multipoint observations of a small substorm in this paper the authors present multipoint observations of a small substorm which occurred just after 0110 ut on april 25 1985 the observations were made by spacecraft ampte cce ampte irm dmsp f6 and dmsp f7 ground auroral stations eiscat magnetometer cross syowa narssarssuaq great whale river and fort churchill and mid latitude stations furstenfeldbruck toledo and argentine island these data provide them with a broad range of observations including the latitudinal extent of the polar cap visual identification of substorm aurorae and the magnetic perturbations produced directly beneath them in situ magnetic field and energetic particle observations of the disruption of the cross tail current sheet and observations concerning the spatial expansion of the current disruption region from two radially aligned spacecraft the dmsp data indicate that the event took place during a period when the polar cap was relatively contracted yet the disruption of the current sheet was observed by cce at 8 56 r sub e they have been able to infer a considerable amount of detail concerning the structure and westward expansion of the auroral features associated with the event and they show that those auroral surges were located more than 10 degree equatorward of the boundary between open andmore closed field lines moreover they present evidence that the current sheet disruption observed by cce in the neutral sheet was located on field lines which mapped to the westward traveling surge observed directly overhead of the ground station at syowa furthermore the observations strongly imply that disruption of the cross tail current began in the near earth region and that it had a component of expansion which was radially antisunward less

microstructural investigation of the interaction and interdependence of cataclastic and plastic mechanisms in feldspar crystals deformed in the semi brittle field abstract plagioclase feldspar an 50 single crystals from the hogarth range nsw australia have been deformed in the brittle to semi brittle regime at 500 800 c 90 300 mpa effective pressure and strain rate between 10 5 and 10 6 this gem quality feldspar is both macroscopically and microscopically devoid of any microstructure other than very rare growth twins so all microstructures produced by experimental deformation can be easily identified the specimens were deformed to failure which occurred at strains of

tailward leap of multiple expansions of the plasma sheet during a moderately intense substorm themis observations a moderately intense substorm on 1 march 2008 from 0830 to 1000 ut observed by themis probes and the ground based observatory gbo is examined to investigate the global evolution of substorm phenomena during this interval all five themis probes are closely aligned along the tail axis near midnight covering a radial range from similar to 9 re to similar to 18 re after the substorm onset plasma sheet expansions take place successively at multiple locations in the magnetotail as measured by different probes the positions of the plasma sheet expansions have a tailward leap progression with an average velocity of similar to 36 km s there are two types of dipolarization detected in this substorm the first type is the dipolarization front which is associated with the bursty bulk flow bbf while the second type which we call global dipolarization is associated with plasma sheet expansions in the substorm studied there are four intensifications as shown in the themis ae index we can detect the effects of localized and short lived magnetic energy release processes occurring in the magnetotail corresponding to each of the four ae intensifications furthermore the inner four probes can detect the global dipolarization signatures similar to 4 15 min earlier than plasma sheet expansions while the outermost probe p1 cannot detect this before the plasma sheet expansion these two phenomena are caused by the same process magnetic energy release process but the effects detected by probes locally appear delayed the observations in this case are not sufficient to distinguish between the two competing substorm models

substorms fundamental observational features distinction from other disturbances and external triggering fundamental observational signatures of substorms and other auroral zone disturbances are summarized in this review it is shown that at least three distinct types of major geomagnetic disturbance have been identified in the auroral zones and it is suggested that care should be taken in distinguishing substorms from the other types of disturbance when reaching observational conclusions concerning substorms substorms can be uniquely identified by the formation of the current wedge and by the associated westward electrojet and region of active aurora that move poleward within the ionosphere during the expansion phase periods of enhanced convection have been observed to continue without substorms for as long as the interplanetary magnetic field imf remains stably southward they can have al signatures that look very much like intense substorms but these signatures result from strong dp 2 ionospheric currents driven by enhanced convection rather than from a nightside westward electrojet a third type of disturbance is characterized by short intervals of enhanced equatorward flow within the ionosphere and enhanced auroral emissions these enhancements move equatorward from near the polar cap boundary and they may be associated with several minute long enhancements of earthward flow within the plasma sheet studies that have examined the triggering of substorm expansions by imf changes are also reviewed when considered together and substorms are distinguished from the other disturbances these studies imply that most and perhaps all expansions are triggered by imf changes these changes include both northward turnings and reductions in the magnitude of the y component if most substorms are indeed externally triggered as suggested here then substorms must generally not be the result of an internal magnetospheric instability

