field evidence for flank instability basal spreading and volcano tectonic interactions at mt cameroon west africa the mt cameroon volcano is the highest and most active volcano of the cameroon volcanic line little geological information is available for improving the understanding of the structure of this large volcanic system and its relationship to regional tectonics after reviewing the tectonic evolution of the region the analysis of a digital elevation model and results from a field campaign dedicated to mapping geological structures in the summit area and at the se base of mt cameroon are presented mt cameroon is a lava dominated volcano with long steep over 30 flanks it is elongate parallel to its well defined rift zone the summit plateau is bordered by 10 m high cliffs formed by summit subsidence along normal faults geological profiles were measured along rivers cutting through a topographic step at the se base of mt cameroon this step is associated with deformed miocene sediments from the douala basin that are overlain by volcanic products weak sediments of this area are deformed by 050 060 and 020 030 trending asymmetrical folds verging toward the se and thrusts faults related to the spreading of the volcano over its mechanically weak substratum combined remote sensing and field observations suggest that spreading is accommodated by summit subsidence and flanks sliding both slow spreading movements and catastrophic collapses of the steep flanks are interpreted to result from complex interactions between the growing edifice repeated dyke intrusions the weak sedimentary substratum and tectonic structures

a sagging spreading continuum of large volcano structure gravitational deformation strongly infl uences the structure and eruptive behavior of large volcanoes using scaled analog models we characterize a range of structural architectures produced by volcano sagging and volcano spreading these arise from the interplay of variable basement rigidity and volcano basement de coupling from comparison to volcanoes on earth la reunion and hawaii and mars elysium and olympus montes the models highlight a structural continuum in which large volcanoes throughout the solar system lie

directional flank spreading at mount cameroon volcano evidence from analogue modeling mount cameroon is characterized by an elongated summit plateau steep flanks and topographic terraces around its base although some of these features can be accounted for by intrusion induced deformation we here focus on the contribution of edifice scale gravitational spreading in the structure of mount cameroon we review the existing geological and geophysical data and morphostructural features of mount cameroon and surrounding sedimentary basins volcanic ridge gravitational spreading is then simulated by scaled analogue models on which fault formation is recorded using digital image correlation three sets of models are presented i models recorded in cross section type i ii models recorded from above with a uniform type iia and iii nonuniform ductile layer type iib type i models illustrate the formation of faults accommodating summit subsidence and lower flank spreading type iia models favor displacement perpendicular to the long axis with formation of a summit graben and basal folds but fail to reproduce the steep flanks type iib models investigate the effect of spatial variations in sediment thickness and or properties consistent with geological evidence directional spreading of the volcano s central part perpendicular to the long axis is accounted for by a sediment layer with restricted lateral extent and increasing thickness away from the volcano axis the later model closely reproduces key features observed at mount cameroon steep upper flanks are accounted for by enhanced lateral spreading of the lower flanks relative to the summit the relevance of these findings for understanding flank instabilities at large oceanic volcanoes is finally highlighted

vertical and lateral collapse of tharsis tholus mars tharsis tholus a more than 3 9 ga old composite shield volcano to the east of the major tharsis montes has experienced a complex history of growth and destruction on the basis of new high resolution images we analysed the morphology as well as the tectonic structures of the tharsis tholus volcano in detail from morphological data cross cutting relations of the surface structures and crater modelling ages we propose a chronostratigraphy for the volcano tectonic history of tharsis tholus the strongly faulted volcano reveals two large scale landslide events followed by two subsequent shield re growth phases between 3 8 and 1 7 ga and two caldera collapses tharsis tholus was also affected by regional extensional tectonics between 1 7 ga and 0 4 ga recorded by sub parallel sets of ne trending graben structures the steep and up to 5 4 km high landslide scarps on tharsis tholus suggest deep faulting of the edifice in order to confirm this hypothesis we used analogue sand box models in which we demonstrated that gravitational flank movement on top of weak basal substrata may have produced the deformation structures as observed on tharsis tholus c 2011 elsevier b v all rights reserved

