multistage damming of the rhine river by tephra fallout during the 12 900 bp plinian laacher see eruption germany syn eruptive rhine damming i abstract the rhine the largest river in western europe was dammed during the plinian laacher see eruption lse 12 900 bp damming during the climactic plinian episode of lse occurred both upstream and downstream of the broad tectonic lower neuwied basin lnb that interrupts the narrow rhine canyon we here document details of the upstream damming at the bottleneck entrance to the lnb near the present city of koblenz our reconstruction is based on a high resolution analysis and correlation of the complex intercalation of primary fallout tephra relics with fluvially reworked laacher see tephra in the lnb tephra units representing complete eruptive cycles repeatedly fell on drained ground in between one minor and 4 major flooding events even at the base of side channels that had been active prior to the lse and that had been flooded by a preceding flooding event this demonstrates that flooding occurred generally during breaks and not during fallout events the repeated formation and breach of a dam at the upstream entrance of the lnb koblenz dam consisting of fallout components and driftwood washed together convincingly explains the multiple repetition of the drainage of the channels in the lnb followed by large magnitude flooding in rapid succession the strongly pulsating nature of the lse reflected in multiple interruptions of eruptive activity fundamentally controlled the damming and flooding dynamics the rhine became completely blocked during distinct fallout phases due to overloading with pumice that had fallen into the river and its major tributaries the temporary dam collapsed during eruptive breaks this is the first recorded example of tephra fallout damming a major watercourse the extremely low gradient of the rhine river allowed the repeated accumulation of large volumes of water in a long multi phase dammed up lake lake koblenz that extended along the upstream course of the river for up to c 30 km despite the low height damming of a major river while a large plinian eruption is in full progress represents an extraordinary challenge for hazard mitigation this is especially pertinent for an area close to and downwind from the vent and therefore simultaneously affected by massive plinian fall such as in the lnb

incipient granular mass flows at the base of sediment laden floods and the roles of flow competence and flow capacity in the deposition of stratified bouldery sands extraordinary floods commonly produce thick deposits as a result of the entrainment and transport often at high concentrations of large volumes of clastic detritus this paper describes the sedimentology of a stratified bouldery sand deposit generated by a 20 km3 breakout flood from intracaldera lake taupo following the 1 8 ka taupo eruption the proximal deposit is characterised by a strongly bimodal grain size distribution with downstream fining boulders enclosed in a poorly sorted and weakly bedded coarse sand to gravel matrix this combination of characteristics is inferred to reflect simultaneous deposition of small and large particles by two distinct mechanisms i accretion of the sandy matrix from an incipient basal granular mass flow as the flood lost capacity and ii progressive emplacement of the boulders as the flood lost the competence to roll them along the channel floor the first mechanism is a product of a nonlinear particle concentration gradient in the sediment laden flow which caused it to segregate into a basal incipient granular mass flow that formed the transient depositional system indicated by the lithofacies the second mechanism is a function of the superjacent turbulent turbid flow that acted as the primary transporting system this pairing of transport and depositional systems is likely to characterise many sediment laden aqueous flows with clay mineral concentrations too low to develop non newtonian rheologies such as jokulhlaups and lahars that entrain unweathered pyroclastic material qualitative and quantitative differences in the way bedload is transported in the presence of this basal granular mass flow as compared to simple water make impossible accurate estimates of flow competence based on shields entrainment functions for beds of both uniform and mixed grain size distribution

the formation and failure of natural dams of the numerous kinds of dams that form by natural processes dams formed from landslides glacial ice and late neoglacial moraines present the greatest threat to people and property landslide dams form in a wide range of physiographic settings the most common types of mass movements that form landslide dams are rock and debris avalanches rock and soil slumps and slides and mud debris and earth flows the most common initiation mechanisms for dam forming landslides are excessive rainfall and snowmelt and earthquakes landslide dams can be classified into six categories based on their relation with the valley floor type i dams 11 of 184 landslide dams from around the world that we were able to classify do not reach from one valley side to the other type ii dams 44 span the entire valley floor in some cases depositing material high on opposite valley sides type iii dams 41 move considerable distances both upstream and downstream from the landslide failure type iv dams 1 are rare and involve the contemporaneous failure of material from both sides of a valley type v dams 1 also are rare and are created when a single landslide sends multiple tongues of debris into a valley and forms two or more landslide dams in the same reach of river type vi dams 3 involve one or more failure surfaces that extend under the stream or valley and emerge on the opposite valley side many landslide dams fail shortly after formation in our sample of 73 documented landslide dam failures 27 of the landslide dams failed less than 1 day after formation and about 50 failed within 10 days over topping is by far the most common cause of failure the timing of failure and the magnitude of the resulting floods are controlled by dam size and geometry material characteristics of the blockage rate of inflow to the impoundment size and depth of the impoundment bedrock control of flow and engineering controls such as artificial spill ways diversions tunnels and planned breaching by blasting or conventional excavation glacial ice dams can produce at least nine kinds of ice dammed lakes the most dangerous are lakes formed in main valleys dammed by tributary glaciers failure can occur by erosion of a drainage tunnel under or through the ice dam or by a channel over the ice dam cold polar ice dams generally drain supraglacially or marginally by downmelting of an outlet channel warmer temperate ice dams tend to fail by sudden englacial or subglacial breaching and drainage late neoglacial moraine dammed lakes are located in steep mountain areas affected by the advances and retreats of valley glaciers in the last several centuries these late neoglacial dams pose hazards because 1 they are sufficiently young that vegetation has not stabilized their slopes 2 many dam faces are steeper than the angle of repose 3 these dams and lakes are immediately downslope from steep crevassed glaciers and near vertical rock slopes and 4 downstream from these dams are steep canyons with easily erodible materials that can be incorporated in the flow and increase flood peaks the most common reported failure mechanism is overtopping and breaching by a wave or series of waves in the lake generated by icefalls rockfalls or snow or rock avalanches melting of ice cores or frozen ground and piping and seepage are other possible failure mechanisms natural dams may cause upstream flooding as the lake rises and downstream flooding as a result of failure of the dam although data are few for the same potential energy at the dam site ownstream flood peaks from the failure of glacier ice dams are smaller than those from landslide moraine and structed earth fill and rock fill dam failures moraine dam failures appear to produce some of the largest downstream flood peaks for potential energy at the dam site greater than 1011 1012 joules differences in flood peaks natural dam failures appear to be controlled by dam characteristics and failure mechanisms

paleohydrology and sedimentology of a post 1 8 ka breakout flood from intracaldera lake taupo north island new zealand sudden releases of impounded water from lakes in volcanic regions constitute a major and frequently repeated hazard an outburst flood from taupo caldera new zealand released 20 km 3 of water within decades following an ignimbrite emplacing eruption ca 1 8 ka paleohydrologic reconstruction of the taupo flood provides estimates of peak discharge at the outlet in the range 17 000 35 000 m 3 s dimensionless analysis demonstrates that 1 failure of the barrier was essentially instantaneous 2 the event may be treated hydraulically as a dam break and 3 the peak discharge was a function of outlet geometry rather than lake volume or breach formation rate paleohydraulic reconstructions based on empirical relations derived from historic dam breaches yield only order of magnitude estimates of peak discharge calculations based on the physical dimensions of the outlet channel and hydraulic principles are likely to be more accurate and are in close agreement with computer implemented dynamic flow routing models the latter give peak discharges and maximum stage levels similar to constraints imposed by field evidence and estimates of flow depth and velocity the long duration of the taupo flood and the relatively narrow confined flood route resulted in minimal attenuation of the flood wave compared with modern dam breach events and flood deposits can be traced as far as 232 km downstream caldera lake breakout floods may be among the most far reaching hazards associated with volcanism

