patterns of family extinction depend on definition and geological timescale the recent claim1 of an approximate 26 myr periodicity in the pattern of mass species extinction over the past 250 myr has already stimulated much astrophysical speculation and debate on causal mechanisms2 13 however as shown here the evidence for that claim is strongly contingent on arbitrary decisions concerning the absolute dating of stratigraphical boundaries14 the culling of the database and the definition of what is mass extinction as opposed to background extinction this evidence becomes insufficient under other plausible geological timescales and other acceptable definitions of mass extinction analysis of the nonculled database shows that the reliability of identification of mass extinctions and their timing is at present extremely limited it also suggests that the apparent periodicity of mass extinctions results from stochastic processes

simulation of seismicity due to ruptures on noncoplanar interactive faults we study the statistical property and temporal variation of activity due to ruptures on two noncoplanar interactive faults each fault is assumed to consist of cracks and intervening asperities the ruptures are assumed to occur in the form of catastrophic coalescence of the coplanar cracks on each fault we apply fracture mechanics to the analysis of the interactions among cracks and to the consideration of fracture condition our fault model gives rise to a complex frequency distribution of event sizes even if no externally imposed spatial inhomogeneity is assumed for the distributions of cracks and asperities and a homogeneous statistical distribution is assumed for fracture strengths for the smaller events the frequency distribution of the event sizes obeys the gutenberg richter magnitude frequency law it is also shown that the number of smaller events increases as the contribution of noncoplanar interactions increases we generally observe the periodic recurrence of events after transient change at the beginning of each sequence of activity the length of the transient part increases considerably as the contribution of noncoplanar interactions increases the recurrence period at the steady periodic state also lengthens with increasing contribution of noncoplanar interactions hence if the temporal variation of activity is observed in a fixed length of time then the activity appears to be much more complex when the contribution of noncoplanar interactions is larger it is also observed that events with smaller recurrence intervals are more abundant when the influence of noncoplanar interactions is large

earthquake rupture complexity due to dynamic nucleation and interaction of subsidiary faults we numerically study the dynamic interaction of propagating cracks it is assumed that propagating cracks can nucleate and drive subsidiary cracks because of shear strain enhancement near the propagating crack tips the critical strain fracture criterion is assumed in the analysis intense interaction is expected to occur among the cracks all the cracks are assumed to be parallel and antiplane strain deformation is assumed in the computation

glacier retreat during the recent eruptive period of popocatepetl volcano mexico popocatepetl 19802 n 98862 w 5424 m is one of the largest active stratovolcanoes in the transmexican volcanic belt a glacier located on the north side has undergone severe ablation since the volcano reinitiated eruptive activity in december 1994 in our study we calculate the extent of the glacier recession and the loss in glacial mass balance during the period of greatest laharic activity 1994 2002 using photogrammetric treatment of 20 pairs of aerial photographs the results indicate that from november 1997 to december 2002 the glacier released approximately 3 967 000 m 3 of water a period of intense glacier melting occurred from 4 november 2000 to 15 march 2001 during which time 717 000 m 3 of water was released much of the melting was attributed to the pyroclastic flow that took place on 22 january 2001 and produced a 14 2 km lahar with 68 000 m 3 of water among the many types of volcanic events pyroclastic flows were the most effective in causing sudden snowmelt although small explosions were also effective since they deposited incandescent material on the glacier the collapse of the plinian columns covered the glacier with pyroclasts and increased its volume the existence of control points for georeferencing and a knowledge of the topography underlying the glacier previous to the eruption would have provided more accurate and useful results for hazard prevention

seismic evidence for an earthquake nucleation phase near source observations show that earthquakes initiate with a distinctive seismic nucleation phase that is characterized by a low rate of moment release relative to the rest of the event this phase was observed for the 30 earthquakes having moment magnitudes 2 6 to 8 1 and the size and duration of this phase scale with the eventual size of the earthquake during the nucleation phase moment release was irregular and appears to have been confined to a limited region of the fault it was characteristically followed by quadratic growth in the moment rate as rupture began to propagate away from the nucleation zone these observations suggest that the nucleation process exerts a strong influence on the size of the eventual earthquake

geomorphic effects of large debris flows on channel morphology at north fork mountain eastern west virginia usa extreme rainfall in june 1949 and november 1985 triggered numerous large debris flows on the steep slopes of north fork mountain eastern west virginia detailed mapping at four sites and field observations of several others indicate that the debris flows began in steep hillslope hollows propagated downslope through the channel system eroded channel sediment produced complex distributions of deposits in lower gradient channels and delivered sediment to floodwaters beyond the debris flow termini based on the distribution of deposits and eroded surfaces up to four zones were identified with each debris flow an upper failure zone a middle transport erosion zone a lower deposition zone and a sediment laden floodwater zone immediately downstream from the debris flow terminus geomorphic effects of the debris flows in these zones are spatially variable the initiation of debris flows in the failure zones and passage through the transport erosion zones are characterized by degradation 2300 to 17 000 m3 of sediment was eroded from these zones the total volume of channel erosion in the transport erosion zones was 1 3 to 1 5 times greater than the total volume of sediment that initially failed indicating that the debris flows were effective erosion agents as they travelled through the transport erosion zones the overall response in the deposition zones was aggradation however up to 43 per cent of the sediment delivered to these zones was eroded by floodwaters from joining tributaries immediately after debris flow deposition this sediment was incorporated into floodwaters downstream from the debris flow termini causing considerable erosion and deposition in these channels 1998 john wiley sons ltd

sedimentology and clast orientations of the 18 may 1980 southwest flank lahars mount st helens washington abstract three lahars that resulted from the flow transformation of an milled pyroclastic surge caused by ejected lithic debris and hydrothermal water during the cataclysmic mount st helena eruption of 18 may 1980 deposited about 1 106 m3 of massive poorly sorted poorly graded volcaniclastic sediment on the southwest flank swf mean peak flow velocities over a 5 km radius ranged from more than 30 m s in the proximal area mean slope angle of 13 to about 5 m s in the distal area mean slope angle of 4 9 the lahars rapidly decelerated as deposition occurred on forested slopes average yield strength of the swf lahars ranged from 9 000 to 12 000 dn cm2 bingham viscosity ranged from 2 8 103 to 50 103 poises average flow density of the swf lahars was about 2 0 g cm3 to determine the grain size distribution of the swf lahars spot sampling of deposit matrix from vertical exposures was supplemented with point count sampling of larger particles results from these analyses indicate that downflow changes in mean gram size and sorting occur only in the coarse fraction of the deposits larger than a critical diameter of about 4 mm and occur only in the basal portion of the deposits both mean grain size and sorting coefficient increase slightly with distance the deposits show weak inverse grading with respect to mean grain size and exhibit a weak trend of upwardly poorer sorting fabric analyses of clast long axes indicate strong planar orientations parallel to flow surfaces an eigenvalue analysis demonstrates the occurrence of both flow parallel long axis orientations with upstream dips and flow transverse orientations similar clast fabric results were obtained for deposits of the north fork toutle river lahar however the mean clast long axis dip in the swf lahars is significantly shallower than the mean clast long axis dip in the north fork toutle lahar and is more like clast long axis dips reported for tills and other sediment flows the data from the swf lahars indicate that lahar clast fabric may be more complex and variable than previously suggested

bolides and biostratigraphy address as retiring president of the geological society of america october 1982 the stratigraphic record is punctuated by episodes of mass extinction these must be accounted for by major changes in the evolution of the earth three groups of forces have acted on the earth through time and may be adduced as the ultimate causes of many geological phenomena including extinctions these are internal earth changes solar system changes and astronomical accidents immediate causes are many and may stem from more than one ultimate cause among astronomical accidents asteroid or cometary impacts by bodies ranging up to 10 km in diameter or more occur at a known rate and must be accepted as an ongoing phenomenon evidence for large body impacts exists locally in the presence of craters and ejecta but regional or worldwide evidence is harder to assess it is probable that geochemical sedimentological and biological effects will prove the most reliable indicators although each involves difficulties in preservation and interpretation mass extinctions and animal or plant diversity are commonly demonstrated by plotting numbers of taxa that disappeared against an interval of time for example genera or families per stage or series a better measure of the real effect on life of an extinction event is to estimate changes in the biomass a measure of the number or volume of organisms and their extent the difference in biomass between two taxa of equal rank can be and frequently is many orders of magnitude changes in the physical environment are seen as the driving force of evolution and may be slow acting or sudden only accurate biochronological and sedimentological studies can determine how sudden an event has been evidence for some extinctions is found by changes at a bedding plane in each section examined if the possibility of gaps in sedimentation in such sections can be eliminated and the horizon accurately correlated then this constitutes strong evidence for a sudden event and an astronomical accident must be considered the phanerozoic record contains at least five major extinctions late ordovician late frasnian mid late devonian late permian late trias and late cretaceous detailed evidence on the time of disappearance of the taxa affected is hard to come by and much more work is required before any hypotheses of extinction can be substantiated although the cretaceous tertiary extinction is marked by noble metal anomalies at many sections the reason for such anomalies remains controversial there appears to be a substantial extinction of biomass at the same horizon in considering the five mass extinctions the likelihood that a large body impact has contributed to each extinction event is assessed as follows probability high enough to take seriously late frasnian late cretaceous possibility late ordovician late trias least likely late permian