dynamics of the substorm expansive phase meridian scanning photometer msp and magnetometer data from the canadian auroral network for the open program unified study canopus ground based array have been used to study the dynamics of the substorm expansive phase the relative latitudinal motions of the msp 630 0 557 7 and 486 1 nm emissions for eight isolated events have been studied the data show the expansive phase to comprise three stages an explosive tens of seconds onset rapid poleward motion of 557 7 nm emissions of the order of a few minutes and a period of slower tens of minutes poleward moving 630 0 nm emissions we interpret the rapid poleward motion of the 557 7 nm data in terms of a region of instability expanding rapidly down the magnetotail possibly accelerating plasma sheet electrons as it proceeds all events show that lobe flux reconnection occurs after near earth onset with typical time delays between 1 and 5 min the extremely short time interval between near earth onset and the beginning of lobe flux reconnection presents a severe observational constraint on any substorm model which attempts to explain the substorm expansive phase magnetometer data from the canopus churchill line have been used to study the dynamics of the substorm electrojets during the growth and early expansive phases of the substorm the borders of these electrojets closely follow the motion of the msp data during the growth and expansive phases at the instant of expansive phase onset the growth phase electrojets as observed along the churchill meridian line disappear being replaced by the substorm westward electrojet associated with the current wedge the equatorward border of the substorm westward electrojet is embedded within the proton aurora emissions and appears at expansive phase onset 2 equatorward of the growth phase electrojets further showing the near earth proximity of expansive phase onset no indication of reconnection on closed field lines prior to expansive phase onset was observed in either the magnetometer or the photometer data

substorm onset by new plasma intrusion themis spacecraft observations 1 recent observations from the 2 d array of high resolution themis all sky imagers asis during the 2008 tail season have shown that the majority of substorm onsets are preceded by equatorward extension of an auroral arc initiated at the poleward boundary of the aurora zone and that the aurora breakup is likely triggered when this precursor arc reaches the onset region these observations were used to suggest that substorm onset is driven by a new population of plasma having lower global entropy content than that of the preexisting plasma intruding into the inner plasma sheet we have taken advantage of conjunctions between the location of the precursor arc observed by the themis asi and themis spacecraft and identified four events where clear signatures of the proposed new plasma intrusion were observed simultaneously in both the magnetotail and ionosphere in two of the events the intrusion plasma was captured more than 10 min before substorm onset in the midtail plasma sheet the intruding plasma is characterized as the reduction of the pressure and entropy parameter associated with more dipolarized magnetic field as well as the enhancement of the duskward electric field and earthward flow in two other events the plasma intrusion was found within 2 min before onset in the inner plasma sheet region r 11 re which is very close to the onset locations these observations are consistent with the proposal that intrusion of low entropy content plasma to the onset region may provide a trigger for substorm onset we also found two plasma intrusion events not associated with substorm onset in the plasma sheet which indicates that the new plasma intrusion is not a sufficient condition for substorm onset we suggest that sufficient pressure and azimuthal pressure gradient buildup in the inner plasma sheet during growth phase should be another necessary condition

organic facies variations in the triassic shallow marine and deep marine shales of central spitsbergen svalbard abstract shallow marine shales of the lower triassic sticky keep member and deep marine shales of the middle triassic botneheia member of the barentsoya formation central spitsbergen have been subjected to detailed organic petrological and organic geochemical study an assessment based on organic facies characteristics has been carried out on these sediments in order to distinguish between those deposited in shallow vs deep marine depositional settings the methods employed include evaluation of organic carbon content rock eval pyrolysis extractable organic matter biomarker distributions and petrographic data the maturity assessment of the samples analysed is mainly based on vitrinite reflectance and tmax data organic facies criteria such as toc hi pp petroleum potential hydrocarbon yield kerogen assemblage and ratios of pristane n c17 phytane n c18 tm ts c24 tetracyclic c26 tricyclic c34 c29 hopane c35 c34 hopane hopanes steranes diasteranes steranes c21 c22 sterane and c21 c21 sterane were found to be useful in differentiating between the two depositional environments the biomarker ratios in particular are thought to reflect more of the depositional condition such as the extent of anoxicity oxicity and less on source input probably as a result of enhanced microbial degradation on the organic matter deposited

