three dimensional inversion of large scale earthscope magnetotelluric data based on the integral equation method geoelectrical imaging of the yellowstone conductive mantle plume 1 interpretation of the earthscope mt magnetotelluric data requires the development of a large scale inversion method which can address two common problems of 3d mt inversion computational time and memory requirements we have developed an efficient method of 3d mt inversion based on an ie integral equation formulation of the mt forward modeling problem and a receiver footprint approach implemented as a massively parallel algorithm this method is applied to the mt data collected in the western united states as a part of the earthscope project as a result we present one of the first 3d geoelectrical images of the upper mantle beneath yellowstone revealed by this large scale 3d inversion of the earthscope mt data these images show a highly conductive body associated with the tomographically imaged mantle plume like layer of hot material rising from the upper mantle toward the yellowstone volcano the conductive body identified in these images is west dipping in a similar way to a p wave low velocity body

eruptive processes and caldera formation in a nested downsagcollapse caldera cerro panizos central andes mountains abstract the cerro panizos ignimbrite center in the central andes mountains produced two ignimbrite sheets and many lava flows the ignimbrite of quebrada cienago was erupted at 7 9 ma and effusive eruptions continued until the two cooling units of the crystal rich dacitic cerro panizos ignimbrite were emplaced at 6 7 ma the lower unit has no laterally continuous flow breaks and was erupted from a single vent or small cluster of vents with limited fluctuation in discharge lithic fragments reach significant concentrations 5 only in the uppermost two meters of this cooling unit where they document vent wall collapse or the opening of a new vent the upper cooling unit contains many flow units with variations in welding thicknesses and lithic fragment concentrations implying an unsteady eruption column the opening of many vents and probable caldera collapse triangulation of anisotropy of magnetic susceptibility flow direction measurements locate a single vent for the lower cooling unit whereas the upper cooling unit had many vents within the present dome cluster a 15 km diameter topographic depression marked by inward dips of 4 8 at the cooling unit contact is centered on the vent area of the lower cooling unit the depression is interpreted as a downsag caldera formed during emplacement of the lower cooling unit collapse began late in the eruption of the lower cooling unit and continued through the emplacement of the upper cooling unit resurgent magmatism occurred as lava flows that mundated the caldera area a ring of dacite domes erupted until at least 6 1 ma in the northern half of the downsag caldera traces the margin of a collapse caldera associated with the upper cooling unit maximum caldera subsidence 353 km 3 is not enough to account for the erupted volume 652 km 3 dre minimum the ash poor crystal rich nature up to 50 crystals in the pumice 75 in the matrix of the cerro panizos pyroclastic flows resulted in poor retention of gases and there is little evidence of fluidization the high crystal content however favored modified grain flow processes in which several particle support mechanisms combine with grain dispersive forces it is postulated that the flows were initially partially fluidized but rapidly lost their gases and grain flow processes grew in relative importance grain flow processes destroyed evidence of fluidization before deposition

younger dryas interval and outflow from the laurentide ice sheet a boxmodel of the great lakes is used to estimate meltwater flow into the north atlantic between 8000 and 14 000 calendar years b p controls on the model include the oxygen isotopic composition of meltwaters and lake waters as measured in the shells of ostracodes outflow rates are highest when oxygen isotopic values of the lake waters are most negative denoting a maximum glacial meltwater component flow rates reach maximum values before the onset of the younger dryas and after it ends these maxima appear to be correlative with the major meltwater pulses mwp 1a and 1b although the resumption of north atlantic deep water formation may be tied to the reduction in ice sheet melting neither the onset nor the end of the younger dryas as recorded in the greenland ice sheet project gisp2 records appear tied to maxima in meltwater outflow from the laurentide ice sheet

response of interior north america to abrupt climate oscillations in the north atlantic region during the last deglaciation several broad scale climatic oscillations during the last deglaciation are well documented in regions around the north atlantic ocean this paper reviews empirical evidence for these deglacial climatic oscillations from non coastal north america and discusses implications for testing climatic simulations and for understanding the cause and transmittal mechanisms paleoclimatic interpretation of oxygen isotope records from several small sites in the eastern great lakes region indicates a classic deglacial climatic sequence that is comparable with records from europe and greenland the climatic events as interpreted from crawford lake oxygen isotopes include the bolling allerod boa warming at 12 700 14c bp a warm boa at 12 500 10 920 14c bp an intra allerod cold period shortly before 11 000 14c bp a cold younger dryas yd climate reversal at 10 920 10 000 14c bp the holocene warming at 10 000 14c bp a brief preboreal oscillation pb at 9650 14c bp and an early holocene cold event at 7500 14c bp 8200 cal bp some of these events were also evident from changes in upland and aquatic vegetation and sediment lithology the pronounced yd climatic reversal has been documented from pollen records along the ecotones at this time and from glacier readvances in the great lakes region along the rocky mountains from alberta to colorado the yd event is indicated by alpine glacier advance and or shift in timberline vegetation in minnesota and upper new york early holocene climatic instability is also suggested by oxygen isotope records and or varve thickness the regional variations in evidence for the yd and other events in north america suggest that climatic oscillations may have different expressions in paleo records depending on geographic location and characteristics of a particular site the extent and magnitude of these climatic oscillations across north america suggest that these oscillations are an expression of climatic change that was probably widespread rather than locally induced by a nearby glacier the location of these sites implies that climatic signals were likely carried over the northern hemisphere through the atmosphere as indicated by general circulation models we hypothesize that the lack of evidence for a cold yd in interior north america west of the great lakes region and east of the rocky mountains was caused by the trapping of cold arctic air mass north of the laurentide ice sheet and by uninterrupted northward expansion of warm caribbean air this strongly contrasted climate also produced non analogous biological assemblages

the pattern of circumferential and radial eruptive fissures on the volcanoes of fernandina and isabela islands galapagos maps of the eruptive vents on the active shield volcanoes of fernandina and isabela islands galapagos made from aerial photographs display a distinctive pattern that consists of circumferential eruptive fissures around the summit calderas and radial fissures lower on the flanks on some volcano flanks either circumferential or radial eruptions have been dominant in recent time the location of circumferential vents outside the calderas is independent of caldera related normal faults the eruptive fissures are the surface expression of dike emplacement and the dike orientations are interpreted to be controlled by the state of stress in the volcano very few subaerial volcanoes display a pattern of fissures similar to that of the galapagos volcanoes some seamounts and shield volcanoes on mars morphologically resemble the galapagos volcanoes but more specific evidence is needed to determine if they also share common structure and eruptive style

late quaternary seismo stratigraphy of lake wanapitei sudbury ontario canada arguments for a possible meteorite impact origin abstract lake wanapitei 132 75 km 2 fills what has been identified as an eocene c 37 ma meteorite impact basin in the canadian shield near sudbury ontario canada the area was glaciated many times during the pleistocene and the basin lies immediately north of the prominent cartier moraine built during the last glaciation by the retreating laurentide ice sheet some 11 000 years ago study of the deeper geophysics of the basin using magnetic and gravity techniques and confirmation of its origin is hampered by lack of data regarding water depths the form of the bedrock surface and the thickness and character of glacial and postglacial sediment to this end more than 300 km of high resolution single channel seismic chirp and 200 khz bathymetric data were collected from the basin in the summer of 2002 water depths reach a maximum of 118 m and acoustic basement is defined by a glacially scoured bedrock surface the overlying pleistocene sediment fill exceeds 35 m in thickness and consists of a lowermost late glacial succession of rhythmically laminated silty clays deposited when the basin was flooded by a deep and regionally extensive ice dammed water body glacial lake algonquin truncation of the upper surface of this succession across large parts of the lake floor records the drainage of lake algonquin and the isolation of wanapitei lake as a separate water body overlying holocene sediment is up to 10 m thick but is markedly discontinuous and commonly occurs as mounded drifts reflecting strong bottom currents and low inputs of modern sediment the presence of apparently undisturbed precambrian bedrock below large portions of the lake basin places significant constraints on the dimensions of any meteorite impact structure

