turbidites associated with bentonites in the upper cretaceous of the campos basin offshore brazil abstract more than fifteen bentonite beds have been identified in the santonian campanian section of the campos basin offshore brazil these range in thickness up to 1 m and are mainly composed of interstratified illite smectite relict textural features can still be observed identifying them as former volcanic ash layers they also display gradational or sharp contacts with the associated lithologies in the carapeba field pure bentonite beds formed mainly by expandable clay minerals occur intercalated with pelitic rocks bentonite beds with up to 20 terrigenous silty material micas quartz and feldspars here classified as impure are found directly overlying siliciclastic turbidite sandstones these bentonite beds incorporate constituents of turbiditic dust and are interpreted as equivalent to the bouma e division of the underlying turbidite layers this fact suggests a contemporaneity of the turbidite and the volcaniclastic processes therefore the volcanism and associated tectonic seismicity are considered a potential mechanism for the onset of turbidite sedimentation

stratigraphy of the 1902 and 1929 nuee ardente deposits mt pelee martinique mild nuee ardentes in 1902 1903 and 1929 1930 at mt pelee formed high aspect ratio h a r deposits i e deposits with a high thickness extent ratio in the riviere blanche channel several violent nuees ardentes from may to august 1902 yielded low aspect ratio l a r deposits unevenly blanketing an area of 58 km2 on the southwestern flank of the volcano three closely spaced nuees ardentes on august 30 additionally affected 56 km2 on the southeastern flank the l a r deposits are divided into eight stratigraphic units u1 u8 either compound or consisting of a single flow unit the deposits of the largest nuees ardentes on may 8 20 and august 30 1902 are identified as u1 u3 and u8 respectively and can be traced over most of the area individual l a r flow units are lenticular and show large erratic lateral variations of thickness and grain size most flow units consist of three distinct beds bed 1 is a few centimeters thick lenticular fines depleted layer bed 2 is the thickest layer and contains more silt sized ash than bed 1 it is normally graded as a whole though reverse grading may occur in the lower part bed 2 generally consists of two parts bed 2a nonbedded and bed 2b finer grained and laminated with cross bedding and duned structures bed 3 is a thin silty capping layer which contains accretionary lapilli the distribution of the l a r flow units especially of the coarse grained facies suggests that they were formed from overpressured blast flows expanding radially from the vent at a high rate in the same manner as the laterally directed blast of mount st helens on may 18 1980 the transport system of the major 1902 nuees ardentes was an inflated relatively low concentration turbulent cloud as suggested by several lines of evidence 1 the transported material was able to cross ridges up to 300 m high 2 many scour and fill and other erosional features can be observed in the deposits at any distance from the vent 3 survivors within the devastated area on may 8 did not experience a dense low profile cloud but instead a choking hot ash laden cloud 4 many human corpses in st pierre remained where they fell instead of having floated away if the current flow surge terminology were to be used the 1902 blast flows would be better considered as pyroclastic surges the normally graded multi layered structure of the flow units is also consistent with deposits primarily formed from relatively low concentration flows

prediction of alluvial channel pattern of perennial rivers abstract purely braided meandering and straight channels can be considered as end members of a continuum of alluvial channel patterns several researchers have succeeded in separating channel patterns in fields defined by flow related parameters however the discriminators of the principal channel patterns derived from these diagrams all require some a priori knowledge of the channel geometry in this paper a method is presented which enables prediction of the equilibrium conditions for the occurrence of braided and high sinuosity meandering rivers in unconfined alluvial floodplains the method is based on two almost channel pattern independent boundary conditions median grain size of the river bed material and a potential specific stream power parameter related to bankfull discharge or mean annual flood and valley gradient this can be regarded as a potential maximum of the available flow energy corresponding to the minimum sinuosity condition p 1 based on an analysis of 228 datasets of measurement sites along rivers from many parts of the world an independent discriminating function was found that separates the occurrence of braided rivers and meandering rivers with p 1 5 the function applies to equilibrium conditions of rivers that neither incise nor show rapid aggradation with a bankfull or mean annual flood discharge above 10 m3 s and a median bed material grain size between 0 1 and 100 mm

conceptual framework for assessing the response of delta channel networks to holocene sea level rise abstract recent research has identified two fundamental unit processes that build delta distributary channels the first is mouth bar deposition at the shoreline and subsequent channel bifurcation which is driven by progradation of the shoreline the second is avulsion to a new channel a result of aggradation of the delta topset the former creates relatively small branching networks such as wax lake delta the latter generates relatively few long distributaries such as the mississippi and atchafalaya channels on the mississippi delta the relative rate of progradation to aggradation and hence the creation of accommodation space emerges as a controlling parameter on channel network form field and experimental research has identified sea level as the dominant control on holocene delta growth worldwide and has empirically linked channel network changes to changes in the rate of sea level rise here i outline a simple modeling framework for distributary network evolution and use this to explore large scale changes in holocene channel pattern that have been observed in deltas such as the rhine meuse and mississippi rapid early to mid holocene sea level rise forced many deltas into an aggradational mode where i hypothesize that avulsion and the generation of large scale branches should dominate slowing of sea level rise in the last 6000 yr allowed partitioning of sediment into progradation facilitating the growth of smaller scale distributary trees at the shorelines of some deltas and a reduction in the number of large scale branches significant antecedent topography modulates delta response the filling of large incised valleys for example caused many deltas to bypass the aggradational phase human effects on deltas can be cast in terms of geologic controls affecting accommodation constriction of channels forces rapid local progradation and mouth bar bifurcation while accelerated sea level rise increases aggradation and induces more frequent channel avulsion

an appraisal of melting processes and the galapagos hotspot major and trace element evidence abstract the variation of nd sr and pb isotopes in lavas from the galapagos islands is thought to be the result of contamination of a mantle plume by the upper mantle in order to evaluate the control of source mixing on the compositions of the lavas the major and incompatible trace element compositions of parental melts are calculated by normalizing their values in aphyric unaltered lavas to mgo 8 along calculated liquid lines of descent the galapagos lavas have approximately half the worldwide major element compositional range of morbs of all the elements and ratios evaluated only volcano averaged k and la sm correlate with the isotopic ratios this must indicate that differences in the melting process such as depth extent of melting or both overwhelm any compositional difference between the plume and upper mantle several of the volcanoes have exceedingly large ranges in the abundances of incompatible trace elements but the range of isotopic values from single volcanoes is small and best explained by large ranges in the extents of melting inferred parental lavas also have a large range of si and fe concentrations which may result from extraction of the melts over a range of depths element abundances and isotopic ratios have geographic patterns which indicate that magmas from the center of the archipelago are derived by relatively lower average extents of melting deeper average extraction depths and less contribution of plume material in general however the mantle beneath the central islands experienced wide ranges in temperature extent of melting and the melts were extracted from widely different depths the lavas of the western and northern islands are more uniform probably controlled by consistent temperatures in the underlying mantle and depth of extraction these relationships are not explainable in terms of simple two stage melting and mixing models the compositions of parental galapagos magmas are controlled by a complex sequence of processes which have different effects on the isotopic major element and trace element compositions of the magmas

multivariate characterization of meandering abstract a suite of 40 morphometric variables has been measured on 57 sections of freely meandering channels from 33 rivers these objective variables are computed from digitized traces of the channel centerline and include measures of sinuosity meander wavelength curvature moments meander asymmetry and pattern irregularity several variables are measured on the half meanders defined by successive inflection points principal components analysis reveals that morphological variations between freely meandering streams can be resolved into four factors related to meander wavelength meander irregularity sinuosity and large scale wandering of the meander axis discriminant analysis using the morphometric variables results in almost perfect separation between natural streams and two theoretical models of meander planform ferguson s disturbed periodic model and howard and knutson s simulation model despite visual similarities the simulated meanders are too regular in size and shape in relation to natural ones perhaps because the interaction of effects of curvature and alternate bars upon topography and flow structure in natural channel bends produces multiple scales of bank erosion not accounted for in present simulation models

controls on the transition from meandering to straight channels in the wetlands of the okavango delta botswana tooth s mccarthy t s 2004 controls on the transition from meandering to straight channels in the wetlands of the okavango delta botswana earth surface processes and landforms 29 13 1627 1649 rae2008

mount st helens 1980 and mount pelee 1902 flow or surge the far reaching directed blast of mount st helens and the devastating nuees ardentes of mount pelee produced deposits that in many places consist of three main layers a basal gravelly or sandy layer 1 a massive or bedded ash layer 2 and a capping ash cloud layer 3 these thin and in part landscape mantling deposits are generally ascribed to pyroclastic surges and the dune bedding seen in parts of layer 2 has reinforced this interpretation there are several reasons however for preferring a pyroclastic flow origin 1 much of layer 2 is unequivocal pyroclastic flow 2 the tripartite subdivision and landscape mantling habit is similar to that displayed by ignimbrites of the low aspect ratio type 3 the deflation of a pyroclastic surge and the subsequent deposition of particulate material will tend not to conserve the fine ash and dust that are abundant in valley ponded parts of layer 2 and cannot explain the observed tendency for layer 2 to show stronger fines depletion on ridges than in valley bottoms and 4 the coarseness and the variance of grain size found within dune bedded bed sets in mount st helens layer 2 are like those of very weak pyroclastic surges and the dune bedding most likely resulted from local minor turbulence as could have been caused by surface roughness in a mountainous terrain littered with tree stumps and fallen trees in a thin depositing pyroclastic flow the directed blast and nuees ardentes at mount st helens and mount pelee are interpreted to have been violently emplaced pyroclastic flows producing deposits of low aspect ratio type the characteristics of which stem from an exceptionally high flow velocity

