paleo glacier reconstruction in southwestern british columbia canada a glaciovolcanic model abstract volcanic rocks erupted within glacial environments offer powerful paleoenvironmental insights these glaciovolcanoes and their deposits serve as proxies that inform on the paleo presence distribution and thickness of past glaciers we investigate the paleoenvironmental implications of three mid to late pleistocene volcanic deposits in the garibaldi volcanic belt of southwestern british columbia canada we use these data to inform a simple geometric model that constrains paleo glacier distributions in the southern canadian cordillera the three volcanoes are used to recover i a coalesced mountain ice sheet in late marine isotope stage mis 4 and possibly into mis 3 up to 49 1 5 5 ka ii major and rapid degradation of glaciers during the late mis 6 to 5e transition by 141 12 9 ka and iii a coalesced mountain ice sheet or continental ice sheet that existed during mis 15 598 7 5 ka

history and isostatic effects of the last ice sheet in southern british columbia abstract the late wisconsinan cordilleran ice sheet covered british columbia southern yukon territory and parts of alaska washington idaho and montana its major source areas were the high mountain ranges of the canadian cordillera and flow was strongly controlled by topography the last ice sheet began to develop about 30 000 25 000 14c yr ago but it did not achieve its maximum extent until 15 000 14 000 14c yr bp ice sheet growth was interrupted locally by stillstands and retreat ice sheet decay was rapid and was characterized by complex frontal retreat at the periphery accompanied locally by brief readvances and by downwasting and stagnation by 10 000 14c yr bp

a middle pleistocene isotope stage 10 glacial sequence in the stikine river valley british columbia in the stikine river valley northwestern british columbia glacial and nonglacial sediments are preserved beneath middle pleistocene basalt flow remnants that originated from mount edziza the magnetic polarity is consistently normal indicating that the sediment and the basalts were probably deposited within the bruhnes normal polarity chron 780 ka the sediments record a regional glacial advance initial ice advance in the coast mountains blocked the westward drainage of the stikine river and formed an advance phase glacial lake sediments deposited in this lake form a coarsening upwards sequence debris flow diamicton units that originated from the valley sides are common the lacustrine sequence culminates in a poorly sorted ice marginal gravel deposited as ice encroached upon the study area there is little record of ice retreat the basalts are deposited on fluvial and or glaciofluvial gravels indicating that postglacial reincision was taking place at the time of eruption hence the sediment

products of subglacial volcanic eruptions under different ice thicknesses two examples from antarctica late cenozoic subglacially erupted volcanic sequences are scattered throughout the antarctic peninsula two of the best preserved examples at mount pinafore alexander island c 5 5 6 ma and brown bluff graham land c 1 ma are complete enough to be regarded as sequence holotypes for this uncommonly preserved eruptive depositional setting despite a common glacial association the sedimentary lithofacies in the two outcrops suggest flowing and ponded water conditions respectively indicating significant differences in the depositional palaeoenvironments the original ice thicknesses exerted a major control on the lithofacies which resulted from each eruptive phase at mount pinafore the lithofacies were confined within a steep sided valley during successive eruptions beneath thin 100 150 m wet based ice the much thicker succession at brown bluff is a tindar tuya edifice which formed within a small basin probably 15 km across confined by ice 400 m thick

the formation of helgafell southwest iceland a monogenetic subglacial hyaloclastite ridge sedimentology hydrology and volcano ice interaction abstract helgafell a little eroded basaltic hyaloclastite ridge in southwest iceland formed in a single eruption under a pleistocene ice sheet the ice thickness at the eruption site was at least 500 m and it was probably located some 15 km from the glacier s snout the eruption created a 2 km long 0 8 km wide and initially 300 m high ridge gravity modelling indicates that helgafell has a bulk density of 1800 kg m 3 and that holocene lavas around it are 40 80 m thick this confirms that helgafell is predominantly made of hyaloclastite with pillow lavas and intrusions only making up a few percent of the total volume the southeast side is made of unsorted eruption fed hyaloclastites considered to have been piled up against an ice wall while moderately to well sorted water transported material is found on the northwest side glacier flow and meltwater drainage was towards the northwest the absence of basal pillow lavas suggests that magma fragmentation occurred from the onset of the eruption until its end the lithofacies preserved indicate a fully subglacial eruption although a final subaerial eruptive phase may have taken place through an ice chimney the volatile contents h 2 o 0 26 0 37 wt of several glass samples from the southeast side of the mountain indicate water pressures of 1 mpa throughout the eruption efficient syn eruptive drainage of meltwater coupled with rapid ice subsidence probably led to partial dynamic support of the ice causing water pressure in the vault to be much lower than the static load of the overlying ice observed lack of correlation between elevation and volatile content may be a consequence of gradual reduction in dynamic support as the eruption rate declined and the edifice grew higher helgafell demonstrates that explosive activity may occur under 500 m thick ice and it may be an analogue to the ridge formed in the gjalp eruption in 1996

lillooet glacier basalts southwestern british columbia canada products of quaternary glaciovolcanism the retreat of lillooet glacier lg has exposed a succession of pillow basalt and subordinate amounts of breccia and hyaloclastite the lithofacies and physiographic setting suggest that the deposits have a glaciovolcanic origin and represent a partially dissected basaltic pillow dominated tindar chemically the lg volcanic rocks are basalt to basaltic andesite and as a group they represent the highest silica quaternary mafic products in the garibaldi volcanic belt gvb like other northern gvb alkaline basalts they lack the nb ta depletion signature typically associated with subduction related products geochemical and petrologic analysis indicates that the lg basalts are comagmatic and that chemical variations within the suite are consistent with sorting of the observed phenocryst assemblage olivine plagioclase thermodynamic modeling establishes shallow crustal pre eruptive storage conditions at 2 kbar 1 kbar 100 mpa or 7 5 km and an h2o content of 0 5 1 wt we estimate that the

