influences of quaternary climatic changes on processes of soil development on desert loess deposits of the cima volcanic field california summary soils formed in loess are evidence of both relict and buried landscapes developed on pliocene to latest pleistocene basalt flows of the cima volcanic field in the eastern mojave desert california the characteristics of these soils change systematically and as functions of the age and surface morphology of the lava flow four distinct phases of soil development are recognized phase 1 weakly developed soils on flows less than 0 18 m y old phase 2 strongly developed soils with thick argillic horizons on 0 18 0 7 m y old flows phase 3 strongly developed soils with truncated argillic horizons massively impregnated by carbonate on 0 7 to 1 1 m y old flows and phase 4 degraded soils with petrocalcic rubble on pliocene flows a critical aspect of the development of stage 1 soils is the evolution of a vesicular a horizon which profoundly affects the infiltration characteristics of the loess parent materials laboratory studies show that secondary gypsum and possibly other salt accumulation probably occurred during the period of phase 1 soil development slight reddening of the interiors of peds from vesicular a horizons of phase 1 soils and presence of weakly developed b horizons indicates a slight degree of in situ chemical alteration however clay and fe oxide contents of these soils show that these constituents as well as carbonates and soluble salts are incorporated as eolian dust in contrast to phase 1 soils chemical and mineralogical analysis of argillic horizons of phase 2 soils indicate proportionally greater degrees of in situ chemical alteration these data the abundant clay films and the strong reddening in the thick argillic horizons suggest that phase 2 and phase 3 soils formed during long periods of time and periodically were subjected to leaching regimes more intense than those that now exist flow age data and soil stratigraphic evidence also indicate that several major loess deposition events occurred during the past 1 0 m y loess events are attributed to past changes in climate such as the pleistocene to holocene climatic change that periodically caused regional desiccation of pluvial lakes reduction of vegetational density and exposure of loose unconsolidated fine materials during times of warmer interglacial climates precipitation infiltrates to shallower depths than during glacial periods extensive saline playas which developed in the mojave desert during the holocene are a likely source of much of the carbonates and soluble salts that are accumulating at shallow depths both in phase 1 soils and in the formerly noncalcareous nongypsiferous argillic horizons of phase 2 and 3 soils

desert pavements and associated rock varnish in the mojave desert how old can they be desert pavements are common features of arid landscapes and have been widely used as a relative age indicator of the geomorphic surfaces upon which they are developed in this study i examined the patterns of pavement development as a function of elevation in the mojave desert as well as the causes for the gradual disappearance of pavement at high elevations pavement density as measured by percentage of pebble coverage decreases systematically with elevation gain by 3 per 100 m from 95 coverage below 500 m to less than 60 at 1700 m plants appear to be the main agent of pavement disruption plant density decreases as pavement density increases burrowing by rodents and crusting by cryptobiota also disrupt pavement development at higher elevation during the last glacial maximum plant communities were displaced 1000 1400 m downward in the mojave desert pavements today generally do not survive above the blackbush coleogyne ramossisma sagebrush artemisia tridentata zone evidence from packrat middens shows that these and other plants typical of high elevations today grew as low as 300 400 m during the last glacial maximum i suggest that during the last glacial maximum desert pavements were confined to the lowest alluvial fans of death valley and adjoining low valleys no alluvial desert pavements above 400 m in the region are older than the latest pleistocene by the same reasoning desert varnish on desert pavements above 400 m may all be holocene in age except where developed on stable boulders

