three hours six days and one year were used to reconstruct spatial distributions of lava velocity and depth and estimate rheological parameters three dimensional reconstructions of flowfronts indicate that lateral extension of the rubbly a like flowfield was accompanied by vertical inflation which differed both spatially and temporally as a function of the underlying topography and localised supply of lava beneath the cooled upper carapace compressive processes also drove the formation of extensive surface ridges across the flowfield continued evolution of the flowfield resulted in the development of a compound flowfield morphology fed by iterative emplacement of breakout lobes the thermal evolution of flow units was modelled using a one dimensional finite difference method which indicated prolonged residence of magma above its glass transition across the flowfield we compare the estimated apparent viscosity 1 21 4 03 1010 pa s of a breakout lobe based on its advance rate over a known slope with plausible lava viscosities from published non arrhenian temperature viscosity models and accounting for crystallinity 50 vol there is an excellent correspondence between viscosity estimates when the lava temperature is taken to be magmatic despite the breakout being located 3 km from the vent and advancing approximately nine months after vent effusion ceased this indicates the remarkably effective insulation of the lava flow interior providing scope for significant evolution of rhyolitic flow fields long after effusive activity has ceased

microcomputed tomography and shock microdeformation studies on shatter cones one of the aspects of impact cratering that are still not fully understood is the formation of shatter cones and related fracturing phenomena yet shatter cones have been applied as an impact diagnostic criterion for decades without the role of shock waves and target rock defects in their formation having been elucidated ever we have tested the application of the nondestructive microcomputed tomography ct method to visualize the interior of shatter cones in order to possibly resolve links between fracture patterns and shatter cone surface features striations and intervening valleys shatter coned samples from different impact sites and in different lithologies were investigated for their ct suitability with a shatter cone in sandstone from the serra da cangalha impact structure brazil remaining as the most promising candidate because of the fracture resolution achieved to validate the obtained ct data the scanned specimen was cut into three orthogonal sets of thin sections scans with 13 m resolution were obtained ct scans and microscopic analysis unraveled an orientation of subplanar fractures and related fluid inclusion trails and planar fracture pf orientations in the interior of shatter cones planar deformation features pdf were observed predominantly near the shatter cone surface previously undescribed varieties of feather features ff in the form of lamellae emanating from curviplanar and curved fractures as well as an arrowhead like ff development with microlamellae originating from both sides of a pf were observed the timing of shatter cone formation was investigated by establishing temporal relations to the generation of various shock microscopic effects shatter cones are thus generated post or syn formation of pf ff subplanar fractures and pdf the earliest possible time for shatter cone formation is during the late stage of the compressional phase that is shock wave passage of an impact event

high effusion rates of the cordon caulle 2011 2012 eruption southern andes and their relation with the quasi harmonic tremor high effusion rates of intermediate to high silica lavas seem to be less uncommon than previously thought in particular during their initial eruptive stages in this study we report satellite based time averaged discharge rates for the 2011 2012 effusive phase at cordon caulle which are well correlated with the evolution of the quasi harmonic tremor the most significant seismic signal after the initial explosive stage such a correspondence could become a key method for detection of the onset of effusive phases especially in remote and or very cloudy areas supplying an additional tool for effective warnings and near real time hazards appraisal