dynamic nucleation process of shallow earthquake faulting in a fault zone two distinct phases are commonly observed at the initial part of seismograms of large shallow earthquakes low frequency and low amplitude waves following the onset of a p wave p1 are interrupted by the arrival of the second impulsive phase p2 enriched with high frequency components this observation suggests that a large shallow earthquake involves two qualitatively different stages of rupture at its nucleation we propose a theoretical model that can naturally explain the above nucleation behaviour the model is 2 d and the deformation is assumed to be anti plane a key clement in our model is the assumption of a zone in which numbers of pre existing cracks are densely distributed this cracked zone is a model for the fault zone dynamic crack growth nucleated in such a zone is intensely affected by the crack interactions which exert two conflicting effects one tends to accelerate the crack growth and the other tends to decelerate it the accelerating and decelerating effects are generally ascribable to coplanar and non coplanar crack interactions respectively we rigorously treat the multiple interactions among the cracks using the boundary integral equation method biem and assume the critical stress fracture criterion for the analysis of spontaneous crack propagation our analysis shows that a dynamic rupture nucleated in the cracked zone begins to grow slowly due to the relative predominance of non coplanar interactions this process radiates the p1 phase if the crack continues to grow coalescence with adjacent coplanar cracks occurs after a short time then coplanar interactions suddenly begin to prevail and crack growth is accelerated the p2 phase is emitted in this process it is interpreted that the two distinct phases appear in the process of the transition from non coplanar to coplanar interaction predominance

the 1997 and 2001 lahars of popocatepetl volcano central mexico textural and sedimentological constraints on their origin and hazards abstract popocatepetl volcano is the most active volcano in central mexico and represents a high risk for more than 40 million people including mexico city in 1994 volcanic activity at popocatepetl renewed with the formation of ash rich plumes up to 7 km high in april 1996 lava emissions filled the crater and were accompanied by a series of explosions that produced eruptive columns up to 8 km high associated with explosive events in 1997 and 2001 two major lahars events occurred leaving debris flow deposits along huiloac gorge for as far as 15 km to the town of santiago xalitzinta the 1997 debris flow deposit originated after a prolonged emission of ash which caused glacier melt and a rapid release of water 1 107 m3 the amount of melting water was sufficient to gradually erode the river bed causing a flood that gradually transformed from a debris flow to a hyperconcentrated flow in contrast the 2001 debris flow that originated from a post depositional remobilization of a pumice flow deposit did not experience any flow transformation and carried 25 water at maximum the different behavior of these two lahars has important hazard implications both lahars reached xalitzintla town but at that point the 1997 lahar had already transformed into a sediment loaded stream flow the 2001 lahar by contrast maintained the characteristics of a debris flow being more competent and with greater destructive power what happened with these lahars demonstrates how important it is to take into consideration secondary volcanic phenomena even though they were not large flows they were capable of threatening populated areas even during periods of volcanic quiescence or reduced magmatic activity

randomness in the pattern of mass extinctions and waves of origination mass extinctions and waves of origination are defined as peaks in the average probability of family extinction and origination per geologic stage respectively a simple neutral model is proposed assuming only that the average probabilities of family extinction and origination have varied independently and had equal chances of going up and down from one stage to another as a null hypothesis this model is not rejected by the available palaeontologic data on marine animal and nonmarine invertebrate families in the phanerozoic the only discrepancy between the model and the empirical pattern is due to a systematic bias introduced by fossil lagerstatten the alleged periodicity of late phanerozoic mass extinctions follows inevitably from the model

a model of correlated episodicity in magnetic field reversals climate and mass extinctions correlated periodicities of mass extinctions climate magnetic field reversals and other geological phenomena may best be explained by cycles of activity within the mantle and core we propose that these cycles are due to variations in the thickness of the thermal boundary layer at the base of the mantle as the layer alternates between two phases of activity in the quiescent phase little flow occurs and the layer thickens with time by thermal diffusion the active phase begins when the thickening layer becomes dynamically unstable hot material erupts from the layer causing it to become thinner as the layer thins the rate of energy supply to the geodynamo and hence its reversal frequency increases the hot material rises to the surface where it causes widespread basaltic volcanic eruptions these eruptions release large amounts of co 2 and sulfates which have a pronounced effect on the climate and biota

cometary impacts into ocean their recognition and the threshold constraint for biological extinctions the montagnais impact crater is presently the only site in the ocean where the effect of a meteorite fall on marine organisms has been studied the impact crater is 45 km in diameter and was formed at 50 8 ma by a fall of probably an old cometary nucleus 3 4 km in diameter into shallow 600 m ocean comparison of the impact structure and related deposits with those on land shows several major differences of which the most significant is the absence of an elevated crater rim instead the crater perimeter is bevelled and eroded as a consequence of impact induced bottom currents and turbulent return water flow into the excavating cavity by this process most of the fall out breccia is reworked back into the crater cavity where it accumulates in much larger thickness than in impact craters on land at a microscopic scale the shock metamorphism features are about the same as those for land impacts geochemically impacts of comet nucleii may not leave a recognizable signature at the impact horizon except for a minor increase in iridium thus stratigraphic horizons associated with extinctions and or major changes in biota have to be closely examined for other impact indicators like the presence of tectites glass spherules and quartz grains with shock features occurrence of megatsunami wave deposits extensive erosion on continental margins margin failures and faunal mixing above erosional unconformities are other potential impact indicators there is no single indicator that can provide sufficient proof of an impact event such interpretations have to be based on multiparameter studies of global extent since many of the impact indicators are only of regional extent the lack of extinction of any marine plankton genera or of bottom dwellers at the montagnais impact site allows us to place a lower limit for biological extinctions caused by cometary impacts on those with nucleus 4 km in diameter the calculated frequency for a cometary impact which could result in a 10 extinction of marine genera is about 6 10 7 yr 1 and for the k t boundary type of extinctions about 2 10 8 yr 1 even allowing for a large degree of uncertainty in these estimates it is unlikely that the biological extinction events for the last 250 ma identified by sepkoski 1990 could have been all caused by meteorite impacts

detritus feeding as a buffer to extinction at the end of the cretaceous at the end of the cretaceous the principal animals that became extinct such as dinosaurs marine animals that lived in the water column and benthic filter feeders were in food chains tied directly to living plant matter animal groups less affected by extinction including marine benthic scavengers and deposit feeders small insectivorous mammals and members of stream communities were in food chains dependent on dead plant material the proposal that an asteroid or comet impact at the end of the cretaceous produced a dust cloud that cut off photosynthesis for several months is consistent with this pattern of extinction food chains dependent on living plant matter crashed while food chains based on detritus were buffered from extinction because there was a food supply adequate for the interval when photosynthesis was halted

empirical green s function study of the january 17 1994 northridge california earthquake the january 17 1994 northridge main shock was analyzed using an empirical green s function approach to estimate the spatial and temporal distribution of fault slip far field source time functions obtained from seismograms recorded by the berkeley digital seismic network bdsn and terrascope stations reveal the distributed nature of the source and the presence of a second large subevent the location of the fault beneath the san fernando valley as well as the updip directivity of the rupture contributed to the locally heavy damage due to this earthquake

