volcano ice interactions precursory to the 2009 eruption of redoubt volcano alaska abstract in late summer of 2008 after nearly 20 years of quiescence redoubt volcano began to show signs of abnormal heat flow in its summit crater in the months that followed the excess heat triggered melting and ablation of redoubt s glaciers beginning at the summit and propagating to lower elevations as the unrest accelerated a variety of morphological changes were observed including the creation of ice cauldrons areas of wide spread subsidence punctures in the ice carved out by steam and deposition from debris flows in this paper we use visual observations satellite data and a high resolution digital elevation model of the volcanic edifice to calculate ice loss at redoubt as a function of time our aim is to establish from this time series a proxy for heat flow that can be compared to other data sets collected along the same time interval our study area consists of the drift glacier which flows from the summit crater down the volcano s north slope and makes up about one quarter of redoubt s total ice volume of 4 km 3 the upper part of the drift glacier covers the area of recent volcanism making this part of ice mass most susceptible to the effect of volcanic heating moreover melt water and other flows are channeled down the drift glacier drainage by topography leaving the remainder of redoubt s ice mantle relatively unaffected the rate of ice loss averaged around 0 1 m 3 s over the last four months of 2008 accelerated to over twenty times this value by february 2009 and peaked at greater than 22 m 3 s just prior to the first major explosion on march 22 2009 we estimate a cumulative ice loss over this period of about 35 million cubic meters m m 3

record of late holocene debris avalanches and lahars at iliamna volcano alaska abstract iliamna volcano is a 3053 meter high glaciated stratovolcano in the southern cook inlet region of alaska and is one of seven volcanoes in this region that have erupted multiple times during the past 10 000 yr prior to our studies of iliamna volcano little was known about the frequency magnitude and character of holocene volcanic activity here we present geologic evidence of the most recent eruptive activity of the volcano and provide the first outline of late holocene debris avalanche and lahar formation iliamna has had no documented historical eruptions but our recent field investigations indicate that the volcano has erupted at least twice in the last 300 yr clay rich lahar deposits dated by radiocarbon to 1300 and 90 yr bp are present in two major valleys that head on the volcano these deposits indicate that at least two large possibly deep seated flank failures of the volcanic edifice have occurred in the last 1300 yr noncohesive lahar deposits likely associated with explosive pyroclastic eruptions date to 2400 1300 1500 300 and debris avalanche deposits from recent and historical small volume slope failures of the hydrothermally altered volcanic edifice cover most of the major glaciers on the volcano although these deposits consist almost entirely of hydrothermally altered rock debris and snow and ice none of the recently generated debris avalanches evolved to lahars a clay rich lahar deposit that formed

jokulhlaups a reassessment of floodwater flow through glaciers 1 in glaciated catchments glacier generated floods jokulhlaups put human activity at risk with large sporadic jokulhlaups accounting for most flood related fatalities and damage to infrastructure in studies of jokulhlaup hydrodynamics the view predominates that floodwater travels within a distinct conduit eroded into the underside of a glacier however some jokulhlaups produce subglacial responses wholly inconsistent with the conventional theory of drainage by focusing on icelandic jokulhlaups this article reassesses how floodwater flows through glaciers it is argued that two physically separable classes of jokulhlaup exist and that not all jokulhlaups are an upward extrapolation of processes inherent in events of lesser magnitude and smaller scale the hydraulic coupling of multiple nonlinear components to the flood circuit of a glacier can induce extreme responses including pressure impulses in subglacial drainage representing such complexity in mathematical form should be the basis for upcoming research as future modeling results may help to determine the glaciological processes behind heinrich events moreover such an approach would lead to more accurate predictive models of jokulhlaup timing and intensity

structural geological and metamorphic significance of the kaapvaal craton limpopo belt contact abstract the hout river shear zone forms the terrane boundary between the granite greenstone terrane of the kaapvaal craton cratonic terrane and the high grade granulite terrane high grade gneiss province of the limpopo belt this structure is superimposed onto a thin skinned northward verging thrust system which exercises a significant control on the distribution of the ene wsw trending greenstone belts the hout river shear zone however dips steeply to the north and flattens out toward the south this structure was responsible for the emplacement of hot granulites against and over the cooler granite greenstone terrane and strongly influenced the metamorphic evolution of the area the shear zone acted as a conduit for infiltrating co2 rich h2o bearing fluids into still hot crust producing a retrograde isograd and associated zone of rehydration in the granulitic hanging wall of the structure retrogressed granulites in the hanging wall are characterized by fine grained kyanite as part of the retrograde assemblages while the greenstones in the footwall are characterized by coarse grained prograde kyanite the terrane boundary appears to be in thermal equlibrium with the lithologies in both the footwall and hanging wall so that tectonically mixed prograde greenstones and retrogressed granulites in and adjacent to the shear zone are apparently at a similar metamorphic grade the granulites appear to have had a hot iron effect on the cooler footwall rocks over which they have been displaced

the structural framework of the southern margin of the limpopo belt south africa abstract the southern marginal zone is characterized by two prominent regional structural features i massive crustal wedges 20 km 50 km containing large oval shaped closed fold structures which are related to an early d 1 phase of deformation and which are bounded by ii major ductile shear zones that strike in a general e w direction the hout river shear zone forms the southern boundary of the southern marginal zone where granulites are juxtaposed against lower grade granite greenstones of the kaapvaal craton this fault system also acted as the sole thrust for the other high grade shear zones in the southern marginal zone and played a major role in the development of the zone of retrogression and the establishment of the orthoamphibole isograd in the southern marginal zone the hout river shear zone and related faults within the southern marginal zone clearly post date d 1 and a period of peak metamorphism m 1 they are composite structures showing dominantly reverse sense displacements and are responsible for the emplacement of the granulite terrane over and onto the kaapvaal craton in contrast to these older structures are the reactivated shear zones these structures clearly cross cut and displace the high grade shear zones in the southern marginal zone as well as the retrograde orthoamphibole isograd the fact that the hout river and related shear zones are in metamorphic equilibrium with the surrounding wall rocks suggests their age is that of the metamorphism i e 2670 ma barton and van reenen 1992

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

the dynamics and thermodynamics of volcanic clouds theory and observations from the april 15 and april 21 1990 eruptions of redoubt volcano alaska on april 15 1990 and april 21 1990 two relatively small explosive eruptions occurred at redoubt volcano alaska lasting about 4 and 8 minutes respectively on both occasions the erupted material travelled as a pyroclastic flow down an ice canyon on the north flank of the volcano using slow scan television recordings of the eruption and the seismic record in both the near and far field we deduce that after a few minutes the upper parts of these pyroclastic flows became buoyant and a large and hot ash cloud ascended off the flow these thermals rose to a height of about 12 km at which point they began to spread laterally as umbrella clouds using a simple thermodynamic model we estimate that the clouds had a temperature in the approximate range 600 700 k as they rose buoyantly from the flow after entraining and heating air and melting and vaporizing ice we also estimate that in each eruption approximately 109 kg of fine ash was injected into the atmosphere during the april 15 eruption a slow scan television camera recorded the ascent of the cloud these observations are described analysed and compared with the predictions of a new model for the dynamics governing the ascent of such clouds in accord with the observations our model predicts that the cloud initially ascended rather sluggishly since it is only just buoyant on rising from the pyroclastic flow as it ascends it entrains and heats up more air and hence generates more buoyancy therefore it accelerates upwards only much higher in the cloud does the velocity decrease again as the thermal energy of the cloud becomes exhausted the model also predicts that the height of rise of such coignimbrite thermals is a function of the initial mass and temperature of the cloud but is almost independent of the initial velocity during the april 21 eruption a sequence of photographs recorded the lateral spreading of the umbrella cloud during an interval of about 10 minutes after the eruption these photographs are analysed and successfully compared with a simple model for the spreading of the umbrella cloud as a gravity current in a stratified environment using an avhrr satellite image of the air fall deposit from the april 15 eruption combined with a simple model we suggest that the primary mechanism of ash dispersal was transport by the ambient wind

