geological and geochemical characteristics and ore genesis of the keketale vms pb zn deposit southern altai metallogenic belt nw china article i nfo article history the keketale pb zn volcanogenic massive sulfide vms deposit occurs in the early devonian sequence of the kangbutiebao formation of the southern altai metallogenic belt samb northern xinjiang china all the orebodies are stratabound and hosted by a suite of meta sedimentary rocks intercalated with volcanic rocks major sulfide ores are pyrite sphalerite galena with minor pyrrhotite and rare chalcopyrite massive banded and disseminated ores are the three main types a typical vms type hydrothermal alteration zone is developed around the orebodies because of the overturned strata an intense chlorite pyrite rich crosscutting feeder zone is present immediately above the orebody a sericite zone envelopes the chlorite zone the meta volcanic rocks show decoupled characteristics between the high field strength elements ta zr and hf and large ion lithophile elements rb ba and th all samples plot in the volcanic arc field in ta vs yb discrimination diagrams the 34 s values of the stockwork zone are 1 0 to 4 5 and those of banded or massive ores range from 11 1 to 8 8 indicating that hydrothermal fluids may have had multiple sources and that bacterial sulfate reduction probably played an important role initial 87

geology of el misti volcano near the city of arequipa peru approximately 750 000 people live at risk in the city of arequipa whose center lies 17 km from the summit 5820 masl meters above sea level of the active el misti volcano the composite edifice comprises a stratovolcano designated misti 1 ca 833 112 ka partially overlapped by two stratocones designated misti 2 and misti 3 112 ka and younger and a summit cone misti 4 11 ka and younger eight groups of lava flows and pyroclastic deposits indicate the following volcanic history 1 three cones have been built up since ca 112 ka at an average eruptive rate of 0 63 km3 k y 2 several episodes of growth and destruction of andesitic and dacitic domes triggered dome collapse avalanches and block and ash flows deposition of these flows alternated with explosive events which produced pyroclastic flow deposits and tephra fall and surge deposits 3 nonwelded dacitic ignimbrites may reflect the formation of a 6 5 km incremental caldera collapse on misti 2 ca 50 000 and 40 000 yr b p and a 2 1 5 km summit caldera on misti 3 ca 13 700 to 11 300 yr b p 4 tens of pyroclastic flows and at least 20 tephra falls were produced by vulcanian and sub plinian eruptions since ca 50 ka on average ash falls have occurred every 500 to 1500 yr and pumice falls every 2000 to 4000 yr 5 misti erupted relatively homogeneous andesites and dacites with a few rhyolites but misti 4 reveals a distinct mineral suite less evolved andesites prevail in scoriaceous products of group 4 1 including historical ash falls scoriae of misti 4 and the ca 2300 2050 yr b p banded pumice commonly show heterogeneous textures of andesite and rhyolite composition this heterogeneity may reflect changes in physical conditions and magma mixing in the reservoir 6 deposits emplaced during the vulcanian a d 1440 1470 event and the sub plinian eruption s at ca 2050 yr b p are portrayed on one map the extent and volume of these deposits indicate that future eruptions of el misti even if moderate in magnitude will entail considerable hazards to the densely populated area of arequipa

