reinterpretation of moraines at 4000 m in the mount kenya afroalpine area abstract moraines at 4000 m on mount kenya have been assigned different radiocarbon ages ranging from the late pleistocene to the mid holocene by various investigators some workers consider that these moraines result from advances of holocene ice out of local cirques and still others from stillstand following recession of late pleistocene glaciers new evidence presented here shows that moraines at 4000 m a s l are the result of stillstand following recession of liki wurm wisconsinan ice on mount kenya the minimum radiocarbon ages of bog and lacustrine sediments associated with several moraines in teleki hausberg and liki north valleys are approximately 12 000 yr b p placing them within the late glacial the implications for reconstruction of late pleistocene and holocene paleoclimate are that no early or mid holocene glacial advances occurred and equilibrium line altitudes ela s of most cirque glaciers fluctuated by no more than 100 m during the neollacial which began at 1000 yr b p

proglacial lacustrine sediments from el altar ecuador evidence for late holocene climatic change late holocene glacier variations on el altar ecuador have been reconstructed from proglacial lacustrine sediments variations in bulk density and organic content in sediment cores from laguna verde are interpreted as representing a record of the neoglacial activity of the obsipo glacier two sedimentological units of laminated silt and clay represent two late holocene episodes of glacier expansion which probably occurred during the last millennium and tentatively suggest a double little ice age signal reconstructed equilibrium line altitudes on the obispo glacier seem to reflect changes in both precipitation and temperature and the glacial fluctuations documented on el altar are consistent with other evidence of environmental change from northern south america

quaternary glaciation in africa key chronologies and climatic implications multiple episodes of quaternary glaciation are evidenced on 10 distinct mountain localities throughout africa with the best dated sites from kilimanjaro and mt kenya in equatorial east africa a general paucity of radiogenic dates constrains the glacial chronology and regional sequences have largely been based on correlations by relative weathering of features excellent glacial moraine preservation and other features of erosion put limits on the spatial extent of palaeoglaciers and have facilitated advances in palaeo equilibrium line altitude ela analyses all radiocarbon dates n 30 provide minimum ages of deglaciation and are consistent with a widespread synchronous advance during the last glacial maximum lgm 21 2 ka k ar dates on kilimanjaro lava flows initially recognised up to three glaciations prior to the lgm over 100 cosmogenic radionuclide crn 36 cl exposure ages on glacial erratics and bedrock from kilimanjaro and mt kenya date 16 landforms confirming maximum glacial extent prior to marine isotope stage mis 10 no glacial geological features have been dated to mis 6 while on kilimanjaro the crn ages confirm a clear lgm mis 2 maximum mt kenya lacks definitive lgm moraines lateglacial advances or stillstands on both mountains are suggested by low relief moraines dating to 14 6 1 2 ka two boulders on mt kenya and 16 3 1 9 ka and 15 8 2 5 ka on kilimanjaro multiple moraines upvalley have early holocene ages ranging from 11 2 0 8 to 8 6 0 2 ka isolated and equivocal dates on both mountains also provide speculative evidence that the maximum extent since mis 5e may have occurred close to 30 ka similar to sites in south america new palaeoclimatic understanding from lake and ocean proxies suggests hypotheses for the source of low latitude climate changes causing the advances that can be tested with improved dates a high degree of intra regional variance in ela is observed with differences by aspect reflecting precipitation gradients future work should include more crn dates improved mapping and inverse climate mass balance modelling copyright 2008 john wiley sons ltd