isotopic evolution of mauna loa volcano abstract in an effort to understand the temporal helium isotopic variations in mauna loa volcano we have measured helium strontium and lead isotopes in a suite of mauna loa lavas that span most of the subaerial eruptive history of the volcano the lavas range in age from historical flows to ninole basalt which are thought to be several hundred thousand years old most of the samples younger than 30 ka in age kau basalt are radiocarbon dated flows while the samples older than 30 ka are stratigraphically controlled kahuku and ninole basalt the data reveal a striking change in the geochemistry of the lavas approximately 10 ka before present the lavas older than 10 ka are characterized by high 3 he 4 he 16 20 times atmospheric higher 206 pb 204 pb 18 2 and lower 87 sr 86 sr 0 70365 ratios than the younger kau samples having he pb and sr ratios of approximately 8 5 atmospheric 18 1 and 0 70390 respectively the historical lavas are distinct in having intermediate sr and pb isotopic compositions with 3 he 4 he ratios similar to the other young kau basalt 8 5 atmospheric the isotopic variations are on a shorter time scale 100 to 10 000 years than has previously been observed for hawaiian volcanoes and demonstrate the importance of geochronology and stratigraphy to geochemical studies the data show consistency between all three isotope systems which suggests that the variations are not related to magma chamber degassing processes and that helium is not decoupled from the other isotopes however the complex temporal evolution suggests that three distinct mantle sources are required to explain the isotopic data most of the mauna loa isotopic variations could be explained by mixing between a plume type source similar to loihi and an asthenospheric source with helium isotopic composition close to morb and elevated sr isotopic values an asthenospheric source or variation within the plume source is considered more likely than lithospheric sources due to the elevated 87 sr 86 sr ratios in the recent kau basalts however the distinct isotopic characteristics of the historical lavas are inferred to be related to lithospheric involvement in the latest stages of shield building volcanism

forms and processes of two highly contrasting rivers in arid central australia and the implications for channel pattern discrimination and prediction tooth s nanson g c 2004 forms and processes of two highly contrasting rivers in arid central australia and the implications for channel pattern discrimination and prediction geological society of america bulletin 116 7 8 802 816 rae2008

dynamics of shallow hydrothermal eruptions new insights from vulcano s breccia di commenda eruption understanding the dynamics and effects of hydrothermal eruptions is crucial to the hazard assessment in both volcanic and geothermal areas eruptions from hydrothermal centres may occur associated with magmatic phases but also as isolated events without magmatic input with the most recent examples being those of te maari tongariro new zealand in 2012 and ontake japan in 2014 the most recent caldera of the island of vulcano southern italy hosts in its centre the la fossa cone active since 5 5 ka and now characterised by continuous fumarolic degassing in historical times la fossa cone has experienced several hydrothermal eruptions with the most violent event being the breccia di commenda eruption that occurred during the thirteenth century ad based on analysis of 170 stratigraphic logs we show that the breccia di commenda eruption occurred in three main phases after an opening low intensity ash emission phase phase 1 the eruption energy climaxed during phase 2 when a series of violent explosions produced an asymmetric shower of ballistic blocks and the contemporaneous emplacement of highly dispersed lithic rich blast like pyroclastic density currents pdcs the tephra units emplaced during phase 2 ranging in volume from 0 2 to 2 7 105 m3 were covered in turn by thin ash fall deposits phase 3 the dynamics of the most violent and intense stage of the eruption phase 2 was investigated by numerical simulations a three dimensional numerical model was applied describing the eruptive mixture as a eulerian eulerian two phase non equilibrium gas particle fluid plus a one way coupled lagrangian ballistic block fraction at the initial simulation time a mass of about 109 kg with initial overpressure above 10 mpa and a temperature of 250 c was suddenly ejected from a 200 m long eastward inclined nne ssw trending fissure the mass release formed blast like pdcs on both sides of the fissure and launched ballistic blocks eastwards field investigations and numerical simulations confirm that hydrothermal explosions at la fossa cone include intense ballistic fallout of blocks emission of pdcs potentially travelling beyond the la fossa caldera and significant ash fallout the hazard associated with both ballistic impact and pdc ingress as associated with hydrothermal eruption is significantly larger with respect to that associated with vulcanian type events of la fossa

hf isotope geochemistry of the galapagos islands 1 we report 176hf 177hf ratios determined by multicollector inductively coupled plasma mass spectrometry on 49 basalts from the galapagos islands the hf values range from about 8 a value typical of many oceanic island basalts to about 15 a value typical of mid ocean ridge basalt the hf shows the expected correlation with nd and inverse correlations with sr and pb isotope ratios the range of values and correlations observed is consistent with galapagos mantle plume asthenosphere mixing inferred from previous geochemical studies while oceanic island basalt isotopic arrays suggest that multicomponent mixing is common in the sources of such basalts the galapagos is unusual in that depleted upper mantle appears to be a principal component thermal and dynamic interaction between the galapagos mantle plume and the lithosphere and shallow asthenosphere is the most likely cause of mixing and of the geographic pattern of geochemical variation the galapagos hf nd sr pb isotopic array diverges toward the plume end indicating the plume is heterogeneous this heterogeneity is geographic with distinct northern central and southern components the central component and to a lesser degree the northern component are compositionally similar to the common component of plumes variously called phem fozo and c while the southern component is unique with relatively high hf and 206pb 204pb compared to 87sr 86sr and nd

grain size sediment transport regime and channel slope in alluvial rivers the general relationship between channel morphology and the grain size of sediment in the channel bed is an important but poorly known aspect of alluvial rivers an analysis of an equation for total sediment flux in the limits of suspension bedload and mixed modes of transport indicates distinct steady state regimes of channel morphology such regimes are readily seen in published data for modern alluvial rivers by way of a conventional shields plot or a plot of channel slope as a function of relative grain size d h and the ratio ws u where d and ws are respectively mean diameter and fall speed of bed sediments and h and u are respectively mean depth and friction velocity of the flow with slope and mode of transport in an alluvial river constrained by grain size and channel depth alone estimates of discharge and sediment flux follow directly introduction of the sediment flux relationship into conventional diffusion models for the evolution of an alluvial system provides nominal estimates of

a sequence of a plinian eruption preceded by dome destruction at kelud volcano indonesia on february 13 2014 revealed from tephra fallout and pyroclastic density current deposits abstract a plinian style eruption with a radially spreading umbrella cloud occurred on february 13 2014 at kelud volcano indonesia we present the sequence of this plinian event based on a geological survey of the eruption products and analysis of satellite images the eruptive deposits were divided into four major depositional units units a b c and d and used to determine the sequence of events the plinian phase was preceded by partial destruction of the existing lava dome and generation of high energy pyroclastic density currents pdcs mainly toward the ne that produced a series of depositional subunits units a 0 2 and blown down trees stage 1 in the main phase of the plinian eruption tephra fallout unit b was widely distributed over east java stage 2 the winds above the volcano significantly affected the tephra dispersal process in stage 3 the plinian column collapsed generating dense pdcs that flowed down the volcano valleys producing pumiceous lobate deposits unit c the declining phase of the eruption produced fine rich fallout tephra layers unit d 1 2 from low level eruption plumes and or ash lofted from pdcs the eruption sequence constructed from field observations is supported by geophysical observations the deposit features and componentry suggest that newly ascended magma triggered the destruction of the lava dome and the generation of high energy pdcs and during the subsequent climactic phase the dome was completely destroyed we estimate a total erupted volume of 0 25 0 50 km 3 bulk deposit volume 0 14 0 28 km 3 in dense rock equivalent and a mass eruption rate of 6 5 2 8 10 7 kg s with a volcanic explosivity index of 4 the eruption sequence and physical parameters of the 2014 eruption will help assess future volcanic activity and hazards at kelud volcano

fluvial responses to the weichselian ice sheet advances and retreats implications for understanding river paleohydrology and pattern changes in central poland the evolution of the fluvial systems during the weichselian pleniglacial in the torun basin central poland was investigated through sedimentological investigation and paleohydraulic analysis within the basin three fluvial cycles deposited after successive phases of the ice advance which took place 50 28 and 20 ka ago successions of four fluvial lithotypes characterize each fluvial formation that are related to the paleoenvironmental changes e g climate instability and changes in the river regime which affected the channel hydraulics and morphology the successions comprise river style metamorphosis between high energy sand bed meandering rivers lithotype m1 high energy sand bed braided rivers lithotype b1 and medium energy sand bed braided rivers with either unit bars lithotype b2 or compound bars lithotype b3 reflects the maturity stage of sand bed braided river evolution in the basin the assessment of the fluvial sedimentary environments enabled the construction of a quantitative model of the changes in the river channel pattern in relation to the climate oscillation both the paleohydrological controls and their sedimentary consequences are discussed in the article lithotypes m1 and b1 represent riverbed modeled under supercritical flow condition deposition of lithotype b2 corresponded to the river channel pattern transformation and was manifested by decreasing flow velocity energy losses associated with bedform roughness and with the transportation of coarser particles the flow velocity was generally greater in rivers of lithotype b3 and energy of sedimentary environment was more stable than during the deposition of lithotype b2

conditions for branching in depositional rivers it is often taken for granted that rivers organize transport into a single active channel in some net depositional environments however flow of water and sediment is distributed in several stable channels such branching rivers may be confined in valleys anabranching or anastomosed or unconfined on deltas distributaries and their existence confronts us with the very basic question of what governs the spatial organization of channel patterns in sedimentary landscapes current models for equilibrium channel morphology cannot predict the occurrence of branching rivers because they do not consider dynamical processes such as avulsion i e the rapid abandonment of a channel in favor of a new path at lower elevation the requisite conditions for avulsion have been the subject of ongoing debate here we resolve the conditions leading to channel avulsion and show that branching rivers occur when avulsion is the dominant mechanism of lateral channel motion a compilation of field and laboratory data demonstrates that avulsion frequency scales with the time required for sedimentation on channel beds to produce a deposit equal to one channel depth from the relative rates of bank erosion and channel sedimentation we derive a dimensionless mobility number that accurately predicts the conditions under which anabranching and distributary channels occur results may be directly applied to modeling landscape evolution over human and geologic time scales and for inverting formative environmental conditions from channel deposits on earth and other planetary surfaces