reconstruction of the ad 1600 huaynaputina eruption based on the correlation of geologic evidence with early spanish chronicles the largest historical eruption vei 6 in the andes began on february 19 and continued until march 6 or 15 ad 1600 at huaynaputina a dacitic stratovolcano located on a high volcanic plateau in south peru tephra falls pyroclastic flows and surges disrupted life in an area of 4900 km2 around the volcano and ash fall was reported 200 500 km away in south peru west bolivia and north chile the aftermath of the large scale eruption was severe and protracted for the people and colonial economy of south peru by linking up the series of events inferred from spanish chronicles with the lithofacies and composition of the tephra bulk volume 11 4 12 1 km3 dense rock equivalent dre 4 6 4 95 km3 we distinguish five eruptive phases 1 during the plinian phase a sustained plinian column 27 35 km high on february 19 20 delivered a dacitic pumice fall of 3 1 km3 dre volume the plinian pumice formed a widespread lobe of 95 000 km2 within the 1 cm isopach strong winds carried fine ash 500 km to the west and west northwest into the pacific ocean the computed volumetric eruption rate was in the range of 5 4 6 6 104 to 1 105 m3 s and the mass eruption rate 1 3 1 6 108 kg s the onset and high discharge of the sustained plinian eruption was fueled by the disruption of an active hydrothermal system enclosed in the pre ad 1600 amphitheater the plinian column shut off as the vent was choked when the fragmentation focus deepened to beneath the weathered bedrock 1600 m below the vent area 2 during the second phase a dwindling column sent ash falls on proximal to medial areas and possibly pyroclastic surges on proximal slopes 3 during the third ignimbrite forming phase with interspersed hydromagmatic events pyroclastic flows 1 5 2 km3 in volume were channeled into the rio tambo canyon and tributaries the flows with interbedded base surge deposits in proximal tributaries probably produced vigorous columns over high rugged relief around the huaynaputina plateau winds winnowing the columns dispersed a widespread co ignimbrite ash probably mixed with co plinian ash over an area of 265 000 km2 4 during the fourth phase an unusual crystal ash fall was deposited when the residual magma with a crystal content as high as 80 was tapped near the end of the eruption 5 during the fifth phase ash flows produced surge deposits and lag fall breccias near vent small volume ash flow deposits in proximal catchments and a thin ash fall layer in medial to distal areas the proximal deposits were also produced by diluted flows able to surmount ridges 1000 m high as far as 15 km east from the vent the ignimbritic and hydromagmatic phases greatly modified the 400 m diameter plinian vent tapping of the crystal rich magma and ash flows towards the end of the eruption led to the formation of two youthful vents in domes geochemistry and mineralogy of the plinian and post plinian units point to an unusual zoned magma sequence the ignimbrite forming phase tapped a magma batch richer in silica than the less differentiated plinian magma the crystal rich magma of unit 4 was fed by crystal mush in a layered magma reservoir and or from two magma reservoirs at distinct depths the geochemical and mineralogical trend throughout the eruption and preliminary measurements of geobarometers suggest a complex model linking a shallow 6 7 km magma reservoir to a deeper 15 km reservoir the total dre volume 4 6 4 95 km3 of erupted tephra did not lead to caldera collapse ring fractures cutting multiple vents are associated with a dyke swarm intruding the weathered volcanic bedrock this suggests the onset of a funnel type or piecemeal collapse

the surges of variegated glacier alaska u s a and their connection to climate and mass balance one of the questions still unanswered concerning the surge behavior of glaciers concerns their quasi periodic occurrence some results on the phenomenological connection between local cumulative balance and surge initiation of variegated glacier alaska u s a are discussed here based on climate data from neighboring weather stations an empirical relation between precipitation temperature and local mass balance is established and used to reconstruct the annual balance at a location in the accumulation area back to 1905 between the last four surges in 1946 47 1964 65 1982 83 and 1994 95 the ice equivalent cumulative balance was 43 5 m on average with a 1 error of 1 2 m although the existence of a surge level cannot be directly interpreted in physical terms it explains the variable length of the quiescent periods of variegated glacier by variations in the accumulation rate prior to the surge we use the surge level to hindcast former unobserved surges to compare the results with other surge datings obtained from photographs and to establish a complete surge history for variegated glacier for the 20th century