in situ spatiotemporal measurements of the detailed azimuthal substructure of the substorm current wedge the substorm current wedge scw is a fundamental component of geomagnetic substorms models tend to describe the scw as a simple line current flowing into the ionosphere toward dawn and out of the ionosphere toward dusk linked by a westward electrojet we use multispacecraft observations from perigee passes of the cluster 1 and 4 spacecraft during a substorm on 15 january 2010 in conjunction with ground based observations to examine the spatial structuring and temporal variability of the scw at this time the spacecraft traveled east west azimuthally above the auroral region we show that the scw has significant azimuthal substructure on scales of 100 km at altitudes of 4000 7000 km we identify 26 individual current sheets in the cluster 4 data and 34 individual current sheets in the cluster 1 data with cluster 1 passing through the scw 120 240 s after cluster 4 at 1300 2000 km higher altitude both spacecraft observed large scale regions of net upward and downward field aligned current consistent with the large scale characteristics of the scw although sheets of oppositely directed currents were observed within both regions we show that the majority of these current sheets were closely aligned to a north south direction in contrast to the expected east west orientation of the preonset aurora comparing our results with observations of the field aligned current associated with bursty bulk flows bbfs we conclude that significant questions remain for the explanation of scw structuring by bbf driven wedgelets our results therefore represent constraints on future modeling and theoretical frameworks on the generation of the scw

the double auroral oval in the dusk midnight sector formation mapping and dynamics 1 the present study observationally examines the double auroral oval structure in the evening premidnight sector the results are summarized as follows 1 the poleward auroral branch can be attributed to accelerated auroral precipitation occasionally accompanied by alfvenic precipitation and it is collocated with the upward r1 current 2 the equatorward auroral branch which is embedded in the downward r2 current very often if not always consists of the diffuse precipitation of energetic electrons and extends mostly poleward of the b2i boundary collocated with energetic ion precipitation 3 the associated ionospheric convection is consistent with the two cell pattern and the poleward branch is located at the zonal flow shear it is inferred that the equatorward branch is mapped to the geosynchronous region and outside whereas the poleward branch is mapped farther down the tail to the source region of the r1 system it is suggested that the intense ring current and injection of energetic electrons are necessary for the equatorward branch to be distinct but the substorm expansion phase is not favorable for the formation of the emission gap this explains why the double oval often takes place during magnetospheric storms and during the recovery phase of intense substorms the equatorial magnetic field is expected to be strong in the mid tail region which might be related to the auroral gap region it is also found that the poleward branch occasionally intensifies in an extended mlt sector in a few minutes which may be explained in terms of the sudden enhancement of the return sunward convection flow

magnetic dipolarization with substorm expansion onset 1 we compare the timing and spatial extent of dipolarization with auroral breakup by using magnetic field data 1 min time resolution from goes 8 and goes 9 and global auroral images 37 s frame rate from the polar ultraviolet imager we survey the entire year of 1997 data and obtain 32 unambiguous dipolarization events that had ionospheric footprints within 2 hours in magnetic local time mlt from their associated auroral breakups it is found that the auroral breakup preceded the arrival of dipolarization at goes by an average of 1 7 2 7 min 1 0 2 1 min for events 1 hour from the auroral breakup the delay of geosynchronous dipolarization represents a propagation time as the substorms rarely initiated at the goes location propagation velocities derived from the propagation time are 60 km s 1 eastward 75 km s 1 westward and 270 km s 1 earthward at geosynchronous orbit we also show for the first time that auroral bulge and dipolarization region closely map the bulge is the region of currents out of the ionosphere the eastern edge of the dipolarization region where field aligned currents are flowing into the ionosphere is based on the t89 magnetic field model located at 0 5 hour in mlt eastward of the eastern edge of the auroral bulge within the initiation site meridian the dipolarization onset was observed 1 min prior to the auroral breakup at the goes altitude this time is associated with alfven transit time and therefore depends on the radial location of the onset the azimuthal scale of the initiation site is found to be 0 5 hour in mlt in the ionosphere or less than 1 earth s radius at 6 6 re these results support current diversion into the ionosphere as a critical part of substorms finally a simple timing analysis of specific substorm onset time features suggests that near earth reconnection cannot occur before current disruption and therefore cannot be the cause of substorm