eruptive and earthquake activities related to the 2000 eruption of mount cameroon volcano west africa abstract mount cameroon is an active volcano located in the gulf of guinea west of central africa after the march april 1999 eruption on the sw flank another eruption of the volcano occurred in 2000 it took place from three sites on the southwest flank and near the summit the first eruptive site was located 500 m to the southwest of the summit at 3900 m altitude activity on this site was mainly explosive with no lava flow the second site was located between 3220 and 3470 m altitude lava was emitted along nne sse fissures from this site and flew towards buea the main city of the area stopping 4 km from the first houses the last site was located in the south western flank at 2750 m altitude the lava ejected from an old cone near the first 1999 eruptive site was divided into two branches for a total length of around 1 km the location of active volcanic cones in 1999 and 2000 seems to be linked to the local tectonics the pre eruptive period was characterized by a seismic swarm which may be a precursor recorded in march 2000 by an analogue seismic station the main shock was a magnitude 3 2 event and was felt by the population in ekona town located on the eastern flank it had a modified mercalli intensity of iii iv when the eruption started a temporary network of short period 3 component seismic stations was set up around the volcano to improve the monitoring of seismic activity the co eruptive period from late may to september was characterized by sequences of earthquake swarms volcanic tremor and a family of earthquakes having similar waveform and appearing regularly in august and early september some of the earthquakes were felt by the population in buea and its environments the largest seismic event recorded had a magnitude of 4 during the post eruptive period from mid september to december seismicity returned to its background level of 1 3 earthquakes per 3 days hypocenter locations reveal a linear narrow structure under the summit zone which could represent the magmatic conduit of the volcano the frequency magnitude relationship revealed a b value of 1 43 higher than those previously determined but more representative of volcanic media seismic energy release was gradual after the 2000 eruption started

the influence of edifice slope and substrata on volcano spreading gravitational volcano spreading is caused by flow of weak substrata due to volcanic loading and is now a process known to affect many edifices the process produces extension in the upper edifice evidenced by graben and normal faults and compression at the base seen in strike slip faults and thrusts where spreading is identified host volcanoes have a range of fault densities variable rift and graben shapes and different degrees of structural asymmetry previous studies have suggested a link between edifice shape and structure and the proportion of brittle to ductile material in the substrata or lower edifice we study this link using refined sand cone analogue models standing on a brittle ductile sand silicone substrata two scenarios have been investigated the first mainly represents oceanic volcanoes with a ductile layer within the edifice type i where there is an outer ductile free surface the second represents most continental volcanoes that have ductile substrata type ii we apply the model results to natural examples and develop quantitative relationships between slope brittle ductile ratio fault density spreading rate and structural style displacement fields calculated from stereophotogrammetry show significant differences between different slope models we find that more faults are produced when the cone is initially steeper or when the brittle substratum is thinner however the effect of the brittle layer dominates over that of slope the strike slip movements are found to be an essential feature in the spreading mechanism and the graben are in fact transtensional features strike slip and graben faults make a conjugate flower pattern the structures produced are well organised for type ii edifices but they are poorly organised for type i models type i models represent good analogues for oceanic volcanoes that are commonly affected by large slumps bounded by an extensional zone and lack of well formed sector graben the well observed connection between oceanic volcano rifts and large landslide slumps is confirmed to be a consequence of spreading