stratocumulus cloud top height estimates and their climatic implications a depth dependent boundary layer lapse rate was empirically deduced from 156 radiosondes released duringsixmonth longresearchcruisestothe southeastpacificsamplingavarietyofstratocumulusconditions thelapse ratedependenceonboundarylayerheightisweak decreasingfromabestfitof7 6to7 2k km 21 as theboundarylayerdeepensfrom800mto2km ship basedcloud baseheightsupto800mcorrespondwellto liftingcondensationlevels indicatingwell mixedconditions withcloud bases 800moften 200 600m higher than the lifting condensation levels the lapse rates were combined with moderate resolution imaging spectrometer 11 mm derived cloud top temperatures and satellite microwave derived sea surface temperatures to estimate stratocumulus cloud top heights the october mean cloud top height structure of the southeast pacific was then spatially and diurnally characterized coastal shoaling is apparent but so is a significant along coast cloud top height gradient with a pronounced elevation of the cloud top heights above thearicabightat 208s diurnalcloud topheightvariations inferredfromirregular4 times dailysampling can locally reach 250 m in amplitude and they can help to visualize offshore propagation of free tropospheric vertical motions a shallow boundary layer associated with the chilean coastal jet expands to its north and west in the afternoon cloud top heights above the arica bight region are depressed in the afternoon which may mean that increased subsidence from sensible heating of the andes dominates an afternoon increase in convergence upward motion at the exit of the chilean coastal jet in the southeast atlantic during october thestratocumuluscloud topheightsaretypicallylowerthanthoseinthesoutheastpacific acoastaljetregion can also be identified through its low cloud top heights coastal shoaling of the south atlantic stratocumulus region is mostly uniform with latitude in keeping with the more linear namibian angolan coastline the southeast atlantic shallow cloudy boundary layer extends farther offshore than in the southeast pacific particularly at 158s

a new approach to retrieve cloud base height of marine boundary layer clouds 1 a novel approach for estimating marine boundary layer cloud base height cbh is proposed based on calculated boundary layer lapse rates collocated cloud top height cth cloud top and ocean surface temperatures from the a train satellite constellation the method takes advantage of the assumption that decreases of temperature within and below water clouds may follow the different constant apparent lapse rates in the same region respectively the cbhs derived from the new method compare well with the coincident cbh product from the active sensors of cloud aerosol lidar and infrared pathfinder satellite observation calipso and cloudsat the correlation coefficient the mean difference and the standard deviation are 0 79 0 54 0 02 km 0 03 km and 0 35 km 0 54 km respectively when cth is derived from calipso data or moderate resolution imaging spectroradiometer retrieval besides the relatively small bias the most important advantage of this method compared to previous cbh retrieval techniques is that it is independent of boundary layer cloud types optical thickness and illumination

northern cordilleran volcanic province a northern basin and range introductionthe western margin of north america hosts adiverse assemblage of large cenozoic igneousprovinces fig 1 inset the temporal spatial andpetrological characteristics of these provinces areattributed to a variety of magmatic processes andtectonic environments including lithosphericextension e g basin and range migration ofmantle plumes e g snake river plain and sub duction e g cascade magmatic arc however neogene alkaline volcanic rocks in the cordilleraof northwestern canada between 55 and 64 n fig 1 currently lack a clear linkage to tectonicprocesses these volcanic centers constitute thenorthern cordilleran volcanic province and repre sent a significant gap in our understanding of thecenozoic tectonomagmatic activity in westernnorth america fig 1 inset our objective is to demonstrate that an exten sional tectonic model adequately explains thepetrologic radiometric and tectonic data forneogene quaternary volcanic rocks in the north ern cordillera furthermore the model we pro pose is consistent with models for neogene quaternary interactions between the pacific andnorth american plates northern cordilleran volcanicprovincethe northern cordilleran volcanic provinceencompasses a broad area of neogene to qua ternary volcanism approximately 1200 km longand 400 km wide in the northern part of west ern canada fig 1 the majority of the vol canoes within this region formed east of thedenali fault system and west of the tintinatrench fig 1 volcanic expressions of magma tism in the province range from large basalticplateaus and stratovolcanoes to individual cin der cones lava flows and eroded vents petrologyneogene to quaternary volcanic rocks withinthe northern cordilleran volcanic province arealkaline and chemically bimodal at least 70 ofthe volcanic rocks comprise alkali olivine basaltand hawaiite fig 2a basaltic lavas are gener ally olivine and plagioclase phyric with plagio clase olivine titanaugite and magnetite in thegroundmass the province also hosts a variety of other alka line and peralkaline rock types that are uncom mon in the north american cordillera such asnephelinite basanite phonolite trachyte and

multiphase flow of the late wisconsinan cordilleran ice sheet in western canada in central british columbia ice flow during the late wisconsinan fraser glaciation ca 25 10 ka occurred in three phases the ice expansion phase occurred during an extended period when glaciers flowed westward to the pacific ocean and east southeastward onto the nechako plateau from ice centers in the skeena hazelton coast and omineca mountains initially glacier flow was confined by topography along major valleys but eventually piedmont and montane glaciers coalesced to form an integrated glacier system the cordilleran ice sheet in the maximum phase a cordilleran ice divide developed over the nechako plateau to 300 km inland from the pacific coast at this time the surface of the ice sheet extended well above 2500 m above sea level and flowed westward over the skeena hazelton and coast mountains onto the continental shelf and eastward across the rocky mountains into alberta in the late glacial phase a rapid rise of the equilibrium line caused ice lobes to stagnate in valleys and restricted accumulation centers to high mountains discordant directions in ice flow are attributed to fluctuations of the ice divide representing changes in the location of accumulation centers and ice thickness ice centers probably shifted in response to climate irregular growth in the ice sheet rapid calving ice streaming and drainage of proglacial and subglacial water bodies crosscutting ice flow indicators and preservation of early valley parallel flow features in areas exposed to later cross valley glacier erosion indicate that the ice expansion phase was the most erosive and protracted event

changes in the extent of north american ice sheets during the late cenozoic magnetostratigraphy indicates that early pleistocene glaciations in north america instead of forming one continuous ice mass from atlantic to pacific as they did in the late pleistocene were characterized by eastern and western ice masses separated by a 2000 km wide north south ice free corridor down the centre of the continent we argue therefore that the area covered by ice during periods of glaciation and hence probably ice volume in north america was considerably less in the first 2 ma of the late cenozoic than it was in the last 0 7 ma this is consistent with delta 18o records of ocean cores indicating the ice volumes were much less in the earlier than in the later part of the cenozoic ice age