melt assisted interior to margin switch from dislocation to diffusion creep in coarse grained plagioclase evidence from a deformed anorthosite pluton abstract at bolangir eastern india massive anorthosites in the pluton interior grade into anorthosites with outward dipping margin parallel foliation neighboring the pluton margin sheets and veins of ferrodiorites residual melts of anorthosite crystallization concordant and discordant to the margin parallel foliation suggest pluton deformation at near magmatic conditions t 950 c and p 6 12 kbar in the pluton interior the larger than centimeter sized magmatic plagioclase grains are replaced by aggregates of smaller 100 600 m dynamically recrystallized internally strained grains with un equilibrated boundaries neighboring the pluton margin poly sized 200 2500 m plagioclase grains in anorthosites are of two types strain free rectangular shaped plagioclase grains with high energy an richer margins indenting neighboring plagioclase grains formed by diffusion creep whereas unstrained end to end touching euhedral plagioclase grains showing tilling represent magmatic flow textures the pluton interior to margin switch in plagioclase deformation from grain boundary migration accommodated dislocation creep to grain boundary diffusion creep is attributed to the increasing melt fraction melt crystal ratio during syn deformation pluton emplacement plagioclase grains in the bolangir pluton are significantly coarser compared to plagioclase aggregates

reaction induced weakening of plagioclase olivine composites abstract the localisation of strain into natural ductile shear zones is often associated with the occurrence of metamorphic reactions in order to study the effects of solid solid mineral reactions on plastic deformation of rocks we have investigated the shear deformation of plagioclase olivine composites during the reaction plagioclase olivine orthopyroxene clinopyroxene spinel garnet microstructures of plagioclase olivine composites were studied after shear deformation experiments in a griggs apparatus experiments were performed on anorthite forsterite an fo and labradorite forsterite lab fo composites at 900 c confining pressures between 1000 1600 mpa and with constant shear strain rates of 5 10 5 s 1 in absence of reaction lab fo composites are stronger than pure olivine and labradorite end members that deform with a high temperature plasticity mechanism lab fo composites strain harden due to the inhibition of extensive recrystallisation by interphase boundaries in an fo composites the reaction induces strain weakening by a switch from dislocation creep to grain size sensitive deformation mechanisms through the development of fine grained size

detailed analysis of a substorm event on 6 and 7 june 1989 2 stepwise auroral bulge evolution during expansion phase 1 we have analyzed in detail the auroral bulge evolution during the expansion phase of an isolated substorm which was observed by the uv imager aboard the akebono satellite it was found that there were three distinct stages in the evolution stage 1 was characterized by rapid poleward and azimuthal predominantly westward expansions in a short time about 2 min stage 2 was characterized by a very slow poleward and slower and continuous azimuthal expansions there was a certain period for transition between stage 1 and stage 2 and it was characterized by a very slow poleward and rapid eastward expansions stage 3 started about 11 min after the onset and was characterized by a sudden reactivation of the rapid poleward and azimuthal expansions the reactivation started around the initial onset meridian and then spread both eastward and westward at the azimuthal front the expansion first occurred at the lowest latitudes spread poleward to around the highest latitudes of stage 1 and then spread further poleward after a brief interval hence the local expansion also had three distinct stages similar to the global one the ground based observations showed that the highest latitude of the local first stage was very close to the latitude of auroral activity that appeared near the ionospheric plasma sheet boundary layer psbl region a few minutes before the onset the further poleward expansion during the local third stage started with a significant intensification of the poleward most auroral activity during the local third stage the bright electron auroral region was bifurcated into a poleward expanding part and an equatorward moving part the proton auroral emission coexisted in the bulge during the local first and second stages and almost disappeared soon after the bifurcation during the local third stage based on these observations we discuss the evolution in the magnetosphere during the expansion phase

radial expansion of the tail current disruption during substorms a new approach to the substorm onset region the substorm onset region and the radial development of the tail current disruption are examined from a new viewpoint the reconfiguration of the magnetotail field at substorm onset can be understood in terms of a sudden decrease disruption in tail current intensity the north south component bz is very sensitive to whether the spacecraft position is earthward or tailward of the disruption region while the change in the sun earth component bx is most sensitive to the change in tail current intensity near the spacecraft if the current disruption starts in a localized range of radial distance and expands radially a distinctive phase relationship between the changes in bx and bz is expected to be observed this phase relationship depends on whether the current disruption starts on the earthward side or the tailward side of the spacecraft thus it is possible to infer the direction of the radial expansion of the current disruption from magnetic field data of a single spacecraft this method is applied to isee observations of a tail reconfiguration event that occurred on march 6 1979 the phase relationship indicates that the disruption region expanded tailward from the earthward side of the spacecraft during the event this model prediction is consistent with the time lag of magnetic signatures observed by the two isee spacecraft the expansion velocity is estimated at 2 re min 200 km s for this event furthermore it is found that the observed magnetic signatures can be reproduced to a good approximation by a simple geometrical model of the current disruption the method is used statistically for 13 events selected from the isee magnetometer data it is found that the current disruption usually starts in the near earth magnetotail x 20re and often within 15 re of the earth the phase relationship between bx and bz is discussed in terms of the near earth neutral line model and the result is compared with previous studies on the substorm onset region