the impact of present lahars on the geomorphologic evolution of proglacial gorges popocatepetl mexico abstract popocatepetl 19 03 n 98 35 w 5450 m is located 70 km southeast of mexico city the volcano resumed its eruptive activity in 1994 with emissions of pyroclasts that partially melted the glacier located on the north face the sudden release of water generated large lahars that flowed downslope though proglacial gorges this paper examines the geomorphologic evolution of these gorges since 1992 and focuses particularly on the landforms created by the most recent lahars of july 1997 the research suggests that the transport capacity of the lahars has increased despite the fact that the sediments they carry originate in deposits formed by earlier processes the presence of continuous permafrost in the area is a key factor in the formation of lahars

how many impact generated microspherule layers in the upper eocene abstract upper eocene microspherules microtektites and microkrystites have been found in deep sea sediments from the western north atlantic caribbean gulf of mexico equatorial pacific and eastern equatorial indian ocean recent studies proposed that there are three or four microspherule layers the north american microtektite and microkrystite couplet in chron c15 and one or two significantly older layer s in chron c16 in dsdp sites 216 292 and 612 based on calcareous nannofossil biochronologic and morphometric data i suggest that the microspherules in the latter sites correlate with the north american microtektite microkrystite couplet my results as well as previously published data are all compatible with the model of one couplet of microtektites microkrystites the inference of three or four microspherule layers in the upper eocene is shown to be an artifact of interpretation

violent volcanism stagnant oceans and some inferences regarding petroleum strata bound ores and mass extinctions past changes from a well mixed aerated ocean to a stratified stagnant ocean are indicated by decreasing 13c of limestone opposite to prior conclusions as well as by increasing 34s of marine sulfates the proposed stagnant ocean model includes a warm co2 enriched atmosphere and a bathyal to abyssal marine system dominated by bacterial components in the food web and by consequent light carbon in organic material and biogenic carbonates culminations of prolonged stagnant episodes correspond with marine faunal extinctions of late permian and late cretaceous time and the protracted changes and correlations provide evidence against any hypothesis of mass extinction by sudden worldwide catastrophe including the asteroid impact hypothesis the cretaceous stagnant ocean taken as the prime example is attributed to climatic warming triggered by volcanic co2 greenhouse effect and several feedbacks including decreased earth albedo and increased sinking of warm evaporite brines instead of aerated polar waters marine extinctions are attributed to upward expansion of the oxygen minimum zone and to catastrophic mixing of surface waters with poisonous sulfidic waters of the deeps the stagnant ocean provided a counterbalance between deep reduction and shallow oxidation conditions that favored maximal formation of black sulfidic shales protopetroleum and sedimentary sulfide ores and shallow to mid depth barite phosphorite iron ore cherty iron formation and sulfate bearing red beds trace elements concentrated in stagnant ocean sediments include chalcophile and pt group metals negating claims that ir provides a unique fingerprint of meteorite impact and cosmic accretion

assessing the record and causes of late triassic extinctions accelerated biotic turnover during the late triassic has led to the perception of an end triassic mass extinction event now regarded as one of the big five extinctions close examination of the fossil record reveals that many groups thought to be affected severely by this event such as ammonoids bivalves and conodonts instead were in decline throughout the late triassic and that other groups were relatively unaffected or subject to only regional effects explanations for the biotic turnover have included both gradualistic and catastrophic mechanisms regression during the rhaetian with consequent habitat loss is compatible with the disappearance of some marine faunal groups but may be regional not global in scale and cannot explain apparent synchronous decline in the terrestrial realm gradual widespread aridification of the pangaean supercontinent could explain a decline in terrestrial diversity during the late triassic although evidence for an impact precisely at the boundary is lacking the presence of impact structures with late triassic ages suggests the possibility of bolide impact induced environmental degradation prior to the end triassic widespread eruptions of flood basalts of the central atlantic magmatic province camp were synchronous with or slightly postdate the system boundary emissions of co2 and so2 during these eruptions were substantial but the contradictory evidence for the environmental effects of outgassing of these lavas remains to be resolved a substantial excursion in the marine carbon isotope record of both carbonate and organic matter suggests a significant disturbance of the global carbon cycle at the system boundary release of methane hydrates from seafloor sediments is a possible cause for this isotope excursion although the triggering mechanism and climatic effects of such a release remain uncertain d 2003 elsevier b v all rights reserved

the paradox of drowned reefs and carbonate platforms shallow water carbonate platforms and reefs are drowned when tectonic subsidence or rising sea level outpaces carbonate accumulation and benthonic carbonate production ceases drowned platforms are common in the geologic record but they present a paradox if one considers rates of processes involved during the early holocene when sea level rose at rates of 6 000 to 10 000 m yr mm 1 000 yr most reefs and platforms were outpaced by the rising sea during the late holocene with sea level rising 500 to 3 000 m yr in the atlantic caribbean area reefs and platforms started to recover built to sea level and prograded seaward 1 000 m yr is thus a conservative estimate of the average growth potential of modern reefs and platforms independently accumulation rates of prograding platforms in the geologic record suggest growth potential in excess of several hundred microns per year the growth potential of 1 000 m yr exceeds any relative rise of sea level caused by long term processes in the geologic record newly formed ocean crust subsides at a maximum of 250 m yr basin subsidence averages 10 to 100 m yr and sea level rises due to increased sea floor spreading amount to less than 10 m yr rapid pulses of relative rise of sea level or reduction of benthic growth by deterioration of the environment remain the only plausible explanations of drowning the geologic record shows examples of both of these processes global mass extinctions of reefs and platforms occurred in the middle cretaceous eustatic rise due to submarine volcanism or desiccation of a small ocean basin and the late devonian global crisis of ocean environment extraterrestrial cause drowning controlled by regional tectonics prevailed in the jurassic and early cretaceous of the tethyan realm and the drowning of mesozoic platforms in the western north atlantic seems to have been dictated by plate tectonic drift to higher latitudes

the rupture process and asperity distribution of three great earthquakes from long period diffracted p waves moment tensor source mechanisms for the largest twenty nine shallow earthquakes from 1981 m s 6 5 have been determined using long period rayleigh wave spectra and p wave first motions recorded by the international deployment of accelerograms ida global digital seismic network gdsn and world wide standardised seismic network wwssn these are compared with the centroid moment tensor mechanisms presented by dziewonski and woodhouse which were obtained in a contrasting manner in most cases the source is well represented by a single double couple mechanisms with good agreement of scalar moment and orientation between the two studies apparent mismatches for other cases may be interpreted in terms of source complexity we emphasise the objectivity of comparing moment tensor elements apparent discrepancies in the moment and orientation of double couple fault plane solutions are usually due to differences in the mxz and myz moment tensor elements which are poorly constrained by long period data

disruption of drift glacier and origin of floods during the 1989 1990 eruptions of redoubt volcano alaska abstract melting of snow and glacier ice during the 1989 1990 eruption of redoubt volcano caused winter flooding of the drift river drift glacier was beheaded when 113 to 121 10 6 m 3 of perennial snow and ice were mechanically entrained in hot rock avalanches and pyroclastic flows initiated by the four largest eruptions between 14 december 1989 and 14 march 1990 the disruption of drift glacier was dominated by mechanical disaggregation and entrainment of snow and glacier ice hot rock avalanches debris flows and pyroclastic flows incised deep canyons in the glacier ice thereby maintaining a large ice surface area available for scour by subsequent flows downvalley flow rheologies were transformed by the melting of snow and ice entrained along the upper and middle reaches of the glacier and by seasonal snowpack incorporated from the surface of the lower glacier and from the river valley the seasonal snowpack in the drift river valley contributed to lahars and floods a cumulative volume equivalent to about 35 10 6 m 3 of water which amounts to nearly 30 of the cumulative flow volume 22 km downstream from the volcano the absence of high water marks in depressions and of ice collapse features in the glacier indicated that no large quantities of meltwater that could potentially generate lahars were stored on or under the glacier the water that generated the lahars that swept drift river valley was produced from the proximal eruption induced volcaniclastic flows by melting of snow and ice

recent geomorphologic evolution of a glaciovolcanic active stratovolcano popocatepetl mexico abstract the recent geomorphologic evolution of the statovolcano popocatepetl 19 03 n 98 35 w including the volcanic activity and the evolution of the glacier that still exists on the north face was assessed most of the data were obtained from references in historical sources and aerial photographs dating from 1945 this information was supplemented with data from fieldwork conducted between 1992 and 1995 landform typology shows volcanic and glacial interactions the conclusions affirm that a geomorphologic evolution of the volcano occurred since the maximum glacier advance in the little ice age lia and the formation of a proglacial ramp later fluvioglacial gorges cut into the ramp especially during the 1921 27 eruption once the glacier had shrunk to a small size and was left hanging above the steep slope the streams disappeared and the gorges filled with blocks that fell in snow and ice avalanches