distal fluvio lacustrine volcaniclastic resedimentation in response to an explosive silicic eruption the pliocene mushono tephra bed central japan the pliocene mushono tephra bed central japan is characterized by fluvio lacustrine volcaniclastic resedimentation at distal locations from the volcanic source sedimentary responses to an explosive eruption are well recorded in the resedimented mushono tephra bed immediately after the eruption large volumes of primary volcaniclastic material were reworked from proximal areas and discharged into the distal basin overloading of fluvial systems by pyroclastic debris resulted in the development of low sinuosity active fluvial channel systems that carried sediment laden hyperconcentrated flows the fluvial system flowed into a lake more than 150 km from the inferred source location resulting in delta formation and progradation primary pyroclastic fall deposits are only a few centimeters thick but are overlain by reworked volcaniclastic deposits more than 5 m thick which show coarsening upward successions representing fluvio lacustrine deltaic progradation the reworked succession consists of six sedimentary facies which can be interpreted as prodelta mouth bar distributary channel hyperconcentrated flow inter distributary lowland or floodplain and slump deposits rounded cobble sized pumice clasts in prodelta and distributary channel deposits and thick reworked deposits imply that the majority of resedimented volcaniclastic material was derived from voluminous proximal ignimbrite rather than thin fine grained distal fallout ash material vertical facies changes are concordant with a decrease in sediment yields with time lateral facies distributions correspond to episodic influxes of volcaniclastic sediments that controlled the morphology of discrete fluvio lacustrine deltas the distal sedimentary response to an explosive silicic eruption is thus clearly recorded in vertical and lateral facies distribution of reworked volcaniclastic sediments

petrologic variations in quaternary volcanic rocks british columbia and the nature of the underlying upper mantle volcanic activity has produced late tertiary and quaternary cinder cones and flows between the snake river plain u s a and the yukon territory canada the rock types include basanites alkali olivine basalts high iron basalts hawaiites ankaramites nephelinites and olivine tholeiites the alkali olivine basalts basanites and hawaiites sampled are chemically similar to rocks from the mid atlantic islands associated with the volcanic rocks are xenoliths of ultramafic rocks gabbros granites and granulites

impacts of explosive volcanism on distal alluvial sedimentation examples from the pliocene holocene volcaniclastic successions of japan abstract volcanic activities can create cataclysmic hazards to surrounding environments and human life not only during the eruption but also by hydrologic remobilisation lahar processes after the cessation of eruptive activity although there are many studies dealing with the assessment and mitigation of volcanic hazards these are mostly concentrated on primary eruptive processes in areas proximal to active volcanoes however the influence of volcaniclastic resedimentation may surpass the impacts of primary eruptive activity in terms of both extent and persistence and can ultimately result in severe hazards in downstream areas examination of the volcaniclastic successions of non marine pliocene holocene sedimentary basins in japan has revealed hydrological volcaniclastic sedimentation in fluvial and lacustrine environments hundreds of kilometres from the inferred source volcano impacts on these distal and often spatially separated basins included drastic changes in depositional systems caused by sudden massive influxes of remobilised pyroclastic material typical volcaniclastic beds comprise centimetre to decimetre thick primary pyroclastic fall deposits overlain by metre to 10s of metres thick resedimented volcaniclastic deposits intercalated in sedimentary successions of non volcanic provenance the relatively low component of primary pyroclastic fall deposits in the volcaniclastic beds suggests that 1 potential volcanic hazards would be underestimated on the basis of primary pyroclastic fall events alone and 2 the majority of resedimented material was likely derived from erosion of non welded pyroclastic flow deposits in catchment areas rather than remobilisation of local fallout deposits from surrounding hillslopes the nature distribution and sequence of facies developed by distal volcaniclastic sediments reflect the influence of 1 proximity to ignimbrite but not directly with the distance to the eruptive centre 2 ignimbrite nature non welded or welded and volume 3 temporal changes in sediment flux from the source area 4 the physiography and drainage patterns of the source area and the receiving basin and any intervening areas and 5 the formation of ephemeral dam lakes and intra caldera lakes whose potential catastrophic failure can impact distal areas models of the styles and timing of distal volcaniclastic resedimentation are thus more complicated than those developed for proximal settings of stratovolcanoes and their volcaniclastic aprons and hence present different challenges for hazard assessment and mitigation

depositional processes of primary to reworked volcaniclastics on an alluvial plain an example from the lower pliocene ohta tephra bed of the tokai group central japan abstract a change from primary to reworked volcaniclastic deposits in the lower pliocene ohta tephra bed has been identified using facies analysis the ohta tephra bed is a widespread marker tephra distributed over a distance of more than 130 km the source volcano of this tephra is inferred to lie 50 to 100 km north of the most proximal ohta tephra bed the ohta tephra bed shows a change in volcaniclastic facies in distal areas eight sedimentary facies are recognized these are interpreted as pyroclastic fall pyroclastic flow and reworked mudflow flood flow channel fill swamp or backswamp lacustrine and ephemeral pond deposits in underlying and overlying non volcaniclastic beds four sedimentary facies are recognized indicating a fluvio lacustrine depositional system on an alluvial plain from the successions of volcaniclastic sedimentary facies deposition of the ohta tephra bed can be divided into stages i to vi each depositional stage is intimately related to volcanic activity stage i is the first eruption during which only the northern part of the study area was covered with fallout ash the second eruption stage of stage ii involved fallout ash distributed over all the study area these two fallout ash beds are derived from precursor eruptions stage iii reflects a period of volcanic quiescence and development of widespread lacustrine conditions the fallout ash may have destroyed vegetation and prompted greater discharge to form the large lacustrine deposits of stage iii a pyroclastic flow stage in stage iv is associated with an explosive eruption and resulted in pyroclastic flow deposits reaching the northern part of the study area stage v is characterized by mudflow deposits widespread mudflow deposits also originated from high water and clastics discharge on the unvegetated surface mudflows extend more than 180 km from the source volcano abundant supply of volcaniclastic material and high water and clastic discharge enhanced a rapid progradation of alluvial plains final reworking occurred during stage vi when volcaniclastic sediments were deposited from a fluvio lacustrine system the reworking system comprises initial mudflows followed by a normal fluvial system normal stream flow the mudflow deposits of stage v are traceable with roughly constant thickness but the volcaniclastic content of the deposits decreases in distal areas the fluvial system deposits of stage vi are thicker and have less abundant volcaniclastics in distal areas fluvial systems were braided streams promoted by volcanic eruption while they were meandering streams during the pre eruption period some eruption induced braided streams remained for a while following the cessation of volcanic activity