breakage and shear behaviour of intermittent rock joints abstract the breakage and shear behaviour of intermittent rock joints have been investigated in a series of direct shear tests with a new shear device specifically designed for this purpose the tests have been performed on specimens of rock like material or hard rock respectively incorporating idealized non persistent joints made up of a number of short cracks in an en echelon arrangement along the central shear axis the shear behaviour of such a joint constellation has been found to be composed of different phases the first phase of shearing is that of the actual rupture initiated by the formation of wing cracks starting from the existing cracks and growing into the material bridges and concluded by the generation of additional new fractures connecting the initial cracks in the zone between the wing cracks the second phase of shearing is characterized by friction processes and volume increase in the then continuous shear zone finally the third phase of shearing reached after large shear displacements is determined by sliding processes inside the strongly fractured shear zone in a large number of shear tests the geometrical parameters of the discontinuous joints as well as the loading conditions have been found to influence the activated shear resistance in each phase of shearing to a noticeably different extent the orientation of the initial cracks and the normal stress however have been identified as the most influential parameters depending on the test conditions an initially discontinuous rock joint can activate the largest shear resistance not just before rupture but in one of the two subsequent phases of shearing as well the mechanisms which govern the different shear phases could be identified as 1 tensile rupturing 2 rolling and sliding friction of dilatant joint zones and 3 sliding within the joint filling composed of brecciated material

numerical analysis of initiation and progressive failure in natural rock slopes the 1991 randa rockslide the 1991 randa rockslide in the swiss alps involved several complex mechanisms relating to geological mechanical and hydrological processes for which no clear trigger can be asserted this paper investigates the concept of progressive failure and the numerical modelling of rock mass strength degradation in natural rock slopes using the randa rockslide as a working example results from continuum i e finite element modelling are presented to illustrate a hypothesis suggesting that initiation of a progressive rock mass degradation process ultimately leading to failure began following deglaciation of the valley below discontinuum distinct element modelling is then applied to investigate the underlying mechanisms contributing to the episodic nature of the rockslide finally the use of a hybrid method that combines both continuum and discontinuum techniques to model fracture propagation are discussed in the context of modelling progressive slide surface development linking initiation and degradation to eventual catastrophic failure

effect of heterogeneity of brittle rock on micromechanical extensile behavior during compression loading a grain based universal distinct element code model was developed to generate a deformable polygonal grain like structure to simulate the microstructure of brittle rock it takes into account grain scale heterogeneity including microgeometric heterogeneity grain scale elastic heterogeneity and microcontact heterogeneity the microgeometric heterogeneity can be used to match the grain size distribution of the rock the discrete element approach is able to simulate the microheterogeneity caused by elastic variation and contact stiffness anisotropy the modeling approach was evaluated using lac du bonnet granite and a aspo diorite the microheterogeneity played an important role in controlling both the micromechanical behavior and the macroscopic response when subjected to uniaxial compression loading the crack initiation stress was found to be controlled primarily by the microscale geometric heterogeneity while the microcontact heterogeneity controlled the strength characteristics the effect of heterogeneity on the distribution and evolution of tensile stresses and associated extension cracks was also examined

past summer temperatures inferred from dendrochronological records of fitzroya cupressoides on the eastern slope of the northern patagonian andes

holocene glaciations and paleoclimate of mount kenya and other east african mountains abstract this paper summarizes available glacial geological data including sediments landforms paleosols and paleoclimate for major mountain areas in east africa from 1890 to the present a number of workers have tried with varying degrees of success to reconstruct holocene glacial and paleoclimatic records most of the mountains are surfaced with late pleistocene and late holocene glacial sediments which are poorly dated and in many cases stratigraphic units are only recognized by their topographic positions only on mount kenya is there an emerging glacial stratigraphy based on dating both holocene and pleistocene glacier fluctuations appear to have been broadly synchronous in the east african mountains terminal moraines on mount kenya extend to 3200 m in most drainage basins and yield minimum 14 c ages of 15 ka bp recessional moraines of liki iii age at 3700 4200 m appear to date from the late glacial later pleistocene see coetzee 1967 for a discussion while younger neoglacial see mahaney 1982 1984 for a discussion moraines are either little ice age in age or are older late holocene deposits the neoglacial sequence on mount kenya is younger than 1 ka bp while on other east african mountains it may have commenced about 2 ka bp most of holocene time appears to have been warmer and perhaps wetter than today when glaciers disappeared altogether or were distributed much as they are today

