The influence of climate and microclimate (aspect) on soil creep efficiency: Cinder cone morphology and evolution along the eastern Mediterranean Golan Heights

Although hillslope evolution has been subject to much investigation for more than a century, the effect of climate on the morphology of soil‐mantled hillslopes remains poorly constrained. In this study, we perform numerical simulations of volcanic cinder cones in the Golan Heights (eastern Mediterranean) to estimate soil transport efficiency across a significant north–south gradient in mean annual precipitation (1100 to 500 mm). We use the initial cinder cone morphology (constrained by stratigraphy), the modern hillslope form (surveyed with sub‐meter accuracy) and the eruption age (based on ⁴⁰Ar–³⁹Ar chronology) to predict the best‐fit value of the soil transport coefficient (‘diffusivity’) based on a nonlinear transport model. Our results indicate that the best‐fit diffusivity (K ) varies from 1 to 6 m² ka^?1 among the five cinder cones in our field area. Diffusivity (K ) values vary systematically with precipitation and hillslope aspect; specifically, K is higher on south‐facing (drier) hillslopes and decreases with mean annual precipitation. We interpret this climate dependency to reflect vegetation‐driven variations in apparent soil cohesion, which increases with root network density, and attenuation of rain splash and overland flow erosion, which increases with vegetative ground cover. To assess how vegetative root mass and ground cover vary with precipitation and aspect, we quantified the spatial distribution of NDVI (normalized difference vegetation index) from ASTER satellite images and observed spatial variations that correlate with our calibrated values of K . Analysis of previously studied cinder cones in the USA can be used to extend our framework to arid domains. This endeavor suggests a humped relationship between K and precipitation with maximum diffusivity at mean annual precipitation of 400–600 mm. Copyright © 2017 John Wiley & Sons, Ltd.     

Computer-simulation models of scoria cone degradation

Long-term erosional modifications of the relatively simple morphology of scoria (‘cinder') cones are ideally suited for study by field and computer-simulation methods. A series of temporally-distinct cones in the San Francisco and Springerville volcanic fields of Arizona provides the foundation for documenting the degradational evolution of scoria cones in a semi-arid climate. Progressive changes due to erosion are illustrated by the systematic decrease with increasing age of various morphometric parameters, including scoria cone height, cone height/width ratio (H_co/W_co), crater depth/width ratio, and slope angle. For example, Holocene–latest Pleistocene cones in the San Francisco field have a mean H_co/W_co value of 0.178±0.041, a mean maximum slope angle of 29.7±4.2°, and a mean average slope angle of 26.4±7.3°, whereas the group of Pliocene cones have values of 0.077±0.024, 20.5±5.8°, and 8.7±2.7°, respectively. Comparative morphology of scoria cones is a potentially useful dating tool for mapping volcanic fields.In order to better understand the degradational modifications of these volcanic landforms, we have developed a numerical approach to simulate the surficial processes responsible for the erosion of a typical scoria cone. The simulation algorithm can apply either a linear diffusion-equation model or a model with a nonlinear transport law. Using a finite-difference formulation, the simulation operates upon a three-dimensional scoria cone input as a matrix of elevation values. Utilizing both field and model results, the correlation between changing H_co/W_co value, cone age, and computer time step was expressed graphically to derive comprehensive values of the transport or diffusion coefficient (D_f) for both volcanic fields. For the San Francisco volcanic field, D_f had a calculated value of 21.4 m²/kyr for the linear model and 5.3 m/kyr for the nonlinear model, while for the Springerville volcanic field D_f had a calculated value of 24.4 m²/kyr for the linear model and 6.3 m/kyr for the nonlinear model.     