desert pavement coated surfaces in extreme deserts present the longest lived landforms on earth all exposed rocks on earth s surface experience erosion the fastest rates are documented in rapidly uplifted monsoonal mountain ranges and the slowest occur in extreme cold or warm deserts millennial submeterscale erosion may be approached only in the latter the oldest previously reported exposure ages are from boulders and clasts of resistant lithologies lying at the surface and the slowest reported erosion rates are derived from bedrock outcrops or boulders that erode more slowly than their surroundings thus these oldest reported ages and slowest erosion rates relate to outstanding features in the landscape while the surrounding landscape may erode faster and be younger we present erosion rate and exposure age data from the paran plains a typical environment in the near east where vast abandoned alluvial surfaces 10 2 10 4 km 2 are covered by well developed desert pavements these surfaces may experience erosion rates that are slower than those documented elsewhere on our planet and can retain their original geometry for more than 2 m y major factors that reduce erosion converge in these regions extreme hyperaridity tectonic stability fl at and horizontal surfaces i e no relief and effective surface armoring by a clast mosaic of highly resistant lithology the 10 be concentrations in amalgamated desert pavement chert clasts collected from abandoned alluvial surfaces in the southern negev israel representing the sahara arabia deserts indicate simple exposure ages of 1 5 1 8 ma or correspond to maximum erosion rates of 0 25 0 3 m m y 1 the 36

cracks in desert pavement rocks further insights into mechanical weathering by directional insolation the formation of cracks is a fundamental first step in the physical weathering of rocks in desert environments in this study we combine new field data from the mojave u s gobi mongolia and strzelecki australia deserts that collectively support the hypothesis that meridional cracks cracks with orientations not readily attributable to rock anisotropies or shape in boulders or cobbles form due to tensile stresses caused by directional heating and cooling during the sun s daily transit the new studies indicate that rock size surface age and latitude play important roles with respect to their influence on rock fracture rock size and pavement surface age exert an influence on the development of rock cracks as the average clast size of mature desert pavements may be at or below the threshold clast size for thermal cracking of rocks latitude controlled seasonal temperature variations play a key role as demonstrated by 1 tightly clustered mean resultant orientations that differ by latitude as predicted in mcfadden et al 2005 and 2 very cold wintertime temperatures and strong diurnal gradients that may favor crack development in wintertime given the likelihood for strong clast heating during early morning hours the consistent evidence for meridional cracks in surfaces of diverse age and desert environments climate vegetation and distance of clast transport indicate that directional insolation may play the key role in initially generating and propagating rock fractures rather than a secondary role as implied in recent field and modeling studies of physical weathering in deserts

surface control of desert pavement pedologic process and landscape function cima volcanic field mojave desert california desert pavement is a distinctive feature widespread across arid lands of the world it plays a dynamic role in geomorphic hydrologic and ecologic processes where desert pavement predominates infiltration is limited and rainfall is delivered as runoff to nearby bare ground areas where shrubs cluster desert pavement surfaces may appear monotonously flat and barren but we have found instead that they are a complex association of landscape and hydrologic elements governed by their surface characteristics previously we identified six unique surface mosaic types that accurately capture the subtle but distinct variations in surface clast arrangements for a desert pavement landscape formed on a single aged basalt flow in the mojave desert we now report that these surface mosaics predict the spatial distribution of fundamental desert vegetation and soil characteristics characteristics of soil morphology and texture the leaching depth of soluble salts percent plant cover and shrub species diversity are remarkably consistent for each mosaic type across a 580 000 year old basalt flow even when measured 1 km apart hydrologic character is distinctly different between desert pavement and bare ground regions and vegetation distributions reflect the spatially heterogeneous soil moisture where desert shrubs cluster on the three bare ground surface mosaics leaching is deep removing most soluble salts to below the 50 cm depth where shrubs are absent or few on the three desert pavement mosaics leaching depths are shallow with soluble salt depth