seismic stratigraphy of lake huron georgian bay and postglacial lake level history five seismic reflectors in the basinal sedimentary section have been identified as representing erosional surfaces in shoaler regions of the lake basins and can be traced throughout most of the study area the reflectors representing these erosional surfaces and their conformable equivalents in the deep lake basins are caused by intervals of coarser grain size in the sediments and are thought to represent lowering of lake levels prior to and following the main algonquin highstand about 10 2 11 0 ka these reflectors are used to define seismic sequences in the postglacial sedimentary fill of the basins two additional erosional surfaces which closely follow the initial fall of the main algonquin lake level are most clearly seen in thicker sections found in the northern part of the basins the two oldest pre main algonquin sequences appear to onlap highs and fill in lows within the basins whereas the younger sequences associated with the main algonquin through mattawa times tend to drape the

lithic breccia and ignimbrite erupted during the collapse of crater lake caldera oregon abstract the climactic eruption of mount mazama 6845 y b p vented a total of 50 km3 of compositionally zoned rhyodacitic to basaltic magma from a a single vent as a plinian pumice fall deposit and the overlying wineglass welded tuff and b ring vents as ignimbrite and coignimbrite lithic breccia accompanying the collapse of crater lake caldera new field and grain size data for the ring vent products are presented in this report the coarse grained poorly bedded clast supported lithic breccia extends as far as 18 km from the caldera center like the associated ignimbrite the breccia is compositionally zoned both radially and vertically and silicic mixed and mafic types can be recognized based on the proportion of rhyodacitic pumice matrix fractions in silicic breccias are depleted of fines and are lithic and crystal enriched relative to silicic ignimbrite due to vigorous gas sorting during emplacement ignimbrite occurs as a proximal veneer deposit overlying the breccia a medial 8 to 25 km from the caldera center compositionally zoned valley fill as much as 110 m thick and an unzoned distal 20 km facies which extends as far as 55 km from the caldera breccia within 9 km of the caldera center is interpreted as a coignimbrite lag breccia formed within the deflation zone of the collapsing ring vent eruption columns expanded pyroclastic flows of the deflation zone were probably vertically graded in both size and concentration of blocks as recently postulated for some turbidity currents an inflection in the rate of falloff of lithic clast size within the lithic breccia at 9 km may mark the outer edge of the deflation zone or may be an artifact of incomplete exposure the onset of ring vent activity at mt mazama was accompanied by a marked increase in eruptive discharge pyroclastic flows were emplaced as a semicontinuous stream as few ignimbrite flow unit boundaries are evident as eruption from the ring vents progressed flow runout distance and the extent of breccia deposition decreased due to a greater internal flow friction and b decreasing eruption column heights effect b probably resulted from a progressive decrease in magmatic gas content and discharge rate waning discharge may have been promoted by the tapping of more viscous crystal rich magma collapse of conduit walls and declining caldera collapse rate

structure of la primavera caldera jalisco mexico deduced from gravity anomalies and drilling results abstract previous studies of la primavera caldera have mostly been based on surface geology and topography since 1980 many wells exploring for geothermal energy have reached depths of about 2 to 3 km at the center of the caldera the results of the drillings together with those of the gravity surveys provide information about the subsurface structure of the caldera and shed light on its formation the drilling results and gravity anomalies at la primavera caldera and san marcos located at about 40 km distance from the caldera suggest that regional gravity anomalies can be interpreted in terms of depths of the granitic basements the basement beneath la primavera caldera is about 3 km deep and consists of roughly the same horizon as that beneath san marcos the drilling results within the caldera reveal that the depth of the caldera fills ranges from 0 3 to 1 km at the drilling sites the andesite basement about 1 km deep remains approximately horizontal and the granitic basement has a depth of about 3 km the surface topographies such as the postcaldera domes scarcely disturb the subsurface strata the local gravity anomalies show two lows within the caldera reflecting the configuration of caldera bottom two funnel shaped depressions one of which corresponds to a vent of the tala tuff deduced from geological observations the mass deficiency within the caldera estimated from the gravity anomaly satisfies the general relationship that the mass deficiency is proportional to the caldera diameter cubed this means that caldera structure is three dimensional the larger the diameter the deeper the funnel shape at present this argument may be limited to funnel shaped calderas

evolution of the caldera forming eruption at crater lake oregon indicated by component analysis of lithic fragments crater lake caldera 8 10 km formed 6845 years b p 14c age during the climactic eruption of the volcanic edifice known as mount mazama is intermediate in size between small calderas associated with central vent eruptions and large calderas that have ring fracture vent systems our quantitative study of lithic fragments in the ejecta confirms the existing model of changes in vent configuration during the climactic eruption of mount mazama initial activity was from a single vent that produced a rhyodacite pumice fall from a plinian column altered preexisting volcanic rocks are the predominant lithic type in the plinian deposit and their extensive hydrothermal alteration is considered as evidence of their relatively deep origin the wineglass welded tuff lies atop the plinian deposit and contains a higher proportion of fresh volcanic rocks suggesting enlargement of the single vent by slumping of its walls this same vent enlargement caused the plinian eruption column to collapse and feed valley hugging pyroclastic flows that deposited the wineglass welded tuff when enough material was erupted from the shallow magma chamber that its roof was no longer adequately supported mount mazama collapsed to form the caldera while highly energetic pyroclastic flows produced the climactic ignimbrite a lag breccia that represents the proximal facies of the compositionally zoned climactic ignimbrite lies atop the wineglass welded tuff and contains predominantly altered volcanic rocks of deeper origin accompanied by minor granitoids from the magma chamber walls azimuthal differences in lithic component proportions in the lag breccia correlate well with the geology of the caldera walls indicating that the climactic ignimbrite was ejected by multiple vents along a ring fracture system systematic lithic component changes within the lag breccia suggest different quarrying levels that reflect waxing and waning of the discharge rate during the caldera collapse phase of the climactic eruption our lithic component analysis demonstrates that calderas that may be too small to experience structural resurgence such as crater lake nevertheless may form by syneruptive subsidence along ring fractures

the lake superior varve stratigraphy and implications for eastern lake agassiz outflow from 10 700 to 8900 cal ybp 9 5 8 0 14c ka abstract glaciolacustrine rhythmites within sediment cores from lake superior record the regional recession of the laurentide ice sheet lis from 10 700 to 8900 cal ybp ca 9 5 8 0 14 c ka lis retreat from superior opened eastern lake agassiz outlets so that the rhythmites reflect the combined impacts of sediment laden meltwater and lake agassiz discharge multiple rhythmite stratigraphies a time series analysis of the thickness measurements and high resolution inorganic carbonate data demonstrate that this is an annual record varved the varve thickness records primarily document regional ice margin dynamics correlative thick varve sequences at 9100 cal ybp 8 1 14 c ka and 10 400 10 200 cal ybp 9 2 9 0 14 c ka record two periods of enhanced glaciofluvial discharge most likely moraine formation the nakina and nipigon general varve cessation is associated with the circumvention of lake agassiz and glacial meltwater into lake ojibway at 9040 cal ybp 8 1 14 c ka although adjacent to the inlets from lake nipigon rhythmic sedimentation persisted for 200 years positively identifying lake agassiz catastrophic discharge events remains speculative but seems feasible following retreat of marquette ice that had re advanced to fill the basin the initial influx of lake agassiz water is expected at around 10 600 cal ybp 9 4 14 c ka but at this time most of northeastern lake superior was covered by ice three sets of thick thin varves in western lake superior perhaps record influxes of lake agassiz at around 10 630 10 600 and 10 570 cal ybp 9 4 14 c ka varve formation in superior coincides with high lake levels in lake huron suggesting that high lake levels in huron correspond to periods of high agassiz and or meltwater flow into lake superior