subaqueous eruption fed density currents and their deposits density currents fed directly from subaqueous eruptions can be divided conceptually into three groups based on modes of fragmentation and transport i explosive fragmentation with deposition from a gas supported current ii explosive fragmentation with deposition from a water supported current iii fragmentation of flowing lava with deposition from a water supported current fig 1 group i products include subaqueously emplaced welded ignimbrite and other high temperature emplaced subaqueous pyroclastic flow deposits group ii products are the most varied and include representatives of both high and low concentration turbidity currents grainflows and debris flows and are termed eruption fed aqueous density currents some clasts in such currents are transported and even deposited at high temperature but the transporting currents ranging from grain flows to dilute turbidity currents are water dominated even though steam may be developed along large clasts margins group iii products formed from lava flow fed density currents tend to be weakly dispersed down gradient along the seafloor and generally consist largely of fragments formed by dynamo thermal quenching and spalling bursting of bubbles formed by vapor expansion probably contributes to some group iii beds distinctive column margin fall deposits may form in water excluded zones that developed very locally around vents in association with group i and ii deposits and are distinguished by heat retention structures indicators of gas phase transport and absence of current formed depositional features 2000 elsevier science b v all rights reserved

insights into the dynamics of mafic magmatic hydromagmatic eruptions from volatile degassing behaviour the hverfjall fires iceland abstract the style and intensity of hydromagmatic activity is governed by a complex interplay between the relative volumes of magma and water that interact their relative viscosities the depth of subsurface explosions the substrate properties and the vent geometry fundamental questions remain however regarding the role of magmatic vesiculation in determining the dynamics of magma water interaction mwi petrological reconstructions of magmatic degassing histories are commonly employed to interpret the pre and syn eruptive conditions during dry magmatic eruptions but the application of similar techniques to hydromagmatic activity has not yet been fully explored in this study we integrate glass volatile measurements s cl h2o and co2 with field observations and microtextural measurements to examine the relationship between degassing and eruptive style during the hverfjall fires fissure eruption iceland here coeval fissure vents produced both dry magmatic jar ba sholar scoria cone complex and variably wet hydromagmatic hverfjall tuff ring activity generating physically distinct pyroclastic deposits with contrasting volatile signatures matrix glass volatile concentrations in hydromagmatic ash 883 172 1 ppm s 0 45 0 03 1 wt h2o 20 ppm co2 are consistently elevated relative to magmatic ash and scoria lapilli 418 93 1 ppm s 0 12 0 48 1 wt h2o co2 below detection and overlap with the range for co erupted phenocryst hosted melt inclusions 1522 127 1 ppm s 165 27 1 ppm cl measurements of hydromagmatic glasses indicate that the magma has degassed between 17 and 70 of its initial sulfur prior to premature quenching at variably elevated confining pressures by comparing volatile saturation pressures for both magmatic and hydromagmatic glasses and how these vary through the eruptive stratigraphy we place constraints on the conditions of mwi crucially our data demonstrate that the magma was already vesiculating when it encountered groundwater at depths of 100 200 m and that the external water supply was sufficient to maintain mwi throughout the eruption with no significant vertical or lateral migration of the fragmentation surface we propose that development of an in vent water sediment slurry provides a mechanism through which the elevated confining pressures of 1 6 2 6 mpa or up to 6 mpa accounting for uncertainty in co2 below analytical detection could be maintained and buffered throughout the eruption whilst enabling vertical mixing and ejection of fragmented juvenile and lithic material from a range of depths importantly these results demonstrate that the volatile contents of hydromagmatic deposits provide valuable records of 1 the environment of mwi e g groundwater versus surface water vertical migration of the fragmentation level and 2 the state of the magma at the time of fragmentation and quenching we further suggest that the volatile content of tephra glasses provides a reliable alternative or additional indicator of a hydromagmatic origin particularly for reduced ocean island basalts where late stage volatile saturation and degassing s h2o occurs over a pressure range relevant to typical mwi environments

a record of early pleistocene glaciation on the mount edziza plateau northwestern british columbia mount edziza is a plio pleistocene volcanic complex that is located in the stikine terrane in northwestern british columbia a sequence of diamictites preserved between ice peak formation basalts on the northwestern blank of mount edziza records an early pleistocene regional glaciation the lowest ice peak formation basalt flow ip1 about 1 ma was probably extruded onto glacial ice because it is deformed and brecciated it is pillowed at the base it lies directly on hyaloclastite deposits and there is a lack of fluvial and lacustrine sediments at the base fabric measurements from the underlying diamictites are consistent with lodgement processes and indicate northwest and southwest transport directions these data and an abundance of striated exotic cobbles indicate that the sediment was deposited by coast mountain ice radiometric paleomagnetic and stratigraphic data all support the interpretation that diamictites at the section are the sedimentary record of an early pleistocene about 1 1 ma i

solidification and morphology of submarine lavas a dependence on extrusion rate the results of recent laboratory experiments with wax extruded beneath relatively cold water may be extrapolated to predict the surface morphology of submarine lavas as a function of the extrusion rate and melt viscosity the experiments with solidifying wax indicated that the surface morphology was controlled by a single parameter the ratio of the time taken for the surface to solidify and a time scale for lateral flow for submarine basalts a solution of the cooling problem which is dominated by conduction in the lava but convective heat transfer in the water and estimates of lava viscosities place this parameter within the empirically determined pillowing regime over a wide range of extrusion rates this result is consistent with the observation that pillow basalts are the most common products of submarine eruptions smoother surfaces corresponding to the various types of submarine sheet flows are predicted for sufficiently rapid extrusions of basaltic magma still higher eruption rates in regions of low topographic relief may produce submarine lava lakes minimum emplacement times can be calculated for submarine volcanic constructs of a single lava flow type