modeling the evolution of natural cliffs subject to weathering 2 discrete element approach the evolution of slopes subjected to weathering has been modeled by assuming mohr coulomb behavior and by using a numerical approach based on the discrete element method dem according to this method soil and or rock are represented by an assembly of bonded particles particle bonds are subject to progressive weakening and so the material weathering and removal processes are modeled slope instability and material movement follow the decrease of material strength in space and time with the only assumption concerning the weathering distribution within the slope first the case of cliffs subject to strong erosion weathering limited conditions and uniform weathering was studied to compare the results of the dem approach with the limit analysis approach second transport limited slopes subject to nonuniform slope weathering were studied results have been compared with experimental data and other geomorphologic models from the literature fisher lehmann and bakker le heux the flux of material from the slope is modeled assuming degradation both in space and time

rock and soil avalanches theory and simulation 1 we present a 2 d contact dynamics discrete element model for simulating initiation and motion of rock avalanches integrating hillslope geometry mohr coulomb rock behavior pore pressure before avalanche triggering and avalanche trigger avalanche motion is modeled as a dense granular flow of dry frictional and cohesive particles on the basis of granular physics and shear experiments we review some of the theories for the unexpectedly long runout of rock avalanches different causes are evoked according to the strength strong or weak of the slip surface relative to the bulk mechanical fluidization and acoustic fluidization theories state that agitation of rock particles reduces frictional strength increasing runout conversely granular mechanics suggests that as shear strain rate increases granular material becomes more agitated more dissipative and more resistant another theory states that dynamic fragmentation of clasts creates an isotropic pressure that drives longer runout in contrast granular mechanics suggests that fragmentation may induce fluidization and strengthening of the granular material while particle size reduction among others induces weakening of the granular flow and enhances long runout runout is also enhanced for column like rock masses collapsing from steep hillslopes long runout may also be linked to thermal weakening mechanisms at the slip surface e g thermal pressurization and shear melting which may lower drastically the shear strength the model is illustrated with a hypothetical example of a rain triggered avalanche mobilizing shallowly dipping layers several phases are identified including slope failure avalanche triggering resulting from slip weakening and avalanche motion in which rocks are folded and sheared

glacial and periglacial geomorphology and its paleoclimatological significance in three north ethiopian mountains including a detailed geomorphological map abstract geomorphological investigations and detailed mapping of past and present peri glacial landforms are required in order to understand the impact of climatic anomalies the ethiopian highlands show a great variety in past and contemporary climate and therefore in the occurrence of glacial and periglacial landforms however only a few mountain areas have been studied and detailed geomorphological understanding is lacking in order to allow a fine reconstruction of the impact of the past glacial cycle on the geomorphology vegetation complexes and temperature anomalies a detailed geomorphological map of three mountain areas mt ferrah amba 12 51 n 39 29 e mt lib amba 12 04 n 39 22 and mt abuna yosef 12 08 n 39 11 e was produced in all three study areas inactive solifluction lobes presumably from the last glacial maximum lgm were found in the highest study area of abuna yosef three sites were discovered bearing morainic material from small late pleistocene glaciers these marginal glaciers occurred below the modeled snowline and existed because of local topo climatic conditions evidence of such pleistocene avalanche fed glaciers in ethiopia and africa has not been produced earlier current frost action is limited to frost cracks and small scale patterned ground phenomena the depression of the altitudinal belts of periglacial and glacial processes during the last cold period was assessed through periglacial and glacial landform mapping and comparisons with data from other mountain areas taking latitude into account the depression of glacial and periglacial belts of approximately 600 m implies a temperature drop around 6 c in the last cold period this cooling is in line with temperature depressions elsewhere in east africa during the lgm this study serves as a case study for all the intermediate mountains 3500 4200 m of the north ethiopian highlands

geomorphic evidence for the late quaternary palaeoclimates of the middle kalahari of northern botswana summary geomorphological studies of caves and lakes in northern botswana since 1973 have yielded a suite of 80 radiometric dates which permit palaeoenvironmental reconstruction of the landscape over the last 45 000 years the unique characteristics of the landforms when interpreted with caution yield palaeoclimatic information on both northern botswana and the catchments of the okavango river in the angolan highlands several major shifts in precipitation are indicated for the period 45 000 20 000 bp with dry conditions prevailing at 25 000 bp the late glacial was characterised by humid conditions especially from 16 000 13 000 bp whilst the early holocene was semi arid with precipitation peaks circa 6 000 5 000 bp and 2 000 bp the results contrast sharply with some southern african palaeo climatic descriptions in print where many of the inferences have been drawn from studies of the northern hemisphere inter tropical zone the implications of the study are briefly discussed in relation to palaeoclimatic models of the whole african continent