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

an overview of the 2009 eruption of redoubt volcano alaska abstract in march 2009 redoubt volcano alaska erupted for the first time since 1990 explosions ejected plumes that disrupted international and domestic airspace sent lahars more than 35 km down the drift river to the coast and resulted in tephra fall on communities over 100 km away geodetic data suggest that magma began to ascend slowly from deep in the crust and reached mid to shallow crustal levels as early as may 2008 heat flux at the volcano during the precursory phase melted 4 of the drift glacier atop redoubt s summit petrologic data indicate the deeply sourced magma low silica andesite temporarily arrested at 9 11 km and or at 4 6 km depth where it encountered and mixed with segregated stored high silica andesite bodies the two magma compositions mixed to form intermediate silica andesite and all three magma types erupted during the earliest 2009 events only intermediate and high silica andesites were produced throughout the explosive and effusive phases of the eruption the explosive phase began with a phreatic explosion followed by a seismic swarm which signaled the start of lava effusion on march 22 shortly prior to the first magmatic explosion early on march 23 2009 utc more than 19 explosions or events were produced over 13 days from a single vent immediately south of the 1989 90 lava domes during that period multiple small pyroclastic density currents flowed primarily to the north and into glacial ravines three major lahars flooded the drift river terminal over 35 km down river on the coast tephra fall deposited on all aspects of the edifice and on several communities north and east of the volcano and at least two and possibly three lava domes were emplaced lightning accompanied almost all the explosions a shift in the eruptive character took place following event 9 on march 27 in terms of infrasound signal onsets the character of repeating earthquakes and the nature of tephra ejecta more than nine additional explosions occurred in the next two days followed by a hiatus in explosive activity between march 29 and april 4 during this hiatus effusion of a lava dome occurred whose growth slowed on or around april 2 the final explosion pulverized the very poorly vesicular dome on april 4 and was immediately followed by the extrusion of the final dome that ceased growing by july 1 2009 and reached 72 m m 3 in bulk volume the dome remains as of this writing effusion of the final dome in the first month produced blocky intermediate to high silica andesite lava which then expanded by means of lava injection beneath a fracturing and annealing cooling surface crust in the first week of may a seismic swarm accompanied extrusion of an intermediate to high silica andesite from the apex of the dome that was highly vesicular and characterized by lower p 2 o 5 content the dome remained stable throughout its growth period likely due to combined factors that include an emptied conduit system steady degassing through coalesced vesicles in the effusing lava and a large crater pit created by the previous explosions we estimate the total volume of erupted material from the 2009 eruption to be between 80 m and 120 m m 3 dense rock equivalent dre the aim of this report is to synthesize the results from various datasets gathered both during the eruption and retrospectively and which are represented by the papers in this publication we therefore provide an overall view of the 2009 eruption and an introduction to this special issue publication

geochronology of the hout river shear zone and the metamorphism in the southern marginal zone of the limpopo belt southern africa abstract in this paper monazite u pb and zircon evaporation dates stepleaching pb pb results on garnet staurolite and kyanite and hornblende ar ar data are presented which constrain the timing of granulite facies metamorphism in the southern marginal zone of the limpopo belt and its thrusting onto the kaapvaal craton the southern marginal zone of the limpopo belt is considered to be a lower crustal equivalent of the northern kaapvaal craton granulite exhumation is associated with southward thrusting along the hout river shear zone which is a set of thrust and strike slip shear zones zircon ages for the matok intrusive complex which was emplaced within the zone during this thrusting charno enderbites 2671 4 ma granodiorites between 2667 and 2664 ma have previously been interpreted as evidence for rapid exhumation of the southern marginal zone within only 7 ma we have obtained a u pb date of 2691 7 ma for monazite from the bandelierkop quarry in the southern marginal zone interpreted as the age of high grade metamorphism a single zircon evaporation pb pb date of 2643 1 ma from a leucosome band at the same locality may indicate longer lasting metamorphism or decompression melting during exhumation anatexis of metapelitic xenoliths within the matok intrusive complex was dated at 2663 4 ma by u pb on monazite indistinguishable from existing zircon ages for this complex pb pb step leaching dates obtained on synkinematically grown garnet 2691 20 ma staurolite 2712 37 ma and kyanite 2672 51 ma from the khavagari hills in the giyani greenstone belt in the immediate footwall of the hout river shear zone indicate that early thrusting was contemporaneous with peak metamorphism in the southern marginal zone ar ar dating and geochemistry on syntectonic hornblende separates from the same shear zone system yielded disturbed spectra and indicated multiple populations probably reflecting repeated or continuous tectonic activity of the hout river shear zone up to about 2600 ma

tectono metamorphic evolution of the simatic basement of the pietersburg greenstone belt relative to the limpopo orogeny evidence from serpentinite abstract detailed studies of low grade ultramafic rocks are integrated with regional structural field observations in the pietersburg greenstone belt these results show that petrographic investigations of serpentinite can reveal more about the tectonometamorphic history of greenstone belts than has previously been appreciated the ultramafic rocks of the pietersburg greenstone belt were affected by an early deformation event d 1 which is reflected by the presence of a relic magnetite fabric s 1 in serpentinite the s 1 fabric is crosscut by the peak metamorphic assemblage of antigorite tremolite talc chlorite both s 1 and the peak metamorphic assemblage are deformed by a second deformation event d 2 on a macroscale d 2 is manifested as a series of northward verging thrust faults that affected the entire belt published u pb zircon data show that d 2 was active between 2 9 and 2 7 ga prior to uplift and southward thrusting of the limpopo belt lb major carbonatization of the simatic basement postdates peak metamorphism and occurred towards the end of d 2 coeval with the late evolution of the limpopo orogeny the alteration assemblages were subsequently deformed the sequence of events that affected the pietersburg greenstone belt is similar to that established for the high grade southern marginal zone of the limpopo belt but their absolute timing is different the results of this study demonstrate the important contribution that low grade ultramafic rocks can make to the understanding of the tectono metamorphic history of the archaean cratons and terrane accretion

monazite behaviour and age significance in poly metamorphic high grade terrains a case study from the western musgrave block central australia abstract integrated in situ textural chemical and electron microprobe age analysis of monazite grains in a migmatitic metapelitic gneiss from the western musgrave block central australia has identified evidence for multiple events of growth and recrystallisation during poly metamorphism in the mesoproterozoic garnet sillimanite bearing metapelite underwent partial melting and segregation to palaeosome and leucosome during metamorphism between 1330 and 1296 ma with monazite grains in leucosome recording crystallisation at 1300 ma monazite breakdown during melting is inferred to have occurred in the palaeosome during a subsequent granulite facies event at 1200 ma deformation and metamorphism of leucosome and palaeosome resulted in partial disturbance of ages and potential minor growth on 1300 ma monazite in leucosome growth of new high y hree monazite in palaeosome domains occurred during garnet breakdown in the presence of sillimanite to cordierite and spinel as a result of post peak isothermal decompression diffusive enrichment of resorbed garnet rims in y hree suggests garnet breakdown occurred slower than volume diffusion of ree monazite in both palaeosome and leucosome were subsequently partially to penetratively recrystallised during a retrogression event that is suggested to have occurred at 1150 1130 ma the intensity of recrystallisation and disturbance of ages appears linked to proximity to retrogressed garnet porphyroblasts and their occurrence in the relatively reactive or fertile local environments provided by the palaeosome mesosome volumes which caused localised changes in retrogressive fluids towards compositions more aggressive to monazite like reaction textures it is apparent that domainal equilibrium and reaction may control or at least strongly influence monazite ree and u th pb chemistry and hence ages