the age of lathrop wells volcanic center an 40ar 39ar dating investigation the sup 40 ar sup 39 ar age spectrum analyses of basalt whole rock and partially degassed sanidine xenocrysts for the lathrop wells volcanic center nevada yield eruption ages of 77 3 plus minus 6 0 and 76 6 plus minus 4 9 ka 2 sigma for the ql sub 1 and ql sub 2 flows respectively basaltic samples were analyzed in four to six replicate runs and yield normally distributed plateau ages miocene ash flow tuff xenoliths were collected from the basaltic units with the majority of samples coming from the ql sub 2 the sanidines from the ql sub 2 xenoliths yielded apparent plateau ages n 10 ranging from 69 plus minus 17 to 309 plus minus 11 ka with a strong cluster n 6 at approximately 77 ka because the diffusion dimension of sanidine is approximated by the physical crystal size cracking of the sanidine following the development of an argon concentration profile may result in complex argon age spectra experimental studies exploring the natural and laboratory argon systematics of degassed sanidine reveal that crystal breakage and hf acid etching lead to anomalously old apparent ages these new ages for the ql sub 1 and ql sub 2 flows are analytically indistinguishable and agree well with published eruption ages determined by sup 3 he and sup 36 cl cosmogenic surface exposure dating methods we conclude that the ql submore 1 and ql sub 2 flows are temporally equivalent additionally the agreement between our eruption ages for these units and the published 76 plus minus 20 ka ital zreda ital et al 1993 sup 36 cl exposure age for the lathrop wells scoria cone qs sub 3 suggests that the cone may be temporally equivalent to the ql sub 1 and ql sub 2 flows we believe the simplest and most reasonable interpretation is that the volcanic center is monogenetic copyright 1999 american geophysical union less

variable deceleration of whillans ice stream west antarctica the whillans ice stream ice plain wip has been slowing since at least 1963 prior constraints on this slowdown were consistent with a constant long term deceleration rate new observations of ice velocity from 11 continuous and 3 seasonal gps sites indicate the deceleration rate varies through time including on interannual time scales between 2009 and 2012 wip decelerated at a rate 6 1 to 10 9 2 m yr2 that was double the multidecadal average 3 0 to 5 6 2 m yr2 to identify the causes of slowdown we used new and prior velocity estimates to constrain longitudinal and transverse force budget models as well as a higher order inverse model all model results support the conclusion that the observed deceleration of wip is caused by an increase in basal resistance to motion at a rate of 10 to 40 pa yr subglacial processes that may be responsible for strengthening the ice stream bed include basal freeze on changes in subglacial hydrology or increases in the area of resistant basal substrate through differential erosion the observed variability in wip deceleration rate suggests that dynamics in subglacial hydrology plausibly driven by basal freeze on and or activity of subglacial lakes plays a key role in modulating basal resistance to ice motion in the region

physical weathering in arid landscapes due to diurnal variation in the direction of solar heating despite the prominent role of physical weathering in arid and semi arid landscapes there has been little study of the specifi c processes responsible for the rapid breakdown of subaerially exposed rocks for example many boulders and cobbles in deserts exhibit fi ne near vertical cracks although workers have hypothesized that these and other cracks are initiated by diurnal heating and cooling no convincing specifi c mechanism for their formation has been proposed we have characterized these cracks at eight sites on surfaces of different ages in the mojave sonoran and chihuahuan deserts and the high desert of central new mexico our data reveal four basic types of cracks longitudinal surfaceparallel fabric related and meridional the orientations of the fi rst three types are associated with clast shape and rock fabric the azimuths of meridional cracks however are preferentially aligned north south typically with a nonrandom multimodal distribution we propose that these cracks are caused by tensile stresses that arise in the interior of clasts due to strong radial gradients in temperature that evolve and rotate in alignment with the sun s rays we suggest that the multimodal nature of crack orientations may be in part attributable to the seasonally varying latitude dependent solar elevation angle over millennial time scales we suggest that this thermal cracking is an effi cient weathering process that together with cumulic soil epipedon development creates the key attributes of most desert pavements in addition to individual clasts exposed on desert surfaces this mechanism of cracking is potentially signifi cant in other climates and on other planets as well as for rock outcrops and for man made structures