21st century increase in glacier mass loss in the wrangell mountains alaska usa from airborne laser altimetry and satellite stereo imagery alaskan glaciers are among the largest regional contributors to sea level rise in the latter half of the 20th century earlier studies have documented extensive and accelerated ice wastage in most regions of alaska here we study five decades of mass loss on high elevation land terminating glaciers of the wrangell mountains 4900 km2 in central alaska based on airborne center line laser altimetry data from 2000 and 2007 a digital elevation model dem from aster and spot5 and us geological survey topographic maps from 1957 the regional mass balance estimates derived from center line laser altimetry profiles using two regional extrapolation techniques agree well with that from dem differencing repeat altimetry measurements reveal accelerated mass loss over the wrangell mountains with the regional mass balance rate evolving from 0 07 0 19 m w e a 1 during 1957 2000 to 0 24 0 16 m w e a 1 during 2000 07 nabesna the largest glacier in this region 1056 km2 lost mass four times faster during 2000 07 than during 1957 2000 although accelerated the mass change over this region is slower than in other glacierized regions of alaska particularly those with tidewater glaciers together our laser altimetry and satellite dem analyses demonstrate increased wastage of these glaciers during the last 50 years

pattern and rate of erosion inferred from inca agricultural terraces in arid southern peru abstract earthworks of assumed age and their initial and current morphologies provide an ideal basis for developing and testing models for long term landform erosion inca agricultural terraces abandoned at 1532 a d in the drylands of southern peru may be used to document morphological changes since the abandonment the objective of this research is to determine the erosion pattern and process to estimate the erosion rate the development of rills and channels on the inca agricultural terraces is evidence for erosion by wash processes on slopes where the anchoring effect of vegetation is absent and loose material is available for removal the pattern and amount of erosion from 1532 2005 a d is estimated by comparing elevation models of the observed morphology and reconstructed models of the original morphology of the inca terraces the results show that in areas of sediment accumulation surface elevation increased up to 0 5 m elevation lowering on the terrace treads was 0 7 m at maximum and a temporally and spatially averaged lowering rate was 0 094 mm yr 1 this gives insights about how the rate of erosion occurs on currently disturbed lands in arid environments where soil resources are scarce and lands are prone to desertification

derivation and analysis of a complete modern date glacier inventory for alaska and northwest canada we present a detailed complete glacier inventory for alaska and neighboring canada using multi sensor satellite data from 2000 to 2011 for each glacier we derive outlines and 51 variables including center line lengths outline types and debris cover we find 86 723 km 2 of glacier area 27 109 glaciers 0 025 km 2 similar to 12 of the global glacierized area outside ice sheets of this area 12 0 is drained by 39 marine terminating glaciers 74 km of tidewater margin and 19 3 by 148 lake and river terminating glaciers 420 km of lake river margin the overall debris cover is 11 with considerable differences among regions ranging from 1 4 in the kenai mountains to 28 in the central alaska range comparison of outlines from different sources on 2500 km 2 of glacierized area yields a total area difference of similar to 10 emphasizing the difficulties in accurately delineating debris covered glaciers assuming fully correlated systematic errors uncertainties in area reach 6 for all alaska glaciers but further analysis is needed to explore adequate error correlation scales preliminary analysis of the glacier database yields a new set of well constrained area length scaling parameters and shows good agreement between our area altitude distributions and previously established synthetic hypsometries the new glacier database will be valuable to further explore relations between glacier variables and glacier behavior

two mica rhyolitic tephra in the east pisco basin peru new age and dispersion constraints for the eruptions of the eastern cordillera of central andes wo mica biotite and muscovite volcanics are particularly rare in the geological record one of the several dozen volcanic ash layers from central andes volcanoes found in the upper miocene marine succession of the pisco formation ica desert peru contains juvenile biotite and muscovite sillimanite andalusite feldspars and rhyolitic glass 39ar 40ar dating on biotite and muscovite concordantly constrain an age of 7 96 ma for this two mica ash layer a second tephra in the pisco formation has a similar biotite composition and an age between 7 45 and 6 93 ma the peculiar mineral assemblage and the chemical composition of biotite indicate a strongly peraluminous composition of the erupted magmas and together with the 39ar 40ar ages suggest to consider a correlation of these ash layers to the eruptions of the miocene macusani peru or morococala bolivia volcanic complexes in the eastern cordillera of central andes the major and trace element composition of glass supports the correlation with macusani a provenance from morococala seems less likely given the large distances involved these results provide new data on the volcanic activity of the eastern cordillera revealing ash that dispersed to over 500 km to the west in the forearc marine basins this finding highlights that the exhumed forearc east pisco basin is highly promising as an archive of distal ash for the reconstruction of the volcanic activity of central andes during the miocene silicic flare ups