the 1989 1990 eruptions of redoubt volcano an introduction abstract redoubt volcano located on the west side of cook inlet in south central alaska erupted explosively on over 20 separate occasions between december 14 1989 and april 21 1990 fourteen lava domes were emplaced in the summit area thirteen of which were subsequently destroyed the eruption caused economic losses estimated at over 160 000 000 making this the second most costly eruption in u s history this economic impact provided the impetus for a integrated comprehensive account of an erupting volcano using both modern and classical research and modern techniques which in turn led to advances in eruption monitoring and interpretation research on such topics as dome formation and collapse and the resulting pyroclastic flows elutriated ash lightning tephra and flooding was blended with the rapid communication of associated hazards to a large user group the seismology successes in predicting and monitoring eruption dynamics were due in part to 1 the recognition of long period seismic events as indicators of the readiness of the volcano to erupt and 2 to the development of new tools that allowed the seismicity to be assessed instantaneously integrated studies of the petrology of erupted products and volatile content over time gave clues as to the progress of the eruption towards completion

contribution of mineral tunneling to total feldspar weathering in this study we quantified the contribution of mineral tunneling by fungi to weathering of feldspars and ecosystem influx of k and ca we studied the surface soils of 11 podzols across a lake michigan sand dune chronosequence with soil ages between 450 and 5000 years weathering by tunneling was quantified in thin sections by image analysis total mineral weathering was quantified by comparing the mineralogy of the surface soil with the underlying parent material mineralogy was characterized using x ray fluorescence spectroscopy xrfs followed by a normative mineralogical calculation tunnels were observed only in soils older than 1650 years throughout the chronosequence the contribution of tunneling to mineral weathering in the upper mineral soil expressed as tunnel volume divided by volume of weathered feldspar was less then 1 contribution of tunneling to na ca feldspar weathering was higher than the contribution of tunneling to k feldspar weathering feldspar tunneling equals an average ecosystem influx of 0 4 g ha 1 year 1 for k and 0 2 g ha 1 year 1 for ca over 5000 years of soil development intensity of mineral tunneling determined as fraction weathered feldspars was higher than in a previously described north swedish podzol chronosequence the presented data suggest that the contribution of tunneling to weathering becomes more important in older soils but remains low

role of fluids in the exhumation of the southern marginal zone of the limpopo complex south africa abstract the limpopo complex lc of southern africa displays structural metamorphic and geochronological evidence for a complex exhumation controlled evolutionary history that followed after burial and attainment of peak metamorphic conditions p 9 kbar t 900 c in the root zone central zone cz of the orogen before 2 72 ga the southern marginal zone smz of the lc in addition provides evidence for thrust controlled emplacement of a hot granulite facies nappe onto the adjacent low grade granite greenstone terrane of the northern kaapvaal craton nkvc in the interval 2 69 2 62 ga a tectono metamorphic scenario in which initial near vertical channeling exhumation at 2 69 ga was transferred into horizontal channeling thrusting at 2 67 2 64 ga is interpreted to have commenced with in situ partial melting of crustal rocks in the root zone of the orogen cz this resulted in formation of melt weakened crust migmatized gneisses that with continuous shortening led to vertical transfer of lower crustal material accompanied by fluid fluxed decompression melting that resulted in production of large volumes of leucocratic granitic material this material detached to form buoyant diapirs that accompanied the emplacement at 2 65 ga of mega sheath folds at the middle crustal level throughout the entire cz of the lc this early stage of near vertical exhumation is reflected i by the fact that linear elements of sheath folds and shear zones plunge consistently at steep angles to the sw throughout the entire cz and ii by decompression cooling dc segments of p t paths constructed for metapelitic rocks dc paths in the far northern part of the smz that are associated with the steep north dipping portion of the hout river shear zone hrsz traversed from p 8 5 kbar t 850 c to p 6 5 kbar t 720 c early near vertical exhumation transferred into horizontal channel flow that controlled emplacement at the middle crustal level of the granulite facies nappe smz horizontal channeling thrusting is recorded in metapelitic granulite by near isobaric cooling p 6 kbar t 700 550 c segments of earlier dc p t paths that are associated with the shallow north dipping portion of the hrsz devolitilization of underthrusted greenschist due to thrusting hot granulite over cool greenschist released large volumes of high temperature aqueous fluids from the underthrusted rocks that constantly infiltrated the overriding granulite during southwards migration of the nappe immiscible co 2 rich and brine fluids thus derived persistently infiltrated into and reacted with cooling granulite and triggered the main pulse of anatexis at p 7 5 kbar t 750 c similar fluids established a regional retrograde isograd and associated zone of 4500 km 2 retrograde hydrated granulite at p 6 kbar t 600 630 c located in the hanging wall of the shallow north dipping hrsz that bounds the smz in the south shear zone hosted metasomatic alteration including formation of lode gold deposits occurred at t 600 630 c p 6 kbar in the zone of rehydrated granulite and at higher p t conditions in the granulite zone the granulite facies smz of the lc thus preserves clear evidence for a direct link between persistent fluid flux and fluid rock interaction as an integral component of a conveyor like tectonic mechanism driven by vertical tectonics that operated from the core region of the orogen cz this mechanism explains the important link between the thrust controlled emplacement of the smz onto the nkvc and deep crustal fluid flow

greenstone belts at the northernmost edge of the kaapvaal craton timing of tectonic events and a possible crustal fluid source abstract the hout river shear zone a complex thrust system marks the boundary between the low to medium grade metamorphic granitoid greenstone northern kaapvaal craton province and the high grade southern marginal zone of the limpopo complex to the north of it the giyani greenstone belt located on the northern kaapvaal craton immediately adjacent to the hout river shear zone is in a key position for studying the tectonics of this shear zone we have mapped two central transects through the greenstone belt dated metamorphic minerals by 207 pb 206 pb stepwise leaching and 40 ar 36 ar and compared our results with data from the pietersburg polokwane and rhenosterkoppies greenstone belts which are close to the hout river shear zone further west the se domain of the ne sw striking giyani greenstone belt appears to constitute a nw vergent nappe movement along the hout river shear zone is consistently sw vergent varying in character between frontal and lateral ramps and nappe the nw domain of the giyani greenstone belt is dominated by ultramafic schists with mafic and banded iron formation bif intercalations it is considered a separate tectonic unit from the se domain in which mafic intrusive and pillow volcanic rocks are prevalent with intercalated clastic sediments correlations between this se domain and the lower mafic unit of the pietersburg greenstone belt and with the mafic bif association of the rhenosterkoppies greenstone belt are possible ages for peak metamorphism in the immediate footwall of the hout river shear zone from this and previous work yield a weighted mean age of 2717 28 ma 95 confidence documenting a hot iron metamorphism a garnet age of 2833 38 ma in the se domain of the giyani greenstone belt overlaps with ages from 2854 to 2870 ma for youngest zircons in syntectonic intrusions within the giyani and the pietersburg greenstone belts as well as youngest detrital zircons in the probably syntectonic clastic sedimentary uitkyk formation of the latter these ages constrain a tectonometamorphic event in both belts termed here the pgb lwaji orogeny amphibole 40 ar 39 ar dates from the hout river shear zone from this and previous work and shear zones within the giyani greenstone belt yield disturbed age spectra with step ages mainly in the 2550 2670 ma range the amphiboles are chemically heterogeneous and the ages can be shown to reflect mixtures of older and younger amphibole generations due to reactivation of the shear zones and fluid activity after the main event in the hout river shear zone thermal modelling shows that in spite of the very low content of radioactive heat producing elements in the southern marginal zone its lower crust after moderate crustal thickening could be heated to granulite facies conditions in 30 60 ma a much shorter time than the 120 ma age difference between the pgb lwaji orogeny and the hout river shear event a causal link between the two tectonic events is thus precluded on the other hand previously low grade supracrustal rocks from the kaapvaal craton buried at lower crustal levels during hout river shear event could provide a source for fluids up to about 80 ma after the event