temperature and biotic crises in the marine realm climatic change has been a prominent cause of marine mass extinction but areal restriction of seafloor during global regression has not late eocene and pliocene pleistocene cooling for example caused major extinctions but profound global oligocene and pleistocene regressions had little or no direct effect on benthic diversity recurrent themes of pre cenozoic marine crises suggest that global temperature change also served as a major and perhaps dominant agent of extinction in these events 1 mass extinctions have frequently been concentrated in the tropics which seem to have become a refrigerated trap from which there has been no escape biotas previously occupying high latitudes have shifted equatorward to replace disappearing tropical biotas 2 some crises were not instantaneous but followed protracted and pulsatile temporal patterns as would be predicted for complex global climatic crises 3 several mass extinctions coincided with recognized intervals of climatic cooling

a mesozoic time scale we present an integrated geomagnetic polarity and stratigraphic time scale for the triassic jurassic and cretaceous periods of the mesozoic era with age estimates and uncertainty limits for stage boundaries the time scale uses a suite of 324 radiomenc dates including high resolution 40 ar 39 ar age estimates this framework involves the observed ties between 1 radiometric dates biozones and stage boundaries and 2 between biozones and magnetic reversals on the seafloor and in sediments interpolation techniques include maximum likelihood estimation smoothing cubic spline fitting and magnetochronology

swift snowmelt and floods lahars caused by great pyroclastic surge at mount st helens volcano washington 18 may 1980 the initial explosions at mount st helens washington on the moring of 18 may 1980 developed into a huge pyroclastic surge that generated catastrophic floods off the east and west flanks of the volcano near source surge deposits on the east and west were lithic sorted lacking in accretionary lapilli and vesiculated ash not plastered against upright obstacles and hot enough to char wood all attributes of dry pyroclastic surge material deposited at the surge base on steep slopes near the volcano transformed into high concentration lithic pyroclastic flows whose deposits contain charred wood and other features indicating that these flows were hot and dry stratigraphy shows that even the tail of the surge had passed the east and west volcano flanks before the geomorphically distinct floods lahars arrived this field evidence undermines hypotheses that the turbulent surge was itself wet and that its heavy components segregated out to transform directly into lahars nor is there evidence that meters thick snow slab avalanches intimately mixed with the surge to form the floods the floods must have instead originated by swift snowmelt at the base of a hot and relatively dry turbulent surge impacting hot pyroclasts probably transferred downslope momentum to the snow surface and churned snow grains into the surge base melting snow and accumulating hot surge debris may have moved initially as thousands of small thin slushflows as these flows removed the surface snow and pyroclasts newly uncovered snow was partly melted by the turbulent surge base this and accumulating hot surge debris in turn began flowing a self sustaining process feeding the initial flows the flows thus grew swiftly over tens of seconds and united downslope into great slushy ejecta laden sheetfloods gravity accelerated the floods to more than 100 km h as they swept down and off the volcano flanks while the snow component melted to form great debris rich floods lahars channeled into valleys

mapping of the high frequency source radiation for the loma prieta earthquake california waveform inversion has been successfully applied to map the spatial distribution of earthquake fault slip from strong motion records but its application has been restricted to a relatively lower frequency range of the data 5 hz generation is incoherent between neighboring patches on the fault we formulated an analytical model for calculating the envelope time history of the squared high frequency displacement seismogram this envelope time history is found through the line integration of the squared energy radiation intensity over the isochron using a recursive stochastic inversion scheme we determined both the lower frequency slip distributions from the low pass filtered strong motion displacement data and the higher frequency energy radiation intensity from the envelope of the squared high frequency displacement data on an extended fault for the 1989 loma prieta earthquake respectively our resulting lower frequency slip model shows that there are two main slip patches and little slip directly updip of the hypocenter one major slip zone located 10 km northwest of the hypocenter with predominant dip slip and the other located 12 km southeast of the hypocenter with predominant oblique slip our preliminary results of the high frequency energy radiation intensity inversion indicate three large high frequency sources and few smaller ones located on the outer periphery of the large slip zones this suggests that the high frequency energy sources are located along or near the boundaries of large slip zones which is consistent with the theoretical consideration that high frequencies are primarily generated from the rupture stopping areas or places with large slip variation

the k t boundary at beloc haiti compared stratigraphic distributions of the boundary markers at beloc haiti the cretaceous tertiary boundary ktb is characterized by a spherule bed containing glass particles these particles are considered by some authors as remains of tektites resulting from a nearby impact however because of the stratigraphic complexity of the beloc sections the genetic link between the ktb cosmic event and the spherule bed is not obvious in this paper we report new data on shocked quartz and ni rich spinels at haitian ktb sites the detailed stratigraphy of these minerals shows that there is no empty gap in the sedimentary sequence the first and largest shocked quartz is found in the upper part of the spherule layer they are abundant and size graded over the 25 30 cm of carbonate rich sediments overlying this layer the first ni rich spinels which are also rich in cr appear in the carbonate sediments the size grading of the spherules and shocked quartz and the stratigraphical overlapping of their distributions suggests that these two components were derived from the same event although different from typical impact glasses tektites beloc glass particles must be considered as impact derived products the enormous fluence of shocked quartz 104 grains cm2 is consistent with a proximal event the upper part of the sequence is more complex a second distribution of shocked quartz associated with ni rich spinels of different compositions appears in the layer containing the maximum ir concentration we propose that these features which are not easily explained by a sedimentary artifact result from a second collisional event

the glaciers of popocatepetl volcano mexico changes and causes an updated inventory of popocatepetl volcano s glaciers and field observations for 1977 1992 is presented two glaciers 0 559 km 2 and four permafrost fields 0 239 km 2 are reported here comparing new descriptions of the glaciar del ventorrillo with previous descriptions a systematic retreat pattern is identified the average retreat rate in terms of terminus altitude for 1906 1968 is nearly 7 m year advance rate 10 m year for 1968 1978 and a response of glacier terminus is documented for 1978 1982 at 40 m year morphological changes are a reduction of 25 in the glaciated area and strong retreat of the tezcalco tongue glaciar noroccidental is a stationary glacierette whose area was reduced by 12 in 34 years total glaciated area was reduced by 22 permafrost fields are located around the glaciers and are the remains of larger glaciers fluctuations are explained in terms of a an increase of heat flow b local climatic changes c global climatic changes and d a combination of these further work is needed to evaluate the role of the glaciers in the local environment the impact of the local and global climate on them the role they would play in the case of explosive activity of popocatepetl and its consequences

volcanogenic upwelling of anoxic nutrient rich water a possible factor in carbonate bank reef demise and benthic faunal extinctions the middle cretaceous oceans were characterized by widespread anoxia high sea levels reef carbonate bank extinctions and at least in the pacific by intense widespread mid plate volcanism the demise of reefs on numerous pacific atolls has left a large number of guyots this synthesis explores the possibility that the middle cretaceous demise of mid pacific and other carbonate bank communities and related extinctions elsewhere reflects sporadic possibly volcanogenic upwelling of anoxic water the upwelling would have had to be rapid enough to overwhelm surface reoxygenation and frequent widespread enough to suppress recolonization according to the hallock schlager hypothesis upwelling of nutrient rich waters destroyed the carbonate bank communities nutrient excess and anoxia may well have worked together the globally widespread presence of at least mid depth anoxia h 2 s is reasonably well established the cenomanian turonian oceanic anoxic event perhaps was an extreme example as is the mortality of modern benthic invertebrates at o 2 levels below about 0 15 ml l recolonization times for modern reef corals imply that upwelling catastrophes if localized need have recurred on approximately decade time scales previously postulated cretaceous sea level fluctuations raising sea level at rates on the order of 5 10 m in 10 4 10 5 yr would then have sufficed to carry the intermittently stressed reef surfaces below the 5 to 10 m deep high carbonate production zone below which reef recovery chances diminish thus the episode of frequent localized but widespread upwelling events need have lasted only about 10 4 10 5 yr although intermittent deep water ventilation is known to have occurred on milankovitch time scales during the cretaceous thereby bringing anoxic and or nutrient rich waters to the surface geologic and ocean modeling evidence is necessary to establish whether a significant amount of rapid upwelling did or could occur because mid plate submarine volcanism was widespread and intense in the middle cretaceous pacific volcanogenic hydrothermal heating and entrainment of nutrient rich anoxic waters is one possible mechanism for catastrophic buoyant upwelling rudistid bank extinction and guyot formation the efficacy of this process depends principally on the buoyancy that is heat energy flux and the buoyancy brunt vaisala frequency typical hydrothermal plumes along the mid oceanic ridge axis rise only to 150 to 400 m heights above the sea floor but megaplumes rising 1 000 m are known simple plume models imply that only basalt eruptions comparable to the largest historically known would be sufficiently energetic 10 12 10 14 j s to bring bottom water to the ocean surface from normal mid oceanic ridge axial depths 3 km over much of the present ocean however these models are conservative through neglect of momentum overshoot and gas exsolution to the extent that higher eruption rates and lower brunt vaisala frequencies due to lower vertical temperature gradients prevailed in the middle cretaceous pacific anoxic deep waters would more readily have reached the ocean surface most cretaceous guyots are now in the north pacific but were carried across the equator by pacific plate motion normal tendencies to upwelling and oxygen deficiency along the equator would have made the carbonate banks most vulnerable during their equatorial transit