analysis of sediment from steps in a small catchment in the polish carpathians in relation to the transition zone between the hillslope and fluvial system abstract an investigation to determine the transition zone between the hillslope system and the fluvial system in connection with the degree of differentiation of sediment located upstream and downstream of log steps ls and boulder steps bs was undertaken in the channel of a stream located in a small mountain catchment area in the silesian beskidy mts polish carpathians a stream channel into which large wood was introduced artificially several years ago granulometric analyses of 41 samples taken from upstream 21 and downstream 20 of steps were performed to analyze the effectiveness of log steps and boulder steps in the differentiation of mineral sediment it was demonstrated that log steps have a significant impact on the functioning of the channel by shaping its longitudinal profile and affecting the transport of mineral material whereas boulder steps retain less sediment showing the beneficial effects of artificial creation of log steps in mountain stream channels results of granulometric analyses suggested the location of the theoretical transition zone between the hillslope system and the fluvial system suggesting the location and range of this zone in small mountain catchment areas can change depending on the intensity and range of slope processes in the longitudinal profile of the channel

morphological and geochemical variations along the eastern galapagos spreading center 1 as the eastern galapagos spreading center gsc shallows westward toward the galapagos archipelago axial morphology evolves from a low relief valley and ridge terrain to an increasingly prominent axial ridge closely mirroring the western gsc between the inca transform 85 5 w and its western termination near 91 w the eastern gsc comprises seven morphological segments separated by five morphological discontinuities and the eastward propagating 87 w overlapping spreading center combined morphologic and geochemical data divide the eastern gsc into two domains independent of the fine scale morphologic segmentation the western domain is defined by its axial ridge morphology and highly variable lava population elemental data define steep along axis gradients reflecting a complex source that includes one or more hot spot related components in addition to a highly depleted component the eastern domain is defined by transitional valley and ridge morphologies and a surprisingly invariant lava population this population is dominated by shallow crystal fractionation processes and displays significantly less variability attributable to multiple source components the galapagos hot spot has long been known to have a symmetrical long wavelength influence on crustal accretion along the gsc existing isotopic and new elemental data define twin geochemical peaks that we interpret as loci for transfer of distinct source components from the galapagos plume to the gsc although na8 and fe8 values lie within the negatively correlated global array na8 increases with decreasing axial depth contrary to global trends and consistent with emerging deep hydrous melting models that predict decreasing overall extent of melting despite increasing melt production support for hydrous melting comes from decreasing heavy ree increasing la sm and la yb and the systematics of decreasing feo and increasing cao and al2o3 with decreasing distance to the hot spot overall an enriched deep melt component appears to coexist in the shallow mantle with a ubiquitous depleted primitive melt component consistent with new models for channelized melt flow connecting a deep hydrous melt regime with the dry shallow mantle nevertheless an absence of low fe lavas suggests that hydrous melting is strictly limited beneath the eastern gsc becoming dominant only near the western geochemical peak where input from a hydrous northern or wolf darwin plume component is inferred

from river to shelf anatomy of a high frequency depositional sequence the late pleistocene to holocene tiber depositional sequence the late pleistocene holocene tiber delta succession represents the most recent and one of the best preserved high frequency low rank depositional sequences developed along the latium continental margin of the italian peninsula several previous studies have established a robust data set from which it has been possible to describe the stratigraphic architecture of the entire tiber depositional sequence from the landward to seaward sectors and over a distance of 60 km the tiber depositional sequence shows many characteristics found in other late pleistocene to holocene deltaic and coastal successions of the mediterranean area the stratigraphic architecture of the tiber depositional sequence is controlled mainly by glacioeustasy although factors such as tectonic uplift volcanism and subsidence exert an influence at a local scale the resulting depositional model allowed discussion of some important points such as 1 the genesis of the tiber mixed bedrock alluvial valley extending from the coastal plain to the innermost portion of the shelf recording i multiple episodes of incision during relative sea level fall and ii a downstream increase of depth and width of the valley during the base level fall and the subsequent base level rise 2 the different physical expression of the tiber depositional sequence boundary from landward to seaward and its diachronous and composite character 3 the maximum depth reached by the tiber early lowstand delta at the end of the sea level fall is estimated at ca 90 m below the present sea level and not at 120 m as suggested by previous works 4 the backward position of the tiber late lowstand delta relative to the deposit of early lowstand 5 the change of the channel pattern and of the stacking pattern of fluvial deposits within the lowstand systems tract transgressive systems tract and highstand systems tract all of these features indicate that the late pleistocene holocene tiber delta succession even if deposited in a short period of time from a geological point of view represents the result of the close interaction among many autogenic and allogenic factors however global eustatic variations and sediment supply under the control of climatic changes can be considered the main factors responsible for the stratigraphic architecture of this sedimentary succession which has been heavily modified by human activity only in the last 3000 years

petrology of lavas from sierra negra volcano isabela island galapagos archipelago sierra negra volcano is a voluminous active basaltic shield situated in the western galapagos archipelago a hotspot related chain adjacent to the galapagos spreading center the volcano has erupted fe rich tholeiitic basalts of very limited compositional range comparison of lavas from each of five stratigraphic units covering the entire volcano indicates that the lavas are mineralogically similar and compositionally restricted 4 4 to 6 9 wt mgo the lavas are fe rich and hypersthene normative and plot in both the alkaline and tholeiite fields of the alkali silica diagram rare earth element ree patterns are steep and parallel the major and trace element data indicate a comagmatic relationship by fractional crystallization of the observed phenocryst phases projections into pseudoternary phase diagrams suggest that the magmas cooled and crystallized at pressures between 1 and 3 kbar calculated melt densities range from 2 73 to 2 77 g cm3 which are higher than those of typical mid ocean ridge basalt morb hawaiian tholeiite and other galapagos lavas the high calculated melt densities may be caused by fractionation at greater depths than is ordinary for the other basaltic magmas suppression of plagioclase and olivine crystallization in favor of augite may eliminate the minimum density window that has been proposed for morb fractionation sierra negra lavas have the most radiogenic lead and strontium isotopic ratios in the western galapagos indicating that the magmas have a relatively large contribution of plume material and have been minimally contaminated by entrainment of morb producing mantle magmatic 3he 4he isotopic ratios from sierra negra are approximately 15 times the atmospheric ratio although these ratios clearly indicate plume helium they are not the highest 3he 4he in the archipelago suggesting that helium is decoupled from the heavier isotopes high sm yb ratios light rare earth element lree enrichment and a steep ree slope are consistent with an origin by moderate extents 5 15 of partial melting of a garnet lherzolite source with ree characteristics that are between chondrite and depleted earth mantle sources sierra negra lavas are compositionally monotonous because homogeneous plume rich material has experienced long term exposure to a well regulated sublithospheric thermal environment with additional compositional restrictions imposed in the lithosphere by high magma flux

mt pelee martinique may 8 and 20 1902 pyroclastic flows and surges abstract the distribution stratigraphic relationships and fragmental components of the may 8 and 20 1902 pyroclastic flows from mt pelee martinique together with eyewitness accounts suggest the following explanation for those eruptions the eruptions were vertically directed magmatic perhaps initiated phreatically and contained abundant juvenile lithics from congealed magma of the dome and neck this resulted in a two part eruption column having 1 a dense lithic charged part which collapsed into the crater and flowed out of a pre existing notch in its side giving rise to pyrochlastic flows and 2 a magmatically derived column containing gases juvenile vitric material and crystals which largely by passed the neck and dome and escaped into the atmosphere all of the energy of the flows was apparently focused through the notch they emerged fully turbulent and flowed down riviere blanche gravity segregation of large and abundant fragments soon resulted in a dense high concentration poorly fluidized block and ash flow confined to the valley leaving above a fully turbulent high energy ash cloud surge as the ash cloud surge moved down the mountain it continued to expand outward the process of gravity segregation continued as the ash cloud surge expanded resulting in secondary block and ash underflows toward st pierre the secondary block and ash flows developed on a gently sloping upland surface 100 m or more above the valley of riviere blanche the turbulent fragment depleted surges above the secondary block and ash flows maintained sufficient energy to devastate the landscape outward to about 3000 m including st pierre the surges refracted around obstacles and in one place moved up a small valley in a direction opposite to the main flows

the meandering braided river threshold a reappraisal abstract the concept of a threshold discharge slope q s combination that separates braided from meandering streams is critically re examined partly by review of previous data and partly by analysis of new data from south island new zealand it is argue d that discriminant functions that use mean annual discharge provide a poor physical basis for interpretations based on natural processes while bankfull discharge is also deficient because it is not a fully independent variable the common use of channel rather than valley slope adds a further misleading component because channel slope is partly dependent on channel pattern sinuosity most previous studies have inadequately treated the role of bed particle size in pattern discrimination failing to recognize that active gravel streams must plot higher on a q s chart than sand bed channels irrespective of pattern because of the greater power requirements for bed material movement within any one size class of bed material there is no evidence to indicate a clear discrimination between braiding and meandering only a weak statistical association between pattern and slope discharge values while this association may be explicable in terms of the high shear stress and stream power that accompany high qs values promoting braiding the real prerequisite for braiding appears to be high loads of bed calibre material producing wide shallow channels a factor that is only partly controlled by stream power accordingly the search for a pattern threshold based on discharge and slope seems to be a futile exercise an interpretation of existing pattern discriminant functions from the perspective of the threshold shear stress for particle movement is made it is concluded that in effect these functions merely state that gravel bed streams are more likely to be braided than channels in finer sediment statistically this appears to be valid and reasons for that difference are offered based on the balance between imposed bed material load and channel capacity