sedimentary records and luminescence chronology of late holocene palaeofloods in the luni river thar desert northwest india river luni is the only well integrated river system in the thar desert of india this river catastrophically flooded due to unusually heavy rainfall in the catchment area during july 1979 in order to establish whether floods of this magnitude have occurred in the recent geological past sedimentary records of palaeofloods occurring in the sindari gorge in the lower luni basin were investigated the principal slackwater flood deposits were observed in a back flooded tributary near bhuka analysis of the slackwater flood deposits preserved in the back flooded tributary and their luminescence dating suggests that the luni river has experienced at least 17 extreme floods during the past millennium evidence from the bhuka site also suggests that no floods comparable in magnitude to the july 1979 megaflood have occurred during this period this observation is in conformity with the palaeoflood record of central india comparison of the long term monsoon rainfall series for the luni basin and the indian region reveals a clear link between the two and indicates that the clustering of large floods in the last few decades and during the medieval warming period is a regional phenomenon associated with wetter conditions this correlates with a regionally extended episode of landscape stability denoted by stabilization of dunes in this region

the 26 may 1982 breakout flows derived from failure of a volcanic dam at el chichon chiapas mexico the eruptions of el chichon between 28 march and 4 april 1982 produced a variety of pyroclastic deposits the climactic phase on 3 april at 07 35 4 april at 01 35 gmt destroyed the central andesitic dome and fed pyroclastic surges and flows that dammed nearby drainages including the magdalena river by late april a lake had formed 4 km long and 300 400 m wide containing a volume of 26 10 6 m 3 of hot water at 01 30 on 26 may the pyroclastic dam was breached and surges of sediment and hot water soon inundated the town of ostuacan 10 km downstream this hot flood was finally contained at penitas hydroelectric dam 35 km downstream where one fatality occurred and three workers were badly scalded stratigraphic and sedimentologic evidence indicates that the rapidly draining lake initially discharged two debris flows followed by five smaller debris flows and water surges the main debris flows became diluted with distance and by the time they reached ostuacan they merged into a single hyperconcentrated flow with a sediment concentration of 30 vol deposits from this hyperconcentrated flow were emplaced for 15 km as far as the confluence with another river the mas pac below which the flow was diluted to sediment laden streamflow the minimum volume of the breakout flow deposits is estimated at 17 10 6 m 3 from high water marks flow profiles and simulations utilizing the dambrk code from the national weather service we calculated a maximum peak discharge of 11 000 m 3 s at the breach this maximum peak discharge occurred 1 h after initial breaching the calculations indicated that 2 h were required to drain the lake

sedimentology and event timing of a catastrophic volcaniclastic mass flow volcan hudson southern chile eruption of volcan hudson aisen region southern chile on 12 august 1971 melted up to 80 of ice within the 9 km wide hudson caldera rapid mobilisation of abundant fluvio glacial sediment within the adjacent huemules valley led to the generation of a catastrophic mass flow which swept westwards along the valley eventually entering the fjord some 40 km downstream the mass flow developing from one source and being unaffected by tributary input was characterised by three events within the flood hydrograph 1 a precessional flood possibly linked to breakage of an early formed dammed lake 2 a main mass flow which varied in its rheological behaviour from plug flow to hyperconcentrated flow and 3 a recessional flood at the tail of the main flow which diluted towards a concentrated streamflow the main debris flow displayed characteristics of pulsing internal segregation density stratification significant downstream sediment assimilation cohesive frictional freezing with ensuing fluidisation and development of an advancing runout phase consideration of these sedimentary characteristics and spatial temporal flow transformations are used to propose a schematic model of event timing within this catastrophic volcaniclastic flow

controls on accumulation of a volcaniclastic fan ruapehu composite volcano new zealand the whangaehu fan is the youngest sedimentary component on the eastern ring plain surrounding ruapehu volcano fan history comprises constructional 830 200 years bp and dissectional 200 years bp phases the constructional phase includes four aggradational periods associated with both syneruptive and inter eruptive behavior all four aggradational periods began when deposition by large lahars changed flow conditions on the fan from channelized to unchannelized subsequent behavior was a function of the rate of sediment influx to the fan the rate of sediment influx in turn was controlled by frequency and magnitude of volcanic eruptions short term climate change and the amount of sediment stored on the volcano flanks fanwide aggradation occurred when rates of sediment influx and deposition on the fan were high enough to maintain unchannelized flow conditions on the fan surface maintenance of an undissected surface required sedimentation from frequent and large lahars that prevented major dissection between events these conditions were best met during major eruptive episodes when high frequency and magnitude eruptions blanketed the volcano flanks with tephra and rates of lahar initiation were high during major eruptive episodes volcanism is the primary control on sedimentation climatic variations do not influence sediment accumulation local aggradation occurred when lahars were too small to maintain unchannelized flow across the entire fan in this case only the major channel system received much sediment following the deposition from the initial lahar this localized aggradation occurred if 1 the sediment reservoir on the flank was large enough for floods to bulk into debris flows and 2 sedimentation events were frequent enough to maintain sediment supply to only some parts of the fan these conditions were met during both minor eruptive and inter eruptive episodes in both cases a large sediment reservoir remained on the volcano flanks from previous major eruptive intervals periods of increased storm activity produced floods that bulked to relatively small debris flows when the sediment reservoir was depleted the fan entered the present dissectional phase syneruptive and noneruptive lahars are mostly channelized and sediment bypasses the fan fan deposits are rapidly reworked this is the present case at ruapehu even though the volcano is in a minor eruptive episode and the climate favors generation of intense storm floods

petrology of recent lava flows volcano mountain yukon territory canada abstract volcano mountain yukon territory a quaternary edifice located near the junction of the pelly and yukon rivers erupted two recent nephelinite lava flows between 3000 and 7300 years bp and a third recent lava flow that is probably younger the porphyritic nephelinites have olivine phenocrysts fo 91 fo 79 and ti rich diopside microphenocrysts the groundmass consists of olivine fo 83 fo 80 ti rich diopside ulvospinel magnetite solid solutions nepheline leucite patches of residual glass and traces of calcite the nephelinites also carry spinel lherzolite xenoliths and xenocrystic olivine fo 87 fo 90 the xenocrysts have lower cao contents than the phenocrysts samples from the two older flows have similar thompson space representations whereas the youngest flow has a distinctly different representation the thompson space representations of the volcano mountain nephelinites are different from the representations of nephelinites from other centers in the cordillera thompson space representations and pearce element ratio diagrams suggest the two older flows are comagmatic whereas the youngest flow is from a separate magma batch thermodynamic modeling suggests the magmas that supplied the lava flows originated at pressures near 2 5 gpa left behind a residue containing olivine and rose along an isenthalpic path after separating from the source rocks

breakout flood from an ignimbrite dammed valley after the 5 ka numazawako eruption northeast japan numazawa volcano in northeast japan erupted most recently at ca 5 ka forming a 2 km diameter caldera and emplacing at least 4 km 3 of valley confined ignimbrite the ignimbrite dammed the tadami river to a depth of 100 m temporarily impounding 1 6 km 3 of water overtopping of the barrier triggered breaching and catastrophic release of the dam lake pyroclastic material redeposited by the ensuing flood is widely distributed along the tadami and agano rivers in deposits tens of meters thick as far as the coastal niigata plain 150 km downstream of the volcano evidence for damming and flooding found along the rivers includes 1 fine grained thinly laminated lacustrine deposits upstream of the pyroclastic blockage 2 30 m thick pumiceous debris flow and hyperconcentrated flow deposits that show continuous sedimentation with no major hiatus downstream of the volcano 3 fine grained slackwater deposits at tributary river mouths and 4 large flood boulders that form lags on stripped ignimbrite surfaces and younger terrace surfaces or cropping out within hyperconcentrated flow deposits paleohydraulic estimation techniques indicate a peak discharge of 30 000 50 000 m 3 s at the breach point burial of neolithic jomon settlements by flood and volcaniclastic aggradational deposits at distal locations indicates that such indirect volcanogenic hazards are significant even where direct primary consequences of volcanic eruptions are minor or absent