marine sediment classification using the chirp sonar the chirp sonar is a calibrated wideband digital fm sonar that provides quantitative high resolution low noise subbottom data in addition it generates an acoustic pulse with special frequency domain weighting that provides nearly constant resolution with depth the chirp sonar was developed with the objective of remote acoustic classification of seafloor sediments in addition to producing high resolution images the calibrated digitally recorded data are processed to estimate surficial reflection coefficients as well as a complete sediment acoustic impulse profile in this paper surficial sediments in narragansett bay ri are used to provide ground truth for an acoustic model quantitative acoustic returns from the chirp sonar are used to estimate surficial acoustic impedance and to predict sediment properties a robust acoustic sediment classification model that uses core samples to account for the local depositional environment has been developed the model uses an estimate of acoustic impedance t

ice melt collapse pits and associated features in the 1991 lahar deposits of volcan hudson chile criteria to distinguish eruption induced glacier melt in subaerial volcaniclastic sequences structures formed by ice blocks can provide information about a volcano s history of lahar generation by glacier melt at volcan hudson in chile catastrophic lahars were initiated by eruption induced melting of glacier ice in august and october 1991 they transported large ice blocks 50 km down the rio de los huemules valley to the sea large current crescents with lee side lenses were formed where ice blocks were deposited during waning stages of the flood when stranded blocks of ice melted they left cone shaped and ring shaped heaps of ice rafted debris on the sediment surface several hundred ice blocks were completely buried within the aggrading lahar sediment and when these melted circular collapse pits formed in the sediment collapse types included subsided coherent blocks of sediment bounded by an outward dipping ring fracture trapdoor structures with horseshoe shaped fractures downsag pits with centroclinal dips locally up to 60 pits with peripheral graben and crevasses piecemeal highly fragmented collapse structures and funnel shaped pits containing disaggregated sediment a sequence of progressive collapse is inferred in which initial downsag and subsidence on an outward dipping ring fracture produces a small diameter pit this is followed by widening of the pit by progressive development of concentric ring fractures and downsag outside the early formed pit and by collapse of overhanging pit walls to produce vertical to inward dipping walls and aprons of collapse debris on the pit floor the various structures have potential for preservation even in regions prone to high rainfall and flooding and they can be used to indicate that former lahars contained abundant blocks of ice

lake level reconstruction and paleohydrology of birch lake central alaska based on seismic reflection profiles and core transects lake level history for birch lake alaska was reconstructed using seismic profiles and multiproxy sedimentary analyses including sedimentology geochemistry magnetic susceptibility and palynology twenty two seismic profiles 18 km total and eight sediment cores taken from the lake margin to its depocenter at 13 5 m provide evidence for low lake stands during the late pleistocene and holocene thirty one ams radiocarbon dates of macrofossils and pollen provide a century scale chronology prior to 12 700 14c yr b p the lake which now overflows was either seasonally dry or desiccated for prolonged periods indicating a severe period of aridity lake level rose more than 18 m between 12 700 and 12 200 14c yr b p before falling to 17 m below the level of overflow between 11 600 and 10 600 14c yr b p the water remained between 14 and 17 m below the overflow level onlap sedimentary sequences were formed during a transgression phase between 10 600 and 10 000 14c yr b p between 10 000 and about 8800 14c yr b p the lake was between 6 and 9 m below the overflow level lake level again rose approaching the overflow level between 8800 and 8000 14c yr b p seismic and core evidence of minor erosional events suggest lowstands of 2 6 m until 4800 14c yr b p there have been no prolonged periods of lake level depression since that time the major restructuring of the climate system during deglaciation evidently generated a complex set of fluctuations in effective moisture in interior alaska which likely affected eolian processes and vegetation development as well as lake levels

glacial meltwater in lake huron during early postglacial time as inferred from single valve analysis of oxygen isotopes in ostracodes abstract 18o measurements of benthic ostracodes are used to reconstruct the 18o history of lake huron and georgian bay water between 10 600 and 7600 14c yr b p this 18o record was calibrated using a comparison of the 18o values of modern ostracodes and lake huron water where a fractionation of 1 0358 was measured between the oxygen isotope ratios of the most isotopically positive ostracode candona subtriangulata and lake water the most positive shell 18o value was used because it is precipitated in the cold 0 to 2 c water common to both deep and shallow environments the 18o of lake huron water reflects a large glacial meltwater component for much of its history before 7600 14c yr b p times of low lake level correlate with the highest ratio of glacial meltwater to local precipitation in the basin most negative 18o values georgian bay water was more negative in 18o than lake huron water of the same age this reflects a higher proportion of glacial meltwater in georgian bay and its separation from lake huron during times of low lake level

sediment sequences and palynology of outer south bay manitoulin island ontario connections to lake huron paleohydrologic phases and upstream lake agassiz events abstract south bay on the southern coast of manitoulin island is a fjord like embayment connected to lake huron by a natural narrow gap in the bay s outer sill 6 5 14 m above the lake a seismic profile pollen plant macrofossil grain size analyses and other sediment properties of two piston cores from a shallow outer basin of the bay document a 9 m thick sediment section comprising rhythmically laminated clay under silty clay containing zones with small molluscan shells and marsh detritus a sandy pebbly layer under soft silty clay mud overlies these sediments this stratigraphy represents inundation by deep glacial lake algonquin followed by the shallowing post algonquin series of lakes and exposure in the early holocene by 5 lake stanley lowstands in the lake huron basin separated by 4 lake mattawa highstands overflow from south bay in the first lowstand is thought to have eroded the outer sill gap marsh environments are inferred to have formed in the bay during subsequent lowstands the lake mattawa highstands are attributed to outburst floods mainly from glacial lake agassiz palynological evidence of increased spruce occurrence an apparent regional climate reversal during the dry pine period is attributed to cold northwest winds from the lake superior basin and a lake effect from the mattawa highstands in the lake huron basin lake waters transgressed south bay following the pine period to form the nipissing shore on manitoulin island transfer of lake huron basin drainage to southern outlets and continued glacioisostatic uplift of the region led to the present configuration of south bay and lake huron

late pleistocene and holocene depositional history of sediments in flathead lake montana evidence from high resolution seismic reflection interpretation abstract this paper presents an interpretation of 3 5 khz reflection seismic data together with information from lower frequency seismic data and sediment cores from flathead lake in northwestern montana flathead lake is located at the margin of the cordilleran ice sheet during the last glacial maximum and is the repository for about 160 m of syn and post glacial sediment six seismic stratigraphic units a f are identified in flathead lake above the acoustic basement reflector the deepest unit is interpreted to be late pleistocene glacial till unit a overlain by well layered glaciolacustrine sediments unit b overlying seismic reflectors unit c include slump deposits turbidites fed by meltwater surges associated with retreat of the flathead glacial lobe and homogeneous silty mud preliminary age models suggest that the transition from glacial rhythmites unit b to more homogenous lake sediments unit c occurred approximately between 14 500 and 13 180 cal y b p parallel high amplitude reflectors of the oldest holocene unit unit d drape pleistocene stratigraphy sediment core analyses suggest that these fine sediments were deposited below wave base by suspension settle out in a lake similar to today s the mt mazama tephra 7630 80 cal y b p is interpreted as the top reflector of unit d onlap geometries between units d and e imply a significant drop in lake level before deposition of unit e sedimentation of unit e was confined to several sub basins that were separated by bedrock highs lake level rose gradually during deposition of unit e to form the well developed basal onlap geometry parallel high amplitude reflectors of the youngest holocene sediments unit f drape all of the older stratigraphy and bedrock highs across the lake basin suggesting a late holocene lake highstand