impacts of pre vs postcolonial land use on floodplain sedimentation in temperate north america abstract this paper evaluates the relative importance of anthropogeomorphic sedimentation on floodplains in the mid latitudes of north america before and after the arrival of euroamericans geohistorical and geoarchaeological theories have emerged that have conflicts concerning the relative effectiveness of pre and postcolonial anthropogeomorphic change in north america many geoarchaeologists paleoecologists historians and cultural geographers have concluded that intensive precolonial land use resulted in substantially humanized landscapes prior to the arrival of colonists this presents a geomorphic paradox to fluvial geomorphologists and river restorationists who interpret buried floodplain soils as evidence of landscape stability evidence for these theories is reviewed from a geomorphic perspective while making distinctions between ecological and geomorphic change and recognizing spatial and temporal variability in some locations mid millennial land use generated erosion and floodplain sedimentation subsequent indigenous population declines and reductions in land use intensity may have been followed by precolonial geomorphic stability and pedogenesis an abandoned landscape hypothesis that floodplain soils developed on precolonial anthropogenic alluvium and were subsequently buried should be considered where indigenous populations and land use activities were high variability is also noted in postcolonial floodplain sedimentation many stratigraphic studies show that rates of sedimentation increased greatly after colonization in response to land clearance for agriculture timbering and mining an inventory of studies that document postcolonial floodplain sediment shows strong spatial patterns most studies of postcolonial sedimentation cluster in the eastern usa from the mid atlantic southward and in the mississippi valley from wisconsin to mississippi but many occurrences of mining sediment are scattered throughout the west few of the studies reviewed in the west document anthropogenic floodplain sediment from agriculture or logging although more thorough regional inventories could change that vast areas of temperate north america do not have historical deposits that are well represented in the scientific literature so assumptions that legacy sediment on floodplains is ubiquitous are ill founded spatial and temporal variations in pre and postcolonial floodplain sedimentation are complex and more research is needed to fill in the knowledge gaps

subglacial volcanism in north central british columbia and iceland the subglacial genesis of five volcanoes in british columbia has been confirmed by field and laboratory studies and by comparison with the stapar table mountains of iceland two of the canadian tuyas are flat topped piles of hyaloclastite overlain by basalt flows these mountains are almost identical in form to their icelandic counterparts three other tuyas are large conical piles of unconsolidated lapilli two of which overlie vitric tuff and pillow basalt layers and may be analogous to relatively rare icelandic subglacial scoria cones all five volcanoes can be accounted for by eruption into englacial meltwater lakes the differences in gross morphology and physical composition can be accounted for by minor variations in the relative importance of magmatic versus phreatomagmatic eruption

chronology of the peralkaline late cenozoic mount edziza volcanic complex northern british columbia canada the late cenozoic mount edziza volcanic complex covers an area of about 1 000 km 2 in north central british columbia 300 km east of the transcurrent boundary between the north american and pacific plates it is made up of a group of overlapping basaltic shields and intermediate to salic peralkaline composite domes flows and central volcanoes that are associated with extensional structures in the underlying basement new k ar and fission track dates 45 and rb sr and sr isotope analyses 12 from the mount edziza volcanic complex are reported the age dates are for the most part consistent with the stratigraphy and indicate that frequent eruptive activity occurred during the past 8 m y five major magmatic cycles each began with the eruption of basalt and culminated with the eruption of oversaturated peralkaline magma low 87 sr 86 sr initial ratios 0 7028 0 0001 indicate a mantle source for the basalts low sr contents and high 87 sr 86 sr ratios in the salic end members suggest that the oversaturated rocks were derived from the basaltic magma by crystal fractionation in crustal reservoirs rb sr isochrons suggest that residence times for the fractionating magma were about 0 7 to 1 m y early removal of large amounts of plagioclase followed by fractionation of potash feldspar can account for most of the observed petrological and isotopic relationships a few individual compositions and one suite of mainly intermediate samples contain anomalously large amounts of both 87 sr and radiogenic argon this indicates that contamination with crustal material and possibly mixing of parental basalt with partly fractionated magma from previous events may have produced the relatively small volume of intermediate rocks

basanite glaciovolcanism at llangorse mountain northern british columbia canada the llangorse volcanic field is located in northwest british columbia canada and comprises erosional remnants of miocene to holocene volcanic edifices lava flows or dykes the focus of this study is a single overthickened 100 m thick valley filling lava flow that is middle pleistocene in age and located immediately south of llangorse mountain the lava flow is basanitic in composition and contains mantle derived peridotite xeno liths the lava directly overlies a sequence of poorly sorted crudely bedded volcaniclastic debris flow sediments the debris flow deposits contain a diverse suite of clast types including angular clasts of basanite lava blocks of peridotite coated by basanite and rounded boulders of granodiorite many of the basanite clasts have been palagonitized the presence and abundance of clasts of vesicular to scoriaceous palagonitized basanite and peridotite suggest that the debris flows are syngenetic to the overlying lava flow and sampled the same volcanic vent during the early stages of eruption they may represent lahars or outburst floods related to melting of a snow pack or ice cap during the eruption the debris flows were water saturated when deposited the rapid subse quent emplacement of a thick basanite flow over the sediments heated pore fluids to at least 80 100 c causing in situ palagonitization of glassy basanite clasts within the sediments the over thickened nature of the llangorse mountain lavas suggests ponding of the lava against a down stream barrier the distribution of similar aged glaciovolcanic features in the cordillera suggests the possibility that the barrier was a lower elevation valley wide ice sheet

physical controls and resulting morphological forms of quaternary ice contact volcanoes in western canada abstract volcanism in association with large quantities of ice manifests itself in a variety of morphological forms developed under differing physical conditions these physical conditions include the location amount thickness and type of confining ice the location and quantity of trapped water and the surrounding topography ice in the form of thick continental ice sheets thinner alpine glaciers or even perennial accumulations of snow can influence the resulting morphological form of the volcano the chemical composition of the erupting magma effusion rate and total erupted volume will also affect the shape of the resulting edifice quaternary volcanoes influenced by many of these factors are found in western canada the morphological forms range from tuyas to subglacial mounds to stratovolcanoes this paper provides a review of ice contact volcanism and volcanic products formed in these environments using canadian examples for illustration