Morphometric analysis of cinder cone degradation

Measurements of Vie geometry of cinder cones can be used to determine the morphological effects and rates of degradation. Cinder cones in the San Francisco volcanic field, Arizona (where radiometric dates and stratigraphic studies have determined cone ages) decrease in height, height/width ratio and slope through time. The ratio of crater diameter to cone basal diameter does not appear to change with degradation, nor, as suggested previously, with chemical composition or particle size. Similar results obtained for cinder cones in Nevada, Oregon, Manchuria, Italy and Reunion suggest that the morphometric patterns of degradation are similar for all cinder-cones. The rates of degradation vary tremendously however, with rainfall and temperature being perhaps the most important factors. Since the initial geometries of cinder cones are remarkably similar, degraded cones may be ideal gauges of long-term climatic change.Degradation can be readily modelled for two cases: burial of cinder cone flanks by subsequent lava flows, and erosion and mass wasting. Although the former is locally important, degradation appears to occur principally by the second process: cinders weather to clay, which is gullied by rainfall, with the debris sliding downslope. Such erosion and mass wasting produces a degradation curve in general agreement with observations. Erosion rates can be accelerated orders of magnitude, however, by the mantling of old cones with easily eroded ash deposited during nearby eruptions. Comparison of cinder zone isopach radii and cone separation distances suggests it to be a common effect.     

Turbulent mixing in the Northern North Sea: a numerical model study

The aim of this study is to intervalidate observations and numerical simulation results for the turbulent dissipation rate under strong wind conditions in the Northern North Sea during one week in October 1998. The observations were obtained by spatially and temporally averaging measurements of small-scale shear with a free-falling shear probe. The 1D numerical model used for this study is based on a state-of-the-art two-equation k− turbulence model with an algebraic second-moment closure scheme. It is discussed by means of annual and seasonal model simulations how the influence of heat and salt advection and internal waves can be accounted for. After these precautions, the agreement between observations and simulations of the turbulent dissipation rate are fairly good. Remaining differences cannot only be explained by problems such as undersampling and noise level, but also by idealising model assumptions.     

A transition from strombolian to phreatomagmatic activity induced by a lava flow damming water in a valley

The Golan Heights is a Plio-Pleistocene volcanic plateau. Cinder cones of Late Pleistocene age are very common in the eastern and northern Golan, while phreatomagmatic deposits are relatively rare and occur just in two structures — the maar of Birket Ram and the tuff ring of Mt. Avital. The complex of Mt. Avital includes two large cinder cones, a tuff ring with an elongated central depression and several basaltic flows, some of them breach the cinder cones. The (exposed) eruptive history of the complex includes (1) an early stage of basaltic lava flows, (2) strombolian activity and the buildup of the southern cinder cone, (3) a second stage of basaltic flows and the buildup of the northern cinder cone, and then a transition to (4) phreatomagmatic explosions. The phreatomagmatic deposits include surges, lapilli fallout deposits and coarse-grained lithic tuff breccias, which were found up to 200 m above the central depression. Basaltic and scoriaceous clasts are the main component of all deposits, while juvenile material is usually a minor component, almost absent in the lapilli deposits.It is suggested that the phreatomagmatic events in Mt. Avital were induced by the infiltration of water from a lake that existed in a nearby topographic low (Quneitra Valley). The lake was formed or significantly expanded at about 300 ka due to a lava flow that blocked the drainage of the valley to the west. The interlayering of tuff and scoria at the top of the northern cinder cone and the good preservation of a lava flow top breccia under the surges imply that the phreatomagmatic activity immediately followed and even coincided with the last stages of strombolian activity. It is suggested that the dry–wet transition was triggered by the effusion of the second stage lavas and the buildup of the northern cinder cone, which probably caused a reduction of pressure in the magmatic system and allowed the lake water an access to the magmatic system. The minimum age of the phreatomagmatic events is determined by a 54 ka Musterian site which lies directly on top of the tuff in the Quneitra Valley.     

Diffusion models of slope development

This paper represents a systematic investigation of slope evolution diffusion models and has the following sections: (1) The model of slope development with linear coefficient k = k₀x; (2) The model of slope development with quadratic coefficient in x; (3) Slope development model with vertical lowering of base level (downcutting); (4) Slope development model with the base level a horizontal variable (undercutting); (5) Steady-state regime of undercut slopes; (6) Model of a pediment and scree slope formation. The comparison is made between mathematical and classical methods of slope evolution analysis.     