the historical 218 14 abp explosive eruption of tutupaca volcano southern peru the little known tutupaca volcano 17 01 s 70 21 w located at the southern end of the peruvian arc is a dacitic dome complex that experienced a large explosive eruption during historical times based on historic chronicles and our radiometric data this eruption occurred 218 14 abp probably between 1787 and 1802 ad this eruption was characterised by a large sector collapse that triggered a small debris avalanche 1 km3 and an associated pyroclastic eruption whose bulk volume was 6 5 7 5 107 m3 both units were emplaced synchronously and spread onto the plain situated to the northeast of tutupaca volcano the spatial and temporal relationship between the debris avalanche and the pyroclastic density current deposits coupled with the petrological similarity between the juvenile fragments in the debris avalanche the pyroclastic density current deposits and the pre avalanche domes indicates that juvenile magma was involved in the sector collapse large amounts of hydrothermally altered material are also found in the avalanche deposit thus the ascent of a dacitic magma coupled with the fact that the tutupaca dome complex was constructed on top of an older altered volcanic sequence probably induced the destabilisation of the hydrothermally active edifice producing the debris avalanche and its related pyroclastic density currents this eruption probably represents the youngest debris avalanche in the andes and was accompanied by one of the larger explosive events to have occurred in southern peru during historical times

largest explosive eruption in historical times in the andes at huaynaputina volcano a d 1600 southern peru the largest explosive eruption volcanic explosivity index of 6 in historical times in the andes took place in a d 1600 at huaynaputina volcano in southern peru according to chronicles the eruption began on february 19 with a plinian phase and lasted until march 6 repeated tephra falls pyroclastic flows and surges devastated an area 70 40 km2 west of the vent and affected all of southern peru and earthquakes shook the city of arequipa 75 km away eight deposits totaling 10 2 13 1 km3 in bulk volume are attributed to this eruption 1 a widespread 8 1 km3 pumice fall deposit 2 channeled ignimbrites 1 6 2 km3 with 3 ground surge and ash cloud surge deposits 4 widespread co ignimbrite ash layers 5 base surge deposits 6 unconfined ash flow deposits 7 crystal rich deposits and 8 late ash fall and surge deposits disruption of a hydrothermal system and hydromagmatic interactions are thought to have fueled the large volume explosive eruption although the event triggered no caldera collapse ring fractures that cut the vent area point to the onset of a funnel type caldera collapse

quiescent phase evolution of a surge type glacier black rapids glacier alaska u s a black rapids glacier a surge type glacier in the alaska range most recently surged in 1936 37 and is currently in its quiescent phase mass balance ice velocity and thickness change have been measured at three to ten sites from 1972 to 1994 the annual speed has undergone cyclical fluctuations of as much as 45 about the mean speed ice thickness and surface slope did not change enough to cause the speed fluctuations through changes in ice deformation which indicates that they are being driven by changes in basal motion the behavior of black rapids glacier during this quiescent phase is significantly different from that variegated glacier another well studied surge type glacier in alaska the present medial moraine configuration of black rapids glacier indicates that a surge could occur at any time however ice velocity data indicate that the next surge may not be imminent we believe that there is little chance that the next surge will cross and dam the delta river

airborne surface profiling of glaciers a case study in alaska a relatively lightweight and simple airborne system for surface elevation profiling of glaciers in narrow mountain valleys has been developed and tested the aircraft position is determined by kinematic global positioning system gps methods the distance to the glacier surface is determined with a laser ranger the accuracy is about 0 3 m sufficient to permit future changes to be observed over short time intervals long term changes can be estimated by comparison of profiles with existing maps elevation profiles obtained in 1993 94 from three glaciers in central and south central alaska are compared with maps made about 1950 the resulting area averaged seasonally corrected thickness changes during the interval are gulkana glacier central alaska range 11 m worthington glacier central chugach mountains 7 m and bear lake glacier kenai mountains 12 m all three glaciers retreated during the interval of comparison the estimated uncertainty in the average thickness change is 5 m which is mainly due to errors in the existing maps constraints on the accuracy of the maps are obtained by profiling in proglacial areas