volcanism and related slope to shallow marine volcaniclastic sedimentation an archean example near chibougamau quebec canada abstract volcaniclastic sedimentary rocks and k rich lava flows of the archean hauy formation are discontinuously exposed in a 850 m thick sequence in the waconichi synclinorium northwest of chibougamau quebec the interstratified sedimentary rocks comprise two major sedimentary facies associations the graded bedded facies association wbfa indicative of deposition by sediment gravity flows and the wavy bedded facies association wbfa defined by wave induced structures the gbfa is near the base of the stratigraphic section and is interstratified up section with the wbfa in the gbfa the abundance of synsedimentary folding truncated slump folds load casts and low angle erosional surfaces as well as sediments deposited by turbidity currents and debris flows support designation as a volcanic slope deposit in the wbfa trough crossbeds wavy bedded gravels and plane bedded volcaniclastic sandstones are indicative of a nearshore setting the abundance of angular to subangular lithic volcanic fragments and broken and euhedral pyroxene crystals in the sediments is suggestive of a pyroclastic origin some beds may represent primary pyroclastic material but unequivocal petrographic recognition of pumice or glass shards is not possible because of metamorphic overprint composition suggests an initial magmatic or phreatomagmatic eruption subsequently reworked along the shoreline and transported downslope lava flows comprising massive to lobate andesites and massive to blocky basalts constitute most of the stratigraphic sequence pillowed flows are strikingly absent from the section both compositional types have an increase in vesicularity and change in flow form up section from massive to lobate or blocky suggesting progressively shallower water depths the composite character represented by hauy lava flows and associated volcaniclastic sediments in characteristic of stratovolcanoes the studied section is an integral part of an evolved arc with an adjacent backarc basin the volcano sedimentary facies indicate the development of a volcanic island in one of these basins

decadal scale morphological adjustment of a lowland tropical river abstract compared with temperate regions much less is known about the dynamics of tropical river systems tropical rivers are typically characterised by pronounced seasonal changes in precipitation large sediment loads and high rates of lateral channel migration across often very low gradient and densely populated floodplains understanding the controls on channel migration or change is integral to our ability to fully predict and build resilience against flood risk and wider river related hazards here we analyse channel and confluence migration over the last 40 years along a 85 km reach of the cagayan river and one of its tributaries the pinacanauan de ilagan luzon philippines using optical satellite imagery captured during this period combining this with spatial variations in channel pattern valley width and new bed material grain size data we demonstrate that sediment transport and deposition are key drivers of the observed tropical channel morphodynamics in this region the high sediment supply generated in the catchment headwaters by mass wasting of hillslopes triggered especially in typhoons results in high aggradation rates and channel widening on the lower gradient alluvial plain we suggest that this aggradation enhances local confluence and lateral channel migration rates which can reach 300 m per decade and that lateral migration rates of tropical rivers are typically greater than those of temperate rivers channel morphodynamics have implications for how to best manage these types of tropical river systems where hard bank protection structures may result in a complex geomorphic response and flood risk mapping may need to include assessment of sensitivity to varying channel position and topography

a 1 5 ma record of plume ridge interaction at the western galapagos spreading center 91 40 92 00 w abstract shallow elevated portions of mid ocean ridges with enriched geochemical compositions near hotspots document the interaction of hot geochemically enriched plume mantle with shallow depleted upper mantle whereas the spatial variations in geochemical composition of ocean crust along the ridge axis in areas where plume ridge interaction is taking place have been studied globally only restricted information exists concerning temporal variations in geochemistry of ocean crust formed through plume ridge interaction here we present a detailed geochemical study of 0 1 5 ma ocean crust sampled from the western galapagos spreading center wgsc axis to 50 km north of the axis an area that is presently experiencing a high influx of mantle material from the galapagos hotspot the tholeiitic to basaltic andesitic fresh glass and few bulk rock samples have incompatible element abundances and sr nd pb isotopic compositions intermediate between depleted normal mid ocean ridge basalt n morb from 95 5 w along the wgsc and enriched lavas from the galapagos archipelago displaying enriched e morb type compositions only limited and no systematic geochemical variations are observed with distance from the ridge axis for

sedimentology of the minoan deep sea tephra layer in the aegean and eastern mediterranean the minoan eruption of santorini resulted in deposition of pyroclastic material over a large area of the aegean sea and eastern mediterranean the eruptive activity commenced with a plinian phase which was followed by a phreatomagmatic phase and ended with numerous pyroclastic flows we present a sedimentological study of the minoan ash deposits found in cores taken throughout the region the cores are divided into three groups 1 those from the sub marine slopes immediately surrounding santorini 2 those from the aegean sea where the topography consists of many steep sided basins and 3 those from the eastern mediterranean where the topography is relatively flat and smooth this last group is designated the distal ash zone the deposits from the aegean sea have been remobilised as turbidity currents grain size distribution analysis reveals that these deposits are bimodal the mdo of the coarse population increasing away from source whilst that of the fine population remains uniform irrespective of distance from source the bimodality is caused by a combination of aggregation of fine particles in ash clouds produced during the last two phases of the eruptive activity and mixing of the coarse plinian and fine phreatomagmatic and co ignimbrite deposits within the turbidity current an unusual feature found in two of the cores is a coarse density sorted layer beneath the bouma a division of several turbidite units this is interpreted as an example of a flow head deposit as described by sparks and wilson 1983 the deposits from the distal ash fall zone are fine grained and unimodal containing phreatomagmatic and co ignimbrite material they show only very minor remobilisation of the original air fall ash due to the flat topography of the area the mdo of the deposit remains uniform irrespective of distance from source and is very similar to that of the fine population found in the bimodal aegean deposits this is interpreted as the result of aggregation of fines in the phreatomagmatic and co ignimbrite ash clouds the deposits in both areas have been extensively bioturbated a study of dispersed ash in the cores to reconstruct the original thickness of the ash layers suggests that ash layers less than 0 62 cm thick will be completely dispersed in the surrounding sediment whilst those thicker than 2 5 cm will be preserved as discrete layers the base being unaffected by bioturbation the deposits on the submarine slopes of santorini were not originally deposited as air fall ash but are derived from the erosion of coastal exposures of non welded ignimbrite they form storm sand layers and are typical of the type of deposit that may be formed by rapid erosion of fresh pyroclastic material after the eruption of an island arc volcano

evolution of mauna kea volcano inferences from lava compositions recovered in the hawaii scientific drilling project the lower 776 m of core recovered during the initial phase of the hawaii scientific drilling project hsdp contains lavas erupted from mauna kea volcano tholeiitic and alkalic basalts including an fe ti rich flow are intercalated in the upper 58 m of mauna kea lavas similar basaltic sections are subaerially exposed on the lower east flank of mauna kea the fe ti rich lavas reflect large amounts of clinopyroxene plagioclase and olivine fractionation within the crust and upper mantle but the range from tholeiitic to alkalic compositions reflects variable extents of melting of a garnet bearing source based on abundances of incompatible elements the extent of melting for a basanitoid was a factor of 2 less than that for nearly coeval tholeiitic lavas all flow units in the lower 718 m of the hsdp core are tholeiitic lavas their variability in major element compositions reflect variable accumulation of olivine incompatible element abundance ratios in these lavas reflect a complex temporal variation in extent of melting within the tholeiitic part of the core lavas from 800 m to 950 m formed by the largest extent of melting whereas tholeiitic lavas from the bottom of the core and from just below the tholeiitic to alkalic transition formed by lower degrees of melting inferred melt compositions at 16 mgo show that the 200 to 400 ka mauna kea lavas from the hsdp core and the 250 ka subaerial exposures define an inverse correlation between sio2 and feo contents based on experimental studies this correlation is caused by differing pressures of melt segregation furthermore abundances of nb and sio2 are also inversely correlated in these calculated melts in general the younger lavas are relatively enriched in feo and incompatible elements but are depleted in sio2 these trends are interpreted to reflect an overall trend of increasing pressure of melt segregation and decreasing extent of melting with decreasing eruption age there are however geochemical variations which indicate short term reversals in this long term trend previously the geochemical trends accompanying the transition from tholeiitic to alkalic volcanism at hawaiian volcanoes have been interpreted as reflecting the effects of increasing distance from the plume axis the long term geochemical trends of tholeiitic lavas in the hsdp core also reflect migration of mauna kea away from the hawaiian plume

evolution of galapagos magmas mantle and crustal fractionation without assimilation summary of helium isotopic data from the gala pagos islands and dredged samples from the gala pagos platform data from graham et al 1993 and kurz et al 1993 data are normalized to the atmospheric value r ra most gala pagos samples are intermediate betweenmorb value of 8 1 and the fernandina value 22 material in those magmas the exception is floreana structure first the depth to the magma chambers mayisland which ranges from 14 5 to 7 7 ra it is possible be due to the rate at which magma is supplied from thethat the low value at floreana re ects assimilation of mantle as the volcano is carried away from the focus ofmaterial with excess radiogenic 4 he by a magma with the hotspot the volcanoes in a juvenile growing stage 3 he 4 he ra 14 most floreana lavas contain abundant of evolution those that have had numerous historicdisaggregated debris from ultrama c xenoliths bow eruptions are dominated by lavas with mgo con geist 1992 a potential source of helium contamination centrations between 5 and 6 e g reynolds geist 1995 a narrow deep caldera characterizes the juvenilestage e g cerro azul wolf and fernandina and iteventually broadens and shallows because of lling with

grain size and sediment concentration effects on channel patterns of silt laden rivers abstract the yellow river in china is characterized by channel erosion and a meandering channel pattern at both low sediment concentrations and at hyperconcentrations whereas deposition and braiding tends to dominate at intermediate sediment concentrations a 3d numerical model delft3d is used to analyze the effect of sediment induced density effects on the formation of channel patterns using a highly simplified river geometry results show that the effect of vertical sediment density stratification and hindered settling significantly influence river channel patterns at some critical concentration the saturation concentration vertical mixing is totally damped by the density gradients at the lutocline a strong vertical gradient in sediment concentration this increases the secondary circulation in the river channel significantly which in turn promotes meander development when the flow is not close to saturation the main channel forming process seems to be channel avulsion through blocking of old channels and subsequent channel cut off when the concentrations are so high that sediment is primarily held in suspension by hindered settling sedimentation is so much reduced that channels are no longer blocked and therefore the river course becomes confined to a single meandering channel