timing of substorm signatures during the november 24 1996 geospace environment modeling event an excellent data set has been gathered for the november 24 1996 geospace environment modeling substorm interval that included an 95 min interval of strongly southward interplanetary magnetic field there were two expansion phase onsets and a pseudobreakup during this period for both onsets the classical signatures of onset in the auroral ionosphere ground magnetometer signatures of electrojet formation ground observations of pi2 pulsations and auroral brightening all occurred within less than a minute of each other indicating consistency between these onset indicators and giving reliable identification of onset times on the other hand low latitude positive bay observations that were most likely made near or within the longitude range of substorm onset did not provide accurate or consistent onset timing low latitude pi2s identified both onsets though one was delayed by l min from the auroral zone onset our most important and unexpected result is that particle injection at synchronous orbit was observed to initiate 2 5 min prior to both expansion onsets in the auroral ionosphere we suggest that the early detection of substorm onset at synchronous orbit was at least in part due to the unusually low latitude 63 magnetic of the auroral zone onsets which maps closer to synchronous orbit than usual onsets more often occur at somewhat higher latitudes which map to a few re beyond synchronous orbit as a result particle injections at synchronous orbit are generally delayed with respect to the time of current wedge initiation within the plasma sheet we do not currently know whether or not onset within the near earth plasma sheet generally occurs 2 5 min prior to expansion onset in the auroral ionosphere while we have no reason to believe that the timing we have observed is unique other studies of low latitude substorm onsets will be needed to ascertain the generality of this result

deformation of granitic rocks experimental studies and natural examples crustal deformation is inherently complex because the crust is inhomogeneous and anisotropic on a wide variety of scales advances in our understanding have come from a combination of field observations laboratory experiments and theoretical models what necessarily links all these investigations is a focus on process crystal plasticity dislocation creep is one of several grain scale deformation processes or mechanisms each process is dominant over a certain range of conditions in the crust has a particular form of the flow law and produces a characteristic set of microstructures characterizing the deformation mechanisms for different minerals over a range of conditions allows one to link the mechanical behavior or flow law associated with that process to the microstructures it produces the information on microstructures can be used to help interpret the thermomechanical history of naturally deformed rocks and flow laws can be used to create more realistic models predicting crustal behavior in different tectonic settings iterations among field theoretical and experimental approaches are critical for making progress in our understanding the deformation process of crystal plasticity is the main focus of this volume crystal plastic deformation of the crust is in many ways more complex than that of the mantle because most crustal rocks are polyphase aggregates and the strengths of common crustal silicates such as quartz feldspar and mica are very different at a given set of conditions yield strength envelopes for the crust as shown schematically in figure 1 are commonly plotted using the dislocation creep flow law for quartz which inherently assumes that quartz is the controlling phase and undergoes steady state flow but is the flow law for dislocation creep of quartz a good approximation for the deformation of a granite let alone a crustal plum pudding of regions and layers of different lithologies the crust

a statistical relationship between the geosynchronous magnetic field and substorm electrojet magnitude in this paper the authors examine the relationship between geosynchronous magnetic field variations during substorms measured by goes 5 and the auroral electrojet as measured by ae and poste de la baleine as in previous studies the authors find that the more taillike the field prior to the local onset the greater the dipolarization of the field during the substorm they also find that the greater the deviation of the field from a dipolar configuration the larger the change in ae during the event this implies that stronger cross tail currents prior to the substorm are associated with larger substorm associated westward electrojets and thus more intense substorms previous work has shown that in order to produce the observed taillike fields at geosynchronous altitude the intense cross tail current that builds up during the growth phase must be localized in the near earth le 10 r sub e region since the westward electrojet is the ionospheric leg of the substorm current wedge this result implies that the substorm associated westward electrojet is drawn from the near earth region in fact the authors find that most of the current diversion occurs in the near earth magnetotail furthermore they estimate that a diversion about half of the near earth cross tail currentmore can account for the current in the northern and southern westward electrojets associated with the substorm current wedge 25 refs 9 figs less