late quaternary lowstands of lake bosumtwi ghana evidence from high resolution seismic reflection and sediment core data results from the first high resolution single channel seismic reflection survey of tropical lake bosumtwi ghana and sedimentological data from a 14c dated sediment piston core were used to revise and extend the basin s late quaternary lake level history we report four seismic sequence boundaries and an exposure surface from a sediment core which are interpreted as erosional surfaces formed at times of drastic low lake level the youngest erosional surface occurs as much as 31 m below present lake level bpll and up to 0 7 m below the present sediment water interface this most recent unconformity observed in the seismic data is interpreted to be coeval with the basin wide late holocene dry period between 0 5 and 1 cal ky bp calendar years before present another exposure surface observed in a sediment core is based on an abrupt contact separating low density wet clay rich sediments from underlying high density compact silt rich and rootlet rich sediments and is interpreted to have developed prior to 16 8 cal ky bp when the lake was 60 m bpll three older erosional surfaces occur at depths of 92 3 102 3 and 107 4 m bpll suggesting numerous lowstands in lake bosumtwi during the late pleistocene by extrapolation of average sedimentation rates 0 41 m ky from the upper 10 5 m of sediment we estimate the ages of these older lowstands to be 65 86 108 cal ky bp the lowstands of lake bosumtwi evidenced from the seismic and sediment core data are interpreted as a response to increased aridity in this part of the equatorial tropics and may correlate to other observed continent wide shifts in african climate over the past 100 ky and possibly to rapid climatic shifts observed at high latitudes determining the precise timing of these lowstands will ultimately reveal much about the drought dynamics of tropical and subtropical africa

outward dipping ring fault structure at rabaul caldera as shown by earthquake locations the locations of a large number of earthquakes recorded at rabaul caldera in papua new guinea from late 1983 to mid 1985 have produced a picture of this active caldera s structural boundary the earthquake epicenters form an elliptical annulus about 10 kilometers long by 4 kilometers wide centered in the southern part of the rabaul volcanic complex a set of events with well constrained depth determinations shows a ring fault structure that extends from the surface to a depth of about 4 kilometers and slopes steeply outward from the center of the caldera this is the first geophysical data set that clearly outlines the orientation of an active caldera s bounding faults this orientation however conflicts with the configuration of many other calderas and is not in keeping with currently preferred models of caldera formation

the hechiceros caldera a recently identified mid tertiary caldera in eastern chihuahua mexico geologic mapping of tertiary volcanic rocks in the sierra hechiceros 50 to 80 km south of big bend national park has delineated a 20 by 25 km mid tertiary caldera herein named the hechiceros caldera this caldera was formed by the eruption of the palomas tuff which has a minimum volume of approximately 170 km3 and is distributed radially around the caldera the caldera was filled with more than 500 meters of postcaldera rhyolitic lavas which overflowed the caldera margin and caused additional subsidence the rocks of the sierra hechiceros are subalkalic and include rhyolites and dacites the chemistry of these rocks is transitional between the calc alkalic province of the sierra madre occidental and the alkalic rocks of the eastern trans pecos magmatic province the potassium argon ages both the palomas tuff and the postcaldera el jobero rhyolite are about 29 0 6 ma

experimental studies of collapse calderas we present experiments to investigate the formation of calderas using inflatable balloons in a medium of fused alumina powder this system provides a model of a magma chamber in an homogeneous elastic media which scales to volcanic systems in terms of geometry and strength of the media three kinds of experiment were performed i the balloon was inflated and then deflated simulating pre emption tumescence and caldera collapse ii the balloon was deflated without prior inflation simulating caldera formation without pre eruption tumescence iii three balloons were inflated and deflated in a line simulating multiple magma chambers the influence of magma chamber shape was investigated using different shaped balloons balloon inflation formed a dome with a surface pattern of shallow polygonal fractures centrally and radial fractures peripherally uplift occurs along deep inward dipping concentric reverse faults deflation generated depressions with an outer set of concentric ring fractures surrounding a central funnel formed by sagging along numerous minor faults some of which formed during doming during collapse the concentric reverse faults were converted to normal faults and the radial normally faulted fractures were converted to reverse faults without prior tumescence an outer set of near vertical ring faults surround a flat central depression due to collapse of a coherent block an inner set of outward dipping reverse ring faults also develop the experimental caldera area increased with balloon size and increased as the balloon depth decreased for a given balloon volume and depth collapse area is significantly smaller if doming had occurred prior to collapse arcuate fractures can provide the pathways for magma ascent during pre emption tumescence smith bailey 1968 but the shallow radial and polygonal fracture systems may also influence vent location changes in fault geometry and sense of movement extension to compression and vice versa occur when subsidence initiates and can account for changes in vent location that occur in many eruptions multiple overlapping collapse calderas are associated with magma chamber migration whereas nested calderas can result from the activity of a single magma chamber

experiments on cauldron formation a polygonal cauldron and ring fractures abstract the sequence of formation of the motojuku type cauldron fujita et al 1970 is summarized as follows 1 doming due to ascent of a magma body 2 development of normal faults which produce a polygonal cauldron and 3 eruption of acidic to intermediate pyroclastic material model experiments on a scale of 1 200 000 reveal that ascent of the magma body which was imitated by a ball of hardened putty produced a polygonal cauldron composed of radial and concentrically arranged short fractures on the roof of a dome no ring fractures were formed this type of cauldron develops near the surface on the other hand emission of magma which was imitated by evaporation of a ball of dry ice in brittle powdered material caused ring faults dipping outward and a circular cauldron without up or downwarping this type of cauldron develops upward from the magma body the former model is equated to the motojuku type cauldron and the latter model to the other types of cauldrons with ring fractures

numerical and experimental models on the formation mechanism of collapse basins during the green tuff orogenesis of japan the geological model about volcanism of the green tuff geosyncline deduced from the field observations consists of the following processes 1 dome shaped uplift with a mean diameter of 30 km 2 collapse of the central part of the domes forming basins with a mean diameter of 10 km 3 volcanic activity inside the collapse basins it is considered that these consecutive processes resulted from the magmatic uplift from a deep part of the crust

the tavua caldera fiji a complex shoshonitic caldera formed by concurrent faulting and downsagging the shoshonitic tavua volcano in viti levu fiji developed from a submarine to a subaerial environment and comprises lava flows dominantly of absarokite composition with debris flows and conglomerates on its flanks the tavua caldera formed in the centre of the volcano within the caldera avalanche deposits are overlain by the turtle pool formation which consists of lacustrine sediments shoshonite lavas and hydrovolcanic pyroclastic lapilli tuffs subsequently the inner caldera formed in the centre of the tavua caldera this was infilled by the morrisons pool formation which comprises lavas and pyroclastic rocks of banakite composition with minor intercalated sediment thus the magmas became progressively more evolved in addition to normal movement along regional and concentric faults subsidence which exceeds 3 km was accomplished via substantial downsagging evidence for this is the consistent centripetal dip of caldera fill material which locally exceeds 50 in parts of the turtle pool formation centripetal dips are also recognized in pre caldera rocks immediately outside the tavua caldera subsidence was episodic caused by periodic non explosive withdrawal of underlying magma to erupt on the flanks and in part within the caldera downsagging and fault movement were responsible for approximately equal amounts of subsidence and their interaction produced a caldera with the shape of a stepped funnel

shishimuta caldera the buried source of the yabakei pyroclastic flow in the hohi volcanic zone japan drill hole geochronologic and gravity data identify the buried shishimuta caldera beneath post caldera lava domes and lacustrine deposits in the center of the hohi volcanic zone the caldera is the source of the yabakei pyroclastic flow which erupted 1 0 ma ago with a bulk volume of 110 km3 the caldera is a breccia filled funnel shaped depression 8 km wide and 3 km deep with a v shaped negative bouguer gravity anomaly up to 36 mgal neither ring vents nor resurgence was recognized instead post caldera monogenetic volcanism in an extensional setting dominated the area the andesitic breccia has a relatively low density and fills the caldera it possibly formed by fragmentation of disrupted roof rock during the violent yabakei eruption and related collapse fewer normal faults and shallow microearthquakes occur inside the caldera than around it possibly because rocks beneath the caldera are structurally incoherent a profile of shishimuta caldera may be more elongated vertically and have a more intensely fractured zone than that of a valles type caldera