interactions between lava and snow ice during the 2010 fimmvorðuhals eruption south central iceland 1 the 20 march 12 april basaltic effusive eruption at fimmvor uhals southern iceland was an important opportunity to directly observe interactions between lava and snow ice the eruption site has local perennial snowfields and snow covered ice and at the time of eruption it was covered with an additional 1 3 m of seasonal snow syn eruption observations of interactions between lava and snow ice are grouped into four categories 1 lava advancing directly on top of snow 2 lava advancing on top of tephra covered snow 3 snow ice melting at lava flow margins and 4 lava flowing beneath snow based on syn and post eruption observations in 2010 11 we conclude that the features seen in the lava flow field show only limited and localized evidence for the influence of snow ice presence during the eruption estimated melting rates from radiant and conductive heating at the flow fronts are too slow on the order of 5 m hr to allow for complete melting of snow ice ahead of the advancing lava flows at least during periods of observed rapid lava advance rates 15 55 m hr thus we conclude that during those periods which largely established the aerial extent of the lava flow field lava advanced on top of snow that this likely was the predominant mode of lava emplacement for much of the eruption is supported by many syn eruption field observations examination of the lava flows subsequent to the eruption has so far only found subtle evidence for interactions between lava and snow ice for example locally lava flows have fractured and are collapsing or have developed marginal rubble aprons from melting of snow banks that were partly covered by lava flow margins

voluminous ice rich and water rich lahars generated during the 2009 eruption of redoubt volcano alaska abstract redoubt volcano in south central alaska began erupting on march 15 2009 and by april 4 2009 had produced at least 20 explosive events that generated multiple plumes of ash and numerous lahars the 3108 m high snow and ice clad stratovolcano has an ice filled summit crater that is breached to the north the volcano supports about 4 km 3 of ice and snow and about 1 km 3 of this makes up the drift glacier on the north side of the volcano explosive eruptions between march 23 and april 4 which included the destruction of at least two lava domes triggered significant lahars in the drift river valley on march 23 and april 4 and several smaller lahars between march 24 and march 31 mud line high water marks character of deposits areas of inundation and estimates of flow velocity revealed that the lahars on march 23 and april 4 were the largest of the eruption in the 2 km wide upper drift river valley average flow depths were at least 2 5 m average peak flow velocities were likely between 10 and 15 ms 1 and peak discharges were on the order of 10 4 10 5 m 3 s 1 the area inundated by lahars on march 23 was at least 100 km 2 and on april 4 about 125 km 2 two substantial lahars emplaced on march 23 and one on april 4 had volumes on the order of 10 7 10 8 m 3 and were similar in size to the largest lahar of the 1989 90 eruption the two principal march 23 lahars were primarily flowing slurries of snow and ice derived from drift glacier and the drift river valley where seasonal snow and tabular blocks of river ice were entrained and incorporated into the lahars despite morphologic evidence of two lahars only a single deposit up to 5 m thick was found in most places and it contained about 80 95 of poorly sorted massive to imbricate assemblages of snow and ice clasts the deposit was frozen soon after it was emplaced and later eroded and buried by the april 4 lahar the lahar of april 4 in contrast was primarily a hyperconcentrated flow as interpreted from 1 to 6 m thick deposits of massive to horizontally stratified sand to fine gravel rock material in the april 4 lahar deposit is predominantly juvenile andesite whereas rock material in the march 23 deposit is rare and not obviously juvenile we infer that the lahars generated on march 23 were initiated by a rapid succession of vent clearing explosions that blasted through about 50 100 m of crater filling glacier ice and snow producing a voluminous release of meltwater from drift glacier the resulting surge of water entrained snow fragments of glacier and river ice and river water along its flow path small volume pyroclastic flows possibly associated with destruction of a small dome or minor eruption column collapses may have contributed additional meltwater to the march 23 lahars meltwater generated by subglacial hydrothermal activity and stored beneath drift glacier may have been ejected or released rapidly as well the april 4 lahar was initiated when hot dome collapse pyroclastic flows entrained and swiftly melted snow and ice on drift glacier the resulting meltwater incorporated pyroclastic debris and rock material from drift glacier to form the largest lahar of the 2009 eruption the peak discharge of the april 4 lahar was in the range of 60 000 160 000 m 3 s 1 for comparison the largest lahar of the 1989 90 eruption had a peak discharge of about 80 000 m 3 s 1 lahars generated by the 2009 eruption led to significant channel aggradation in the lower drift river valley and caused extensive inundation at an oil storage and transfer facility located there the april 4 2009 lahar was 6 30 times larger than the largest meteorological floods known or estimated in the drift river drainage

single mineral pbpb dating a new stepwise leaching procedure applied to ca bearing silicates enables the selective recovery of radiogenic and common pb from a mineral and thus makes single phase pbpb dating possible this permits independent dating of rock forming minerals within a paragenetic sequence and the direct combination of age data with petrological results furthermore the stepwise leaching technique has the advantage over multi phase pbpb dating in that it does not require initial pb isotope equilibrium among the phases but may be used to test it the presence of microscopic solid inclusions which complicate dating of single phases if they are not in equilibrium with the host can be traced via the behaviour of the 208pb 204pb ratio through the various leaching steps this constitutes a major advantage over conventional bulk phase pbpb dating in addition the precision of an isochron age can be improved relative to that of the bulk mineral approach since the 207pb 204pb and 206pb 204pb data spread is significantly enhanced therefore the technique may both be used to date younger phases and or minerals with less favourable u pb ratios and to extend the applicability of self consistent p t t modelling