volcanic mixed avalanches a distinct eruption triggered mass flow process at snow clad volcanoes a generally unrecognized type of pyroclastic deposit was produced by rapid avalanches of intimately mixed snow and hot pyroclastic debris during eruptions at mount st helens nevado del ruiz and redoubt volcano between 1982 and 1989 these mixed avalanches traveled as far as 14 km at velocities up to 27 m s involved as much as 10 7 m 3 of rock and ice and left unmelted deposits of single flow units as thick as 5 m during flow downslope heat transfer from hot rocks to snow produced meltwater that partially saturated the mixtures apparently giving these mixed avalanches mobilities equal to or greater than those of dry debris avalanches of similar volume the avalanches studied for this report began as snow avalanches triggered during explosive phases of the eruptions by ballistic bombardment of snow slopes by hot pyroclastic debris erosion by the avalanches incorporated additional snow fragments of glacier ice and other rock debris mixed avalanche deposits were massive very poorly sorted and ungraded to inversely graded prior to melting although rock debris composed as much as 70 wt of the frozen samples the bulk volumetric content of rock debris was only 9 to 36 snow and ice composed between 36 and 72 of the frozen samples with void space making up the remainder of the volume deformation and consolidation after melting of the ice content reduced the deposits to loose porous layers that were only a fraction of their original thickness and that later gave little hint of their origin after melting and desiccation the deposits are highly susceptible to erosion and unlikely to be well preserved in the stratigraphic record

periodicity in extinction and the problem of catastrophism in the history of life the hypothesis that extinction events have recurred periodically over the last quarter billion years is greatly strengthened by new data on the stratigraphic ranges of marine animal genera in the interval from the permian to recent these data encompass some 13 000 generic extinctions providing a more sensitive indicator of species level extinctions than previously used familial data extinction time series computed from the generic data display nine strong peaks that are nearly uniformly spaced at 26 ma intervals over the last 270 ma most of these peaks correspond to extinction events recognized in more detailed if limited biostratigraphic studies these new data weaken or negate most arguments against periodicity which have involved criticisms of the taxonomic data base sampling intervals chronometric time scales and statistical methods used in previous analyses the criticisms are reviewed in some detail and various new calculations and simulations including one assessing the effects of paraphyletic taxa are presented although the new data strengthen the case for periodicity they offer little new insight into the driving mechanism behind the pattern however they do suggest that many of the periodic events may not have been catastrophic occurring instead over several stratigraphic stages or substages

half a century of progress in research on terrestrial impact structures a review abstract the author who investigated the wolfe creek australia in 1962 and edited two benchmark sets of readings on meteorite craters and possible astroblemes in 1977 and 1979 reviews the state of knowledge at the present time the text is concerned with terrestrial impact structures geological features without any consideration of extraterrestrial analogues a handful of definitive publications are drawn on to present the story of terrestrial impact in a single article the text covers historical aspects briefly the effect of target variations the paucity of human observation of such large scale events distinction from volcanic endogenous structures modification by geological processes the transience of the crater initially formed on the target and its subsequent modifications the global geographic distribution of the 174 structures now listed of which a number are dubious attributions their distribution in geological time many ages being known only known to wide limits maximum or minimum values their size distribution calculations of impact frequencies shock effects processes on impact the stages of formation impact into shallow marine and deep sea targets impacts on ice about which little is known and finally the input of impact into biotic extinctions in this last lengthy section the summaries of the conclusions of scientists researching impact on earth and palaeontologists researching biotic impact are set side by side it is concluded that if the recent foraminiferal evidence obtained by gerta keller and associates is taken at its face value the case of impact as a sole agent in extinction is non existent biotic extinction is clearly a complex process involving a number of causes in some cases it was staggered in time and different sets of organisms responded quite differently and surprisingly even in the same extinction event extraterrestrial impact may have been one of the causes in some cases but it may have been regional rather than global in its effects we may never know how much input it had into the record of biotic extinction on earth an enormous amount of new knowledge has arisen from detailed studies of this new family of remarkable geological structures

distribution of iridium and associated geochemistry across the triassic jurassic boundary in sections at kuhjoch and kendlbach northern calcareous alps austria abstract samples from strata spanning the triassic jurassic boundary at the gssp at kuhjoch and at kendlbachgraben were studied by naa xrf and combustion analysis to determine ir levels and associated geochemistry the results are compared to previously determined carbon isotope stratigraphy at these sections ir concentrations in the limestones of the kossen formation at kuhjoch are very low

geomorphological evolution of a fluvial channel after primary lahar deposition huiloac gorge popocatepetl volcano mexico abstract popocatepetl volcano 19 02 n 98 62 w 5424 m began its most recent period of volcanic activity in december 1994 the interaction of volcanic and glacier activity triggered the formation of lahars through the huiloac gorge located on the northern flank of the volcano causing significant morphological changes in the channel the most powerful lahars occurred in april 1995 july 1997 and january 2001 and were followed by secondary lahars that formed during the post eruptive period this study interprets the geomorphological evolution of the huiloac gorge after the january 2001 lahar variations in channel morphology at a 520 m long research site located mid way down the gorge were recorded over a 4 year period from february 2002 to march 2005 and depicted in five geomorphological maps scale 1 200 for 14 february and 15 october 2002 27 september 2003 9 february 2004 and 16 march 2006 a gis was used to calculate the surface area for the landforms identified for each map and detected changes and erosion deposition processes of the landforms using the overlay function for different dates findings reveal that secondary lahars and others types of flows like sediment laden or muddy streamflows caused by precipitation rapidly modified the gorge channel following the january 2001 non eruptive lahar a period associated with volcanic inactivity and the disappearance of the glacier once located at the headwall of the gorge field observations also confirmed that secondary flows altered the dynamics and geomorphological development of the channel these flows incised and destroyed the formations generated by the primary lahars 1997 and 2001 causing a widening of the channel that continues today after february 2004 a rain triggered lahar and other flows infilled the channel with materials transported by these flows the deposits on the lateral edges of the channel form terraces a recent lull in lahar activity contrasts with the increasing instability of the edges of the channel and the continuous edification and destruction of recent lahar terraces

geological and biological consequences of giant impacts it seems inescapable that large impacts have been an ongoing phenomenon throughout geological time in the phanerozoic the estimated frequency of impact by bodies of 5 km diameter or more is on the order of 1 0 every 1 00 myr the 5 km lower limit is chosen arbitrarily but it is shown in this review that such a body is certainly large enough to cause very large terrestrial effects a small decrease in this size limit would lead to a large increase in the frequency of occurrence it may be claimed that bodies of a size likely to have drastic and sudden effects on niany physical and biological features of the earth have arrived with a frequency comparable to the average duration of a biostratigraphic stage nothing is said of the interval between arrivals which may be random i e stochastic currently there is not enough geological evidence to allow recognition of other than random spacing between a few positively identified events during the phanerozoic since impacts by extraterrestrial bodies have been clearly established it is reasonable to postulate that they must leave a record in the stratigraphic column why then do we currently have so few horizons that have been claimed to contain evidence of a global impact event since the effects we are considering are unquestionably global in this paper we bring together evidence from several disciplines that