landslides and spreading of oceanic hot spot and arc shield volcanoes on low strength layers lsls an analogue modeling approach during their growth and evolution oceanic hot spot and arc shield volcanoes are destabilized and often dismantled by landslides the origin of these phenomena can be found in the structure of volcanic islands marked by several low strength horizons such as a marine substratum hyaloclastites hydrothermally altered rocks or volcaniclastic deltas that weaken the edifice we have tested the role of these low strength layers lsls with analogue modeling we conclude that destabilization on these levels can lead to landslides or gravitational spreading in our experiments slides generated are only slump type events but they can evolve in nature into debris avalanches through mechanisms that could not be modeled in the laboratory the role of fan shaped volcaniclastic deltas included in the edifice structure is critical to slump formation this process is more efficient if deltas are associated with another upper lsl such as weak hyaloclastites or hydrothermally altered rocks in this case recurrent events generating collapses with increasing width are observed gravitational spreading of a volcanic island occurs when it is underlain by a ductile marine substratum or includes an intra edifice unconstrained hyaloclastic layer 1 3 as thick as the height of the subaerial edifice spreading is characterized by edifice stretching and subsidence and does not generate major landslides we also show that gravitational spreading on a marine substratum tends to inhibit slide generation especially when the thickness of the sedimentary layer is larger than 1 10 of the edifice total height however if this value is lower spreading occurs later and slumps can develop initially we describe natural examples of oceanic hot spot or arc shield volcanoes affected by both spreading and landslides such as reunion island hawaii island james ross island antarctica and martinique island antilles the main structural features on these volcanoes can be explained solely by deformation on lsls we therefore propose a general model for the destabilization of volcanic islands that is applicable to all shield like edifices

morphometry and evolution of arc volcanoes volcanoes change shape as they grow through eruption intrusion erosion and deforma tion to study volcano shape evolution we apply a comprehensive morphometric analysis to two contrasting arcs central america and the southern central andes using shuttle radar topography mission srtm digital elevation models we compute and defi ne parameters for plan ellipticity irregularity and profi le height width summit basal width slope shape as well as size height width volume we classify volcanoes as cones sub cones and massifs and recognize several evolutionary trends many cones grow to a critical height 1200 m and volume 10 km 3 after which most widen into sub cones or massifs but some grow into large cones large cones undergo sector collapse and or gravitational spreading without sig nifi cant morphometry change other smaller cones evolve by vent migration to elliptical sub cones and massifs before reaching the critical height the evolutionary trends can be related to magma fl ux edifi ce strength structure and tectonics in particular trends may be controlled by two balancing factors magma pressure versus lithostatic pressure and conduit resistance versus edifi ce resistance morphometric analysis allows for the long term state of individual or volcano groups to be assessed morphological trends can be integrated with geological geophysical and geochemical data to better defi ne volcano evolution models

importance of gravitational spreading in the tectonic and volcanic evolution of mount etna the interaction between gravity and thermal effects largely determines the structural and magmatic evolution of volcanic constructs at scales spanning several orders of magnitude from small cones to the oceanic crust1 3 although gravitational spreading is a direct consequence of this interaction and a fundamental process in volcano growth it is rarely recognized as such here we describe a striking example of this process at etna volcano in italy the volcano and its clay rich substratum are slowly spreading towards the east and south driven by gravity spreading produces extensional structures in the summit region and contpressional structures at the base of the volcano eastward movement of the volcanic edifice over a stationary magma supply may be the cause of an apparent westward migration of volcanic activity as gravitational spreading seems to control the location and magnitude of shallow seismicity and flank eruptions an appreciation of its effects could become an essential element of future volcanic hazard evaluation our model proposed here for etna may also be relevant to a reinterpretation of the geological history of a number of other volcanoes

scaled experiments of volcanic spreading experiments were conducted to study the spreading of volcanic constructs volcanoes are simulated by a sand cone and the volcanic substratum is simulated by a sand layer brittle substratum overlying a silicone layer ductile substratum similarity conditions between natural volcanoes and experimental prototypes led to the definition of dimensionless numbers experiments determine values which predict whether or not spreading takes place of particular importance are the ratio between the thickness of the brittle substratum and the height of the volcano 2 and the brittle ductile ratio of the substratum 3 2 indicates that the volcano must be large enough to break the substratum before spreading occurs whereas 3 controls the style of deformation during spreading these dimensionless numbers change with time reaching values that tend toward those observed for stable configurations experimental values are compared with those from well constrained natural examples it is found that an essential requirement for volcanic spreading is the presence of a low viscosity layer within the substratum flow of the weak layer away from the excess load is responsible for the spreading the overlying edifice displays radial intersecting grabens due to concentric stretching dissected summit areas concentric zones of thrusts and folds form in the substratum around the edifice and diapirs of the ductile substratum rise within the fault zones