tholeiitic alkalic transition at subglacial volcanoes tuya region british columbia canada ash mountain south tuya and tuya butte are three small basaltic volcanoes in the stikine volcanic belt of northern british columbia the volcanoes rise 700 500 and 400 m above their bases and are about 3 2 1 6 and 2 6 km3 in volume respectively they began eruptive activity under several hundred meters of overlying glacial ice or water in an ice impounded lake and undegassed pillow lava was erupted and forms the bases of all three later as the vents grew into shallow water explosive phreatomagmatic activity erupted partly degassed glassy tuffs finally when the volcano emerged through the surface of the ice or water or the water was drained degassed subaerial lava flows were erupted and were converted to assemblages of foreset bedded pillow breccia and pillow lava when subaerial flows crossed a shoreline and flowed into meltwater lakes the undegassed subglacial pillow base of ash mountain is overlain by partly degassed pillows and hyaloclastite tuff cut by dikes at south tuya the pillow base is overlain by hyaloclastite tuffs and lenses of pillow lava at tuya butte the pillow base is overlain by foreset bedded pillow lava pillow breccias and hyaloclastite tuffs which in turn are overlain by subaerial lava flows composing a small shield volcano the undegassed basal subglacial pillow lava of the three volcanoes contain 0 10 0 01 wt sulfur and 0 5 wt h2o the overlying partly degassed assemblages contain 0 06 0 02 sulfur and 0 2 h2o at ash mountain 0 07 0 01 sulfur at south tuya and 0 03 0 01 sulfur at tuya butte the differences in the degree of degassing can be related to the nature of eruption and quenching and the distance of flow of the subaerial lava when the volcanoes switched from subglacial to shallow water or subaerial eruptions as shown by change to more explosive activity and then to subaerial lava flows and by a marked reduction of sulfur in volcanic glass the magma shifted from tholeiitic to alkalic composition this transition occurs at each of the three volcanoes the tholeiitic and alkalic magmas cannot be related by shallow crystal fractionation and apparently originated by differing degrees of deep melting at a mantle source prior to eruption the tholeiitic melts overlay alkalic melts in shallow chambers underlying each of the volcanoes because of their lower density and were therefore the first to erupt under subglacial conditions as the volcano grew through the ice or ice impounded water the volcanic conduit vented to the atmosphere producing a partial depressurization of the conduit and the subsurface chamber this sudden reduction in confining pressure caused enhanced vesiculation of volatile saturated melts particularly of the more volatile rich alkalic melts causing them to rise to the top of the chamber and erupt

evolution of an englacial volcanic ridge pillow ridge tindar mount edziza volcanic complex ncvp british columbia canada abstract glaciovolcanic deposits are critical for documenting the presence and thickness of terrestrial ice sheets and for testing hypotheses about inferred terrestrial ice volumes based on the marine record deposits formed by the coincidence of volcanism and ice at the mount edziza volcanic complex mevc in northern british columbia canada preserve an important record for documenting local and possibly regional ice dynamics pillow ridge located at the northwestern end of the mevc formed by ice confined fissure fed eruptions it comprises predominantly pillow lavas and volcanic breccias of alkaline basalt composition with subordinate finer grained volcaniclastic deposits and dykes the ridge is presently 4 km long 1000 m in maximum width and 600 m high fifteen syn and post eruptive lithofacies are recognized in excellent exposures along the glacially dissected western side of the ridge we recognize five lithofacies associations 1 poorly sorted tuff breccia and dykes 2 proximal pillow lava dykes and tuff breccia 3 distal pillow lava poorly sorted conglomerate and well sorted volcanic sandstone 4 interbedded tuff lapilli tuff and tuff breccia units and 5 heterolithic volcanogenic conglomerate and sandstone given the abundance of pillow lavas and the lack of surrounding topographic barriers capable of impounding water we agree with souther souther j g 1992 the late cenozoic mount edziza volcanic complex geol soc can mem vol 420 320 pp that the bulk of the edifice formed while confined by ice but have found evidence for a more complex and variable eruption history than that which he proposed preliminary estimates of water ice depths derived from ftir analyses of h2o give ranges of 300 to 680 m assuming 0 ppm co2 and 857 to 1297 m assuming 25 ppm co2 variations in depth estimates among samples may indicate that water ice depths changed during the evolution of the ridge which is consistent with our interpretations for the origins of different lithofacies associations given that the age of the units are likely to be ca 0 9 ma souther j g 1992 the late cenozoic mount edziza volcanic complex geol soc can mem vol 420 320 pp pillow ridge may be the best documentation of a regional high stand of the cordilleran ice sheet cis in the middle pleistocene and an excellent example of the lithofacies and stratigraphic complexities produced by variations in water levels during a prolonged glaciovolcanic eruption

crystal fractionation of the basalt comendite series of the mount edziza volcanic complex british columbia major and trace elements abstract the mount edziza volcanic complex in north central british columbia includes a group of overlapping basaltic shields salic composite volcanoes domes and small calderas that range in age from 7 5 ma to less than 2000 years b p the volcanic assemblage is chemically bimodal comprising voluminous alkali olivine basalt and hawaiite a salic suite of mainly peralkaline trachyte and comendite plus a relatively small volume of intermediate rocks trachybasalt tristanite mugearite benmoreite the complex is the product of five cycles of magmatic activity each of which began with alkali olivine basalt and culminated with the eruption of salic magma the regular chemical variation shown by almost 100 major and trace element analyses suggests a genetic lineage between the basic and salic members of each cycle least squares mathematical modelling indicates that the salic rocks trachyte and comendite have formed by fractionation of observed phenocryst and cumulate nodule mineral phases from a common alkali olivine basalt parent magma hawaiite is thought to be a cumulate rock formed by partial fractionation and feldspar accumulation within rising columns of primary alkali olivine basalt fractionation leading from alkali olivine basalt through trachybasalt and trachyte to comendite is believed to have taken place where primary basalt became trapped in large crustal reservoirs the early removal of olivine clinopyroxene and plagioclase leading to a trachytic residuum and subsequent fractionation of mainly alkali feldspar leading to the peralkaline end members is consistent with major and trace element variation and with isotopic and ree data the chemical diversity of the complex is attributed to its location over a zone of crustal extension where mantle derived basalt trapped in large high level reservoirs underwent prolonged fractionation