mineralogy and petrology of the haiti cretaceous tertiary section abstract the cretaceous tertiary sections in haiti consist of a sequence of graded beds of turbidity current or gravity flow origin each section may consist variously of five major constituents 1 black glass 2 brown globules veinlets and blobs 3 argillaceous limestone clasts 4 white clasts and shards and 5 a fine grained matrix rich in smectite zeolites and calcite in addition there are trace amounts of a high calcium yellow glass the black glass particles are vesicular apparently unaltered with an andesitic to dacitic composition and in sharp contact with the secondary matrix and veinlet alteration products the brown globules veinlets and blobs consist of a palagonite smectite mix of a distinctly different composition than the black glasses white clasts and shards of a still different more hydrated composition are also present all of these materials are sometimes associated with accretionary lapilli the hydration or alteration products brown globules white clasts and shards and matrix amount to something in excess of 95 of the total deposits the sedimentology mineralogy and petrology of the haiti sections has provided a record of a sequence of probable volcanic events

model for intermediate term precursory clustering of earthquakes a causal sequence of an earthquake swarm extended quiescence foreshocks and main shock can be understood in terms of an aggregation model of crack growth and ultimate fusion on a multiplanar earthquake fault system the rheological model is one of accelerated crack growth under conditions of slip weakening coupled with a slow recovery of static friction after fracture once a slip feature has been generated that spans a large part of the available geometry a stress shadow is cast on other neighboring cracks that strongly retards and inhibits crack growth thus initiating the quiescent phase abruptly the success of the model depends on the assumption that nonlinear rheology regulates the rate of slip and that earthquake faults are not simply connected surfaces but instead have many strands

the cretaceous tertiary boundary at beloc haiti no evidence for an impact in the caribbean area abstract the k t boundary at beloc in haiti is characterized by a bed containing more or less spheroidal globules composed of either smectite or calcite a small fraction of these objects still contains well preserved glass particles which have been interpreted as the remains of tektites resulting from a bolide impact in the caribbean area however beloc glasses are markedly different from tektites and microtektites their structure and composition suggest that they are not impact generated products but are likely of volcanic origin on the other hand in sections which have probably preserved their original stratigraphy the globule bed showing well developed grading appears first in the sedimentary column followed by the ubiquitous cretaceous tertiary boundary ktb clay layer containing ir and ni rich spinels granulometric considerations indicate that the globule bed and the ir and spinel rich clay layer correspond to two distinct events we conclude that the globules at the ktb depositional sequence of beloc represent a local volcanic event slightly preceding the great ktb cosmic catastrophe our observations do not provide any unequivocal evidence that a cratering event occurred nearby

tree ring reconstruction of past lahar activity at popocatepetl volcano mexico lahars represent a major threat on the slopes of volcanoes all over the world in order to realistically assess hazards knowledge on the occurrence and timing of past lahar activity is of crucial importance however archival data on past events is usually scarce or completely missing tree ring records have repeatedly proved to be a reliable data source for the reconstruction of past geomorphic events however tree rings have seldom been applied for the identification of past lahars therefore it was the aim of this study i to identify and describe disturbances in tree growth induced by well documented lahar events and on this basis and ii to recognise older unknown lahar events with tree ring analyses based on these goals we collected 140 tree ring series from 62 trees abies religiosa pinus hartwegii and pinus ayacahuite standing inside or adjacent to the lahar channel in the huiloac gorge at popocatepetl volcano central mexico most commonly the known lahar events of 1997 and 2001 resulted in abrupt changes in tree ring width as well as injuries the same growth disturbances could be identified in the tree ring series indicating that five previously unknown lahar events would have occurred during the 20th century popocatepetl is one of the best surveyed volcanoes in the world and past eruptions are precisely noted in archives as most of these unknown events occurred during periods with no volcanic activity we believe that they were rainfall induced rather than related to volcanic activity in order to assess rainfall intensity threshold values for the triggering of events the analyses of meteorological data needs to be integrated in general the investigated tree species proved to be highly suitable for the reconstruction of mass movement processes therefore the applied methods can be transferred to other locations where data on past events are missing

evaluation of aster and srtm dem data for lahar modeling a case study on lahars from popocatepetl volcano mexico abstract lahars are among the most serious and far reaching volcanic hazards in regions with potential interactions of lahars with populated areas and human structures the assessment of the related hazards is crucial for undertaking appropriate mitigating actions and reduce the associated risks modeling of lahars has become an important tool in such assessments in particular where the geologic record of past events is insufficient mass flow modeling strongly relies on digital terrain data availability of digital elevation models dems however is often limited and thus an obstacle to lahar modeling remote sensing technology has now opened new perspectives in generating dems in this study we evaluate the feasibility of dems derived from the advanced spaceborne thermal emission and reflection radiometer aster and the shuttle radar topography mission srtm for lahar modeling on popocatepetl volcano mexico two gis based models are used for lahar modeling laharz and a flow routing based debris flow model modified single flow direction model msf both predicting areas potentially affected by lahars results of the lahar modeling show that both the aster and srtm dems are basically suitable for use with laharz and msf flow path prediction is found to be more reliable with srtm data though with a coarser spatial resolution errors of the aster dem affecting the prediction of flow paths due to the sensor geometry are associated with deeply incised gorges with north facing slopes laharz is more sensitive to errors of the aster dem than the msf model lahar modeling with the aster dem results in a more finely spaced predicted inundation area but does not add any significant information in comparison with the srtm dem lahars at popocatepetl are modeled with volumes of 1 10 5 to 8 10 6 m 3 based on ice melt scenarios of the glaciers on top of the volcano and data on recent and historical lahar events as regards recently observed lahars the travel distance of lahars of corresponding volume modeled with laharz falls short by 2 to 4 km an important finding is that the travel distance of potential lahar events modeled with laharz may differ by about 2 km when using srtm or aster data because of varying lateral flow volume distribution as a consequence verification and sensitivity analysis of the dem is fundamental to deriving hazard maps from predicted modeled inundation areas because of the global coverage of this type of remote sensing data the conclusion that both srtm and aster derived dems are feasible for lahar modeling opens a wide field of application in volcanic hazards studies

sedimentological processes in lahars insights from optically stimulated luminescence analysis abstract a lahar is a sediment laden flow capable of major destructive impacts on infrastructure and human life how lahars transport sediment is thus a key issue for understanding lahar sedimentology and behavior especially in terms of the lahar s hydraulic and rheological properties which can be substantially altered as the lahar gains or loses material during its travel in this research we analyze lahar entrainment processes by evaluating luminescence signals total photon counts from lahar sediments using blue luminescence signals blsl a portable osl reader that analyzes several grams of polymineral and polygrain size samples was applied we use data from three lahars in the tenenepanco and huiloac gorges on popocatepetl volcano mexico to elucidate the ways in which lahars may gain lose and transport sediment during flow sediment samples for luminescence analysis were taken through the full thickness of the lahar deposits at eight different sites for two lahars 1997 and 2001 and at one site for a lahar that occurred 500 yrs ago mean luminescence values obtained for the 1997 lahar help to evaluate the relationship between the lahar entrainment processes and the drainage at the different sites for the 2001 lahar this relationship was also established taking into account the channel slope the main conclusion is that luminescence signals total photon counts can be used to detect entrained material in the flow because the bulked materials modify the final osl signals the application of this type of luminescence analysis thus has the potential to provide a better understanding of sediment entrainment in these sediment laden flows

the impact of the cretaceous tertiary bolide on evaporite terrane and generation of major sulfuric acid aerosol abstract impact glasses from the k t boundary in haiti include high ca glasses with up to 1 wt so3 formed by the fusion of anhydrite or gypsum rich evaporite sediments in the presence of high silica melts derived from melting of continental crust experimental studies have duplicated these two melts by fusion of gypsum and andesite and suggest a formation temperature of 1300 c geochemical evidence from the glasses is consistent with their derivation from the 180 km diameter chicxulub impact crater on the yucatan peninsula mexico situated in a thick late cretaceous evaporite succession this large diameter crater is most likely the result of a cometary impact on the basis of bolide scaling from the ir anomaly in k t boundary sediments the sulfur degassing associated with formation of the high ca glass alone could have formed a2 1016 g h2so4 stratospheric aerosol however the total sulfur degassing from the evaporite rich sediments in the chicxulub impact site may have led to formation of3 8 1018 to1 3 1019 g sulfate aerosol and global atmospheric mass loading from the sulfate aerosol alone is estimated to beof the order of 1 2 6 g cm 2 combined with the great optical depth estimates of pollack et al 1 and covey et al 2 resulting from an impact dust cloud the sulfate aerosol may have contributed not only to a rapid decline in global surface temperatures to near freezing in about one week but also prolonged the cooling for several years because of the time dependent conversion of so2 to h2so4 aerosol in the atmosphere the contribution of sulfur to acid rain following the k t boundary event may have equalled the proposed rainout of nitric acid