downstream dilution of a lahar transition from debris flow to hyperconcentrated streamflow nearly instantaneous melting of snow and ice by the march 19 1982 eruption of mount st helens released a 4 106 m3 flood of water from the crater that was converted to a lahar volcanic debris flow through erosion and incorporation of sediment by the time it reached the base of the volcano over the next 81 km that it traveled down the toutle river the flood wave was progressively diluted through several mechanisms a transformation from debris flow to hyperconcentrated streamflow began to occur about 27 km downstream from the crater when the total sediment concentration had decreased to about 78 by weight 57 by volume the hyperconcentrated lahar runout flood wave transporting immense quantities of sand in suspension continued to experience progressive downstream dilution although turbulence was significantly dampened by the extremely high suspended load very large standing waves and antidune waves were observed the hyperconcentrated lahar runout flow deposited an unusual faintly stratified coarse sand which locally contained small isolated gravel lenses very similar deposits in the quaternary stratigraphy of mount st helens and other cascades volcanoes suggest that lahars may be more frequent than previously recognized

outburst flood evolution at russell glacier western greenland effects of a bedrock channel cascade with intermediary lakes abstract glacial lake outburst floods have produced a distinctive and widespread quaternary record both onshore and offshore via widespread and intense geomorphological impacts yet these impacts remain poorly understood due to a lack of modern analogues this study therefore makes a systematic quantification of the evolution of a bedrock channelled outburst flood channel topography was obtained from digitised aerial photographs a 5 m grid resolution dem and bathymetric surveys flood inundation was measured in the field from dgps measurements flood evolution was analysed with application of a numerical model novel flood metrics including rate of rise to peak and persistence above a value of hydraulics were calculated the key findings were that outburst floods i that enter intermediary lakes are dramatically attenuated in terms of propagation whilst basin filling proceeds to the outlet level dissipated of energy due to considerable flow recirculation during the rising stage of the flood and after overtopping of the outlet are moderated in terms of peak discharge ii can have kinematic waves that are introduced to a flood via hydraulic ponding iii may develop a partitioning of flow regimes for instantaneous peak discharge bed shear stress and froude number due to time transgressive changes in channel cross section specifically between inner channel and overbank regions furthermore normalised change in cross sectional area and normalised change in channel slope can be used to suggest the relative magnitude of instantaneous peak discharge net erosion along a reach can be related to hydraulic persistence above a marker value and net deposition can be related to a time to peak value reconstructions of quaternary outburst floods should consider the importance of including intermediary lakes modern hazard mitigation studies could usefully note the potential use of reservoirs as an outburst flood alleviation resource

stratigraphy of the fort selkirk volcanogenic complex in central yukon and its paleoclimatic significance ar ar and paleomagnetic data brunhes matuyama kaena and mammoth age basaltic lava flows tertiary quaternary selkirk volcanics were sampled in west central yukon the mean characteristic remanent magnetization chrm direction of the flows sampled in this and previous studies has a declination of 348 7 and an inclination of 70 8 n 42 k 99 6 95 2 2 all on lower hemisphere the time range represented in this study ca 3 25 to ca 0 004 ma is great enough to have confidently averaged secular variation sediment associated with the basalt has a mean declination of 7 6 and inclination of 78 8 n 5 k 5 6 95 35 7 a new 40ar 39ar date on the reversely magnetized basal basalts at ne ch e ddhawa places the eruption in the mammoth subchron of the gauss normal chron the newly dated basal basalt at ne ch e ddhawa precedes the initial continental glaciation in yukon and is older than the fort selkirk vent lower mushroom which was previously thought to be the oldest eruption at fort selkirk volcanic complex

formation and failure of volcanic debris dams in the chakachatna river valley associated with eruptions of the spurr volcanic complex alaska abstract the formation of lahars and a debris avalanche during holocene eruptions of the spurr volcanic complex in south central alaska have led to the development of volcanic debris dams in the chakachatna river valley debris dams composed of lahar and debris avalanche deposits formed at least five times in the last 8000 10 000 years and most recently during eruptions of crater peak vent in 1953 and 1992 water impounded by a large debris avalanche of early holocene age may have destabilized an upstream glacier dammed lake causing a catastrophic flood on the chakachatna river a large alluvial fan just downstream of the debris avalanche deposit is strewn with boulders and blocks and is probably the deposit generated by this flood application of a physically based dam break model yields estimates of peak discharge q p attained during failure of the debris avalanche dam in the range 10 4 q p 6 m 3 s 1 for plausible breach erosion rates of 10 100 m h 1 smaller short lived lahar dams that formed during historical eruptions in 1953 and 1992 impounded smaller lakes in the upper chakachatna river valley and peak flows attained during failure of these volcanic debris dams were in the range 10 3 q p 4 m 3 s 1 for plausible breach erosion rates volcanic debris dams have formed at other volcanoes in the cook inlet region aleutian arc and wrangell mountains but apparently did not fail rapidly or result in large or catastrophic outflows steep valley topography and frequent eruptions at volcanoes in this region make for significant hazards associated with the formation and failure of volcanic debris dams

secondary hydroeruptions in pyroclastic flow deposits examples from mount st helens abstract secondary hydroeruptions occur in pyroclastic flow deposits when water or ice is trapped beneath hot pyroclastic debris and rapidly heated to steam these eruptions display various styles of activity including fumarolic degassing tephra fountaining and explosive cratering the deposits which occupy the layer 3 stratigraphic position on the top of pyroclastic flow units can be distinguished from ash cloud material by lateral thickness variation clast composition and other sedimentary features the ejecta of secondary hydroeruptions comprise a subset of hydrovolcanic pyroclastic deposits a small secondary hydroeruption observed on the mount st helens pumice plain in 1981 produced tephra that was emplaced ballistically by deposition from base surges and by fallout from an eruption column stratigraphic descriptions and grain size analysis of the ejecta from several secondary craters on the pumice plain demonstrate that the bedforms produced by a hydroeruption change with crater diameter in particular craters of small diameter are surrounded by interbedded ripple laminated ash horizons and nonstratified fines depleted units large craters have ejecta ramparts comprised of coarse dunes and antidunes these bedform changes are related to a progressive increase in eruptive energy which produces base surges of greater power and eruptive columns of greater height we suggest that the style of activity displayed during a secondary hydroeruption is controlled by both the total thermal energy of the system and the permeability of the pyroclastic overburden

the history and dynamics of a welded pyroclastic dam and its failure the 2 360 bp eruption of mount meager british columbia began as an explosive dacitic sub plinian eruption that waned rapidly to a sustained period of vulcanian eruption triggered dome collapse events producing voluminous block and ash flow baf deposits the earliest baf deposits accumulated rapidly enough immediately downslope of the vent to retain heat and weld using the deposit as a paleoviscometer determines an effective viscosity of 109 1010 pa s during welding this prolific production of hot lava and block and ash flows in a steep mountainous terrain created a 110 m high largely impermeable dam capped by permeable non welded baf deposits and unconsolidated avalanche deposits that blocked the flow of the lillooet river and created a temporary lake the welded pyroclastic dam was compromised and overtopped at least once before the peak dam height was reached renewed eruption caused buildup of the dam to a maximum of 780 m above sea level asl and grew the temporary lake to an elevation of 740 m asl and a minimum volume of 0 55 km3 the rise of lake level led to catastrophic failure of the top of the dam generating an outburst flood that carved a canyon through most of the dam and resulted in a voluminous lahar that is traced at least 65 km downstream based on current flow rates of the lillooet river the lake would have overtopped the final dam at a minimum of 39 65 days after its formation the peak deluge lasted approximately 8 h and eroded a 2 5 km long canyon into the still hot dam core before returning to background flow rates