vent evolution and lag breccia formation during the cape riva eruption of santorini greece the 18 500 yr b p cape riva cr eruption of santorini vented several km3 or more of magma generating four eruption units a basal plinian fall deposit cr a and three pyroclastic flow deposits cr b to cr d upwards cr b and cr d are welded ignimbrites cr c consists predominantly of up to 25 m thick coarse lithic rich co ignimbrite lag breccias resulting from a climactic phase of the eruption the initial plinian phase occurred from a localized vent in n santorini and subsequent column collapse resulted in emplacement of cr b towards the end of cr b new conduits were activated and pyroclastic flows discharged from multiple vents to generate the lag breccias cr c cr d probably records a return to a localized vent as the eruption waned the eruption sampled a zoned magma chamber containing rhyodacite overlying andesite and leaks of these magmas were manifested as the skaros therasia lavas preceding the cr eruption plinian and initial ignimbrite stages occurred while the magma chamber was ove

sedimentary processes in the great lakes sediments in the great lakes are subject to four general categories of processes input transport deposition and postdepositional alteration sedimentary input is from coastal erosion 64 riverine input 26 atmospheric transport 3 and several lesser sources 7 once in the lake sediments are transported to their place of deposition along the bottom and or in suspension depending upon grain size and density the rate of deposition in the great lakes is variable and ranges from zero in some areas to over 6000 g m sup 2 yr sup 1 in the eastern basin of lake erie after deposition sediments undergo bioturbation dissolution of carbonate and opal remobilization of many metals followed by reprecipitation within the surficial oxidized layer and degradation of organic compounds

incremental caldera collapse of suswa volcano gregory rift valley kenya suswa volcano located at 1 10 s 36 20 e is quaternary in age following caldera i collapse phonolite lava flows were erupted from the caldera floor centrally erupted phonolite lava flows led to the construction of ol doinyo onyoke lava cone a pit crater on the cone was a precursor to the collapse of caldera ii both of which were generated entirely by magma withdrawal regional decompression caused ring fault bounded block resurgence of the caldera floor

downsag and extension at calderas new perspectives on collapse geometries from ice melt mining and volcanic subsidence structures at calderas may form as a result of precursory tumescence subsidence due withdrawal of magmatic support resurgence and regional tectonism structural reactivation and overprinting are common to explore which types of structures may derive directly from subsidence without other factors evidence is reviewed from pits caused by the melting of buried ice blocks mining subsidence scaled subsidence models and from over 50 calderas this review suggests that complex patterns of peripheral deformation with multiple ring and arcuate fractures both inside and outside caldera rims topographic embayments arcuate graben and concentric zones of extension and compression may form as a direct result of subsidence and do not require a complex subsidence and inflation history downsag is a feature of many calderas and it does not indicate subsidence on an inward dipping ring fault as has been inferred previously where magmatic inflation is absent or slight initial arcuate faults formed during collapse are likely to be multiple and dip outwards to vertical associated downsag causes the peripheries of calderas undergo radial centripetal extension and this accounts for some of the complex peripheral fractures arcuate crevasses graben and some topographic moats the structural boundary of a caldera defined here as the outermost limits of subsidence and related deformation including downsag commonly lies outside ring faults and outside the embayed topographic wall it is likely to be funnel shaped i e inward dipping even though ring and arcuate fractures within it may dip outward inward dipping arcuate normal faults at shallow levels and steep inward dipping contacts between a caldera s fill and walls may both occur at a caldera that has initially subsided on outward dipping ring faults they arise due to peripheral surficial extension gravitational spreading and scarp collapse topographic enlargement at some calderas and the formation of embayments may reflect general progressive downsag and localized downsag respectively these processes may occur in addition to surficial degradation of oversteep ring fault scarps

structural eruptive and intrusive evolution of the grizzly peak caldera sawatch range colorado volcanic and shallow intrusive features of the deeply dissected 34 ma grizzly peak caldera in west central colorado record evolution of the magmatic center from pre caldera through post resurgent stages pre caldera dikes zones of hydrothermally altered rocks and lava flows formed along a circular swarm of cone sheet fractures around the site of the future caldera early largely rhyolitic uppercrustal magmatism culminated in the caldera forming eruption of the grizzly peak tuff intracaldera tuff is zoned from high silica rhyolite at the base to low silica rhyolite at the eroded top and further contains dacite to mafic latite pumice lumps in two heterogeneous tuff layers in the upper third of the preserved section half of the erupted tuff ponded in the 17 by 23 km 600 km 3 caldera filling the asymmetric depression to a compacted thickness locally 2 7 km including intercalated rock avalanche megabreccias shed from ring fault scarps the asymmetric caldera has an inner ring fracture zone that separates two structural segments that collapsed to different depths caldera fill buried collapse structures as they formed the inner ring fracture zone is a growth or syn depositional fault in the single cooling unit tuff welded tuff ring dikes are locally exposed at erosion levels below the caldera fill these dikes are remnants of fissure vents in the outer ring fracture zone following collapse the grizzly peak caldera was uplifted forming a complexly faulted resurgent dome resurgence resulted partly from emplacement of a composite granodiorite laccolith now exposed in the eroded core of the dome a belt of mafic latite to rhyolite porphyry dikes and small stocks formed across the center of the domed caldera during the waning of the magmatic center latite intrusions in this suite represent the penetration of relatively mafic magma to the surface following solidification of the felsic subcaldera batholith

gross brukkaros namibia an enigmatic crater fill reinterpreted as due to cretaceous caldera evolution at gross brukkaros a central depression has developed within domed nama group sediments and has functioned as a local depocenter with a primary fill deposited during the cretaceous and a small secondary fill by alluvial fans during the tertiary and quaternary the diameter of the entire structure is about 10 km and that of the central depression is about 3 km within this depocenter the sedimentary sequence consists mainly of debris flow and mudflow deposits with minor intercalations of fluviatile braided channel sediments and fossiliferous lacustrine deposits the sedimentary system represents a set of coalesced subaerial fans which formed a fringing sedimentary apron along the margin of the depocenter this sedimentary apron passed distally and centrally into a permanent lake which was characterized by a fluctuating water level facies transitions observed are typical of those described from modern and ancient fan delta systems contact relationships show the gross brukkaros sediments to be about the same age upper cretaceous as the surrounding carbonatitic volcanism an upper cretaceous age is also consistent with the plant fossil association recently recognized within the lacustrine beds of gross brukkaros

lake sediments as natural seismographs earthquake related deformations seismites in central canadian lakes abstract central canada experiences numerous intraplate earthquakes but their recurrence and source areas remain obscure due to shortness of the instrumental and historic records unconsolidated fine grained sediments in lake basins are natural seismographs with the potential to record ancient earthquakes during the last 10 000 years since the retreat of the laurentide ice sheet many lake basins are cut into bedrock and are structurally controlled by the same precambrian basement structures shear zones terrane boundaries and other lineaments implicated as the source of ongoing mid plate earthquake activity a regional seismic sub bottom profiling of lakes gull muskoka joseph rousseau ontario wanapitei fairbanks vermilion nipissing georgian bay mazinaw simcoe timiskaming kipawa parry sound and lake of bays encompassing a total of more than 2000 kilometres of high resolution track line data supplemented by multibeam and sidescan sonar survey records show a consistent sub bottom stratigraphy of relatively thick lowermost lateglacial facies composed of interbedded semi transparent mass flow facies debrites slumps and rhythmically laminated silty clays mass flows together with cratered kettled lake floors and associated deformations reflect a dynamic ice contact glaciolacustrine environment exceptionally thick mass flow successions in lake timiskaming along the floor of the timiskaming graben within the seismically active western quebec seismic zone wqsz point to a higher frequency of earthquakes and slope failure during deglaciation and rapid glacio isostatic rebound though faulting continues into the postglacial lateglacial faulting diapiric deformation and slumping of coeval lateglacial sediments is observed in parry sound lake muskoka and lake joseph which are all located above prominent precambrian terrane boundaries lateglacial sediments are sharply overlain by relatively thin rhythmically laminated and often semi transparent postglacial silty clay laminations postglacial neotectonic activity is recorded by co seismic water escape structures faults slumps and debris flows in lake simcoe above a terrane boundary and especially within lakes timiskaming and kipawa recording ongoing deformation within the wqsz high resolution seismo stratigraphic data presented here support the model that ongoing mid plate earthquake activity is a consequence of brittle deformation of the upper crust of the north american plate such activity appears to have been greatest during deglaciation but continues today