comet showers as a cause of mass extinctions if at least some mass extinctions are caused by impacts why do they extend over intervals of one to three million years and have a partly stepwise character the solution may be provided by multiple cometary impacts astronomical geological and palaeontological evidence is consistent with a causal connection between comet showers clusters of impact events and stepwise mass extinctions but it is too early to tell how pervasive this relationship may be

terrestrial impact structures impact structures are the dominant landform on planets that have retained portions of their earliest crust the present surface of the earth however has comparatively few recognized impact structures this is due to its relative youthfulness and the dynamic nature of the terrestrial geosphere both of which serve to obscure and remove the impact record although not generally viewed as an important terrestrial as opposed to planetary geologic process the role of impact in earth evolution is now receiving mounting consideration for example large scale impact events may have been responsible for such phenomena as the formation of the earth s moon and certain mass extinctions in the biologic record the importance of the terrestrial impact record is greater than the relatively small number of known structures would indicate impact is a highly transient high energy event it is inherently difficult to study through experimentation because of the problem of scale in addition sophisticated finite element code calculations of impact cratering are gen erally limited to relatively early time phenomena as a result of high com putational costs terrestrial impact structures provide the only ground truth against which computational and experimental results can be com pared these structures provide information on aspects of the third dimen sion the preand postimpact distribution of target lithologies and the nature of the lithologic and mineralogic changes produced by the passage of a shock wave they also provide data for cratering rate estimates and in some cases on the nature of the impacting body in this review emphasis is placed on the nature of terrestrial impact structures the criteria for their identification and their contribution to

fault steps and the dynamic rupture process 2 d numerical simulations of a spontaneously propagating shear fracture fault steps may have controlled the sizes of the 1966 parkfield 1968 borrego mountain 1979 imperial valley 1979 coyote lake and the 1987 superstition hills earthquakes this project investigates the effect of fault steps of various geometries on the dynamic rupture process we have used a finite difference code to simulate spontaneous rupture propagation in two dimensions we employ a slip weakening fracture criterion as the condition for rupture propagation and examine how rupture on one plane initiates rupture on parallel fault planes the geometry of the two parallel fault planes allows for stepover widths of 0 5 to 10 0 km and overlaps of 5 to 5 km our results demonstrate that the spontaneous rupture on the first fault segment continues to propagate onto the second fault segment for a range of geometries for both compressional and dilational fault steps a major difference between the compressional and dilational cases is that a dilational step requires a longer time delay between the rupture front reaching the end of the first fault segment and initiating rupture on the second segment therefore our dynamic study implies that a compressional step will be jumped quickly whereas a dilational step will cause a time delay leading to a lower apparent rupture velocity we also find that the rupture is capable of jumping a wider dilational step than compressional step

distinguishing shocked from tectonically deformed quartz by the use of the sem and chemical etching abstract multiple sets of crystallographically oriented planar deformation features pdfs are generated by high strain rate shock waves at pressures of 12 gpa in naturally shocked quartz samples on surfaces pdfs appear as narrow 50 500 nm lamellae filled with amorphosed quartz diaplectic glass which can be etched with hydrofluoric acid or with hydrothermal alkaline solutions in contrast slow strain rate tectonic deformation pressure produces wider semi linear and widely spaced arrays of dislocation loops that are not glass filled etching samples with hf before examination in a scanning electron microscope sem allows for unambiguous visual distinction between glass filled pdfs and glass free tectonic deformation arrays in quartz this etching also reveals the internal pillaring often characteristic of shock induced pdfs this technique is useful for easily distinguishing between shock and tectonic deformation in quartz but does not replace optical techniques for characterizing the shock features

the bright spot of an earthquake abstract umeda y 1992 the bright spot of an earthquake in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 13 22 field surveys in the source region for some large shallow earthquakes reveal that there is a special spot characterized by high acceleration aftershock gap and parallel faults system this spot is called an earthquake bright spot which occupies a relatively small region compared to the final rupture region at least two kinds of phases p1 and p2 which are responsible for the rupture initiation and the bright spot formation can be identified on the broad band seismograms for large earthquakes duration times of preliminary waves p1 p2 are proportional to the earthquake magnitudes by using this evidence from the field and seismograms a model having three rupture stages is proposed for a growth process of large earthquakes the 1st stage is an initiation and propagation of rupture the 2nd stage is formation of the bright spot which produces the high frequency seismic waves in the 3rd stage smooth dislocations again propagate in bi laterally and radiate the long period seismic waves

late cretaceous and paroxysmal cretaceous tertiary extinctions the various geological signatures at cretaceous tertiary time including iridium and other associated elements microspherules and shock deformation features are compatible with the suggestion that the transition is marked by a period of intense volcanism the volatile emissions from this volcanism would lead to acid rain reduction in the alkalinity and ph of the surface ocean global atmospheric temperature changes and ozone layer depletion these environmental effects coupled with those related to the major sea level regression of the late cretaceous provide the framework for an explanation of the selective nature of the observed extinction record

shocked quartz at the triassic jurassic boundary in italy quartz grains that appear to have been shock metamorphosed occur within three closely spaced shale beds from the uppermost triassic rhaetian calcare a rhaetavicula in the northern apennines of italy the upper shale coincides with the abrupt termination of the distinctive uppermost triassic rhaetavicula fauna and is overlain by the hettangian lower jurassic calcare massiccio no extinctions appear to be associated with the two lower layers which occur 1 2 and 2 4 meters below the boundary shale approximately 5 to 10 of the quartz grains within these layers exhibit one or more sets of planar deformational features whose orientations cluster around the rational crystallographic planes basal and most commonly observed in shocked quartz textural and stratigraphic observations support an interpretation of at least three closely spaced impacts at the end of the triassic

the cretaceous tertiary biotic transition mass extinctions are recognized through the study of fossil groups across event horizons and from analyses of long term trends in taxonomic richness and diversity both approaches have inherent flaws and data that once seemed reliable can be readily superseded by the discovery of new fossils and or the application of new analytical techniques herein the current state of the cretaceous tertiary k t biostratigraphical record is reviewed for most major fossil clades including calcareous nannoplankton dinoflagellates diatoms radiolaria foraminifera ostracodes scleractinian corals bryozoans brachio pods molluscs echinoderms fish amphibians reptiles and terrestrial plants macrofossils and palynomorphs these reviews take account of possible biasing factors in the fossil record in order to extract the most comprehensive picture of the k t biotic crisis available results suggest that many faunal and floral groups ostracodes bryozoa ammonite cephalopods bivalves archosaurs were in decline throughout the latest maastrichtian while others diatoms radiolaria benthic foraminifera brachiopods gastropods fish amphibians lepidosaurs terrestrial plants passed through the k t event horizon with only minor taxonomic richness and or diversity changes a few microfossil groups calcareous nannoplankton dinoflagellates planktonic foraminifera did experience a turnover of varying magnitudes in the latest maastrichtian earliest danian however many of these turnovers along with changes in ecological dominance patterns among benthic foraminifera began in the latest maastrichtian improved taxonomic estimates of the overall pattern and magnitude of the k t extinction event must await the development of more reliable systematic and phylogenetic data for all upper cretaceous clades

changes in whangaehu river lahar characteristics during the 1995 eruption sequence ruapehu volcano new zealand abstract during the 1995 ruapehu eruptive sequence multiple lahars occurred in the whangaehu river which drains ruapehu s crater lake during the earlier phreatic and phreatomagmatic eruptions lahars were generated by expulsion of waters from the lake but once the lake had emptied lahars were formed by remobilisation of seasonal snowpack laden with saturated freshly erupted tephra four types of lahars occurred during the eruptive sequence 1 initial snow slurry lahars composed of granular snow and ice incorporated by eruptively expelled crater lake waters which left behind frozen deposits with 2 5 20 clastic sediment 2 large dilute lahars generated as the volumes of ejected lake water increased and removed much of the readily available snow at least one third of the pre eruption crater lake volume was expelled during one day producing the largest lahars of the series these lahars were hyperconcentrated flows for up to 84 km from source leaving extensive deposits along the channel margins 3 concentrated lahars smaller volume lahars generated as the frequency of eruptions and volumes of expelled water declined these lahars were able to maintain high sediment concentrations measured at 46 52 by volume suspended sediment at 42 km from source their high sediment concentrations were maintained by erosion and incorporation of sand from the deposits of earlier flows which were lining the channel margins 4 remobilised tephra lahars generated following the two largest tephra eruptions of the sequence seasonal snowpack was covered by water saturated tephra warmer spring temperatures and heavy rainfall events caused collapse and remobilisation of snow and tephra producing several lahars in catchments draining eastern ruapehu