electron beam analytical instruments and the determination of modes spatial variations of minerals and textural features of rocks in polished section point counts based on x ray intensities collected with an automated electron beam analytical instrument can be used to estimate the accurate modes required by modern physical chemical modelling of rock forming processes in addition the spatial distributions of mineral species and mineral compositions can be extracted from the coordinates of the point counting grid statistical tests of whether the mineral and porosity distributions are uniform can be performed by comparing the results from subareas of the polished section the separate and composite contributions of the mineral phases to the texture and structure of the rock can be visually represented with mineral distribution maps qualitative differences in grain size can be recognized because the nonzero finite electron beam diameter can detect grain boundaries spots on the minerals are identified by comparing the ranking of x ray intensities with the ranking of intensities on a set of standards for example this method identified more than 88 of the spots on the surface of a polished section of an alkali olivine basalt a small additional number of spots approximately 1 were identified because their spectra were dominated by the x ray intensities of two characteristic elements this method was particularly efficient in identifying phases with a small grain size and simple chemistry e g apatite the modal amounts of an alkali olivine basalt obtained with a microprobe are within approximately one standard deviation of the results obtained by point counting the same areas of the polished section under reflected and transmitted light if the minerals are major constituents and occur only in the groundmass accessory and phenocryst phases agree to within approximately three standard deviations

sedimentary and geomorphic responses to ignimbrite emplacement readjustment of the waikato river after the a d 181 taupo eruption new zealand abstract the posteruption readjustment of landscapes to major explosive volcanic eruptions is one of the most dramatic processes in physical sedimentology and geomorphology large ignimbrite emplacing eruptions from rhyolitic caldera volcanoes can form new landscapes over 104 km2 create new depocenters in calderas and bury existing topography and drainage systems beneath hundreds of meters of pyroclastic material subsequent remobilization of this debris constitutes a widespread and enduring hazard in the central north island of new zealand the a d 181 taupo eruption destroyed hydrologic systems over 20 000 km2 when emplacement of the climactic ignimbrite filled valleys and depressions with up to 70 m of pyroclastic material and blocked the waikato river outlet to the intracaldera lake basin posteruption remobilization of this material in the waikato catchment was associated with axial stream reestablishment vertical lateral and proximal distal changes in volcaniclastic lithofacies preserved al

on subsurface stormflow predictions with simple kinematic theory for saturated and unsaturated flows this paper provides analytical solutions for some simple cases of subsurface stormflow the solutions are based on kinematic approximations in both unsaturated and saturated zones solutions for rising falling and partial equilibrium hydrographs are given the analytical model has been applied to data collected on a hillslope of the east twin brook catchment mendip united kingdom measured and predicted hillslope hydrographs were in good agreement

petrologic hypothesis testing with pearce element ratio diagrams derivation of diagram axes in petrology pearce element ratio per diagrams have been used i to determine whether members of a rock suite are co genetic ii to identify the minerals involved in differentiation processes and iii to evaluate the extent to which those mineral are involved the axis coefficients of each diagram are chosen such that sorting of minerals or combinations of minerals will generate unique and predictable trends unfortunately selection of the optimal combination of axis coefficients is a difficult task especially if the system being investigated has a large number of phases or complicated solid solution minerals our work has established a formal set of rules and matrix operations which facilitate the determination of per diagram axes coefficients this methodology can be used to determine the unit molar vector displacement caused by the addition or subtraction of a specific mineral given a set of axis coefficients it can also be used to create per diagrams on which minerals have predetermined vector displacements by designating all vector displacements to be parallel axis coefficients for assemblage test diagrams can be determined to test the following hypothesis the observed chemical variation is due to the addition or removal of a specific set of minerals alternatively by designating all vector displacements to be mutually perpendicular phase discrimination diagrams can be created which test whether the observed chemical variations require a specific phase to be involved in differentiation phase discrimination diagrams also provide a means to estimate the extent of that involvement this methodology facilitates construction of powerful yet simple per diagrams which provide an effective means of testing alternative differentiation hypotheses

coupled fluid dynamics sediment transport modelling of a crater lake break out lahar mt ruapehu new zealand summary lahars and other sudden onset floods are highly dynamic with temporally and spatially evolving sediment loads numerical models of sudden onset flood behaviour particularly those applied to real world events have tended to neglect sediment transport processes this is a serious though perhaps understandable shortcoming given a lack of field measurements on flow bed interactions the extra computational complexity required and the problem of iteratively updating plastic channel boundaries this paper applies a fluid dynamics model with suspended sediment and bedload transport to calculate flow conditions and rapid landscape change due to an initially dilute lahar from mt ruapehu new zealand we take advantage of an unprecedented dataset of net topographic changes determined from pre and post lahar airborne lidar surveys and time series of hydrological measurements to run and validate the model a sensitivity analysis of key controls on water surface elevation and final sediment thickness illustrates how the flow was supply limited in the upper reaches it was capable of transporting a greater volume and calibre of material than that available modelled patterns of net erosion and deposition can replicate those observed r 2 0 79 and are dominantly controlled by channel gradient and topographic

environmental impact of the 1 8 ka taupo eruption new zealand landscape responses to a large scale explosive rhyolite eruption abstract large scale ignimbrite eruptions from rhyolitic caldera volcanoes can trigger geologically instantaneous changes in sedimentary systems over huge areas by either burying existing environments or overloading them with vast quantities of unconsolidated particulate material the post eruption readjustment of the landscape to such perturbations is one of the most dramatic processes in physical sedimentology exemplified here by the 1 8 ka taupo eruption in the central north island of new zealand this eruption generated voluminous fall deposits then climaxed with emplacement of a c 30 km 3 non welded ignimbrite over a near circular area of c 20 000 km 2 approximately 90 of the area but the headwaters of many of the north island s largest rivers were impacted by both the primary pyroclastic fall and flow material large scale post eruption remobilisation of this material coupled with the re establishment of fluvial systems occurred in a distinct sequence as recorded by the evolution of sedimentary facies in different sub environments following an initial period dominated by mass flows re establishment of fluvial systems began with the headward erosion of box canyons through the ponded ignimbrite deposits a process often associated with the break out of temporary lakes aggradational streams developed in these channels rapidly evolved from shallow ephemeral sediment laden outbursts associated with flash flood events to deeper permanent braided rivers before declining sediment yields led to retrenchment of single thread rivers and a return to pre eruption gradients and bedloads years to decades later typically the modern profile of many streams and rivers follow closely their pre eruption profiles and incision and erosion is overwhelmingly confined to the deposits of the eruption itself although the general remobilisation pattern is similar for all impacted river systems detailed studies of the waikato rangitaiki mohaka ngaruroro and whanganui catchments show that the relative timing and scale of each eruption response phase differs between each catchment these reflect differences in catchment physiography and hydrology and the volume and type of pyroclastic material deposited in each ultimately the landscape response reflects the relative spatial distributions of and the volumetric ratios between the volumes of pyroclastic debris water and accommodation space in the basin cf kataoka and manville this volume