thermodynamics and fluid dynamics of effusive subglacial eruptions we consider the thermodynamic and fluid dynamic processes that occur during subglacial effusive eruptions subglacial eruptions typically generate catastrophic floods jokulhlaups due to melting of ice by lava and generation of a large water cavity we consider the heat transfer from basaltic and rhyolitic lava eruptions to the ice for typical ranges of magma discharge and geometry of subglacial lavas in iceland our analysis shows that the heat flux out of cooling lava is large enough to sustain vigorous natural convection in the surrounding meltwater in subglacial eruptions the temperature difference driving convection is in the range 10 100 c average temperature of the meltwater must exceed 4 c and is usually substantially greater we calculate melting rates of the walls of the ice cavity in the range 1 40 m day indicating that large subglacial lakes can form rapidly as observed in the 1918 eruption of katla and the 1996 eruption of gjalp fissure in vatnajokull the volume changes associated with subglacial eruptions can cause large pressure changes in the developing ice cavity these pressure changes can be much larger than those associated with variation of bedrock and glacier surface topography previous models of water cavity stability based on hydrostatic and equilibrium conditions may not be applicable to water cavities produced rapidly in volcanic eruptions energy released by cooling of basaltic lava at the temperature of 1200 c results in a volume deficiency due to volume difference between ice and water provided that heat exchange efficiency is greater than approximately 80 a negative pressure change inhibits escape of water allowing large cavities to build up rhyolitic eruptions and basaltic eruptions with less than approximately 80 heat exchange efficiency cause positive pressure changes promoting continual escape of meltwater the pressure changes in the water cavity can cause surface deformation of the ice laboratory experiments were carried out to investigate the development of a water cavity by melting ice from a finite source area at its base the results confirm that the water cavity develops by convective heat transfer

continuous lake sediment records of glaciation in the sierra nevada between 52 600 and 12 50014c yr b p the chemistry of the carbonate free clay size fraction of owens lake sediments supports the use of total organic carbon and magnetic susceptibility as indicators of stadial interstadial oscillations owens lake records of total organic carbon magnetic susceptibility and chemical composition of the carbonate free clay size fraction indicate that tioga glaciation began 24 500 and ended by 13 60014c yr b p many of the components of glacial rock flour e g tio2 mno bao found in owens lake sediments achieved maximum values during the tioga glaciation when valley glaciers reached their greatest extent total organic carbon and sio2 amorphous concentrations reached minimum values during tioga glaciation resulting from decreases in productivity that accompanied the introduction of rock flour into the surface waters of owens lake at least 20 stadial interstadial oscillations occurred in the sierra nevada between 52 600 and 14 00014c yr b p total organic carbon data from a pyramid lake sediment core also indicate oscillations in glacier activity between 39 500 and 13 60014c yr b p alpine glacier oscillations occurred on a frequency of 1900 yr in both basins suggesting that millennial scale oscillations occurred in california and nevada during most of the past 52 600 yr

stratigraphy and paleomagnetism of brunhes and matuyama 790 ka quaternary deposits at merritt british columbia a succession of quaternary deposits in the merritt basin of south central british columbia contains evidence for four glaciations and two interglaciations paleomagnetic signatures in these sediments are of three types normal polarity proposed to have been acquired during the brunhes normal polarity chron reversed polarity proposed to have been acquired during the matuyama reversed polarity chron and reversed polarity matuyama age all but obscured by a normally magnetized overprint brunhes age reversely magnetized deposits at the base of the succession include glacial lacustrine deposits interpreted as representing two different glaciations and a paleosol and a succession of nonglacial sediments which are evidence of two interglaciations as the reversed polarity of these deposits is proposed to have been acquired during the matuyama reversed polarity chron they are older than 790 ka normally magnetized deposits which make up the rest of the succession contain evidence for only two glaciati

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

early pleistocene volcanism and glaciation in central yukon a new chronology from field studies and paleomagnetism the paleomagnetism of the selkirk volcanics and nearby stratified pleistocene sediments was investigated to resolve the chronology of early pleistocene glaciations in central yukon radiometric dates on these low k basalts have proven to be erroneously old most sampled sediments and all basalts accurately record the paleofield and true reversals the valley filling phase of the selkirk volcanics was in part coeval with the younger pre reid glaciation it was erupted during the matuyarna chron either post cobb mountain subchron or post jararnillo subchron over a period too brief to average secular variation the older pre reid glaciation occurred after ca 1 60 ma and prior to the eruption of the fort selkirk tephra pre jaramillo or pre cobb mountain sediments investigated at revenue creek and braden s canyon are normally magnetized the assigned brunhes age is compatible with their occurrence in valleys that were cut or deepened sometime after the pre reid glaciations

volatiles in basaltic glasses from a subglacial volcano in northern british columbia canada implications for ice sheet thickness and mantle volatiles

models of ice melting and edifice growth at the onset of subglacial basaltic eruptions 1 models of the early stages of basaltic eruptions beneath temperate glaciers are presented that consider the evolving sizes of volcanic edifices emplaced within subglacial cavities the cavity size reflects the competing effects of enlargement by melting and closure by downward ductile deformation of the ice roof which occurs when the cavity pressure is less than glaciostatic due to meltwater drainage eruptions of basaltic magma from fissures and point sources are considered which form either hemicylindrical or hemispherical cavities the rate of roof closure can therefore be estimated using nye s law the cavity size edifice size and depth of meltwater above the edifice are predicted by the model and are used to identify two potential eruption mechanisms explosive and intrusive when the cavity is considerably larger than the edifice hydroclastic fragmentation is possible via explosive eruptions with deposition of tephra by eruptionfed aqueous density currents when the edifice completely fills the cavity rising magma is likely to quench within waterlogged tephra in a predominantly intrusive manner the models were run for a range of magma discharge rates ice thicknesses and cavity pressures relevant to subglacial volcanism in iceland explosive eruptions occur at high magma discharge rates when there is insufficient time for significant roof closure the models correctly predict the style of historic and pleistocene subglacial fissure eruptions in iceland and are used to explain the contrasting sedimentology of basaltic and rhyolitic tuyas the models also point to new ways of unraveling the complex coupling between eruption mechanisms and glacier response during subglacial eruptions