determining coastal mean dynamic topography by geodetic methods in geodesy coastal mean dynamic topography mdt was traditionally determined by spirit levelling technique advances in navigation satellite positioning e g gps and geoid determination enable space based levelling with an accuracy of about 3 cm at tide gauges recent cryosat 2 a satellite altimetry mission with synthetic aperture radar sar and sar interferometric measurements extends the space based levelling to the coastal ocean with the same accuracy however barriers remain in applying the two space based geodetic methods for mdt determination over the coastal ocean because current geoid modelling focuses primarily on land as a substitute to spirit levelling to realize the vertical datum

soil heterogeneity in mojave desert shrublands biotic and abiotic processes 1 geological and ecological processes play critical roles in the evolution of desert piedmonts feedback between fast cyclic biotic and slow cumulative pedogenic processes on arid alluvial fan systems results in a heterogeneous landscape of interspace and canopy microsites defining the spatial extent between these processes will allow a better connection to ecosystem service and climate change we use a soil chronosequence in the mojave desert and high spatial resolution infiltrometer measurements along transects radiating from canopies of perennial shrubs to assess the extent of biotic and abiotic processes and the heterogeneity of soil properties in arid shrublands results showed higher saturated conductivity under vegetation regardless of surface age but it was more conspicuous on older developed soils at proximal locations to the shrub bulk density soil structure grade silt and clay content significantly increased radially from the canopy while sand and organic material decreased soil properties at distal locations 2 5 times the canopy radius had no significant spatial correlation the extent of the biotic influence of the shrub was 1 34 0 32 times the canopy radius hydraulic properties were weakly correlated in space but 75 of the variance could be attributed to sand content soil structure grade mean particle diameter and soil organic material none of which are exclusively biotic or abiotic the fast cyclic biotic processes occurring under vegetation are clearly overprinted on slow cumulative abiotic processes resulting in the deterministic variability observed at the plant scale

desert pavement evolution an example of the role of sheetflood patches of young well developed desert pavement were found atop bare rock on the pisgah basalt flow california these particular stone mosaics formed directly atop the flow no soil is present or ever has been therefore soil expansion and aeolian processes played no significant role in their creation or maintenance the unusual setting at pisgah allows sheetflood to be the sole agent responsible for the lateral movement of surface stones into these mosaics the pisgah mosaics thus represent an end member case of desert pavement types and they may represent a previously unrecognized initial substage of accretionary mantle formation

the vesicular layer and carbonate collars of desert soils and pavements formation age and relation to climate change abstract the vesicular fine grained a horizon av is the widespread ubiquitous surficial horizon of desert soils in diverse landforms and parent materials of varying ages now known to form mostly through accumulation of eolian dust recent studies show that dust accumulation and concomitant soil development are genetically linked to stone pavement formation changes in the magnitude of eolian activity and effective leaching related to quaternary climatic changes are also hypothesized to have influenced the evolution of the av horizon numerical modeling geochronologic and field laboratory studies elucidate the nature of pedogenic processes controlling compositional evolution of av how the changing av horizon increasingly influences soil infiltration and carbonate translocation and accumulation and the control that clasts of the evolving pavement exert on pedogenic processes results of a model that determines soil bulk chemical composition based on mixing of estimated proportions of externally derived eolian material and parent materials imply that the evolution of the soil bulk composition is strongly influenced by av horizon formation the early development of a weakly to moderately developed av horizon directly over gravelly parent material in late and middle holocene soils moderately influences soil infiltration but significant leaching of very soluble materials and some carbonate in dust are permitted in older pleistocene soils however the texturally more mature av and underlying cumulic nongravelly horizons more strongly limit the rate and depth of leaching and soil bulk composition therefore more closely approximates a simple mixture of dust and parent material other aspects of av horizon development and its relations to the pavement are evaluated through studies of pavement clasts with coatings of soil carbonate referred to as carbonate collars development of a numerical model that integrates soil hydrology a co2 production diffusion model calcite kinetics and thermodynamic considerations composition and thermal characteristics of pavement clasts and the textural and structural properties of the surface horizon provides the basis for testing a hypothesis of collar formation model results combined with results of 13c and 18o analyses of collar carbonate demonstrate how precipitation of calcite on pavement clasts and within the av is favored at a depth much shallower than that indicated by the classic carbonate depth climate relationship of jenny and leonard jenny h j leonard c d 1935 functional relationships between soil properties and rainfall soil science 38 363 381 and arkley arkley r j 1963 calculations of carbonate and water movement in soil from climatic data soil science 96 239 248 or simulated by numerical models of carbonate accumulation simultaneous development of thick carbonate collars and the av horizon requires the sustained pavement clast av horizon coupling for at least centuries to possibly millennia new thermoluminescence ages also indicate that much of the av horizon formed in the holocene and that it is certainly much younger than the older pleistocene pavements this supports the previously proposed hypothesis that increased dust flux during the pleistocene to holocene transition triggered and or greatly accelerated av horizon development an understanding of the genesis of collars provides not just an understanding of how carbonate can accumulate in surface environments but it also provides important clues into processes of pavement evolution and preservation of av horizons during long glacial periods the av horizon is not merely an insignificant surficial zone of recent dust accretion instead its development profoundly influences the genesis of desert soils and pavements