explosive rhyolitic volcanism in the jemez mountains vent locations caldera development and relation to regional structure the jemez volcanic field straddles the western margin of the rio grande rift where the rift is intersected by the jemez lineament in north central new mexico the field has a record of volcanism extending back to before 13 ma initial basaltic activity was related to active rifting with minor rhyolitic eruptions occurring along n s rift bounding faults between 10 and 7 ma voluminous andesitic volcanism took place in the central jemez mountains area overwhelming contemporaneous basaltic and rhyolitic magmatism an apparent tectonic lull took place from 7 to 4 ma accompanied by lower eruption rates during this interval dacitic magmas were erupted to form the tschicoma volcanic center but mafic and rhyolitic volcanism virtually ceased since 4 ma accompanying resumption of rifting a growing silicic magma system has been present under the central part of the jemez mountains ultimately evolving to the magma body that produced the voluminous rhyolitic bandelier tuffs explosive rhyolitic eruptions from this large magma body have occurred many times since 3 to 4 ma early eruptions 3 6 2 8 ma produced high silica rhyolite ignimbrites restricted to the southwest part of the jemez mountains formation of these units may have been accompanied by caldera collapse these events were followed by the two caldera forming bandelier tuff ignimbrite eruptions 1 45 and 1 12 ma post caldera explosive and effusive rhyolite eruptions have also tapped the magma body from vents generally located along ring fractures after both bandelier events vent and caldera locations for the rhyolitic eruptions during 3 4 ma have been inferred from grain size characteristics dispersal patterns and facies variations in the plinian deposits and ignimbrites lithic breccia zones of the pre bandelier ignimbrites indicate possible caldera sources in the southwest part of the present valles caldera during the eruption of both bandelier tuffs initial plinian falls and early pyroclastic flows emanated from vents centrally located in the jemez mountains in the lower bandelier tuff eruption a transition to ring fracture vents occurred before the emission of later pyroclastic flows but there is no strong evidence to suggest such a transition occurred during the upper bandelier eruption calderas associated with the lower and upper bandelier tuffs toledo and valles respectively are almost identical in location as are the plinian vent sites for these two large eruptions the toledo embayment northeast of the valles caldera contains lava domes from up to 3 6 ma and may be a caldera or crater associated with early explosive dacitic volcanism in the tschicoma volcanic center post 1 4 ma lava domes also fill this depression the main volcanic features of the jemez mountains field including the valles caldera complex eruption vents and the apical graben of the post valles redondo resurgent block appear to be aligned along the ne sw trending jemez fault zone this zone the local expression of a precambrian basement feature the jemez lineament has exerted strong control on the location and style of eruptions from the bandelier rhyolitic magma system

evolution of the olympus mons caldera mars synoptic images of the martian volcano olympus mons are of a quality and quantity that are unique for mars and somewhat surprisingly are appreciably better than image data that exist for many volcanoes on earth useful information about the evolution of shield volcanoes on earth can thus be derived from the investigation of this extraterrestrial example we have used shadow length measurements and photoclinometrically derived profiles to supplement and refine the topographic map of the olympus mons caldera as much as 2 5 km of collapse took place within the 80 65 km diameter caldera and the elevation of the caldera rim varies by almost 2 0 km low around the oldest collapse events high around the youngest an eight stage evolutionary sequence for the caldera of olympus mons is identified which shows that caldera subsidence was a longterm process rather than the near instantaneous event that has been interpreted from comparable terrestrial examples tectonic features on the caldera floor indicate a transition from an extensional environment graben formation around the perimeter of the caldera to compression ridge formation towards the caldera center this transition from a compressional to extensional environment is surprisingly sudden occurs at a radial distance of 17 km from the caldera center and is import because it can be used to infer that the magma chamber was relatively shallow thought to be at a depth of 16 km beneath the caldera floor zuber and mouginis mark 1990 ample evidence is also found within the olympus mons caldera for solidified lava lakes more than 30 km in width and for the localzed overturning and or withdrawal of lava within these lakes

lakes of the huron basin their record of runoff from the laurentide ice sheet the 189 000 km2 hur n basin is central in the catchment area f the present q s r lanrentian great lakes that now drain via the st lawrence river to the north atlantic ocean during deglaciation from 21 7 5 ka bp and owing to the interactions of ice margin positions crustal rebound and regional topography this basin was much more widely connected hydrologi cally draining by various routes to the gulf of mexico and atlantic ocean and receiving over flows from lakes impounded north and west of the great lakes hudson bay drainage divide early ice marginal lakes formed by impoundment between the laurentide ice sheet and the southern margin of the basin during recessions to interstadial positions at 15 5 and 13 2 ka be in i each of these recessions lake drainage was initially southward to the mississippi river and gulf of mexico in the first recession drainage subsequently switched eastward along the ice margin to the north atlantic ocean in the second recession drainage continued southward through the michigan basin and later eastward via the ontario basin and mohawk river valley to the north atlantic ocean during the final retreat of ice in the huron basin from 13 to 10 ka be proglacial lake drainage switched twice from the michigan basin and the mississippi river system to the i north atlantic via the ontario basin and mohawk river valley finally diverting to the champlain sea in the st lawrence river valley at about 11 6 ka be new seismo and litbo stratigraphic information with ostracode data from the offshore lacustrine sediments were integrated with the traditional data of shorelines uplift histories of outlets and radio carbon dated shallow water evidence of transgressions and regressions to reconstruct the water level history and paleolimnological record for the northern huron basin for the 11 7 ka bp period negative excursions in the 8 80 isotopic composition of ostracodes and bivalves in southern lake michigan southwestern lake huron and eastern lake erie indicate an influx of water from ice marginal lake agassiz in central north america about 11 ka be a major decline in water levels of the huron basin after 10 5 ka bp followed the high level main lake algonquin phase as ice receded and drainage was established through the north bay area to ottawa river valley during the subsequent mattawa stanley phase the lake level history was dominated by fluctuations of tens of meters highstands of the earliest oscillations whose origin is not clear might be related to some of the well known post algonquin shorelines after 9 6 ka be it is suggested that large inflows from lake agassiz and hydraulic damming in downstream outlets were the likely cause of the lake mattawa highstands a lowstand at 9 3 9 1 ka bp occurred when these inflows were diverted or impeded by an ice advance in the nipigon basin area while undiluted meltwater continued to enter the huron basin assemblages and isotopic composition of the ostracode fauna indicate very dilute meltwater during the lowstands as late as 7 5 ka be and precipitation runoff with comparatively higher dissolved solids dur ing the highstands we speculate that the water composition of the lake mattawa highstands was dom inated by the agassiz inflows by that time much of lake agassiz was remote from ice marginal envi ronments and the inflows were drawn from surface water of the southern sector of the lake which was largely supplied by runoff and dissolved solids from the exposed land area of western canada major inflows apparently ended about 8 ka be but northern proglacial lakes apparently continued as melt water persisted in the huron basin until about 7 5 ka bp the cessation of major inflows initiated the final lowstand in the huron basin and the present hydrological regime of local runoff

post younger dryas seasonality in the north american midcontinent region as recorded in lake huron varved sediments a sediment core from northwestern lake huron contains a sequence of red and gray layered sediments that are postglacial and early holocene glaciolacustrine varves two hundred and sixty four couple