the 1996 eruption at gjalp vatnajokull ice cap iceland efficiency of heat transfer ice deformation and subglacial water pressure the 13 day long gjlp eruption within the vatnajkull ice cap in october 1996 provided important data on ice volcano interaction in a thick temperate glacier the eruption produced 0 8 km 3 of mainly volcanic glass with a basaltic icelandite composition equivalent to 0 45 km 3 of magma ice thickness above the 6 km long volcanic fissure was initially 550 750 m the eruption was mainly subglacial forming a 150 500 m high ridge only 2 4 of the volcanic material was erupted subaerially monitoring of the formation of ice cauldrons above the vents provided data on ice melting heat flux and indirectly on eruption rate the heat flux was 5 6 10 5 wm 2 in the first 4 days this high heat flux can only be explained by fragmentation of magma into volcanic glass the pattern of ice melting during and after the eruption indicates that the efficiency of instantaneous heat exchange between magma and ice at the eruption site was 50 60 if this is characteristic for magma frag mentation in subglacial eruptions volcanic material and meltwater will in most cases take up more space than the ice melted in the eruption water accumulation would therefore cause buildup of basal water pressure and lead to rapid release of the meltwater continuous drainage of meltwater is therefore the most likely scenario in subglacial eruptions under temperate glaciers deforma tion and fracturing of ice played a significant role in the eruption and modified the subglacial water pressure it is found that water pressure at a vent under a subsiding cauldron is substantially less than it would be during static loading by the overlying ice since the load is partly compensated for by shear forces in the rapidly deforming ice in addition to intensive crevassing due to subsidence at gjlp a long and straight crevasse formed over the southernmost part of the volcanic fissure on the first day of the eruption it is suggested that the feeder dyke may have overshot the bedrock ice interface caused high deformation rates and fractured the ice up to the surface the crevasse later modified the flow of meltwater explaining surface flow of water past the highest part of the edifice the dominance of magma fragmentation in the gjlp eruption suggests that initial ice thickness greater than 600 700 m is required if effusive eruption of pillow lava is to be the main style of activity at least in similar eruptions of high initial magma discharge

glaciovolcanic emplacement of an intermediate hydroclastic breccia lobe complex during the penultimate glacial period 190 130 ka ruapehu volcano new zealand

north american ice sheet reconstructions at the last glacial maximum abstract the areal extent of the last glacial maximum lgm ice sheets is well known in north america but there is no direct geological proxy for ice sheet thickness or volume uncertainties associated with glaciological and geophysical reconstructions give widely varying estimates of north american ice sheet nais volume at lgm in an effort to quantify the uncertainty associated with glaciological reconstructions we conducted a suite of 190 numerical simulations of the last glacial cycle in north america prescribing different climatic mass balance glaciologic and isostatic treatments for the least constrained model variables lgm ice sheet reconstructions were evaluated using the well established geologic record of ice sheet area and southern extent at lgm dyke and prest 1987 these constraints give a subset of 33 simulations that produce reasonable lgm ice cover in north america ice sheet dispositions and the associated parameter settings in this subset of tests provide insight into the plausible range of nais thickness form and mass balance regime at lgm ice volume in this subset of tests spans a range of 28 5 38 9 10 15 m 3 at lgm with a predominant cluster at 32 36 10 15 m 3 taking floating ice and displaced continental water into account this corresponds to 69 94 m eustatic sea level msl more than 75 of the accepted tests fall in the range 78 88 msl we argue that this is a plausible estimate of the volume of water locked up in the nais at lgm

volcano ice interactions in the arsia mons tropical mountain glacier deposits fan shaped deposits fsd superposed on the sides of the tharsis montes volcanic edifices are widely interpreted to have been formed by cold based glaciation during the late amazonian a period when the tharsis montes were volcanically active we survey the similar to 166 000 km 2 arsia mons fsd using new high resolution image and topography data and describe numerous landforms indicative of volcano ice interactions these include 1 steep sided mounds morphologically similar to terrestrial tindar that form by subglacial eruptions under low confining pressure 2 steep sided leveed flow like landforms with depressed centers interpreted to be subglacial lava flows with chilled margins 3 digitate flows that we interpret as having resulted from lava flow interaction with glacial ice at the upslope margin of the glacier 4 a plateau with the steep sides and smooth capping flow of a basaltic tuya a class of feature formed when subglacial eruptions persist long enough to melt through the overlying ice and 5 low areally extensive mounds that we interpret as effusions of pillow lava formed by subglacial eruptions under high confining pressure together these eruptions involved hundreds of cubic kilometers of subglacially erupted lava thermodynamic relationships indicate that this amount of lava would have produced a similar volume of subglacial liquid meltwater some of which carved fluvial features in the fsd landforms in the fsd also suggest that glaciovolcanic heat transfer induced local wet based flow in some parts of the glacier glaciovolcanic environments are important microbial habitats on earth and the evidence for widespread liquid water in the amazonian aged arsia mons fsd makes it one of the most recent potentially habitable environments on mars such environments could have provided refugia for any life that developed on mars and survived on its surface until the amazonian

the hyaloclastite ridge formed in the subglacial 1996 eruption in gjalp vatnajokull iceland present day shape and future preservation

the relative importance of supraglacial versus subglacial meltwater escape in basaltic subglacial tuya eruptions an important unresolved conundrum the hydraulic behaviour of meltwater during subglacial basaltic eruptions in temperate ice is of paramount importance in understanding the eruptive processes lithofacies and architecture of the edifices formed hydraulics also determines the timing location and volume of meltwater discharge which may be sudden and catastrophic and via subglacial and or supraglacial routes increasing our knowledge of meltwater hydraulics is therefore important for understanding predicting and mitigating the impact of meltwater release on vulnerable human communities new observations about eruption related meltwater hydraulics are presented for well exposed glaciovolcanic lava fed deltas on james ross island antarctica and accounts of historical eruptions are also re examined to identify the major meltwater discharge routes the study provides the first conceptual model for how meltwater escapes supraglacially in the absence of a crevassed layer which will dominate any meltwater flow overflowing may be initiated by enhanced rates of seepage despite the intrinsically low hydraulic conductivities of snow and firn once overflowing is established the rate of spillway incision is a likely overriding control on the evolution of the system and whether the discharge is unstable fast or stable slower the james ross island sequences demonstrate that meltwater discharge is highly dynamic and may have involved both subglacial and supraglacial escape subglacial discharge probably occurs throughout basaltic tuya eruptions but some periods may be dominated by concurrent overflowing it is still unclear if overflowing systems are sufficiently stable to enable the growth of laterally extensive glaciovolcanic lava fed deltas