大兴安岭焰山、高山火山——一种新的火山喷发型式

在野外地质资料基础上,利用火山形态学方法,探讨了大兴安岭焰山、高山火山的喷发型式。结果表明,大兴安岭哈拉哈河-绰尔河火山群中的焰山和高山火山不同于斯通博利式喷发形成的火山,其早期爆破喷发的火山碎屑形成火山渣锥、空降火山碎屑席和小型火山碎屑流,晚期溢出大量熔岩。两火山具有较高大的锥体(标高200～300m以上),在结构上,松散火山砾、火山弹等构成下部的降落锥,熔结集块岩构成上部的溅落锥。由火山砾和火山灰组成的空降火山碎屑席分布在火山锥体周围。两火山溢出的熔岩经历了从结壳熔岩→翻花石→渣状熔岩的演变。根据喷发产物可推断焰山和高山火山具有以下喷发特征:爆破喷发形成持续的喷发柱→斯通博利式喷发→熔岩喷泉喷溢,其中以持续时间较长的喷发柱区别于典型的斯通博利式喷发。类似焰山、高山火山的喷发特征,在龙岗第四纪火山群、镜泊湖全新世火山群中也都有个例,这是中国大陆火山作用中一种新的喷发型式。     

Relations of cinder cones to the magmatic evolution of Mount Mazama,Crater Lake National Park,Oregon

Cinder cones at Crater Lake are composed of high-alumina basaltic to andesitic scoria and lavas. The Williams Crater Complex, a basaltic cinder cone with andesitic to dacitic lava flows, stands on the western edge of the caldera, against an andesite flow from Mount Mazama. Bombs erupted from Williams Crater contain cores of banded andesite and dacite, similar to those erupted during the climatic eruption of Mount Mazama.Major- and trace-element variations exhibit an increase in incompatible elements and a decrease in compatible elements, consistent with crystal fractionation of olivine, plagioclase, clinopyroxene, orthopyroxene, and magnetite. LREE patterns in the rocks are irregular; each successive basalt is enriched in LREE relative to the preceding andesite.Compositional variations in the magmas of the cinder cones suggest that three magmatic processes were involved, partial melting, fractional crystallization, and magma mixing. Partial melting of more than one source produced primary basaltic magma(s). Subsequent mixing and fractional crystallization produced the more differentiated basaltic to andesitic magmas.     

Transport of suspended sediment under the dam‐break flow on an inclined plane bed of arbitrary slope

The problem of transport of suspended sediment after the break of a dam on an inclined bed is considered. To that end we use the shallow‐water approximation for arbitrary, constant slopes of the bottom, taking into consideration the effect of friction. The numerical technique and the frictional model are validated by comparison with available experimental data and asymptotic analytical solutions, with special attention to the numerical solution near the wetting front. The transport of suspended sediment down the inclined bed is obtained and discussed as a function of the slope of the bed for different values of the parameters characterizing the sediment and its transport properties. For sufficiently large times we always find the formation of roll waves near the water front, which affects the transport of sediments significantly. These strong oscillations are accurately computed with the numerical method used. The relative importance of the bed load (to the suspended load) sediment transport is also discussed as a function of the size of the sediment particles and the slope of the bed for different models on the initiation of sediment suspension from bed load. We also check the dilute sediment approach and characterize the conditions for its failure. Finally, the results of the present simplified model are intended to be used as tests of more complex numerical models. Copyright © 2007 John Wiley & Sons, Ltd.     