volcanic geology of tyrrhena patera mars tyrrhena patera is a large low relief volcano located in the southern cratered highlands of mars on a proposed ring fracture of the hellas basin the volcano is composed of four or possibly five principal units the oldest and most areally extensive of which are suggested to be ash on the basis of their morphology and erosional characteristics lava flows are observed in a flank flow unit extending from the central caldera to the southwest and apparently cap the summit region models derived from the study of terrestrial pyroclastic flows have been applied to the deposits at tyrrhena patera and indicate that the distribution and lengths of the observed deposits are consistent with an origin by the emplacement of gravity driven ash flows generated by hydromagmatic or magmatic explosive eruptions the hydromagmatic case requires considerable quantities of water 7 5 1016 kg however at volumetric water flow rates determined by other investigators 105 m3 s and calculated from the regional slopes in the hesperia planum region 106 m3 s the amounts necessary could be supplied relatively rapidly in geologic time tyrrhena patera represents the earliest central vent volcanism identified on mars and may reflect a transition from the flood style eruptions which dominated early martian history a hydromagmatic origin for tyrrhena patera is in agreement with suggested climatic changes on mars and may explain the absence of this style of volcanism in later eras alternatively a magmatic origin for tyrrhena patera implies an evolution in the types of magmas produced on mars from hesperian to amazonian time during which the prominent shields in the tharsis region formed

paleohydrology of the upper laurentian great lakes from the late glacial to early holocene abstract between 10 500 and 9000 cal yr bp 18o values of benthic ostracodes within glaciolacustrine varves from lake superior range from 18 to 22 pdb in contrast coeval ostracode and bivalve records from the lake huron and lake michigan basins are characterized by extreme 18o variations ranging from values that reflect a source that is primarily glacial 20 pdb to much higher values characteristic of a regional meteoric source 5 pdb re evaluated age models for the huron and michigan records yield a more consistent 18o stratigraphy the striking feature of these records is a sharp drop in 18o values between 9400 and 9000 cal yr bp in the huron basin this low 18o excursion was ascribed to the late stanley lowstand and in the lake michigan basin to lake agassiz flooding catastrophic flooding from lake agassiz is likely but a second possibility is that the low 18o excursion records the switching of overflow from the lake superior basin from an undocumented northern outlet back into the great lakes basin quantifying freshwater fluxes for this system remains difficult because the benthic ostracodes in the glaciolacustrine varves of lake superior and lake agassiz may not record the average 18o value of surface water

stratigraphy and paleolimnologic record of lower holocene sediments in northern lake huron and georgian bay seismic reflection profiling and piston coring identified seismic reflectors in northern lake huron and georgian bay linked with unconformities caused by at least six reductions in lake level in ascending stratigraphic order these lowstands occurred at about 11 200 bp associated with the kirkfield outlet from early lake algonquin 10 200 9900 bp associated with the post algonquin lake level fall 9800 9050 bp the most extreme lowstand associated with the main stanley hough draw down and 7800 7450 bp the concomitant highstands are lake algonquin from about 11 200 10 200 bp early lake mattawa between 9600 and 9350 bp the main mattawa phase 9050 7800 bp and the nipissing highstand at about 4700 bp isotopic and paleoecological data show that all of the lowstands are characterized by cold dilute and isotopically very light 20 waters from the melting laurentian ice cap highstands on the other hand are characterized by higher dissolved solid concentrations and a much smaller

folded cauldrons of the early proterozoic labine group northwestern canadian shield great bear magmatic zone 1 875 1 84 ga is a linear belt 100 km wide by 800 km long of little metamorphosed volcanic rocks and allied plutons of calc alkaline affinity the entire zone was folded shortly after magmatism and sections many kilometers thick through several cauldron complexes are exposed eruptions of rhyolite led to collapse of black bear cauldron in which 1 5 km of tuff ponded the cauldron then became the locus of fluvial and lacustrine sedimentation followed by augite plagioclase porphyritic andesite volcanism both of which also accumulated within the cauldron ash flow eruptions of dacite caused collapse of clut cauldron which was accompanied by landsliding and avalanching of the cauldron walls abundant andesitic and intrusive debris is intercalated with the propylitized intracauldron facies tuff adjacent to the walls and some blocks are as large as 1 km across clut cauldron also became the site for fluvio lacustrine sedimentation after collapse and was likely resurgent resurgence was probably related to the emplacement of a quartz monzonite pluton which occupies the core of the cauldron cornwall cauldron which is exposed entirely in cross section contains 1 2 km of intracauldron facies tuff overlain by a complex of dacite lava flows the cauldron was intruded by a granodiorite monzogranite pluton probably responsible for resurgence data from the area suggest the following 1 the similarity of early proterozoic cauldrons to cenozoic examples indicates that cauldron collapse is a process that has occurred since 1 9 ga 2 batholiths beneath ash flow tuff fields are probably composite bodies made up of many individual plutons 3 most of the plutons are sheetlike in cross section and 4 cauldron collapse begins as soon as ash flow eruptions and relief on the cauldron margin becomes large early in the subsidence history

formation of collapse calderas collapse calderas are common in many volcanic regions but the mechanics of their formation is not well understood i propose that collapse calderas may form as a result of the growth of associated shallow magma chambers excess magmatic pressure in a shallow chamber gives rise to tensile stresses at the surface but the location and magnitude of the maximum tensile stress depend on the shape of and depth d to the top of the chamber this model indicates that penny shaped chambers i e sill like chambers of circular horizontal cross sections are most likely to initiate collapse calderas for such a chamber with horizontal radius a the higher the a d radius depth ratio the greater the probability of forming a collapse caldera

effects of bottom currents and fish on sedimentation in a deep water lacustrine environment sands and muds accumulating under the influence of apparently strong bottom currents in a deep water environment of lake superior were studied in detail using 3 5 khz echo sounding side scan sonar piston gravity and box coring and lake floor photography the study area includes a well defined slope and basin environment bottom currents maintain a scoured trough 20 m deep and 2 km wide at the base of the slope and modify the shapes of large ringlike depressions that are common in many regions of lake superior side scan sonar records reveal lineations subparallel to the direction of bottom current flow that are interpreted in different areas to be sand ribbons slight depressions or depositional furrows the sediments range from silty sand in the scoured trough to sandy clays farther offshore radiocarbon dating and 210 pb dating show the sediments to be postglacial rather than relic glacial lacustrine sediments grain size distributions are affected by bottom currents and a nearby source of sand some sedimentary structures including plane laminations and interbedded sands and muds are due to bottom currents fish create dish laminations however that often obscure the effects of the bottom currents fish and fish scour depressions are common in lake floor photographs whereas evidence for bottom currents is not biological activity therefore appears to erase traces of intermittent currents

evidence of incremental growth in the vulsinian calderas central italy abstract a comparative work on the vulsini calderas has been carried out fundamentally from volcanological field study drilling and gravity anomalies data on the basis of a detailed stratigraphy four volcanic complexes have been recognized in the vulsini volcanic district from the oldest to the youngest paleo bolsena bolsena montefiascone and latera the latter two complexes having a contemporaneous evolution in their activity each complex gave rise to several little calderas caused by piecemeal collapses due to moderate size explosive eruptions or coherent subsidence of a superficial piston like block nevertheless the extensional volcano tectonic regime affecting this area could have accounted for as much of the total subsidence as the caldera forming eruptions bolsena and latera calderas respectively 16 km and 8 km in average diameter represent the main depressions of the vulsini district and their topographic features are still well preserved several collapses forming the main poligenetic calderas of bolsena and latera are masked at present by the bolsena lake the younger volcanic covering the lacustrine deposits and erosion in a minor amount only the little montefiascone and vepe calderas which represent the youngest collapses of the vulsini area have not been masked bolsena caldera could be considered a depression caused mostly by a subsidence hinged to the southwest by down sagging and by collapses which occurred after the most important explosive eruptions of the paleo bolsena bolsena and montefiascone volcanic complexes meanwhile latera caldera is mostly due to a piecemeal collapse linked to several moderate size ignimbrite units which were emplaced throughout the explosive activity of the latera complex therefore the vulsinian calderas did not form by catastrophic eruptions because several and frequent moderate size eruptions downsagging and tectonic hinged subsidence concur indicating an incremental growth