volcanic spreading and lateral variations in the structure of olympus mons mars the olympus mons volcano on mars is notable not only for its immense height and width but also for substantial asymmetries in its structure the gently sloped northwest flank extends to a much greater distance from the central caldera complex than the more steeply sloped southeast flank furthermore the northwest flank exhibits lower flank extensional faults whereas the southeast shows upper flank compressional terraces and lower flank upthrust blocks however both the northwest and southeast flanks exhibit characteristic concave upward profiles and steep bounding scarps in contrast to other sectors the nw se asymmetries are aligned with the regional slope from the tharsis rise but an understanding of the underlying causes has remained elusive we use particle dynamics models of growing spreading volcanoes to demonstrate that these flank structures could reflect the properties of the basement materials underlying olympus mons we find that basal slopes alone are insufficient to produce the observed concave upward slopes and asymmetries in flank extent and deformation style that are observed at olympus mons instead lateral variations in basal friction are required these variations are most likely related to the presence of sediments transported and preferentially accumulated downslope from the tharsis rise such sediments likely correspond to ancient phyllosilicates clays recently discovered by the mars express mission

morphology of pyroclastic cones and tectonics the relationships between morphology and spatial distribution of 1315 quaternary pyroclastic cones and coeval faulting of the volcanic substrate are analyzed in the following regions with different structural settings tepic rift mexico ethiopian rift mexican volcanic belt canary archipelago and mount etna field data and analog experiments of tephra cone emplacement and collapse enable the definition of a number of parameters which can be used to infer the geometry of the fracture feeding the magma to a pyroclastic cone the strike of the feeding plane is directly related to 1 the elongation of cone base and crater 2 the location of depressions on the crater rim and 3 the alignment of pyroclastic cones in relation to a given vent spacing in addition the strike and dip of faults affect the direction of cone breaching these relationships are valid for volcanic substrate topographic surfaces with an inclination of less than 9 and are especially sensitive to fault escarpment and cone height lava and cone density and fault orientation with respect to the dip of the volcanic substrate topography relations 1 and 2 become more pronounced for regions undergoing extensional tectonics where edifices also have a larger dimension whereas breaching in the direction of the fault dip is more widespread in regions under extension breaching along the fault strike as well as the coincidence between fault strike and vent alignment are more frequent in regions with transcurrent or transtensional tectonics

buttressing and fractional spreading of tenerife an experimental approach on the formation of rift zones 1 the island of tenerife is composed of three miocene shields which are centered by the canadas volcano since the pliocene tenerife sits on more than 2 km thick oceanic sediments quaternary volcanism of the canadas series and giant landslides were principally confined to triaxial rift zones the mechanism of triaxial rifting however has remained unclear physical analog models show that these rift zones may have formed by gravity driven lateral escape of island segments induced by loading of the deformable substratum for experiments scaled to tenerife three adjacent sand cones were mounted onto viscous pdms substratum gravitational spreading caused circumferential expansion of each cone until a large edifice canadas was constructed in their center the older cones now acted each as a buttress radial fractures were overprinted by fractional spreading of the canadas edifice this resulted in formation of three main extensional zones resembling the triaxial rifting configuration of tenerife