geological discontinuity persistence implications and quantification abstract persistence of geological discontinuities is of great importance for many rock related applications in earth sciences both in terms of mechanical and hydraulic properties of individual discontinuities and fractured rock masses although the importance of persistence has been identified by academics and practitioners over the past decades quantification of areal persistence remains extremely difficult in practice trace length from finite outcrop is still often used as an approximation for persistence this paper reviews the mechanical behaviour of individual discontinuities that are not fully persistent and the implications of persistence on the strength and stability of rock masses current techniques to quantify discontinuity persistence are then examined this review will facilitate application of the most applicable methods to measure or predict persistence in rock engineering projects and recommended approaches for the quantification of discontinuity persistence furthermore it demonstrates that further research should focus on the development of persistence quantification standards to promote our understanding of rock mass behaviours including strength stability and permeability

holocene vegetation zonation in the eastern sahara recent discoveries of fossil bearing holocene lake sediments from the eastern sahara have brought further confirmation and detail to earlier indirect evidence that a major pluvial episode occurred between 9 500 and 4 500 yr bp1 3 the botanical results mainly palynological show that savanna and desert grassland occupied regions that today are plantless hyperarid deserts we present here the first evidence from the entire saharan region of a latitudinal zonation of holocene vegetation extending across a 500 km wide belt in north west sudan we have completed detailed pollen analyses of three of the several sites discovered so far and the salient features from two the northernmost and the southernmost are presented here to demonstrate a steep gradient of past vegetative cover

holocene and latest pleistocene alpine glacier fluctuations a global perspective abstract alpine glacier fluctuations provide important paleoclimate proxies where other records such as ice cores tree rings and speleothems are not available about 20 years have passed since a special issue of quaternary science reviews was published to review the worldwide evidence for holocene glacier fluctuations since that time numerous sites have been discovered new dating techniques have been developed and refined climatic hypotheses have been proposed that contribute to a better understanding of earth s climate system this special volume includes 12 papers on holocene and latest pleistocene alpine glacier fluctuations that update the seven review papers from 1988 major findings of these 12 papers include the following many but certainly not all alpine areas record glacier advances during the younger dryas cold interval most areas in the northern hemisphere witnessed maximum glacier recession during the early holocene with some glaciers disappearing although a few sites yield possible evidence for advances during the 8 2 ka cooling event in contrast some alpine areas in the southern hemisphere saw glaciers reach their maximum post glacial extents during the early to middle holocene in many parts of the globe glaciers reformed and or advanced during neoglaciation beginning as early as 6 5 ka neoglacial advances commonly occurred with millennial scale oscillations with many alpine glaciers reaching their maximum holocene extents during the little ice age of the last few centuries although the pattern and rhythm of these glacier fluctuations remain uncertain improved spatial coverage coupled with tighter age control for many events will provide a means to assess forcing mechanisms for holocene and latest pleistocene glacial activity and perhaps predict glacier response to future impacts from human induced climate change

modelling progressive failure in fractured rock masses using a 3d discrete element method instabilities in rock structures involve coupled mechanisms related to both deformations along existing discontinuities and brittle fracture of intact rock conventional kinematic and limit equilibrium techniques used to study rock slope stability suffer from oversimplifications mass strength degradation and progressive failure mechanisms in rock bridges cannot be ignored and must be considered to predict the overall slope behaviour a 3d numerical model based on the discrete element method has been developed to overcome these limitations pre existing discontinuities as a discrete fracture network dfn can be initially plugged into a set of discrete elements combined with the use of a modified contact logic which provides an explicit representation of joints both fracturing of intact material and yielding within discontinuities can therefore be reproduced depending on the loading conditions and material strength simulations of referenced experimental tests are presented here to show the capabilities of the model in tackling the failure mechanisms of intact rock in the presence of pre existing discrete fractures with an emphasis on the initiation and propagation processes this model proves to be a promising tool in understanding and predicting instabilities that could lead to the failure of fractured rock slopes