the timing of successive fluid events in the southern marginal zone of the limpopo complex south africa constraints from 40ar 39ar geochronology abstract we have carried out a geochronological study in the high grade metamorphic southern marginal zone of the limpopo complex south africa aimed at providing time constraints on episodes of fluid activity this work chiefly comprises 40 ar 39 ar dating with accompanying petrography of amphibole and mica and further incudes zircon u pb geochronology on a post peak metamorphic trondhjemite intrusion the so called zone of rehydration within 30 km n of the hout river shear zone where the southern marginal zone is thrust over the kaapvaal craton was studied in particular but sampling also included areas of patchy metasomatism in the granulite zone further north where no post peak metamorphic rehydration on a regional scale occurred amphibole in a mafic granulite and an older generation of zircons in the petronella trondhjemite yielded dates in the 2700 2730 ma range corresponding to the published age of peak granulite metamorphism in the southern marginal zone a cluster of 40 ar 39 ar dates on amphiboles from the zone of rehydration indicate an age between 2660 and 2670 ma for this event in accord with results from previous work on the hout river shear zone the main population of u pb zircon dates from the trondhjemite dating its intrusion also falls in this age range as does the published age of the matok pluton these data suggest that rehydration at 2660 2670 ma was a discrete episode a further 40 ar 39 ar age cluster defined by dates on both amphiboles and micas is seen at 2010 2040 ma which is the age of a major tectonometamorphic event in the central zone of the limpopo complex and previously reported from the southern marginal zone as rb sr biotite dates events we found in the 2010 2040 ma bracket are strike slip shearing dated by fabric forming amphibole as well as localized potassic metasomatism dated by both amphiboles and micas the latter was not confined to the zone of rehydration and appears in one case to be associated with gold mineralization a number of 40 ar 39 ar dates intermediate between 2600 and 2000 ma were obtained but these are diverse and can be accounted for by mixing incomplete resetting and or excess 40 ar they do not indicate events in this intermediate age bracket

juvenile crust formation in the northeastern kaapvaal craton at 2 97 ga implications for archean terrane accretion and the source of the pietersburg gold abstract combined u pb and lu hf isotope data of detrital and magmatic zircon grains provide evidence that the murchison greenstone belt ne kaapvaal craton south africa hosts an important archean suture zone which primarily formed by a collision 2 97 ga ago between a back arc system to the north and an evolved terrane the proto kaapvaal craton to the south this model is supported by the observations that igneous rocks of the northern terrane comprising mafic rocks and tonalites of the rooiwater complex and felsic volcanics of the rubbervale formation were both emplaced at about 2 97 ga and show highly superchondritic hft of 4 4 to 5 1 in contrast u pb hf isotope data of detrital zircon grains from quartzitic schists of the murchison unit and the la france formation both part of the southern terrane provide evidence for a southern provenance which was affected by granitoid intrusions accompanied by crust re working at 3 53 3 42 ga hft 1 8 to 4 8 3 30 3 20 ga hft 1 8 to 6 3 and 3 13 3 05 ga hft 1 3 to 5 6 pointing to a connection with the barberton greenstone belt and swaziland a spatial separation of the two terranes prior to 2 97 ga is furthermore supported by the facts that the youngest detrital zircon grains of the southern terrane having ages between 2 99 and 2 97 ga show subchondritic hft between 6 5 and 1 5 in contrast to the highly superchondritic hft obtained from the contemporaneous magmatic rocks of the northern terrane the juvenile character of the rooiwater and rubbervale magmatic rocks along with the occurrence of vms deposits makes them a likely candidate for the source of the gold pyrite bearing conglomerates exposed in the adjacent pietersburg greenstone belt

pb diffusion in monazite a combined rbs sims study abstract we report measurements of pb diffusion in both synthetic cepo4 and natural monazites run under dry 1 atm conditions powdered mixtures of prereacted cepo4 and pbzro3 were used as the source of pb diffusant for in diffusion experiments conducted in sealed pt capsules for durations ranging from a few hours to several weeks following the diffusion anneals pb concentration profiles were measured with rutherford backscattering spectroscopy rbs and supplemented by measurements with secondary ion mass spectrometry sims in order to evaluate potential compositional effects upon pb diffusivity and simulate diffusional pb loss that might occur in natural systems we also conducted out diffusion experiments on pb bearing natural monazites in these experiments monazite grains were surrounded by a synthetic zircon powder to act as a sink monazites from these experiments were analyzed with sims over the temperature range 1100 to 1350 c the arrhenius relation determined for in diffusion experiments on synthetic monazite is given by d 0 94 exp 592 39 kj mol 1 rt m 2 s 1 diffusivities for synthetic and natural monazites are similar as are results of measurements made on the same samples using both rbs and sims the activation energy for pb diffusion we determined is more than three times that for natural monazite reported previously with pb diffusivities at

geomorphic consequences of volcanic eruptions in alaska a review abstract eruptions of alaska volcanoes have significant and sometimes profound geomorphic consequences on surrounding landscapes and ecosystems the effects of eruptions on the landscape can range from complete burial of surface vegetation and preexisting topography to subtle short term perturbations of geomorphic and ecological systems in some cases an eruption will allow for new landscapes to form in response to the accumulation and erosion of recently deposited volcaniclastic material in other cases the geomorphic response to a major eruptive event may set in motion a series of landscape changes that could take centuries to millennia to be realized the effects of volcanic eruptions on the landscape and how these effects influence surface processes has not been a specific focus of most studies concerned with the physical volcanology of alaska volcanoes thus what is needed is a review of eruptive activity in alaska in the context of how this activity influences the geomorphology of affected areas to illustrate the relationship between geomorphology and volcanic activity in alaska several eruptions and their geomorphic impacts will be reviewed these eruptions include the 1912 novarupta katmai eruption the 1989 1990 and 2009 eruptions of redoubt volcano the 2008 eruption of kasatochi volcano and the recent historical eruptions of pavlof volcano the geomorphic consequences of eruptive activity associated with these eruptions are described and where possible information about surface processes rates of landscape change and the temporal and spatial scale of impacts are discussed a common feature of volcanoes in alaska is their extensive cover of glacier ice seasonal snow or both as a result the generation of meltwater and a variety of sediment water mass flows including debris flow lahars hyperconcentrated flow lahars and sediment laden water floods are typical outcomes of most types of eruptive activity occasionally such flows can be quite large with flow volumes in the range of 107 109 m3 a review of the lahars generated during the 2009 eruption of redoubt volcano will illustrate the geomorphic impacts of lahars on stream channels and riparian habitat although much work is needed to develop a comprehensive understanding of the geomorphic consequences of volcanic activity in alaska this review provides a synthesis of some of the best studied eruptions and perhaps will serve as a starting point for future work on this topic

formation of an archaean continent about 30 of the earth is covered by continents but only about 10 small kernels of these continentsae known as archaean cratonsae are continental fragments formed before 2 5 gyr ago the kaapvaal craton of south africa which formed and stabilized between 3 7 and 2 7 gyr ago is one of the oldest reasonably sized examples of these continental fragments it consists of a mosaic of subdomains that have been welded together by processes similar to those of modern day plate tectonics the earliest subdomains may have owed their origin to the onset of efficient recycling from the earth s hydrosphere into the mantle

structure and metamorphism of the archean murchison belt kaapvaal craton south africa critical examination of five popular prejudices applied to greenstone belts in general and the murchison belt in particular show that 1 the term murchison schist belt is preferred to that of murchison greenstone belt which differs significantly from the barberton greenstone belt 2 the murchison belt is tectonized such that no simple stratigraphy may be erected 3 the murchison belt lacks a synclinal keel 4 heterogeneous oblique to dip slip shear characterizes much of the murchison belt but strike slip deformation occurred in the letaba shear zone along the northeastern margin of the belt and 5 contact metamorphism was a limited phenomenon that was not responsible for the highest grade rocks of the murchison belt the belt comprises a number of domains each with its own characteristic lithologies structural style and metamorphic history bounded by tectonic breaks which have juxtaposed rocks from different crustal levels