el nino floods and culture change a late holocene flood history for the rio moquegua southern peru the 1997 1998 el nino generated large floods throughout southern peru especially in inland locations along rio moquegua using remote sensing hydraulic modeling field surveying and stratigraphic analyses we estimate the magnitude and frequency of this flood and determine a late holocene flood history for main stem and tributary sections modeling indicates a peak discharge of 450 m3 s for the 1998 flood with an estimated recurrence interval between 50 and 100 yr flood deposits for two large events dated to a d 690 and a d 1300 respectively exist in a small tributary system tiwanaku site abandonment ca a d 1000 predates the younger flood indicating that stratigraphically recognizable el nino driven floods were not a causal mechanism for abandonment although possessing three flood units main stem alluvium is considerably younger 320 14c yr b p than tributary alluvium evidencing the major channel widening and lateral reworking of the main stem

how much do we really know about glacier surging some of the ideas about glacier surging are considered mainly but not entirely in the light of observations of temperate glaciers in alaska u s a made within the last 15 years climate has an influence on surge recurrence interval climate and weather also affect surge initiation termination and magnitude regional studies lead to the speculation that subglacial till plays a key role in surging and it has been found under all surge type glaciers studied so far including black rapids and variegated glaciers alaska in most of the glaciers studied till deformation processes dominate the motion in quiescence the linked cavity model of surge triggering and rapid motion is not consistent with these observations but the limited coverage of the observations does not rule it out under parts of the glaciers studied the till observations in alaska raise old questions about the interaction between till and the hydraulic systems of temperate glaciers the role of stored water which observations show to be active even in winter on black rapids glacier is noted

late quaternary hydroclimatology of a hyper arid andean watershed climate change floods and hydrologic responses to the el nino southern oscillation in the atacama desert abstract although certain characteristics of the el nino southern oscillation enso are well known on contemporary timescales less is known about the magnitude frequency relationships of this atmospheric phenomenon on longer timescales or its relationship to widespread flooding especially in its core zone along the sub tropical andes where la nina or el nino episodes control regional hydroclimatology using a combination of stratigraphic evidence geochronologic dating 14c and osl stable isotope analyses and water geochemistry along the rio moquegua in the northern fringes of the atacama desert we assemble a paleoflood chronology for mainstem and tributary sections for the past ca 20 ka and ascertain the variation in enso frequency and magnitude because of the inherent watershed structure and regional hydroclimatology mid valley tributaries of the rio moquegua only flood during el nino episodes and thus provide an important proxy of extreme el ninos while mainstem stratigraphy records both la nina and el nino episodes el nino floods appear to have been pronounced during the late pleistocene and up to at least the younger dryas 12 000 cal yr bp while stratigraphic evidence of large el nino floods is lacking in tributary systems during the mid holocene flood stratigraphy in a 2 ka 7 m high terrace along the mainstem indicates an increased frequency and magnitude of large floods between ca 700 and 1610 ad as compared to the period from ca 160 bce to 700 ad with mega ninos occurring ca 1330 ad and ca 1650 ad water geochemistry and radiocarbon dating indicate that at least two major aquifers exist with wells and springs in the mid valley dating to 710 and 3100 14c yr bp respectively while water from a spring in the headwaters dates to 10 320 14c yr bp this range in dates suggests that groundwater flow in the mid valley is neither fossil water nor exclusively recharged from local precipitation while the older date for headwater sections suggests a more fossil groundwater source and lack of contemporary recharge the 18o similarity between groundwater in the mid valley and the regional meteoric water line mwl suggests a pacific moisture source and the role of el nino precipitation in recharging local aquifers thus el ninos and pacific sourced moisture are an important component of the regional hydroclimatology and the variation in enso frequency and intensity has significant social and hydrologic repercussions in these hyper arid settings