the late quaternary glaciation of africa a regional synthesis abstract there are numerous mountain ranges in africa which carry evidence of quaternary glaciation though few have glaciers now and these are rapidly disappearing the best studied sites are in east africa and ethiopia in east africa the three highest mountains kilimanjaro kenya and the rwenzori each have evidence of 3 5 major glaciations dating back to an estimated 400 000 bp one of which is dated to the last glacial maximum lgm by radiocarbon or cosmogenic radionucleide crn methods two other mountains have similar large moraine systems inferred to date to the lgm in ethiopia three mountains have clear evidence of glaciation and on two of them radiocarbon dates give minimum ages which suggest that these are lgm features in north africa the atlas and other groups of mountains have evidence of glaciation but in the absence of any dating it is unclear whether these sites were glaciated at the lgm although geomorphic features resembling moraines and protalus ramparts have been described from the drakensberg mountains south africa it is still unclear whether these features indicate glaciation and can be attributed to the lgm thus although circumstances for fieldwork on african mountains are very difficult in many cases investigations for more than a century provide an extensive picture of quaternary glaciation in some cases there has been an increasingly detailed mapping of field evidence and the first crn dates were carried out recently thus improving the global picture considerably particularly in east africa and ethiopia there is sufficiently reliable field information to make useful estimates of equilibrium line altitude changes at the provisionally identified lgm but this age needs confirmation by further crn dating by making some hypothetical assumptions inferences can be drawn that indicate a temperature lowering of 5 6 c during the lgm elsewhere in africa further field surveys are required

pollen inferred climatic fluctuations in ethiopia during the last 3000 years abstract although lake levels fluctuations are known from ethiopia during the late holocene few continuous paleoclimatic records are available from the high plateau a 3000 yr high resolution pollen record from the arsi mountain in arussi province of southeastern ethiopia provides information on recent changes in the vegetation above the upper tree line at 3600 m elevation the organic sediment was deposited into the dega sala swamp ca 3000 yr b p on top of a coarse gravel reepresenting a solifuxion deposit linked to neoglacial the surrounding vegetation was established as that of the ericaceous belt with ericaceae shrub dominant the pollen diagram registers significant changes in the relative frequencies of the local herbaceous pollen component versus the arboreal taxa from the upper forest limit allowing the high altitude of the site such changes indicate downward movements of the vegetation belts that are due mainly to temperature variations an estimated cooling of 2 c is registered at 560 120 yr b p ams date contemporaneous with the little ice age the successive upward regrowth of the forest is rather indicative of a recent warming than linked to recolonization of the forest following anthropogenic deforestation unlikely at altitude above potential cultivation

monsoon climate of the early holocene climate experiment with the earth s orbital parameters for 9000 years ago values for the precession and obliquity of the earth 9000 years ago indicate that the global average solar radiation for july 9000 years ago was 7 percent greater than at present when the estimated solar radiation values are used in a low resulation climate model the model simulates an intensified continent scale monsoon circulation this result agrees with paleoclimatic evidence from africa arabia and india that monsoon rains were stronger between 10 000 and 5000 years ago than they are today

estimation of the joint roughness coefficient of rock joints by consideration of two order asperity and its application in double joint shear tests abstract joint roughness has a significant influence on the shear behavior of rock joints many different statistical parameters have been used to estimate the joint roughness coefficient jrc of rock joints depending on what is most easily available and convenient six statistical parameters z2 sf rp 1 log z2 logsf and r p 1 of ten typical roughness profiles were calculated at different sampling intervals si the results indicate that the jrc of rock joints could not be accurately estimated by using only a single statistical parameter because the first order and second order asperities of joints have different effects on shear behavior a classified and weighted fitting formula jrc 16 09 logz21st 12 70 logz22nd 33 75 si 5 0 mm 0 5 mm is proposed to estimate the jrc shear tests on sandstone joints indicate that the maximum jrc along the shear direction is appropriate to represent the total joint surface and estimate the shear strength this formula was adopted to estimate the jrc in double joint shear tests and the results show that the mechanical behavior of double parallel joints is closely related to the interlayer rock and the weaker joint under lower normal stress the interlayer rock does not fracture and the weaker joint determines the peak shear strength of the rock specimen in contrast under higher normal stress the peak shear strength is attained when the tensile fractures initiate in the interlayer rock and it has also relevancy to the jrc of double joints and interlayer thickness