fluid mediated partial alteration in monazite the role of coupled dissolution reprecipitation in element redistribution and mass transfer monazite ce lree th u ca p si o4 with complex zoning in th and other elements is commonly observed in metamorphic and igneous rocks the hypothesis that this alteration is a product of fluid mediated element mass transfer has been tested in the piston cylinder press caf2 assembly cylindrical graphite oven at 1 000 mpa and 900 c and in cold seal autoclaves on a hydrothermal line at 500 mpa and 600 c experiments included a relatively homogeneous monazite ce 7 8 wt tho2 from a heavy mineral sand plus a series of alkali bearing fluids including 2n naoh 2n koh and na2si2o5 h2o experiments were conducted using bse imaging emp analysis and both tem and hrtem a subset of monazite grains from each experiment show evidence of partial alteration in the form of areas enriched in th si with sharp curvilinear compositional boundaries extending from the grain rim into the monazite interior these thsio4 enriched textures are similar to those commonly seen in natural examples of metasomatised monazite in both magmatic and metamorphic rocks in the na2si2o5 h2o experiments scarce inclusions of britholite formed in the altered monazite the altered monazite is also characterised by strong depletion in pb ca and y thorium and si mobility coupled with the formation of britholite inclusions during partial alteration in the monazite grain is considered to be the product of fluid aided coupled dissolution reprecipitation as opposed to solid state diffusion since other fluids including nacl and kcl brines do not result in the formation of these textures the experimental replication of thsio4 enriched areas in the monazite strongly suggests that similar textures in monazite observed in nature are fluid induced specifically by alkali bearing fluids if true complex metasomatically induced textures in monazite could yield information concerning the nature of the fluid responsible for their formation as well as allow for the dating of the metasomatic event presuming that all the original radiogenic pb has been removed

the deep structure of the limpopo belt from geophysical studies abstract geophysical studies in the limpopo belt and adjoining cratons contributed to the fundamental understanding of the deep structure in these tectonic provinces the seismic and geoelectrical data sets prove that the terrane boundaries of the limpopo belt dip in towards the belt to give it the appearance of a pop up structure the gravity data show that high grade metamorphic rock with densities that are high compared to the normal low grade upper crustal granitoids form the upper crust in the northern and southern marginal zones of the limpopo belt these rocks also show a low magnetic susceptibility the geophysical data clearly defined the position of the southern boundary of the limpopo belt the northern boundary of the central zone can be related to a southward dipping zone of seismic reflectors while the southern boundary of this terrane as formed by the palala shear zone seems to be near vertical the greenstone belts of the craton have a limited depth extent and are allochtonous the present position of these structures could be related to thin skinned tectonics that affected the upper crust on the kaapvaal craton and which were revealed by seismic reflection data

hydrologic hazards in the lower drift river basin associated with the 1989 1990 eruptions of redoubt volcano alaska abstract the eruptions of redoubt volcano between december 14 1989 and april 26 1990 triggered flows of snow ice water sediment and debris that traveled down the drift river as far as its mouth about 40 km downstream a major explosive eruption and dome collapse on january 2 1990 produced the largest flow the peak discharge of this flow at a location 22 km downstream from the volcano was estimated to be between 12 000 and 60 000 m 3 per second the estimated peak discharge of this event is more than 100 times larger than the 100 year meteorologically generated flood estimated for the drift river pyroclastic flows and hot meltwater scoured the surface of drift glacier on the north flank of the volcano and were transformed into multipulsed multiphased debris flows several other significant flows were generated by eruptions during this period the two largest of these occurred on december 15 1989 and february 15 1990 subsequent channel changes threatened the drift river oil terminal built on an alluvial fan near the mouth of the drift river

transverse architecture of lahar terraces inferred from radargrams preliminary results from semeru volcano indonesia semeru volcano is the highest mountain of java indonesia and a vulcanian explosion occurs every 15 minutes on average since 1967 thus a constantly renewed stock of material and the heavy monsoon rainfall 3700 mm yr 1 at 1500 m above sea level a s l provide a perfect setting for the study of lahars and their deposits hence we examined the architecture of lahars terraces 9 5 km from the summit in the curah lengkong valley we first used ground penetrating radar gpr over vertical exposures of the lahars cut bank terraces this allowed us to better understand transversal radargrams across terraces which are not visually accessible in the field preliminary results from a single radargram are very instructive since 1 they prove that the lateral architecture does not correspond to that observed from banks only 2 we could observe the presence of lenses and stratigraphic discontinuities 3 the setting of the various units can also help reconstruct deposition processes and the chronology of different units in order to finalize these preliminary results we however need to perform multiple gpr radargrams and provide a complete set of results copyright 2010 john wiley sons ltd

ice volcano interactions during the 2010 eyjafjallajokull eruption as revealed by airborne imaging radar 1 during the eruption of the ice covered eyjafjallajokull volcano a series of images from an airborne synthetic aperture radar sar were obtained by the icelandic coast guard cloud obscured the summit from view during the first three days of the eruption making the weather independent sar a valuable monitoring resource radar images revealed the development of ice cauldrons in a 200 m thick ice cover within the summit caldera as well as the formation of cauldrons to the immediate south of the caldera additionally radar images were used to document the subglacial and supraglacial passage of floodwater to the north and south of the eruption site the eruption breached the ice surface about four hours after its onset at about 01 30 utc on 14 april 2010 the first sar images obtained between 08 55 and 10 42 utc show signs of limited supraglacial drainage from the eruption site floodwater began to drain from the ice cap almost 5 5 h after the beginning of the eruption implying storage of meltwater at the eruption site due to initially constricted subglacial drainage from the caldera heat transfer rates from magma to ice during early stages of cauldron formation were about 1 mw m 2 in the radial direction and about 4 mw m 2 vertically meltwater release was characterized by accumulation and drainage with most of the volcanic material in the ice cauldrons being drained in hyperconcentrated floods after the third day of the eruption meltwater generation at the eruption site diminished due to an insulating lag of tephra

tectonic model for the evolution of the limpopo belt abstract constraints on which any model for the tectonic evolution of the limpopo belt must be based include the following data crustal thickening to at least 65 km between 2700 and 2650 ma ago was responsible for the formation of the granulite terrane exposed in the limpopo belt today this crustal thickening probably resulted from the thrusting of the kaapvaal craton over the zimbabwe craton along the south dipping triangle tuli sabi shear zone other northwardverging thrusts related to this event occur in the kaapvaal craton south of the limpopo belt in the central and southern marginal zones and the kaapvaal craton this shortening is directed to the southwest while in the northern marginal zone it was directed to the north northwest peak metamorphism was superimposed on the thickened crust and then nearly isothermal decompression of several kbars occurred throughout the belt during this decompression rocks moved upward and spread outward onto the adjacent cratons from the zone of thickened crust along several inward directed shear zones creating a regional pop up high grade rocks were thrust over low grade rocks on the cratons producing along the terrane boundary retrograde metamorphism in the limpopo belt and prograde metamorphism on the cratons