upper mantle structure beneath cameroon from body wave tomography and the origin of the cameroon volcanic line the origin of the cameroon volcanic line cvl a 1600 km long linear volcanic chain without age progression that crosses the ocean continent boundary in west central africa is investigated using body wave tomography relative arrival times from teleseismic p and s waves recorded on 32 temporary seismic stations over a 2 year period were obtained using a multichannel cross correlation technique and then inverted for mantle velocity perturbations the p and s wave models show a tabular low velocity anomaly directly beneath the cvl extending to at least 300 km depth with perturbations of 1 0 to 2 0 for p and 2 0 to 3 0 for s the s wave velocity variation can be attributed to a 280 k or possibly higher thermal perturbation if composition and other effects on seismic velocity are negligible the near vertical sides of the anomaly and its depth extent are not easily explained by models for the origin of the cvl that invoke plumes or decompression melting under reactivated shear zones but are possibly consistent with a model invoking edge flow convection along the northern boundary of the congo craton lithosphere if edge flow convection in the sublithospheric upper mantle is combined with lateral flow channeled along a fracture zone beneath the oceanic sector of the cvl then the oceanic sector can also be explained by flow in the upper mantle deriving from variations in lithospheric thickness

mechanical behaviour of granular materials used in analogue modelling insights from grain characterisation ring shear tests and analogue experiments abstract the mechanical behaviour of several dry granular materials is investigated through ring shear tests grain characterisation and simple analogue experiments analysed by x ray computed tomography an improved knowledge of granular materials is essential to determine their suitability as analogues for upper crustal rocks in experimental models and to compare analogue and numerical experiments the ring shear tests show that the granular materials have an elastic frictional plastic behaviour with strain hardening preceding failure at peak strength followed by strain softening until a dynamic stable value is reached this is similar to the behaviour exhibited by experimentally deformed rocks the physical characteristics of the grains determine the amount of diffuse deformation before failure the percentage of strain softening and act on the thickness of the shear zones before broadening initial shear zone width in extensional and contractional experiments is between 11 and 16 times the mean grain size the angle of internal friction defining one of the mechanical properties of granular materials and thus fault dip is not only related to physical characteristics of the grains and to the handling technique used e g sieving or pouring but also to the overburden and to the experimental setup used

shear test results for cohesion and friction coefficients for different granular materials scaling implications for their usage in analogue modelling abstract laboratory tests have been carried out on dry granular materials such as quartz sand glass microspheres and sugar with different grain size rounding and sphericity the measurements have been made with a simple shear test machine for different values of normal stress 50 900 pa shear stress has been plotted against normal stress in order to determine the cohesion and coefficient of internal friction for the investigated materials resulting values of cohesion and coefficient of internal friction are mainly dependent on rounding and sphericity while grain size has a less significant influence further the behaviour of the materials for very small normal stresses 0 400 pa is more complex than previously assumed the fracture envelopes for all materials investigated are convex outward for this small range and converge towards a straight failure envelope with increasing normal stress finally in extensional faulting experiments there is no significant change in fault dip with increasing depth therefore the non linear behaviour for small normal stresses is best described as a dependence of the cohesion on the normal stress and not as a dependence of the coefficient of internal friction on the normal stress values for cohesion increase from 0 pa 15 pa at zero normal stress to 137 247 pa 15 pa for normal stresses greater than 250 400 pa the results show that well rounded spherical material is better suited to model brittle behaviour of rocks in crustal and lithospheric scale analogue models than less well rounded material since it has a smaller cohesion and a coefficient of internal friction which is closer to values of natural rocks

gravitational spreading and formation of new rift zones on overlapping volcanoes large volcanic edifices are often shaped by the coalescence of adjacent volcanoes as well as intrusive rift zones and gravitational spreading to better understand the structure of such volcanoes we designed analogue experiments simulating gravitational spreading of an edifice made by overlapping cones of different age and examined the formation of rift zones the results allow distinction of two main rift geometries i spreading edifices of similar age that partly overlap tend to develop a rift zone approximately perpendicular to the boundary of both volcanoes such a rift zone causes two volcanoes to grow together and form an elongated topographic ridge ii partly overlapping volcanoes of different age are spreading at different rates and thus form a rift zone parallel to the boundary of both volcanoes such a rift zone causes two volcanoes to structurally separate the results are widely applicable for large volcanoes subject to rifting and flank spreading which we demonstrate for reunion island and for southern hawaii