initiation and flow behavior of the 1980 pine creek and muddy river lahars mount st helens washington two large high velocity lahars volcanic debris flows were triggered by a pyroclastic surge during the first few minutes of the may 18 1980 eruption of mount st helens the initial surge cloud evolved progressively by gravity segregation from a gas mobilized highly inflated density flow to a dense water mobilized basal debris flow lahar and accompanying ash cloud as it flowed down the east flank of the volcano the main source of the water for the lahars was probably from eroded snow and ice incorporated into the flow by turbulent mixing but ground water expelled together with the rock debris by the initiating volcanic explosions also may have contributed peak lahar discharge from the pine muddy fan upper smith creek and ape canyon probably exceeded 250 000 m 3 s initially but decreased exponentially in the downstream direction total volume of the lahars was in excess of 1 4 10 7 m 3 initial peak flow velocities in excess of 30 m s also decreased markedly downstream where flow was not impeded velocity was strongly related to the depth slope term r 2 3 s 1 2 from the manning uniform flow equation as a power law function during much of the route traveled lahar flow appears to have been supercritical deposits left in channels were generally thin relative to flow depth 0 to 2 5 m particles up to small boulder size were randomly distributed in the poorly sorted nonstratified matrix indicating complete suspension in a fully developed debris flow slurry however much larger clasts were transported as bed load computed sediment concentrations of matrix slurry samples ranged from 84 to 91 solids by weight and were similar for the two lahars two indirect methods for computing peak flow velocity previously only tentatively applied to debris flows were tested for accuracy by comparing computed lahar arrival times with recorded arrival times at swift reservoir the computed velocities appear to be 15 slower than the recorded velocities which is consistent with the restriction that the velocity formulas produce only minimum values

reconstructing 800 years of historical eruptive activity at popocatepetl volcano mexico pictorial and written documents spanning 800 years were analyzed for information about historical eruptions at popocatepetl volcano these documents were prepared by several indigenous groups as well as by the spanish conquistadors and missionaries during their military campaigns and long term evangelization and colonization and later on by indian nobles and spanish historians pre columbian drawings show flames coming out of popocatepetl s crater while later descriptions from the spanish colonial period in mexico 1521 to 1821 refer to ash emission and ballistics lahars and some pumice falls similar to what were depicted in the thirteenth to sixteenth century drawings graphic information from the pre columbian codices colonial maps and paintings referring to the eruptions were correlated with historical accounts and religious chronicles thereby leading to the reconstruction of a more detailed sequence of eruptive events from such information it was possible for us to prepare ash distribution maps for the 1540 1592 and 1664 eruptions most of the known historical eruptions seem to be similar to those that have been occurring at popocatepetl since 1994 indicating the importance of ash emission and crater dome formation throughout its recent eruptive history the strongest eruptions occurred in 1510 1519 1540 1580 1664 and 2001 these produced widespread ash falls that affected both populated and rural areas duration of eruptive episodes during the past 800 years were estimated to have ranged from less than a year to more than 30 years separated by repose periods ranging between 7 and over 100 years

glacial retreat and its geomorphologic effects on mexico s active volcanoes 1994 95 the northern slopes of mexico s great active stratovolcanoes popocatepetl 5450 m a s l and pico de orizaba 5700 m a s l are covered by glaciers this paper analyzes the retreat of the glaciers between february 1994 and october 1995 and its geomorphological consequences the results discussed are a continuation of research conducted on the volcanoes up to 1994 although the glaciers retreated a great deal during the 1980s and early 1990s the process accelerated from 1994 to 1995 the heavy erosion that had been observed on the deglaciated walls of the volcanoes during these decades increased as the glaciers retreated sediments originating from material loosened from the exposed lateral walls added to the supraglacial layer temperature readings taken from boreholes made in the glaciers indicated that ablation had been reduced as a result of the insulating effects of the debris mantle this allowed widespread formation of deposits that buried ice in areas where the glacier had already disappeared these results confirmed the conclusions of earlier studies that deglaciation of stratovolcanoes has resulted in the acceleration of slope processes and the formation of buried ice deposits

volcanism around k t boundary time its role in an impact scenario for k t extinction events abstract a widespread debate on the cretaceous tertiary k t boundary 65 1 ma extinctions has largely revolved around an impact or a volcanic cause evidence for both events is strengthened by recent work impact triggered deccan volcanism or independent impact and mantle plume events have been suggested this review examines evidence for both events and suggests that they overlapped in time but with globally uneven effects the combined effects produced generally inhospitable k t environments leading to mass extinctions although some life forms flourished afterwards volcanic contributions to the k t boundary environment probably include increased platinum group element pge and acid rain fall out particularly from large scale mantle upwellings e g deccan cameroon and coral sea events a previously proposed post k t greenland starting plume apparently already existed in the cretaceous k t features such as the iridium ir anomaly most likely include longer term volcanic as well as sudden meteoritic impact contributions volcanic contributions were greatest from southern hemisphere sources whereas meteorite impacts featured in the northern hemisphere

high amplitude seismic waves radiated from the bright spot of an earthquake abstract in the source region of a large shallow earthquake there is likely to be a small spot characterized by the production of high accelerations and a gap in the aftershocks on the seismograms of most large shallow earthquakes and on some seismograms of moderate main shocks two types of p waves may be detected the relatively low amplitude waves following phase p 1 and the predominant high frequency waves following phase p 2 the duration of the seismic waves following phase p 1 the p 1 p 2 time increases proportionately with the earthquake magnitude this fact strongly suggests that the preliminary rupture period is closely related to the radiation of the predominant high frequency seismic waves to explain the above observation the shear strain waves radiated from a moving rupture are calculated because the seismic wave velocity is generally higher than the rupture propagation velocity maximum shear strains by seismic waves usually precede the rupture front if the total shear strain which is equal to the static pre earthquake shear strain plus dynamic shear strain excited by seismic waves exceeds the critical shear strain 10 4 the secondary ruptures break out in the forward direction from the initial rupture the rupture processes of large or moderate earthquakes occur in two stages one is the initiation of rupture corresponding to phase p 1 and the other is the development of secondary ruptures triggered by seismic shear strain waves the latter process generates an earthquake bright spot which radiates high amplitude seismic waves

using tree ring analysis to evaluate intra eruptive lahar activity in the nexpayantla gorge popocatepetl volcano central mexico abstract lahar activity is one of the most important processes capable of mobilizing large volumes of sediment and producing devastating effects on human populations and infrastructure estimating the frequency distribution and climatic controls on lahar activity is necessary for understanding how sediment is mobilized from hillslopes to channels and how the landscape is modified by this type of process in this study we evaluate the intra eruptive lahar activity in the nexpayantla gorge nw of popocatepetl volcano central mexico by using a dendrogeomorphological analysis of growth ring records of abies religiosa trees n 100 based on the identification of 322 growth disturbances gd and field evidences we identify 23 intra eruptive lahars that occurred from 1959 to 2013 by analyzing the lahar chronology and climatic data we observed that 83 of lahar activity coincides with moderate to strong cold and dry phases of el nino southern oscillation based on the seasonal timing of gd we explored the intra annual lahar activity we found that 43 of the lahars occurred in the latewood season from july to october when the mean summer monthly rainfall is 120 mm approximately 37 of lahars occurred during the first heavy rainfalls from march to june which corresponds to the earlywood season the remaining 20 of the lahar activity corresponds to the dormancy period winter season when snowmelt and or occasional strong rainfalls 100 mm day 1 occur the gd indicate that in nexpayantla gorge intra eruptive lahar activity takes place around every two years we also found that the effects of lahars are concentrated around the transitional and depositional zone of nexpayantla gorge