geology and petrology of floreana island galapagos archipelago ecuador abstract floreana is the sixth largest of the galapagos islands and lies at the extreme southern margin of the galapagos platform in sharp contrast to the other major islands floreana lacks a well developed volcanic center but instead is dominated by pyroclastic vents and derivative ash and cinder deposits the majority of exposed lavas are xenolith bearing alkali olivine basalts that constitute a coherent suite referred to as the main series the flank series comprises a group of isolated vents on the southern flank of the island which erupted xenolith poor basaltic lavas that are generally poorer in incompatible trace elements and richer in plagioclase paleomagnetic measurements supported by k ar age determinations record a relatively limited timespan for subaerial volcanism shield development reached maximum dimensions by 1 0 ma with the final phases of pyroclastic activity at an end by 0 3 ma main series rocks are phenocryst poor and typically contain clusters of xenocrysts in an extremely fine grained granular matrix petrogenetic modelling indicates they represent products of partial melting of a lree enriched spinel lherzolite source little modified by subsequent fractional crystallization porphyritic flank series lavas experienced rather different ascent histories and may have resided in shallow level magma chambers prior to eruption isotopic geochemical and petrographic data demonstrate that flank series lavas are not cogenetic in any simple way with main series flows floreana lavas contain a diverse suite of ultramafic xenoliths al augite group megacrysts are likely to be cognate and comagmatic with host alkali olivine basalts transition metal contents of megacrysts and host lavas however preclude clinopyroxene fractionation as a mechanism of magmatic differentiation it is proposed that megacrysts were products of passive crystallization and were never effectively separated from host magmas cr diopside group nodules may have crystallized at very high temperatures from advanced partial melts and are less likely to bear any direct genetic connection to host basalts

predicting grain size in gravel bedded rivers using digital elevation models application to three maine watersheds riverbed grain size controls suitability of spawning habitat for threatened fish species motivated by this relationship we developed a model that uses digital elevation models dems to predict bed grain size we tested the accuracy of our model and two existing models with channel measurements from high resolution airborne light detection and ranging lidar dems all three models assume that bed grain size is a function of reach average high flow channel hydraulics measured by shear stress or stream power our test data are field measurements of median grain size d 50 at 276 stations along four rivers in maine pleistocene continental glaciation strongly influences the longitudinal profiles which have alternating steep and gradual segments we exploit the resulting variations in sediment supply to understand the controls on model success or failure in predicting bed grain size results show that all three models have 70 success in predicting d 50 within a factor of two overall and better where the rivers are coarse gravel bedded 80 success where d 50 16 mm this similarity is unsurprising given that the models primarily rely on channel gradient s and drainage area as inputs measurements of s from lidar dems yield only a modest improvement in model success over those from topographic maps we find that our model works best in sediment starved steep reaches model failures fall into two broad categories 1 relatively fine grained d 50 d 50 we argue that models based on airborne infrared lidar dems may reach a maximum around 80 85 accuracy due to these sub reach scale factors which cannot be easily measured from dems the overall success of the models in predicting grain size indicates that the morphology of these channels has adjusted to the imposed s and sediment load during the 15 k y since deglaciation and through the period of anthropogenic channel change over the past three centuries

a graded stream response relation for bed load dominated streams 1 the relation between equilibrium stream channel morphology and three independent governing variables discharge sediment supply and valley slope is investigated using a stream table the experiments are based on a generic physical model of a laterally active gravel bed stream and directed by a recently published rational regime model while a variety of channel adjustments are possible the primary adjustment observed was in the channel slope other adjustments such as surface armor composition and cross sectional shape were minor in comparison the equilibrium slope is well predicted by a linear function of sediment concentration which is bounded by two thresholds one associated with the minimum stream power necessary to deform the bed and the other associated with the maximum sediment feed that can be transported by the available flow the results suggest that the system tends to move toward the minimum slope capable of transporting the sediment supply in the process increasing the flow resistance for the system whether or not a minimum slope or a maximum system scale flow resistance is reached cannot be determined given the available data but there appears to be a unique slope or at least a limited range of slopes for which a stable alluvial channel morphology can be established for a given sediment concentration the implications of this behavior are evaluated for the theoretical basis of the extremal hypothesis used to close the formulation of a rational regime model

meander hydromorphology of ephemeral streams similarities and differences with perennial rivers abstract the geomorphology hydrology and processes of ephemeral streams are poorly known and studies on the geomorphic characteristics of ephemeral meandering streams ems are even less investigated we collected geomorphic data namely channel width sinuosity wavelength and curvature radius from 107 ems reaches in different drylands of the world from googleearth pro satellite images geomorphology features processes such as neck cutoffs point scrolls and chute cutoffs were also assessed the main aim of this paper is to investigate similarities and differences between ems and perennial counterparts of more humid areas on which almost all hydrogeomorphic models are based for the purpose of explaining why meandering rivers form in environments where the lack of vegetation sporadic flows and a high sediment supply should favour the development of braided rivers ems showed both geomorphological similarities and differences with perennial rivers point bar chute cutoffs and point bar scrolls are the most common geomorphic features of ems suggesting a certain stability of meander behaviour though within a continuous process of changes towards the most probable form similarly to perennial streams the bimodal distribution of curvature ratio values departs substantially from that of perennial rivers that are commonly in the 2 3 range data from the rivers in this study indicate that their channel pattern is well described by the same sine generated function used for perennial rivers wavelength to channel width ratio and bankfull discharge to streambed gradient of ems unexpectedly plot in different areas of classical diagrams developed for perennial rivers we interpret such differences in terms of excess energy expenditure for high sediment load transport we also propose a new function to discriminate the meandering from the braided pattern of ephemeral streams and present a conceptual model of meander formation in ephemerals streams based on an autogenic process of bank collapse and bar deposition

characteristics of pyroclastic deposits at the margin of fond canonville martinique and implications for the transport of the 1902 nuees ardentes of mt pelee abstract the deposits which were studied at fond canonville represent a well defined record of the 1902 eruptions of the montagne pelee the sections are located at the margin of the valley where six eruption units with thicknesses ranging from 8 cm up to 2 m were identified overlying the pre 1902 paleosol the pyroclasts consist of lapilli coarse and fine ash composed of juvenile andesitic fragments of varying vesicularity dust free crystals dust free transparent vesicular glass and accretionary lapilli paleocurrent measurements on cross bedding support the stratigraphic evidence that the flows came down the valley of fond canonville rather than up as was previously proposed most eruption units are made of two or three beds always in sharp contact the basal bed relatively thin contains the coarsest grain size of the unit is massive or normally graded and fine depleted a thin division of parallel laminae may be present at the top the middle bed the thickest in all eruption units has a mean grain size which is intermediate between the lower and upper beds it may be simple or multiple normal grading is generally present on which may be superimposed in the upper portion of the bed 2 4 cm thick strata parallel to the upper bed surface or dipping from it at angles of less than 10 the upper bed the most regular in all units with thicknesses ranging from 2 to 8 cm is made of accretionary lapilli set in a matrix of fine ash a division of parallel and or oblique laminae is generally present at its top the characteristics of the deposits suggest that all three beds accumulated from a stratified turbulent flow normal grading of both coarsest and mean sizes sequences of internal structures including laminae evince that the flows were of relatively low concentration and that traction was an active process during the accumulation of the deposits for these structures to form the flows had to be turbulent for all three beds of the eruption units the thick strata superimposed on the graded middle bed are interpreted as having formed by the migration and aggradation of long wavelength antidunes the relationship between flow velocity and wavelength suggests that subaerial pyroclastic flows can only produce long wavelength antidunes with resulting internal stratification essentially parallel to bedding short wavelength antidunes in subaerial pyroclastic deposits require very high flow density at least close to the bed surface or low velocity which is unlikely considering the grain size present in the deposits

the width of a bankfull channel lacey s formula explained according to lacey s formula the width of a natural channel at bankfull flow is proportional to the root of the discharge it is a very simple formula that has been confirmed by many authors and which until now has had no physical explanation it appears that lacey s equation is composed of physical and measurable parameters which agree with field observations the equation hinges on the fact that the velocity at bankfull discharge is a sole function of the bed material at bankfull discharge the average velocity is no longer a function of the discharge as is assumed in regime theory at discharges below bankfull level the stream velocity is a function of the discharge to the power 1 6 however at bankfull discharge a singularity occurs where the water slope is forced on the slope of the natural levees the velocity of flow associated with this slope is fully determined by the bed material and is independent of the discharge an analytical expression for lacey s coefficient of proportionality is presented

flow evolution of experimental gravity currents implications for pyroclastic flows at volcanoes abstract gravity current experiments consisting of aqueous glucose flows and turbidity suspensions were performed to investigate mixing and sedimentation processes the evolution from high density to low density flows and the effects of variable grain density the initial solution or suspension was released from a lucite box into a water filled flume 5 5 m long the aqueous glucose solutions had initial densities ranging from 1014 to 1206 kg m 3 the turbidity suspensions consisted of spherical acrylic resins with a narrow grain size of 0 21 0 30 mm 2 25 1 75 and two densities of 1143 and 1196 kg m 3 the initial bulk densities of the turbidity suspensions were 1040 and 1062 kg m 3 both the aqueous glucose flows and the turbidity currents became density stratified with distance with the densest flows exhibiting the largest vertical density gradients for a given initial bulk density the turbidity flows had lower velocities densities viscosities and reynolds numbers due to the effects of sedim