PHT3D‐UZF: A Reactive Transport Model for Variably‐Saturated Porous Media

A modified version of the MODFLOW/MT3DMS‐based reactive transport model PHT3D was developed to extend current reactive transport capabilities to the variably‐saturated component of the subsurface system and incorporate diffusive reactive transport of gaseous species. Referred to as PHT3D‐UZF, this code incorporates flux terms calculated by MODFLOW's unsaturated‐zone flow (UZF1) package. A volume‐averaged approach similar to the method used in UZF‐MT3DMS was adopted. The PHREEQC‐based computation of chemical processes within PHT3D‐UZF in combination with the analytical solution method of UZF1 allows for comprehensive reactive transport investigations (i.e., biogeochemical transformations) that jointly involve saturated and unsaturated zone processes. Intended for regional‐scale applications, UZF1 simulates downward‐only flux within the unsaturated zone. The model was tested by comparing simulation results with those of existing numerical models. The comparison was performed for several benchmark problems that cover a range of important hydrological and reactive transport processes. A 2D simulation scenario was defined to illustrate the geochemical evolution following dewatering in a sandy acid sulfate soil environment. Other potential applications include the simulation of biogeochemical processes in variably‐saturated systems that track the transport and fate of agricultural pollutants, nutrients, natural and xenobiotic organic compounds and micropollutants such as pharmaceuticals, as well as the evolution of isotope patterns.     

A coupled model for simulating surface water and groundwater interactions in coastal wetlands

Coastal wetlands are characterized by strong, dynamic interactions between surface water and groundwater. This paper presents a coupled model that simulates interacting surface water and groundwater flow and solute transport processes in these wetlands. The coupled model is based on two existing (sub) models for surface water and groundwater, respectively: ELCIRC (a three‐dimensional (3‐D) finite‐volume/finite‐difference model for simulating shallow water flow and solute transport in rivers, estuaries and coastal seas) and SUTRA (a 3‐D finite‐element/finite‐difference model for simulating variably saturated, variable‐density fluid flow and solute transport in porous media). Both submodels, using compatible unstructured meshes, are coupled spatially at the common interface between the surface water and groundwater bodies. The surface water level and solute concentrations computed by the ELCIRC model are used to determine the boundary conditions of the SUTRA‐based groundwater model at the interface. In turn, the groundwater model provides water and solute fluxes as inputs for the continuity equations of surface water flow and solute transport to account for the mass exchange across the interface. Additionally, flux from the seepage face was routed instantaneously to the nearest surface water cell according to the local sediment surface slope. With an external coupling approach, these two submodels run in parallel using time steps of different sizes. The time step (Δt_g) for the groundwater model is set to be larger than that (Δt_s) used by the surface water model for computational efficiency: Δt_g = M × Δt_s where M is an integer greater than 1. Data exchange takes place between the two submodels through a common database at synchronized times (e.g. end of each Δt_g). The coupled model was validated against two previously reported experiments on surface water and groundwater interactions in coastal lagoons. The results suggest that the model represents well the interacting surface water and groundwater flow and solute transport processes in the lagoons. Copyright © 2011 John Wiley & Sons, Ltd.     

Vertical Discretization Impact in Numerical Modeling of Matrix Diffusion in Contaminated Groundwater

Understanding the effects of contaminants that can diffuse into low-permeability (“low-k”) zones is crucial for effective groundwater remedial decision-making. Because low-k zones can serve as low-level sources of contamination to more transmissive zones over time, an accurate evaluation of the impacts of matrix diffusion at contaminated sites is vital. This study compared numerical groundwater flow and transport simulations using MODFLOW/RT3D at a hypothetical site using three cases, each with increasing discretization of the vertical 10-m thick domain: (1) a coarse multilayer heterogeneous grid based on one layer for each of four different hydrogeological units, (2) a “low-resolution” discretization approach where the low-k units were divided into several sublayers giving the model 10 layers, and (3) a “high-resolution” numerical model with 199 layers that are a few centimeters thick. When comparing the results of each case, significant differences were observed between the discretizations used, even though all other model input data were identical. The conventional grid models (Cases 1 and 2) appeared to underestimate groundwater plume concentrations by a factor ranging from 1.1 to 36 when compared to the high-resolution grid model (Case 3), and underestimated predicted cleanup times by more than a factor of 10 for some of the hypothetical sampling points in the modeling domain. These results validate the implication of Chapman et al. (2012), that conventional vertical discretization of numerical groundwater flow and transport models at contaminated sites (with layers that are greater than 1 m thick) can lead to significant errors when compared to more accurate high-resolution vertical discretization schemes (layers that are centimeters thick).     