dynamic recrystallization near the brittle plastic transition in naturally and experimentally deformed quartz aggregates the electron backscattering diffraction technique ebsd was used to analyze bulging recrystallization microstructures from naturally and experimentally deformed quartz aggregates both of which are characterized by porphyroclasts with finely serrated grain boundaries and grain boundary bulges set in a matrix of very fine recrystallized grains for the tonale mylonites we investigated a temperature range of 300 380 c 0 25 gpa confining pressure a flow stress range of 0 1 0 2 gpa and a strain rate of 10 13 s 1 were estimated experimental samples of black hills quartzite were analyzed which had been deformed in axial compression at 700 c 1 2 1 5 gpa confining pressure a flow stress of 0 3 0 4 gpa a strain rate of 10 6 s 1 and to 44 to 73 axial shortening using orientation imaging we investigated the dynamic recrystallization microstructures and discuss which processes may contribute to their development our results suggest that several deformation processes are important for the dismantling of the porphyroclasts and the formation of recrystallized grains grain boundary bulges are not only formed by local grain boundary migration but they also display a lattice misorientation indicative of subgrain rotation dynamic recrystallization affects especially the rims of host porphyroclasts with a hard orientation i e with an orientation unsuitable for easy basal slip in addition dauphine twins within porphyroclasts are preferred sites for recrystallization we interpret large misorientation angles in the experimental samples which increase with increasing strain as formed by the activity of fluid assisted grain boundary sliding

a multisatellite study of a pseudo substorm onset in the near earth magnetotail this paper reports the multisatellite and ground observations of two pseudo substorm onset events that occurred successively at 0747 ut and 0811 ut may 30 1985 with more attention to the 0747 ut onset the distinguishing features of the 0747 ut event are as follows 1 the substorm associated tail reconfiguration started in a very localized region in the near earth magnetotail 2 the magnitude of the current disruption decreased markedly as the disruption region expanded tailward 3 on the ground the onset of a very small negative bay 40 nt was observed simultaneously with the onset of the current disruption but over a much wider local time sector than the near earth tail reconfiguration positive bay onsets at mid latitudes also had a longitudinally wide distribution from these features we infer that in the present event the current disruption took place filamentarily near ampte cce at 8 8 re it is also inferred that pseudo substorm onsets are distinguished from standard substorm onsets by the absence of a global expansion of the current disruption and that the spatial scale of the onset region in the magnetosphere is not a major difference between the two the present study suggests that the spatial distribution of the magnetic distortion before onsets is an important factor to determine the expansion scale of the current disruption it is also suggested that the current disruption is basically an internal process of the magnetosphere

microcracks in rocks a review abstract in the past decade the number of studies about microcracks in rocks has rapidly increased this review of recent work concentrates on microcracks in rock as separate entities emphasizing microcrack morphogenesis kinematics dynamics population statistics and observational techniques cracks are produced when the local stress exceeds the local strength the local stress may be augmented by twin lamellae interactions kink bands and deformation lamellae stress concentrations at grain boundary contacts and around intracrystalline cavities local strength may be reduced along cleavage planes along grain boundaries and along any internal surface as a result of corrosion by chemically active fluids dislocations appear not to be a significant factor for crack nucleation below about 500 c in silicates spatial and temporal changes in temperature can also induce microcracking as a result of differential thermal expansion between grains with different thermoelastic moduli and thermal conductivities the amount of quartz in the rock has a significant effect on thermally induced microcracks because of its large and variable thermal expansivity the application of hydrostatic pressure between 100 and 200 mpa effectively closes most cracks but the closure may not be uniform if crack wall asperities exist hydrostatic pressure appears to stabilize cracks and make crack growth more difficult the number and average size of mechanically induced microcracks is greater in rock deformed at higher pressures the application of a deviatoric stress field on the boundaries of a rock mass results on a microscopic scale in a very complex stress system which greatly affects nucleation and propagation paths the relative amount of intragranular and intergranular cracking appears to depend upon mineralogy rock type and stress state the vast majority of stress induced microcracks in rocks appear to be extensional statistically they are predominantly oriented within 30 of the macroscopic maximum stress direction crack densities increase as macroscopic deviatoric stress increases above a threshold level crack size distributions may be either lognormal or exponential fracture in rock under compressive boundary loads is a result of the coalescence of many microcracks not the growth of a single crack some crack configurations are more favorable for coalescence than others as deviatoric stress increases and rock failure is approached the microcrack population changes spatially from random to locally intense zones of cracking away from the fault the crack density dies off rapidly to the background level a few grains away under lesser deviatoric stresses slow subcritical microcrack growth can occur as a result of stress aided corrosion at the crack tip the rate governing mechanism may be either the chemical reaction rate or the rate at which water can get to the crack tip important details still remain to be worked out