analogue benchmarks of shortening and extension experiments we report a direct comparison of scaled analogue experiments to test thereproducibility of model results among ten different experimental modelling laboratories we present results for two experimen

post eruptive seismic activity of mount cameroon cameroon west africa a statistical analysis abstract recent seismological studies of the cameroon volcanic line show that mt cameroon is the most active centre so a permanent seismic network of six seismographs was set up in its region between 1984 and 1986 the network was reinforced with temporary stations up till 1987 and the local seismicity was studied here we emphasise a statistical analysis of seismic events recorded by the permanent seismic stations four swarms lasting 9 to 14 months are identified at intervals of 2 3 years most earthquakes are felt intensity and magnitude respectively less than vi mm and 5 during the first three swarms and a few during repose periods the main focal regions are the northwest and southeast flanks the bimbia and bioko regions in the south of the volcano hypocentres are distributed from the surface to 60 km depth indicating crustal and subcrustal activities the subcrustal events are observed only in the southeast flank they are the most regular earthquakes with a monthly frequency of 9 to 15 events they are characteristic earthquakes with magnitude 2 8 0 1 between 1984 and 1992 their yearly mean time interval between successive events range from 50 to 86 hours for that period their occurrence can be modelled as a stationary renewal process with a 3 day period but the analysis of variance shows possible significant differences among yearly means a weibull s distribution confirms that the time intervals between successive deep events are not independent and in 1993 a swarm of deep earthquakes is recorded hence a non loglinear magnitude frequency relation the deep seismicity is thought to be associated with a zone of weakness perhaps a magmatic conduit and may have some close relationship with the magmatic activity

edifice and substrata deformation induced by intrusive complexes and gravitational loading in the mull volcano scotland it is likely that the structure of a volcanic edifice can be significantly modified by deformation caused by large shallow intrusions such deformation may interact with that caused by volcano loading we explore such intrusion related and loading related deformation with field evidence and analogue models to do this we have chosen the eroded palaeogene mull volcano scotland that had a major edifice has well exposed intrusions and significant deformation there are thin mesozoic sedimentary rocks forming ductile layers below the volcano but their thickness is insufficient to allow the gravitational spreading of the volcanic edifice especially when considering that a thick lava pile covers them thus intrusive push may have been the driving force for deformation the mull activity migrated toward the northwest forming three successive intrusive complexes centres 1 2 and 3 our detailed fieldwork reveals that deformation due to these was accommodated on three levels along thrust planes in lava sequences along a decollement located in a thin clay rich sediment succession and in basement schists a relative chronology has been established between different groups of structures using dyke and sill cross cutting relationships centre 1 is surrounded by a fold and thrust belt leading to radial expansion in contrast centre 2 and 3 are connected to thrusts located to the south and east bounded by strike slip faults leading to expansion to the southeast the migration of centres and the directed sliding of the edifice may be related to the presence to the southeast of low resistance dalradian basement that failed significantly during growth of centres 2 and 3 to study the observed relationships we have carried out scaled analogue models models are made with fine powder intruded by a viscous magma analogue the models show an intimate relationship between intrusion growth uplift of the volcano and subsequent flank sliding the structures produced can be compared with mull and suggest that the centre 1 thrust belt probably formed following edifice gravitational sliding as a consequence of the uplift associated with centre 1 formation centre 2 and 3 are responsible for the sector sliding of the edifice flank toward the southeast as the magmatic complex became more asymmetric the features observed at mull and in the models are similar to those seen on active volcanoes such as etna providing a structural framework for their deformation and evolution