simulation of the sliding process of donghekou landslide triggered by the wenchuan earthquake using a distinct element method in the village of donghekou in qingchuan county sichuan province china a major landslide was triggered by the wenchuan earthquake in 2008 with more than 10 106 m3 of rock displaced the kinematic behavior of this landslide is simulated using a 2d discrete element model the numerical model used in this work is composed of discs bonded together the initial boundary conditions are applied along the ball wall contacts by using an initial velocity estimated from the strong motion data the constraints are mainly issued from the final geometry of the landslide including its capacity to dam the river the modeling thus indicates that a low friction coefficient about 0 1 is required to account for the actual landslide characteristics the runout behaviors are analyzed and some useful conclusions are drawn

stability analysis and design charts for over dip rock slope against bi planar sliding abstract bi planar sliding is one of the main instabilities in over dip rock slope the geometry and strength of the bi planar sliding surface comprised of two sets of cross discontinuities play an important role in slope stability assessment however few studies had been conducted to systematically investigate the influences of them in this study three categories of failure modes related to the failure region namely the shallow intermediate and deep are proposed corrected implicit transfer coefficient method citcm with the modified sliding surface is proposed as an alternative to the limited equilibrium method lem for calculating the slope factor of safety fos the change patterns of slope stability are derived from the fos values calculated via citcm considering different scenarios of geometrical and mechanical parameters of the critical bi planar sliding surface the equivalent slope model for the overlap rock slope under bi planar sliding is proposed to explain slope stability change patterns reasonably the design charts for preliminary slope stability assessment are presented in the over dip rock against bi planar sliding finally compared with other traditional limited equilibrium le approaches case history engineering applications validate the proposed design charts in calculating fos determining the critical sliding surface and recognizing the failure mode is more effectively

the tsaoling landslide triggered by the chi chi earthquake taiwan insights from a discrete element simulation article i nfo in the village of tsaoling in yunlin county taiwan a major landslide was triggered by the chi chi earthquake in 1999 with more than 125 10 6 m 3 of rock displaced the kinematic behaviour of this landslide is simulated using a 2d discrete element model pfc2d code our numerical model is composed of discs bonded together the initial boundary conditions are applied along the ball wall contacts by using derived velocities integrated from the strong motion data with a duration of 160 s including the peak acceleration near tsaoling the constraints are mainly issued from the final geometry of the landslide including its capacity to cross the river valley and reach a significant elevation on the opposite mountain flank they also result from a variety of geological and hydrological observations including the local levels of material disruption and the location of survivors our modelling thus indicates that a low friction coefficient about 0 15 and a medium strength are required to account for the actual landslide characteristics a self lubrication mechanism probably accounts for the low residual friction our model also suggests that the maximum velocity of sliding reached 50 m s a result that cannot be checked in the absence of actual measurements in addition to friction the strength of sliding block is of special importance because it controlled the possibility for the upper layer fragments to roll and get buried and hence the probability of survival

modeling failure of jointed rock slope with two main joint sets using a novel dem bond contact model abstract in order to investigate the failure processes and mechanisms of jointed rock slopes distinct element method dem was applied with a novel bond contact model the bond contact model is validated by a series of numerical direct shear tests on jointed models and the comparisons to laboratory test results dem jointed rock slope models were then generated and two arrangements of joint sets and two types of slope surface were chosen to test the capabilities of the bond contact model the slope models were destabilized progressively by the critical gravity approach and both the failure onset and the slope collapse process were analyzed microscopic information reveals that the initiation of slope failure can be divided into three phases and is induced from stress concentration within the rock bridges occurrence of tension cracks and their propagation and coalescence the slope surface properties reflected by different damping affect both post failure configuration and propagation distance of the rock slope the study demonstrates that the novel bond contact model implemented in the dem code is able to tackle the fundamental problems of jointed rock slope failure and helps to better understand the slope failure mechanisms from a macroscopic and microscopic level