volcaniclastic resedimentation in distal fluvial basins induced by large volume explosive volcanism the ebisutoge fukuda tephra plio pleistocene boundary central japan the ebisutoge fukuda tephra plio pleistocene boundary central japan has a well recorded eruptive style history magnitude and resedimentation styles despite the absence of a correlative volcanic edifice this tephra was ejected by an extremely large magnitude and complex volcanic eruption producing more than 400 km 3 total volume of volcanic materials volcanic explosivity index 7 which extended more than 300 km away from the probable eruption centre remobilization of these ejecta occurred progressively after the completion of a series of eruptions resulting in thick resedimented volcaniclastic deposits in spatially separated fluvial basins more than 100 km from the source facies analysis of resedimented volcaniclastic deposits was carried out in distal fluvial basins the distal tephra 100 300 km from the source comprises two different lithofacies primary pyroclastic fall deposits and reworked volcaniclastic deposits the resedimented volcaniclastic succession shows five distinct sedimentary facies interpreted as debris flow deposits facies a hyperconcentrated flow deposits facies b channel fill deposits facies c floodplain deposits with abundant flood flow deposits facies d and floodplain deposits with rare flood deposits facies e resedimented volcaniclastic materials at distal locations originated from unconsolidated deposits of a climactic large ignimbrite forming eruption factors controlling inter and intrabasinal facies changes are 1 temporal change of introduced volcaniclastic materials into the basin 2 proximaldistal relationship and 3 distribution pattern of pyroclastic flow deposits relative to drainage basins thus studies of the ebisutoge fukuda tephra have led to a depositional model of volcaniclastic resedimentation in distal areas after extremely large magnitude eruptions an aspect of volcaniclastic deposits that has often been ignored or poorly understood

geomorphic and sedimentary evidence of a gigantic outburst flood from towada caldera after the 15 ka towada hachinohe ignimbrite eruption northeast japan abstract this study reports on geomorphic and sedimentary evidence for a gigantic breakout flood from towada caldera honshu island northeast japan as observed along the oirase river downstream from the outlet of the caldera lake a number of features of the oirase river are inconsistent with its present discharge including the occurrence of 1 hanging valleys and a horseshoe shaped waterfall in the upstream gorge area 2 boulder bars and scattered flood boulders 3 a dry valley at a high elevation in the gorge and 4 a large fan the sanbongi fan in the downstream area of the river the sanbongi fan is composed of thick lithic rich hyperconcentrated flow deposits that include pumice clasts derived from the 15 ka towada hachinohe ignimbrite and well rounded meter sized and outsized boulders derived from bedrock of welded ignimbrite the deposits are entirely aggradational with no major channels indicating the absence of a major hiatus during sedimentation the depositional facies also indicate that a single sheet like flood event deposited the sediment within the sanbongi fan area based on the age of the sanbongi fan the flood occurred between 15 and 12 ka after eruption of the towada hachinohe ignimbrite the most probable water source for the flood is towada caldera lake as suggested by landforms and sediments along the lake outlet a paleohydrological analysis indicates that at least 6 km 3 of water was released from the caldera at a peak discharge of 2 10 4 to 3 10 5 m 3 s 1 during the breakout flood although the sanbongi fan was previously considered to be a normal alluvial fan that formed during a stage of low stand sea level the present results show that the formation of the fan was closely related to a catastrophic flood from towada caldera rather than perennial fluvial activity climate change or a change in relative sea level

a catastrophic flood caused by drainage of a caldera lake at aniakchak volcano alaska and implications for volcanic hazards assessment aniakchak caldera located on the alaska peninsula of southwest alaska formerly contained a large lake estimated volume 3 7 10 9 m 3 that rapidly drained as a result of failure of the caldera rim sometime after ca 3400 yr b p the peak discharge of the resulting flood was estimated using three methods 1 flow competence equations 2 step backwater modeling and 3 a dam break model the results of the dam break model indicate that the peak discharge at the breach in the caldera rim was at least 7 7 10 4 m 3 s 1 and the maximum possible discharge was 1 1 10 6 m 3 s 1 flow competence estimates of discharge based on the largest boulders transported by the flood indicate that the peak discharge values which were a few kilometers downstream of the breach ranged from 6 4 10 5 to 4 8 10 6 m 3 s 1 similar but less variable results were obtained by step backwater modeling finally discharge estimates based on regression equations relating peak discharge to the volume and depth of the impounded water although limited by constraining assumptions provide results within the range of values determined by the other methods the discovery and documentation of a flood caused by the failure of the caldera rim at aniakchak caldera underscore the significance and associated hydrologic hazards of potential large floods at other lake filled calderas

distribution nature and origin of neogene quaternary magmatism in the northern cordilleran volcanic province canada the northern cordilleran volcanic province encompasses a broad area of neogene to quaternary volcanism in northwestern british columbia the yukon territory and adjacent eastern alaska volcanic rocks of the northern cordilleran volcanic province range in age from 20 ma to ca 200 yr b p and are dominantly alkali olivine basalt and hawaiite a variety of more strongly alkaline rock types not commonly found in the north american cordillera are locally abundant in the northern cordilleran volcanic province these include nephelinite basanite and peralkaline phonolite trachyte and comendite the most mgo rich nephelinites basanites and alkaline basalts from throughout the northern cordilleran volcanic province show trace element abundances and isotopic compositions that are consistent with an asthenospheric source region similar to that for average oceanic island basalt and for post 5 ma alkaline basalts from the basin and range our petrologic observations help constrain the origin of northern cordilleran volcanic province magmatism as well as lithosphere changes between the four major basement terranes that underlie the province results from phase equilibria calculations and the spatial distributions of volcanic rock types and magmatic inclusions are more consistent with the existence of thicker lithosphere beneath stikinia which underlies the southern part of the northern cordilleran volcanic province than beneath the cache creek and yukon tanana terranes which underlie the northern part of the northern cordilleran volcanic province our results support a model for initiation of northern cordilleran volcanic province magmatism due to incipient rifting of the northern cordillera driven by changes in relative plate motion between the pacific and north american plates ca 15 10 ma

debris flow and hyperconcentrated flood flow deposits in an alluvial fan northwestern part of the cretaceous yongdong basin central korea abstract the alluvial fan deposits in the cretaceous yongdong basin korea consist of conglomerates with a muddy or sandy matrix and sandstones with thick or thin laminations the conglomerates and sandstones occur commonly in couplets constituting apparently single sedimentation units facies transitions in the deposits can be summed up in a tripartite facies sequence 1 a clast supported conglomerate with a muddy or sandy matrix and parallel clast alignment facies a and b 2 a matrix supported coarse tail normally graded conglomerate with random clast orientation facies c and thickly stratified pebbly sandstone facies d and 3 laminated sandstone facies e the clast supported conglomerate is interpreted as deposits of debris flows dominated by frictional grain interactions development of pervasive parallel clast alignment lacking large floating clasts and inverse grading suggests deposition via incremental aggradation rather than en masse freezing the matrix supported conglomerate an