lithostratigraphic geophysical and morpho tectonic studies of the south cameroon shelf abstract a geological and structural sketch of the southern cameroon shelf was drawn on the basis of a high resolution seismic survey dredgings and kullenberg core sampling of the outcropping rocks at sea bed palynological and mineralogical data x ray diffractometry supported by occasional foraminiferal evidence revealed four main lithostratigraphic sets resting on a precambrian basement upper cretaceous albian to senonian paleocene lower eocene miocene pliocene and pleistocene three successive deformational types characterise the structure of this margin 1 n s trending fractures parallel to the onshore precambrian structural directions formed during the rifting period and crosscut by n60 e trending cross faults induced by the transform movement of the south atlantic 2 compressive structures governed by local stresses during the eocene transcurrent reactivation along the n60 e transform faults excluding a salt controlled deformation 3 a set of approximately n s faults affecting the unfolded sequence of the miocene pliocene or the quaternary and indicative of distensive conditions the geodynamic evolution is placed in the general framework of the central and south atlantic rifting and passive margin history interestingly the age of the compressive deformation varies from late albian to mid eocene depending on the regional plate motion events which are generated along the narrow fracture zones extending as far as or even into the continental domain

numerical experiments on the dynamics of channelised lava flows at mount cameroon volcano with the flowgo thermo rheological model abstract as for many other effusive volcanoes mount cameroon mc is a volcano for which only limited information exists on lava flow properties and emplacement dynamics for recent eruptions this study provides new quantitative constraints for rheological and dynamic characteristics of lava flow effusion for the 1982 and 2000 eruptive events used to calibrate the flowgo thermo rheological model for these lava flows the flowgo 1 d physical model is used to simulate down flow evolution of the geometry and rheology of channel contained cooling limited lava flows morphometric data from historical lava flows were acquired from the field e g channel geometry levee and background slope in order to estimate lava yield strength velocity and effusion rate lava density and viscosity were also estimated from compositional data and laboratory methods to account for uncertainty in the input rheological and geometrical data three end member scenarios were used to bracket the potential range in lava channel initial dimension initial lava temperature and phenocryst content for each of these scenarios two crustal growth models were used one assuming strong insulation due to lava flow surface crusting the other a much lower crusting rate twelve numerical simulations were made per flow and the results were compared against the channel geometry microlite content yield strength and viscosity estimates from field and laboratory investigations best fit models where obtained for both the 1982 and 2000 lava flows using a low rate of surface crusting high initial temperature and low phenocryst content model predicted lengths were within 5 of the actual lengths modelled mean effusion rates for the 1982 52 64 m 3 s 1 and 2000 10 m 3 s 1 flows closely matched field data derived estimations 26 68 and 9 5 m 3 s 1 respectively flowgo model results are highly sensitive to initial channel dimensions phenocryst content and the flowgo model is unable to reproduce the observed rapid near vent increase in microlite content near vent rapid cooling induced by lava degassing is yet to be fully documented and integrated into numerical lava flow models such as flowgo but obtained results already provide valuable insights for lava flow hazard assessment at mount cameroon

evolution of coronas in norwegian anorthosites re evaluation based on crystal chemistry and microstructures coarse grained garnet pyroxene coronas in anorthositic gabbros of the bergen and sognefjord areas preserve relics of two stages of reaction during cooling from magmatic t 1 ol plag cpx i opxi spn 2 cpxi opxi spn plag cpxii opxii gnt

a new tectonic model for the cameroon line central africa abstract the cameroon line a major geological feature in central africa has been considered successively as a series of horsts and grabens a continental rift and a mega shear zone it is marked out by about 60 anorogenic complexes and a dozen volcanic centres all of which have alkaline affinity remote sensing allows us recognition of the main lineament trends n70 n s n135 and e w while autocorrelation analysis reveals a major fault zone striking n30 in western cameroon and n15 in the northern region a mega left lateral shear zone is the model that best accounts for the fracture pattern and associated features such as linear and circular structures alignment of subvolcanic complexes syntectonic leucogranites marking out older shear zones and vein dykes the n70 adamawa fault zone a pan african fracture reworked during albian aptian times is the only shear zone of continental scale that could have initiated en echelon mega tension gashes within the cameroon line during a cainozoic left lateral transcurrent movement