quaternary soils and dust deposition in southern nevada and california eoliandustconstitutesmuchofthepedogenic material in late pleistocene and holocene soils of many arid regions of the world comparisonofthecompositionsand influx rates of modern dust with the eolian component of dated soils at 24 sites in southern nevada and california yields informationon 1 thecompositionandinflux rate of dust in late pleistocene and holocene soils 2 paleoclimate and its effects on the genesis of aridic soils especially with regard to dustfall events 3 the timing and relative contribution of dust from playa sources versus alluvial sources and 4 the effects of accumulation of dust in soil horizons the 2mmfractionsofaandbhorizons of soils formed on gravelly alluvial fan deposits in the study area are similar to moderndustingrainsize contentofcaco3and salt majoroxides andclaymineralogy thus they are interpreted to consist largely of dust the major oxide compositions of the shallow soil horizons are nearly identical to that of the modern dust but the compositions of progressively deeper horizons approachthatoftheparentmaterial theclay mineralogyofmoderndustatagivensiteis similar to that of the av horizons of nearby holocene soils but is commonly different from the mineralogies of deeper soil horizonsandoftheavhorizonsofnearbypleistocenesoils theseresultsareinterpretedto indicate that dust both accumulates and is transformed in av horizons with time changes in soil accumulation rates provide insights into the interplay of paleoclimate dustsupply andsoil formingprocesses modern dust deposition rates are more than large enough to account for middle and late holocene soil accumulation rates at nearly all sites however the early holocene soil accumulation rates in areas near late pleistocene pluvial lakes are much higher than modern rates and clearly indicate a dust deflation and deposition event that caused rapid formation offine grained shallow soil horizons on uppermost pleistocene and lower holocene deposits we interpret late pleistocene soil accumulation rates to indicate that dust deposition rates were low during this period but that increased effective moisture during the late wisconsinan favored translocation of clay andcaco3fromnearthesurfacetodeeper inthesoilprofile pre latepleistocenerates are very low in most areas mainly due to a pedogenic threshold that was crossed when accumulations of silt clay and caco3 began to inhibit the downward transport of eolian material but in part due to erosion