the evolution of the samaoding paleolandslide river blocking event at the upstream reaches of the jinsha river tibetan plateau abstract a large number of landslides have occurred in the upstream reaches of the jinsha river tibetan plateau due to the intensity of tectonic movement in the area remote sensing and field investigation indicate that one of them the samaoding paleolandslide previously blocked the river various river blocking phenomena are well preserved including the old landslide dam and deposits fluvial sediments and hydrostatic sandy sediment to better understand the evolution of the samaoding landslide the authors carried out thermoluminescence tl dating and numerical simulations the tl analysis shows that the landslide occurred at 10 6 0 5 ka bp discrete element method dem simulation of the landslide based on landform restoration provided results that are consistent with field observations the simulation indicates that the entire landslide process lasted for 80 s and the sliding mass reached a maximum velocity of 64 m s the landslide formed a landslide dam with a length of 1900 m a width of 600 m and a depth of 200 m the simulation results show that the level of the riverbed at that the time of the landslide was at least 25 m higher than it is today on the combined basis of the simulation results and field observations the authors propose explanation that the following valley evolution sequence occurred after river blocking the landslide dam experienced flood overtopping and then was eroded until it had mostly had been transported away by river flow and the river then rapidly incised the bedrock to form the present day landform based on the field investigations the authors summarize the failure mechanism of steep inclined antidip rockslides and found that tectonics play an important role in the formation of landslide dams or trigger of landslides and the failure of landslide dams in an active tectonic environment of tibetan plateau

coarse grained nonmarine volcaniclastic sediment terminology and depositional process modern and ancient volcaniclastic sedimentary sequences contain depositional units whose features cannot be attributed to fully turbulent dilute stream flow or viscous debris flow the characteristics of these poorly sorted sediments suggest rapid deposition from high concentration dispersions but not en masse sedimentation thus appears related to high discharge flows intermediate in sediment water ratio between stream flow and debris flow the term hyperconcentrated flood flow is proposed for describing this intermediate condition hyperconcentrated flood flow deposits are distinguished from debris flow deposits by lack of matrix support or reverse grading and instead exhibit distribution normal grading and horizontal stratification these deposits are distinguished from normal dilute stream flow deposits by lack of cross stratification in sand facies and by very poor sorting poor imbrication and numerous clasts with long axes oriented parallel to flow direction in gravel facies the horizontal bedding that dominates sandy hyperconcentrated flood flow deposits consists of sediment too coarse grained and strata too thick to have been produced in the boundary layer of the upper flow regime and should not be confused with the more familiar thin graded laminae of fine to medium grained sand often associated with parting lineation hyperconcentrated flood flow deposits are not unique to volcanic settings they also occur in arid alluvial fan sequences debris flow and hyperconcentrated flood flow deposits however are much thicker and more extensive in volcanic regions than on alluvial fans because explosive volcanism leads to rapid mobilization of large volumes of sediment and water on a scale unparalleled in nonvolcanic settings in volcanic regions therefore these deposits have greater preservation potential show greater lateral variability and are more voluminous transformation of channelized debris flow to hyperconcentrated flood flow by dilution with stream water recently observed at mount st helens is recorded in ancient volcaniclastic sequences and may serve as the primary mechanism for generating hyperconcentrated flood flow

delayed sedimentary response to the a d 1886 eruption of tarawera new zealand sedimentary response to tephra deposition from substantial volcanic eruptions is generally immediate and dramatic an important exception is the a d 1886 eruption of mount tarawera new zealand which blanketed 200 km 2 with 50 cm of basaltic scoria and ash fall erupted from a fissure through the tarawera dome complex this fall deposit has suffered very little rilling or other surface erosion and there was no immediate downstream redistribution of tephra by lahars or dilute floods we attribute this lack of early response to the high permeability of the tephra deposit and gentle relief in the areas of substantial accumulation numerous small lakes occur among the dome complexes of okataina caldera and one direct result of the eruption was constriction of lake tarawera9s outflow which caused the lake to rise substantially eighteen years after the eruption collapse of a tephra bank that had controlled lake level triggered a breakout flood from this raised intracaldera lake triggering a decades long period of intense tributary erosion in the upper catchment and damaging stream bed aggradation outside the caldera this abrupt increase in sediment yield from an initially stable posteruptive landscape has no documented precedents yet may be a common posteruptive sedimentary response in caldera complexes of temperate to humid regions

objective delineation of lahar inundation hazard zones a new method of delineating lahar hazard zones in valleys that head on volcano flanks provides a rapid objective reproducible alternative to traditional methods the rationale for the method derives from scaling analyses of generic lahar paths and statistical analyses of 27 lahar paths documented at nine volcanoes together these analyses yield semiempirical equations that predict inundated valley cross sectional areas a and planimetric areas b as functions of lahar volume v the predictive equations a 0 05v 2 3 and b 200v 2 3 provide all information necessary to calculate and plot in

boundary conditions for damming of a large river by fallout during the 12 900 bp plinian laacher see eruption germany syn eruptive rhine damming ii abstract the rhine river germany the largest river in western europe was dammed by pyroclastic material multiple times during the major plinian laacher see eruption 12 900 bp dams formed both upstream and downstream of the broad tectonic lower neuwied basin lnb which interrupts the narrow rhine canyon here we document upstream damming of the rhine river at the entrance to the lnb close to the present city of koblenz due to overloading with tephra fall into the rhine and its major tributaries the moselle and the lahn the dam was formed repeatedly during rapid pumiceous tephra fall events and breached during breaks in eruptive activity causing extensive high energy flooding throughout the entire basin the ephemeral koblenz dams differed significantly from normal volcanically induced dams by consisting principally of washed together pumice clasts and some driftwood the porous nature of pumice and its ability to absorb water were crucial factors thus a large volume percentage of the tephra that had fallen into the rhine floated submerged within the upper part of the water column or swam at the surface moreover the absorption of the river water by the pumice clasts increased the sediment water ratio of the two phase flow considerably we here present a model of dam formation resembling the formation of ice jams we visualize the koblenz dams to have been elongate partly floating and partly grounded permeable plugs many kilometers long and rising no higher than the flood plain damming was most plausibly initiated in the lnb within the area of maximum tephra loading and propagated upstream in a chain reaction comparable to the formation of traffic jams a major dam was finally accumulated at the bottleneck entrance to the lnb a site combining several favorable conditions the upstream multi channel rhine was confined to a single channel change of flow direction by 125 extremely low gradient 0 19 starting already 24 km upstream of the bottleneck constant decrease of flow velocity over many kilometers towards the bottleneck and the moselle river largest tributary of the rhine within the lnb and an important conveyor of additional tephra masses entered the rhine only 700 m upstream of the bottleneck we assume that the koblenz dams could only have formed and been stabilized by an extremely long foot region that extended many kilometers downstream and that was possibly connected to one or several low rise secondary jams dams the backwater of lake brohl that was dammed by pyroclastic flows 7 km downstream of the lnb about halfway through the eruption extended further and further upstream into the lnb during the second plinian stage of the laacher see eruption and was probably a major factor contributing to the formation and large size of koblenz dam no 4 the koblenz dams were probably not completely sealed most of the time this way the major pre eruptive rhine channel received some water an equilibrium condition was established that enabled the dams to remain stable as long as tephra fell into the rhine relatively continuously