the tholeiite to alkalic basalt transition at haleakala volcano maui hawaii previous studies of alkalic lavas erupted during the waning growth stages 0 9 ma to present of haleakala volcano identified systematic temporal changes in isotopic and incompatible element abundance ratios these geochemical trends reflect a mantle mixing process with a systematic change in the proportions of mixing components we studied lavas from a 250 m thick stratigraphic sequence in honomanu gulch that includes the oldest 1 1 ma subaerial basalts exposed at haleakaka the lower 200 m of section is intercalated tholeiitic and alkalic basalt with similar isotopic sr nd pb and incompatible element abundance ratios e g nb la la ce la sr hf sm ti eu these lava compositions are consistent with derivation of alkalic and tholeiitic basalt by partial melting of a compositionally homogeneous clinopyroxene rich garnet lherzolite source the intercalated tholeiitic and alkalic honomanu lavas may reflect a process which tapped melts generated in different portions of a rising plume and we infer that the tholeiitic lavas reflect a melting range of 10 to 15 while the intercalated alkalic lavas reflect a range of 6 5 to 8 melting however within the uppermost 50 m of section 87sr 86sr decreases from 0 70371 to 0 70328 as eruption age decreased from 0 97 ma to 0 78 ma we infer that as lava compositions changed from intercalated tholeiitic and alkalic lavas to only alkalic lavas at 0 93 ma the mixing proportions of source components changed with a morb related mantle component becoming increasingly important as eruption age decreased

origin of espanola island and the age of terrestrial life on the galapagos islands geological studies of espanola hood island galapagos ecuador indicate that the island had a subaerial rather than a submarine origin because the younger lava flows are dated at 3 million years espanola has apparently existed as an island for at least that long thus terrestrial life may have existed or arrived on the galapagos islands at least 3 million years ago more than twice as long as had been assumed

ignimbrite depositional facies the anatomy of a pyroclastic flow a model is presented for the depositional regimes in a pyroclastic flow to explain the origin of facies in the taupo ignimbrite and to compare these with other published examples in this model a pyroclastic flow consists of a head a body and a tail the head which is where fluidization caused by air ingestion occurs generates layer 1 deposits on lithological grounds these are divided into layers 1 p which is fines depleted and rich in pumice and 1 h which is fines depleted and rich in lithics and crystals layer 1 p represents material thrown forwards from the flow head and is termed the jetted deposits layer 1 h represents material sedimented from within the flow head and is termed the ground layer the body represents the bulk of the flow and the tail is its trailing part which is slowed by ground friction these parts generate the layer 2 deposits which include the valley pond ignimbrite and its associated ignimbrite veneer deposit localized depositional modes within the body and tail generate distinctive coarse pumice concentration zones and pumiceous lee side lenses behind obstacles at its outer limits all of the flow on interfluves is affected by air ingestion fluidization producing the distant facies which combines features of both layer 1 and 2 deposits one important conclusion of this work is that the thicknesses and compositions of the various facies are not related in any simple way to the thickness and composition of the parent flow

subaqueous intermediate to silicic composition explosive volcanism a review abstract mass flow deposits of pyroclastic debris commonly consist of two parts a lower coarse grained basal section and an upper finer grained bedded section subaqueous pyroclastic flow deposits particularly those derived from a vertically directed eruption appear better sorted than their subaerial counterparts because of the efficient sorting characteristics of water compared to air subaqueous deposits frequently are depleted in bubble wall pumice that floats off and vitric ash that remains in suspension hence the deposits show a relative enrichment of crystals especially at the bases from 1 flotation and suspension of the vitric material and 2 ingestion of water at the head of the flow a subaqueous volcaniclastic flow deposit may contain evidence indicative of both turbulent and non turbulent movement the flow may have had a turbulent head and non turbulent body or flow transformations may have occurred from 1 collapse of the eruption column 2 changes in slope and in the velocity of the flow and or 3 the amount of water mixed into the flow retention of heat in the subaqueous environment may be enhanced by lateral eruptions high eruption rates large eruptive volumes and little or no mixing of pyroclastic debris with water the lack of welded subaqueous deposits in the geologic record may be a result of 1 effusive rather than explosive eruptions in a deep water environment 2 explosive phreatic disintegration of pyroclastic flows in a shallow subaqueous environment and or 3 the inability of subaerial pumiceous pyroclastic flows to penetrate the land water interface characteristics of explosive intermediate to silicic composition subaqueous volcanic eruptions also have been summarized in terms of 1 environment of the eruption 2 orientation of the eruption and 3 composition of the juvenile debris these three aspects of explosive subaqueous volcanism are ideal end members and gradations no doubt occur

subsidence control on river morphology and grain size in the ganga plain the ganga plain represents a large proportion of the current foreland basin to the himalaya the himalayan sourced waters irrigate the plain via major river networks that support approximately 10 percent of the global population however some of these rivers are also the source of devastating floods the tendency for some of these rivers to flood is directly linked to their large scale morphology in general the rivers that drain the east ganga plain have channels that are perched at a higher elevation relative to their floodplain leading to more frequent channel avulsion and flooding in contrast those further west have channels that are incised into the floodplain and are historically less prone to flooding understanding the controls on these contrasting river forms is fundamental to determining the sensitivity of these systems to projected climate change and the growing water resource demands across the plain here we present a new basin scale approach to quantifying floodplain and channel topography that identifies areas where channels are super elevated or entrenched relative to their adjacent floodplain we explore the probable controls on these observations through an analysis of basin subsidence rates sediment grain size data and sediment supply from the main river systems that traverse the plain yamuna ganga karnali gandak and kosi rivers subsidence rates are approximated by combining basement profiles derived from seismic data with known convergence velocities results suggest a more slowly subsiding basin in the west than the east grain size fining rates are also used as a proxy for relative subsidence rates along the strike of the basin the results also indicate higher fining rates and hence subsidence rates for given sediment supply in the east by integrating these observations we propose that higher subsidence rates are responsible for a deeper basin in the east with perched low gradient river systems that are relatively insensitive to climatically driven changes in base level in contrast the lower subsidence rates in the west are associated with a higher elevation basin topography and entrenched river systems recording climatically induced lowering of river base levels during the holocene

the relationship between flow shear stress and stream pattern based on simple hydraulic reasoning a discharge q slope s diagram for streams is given a physical interpretation permitting the assignment of a relative shear stress value to each stream a sample of 359 streams is used ranging in mean annual discharge from 900 000 to 0 05 ft 3s 1 varying in pattern from highly sinuous p 3 1 to straight and braided sorted according to their relative shear stress values 0 avg streams of high sinuosity p 1 5 are characterized by low relative shear stress median 0 56 which is significantly lower than that characterizing braided streams median 1 70 it is concluded that for natural stream populations the passage from one pattern to another is gradual and the geomorphic thresholds defined by relative shear stress are fuzzy ones

the 1902 peleean deposits in the fort cemetery of st pierre martinique a model for the accumulation of turbulent nuees ardentes abstract the ancient fort cemetery located at the northwestern limit of st pierre has been left undisturbed since 1902 and therefore offers a unique opportunity to study the deposits of the eruptions that destroyed the city six eruption units were found overlying the floor of the cemetery leaving an average 1 6 m thick deposit the city was hit by four major nuees which occurred on may 8 20 june 6 and august 30 the may 20 and june 6 eruption units are each made of three beds the basal bed always lacks fines and is generally massive the middle bed is normally graded on which dune type cross bedding is superimposed in the may 20 deposit the upper bed the thinnest has the finest grain size of the eruption unit and is normally graded true parallel laminae were only observed in the upper bed the may 8 eruption unit is essentially made of fines depleted massive lapilli and tuff the evidence strongly suggests that the deposits of the middle and upper beds accumulated from turbulent suspensions the absence of structures in the basal bed is not conclusive evidence of laminar flow deposition and its characteristics are similar to deposits that have accumulated from highly concentrated turbulent suspensions the lack of fines observed only in the basal bed is attributed to gravity segregation helped by vaporization of water present in the underlying deposits and for the may 8 deposit to escaping gas produced by the destruction of the vegetation during or immediately after deposition which could explain the absence of primary structures in the basal bed the fine grained deposits of mt pelee show a primary structure sequence similar to the turbidite sequence and are therefore interpreted in terms of decreasing flow velocity and grain size with time at a fixed site

geology and petrogenesis of lavas from san cristobal island galapagos archipelago volcanism became emergent on san cristobal island 2 3 m y b p in the eruption of floods of alkaline basalt volcanism then coalesced in a central shield volcano which erupted continually until 0 6 m y b p at 0 9 m y b p activity began to jump to the northeast and the style of volcanism changed to fissure fed eruptions that have continued almost to historical time three distinct magma series occur on san cristobal an alkaline series a series of morb like tholeiites and a series of incompatible element enriched tholeiites that are typical of oceanic islands each series is represented by very primitive basalts that have been little modified since segregation from their sources each series also has more evolved lavas which range from low mg basalts to hawaiites and icelandites these evolved lavas are the result of simple fractional crystallization of magmas which produces major and compatible element compositions similar to those of observed primitive lavas the isotope and trace element data suggest that the different series are generated by different degrees of partial melting and mixing of both a depleted and an enriched source the first source is thought to be the depleted asthenosphere which generates the morb9s of the galapagos spreading center and the enriched source is the mantle plume associated with the galapagos hotspot

lithological and fluvial controls on the geomorphology of tropical montane stream channels in puerto rico an extensive survey and topographic analysis of five watersheds draining the luquillo mountains in north eastern puerto rico was conducted to decouple the relative influences of lithologic and hydraulic forces in shaping the morphology of tropical montane stream channels the luquillo mountains are a steep landscape composed of volcaniclastic and igneous rocks that exert a localized lithologic influence on the stream channels however the stream channels also experience strong hydraulic forcing due to high unit discharge in the humid rainforest environment gis based topographic analysis was used to examine channel profiles and survey data were used to analyze downstream changes in channel geometry grain sizes stream power and shear stresses results indicate that the longitudinal profiles are generally well graded but have concavities that reflect the influence of multiple rock types and colluvial alluvial transitions non fluvial processes such as landslides deliver coarse boulder sized sediment to the channels and may locally determine channel gradient and geometry median grain size is strongly related to drainage area and slope and coarsens in the headwaters before fining in the downstream reaches a pattern associated with a mid basin transition between colluvial and fluvial processes downstream hydraulic geometry relationships between discharge width and velocity although not depth are well developed for all watersheds stream power displays a mid basin maximum in all basins although the ratio of stream power to coarse grain size indicative of hydraulic forcing increases downstream excess dimensionless shear stress at bankfull flow wavers around the threshold for sediment mobility of the median grain size and does not vary systematically with bankfull discharge a common characteristic in self forming threshold alluvial channels the results suggest that although there is apparent bedrock and lithologic control on local reach scale channel morphology strong fluvial forces acting over time have been sufficient to override boundary resistance and give rise to systematic basin scale patterns copyright 2010 john wiley and sons ltd