new insights into eruption source parameters of the 1600 ce huaynaputina plinian eruption peru in the central andes large plinian eruptions volcanic explosivity index 5 occur at a relatively high frequency i e average one every 2000 to 4000 years over the past 50 000 years in peru such recurring explosive activity represents a significant challenge for regions typically hosting several million people e g southern peru western bolivia and northern chile with vei 6 the 1600 ce huaynaputina eruption is considered the largest historical eruption in south america we have re examined the first plinian phase of this eruption in order to better assess critical eruption source parameters i e erupted volume plume height mass eruption rate eruption duration the revised bulk volume of the tephra fall deposit associated with the plinian phase is approximately 13 14 km3 almost twice the previous estimate 7 8 km3 within the 1 cm isopach based on methods including power law weibull function and bayesian linear regression tephra was dispersed by strong winds to the wnw as far as 400 km on peruvian territory and then in the pacific ocean seven villages were buried killing 1500 people the revised plume height estimate 32 2 2 5 km is consistent with the early estimations as a result the huaynaputina 1600 ce first eruption phase lies in the upper part of the plinian field close to the ultra plinian transition making this event one of the largest in the past millennium which coincides with results from recent studies on palaeoclimatic impacts

geodetic mass balance of surge type black rapids glacier alaska 1980 2001 2010 including role of rockslide deposition and earthquake displacement we determine the geodetic mass balance of surge type black rapids glacier alaska for the time periods 1980 2001 and 2001 2010 by combining modern interferometric synthetic aperture radar insar derived digital elevation models dems dems derived from archival aerial imagery laser altimetry data and in situ surface elevation measurements our analysis accounts for both the large rockslides and terrain displacements caused by the 2002 m7 9 earthquake on the denali fault which runs through black rapids glacier to estimate uncertainties we apply monte carlo simulations for the earthquake triggered rockslides we find a volume of 56 62 2 86 106 m3 equivalent to an average debris thickness of 4 44 0 24 m across the 11 7 km2 deposit area on the glacier terrain displacement due to the earthquake corresponds to an apparent glacier volume change of 53 1 106 m3 which would cause an apparent specific mass balance of 0 19 meter water equivalent mwe if not taken into account the geodetic mass balance of black rapids glacier is 0 48 0 07 mwe a 1 for the entire 30 year period but more negative for the period 2001 2010 0 64 0 11 mwe a 1 than the period 1980 2001 0 42 0 11 mwe a 1 in agreement with trends indicated by in situ mass balance measurements elevation data indicate no net thickening of the surge reservoir between 1980 and 2010 in contrast to what is expected during the quiescent phase a surge of black rapids glacier in the near future is thus considered unlikely

the eruptive chronology of the ampato sabancaya volcanic complex southern peru abstract we have reconstructed the eruptive chronology of the ampato sabancaya volcanic complex southern peru on the basis of extensive fieldwork and a large dataset of geochronological 40 k 40 ar 14 c and 3 he and geochemical major and trace element data this volcanic complex is composed of two successive edifices that have experienced discontinuous volcanic activity from middle pleistocene to holocene times the ampato compound volcano consists of a basal edifice constructed over at least two cone building stages dated at 450 400 ka and 230 200 ka after a period of quiescence the ampato upper edifice was constructed firstly during an effusive stage 80 70 ka and then by the formation of three successive peaks the northern southern 40 20 ka and central cones 20 10 ka the southern peak which is the biggest experienced large explosive phases resulting in deposits such as the corinta plinian fallout during the holocene eruptive activity migrated to the ne and constructed the mostly effusive sabancaya edifice this cone comprised many andesitic and dacitic blocky lava flows and a young terminal cone mostly composed of pyroclastic material most samples from the ampato sabancaya define a broad high k magmatic trend composed of andesites and dacites with a mineral assemblage of plagioclase amphibole biotite ortho and clino pyroxene and fe ti oxides a secondary trend also exists corresponding to rare dacitic explosive eruptions i e corinta fallout and flow deposits both magmatic trends are derived by fractional crystallisation involving an amphibole rich cumulate with variable amounts of upper crustal assimilation a marked change in the overall eruptive rate has been identified between ampato 0 1 km 3 ka and sabancaya 0 6 1 7 km 3 ka this abrupt change demonstrates that eruptive rates have not been homogeneous throughout the volcano s history based on tephrochronologic studies the late holocene sabancaya activity is characterised by strong vulcanian events although its erupted volume remained low and only produced a local impact through ash fallout we have identified at least 6 eruptions during the last 4 5 ka including the historical ad 1750 1784 and 1987 1998 events on the basis of this recurrent low to moderate explosive activity sabancaya must be considered active and a potentially threatening volcano