quaternary eruptive history and hazard zone model at nevado del tolima and cerro machin volcanoes colombia the ice clad and fumarolic nevado del tourna volcano 4 39 n 75 20 w south of nevado del ruiz is offset toward the southeast from the axis of the volcanic ruiz tolima massif with respect to the major ne trending strike slip palestina fault it is composed of four units 1 a pre tolima plateau like basement of basaltic andesite lava flows of early quaternary age 2 a dissected ancestral tourna stratovolcano cut by a presumed collapse caldera of middle pleistocene age 3 an older tolima stratovolcano of late pleistocene age partly destroyed by a summit caldera and 4 composite domes of the cone shaped young and present tolima young tolima volcano is an andesitic and dacitic composite cone formed over the past 40 000 years within a 3 km wide caldera that opened around 0 14 ma deposits of welded and nonwelded pumice and scoria flows were emplaced toward the southeast rio combeima and northeast rio totare repeated growth of lava domes over the past 16 000 years is witnessed by thick block lava flows on the southern and eastern flanks and by block and ash or scoria rich pyroclastic flow deposits this activity occurred during at least six eruptives stages as follows el placer ca 16 200 14 000 yr b p romerales ca 13 000 12 300 yr b p canalones ca 11 500 9750 yr b p mesetas ca 7200 4600 yr b p encanto ca 3600 1700 yr b p and nieves historical interactions with the ice cap probably triggered debris flows that partly filled the combeima and totare valleys and formed the holocene terraces on the upper pleistocene volcaniclastic fans of ibagueand venadillo as much as 60 km from the source the latest major activity was a plinian eruption which deposited a pumice fall layer ca 3600 yr b p 0 5 km3 actual volume mainly toward the west and northwest minor tephra falls and debris flows occurred during the historical period before the reported 1918 and 1943 small phreatic events a general hazard zone map shows areas potentially affected by future eruptions both at nevado del tolima and at active cerro machin 12 km southward the extent of areas likely to be affected by tephra falls debris flows pyroclastic flows or surges debris avalanches and lava flows is shown subplinian and plinian eruptions of nevado del tolima were used to represent the moderate and large events to be expected 300 000 people live within a 35 km distance from those volcanoes which have exhibited a behaviour more explosive than nevado del ruiz despite the small sized ice cap debris flows are the most probable hazard for even a minor eruption because of the very steep slope gradient and because of probable interactions of hot eruptive products with ice and snow additionally scoria flows and debris avalanches can be directed toward the southeast and could be transformed into debris flows that would devastate the combeima valley and suburbs of ibaguecity where about 50 000 people live

proximal pyroclastic deposits from the 1989 1990 eruption of redoubt volcano alaska stratigraphy distribution and physical characteristics abstract more than 20 eruptive events during the 1989 1990 eruption of redoubt volcano emplaced a complex sequence of lithic pyroclastic flow surge fall ice diamict and lahar deposits mainly on the north side of the volcano the deposits record the changing eruption dynamics from initial gas rich vent clearing explosions to episodic gas poor lava dome extrusions and failures the repeated dome failures produced lithic pyroclastic flows that mixed with snow and glacial ice to generate lahars that were channelled off drift glacier into the drift river valley some of the dome failures occurred without precursory seismic warning and appeared to result solely from gravitational instability material from the disrupted lava domes avalanched down a steep partly ice filled canyon incised on the north flank of the volcano and came to rest on the heavily crevassed surface of the piedmont lobe of drift glacier most dome collapse events resulted in single monolithologic massive to reversely graded medium to coarse grained sandy pyroclastic flow deposits containing abundant dense dome clasts these deposits vary in thickness grain size and texture depending on distance from the vent and local topography deposits are finer and better sorted down flow thinner and finer on hummocks and thicker and coarser where ponded in channels cut through the glacial ice the initial vent clearing explosions emplaced unusual deposits of glacial ice snow and rock in a frozen matrix on the north and south flanks of the volcano similar deposits were described at nevado del ruiz columbia and have probably been emplaced at other snow and ice clad volcanoes but poor preservation makes them difficult to recognize in the geologic record in a like fashion most deposits from the 1989 1990 eruption of redoubt volcano may be difficult to recognize and interpret in the future because they were emplaced in an environment where glacio fluvial processes dominate and quickly obscure the primary depositional record

lava and ice interaction at stratovolcanoes use of characteristic features to determine past glacial extents and future volcanic hazards structures resulting from lava and ice interaction are common at glaciated stratovolcanoes during summit eruptions at stratovolcanoes meltwater is produced and travels freely down steep slopes and thin permeable valley glaciers eroding the ice and enlarging preexisting glacial drainages as a result eruptions in this environment have produced few catastrophic floods lava flowing into the open channels and voids in the glaciers becomes confined and grows thicker filling the available space and producing steep sided bodies with smooth bulbous contact surfaces quenching of lava against ice or by water forms small scale features such as tensional fractures and glass as the amount of meltwater in contact with the lava increases the type and abundance of smaller scale features become similar to those produced during subglacial eruptions into meltwater lakes identification of large and small scale lava ice contact features in the field can be used to reconstruct paleoglacial extent and combined with geochronology of lavas to determine past paleoclimate an understanding of lava ice interaction allows us to better assess the hazards posed by future eruptions at glaciated volcanoes

isotopic and ree characteristics of the intrusive charnoenderbite and enderbite geographically associated with the matok pluton limpopo belt southern africa abstract intrusive charnoenderbite and enderbite in the southern marginal zone of the limpopo belt are genetically linked with peak metamorphism m1 during the 2700 ma to 2650 ma limpopo orogeny they occur geographically associated with and are intruded by the granodiorite and granite of the matok pluton which were emplaced between the response phase m2 and the rehydration phase m3 of that orogeny rb sr pb ree and sm nd elemental and isotopic data for whole rock samples and u pb data for populations of zircon suggest that the charnoenderbite and enderbite were intruded 2671 ma ago and were derived from a crustal precursor the granodiorite and granite of the matok pluton were intruded at approximately the same time between 2667 ma and 2664 ma ago they also were derived from a crustal precursor nd model ages and ree patterns for the charnoenderbite and enderbite and the granodiorite are distinct this relationship may imply that they are not cogenetic alternatively these rock types could be cogenetic if the magma from which they were derived was assimilating older crustal material as it evolved nd isotopic data indicate that the magma giving rise to the charnoenderbite and enderbite formed by partial melting of rocks compositionally similar to some portions of the baviaanskloof gneiss data for the granodiorite are consistent with the possibility that this unit formed in a similar manner from rocks compositionally similar to other portions of the baviaanskloof gneiss the close temporal relationship between the emplacement of the charnoenderbite and enderbite and the granitic rocks of the matok pluton indicates that the retrograde portion of the limpopo orogeny beginning 2670 ma ago occurred rapidly perhaps over 7 ma

unusual ice diamicts emplaced during the december 15 1989 eruption of redoubt volcano alaska abstract ice diamict comprising clasts of glacier ice and subordinate rock debris in a matrix of ice snow grains coarse ash and frozen pore water was deposited during the eruption of redoubt volcano on december 15 1989 rounded clasts of glacier ice and snowpack are as large as 2 5 m clasts of redoubt andesite and basement crystalline rocks reach 1 m and tabular clasts of entrained snowpack are as long as 10 m ice diamict was deposited on both the north and south volcano flanks on redoubt s north flank along the east side of drift piedmont glacier and outwash valley ice diamict accumulated as at least 3 units each 1 5 m thick two ice diamict layers underlie a pumice lithic fall tephra that accumulated on december 15 from 10 15 to 11 45 ast a third ice diamict overlies the pumiceous tephra some of the ice diamicts have a basal ice sandstone layer the north side icy flows reached as far as 14 km laterally over an altitude drop of 2 3 km and covered an area of about 5 7 km 2 on crescent glacier on the south volcano flank a composite ice diamict is locally as thick as 20 m it travelled 4 3 km over an altitude drop of 1 7 km covering about 1 km 2 the much higher mobility of the northside flows was influenced by their much higher water contents than the southside flow s erupting hot juvenile andesite triggered and turbulently mixed with snow avalanches at snow covered glacier heads these flows rapidly entrained more snow firn and ice blocks from the crevassed glacier on the north flank a trailing watery phase of each ice diamict flow swept over and terraced the new icy deposits the last and perhaps each flood reworked valley floor snowpack and swept 35 km downvalley to the sea ice diamict did not form during eruptions after december 15 despite intervening snowfalls these later pyroclastic flows swept mainly over glacier ice rather than snowpack and generated laharic floods rather than snowflows similar flows of mixed ice grains and pyroclastic debris resulted from the november 13 1985 eruption of nevado del ruiz volcano and from eruptions of snowclad mount st helens in 1982 1984 such deposits at snowclad volcanoes are initially broad and geomorphically distinct but they soon become extensively reworked and hard to recognize in the geologic record