hydrodynamic behaviour of taupo 1800a pumice implications for the sedimentology of remobilized pyroclasts large scale explosive eruptions from silicic caldera volcanoes can generate huge volumes of pyroclastic material in terrestrial and marine environments on land erosion remobilization and redeposition of this debris is predominantly carried out by running water in the form of precipitation run off conversely in the submarine realm both primary emplacement and subsequent remobilization are influenced by the presence of water as a transporting medium despite this and the number of studies devoted to volcaniclastic sedimentation relatively little attention has been paid to the hydrodynamic behaviour of the particles themselves which ought to underpin any assessment of transport or depositional process this is crucial as many volcanic particles exhibit variable density according to composition and as functions of differing degrees of vesiculation and the extent to which pore space is filled by water and or gaseous phases during transport and deposition investigation of the physical and hydrodynamic properties of taupo 1800a pumice with reference to sedimentary facies developed during the eruption aftermath shows that although buoyant when dry when sufficiently waterlogged cool pumice clasts will sink and behave more like quartzo feldspathic material saturation is apparently achieved by a combination of rapid capillary flooding of large interconnected vesicles and slower diffusional air water exchange in smaller pores low saturated pumice densities result in lower settling velocities and easier entrainment by tractional currents than those for equivalent sized quartzo feldspathic or crystal lithic particles fine grained pumice is conversely harder to entrain because of the frictional interlocking of angular particles these unusual properties of temporary buoyancy variable saturation low density and size dependent cohesion complicate interpretations of the depositional setting and energy of pumiceous sediments and give rise to several unique facies these findings have implications not only for the analysis of remobilized pyroclastic facies in terrestrial and marine environments but also for primary depositional processes during subaqueous explosive volcanism

sedimentology of proximal to distal volcaniclastics dispersed across an active foldbelt ellensburg formation late miocene central washington late miocene volcaniclastics of the ellensburg formation washington northwestern usa are exposed in basins within an integrated palaeodrainage over the depositional reach from 15 to 120 km eastward from a cascade range source area two facies associations are recognized the first is composed of laterally restricted well sorted polymictic conglomerates representing a gravel bedload regime during inter eruption periods the second comprises laterally extensive sheets bounded by deeply developed paleosols composed of monomictic pebbly pumiceous dacitic sandstones with intercalated debris flow and hyperconcentrated flood flow deposits these sheets aggraded in response to eruptive events that are sometimes recorded by air fall tephras at the bases of sequences debris flow and hyperconcentrated flood flow deposits occur as far as 120 km from source but are uncommon beyond 50 km hyperconcentrated flood flow deposits initially increase in abundance away from source as debris flow deposits diminish suggesting formation of hyperconcentrated flood flows by dilution of debris flows sandy facies form broad sheets dominated by scour fill bedding in proximal and medial settings and grade to narrower trough cross bedded sheets in distal settings suggesting moderation of flood discharges with distance base level changes associated with episodic sediment influx caused incision aggradation cycles in mainstream settings and episodic impoundment of tributaries to form lakes or rapidly aggrading poorly drained floodplains although volcanism was the primary control on depositional style concurrent development of the yakima fold belt produced a structure consequent drainage pattern that determined sediment dispersal and basin subsidence permitted preservation of both syn eruption and inter eruption facies detritus from rising anticlines was generally diluted by the volumetrically superior extrabasinal volcaniclastics but dominates deposits of small tributary streams flowing toward rather than from the cascades

millennial scale reworking of tephra in alluvial to shallow marine settings distinguishing pseudo isochrons from genuine ones abstract primary tephra layers can provide isochrons in a stratigraphic sequence such isochrons are powerful tools in terms of correlation and chronology of strata and they constrain a chronological framework for geology geomorphology archaeology and palaeo environmental studies in the neogene to quaternary however reworking processes can re distribute tephra materials in both time and space so that a given tephra can form a new discrete layer later in the depositional sequence or it can be distributed in low concentrations no visible layers i e crypto tephras over a range of different stratigraphic horizons the present study based on sedimentological petrographical and geochemical analyses of reworked tephras in japan evaluates the duration and persistence of reworking and the tephra mobilisation and delivery to watersheds furthermore it discusses how a unique isochron can be constructed from tephra distribution in a sedimentary sequence examples presented here are from fluvial and deltaic sequences in the japanese quaternary which contain several visible tephra layers and a certain amount of tephra materials from multiple sources at different stratigraphical horizons in the niigata plain 5 4 ka numazawako tephra material continued to be reworked for more than 4000 years after the eruption of numazawa volcano and was re deposited as reworked pumice grains consisting of a visible tephra layer in delta front deposits mixed tephra materials originating from an earlier eruption of numazawa volcano and eruptions from other calderas were also identified in the tsugaru plain post 2 5 ka subsurface deposits have 10 glass shards content throughout the sequence glass shards were derived from several different tephra materials such as those from the most recent eruption of towada volcano ad 915 and earlier caldera forming eruptions 30 ka 15 ka some of the glass shards probably originated from erosion of exposed pliocene pyroclastic bedrock thus tephra reworking processes in alluvial to shallow marine settings can continue long after an eruption has ended and can persist even during inter eruptive i e background sedimentation periods as tephra reworking involves complicated processes careful observation and interpretations are necessary to evaluate whether tephra materials can truly provide an isochron for the strata in which they are found to exist

analysis of petrologic hypotheses with pearce element ratios pearce element ratios can test whether the members of a rock suite are comagmatic and can illustrate the causes of chemical diversity in comagmatic suites comagmatic rocks have constant ratios for elements conserved in the system during changes that led to the chemical diversity in basaltic systems the incompatible elements ti k and p are often conserved the slope of the trend on a pearce element ratio diagram is sensitive to the stoichiometry of the crystallizing and segregating phases a judicious choice of ratios as axes for the diagram provides a signature for the phases involved and estimates of their compositions in basaltic rocks diagrams with ti k vs p k can provide a test of the comagmatic hypothesis diagrams with 0 5 mg fe k vs si k have trends that are distinct for each comagmatic suite and different mineral assemblage different suites are distinguished by the intercepts in diagrams whereas mineral assemblages are recognized by the slopes of the trends for example if olivine is the sole crystallizing and segregating phase the trend will have a slope of 1 diagrams with 2ca na k vs al k distinguish plagioclase from augite assemblages and in conjunction with 0 5 mg fe k diagrams unravel the crystallization sequences of suites that have suffered three phase crystallization and segregation analyses from the uwekahuna laccolith kilauea the 1955 and 1967 68 eruptions of kilauea diamond craters volcanic field oregon and experimental data on morb glasses provide illustrations of the interpretations that can be obtained from pearce element ratios

methods for predicting peak discharge of floods caused by failure of natural and constructed earthen dams floods from failures of natural and constructed dams constitute a widespread hazard to people and property expeditious means of assessing flood hazards are necessary particularly in the case of natural dams which may form suddenly and unexpectedly we revise statistical relations derived from data for past constructed and natural dam failures between peak discharge qp and water volume released v0 or drop in lake level d but assert that such relations even when cast into a dimensionless form are of limited utility because they fail to portray the effect of breach formation rate we then analyze a simple physically based model of dam breach formation to show that the hydrograph at the breach depends primarily on a dimensionless parameter kv0 gl 2d7 2 where k is the mean erosion rate of the breach and g is acceleration due to gravity the functional relationship between qp and takes asymptotically distinct forms depending on whether 1 relatively slow breach formation or small lake volume or 1 relatively fast breach formation or large lake volume theoretical predictions agree well with data from dam failures for which k and thus can be estimated the theory thus provides a rapid means of predicting the plausible range of values of peak discharge at the breach in an earthen dam as long as the impounded water volume and the water depth at the dam face can be estimated