geology of santa fe island the oldest galapagos volcano abstract santa fe island was a volcanic center when it emerged 3 9 0 6 m y ago later upfaulting of a horst along the central axis of the island dominates its present morphology santa fe is made up of evolved transitional lavas that are not related by fractional crystallization alone source heterogeneties differing degrees of melting or open system magma chambers may explain the observed trace element variations santa fe baltra and espanola make up a geologic subprovince in the central galapagos they are older than the other islands and their lavas are compositionally similar at the time of their emergence the three islands were in a tectonic setting similar to that of the young western and central galapagos island

assessing the effect of vegetation related bank strength on channel morphology and stability in gravel bed streams using numerical models bank strength due to vegetation dominates the geometry of small stream channels but has virtually no effect on the geometry of larger ones the dependence of bank strength on channel scale affects the form of downstream hydraulic geometry relations and the meandering braiding threshold it is also associated with a lateral migration threshold discharge below which channels do not migrate appreciably across their floodplains a rational regime model is used to explore these scale effects it parameterizes vegetation related bank strength using a dimensionless effective cohesion cr the scale effects are explored primarily using an alluvial state space defined by the dimensionless formative discharge q and channel slope s which is analogous to the q s diagrams originally used to explore meandering braiding thresholds the analyses show that the effect of vegetation on both downstream hydraulic geometry and the meandering braiding threshold is strongest for the smallest streams in a watershed but that the effect disappears for q 106 the analysis of the migration threshold suggests that the critical discharge ranges from about 5 m3 s to 50 m3 s depending on the characteristic rooting depth for the vegetation the analysis also suggests that where fires frequently affect riparian forests channels may alternate between laterally stable gravel plane bed channels and laterally active riffle pool channels these channels likely do not exhibit the classic dynamic equilibrium associated with alluvial streams but instead exhibit a cyclical morphologic evolution oscillating between laterally stable and laterally unstable end members with a frequency determined by the forest fire recurrence interval copyright 2008 john wiley sons ltd

longitudinal profile of channels cut by springs we propose a simple theory for the longitudinal profile of channels incised by groundwater flow the aquifer surrounding the stream is represented in two dimensions through darcy s law and the dupuit approximation the model is based on the assumption that everywhere in the stream the shear stress exerted on the sediment by the flow is close to the minimal intensity required to displace a sand grain because of the coupling of the stream discharge with the water table elevation in the neighbourhood of the channel head the stream elevation decreases as the distance from the stream s tip with an exponent of 2 3 field measurements of steephead ravines in the florida panhandle conform well to this prediction

patterns of alluvial rivers the pattern planform of a river can be considered at vastly different scales depending upon both the size of the river and the part of the fluvial system that is under consideration figure 1 for example in the broadest sense river patterns comprise a drainage network dendritic parallel trellis etc figure la the type of pattern is of interest to geomorphologists and geologists who interpret geologic conditions from aerial photographs at another scale a river reach which in figure lb is meandering is of interest to the geomorphologist who is interested in what that pattern reveals about river history and behavior and to the engineer who is charged with maintaining navigation and preventing major instability when a single meander is examined figure 1 c the hydraulics offtow the sediment transport and the potential for bank erosion are of concern in addition the sedimentologist is interested in the distribution of sediment within the bend bed forms within the channel figure ld and sedimentary structures figure ie which also establish a component of roughness for the hydraulic engineer finally the individual grains figure if provide geologic information on the sediment sources the nature of sediment loads and the feasibility of dredging for gravel there is an interaction of hydrology hydraulics geology and geomorphology at all scales which emphasizes the point that the fluvial system as a whole cannot be ignored even though only a component of the system is to be studied in this review only the patterns or planforms of alluvial rivers are discussed although it is apparent that the hydrologic and sediment yield characteristics of the drainage basin figure la as well as its geologic history cannot be ignored in the explanation of the pattern of any river

variations in downstream grain sizes to interpret sediment transport in the middle lower yangtze river china a pre study of three gorges dam abstract in 2000 and 2003 before the closure of 3 gorges dam numerous sediment samples were taken from the middle lower yangtze river channel to examine sediment transport processes and associated hydromorphological nature of the river analytical results show that the riverbeds consist mostly of medium to coarse sands and gravelly sands and fine sand occurs locally especially near the river coast the results further indicate a downstream fining trend in riverbed sediment from yichang to the river mouth totaling about 1900 km long with 12 sediment zones i xii these were identified as alternate coarse and fine grained sediment on the riverbed although the zonation of i iii below three gorges dam site is weaker the mode of sediment transport in the river is dominated by saltation 20 80 followed by bed load transport with 3 15 transport by suspension is quite low grain sizes associated with hydrological parameters have greater values in the jingjiang reaches from yichang to chenglingji unit stream power 5 18 n m 1 s 1 boundary shear stress 14 nm 2 and mean flow velocity 2 3 2 ms 1 whereas the values obtained from chenglingji downstream are considerably low 1 s 1 1 4 nm 2 and 1 these values when compared with on site measured velocity of the adp flow column revealed the erosive riverbed sediment transport in the jingjiang reaches and the accumulative riverbed transport downstream from wuhan to the river coast hydrological parameters together with distribution of grain sizes indicate a coarsening riverbed in the jingjiang river largely because damming peaked since the last half century this corroborates the weakening sediment zonation in the jingjiang reaches which is expected to extend further downstream towards the river coast in response to the potential impact of 3 gorges dam in the coming decades

vertical density currents a review of their potential role in the deposition and interpretation of deep sea ash layers marine tephra layers form important chronostratigraphic markers and may also provide information on eruption dynamics and chronology recent experiments and field data from the 1991 pinatubo eruption challenge tacit assumptions that following atmospheric fallout on the ocean surface marine sedimentation of ash grade material occurs by stokes law settling of individual grains laboratory experiments have demonstrated that vertical density currents can be initiated at representative mass flux rates further experiments described herein involving sustained particle fluxes entering stratified environments demonstrate that ash particles can decouple from the transporting fluid whose vertical motions are limited by the stable density gradient in the ocean vertical density currents generated by ash loading and gravitational destabilization of the water column can overcome the strong stable density gradients in the ocean and transport ash particles vertically 1 3 orders of magnitude faster than possible by stokesian settling greatly reducing their residence time sea bed sedimentation is inferred to involve settling of individual coarse or dense grains through the entire water column plus suspension fallout from the polydisperse turbid layer delivered by convective plumes yielding normal distribution grading the turbid layer may continue laterally as a nepheloid current under the influence of bottom slopes and or geostrophic forces either mechanism possibly modifying the grain size distribution of the suspension and hence the resulting ash layer by transporting selected fractions into different depositional locations consequently and regardless of any subsequent reworking deep sea ash isopachs may not directly reflect an eruptive signal and the ash layers may not directly record the order rate or location of arrival of ash particles at the ocean surface

sediment transport constraints on river response to regulation abstract we demonstrate that alluvial channel response to regulation depends on the characteristic mode of sediment transport in a channel and thus for a channel in supply transport equilibrium on the caliber of sediment mantling the channel bed as well as changes in characteristic water discharge and sediment load we find that regulated coarse gravel and cobble bed alluvial rivers dominated by bedload transport are predicted and observed to preserve the value of a dimensionless ratio of characteristic water discharge channel slope width and size of sedimentary particles mantling the bed associated with the critical conditions required for the onset of significant sediment transport the covariant geomorphic adjustment of such rivers to regulation is independent of the degree of sediment impoundment although the nature and rate of changes in any specific characteristic appear to be sensitive to changes in sediment supply in contrast covariant geomorphic changes in sand and fine gravel bed alluvial rivers dominated by weak suspension transport are predicted and observed to be dependent on the degree of sediment impoundment and commensurate with the change in the sediment concentration of characteristic formative flows in both cases specific geomorphic characteristics of alluvial rivers exhibit varied responses to regulation in different rivers but across all rivers a systematic covariation in channel slope width and grain size is observed that for a given mode of sediment transport preserves a characteristic value of the shields number

how to make a meandering river despite the ubiquity of meandering rivers in nature only recently have appropriate experimental conditions been produced to replicate a stably meandering stream in the laboratory as described in a recent issue of pnas 1 meandering channels occur in a wide variety of sedimentary environments including on deep sea fans formed by turbidity currents 2 as relict meanders on mars 3 fig 1 and as channels formed by flowing alkenes on titan

lithospheric structure and compensation mechanisms of the galapagos archipelago volcanic islands of the galapagos archipelago are the most recent subaerial expression of the galapagos hotspot these islands and numerous seamounts are constructed mainly upon a broad volcanic platform that overlies very young 10 m y oceanic lithosphere just south of the active galapagos spreading center the 91 w fracture zone crosses the platform and creates an estimated 5 m y age discontinuity in the lithosphere major tectonic features of the galapagos include an unusually broad distribution of volcanic centers pronounced structural trends such as the nw se wolf darwin lineament wdl and a steep escarpment along the western and southern margins of the archipelago we use shipboard gravity and bathymetry data along with geosat geoid data to explain the tectonic and structural evolution of the galapagos region we model the gravity anomalies using a variety of compensation models including airy isostasy continuous elastic flexure of the lithosphere and an elastic plate with embedded weaknesses and we infer significant lithospheric strength variations across the archipelago the outboard parts of the southern and western escarpment are flexurally supported with an effective elastic thickness of 12 km this area includes the large shield volcanoes of fenandina and isabela islands where the lithosphere regionally supports these volcanic loads the central platform is weaker with an elastic thickness of 6 km or less and close to airy isostasy the greatest depths to the moho are located beneath eastern isabela island and the central platform thinner lithosphere in this region may account for the broad distribution of volcanoes the extended period of eruption of the central volcanoes and their reduced size the transition from strong to weak lithosphere along the southern escarpment appears to be abrupt within the resolution of our models and can be best represented by a free end or faultlike discontinuity also modeling the wdl as a lithospheric fault increases the match to the observed gravity anomalies the primary feature is the weak central platform whose existence is a logical convergence of several factors the 91 w transform age offset the possible increase in age of the lithosphere along the southern part of the platform due to ridge jumping and the formation of the central platform at a spreading ridge