clast size controls and longevity of pleistocene desert pavements at lathrop wells and red cone volcanoes southern nevada formation of desert pavement and accretionary soils are intimately linked in arid environments well sorted fallout scoria lapilli at lathrop wells 75 80 ka and red cone ca 1 ma volcanoes southern nevada formed an excellent parent material for pavement allowing infiltration of eolian silt and fine sand that first clogged the pore space of underlying tephra and then aggraded and developed vesicular a av horizons variations in original pyroclast sizes provide insight into minimum and maximum clast sizes that promote pavement and soil formation pavement becomes ineffective when clasts can saltate under the strongest winds while clasts larger than coarse lapilli are unable to form an interlocking pavement that promotes silt accumulation necessary for av development contrary to predictions that all pavements above altitudes of 400 m would have been reset in their development after late pleistocene vegetation advances the soils and pavements show clear differences in maturity between the two volcanoes this indicates that either the pavements and or soils develop slowly over many tens of thousands of years and then are very stable or if they are disrupted by vegetation advances subsequent pavements are re established with successively more mature characteristics

influences of eolian and pedogenic processes on the origin and evolution of desert pavements well developed desert pavements are present above eolian deposits that mantle flows of the cima volcanic field located in the mojave desert california soil stratigraphic data and geochemical data demonstrate that eolian and pedogenic processes play major roles in the evolution of these pavements eolian dust 1 accelerates mechanical fragmentation of flow rock providing the source material for pavements and 2 accumulates slowly below basaltic colluvium in flow depressions eventually promoting development of cumulate soils below the evolving stone pavement an increase in dust flux during the holocene has raised ancient pleistocene pavements as much as 20 cm above the former land surface the results of our studies demonstrate for the first time that most desert pavements do not form by deflation by overland flow or by upward migration of stones through a slowly formed clayey argillic horizon desert pavements are born and maintained at the surface

cosmogenic 36cl dating of a young basaltic eruption complex lathrop wells nevada it has been proposed that the lathrop wells volcanic center a late quaternary basaltic complex in southern nevada has erupted more than once in common with most quaternary basalts this volcanic center has proved difficult to date by k ar and other commonly employed methods we have measured the accumulation of 36 cl in 11 samples from lava flows and volcanic bombs and obtained a combined average age of 81 7 9 ka with no systematic differences between sample subsets collected from different volcanic features the 36 cl dates do not support a history of multiple eruptions but neither do they completely preclude the possibility

40ar 39ar age of the lathrop wells volcanic center yucca mountain nevada paleomagnetic and 40 ar 39 ar analyses from the lathrop wells volcanic center nevada indicate that two eruptive events have occurred there the ages 136 8 and 141 9 thousand years ago for these two events are analytically indistinguishable the small angular difference 4 7 between the paleomagnetic directions from these two events suggests they differ in age by only about 100 years these ages are consistent with the chronology of the surficial geological units in the yucca mountain area these results contradict earlier interpretations of the cinder cone geomorphology and soil profile data that suggest that at least five temporally discrete eruptive events occurred at lathrop wells approximately 20 000 years ago

evaluating the life expectancy of a desert pavement abstract this paper integrates prior scholarship on desert pavements with a case study of pavements on stream terraces in the sonoran desert to analyze the processes and site conditions that facilitate the survival of ancient desert pavements this synthesis identifies vital factors key factors and site specific factors promoting pavement stability hyperaridity is the vital factor in pavements surviving for 106 years or more aided by minimal bioturbation and clast size reduction three key factors aid in pavements surviving for 104 to 105 years accumulation of allochthonous dust underneath pavement cobbles a flat topography and a lack of headward retreating swales or gullies a unified explanation for pavement longevity however did not emerge from a literature review because a variety of site specific factors can also promote pavement antiquity including resistant bedrock beneath the pavement disk shaped cobbles to promote dust accumulation and microclimatological and ecological reasons for minimal bioturbation both key and site specific explanations for pavement longevity apply well to a case study of pavements on stream terraces in the sonoran desert central arizona the buildup of cosmogenic 10be and in situ 14c optically stimulated luminescence and varnish microlamination ages reveal stable pavements range in age between 30 and 332 ka with conditions for longevity including flat surface topography pavements underlain by consolidated granitic bedrock a lack of headward retreating gullies and swales 87sr 86sr analyses indicating the infiltration of allochthonous dust floating disk shaped pavement cobbles and a quartzite lithology resistant to disintegration however 10be ages also indicate evidence for the instability of desert pavements on stream terraces underlain by unconsolidated playa clays and unconsolidated fanglomerate these weaker materials allowed the growth of headward retreating swales that in turn promoted exposure of newer gravels by surface erosion