the geology and tectonic evolution of the pietersburg greenstone belt south africa abstract the pietersburg greenstone belt occurs near the northern margin of the low grade granite greenstone terrain of the kaapvaal craton to the north lies the high grade granite greenstone terrain which constitutes the southern marginal zone of the limpopo belt the original relationship between the two terrains has been overprinted by the effects of the limpopo orogeny and are therefore not known a better understanding of the tectonic evolution of the pietersburg greenstone belt may help to clarify this relationship the rocks of the southwestern part of the pietersburg greenstone belt have been mapped in considerable detail the rocks can be subdivided into two sequences the first comprises a sequence of massive to pillowed metabasalts metagabbros and metaperidotites as well as bif fine grained mafic tuffs and sediments the sequence was subjected to extensive hydrothermal metamorphism at low pressures and variable temperatures in the greenschist to amphibolite facies this suggests an oceanic like environment the second sequence known as the uitkyk formation comprises a sequence of terrestrial clastics which overlie the first sequence with a profound unconformity sedimentology indicates that the uitkyk formation formed in a prograding alluvial fan or braided alluvial plain the uitkyk sediments were deformed and infiltrated by fluids that precipitated quartz tourmaline gold and siderite soon after deposition age relationships bedding cleavage relationships and younging directions indicate that both the sediments and simatic rocks were involved in a phase of thrust related deformation d2 that resulted in south over north stratigraphic repetition under generally low grade conditions of metamorphism post d2 granites date this thrusting at pre 2 69 ga d2 was probably diachronous and at surface the deformation was accompanied by syntectonic terrestrial sedimentation and placer gold deposition the tectonic environment was probably one of an intermontane accretionary or terrestrial foreland basin within this tectonic environment the basement and cover became tectonically stacked across evolving shear zones combined field and microstructural relationships within a major fault zone have revealed a complex history of progressive d2 deformation metamorphism mineralisation and associated sedimentary processes the zone represents a deformed syn sedimentary duplex that was carried piggy back above an allochthonous thrust sheet of simatic rocks at the deepest levels this fault and other shear zones were intruded by syntectonic granitoids this late archaean northward verging d2 thrust stack model for the greenstone belt with the syntectonic granites intruding the thrusts from beneath the greenstone belt is in good accord with the recent geophysical observations the age of cover sedimentation is older than 2 69 ga but younger than 2 98 ga in age this supports suggestions that these deposits are age equivalent of the upper gold bearing sediments of the witwatersrand basin and that the northwards thrusting in both basins may be of the same age and related to the same regional tectonic forces the simatic rocks of the pietersburg greenstone belt may have been part of a more extensive allochthonous simatic terrain which includes the similar greenstone remnants of the southern marginal zone of the limpopo belt such a terrain must have been disrupted and metamorphosed during the northward directed imbrication and tectonic burial recorded in the pietersburg belt prior to the climax of the limpopo orogeny the latter subsequently severely disrupted the pietersburg greenstone belt during ventersdorp times 2 6 2 7 ga

dome growth and destruction during the 1989 1990 eruption of redoubt volcano abstract much of the six month long 1989 1990 eruption of redoubt volcano consisted of a dome growth and destructive phase in which 14 short lived viscous silicic andesite domes were emplaced and 13 subsequently destroyed the life span of an individual dome ranged from 3 to 21 days and volumes are estimated at 1 10 6 to 30 10 6 m 3 magma supply rates to the vent area averaged about 5 10 5 m 3 day for most of the dome building phase and ranged from a high of 2 2 10 6 m 3 per day initially to a low of 1 8 10 5 m 3 per day at the waning stages of the eruption the total volume of all domes is estimated to be about 90 10 6 m 3 and may represent as much as 60 70 of the volume for the entire eruption the site of 1989 1990 dome emplacement like that in 1966 was on the margin of a north facing amphitheatre like summit crater the domes were confined on the east and west by steep cliffs of pre eruption cone building volcanic rocks and thus were constrained to grow vertically rapid upward growth in a precarious site caused each dome to spread preferentially to the north resulting in eventual gravitational collapse as long as the present conduit remains active at redoubt volcano any dome formed in a new eruption will be confined to a narrow steeply sloping gorge leading to rapid vertical growth and a tendency to collapse gravitationally repetitive cycles of dome formation and failure similar to those seen in 1989 1990 are probably the norm and must be considered in future hazard analyses of redoubt volcano

the enormous chillos valley lahar an ash flow generated debris flow from cotopaxi volcano ecuador the chillos valley lahar cvl the largest holocene debris flow in area and volume as yet recognized in the northern andes formed on cotopaxi volcano s north and northeast slopes and descended river systems that took it 326 km north northwest to the pacific ocean and 130 km east into the amazon basin in the chillos valley 40 km downstream from the volcano depths of 80 160 m and valley cross sections up to 337 000 m2 are observed implying peak flow discharges of 2 6 6 0 million m3 s the overall volume of the cvl is estimated to be 3 8 km3 the cvl was generated approximately 4500 years bp by a rhyolitic ash flow that followed a small sector collapse on the north and northeast sides of cotopaxi which melted part of the volcano s icecap and transformed rapidly into the debris flow the ash flow and resulting cvl have identical components except for foreign fragments picked up along the flow path juvenile materials including vitric ash crystals and pumice comprise 80 90 of the lahar s deposit whereas rhyolitic dacitic and andesitic lithics make up the remainder the sand size fraction and the 2 to 10 mm fraction together dominate the deposit constituting 63 and 15 wt of the matrix respectively whereas the silt size fraction averages less than 10 wt and the clay size fraction less than 0 5 wt along the 326 km runout these particle size fractions vary little as does the sorting coefficient average 2 6 there is no tendency toward grading or improved sorting limited bulking is recognized the cvl was an enormous non cohesive debris flow notable for its ash flow origin and immense volume and peak discharge which gave it characteristics and a behavior akin to large cohesive mudflows significantly then ash flow generated debris flows can also achieve large volumes and cover great areas thus they can conceivably affect large populated regions far from their source especially dangerous therefore are snow clad volcanoes with recent silicic ash flow histories such as those found in the andes and alaska

when was the limpopo orogeny abstract the term limpopo orogeny refers to the common high grade metamorphic and tectonic event that affected the archaean rocks of the central and southern marginal zones of the limpopo belt and its influence on the adjacent portions of the kaapvaal craton available geochronological data from muscovite kyanite prograde metamorphic assemblages zircon in igneous charnoenderbite and enderbite muscovite in syntectonic pegmatite bodies formed in shear zones partial melts and syn and post tectonic plutons of granodiorite and granite indicate that this orogeny occurred during the time interval from 2700 ma ago to 2650 ma ago the retrograde phase of this orogeny from the onset of the response phase m 2 until the rocks were subjected to rehydration m 3 was very rapid probably lasting 7 ma from 2671 ma to 2664 ma ago similar but limited data from the northern marginal zone and adjacent portions of the zimbabwe craton suggest that rocks there were subjected to a more recent interval of high grade metamorphism and tectonism extending from 2600 ma until 2550 ma