summit lake landslide and geomorphic history of summit lake basin northwestern nevada abstract the summit lake landslide northwestern nevada composed of early miocene pyroclastic debris ashdown tuff and basalt and rhyolite of the black rock range blocked the upper soldier creek snow creek drainage and impounded summit lake sometimes prior to 7840 yr b p the slide covers 8 2 km 2 and has geomorphic features characteristic of long run out landslides such as lobate form longitudinal and transverse ridges low surface gradient 7 1 and preservation of original stratigraphic position of transported blocks however estimated debris volume is the smallest reported 2 5 10 5 m 3 for a landslide of this type the outflow channel of the summit lake basin was a northward flowing stream valley entrenched by mahogany creek subsequent negative tectonic adjustment of the basin by about 35 m accompanied by concommitant progradation of a prominent alluvial fan deposited by mahogany creek argues for a probable diversion of drainage from the alvord basin southward into the lahontan basin the landslide occurred while the creek flowed southward transferring about 147 km 2 of watershed from the lahontan basin back to the alvord basin overflow northward occurred during high stands of pluvial lake parman in the basin otherwise under drier climates the summit lake basin has been closed within large depressions on the slide surface the ca 6800 yr old mazama bed and other sediments have buried a weakly developed soil disseminated humus in the soil yields an age of 7840 310 yr b p absence of older tephra such as st helens m brackets the slide age between 7840 and 19 000 yr b p projectile points found on the highest strandlines of pluvial lake parman suggest a ca 8700 yr b p age by correlation with cultural artifacts and radiocarbon ages from nearby last supper cave nevada organic matter accumulation in landslide soils suggests ages ranging from 9100 to 16 250 yr b p estimation of the age of the slide from morphologic data for the isolated summit lake population of lahontan cutthroat trout does not conflict with the radiometric ages

characteristics of dune bedded pyroclastic surge bedsets abstract the grainsize characteristics of dune bedded pyroclastic surge bedsets are surveyed the variance between coarsest and finest beds ranges from 1 to 6 phi in different surge bedsets and it increases as the grainsize of the coarsest bed increases reflecting an increasing velocity of emplacement deposits of wet surges identified as those which contain accretionary and ash coated lapilli tend to be finer and show less variance this partly because wet ash is cohesive but mainly because wet surges tend to be weaker dry surge bedsets are strongly fines depleted wet ones less so the lack of erosion of underlying ash layers shows that the environment is a strongly depositional one individual bedsets are demarcated by thin intervening fine ash fall layers which are the complementary ash cloud deposits settled or flushed out after the passage and decay of each turbulent surge surge deposits are generally less coarse than the coarsest associated airfall deposits which shows that they are formed by generally weaker events this study helps interpret the dune bedded parts of the landscape mantling may 18th 1980 blast deposit of mount st helens the blast was a very violent event but the variance and the grainsize of the coarsest bed are those of a relatively weak surge this suggests that the dune bedding was produced by a weak effect such as minor turbulence in a thin pyroclastic flow coming to rest in a mountainous terrain roughened by tree stumps and fallen logs

the volcanic geology and petrology of isla pinta galapagos archipelago pinta one of the northern islands in the galapagos archipelago is the top of a shield volcano which became emergent 0 7 m y ago a regional north north west trending fracture system influenced the locus of volcanism pinta9s geologic development can be divided into an older shield stage and a younger fissure stage each stage produced a similar yet distinct series of evolved olivine tholeiites magnesium number 52 the shield series underwent some additional shallow level fractionation in a periodically replenished chamber rare earth element data indicate that the mantle source for both of the series has residual garnet higher concentrations of incompatible trace elements iri the fissure series imply a lower degree of partial melting 143 nd 144 nd and 87 sr 86 sr ratios suggest that both series have an enriched hotspot type source despite close proximity to the galapagos spreading ridge pinta9s lavas lack features characteristic of mid ocean ridge basalts the isotopic and rare earth element data from pinta and the other northern islands are consistent with previous models for the galapagos spreading ridge which require north south axially directed transfer of enriched asthenosphere and or melts from the galapagos hotspot to the galapagos ridge

grain size sediment transport and alluvial channel pattern abstract the shields parameter a dimensionless bed shear stress which can be expressed as a ratio of the slope depth and characteristic properties of bed material in an alluvial channel has been observed in natural rivers to take on modal values for the regimes of bedload mixed load and suspension transport dade w b friend p f 1998 grain size sediment transport regime and channel slope in alluvial rivers j geology 106 661 675 such conditions correspond to channel geometries associated with conserved sediment flux and thus to a state of approximate equilibrium in which the channels are neither aggrading nor degrading to a significant degree these findings are extended here to accommodate the effects of channel width in the context of a comprehensive data set on stable perennial rivers the new analysis reconciles existing empirical and theoretical approaches to predict alluvial channel geometry

the geology and geochemistry of isla marchena galapagos archipelago an ocean island adjacent to a mid ocean ridge abstract isla marchena is the subaerial exposure of a shield volcano located between the galapagos platform and the galapagos spreading center the geologic evolution of the island can be divided into two lava series separated by formation of a cadera and a period of explosive activity both lava series contain evolved alkali olivine and tholeiitic basalts average mg 55 2 and are virtually aphyric with the exception of a plagioclase rich horizon in the pre caldera section although the major element compositions of the two series are similar the abundances of incompatible elements and ratios of highly moderately incompatible elements increase systematically with decreasing stratigraphic age these compositional differences together with textural and mineral chemical evidence for disequilibrium are not easily explained by any single petrologic mechanism the combined effects of a periodically replenished magma chamber assimilation fractionation and in situ crystallization of a solidification zone are possible mechanisms that may be responsible for the temporal changes in lava composition morb like abundances of trace elements in addition to unusually depleted 87 sr 86 sr and 143 nd 144 nd ratios for ocean island basalts suggest that magmas associated with the galapagos plume interacted with those of the galapagos spreading center

fernandina volcano s evolved well mixed basalts mineralogical and petrological constraints on the nature of the galapagos plume fernandina volcano the most active of galapagos volcanoes and the one most directly overlying the galapagos hotspot is one of a small number of active plume related oceanic island volcanoes with a well documented recent eruptive record whole rock and glass analyses of lava and tephra from all 12 known eruptions from 1958 to the present show them to be evolved mg mg mg fe2 of 0 47 0 56 plagioclase phyric clinopyroxene and olivine bearing tholeiites thermodynamic modeling indicates that this phase assemblage erupted at 1100 1130 c and is stable only at low pressures 0 1 0 2 gpa thereby representing crystallization within a shallow magma chamber underlying the well defined 850 m deep 5 by 6 5 km caldera these tholeiites are remarkably homogenous as is shown by narrow ranges in zr y 5 7 6 3 la sm n 1 4 1 7 and la yb n 3 8 4 3 prehistoric fernandina lavas selected for petrographic and geographic variation over the entire subaerial volcano are of remarkably similar composition e g zr y is 5 2 6 1 la sm n is 1 5 1 6 and la yb n is 3 4 4 1 with a single example of a more primitive lava having an mg of 0 63 with 8 5 feo at 8 mgo cr spinel and glass melt inclusions in plagioclase together with zoned olivine give evidence for a range of parental melts in the recent shallow magmatic system with mg as high as 0 62 extensive shallow magma mixing is required to explain these variations in mineral and inclusion chemistry indicating more thermal variation within the shallow magmatic system than is implied by the relatively monotonous whole rock compositions apparently fernandina s shallow magma chamber has physically blocked passage of more primitive magmas to the surface and homogenized the erupted lavas by extensive mixing small variations in lava isotopic and compatible element composition combined with the need to resupply melt to maintain thermal balance indicate that replenishment of the magma chamber is semicontinuous with resupply occurring of the order of a decade or less plagioclase dominated evolved fernandina lavas are in sharp contrast to more primitive olivine dominated hawaiian lavas the total lack of primitive melts in the shallow fernandina magmatic system implies that substantial fractionation and heat loss occurred during melt transportation from the mantle source to the edifice magma chamber despite the fact that these melts were transported through relatively warm young and thin lithosphere adjacent to the cocos nazca ridge the diffuse nature of galapagos volcanism with 20th century eruptions from eight distinct volcanoes over a 20 000 km2 area further supports this interpretation and suggests that the galapagos plume is relatively diffuse weak and significantly less thermally intense than the vigorous hawaiian plume it may in fact be waning

flow head deposits in ash turbidites during the bronze age minoan eruption of santorini volcano greece a volcanic ash layer was spread over the southern aegean and the eastern mediterranean contemporaneous slumping of ash fall deposits which accumulated on the sea floor generated density currents that entered basins in the southern aegean sea beneath the t a divisions of the resulting turbidites we have observed a separate depositional layer this layer is coarser grained than the base of the overlying t a division shows a lower half enriched in crystals and lithic rock and an upper half enriched in pumice shards and is depleted in silt size components compared to the t a division we propose that this layer formed by the incorporation of water into the head of a sediment laden density current resulting in fluidization and expansion of the head and the segregation and sedimentation of the coarser and denser particles this layer was subsequently overridden by the body of the flow that deposits the overlying t a division

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