rock varnish microlamination dating of late quaternary geomorphic features in the drylands of western usa abstract varnish microlamination vml dating is a correlative age determination technique that can be used to date and correlate various geomorphic features in deserts in this study we establish a generalized late quaternary i e 0 300 ka varnish layering sequence for the drylands of western usa and tentatively correlate it with the specmap oxygen isotope record we then use this climatically correlated varnish layering sequence as a correlative dating tool to determine surface exposure ages for late quaternary geomorphic features in the study region vml dating of alluvial fan deposits in death valley of eastern california indicates that during the mid to late pleistocene 5 15 ky long aggradation events occurred during either wet or dry climatic periods and that major climate shifts between glacial and interglacial conditions may be the pacemaker for alteration of major episodes of fan aggradation during the holocene interglacial time however 0 5 1 ky long brief episodes of fan deposition may be linked to short periods of relatively wet climate vml dating of alluvial desert pavements in death valley and the mojave desert reveals that pavements can be developed rapidly

the history and nature of wind erosion in deserts recently the importance of wind erosion has been reevaluated many low angle surfaces developed on susceptible materials possess closed depressions pans and these often have a distinctive morphology and lunette dunes on their lee sides it has also become apparent that in drylands there are extensive areas of stone pavements some of which have been molded by deflation however it is now recognized that other horizontal sorting processes have played a role especially movement of fines by overland flow vertical sorting associated with wetting and drying salt heave bioturbation frost action and dust accretion have also played a role in hyperarid areas especially with unidirectional winds bedrock outcrops and old lake beds have been molded to give aerodynamic forms called yardangs the study of dust storms by analysis of climatological data and remote sensing has revealed the importance of deflation especially in hyperarid areas with centripetal drainage deflation hot spots such as the bo

cosmogenic 3he surface exposure dating of stone pavements implications for landscape evolution in deserts the formation of stone pavements a ubiquitous gravel armor mantling landforms in arid regions of the world has been previously attributed to erosion by wind and water or alternating shrinking and swelling of soil horizons implying that gravel is concentrated at the land surface in a time transgressive manner a newly proposed model for pavement evolution differs from these models in that pavement clasts are continuously maintained atthelandsurfaceinresponsetodepositionandpedogenicmodificationofwindblowndust in situ cosmogenic 3 he surface exposure ages on volcanic and alluvial landforms in the mojave desert of california are used to understand pavement evolution over geologic time scales and to test this new model these exposure ages are stratigraphically consistent show internal consistency at each site and for stone pavements adjacent to pristine continuously exposed volcanic bedrock are indistinguishable at the 1 level we conclude that stone pavements are born at the surface and that pavements may provide one of the longest term records of geologic hydrologic and climatic processes operating on desert surfaces

petrology and geochemistry of hawaiite lavas from crater flat nevada hawaiite type lavas were erupted in three cycles 3 7 1 2 and 0 3 m y at crater flat nevada the compositions of all three cycles considered together form a straddling alkalic series as defined by miyashiro in which the less evolved basalts plot near the normative olivine diopside divide and the more evolved basalts project into hypersthene or nepheline fields fractionation modeling based on the oldest cycle allows the removal of olivine clinopyroxene and amphibole to arrive at the more evolved hawaiite compositions in general fractionation of phlogo pite or feldspar is limited by the fractionation modeling and by eu ree relations in detail all hawaiites within one cycle 3 7 m y need not be derived from a single parent magma varied parentage is more evident between cycles although all cycles are consistently of hawaiite composition basalts of the youngest two cycles are generally enriched in trace elements superimposed on this enrichment is a lack of rb variation leading to rb sr ratios far lower than required to generate the high 87sr 86sr ratio 0 707 typical of basalts in this region the very low rb sr ratios limit processes that may lead to trace element enrichment during magma evolution cyclic recharge of a fractionating magma chamber decreased fractions of mantle melting leaving phlogopite in the residuum or an earlier event of metasomatic transport from phlogopite bearing mantle rocks into a phlogopite absent mantle assemblage might explain the observed trace element enrichment with low rb sr