constraints on the dynamics of subglacial basalt eruptions from geological and geochemical observations at kverkfjoll ne iceland the kverkfjoll area ne iceland is characterised by subglacial basalt pillow lavas erupted under thick ice during the last major glaciation in iceland the water contents of slightly vesiculated glassy rims of pillows in six localities range from 0 85 0 03 to 1 04 0 03 wt the water content measurements allow the ice thickness to be estimated at between 1 2 and 1 6 km with the range reflecting the uncertainty in the co2 and water contents of the melt the upper estimates agree with other observations and models that the ice thickness in the centre of iceland was 1 5 2 0 km at the time of the last glacial maximum many of the pillows in the kverkfjoll area are characterised by vesiculated cores 40 60 vesicles surrounded by a thick outer zone of moderately vesicular basalt 15 20 vesicles the core contains 1 mm diameter spherical vesicles distributed uniformly this observation suggests a sudden decompression and vesiculation of the still molten core followed by rapid cooling the cores are attributed to a jokulhlaup in which melt water created by the eruption is suddenly released reducing the environmental pressure mass balance and solubility relationships for water allow a pressure decrease to be calculated from the observed change of vesicularity of between 4 4 and 4 7 mpa depressurization equivalent to a drop in the water level in the range 440 470 m consideration of the thickness of solid crust around the molten cores at the time of the jokulhlaup indicates an interval of 1 3 days between pillow emplacement and the jokulhlaup upper limits for ice melting rates of order 10 3 m s are indicated this interpretation suggests that jokulhlaups can reactivate eruptions

subglacial intermediate volcanism at kerlingarfjoll iceland magma water interactions beneath thick ice the products of andesite and dacite glaciovolcanism at kerlingarfjoll are unlike others previously described in the literature three sequences of lithofacies are described and interpreted here the andesitic deposits at campsite gully are divided into massive vitriclastic lapilli tuff with intrusions fluidal clast bearing vitriclastic lapilli tuff and stratified pumice rich vitriclastic lapilli tuff at tindur andesitic eruptions produced contorted pillow fragment breccia and the clast supported lithic breccia finally pillow lava with intrusions and crudely bedded vitriclastic lapilli tuff of dacitic composition are described at haraldur abundant vitriclasts the presence of pillow lavas hackly fracturing of lava bodies and lack of oxidation of clasts demonstrate that each of these lithofacies formed in the presence of abundant water this contrasts with all other descriptions of subglacially erupted intermediate magmas which are characterised by jointed and glassy lava flows and domes with a marked scarcity of fragmental material the kerlingarfjoll sequences therefore demonstrate that it is possible for intermediate magmas to generate and interact with significant volumes of water at the base of a glacier preliminary estimates of volatile contents in glassy clasts correspond to quenching pressures equivalent to 500 m water or 550 m ice this is consistent with eruption beneath an ice sheet that was thick enough to overwhelm the underlying topography and where meltwater drainage was controlled by the morphology of the glacier surface it is argued that the drainage of water due to steep topography and or thin and fractured ice as opposed to thermodynamic considerations is the most likely explanation for the absence of evidence for significant magma water interaction in previously described instances of intermediate glaciovolcanism the apparent low viscosity of the kerlingarfjoll magmas may relate to relatively high eruption temperatures and or the inhibition of degassing of the magma due to high ambient pressures and the consequent limitation of groundmass crystallisation

morphometric study of pillow size spectrum among pillow lavas measurements of h and v dimensions in the horizontal and vertical directions of pillows exposed in vertical cross section were made on 19 pillow lavas from the azores cyprus iceland new zealand tasmania the western usa and wales the median values of h and v plot on a straight line that defines a spectrum of pillow sizes having linear dimensions five times greater at one end than at the other basaltic toward the small size end and andesitic toward the large size end the pillow median size is interpreted to reflect a control exercised by lava viscosity pillows erupted on a steep flow foot slope in lava deltas can however have a significantly smaller size than pillows in tabular pillowed flows inferred to have been erupted on a small depositonal slope indicating that the slope angle also exercised a control pipe vesicles generally abundant in the tabular pillowed flows and absent from the flow foot pillows have potential as a paleoslope indicator pillows toward the small size end of the spectrum are smooth surfaced and grew mainly by stretching of their skin whereas disruption of the skin and spreading were important toward the large size end disruption involved increasing skin thicknesses with increasing pillow size and pillows toward the large size end are more analogous with toothpaste lava than with pahoehoe and are inferred from their thick multiple selvages to have taken hours to grow pseudo pillow structure is also locally developed an example of endogenous pillow lava growth that formed intrusive pillows between normal pillows is described from sicily isolated pillow like bodies in certain andesitic breccias described from iceland were previously interpreted to be pillows but have anomalously small sizes for their compositions it is now proposed that they may lack an essential attribute of pillows namely the development of bulbous forms by the inflation of a chilled skin and are hence not true pillows para pillow lava is a common lava type in the flow foot breccias it forms irregular flow sheets that are locally less than 5 cm thick and failed to be inflated to pillows perhaps because of an inadequate lava supply rate or too high a flow velocity

intrusion of basalt into frozen sediments and generation of coherent margined volcaniclastic dikes cmvds coherent margined volcaniclastic dikes cmvds are described for the first time from askja dyngjufjoll iceland these dikes display continuous coherent glassy margins 5 cm thick and have a variety of clastic interiors comprising vitric tephra pillow and pillow fragment bearing lapilli tuffs cmvds are interpreted to form when basaltic dikes interact with subglacial ice cemented volcaniclastic sediments formation requires chilling of dike margins drainage of the dike interior and flooding of the cavity by meltwater followed by non explosive mingling of magma clastics and water evidence for ice cementation includes minimal interaction with the host 3 mm peperite local pillowed dike margins and the preservation of continuous glassy coherent margins along the edge of a drained dike cavity cmvds should be present at other volcanic centers with ice cemented sediments including the martian cryosphere

satellite remote sensing of cloud base height for convective cloud fields a case study 1 a method is proposed for estimating base heights of convective clouds from satellite data the approach takes advantage of the fact that convective water clouds appear as geometrically and optically thin clouds near an approximately constant condensation level in their earliest stage of growth and that deriving geometrical thicknesses for such thin clouds is less error prone striking is the fact that the method also provides the base height for clouds with large vertical extensions and high optical thicknesses the method has been applied to noaa avhrr data of 20 selected cloud scenes for an evaluation satellite retrieved cloud base heights have been compared to surface ceilometer measurements at the same time first results are encouraging the standard deviation of the differences between satellite and ceilometer measurements is 369m with no systematic bias