deformation of the wineglass welded tuff and the timing of caldera collapse at crater lake oregon abstract four types of deformation occur in the wineglass welded tuff on the northeast caldera rim of crater lake a vertical tension fractures b ooze outs of fiamme c squeeze outs of fiamme and d horizontal pull apart structures the three types of plastic deformation b d developed in the lower part of the wineglass welded tuff where degree of welding and density are maximum deformation originated from concentric normal faulting and landsliding as the caldera collapsed the degree of deformation of the wineglass welded tuff increases toward the northeast part of the caldera where plastic deformation occurred more easily because of a higher emplacement temperature probably due to proximity to the vent the probable glass transition temperature of the wineglass welded tuff suggests that its emplacement temperature was 750 c where the tuff is densely welded calculation of the conductive cooling history of the wineglass welded tuff and the preclimactic cleetwood lava flow under assumptions of a initially isothermal sheet and uniform properties suggests that a caldera collapse occurred a maximum of 9 days after emplacement of the wineglass welded tuff and that b the period between effusion of the cleetwood lava flow and onset of the climactic eruption was

stable isotope o and c and pollen trends in eastern lake erie evidence for a locally induced climatic reversal of younger dryas age in the great lakes basin a cool period from about 11000 to 10 500 bp 11 to 10 5 ka is recognized in pollen records from the southern great lakes area by the return of picea and abies dominance and by the persistence of herbs the area of cooling appears centred on the upper great lakes a high resolution record ca 9 mm y from a borehole in eastern lake erie reveals in the same time interval this pollen anomaly isotope evidence of meltwater presence a 3 per mil shift in 18o and a 1 1 per mil shift in 13c increased sand and reduced detrital calcite content all suggesting concurrent cooling of lake erie the onset of cooling is mainly attributed to the effect of enhanced meltwater inflow on the relatively large upstream main lake algonquin during the first eastward discharge of glacial lake agassiz termination of the cooling coincides with drainage of lake algonquin and is attributed to loss of its cooling effectiveness associated with a substantial reduction in its surface area it is hypothesized that the cold extra inflow effectively prolonged the seasonal presence of lake ice and the period of spring overturn in lake algonquin the deep mixing would have greatly increased the thermal conductive capacity of this extensive lake causing suppression of summer surface lakewater temperatures and reduction of onshore growing degree days alternatively a rapid flow of meltwater buoyed on sediment charged denser lakewater may have kept the lake surface cold in summer other factors such as wind shifted pollen deposition and possible effects from the younger dryas north atlantic cooling could have contributed to the great lakes climatic reversal but further studies are needed to resolve their relative significance

rock drumlins and megaflutes of the niagara escarpment ontario canada a hard bed landform assemblage cut by the saginaw huron ice stream abstract the late wisconsin c 18 000 ybp saginaw huron ice stream shis of the laurentide ice sheet was as much as 800 km long and 250 km wide in the great lakes basin of mid continent north america in its onset zone in the upper lake huron basin of ontario canada it flowed south from the high standing canadian shield to move over gently dipping ordovician and silurian dolostones devoid of glacial sediment cover the onset zone is recorded by a distinct hard bed landform assemblage across 3000 km2 of north facing escarpments niagara fossil hill and kagawong and dip slope pavements on the bruce peninsula and manitoulin island and the adjacent floor of lake huron a wide range of glacially streamlined rock landforms were carved into dolostones below fast flowing ice the largest are 30 km wide bullet shaped escarpment promontories that face upglacier superposed on these are swarms of rock drumlins up to 5 km wide with downglacier lengths of as much as 10 km promontories and drumlins record streaming of dirty basal ice around escarpment highs and create a distinct zig zag planform to the escarpments in turn down dip trailing edge dolostone pavements are corrugated by kilometre long megagrooves and megaflutes cut by basal ice flowing around resistant high standing bioherm mounds on dolostone bedding planes the geomorphology of the niagara escarpment does not primarily reflect a lengthy history of preglacial cenozoic fluvial erosion as classically argued but geologically brief episodes of accelerated abrasion and quarrying below ice streams within successive pleistocene ice sheets

calderas of the sierra madre occidental volcanic field western mexico approximately 296 000 km2 of western mexico is covered by a volcanic sequence that has an average thickness of 1 km and is mostly ash flow tuff although some 350 calderas are suggested by this tremendous volume only a few have been located or described in the state of durango the only well documented examples are in the 30 ma old chupaderos caldera complex near durango city although the complex is mostly covered by quaternary basalt and sediment the exposed portion reveals nested calderas that produced two major rhyolitic ash flow tuffs a third major tuff appears to have been erupted from the complex but no caldera has yet been found the caldera formed by the first eruption contains a resurgent dome and a thick sequence of moat sedimentary rocks and lava flows that are hosts for significant iron and tin deposits other possible durango calderas exist at el salto west of durango city at temoaya south of durango city and in the sierra de la silla to the north calderas in chihuahua have beep described from mountain ranges in the central part of the state and from the sierra madre occidental to the west a composite resurgent caldefa 35 ma old and approximately 22 km in diameter has been de scribed from the sierra pastorias immediately south of chihuahua city the main erupted unit is well exposed in the caldera s resurgent dome and a second tuff found outside the caldera was emplaced from a trap door structure nested in the caldera s northwest corner the area also contains a younger smaller resurgentiy domed caldera other possible calderas include a relatively large 35 km in diameter structure in the sierra del nido 75 km north of chihuahua city and a poorly defined structure related to a sequence f older 45 ma silicic volcanic rocks at cerro jesus maria 25 km northwest of chihuahua city calderas in the sierra madre plateau have been described from the tomochic and ocampo areas of western chihuahua the tomochic area records volcanic activity from 34 5 to 28 9 ma and contains at least three calderas these include the tomochic caldera a strikingly well preserved resurgent caldera approximately 20 by 25 km in diameter the north structural margin of the tomochic caldera cuts the intracaldera facies of an ash flow tuff related to the older las varas caldera epithermal precious metal fissure veins are located where the tomochic structural margin cuts this older unit a third structure the corallitos caldera is located only a few kilometers to the northeast reconnaissance studies in far western chihuahua indicate the existence of a large 40 km resurgent caldera centered on the village of ocampo landsat imagery shows a well developed arcuate drainage pattern and maps of the area show a distribution of lithologies that is consistent with the caldera interpretation gold and silver deposits hosted almost exclusively in precaldera rocks are best developed where the older rocks are cut by faults of the caldera s ring fracture zone and resurgent dome in general the calderas thus far mapped in western mexico follow the valles type resurgent caldera model variations however of the ideal caldera cycle are common as are repetitions of the cycle to form caldera complexes all of the better known calderas have associated mineral deposits although they may be best developed in precaldera rocks

the volcanic history of volcan alcedo galapagos archipelago a case study of rhyolitic oceanic volcanism volcan alcedo is one of the seven western galapagos shields and is the only active galapagos volcano known to have erupted rhyolite as well as basalt the volcano stands 4 km above the sea floor and has a subaerial volume of 200 km3 nearly all of which is basalt as volcan alcedo grew it built an elongate domal shield which was partly truncated during repeated caldera collapse and partial filling episodes an outward dipping sequence of basalt flows at least 250 m thick forms the steepest to 33 flanks of the volcano and is not tilted thus a constructional origin for the steep upper flanks is favored about 1 km3 of rhyolite erupted late in the volcano s history from at least three vents and in 2 5 episodes the most explosive of these produced a tephra blanket that covers the eastern half of the volcano homogeneous rhyolitic pumice is overlain by dacite rhyolite commingled pumice with no stratigraphic break the tephra is notable for its low density and coarse grain size the calculated height of the eruption plume is 23 30 km and the intensity is estimated to have been 1 2x108 kg s rhyolitic lavas vented from the floor of the caldera and from fissures along the rim overlie the tephra of the plinian phase the age of the rhyolitic eruptions is 120 ka on the basis of k ar ages between ten and 20 basaltic lava flows are younger than the rhyolites recent faulting resulted in a moat around part of the caldera floor alcedo most resently erupted sometime between 1946 and 1960 from its southern flank alcedo maintains an active transient hydrothermal system acoustic and seismic activity in 1991 is attributed to the disruption of the hydrothermal system by a regional scale earthquake