desert pavement characteristics on wadi terrace and alluvial fan surfaces wadi al bih u a e and oman abstract in arid mountain areas the dating and correlation of alluvial depositional surfaces is often uncertain especially in regions where the geomorphology is not well known surface modification by the development of soil and desert pavement may allow the correlation of geomorphic surfaces and estimation of at least their relative ages pleistocene wadi terraces and associated alluvial fans occur in wadi al bih u a e and oman for which correlations and age relationships are not known three age related groups of fans and terraces have been identified and mapped on the basis of their morphostratigraphic relationships deposition of the oldest terrace sediments and associated fans followed a long period of sustained incision after miocene uplift of the region the younger two groups of terraces and fans are inset within the older group to identify the gross effects of pavement development comparisons have been made between terrace surface and subsurface particle size distributions the older terraces have finer surface sediments and a greater contrast between finer surface and subsurface sediments than the younger terraces this reflects the degree of pavement development particle size on the fan surfaces is comparable with that on the equivalent terrace surfaces criteria for the classification of pavements were developed based on clast fracturing and angularity size sorting packing and surface texture from which a simple index of pavement development has been derived other properties rock varnish and weathering characteristics were also recorded but these proved to be less discriminatory than pavement characteristics the pavement data have been augmented by observations on soils detailed studies of pavements on terraces 8 sites 12 samples covering the three age groups and fans 5 sites 10 samples covering the three age groups allow differentiation between age groups the three terraces show three different age related pavement types expressed by differences in the pavement development index weakly developed pavements little fracturing sub rounded clasts some modification of the depositional fabric incipient soil development stage i caco3 accumulation occur on the youngest terrace and fan surfaces moderately developed pavements clast fracturing sub angular clasts moderate sorting and packing deeper soil development stage ii caco3 accumulation occur on the middle terrace and fan surfaces well developed pavements complete clast fracturing into small angular fragments mature sorting and packing of the pavement surface deep soil development with strong horizonation stage iii caco3 accumulation occur on the highest terrace and oldest fan surfaces there are minor differences between the youngest pavements on terraces and fans which reflect initial sedimentological differences these differences become less as the pavements develop on the basis of comparative studies the oldest terrace is estimated to date from sometime prior to ca 100 ka bp the second terrace and the most extensive fan surface from the late pleistocene and the youngest terrace and fan phase from the latest pleistocene or early holocene

models of maar volcanoes lunar crater nevada usa maar volcanoes are generally understood to be the result of highly energetic explosive interaction between magma and water groundwater or surface water two end member conceptual models have been proposed to explain the dimensions diameter depth of maar craters 1 an incremental growth model where a crater grows due to subsidence and ejection of debris over the course of many explosions and the final size is an integrated result of multiple explosive events 2 a model in which the dimensions of a maar crater are the result of the largest single explosion during the lifetime of the maar major explosion dominated model in the latter case the maar size can be used to estimate the energy and depth of the largest explosion which in turn allows estimation of the magma mass involved this paper describes lunar crater maar nevada usa and tests the two models as explanations for the characteristics of the volcano in particular the major explosion dominated model this model implies magma mass and supply rates that are unrealistic and the tephra at the maar do not contain key features observed in the ejecta at large single explosion craters the incremental growth model seems most suitable based upon geological evidence