a 38 000 year record of floods and debris flows in the ilo region of southern peru and its relation to el nino events and great earthquakes abstract previous work throughout the ilo region of south coastal peru has documented the existence of flood and debris flow deposits produced by two el nino events evidently much more severe than any in recent history these two events have been dated to ca ad 1300 1400 and ad 1607 08 the late pleistocene to holocene record of older sedimentary deposits in this region is dominated by flood and debris flow deposits of similar scale these older deposits have been described and dated from three coastal alluvial fan sites these deposits which are as old as 38 200 years are dominated by massive debris flow deposits several tens of cm thick typically composed of cobble and boulder sized clasts in a matrix of silty sand with characteristics indicating generation by heavy rainfall in an arid environment twenty two radiocarbon dates and a single infrared stimulated luminescence date show that particularly severe el nino events occurred throughout the late pleistocene and two of three divisions of the holocene with significantly different frequencies the period of greatest activity was during the early holocene when at least six such events took place during a period of ca 3600 years beginning near the end of the younger dryas ca 12 000 years ago one of these events produced a debris flow that may have caused abandonment of the paleo indian site at quebrada tacahuay one of the oldest on the andean coast no severe events took place during the middle holocene between ca 8400 and 5300 years ago when a wide variety of other paleoclimate proxy records indicate that the el nino southern oscillation regime was particularly weak since ca 5300 years ago four of these severe events have taken place the late pleistocene sequence is constrained by only two dates which indicate that at least ten severe events took place between ca 38 200 and 12 900 years ago mechanisms probably responsible for generating these large scale deposits include 1 mega ninos that produced anomalously heavy rainfall along most or all of the central andean coast 2 el ninos that occurred shortly after great earthquakes that produced large amounts of sediment or 3 el ninos that produced anomalously heavy local rainfall the existence of these large scale deposits in the ilo region implies a level of hazard much higher than indicated by the historical record alone

timing and emplacement dynamics of newly recognised mass flow deposits at 8 12ka offshore soufriere hills volcano montserrat how submarine stratigraphy can complement subaerial eruption histories this contribution describes two mass movement deposits total volume 0 5 km3 identified in seven marine cores located 8 to 15 km offshore southern montserrat west indies the deposits were emplaced in the last 35 ka and have not previously been recognised in either the subaerial or distal submarine records age constraints provided by radiocarbon dating show that an explosive volcanic eruption occurred at ca 8 12 ka emplacing a primary eruption related deposit that overlies a large 0 3 km3 reworked bioclastic and volcaniclastic flow deposit formed from a shelf collapse between 8 and 35 ka the origin of these deposits has been deduced through the correlation of marine sediment cores component analysis and geochemical analysis the 8 12 ka primary volcanic deposit was likely derived from a highly erosive pyroclastic flow from the soufriere hills volcano that entered the ocean and mixed with the water column forming a water supported density current previous investigations of the eruption record suggested that there was a hiatus in activity at the soufriere hills volcano between 16 and 6 ka the ca 8 12 ka eruptive episode identified here shows that this hiatus was shorter than previously hypothesised and thus highlights the importance of obtaining an accurate and complete marine record of events offshore from volcanic islands and incorporating such data into eruption history reconstructions comparisons with the submarine deposit characteristics of the 2003 dome collapse also suggests that the 8 12 ka eruptive episode was more explosive than eruptions from the current eruptive episode