paleoclimatic estimates from water and energy budgets of east african lakes abstract the turkana nakuru elmenteita and naivasha basins in the rift valley of east africa experienced high water levels during the period of 10 000 7000 yr b p analyses of the modern hydrologic and energy budgets for these basins along with that of lake victoria were used to infer the amount of precipitation that would have been required to maintain the enlarged paleolakes of the early holocene precipitation must have been at least 150 300 mm yr 15 to 35 above the modern average the precipitation estimates were fairly consistent among the various basins but no quantitative estimate was made for the additional precipitation required to account for overflow from the rift valley lake basins discharge from the lake victoria basin around 1880 a d was considerably above the more recent average and the increased discharge into the white nile for that period might have been similar to that of the early holocene a sensitivity analysis showed that temperature changes were probably not too important for changing the hydrologic energy budget changes of albedo bowen ratio and cloudiness were likely to have been of greater importance

quaternary glaciations in africa abstract glaciers were active on the african mountains during several periods of the quaternary evidence exists of three old glaciations before 100 ka bp on kilimanjaro and two on mount kenya and ruwenzori the last glaciation which was synchronous with the last major glacial phase in temperate latitudes affected kilimanjaro mount kenya ruwenzori nyandarua range mount elgon the ethiopian highlands and several of the high mountains within and around the sahara desert the ice disappeared from most of these areas during the early holocene except from kilimanjaro mount kenya and ruwenzori where glaciers exist at the present day

quantitative estimates of full glacial temperatures in equatorial africa from palynological data this paper presents a new reconstruction of the mean annual temperature obtained from a high altitude pollen sequence in equatorial africa 3 28 s 29 34 e 2240 m it has been achieved with an extended modern african reference data set by adding spectra from 228 new sites and using another selection for pollen taxa the purpose of this paper is to test how the obtained temperature value depends upon the availability of modern analogues the results are in good agreement with those previously published reinforcing the validity of the method the mean standard error is reduced by 0 3 c the mean temperature for the holocene appears 1 4 c warmer than the present and the last glacial maximum 25 18 kyrs bp cooling is better specified at 3 1 9 c a conservative value more consistent with reconstructed sea surface temperature in the equatorial ocean

discrete element simulation of the jiufengershan rock and soil avalanche triggered by the 1999 chi chi earthquake taiwan we present contact dynamics discrete element simulations of the earthquake triggered jiufengershan avalanche which mobilized a 60 m thick 1 5 km long sedimentary layer dipping similar to 22 degrees se toward a valley the dynamic behavior of the avalanche is simulated under different assumptions about rock behavior water table height and boundary shear strength additionally seismic shaking is introduced using strong motion records from nearby stations we assume that seismic shaking generates shearing and frictional heating along the surface of rupture which in turn may induce dynamic weakening and avalanche triggering a simple slip weakening criterion was adopted to simulate shear strength drop along the rupture surface we investigate the mechanical processes occurring during triggering and propagation of an avalanche mobilizing shallowly dipping layers incipient deformation forms a pop up structure at the toe of the dip slope as the avalanche propagates the pop up deforms into an overturned fold which overrides the surface of separation along a decollement simultaneously uphill layers slide at high velocity 125 km h and are folded and disrupted as they reach the toe of the dip slope the avalanche foot forms a wedge that is pushed forward as deformed rocks accrete at its rear we simulated five cross sections across the jiufengershan avalanche which differ in the geometry of the surface of separation topographic and simulated surface profiles are similar the friction coefficient at the surface of separation determined from back analysis is abnormally low mu ss 0 2 possibly due to lubrication by liquefied soils the granular deposits of simulated earthquake and rain triggered avalanches are similar