Reconnaissance geophysics and geology of the Pinacate craters,Sonora, Mexico

The Pinacate volcanic field of northwest Sonora, Mexico contains eleven tuff-rimmed explosion craters as well as hundreds of cinder cones. A multiphase eruptive and collapse history for each of the craters is evidenced by: cinder cones exposed in cross-section in crater rims, rim-breaking cinder cones, major eccentric vents, erosion hiatuses between tuff deposition, and coalescing craters exhibiting differeng degrees of erosion. Basaltic intrusions occurred along ring fractures and in the center of at least one crater. Magnetic surveys indicate that explosion craters have tuffaceous feeder vents whereas cinder cones have basaltic, highly magnetic plugs. Computer models illustrate that magnetic terrain effects can contribute to anomalies observed associated with craters, and need to be considered where magnetic surveys include irregularities such as rims or peaks.     

Surface exposure dating of Holocene basalt flows and cinder cones in the Kula volcanic field (Western Turkey) using cosmogenic 3He and 10Be

The Kula volcanic field in Western Turkey comprises about 80 cinder cones and associated basaltic lava flows of Quaternary age. Based on geomorphological criteria and K-Ar dating, three eruption phases, β2–β4, were distinguished in previous studies. Human footprints in ash deposits document that the early inhabitants of Anatolia were affected by the volcanic eruptions, but the age of the footprints has been poorly constrained. Here we use ³He and ¹⁰Be exposure dating of olivine phenocrysts and quartz-bearing xenoliths to determine the age of the youngest lava flows and cinder cones. In the western part of the volcanic field, two basalt samples from a 15-km-long block lava flow yielded ³He ages of 1.5 ± 0.3 ka and 2.5 ± 0.4 ka, respectively, with the latter being in good agreement with a ¹⁰Be age of 2.4 ± 0.3 ka for an augen gneiss xenolith from the same flow. A few kilometers farther north, a metasedimentary xenolith from the top of the cinder cone Çakallar Tepe gave a ¹⁰Be age of 11.2 ± 1.1 ka, which dates the last eruption of this cone and also the human footprints in the related ash deposits. In the center of the volcanic field, a basalt sample and a metasedimentary xenolith from another cinder cone gave consistent ³He and ¹⁰Be ages of 2.6 ± 0.4 ka and 2.6 ± 0.3 ka, respectively. Two β4 lava flows in the central and eastern part of the volcanic province yielded ³He ages of 3.3 ± 0.4 ka and 0.9 ± 0.2 ka, respectively. Finally, a relatively well-preserved β3 flow gave a ³He age of ∼13 ka. Taken together, our results demonstrate that the penultimate eruption phase β3 in the Kula volcanic field continued until ∼11 ka, whereas the youngest phase β4 started less than four thousand years ago and may continue in the future.     

On the incipient formation of bars and channels on alluvial fans

The morphodynamics of topographic expansion has been recently investigated both experimentally, by Sittoni et al., (2014) Shaw et al., (2018), and numerically Sittoni et al., 2014. Here, we study the basic mechanism that governs the evolution of topographic and expansions and explore the instability of the bottom topography under conditions of steady but spatially expanding flow. We model the expanding flow via a by configuration where water and sediments are supplied from a central hole and flow on a cone shaped surface confined by lateral walls. The governing equations are the shallow-water equations coupled with the Exner equation, written in cylindrical coordinates. We initially approach the problem analytically by considering the conditions required for the basic state, consisting of a pure radial flow and bottom profile, to lose stability to small amplitude perturbations. This analysis suggests that more than one mode may be unstable, encouraging us to extend the analysis to the nonlinear regime. We do this through numerical modeling of the full governing equations, which allows us to predict the establishment of a bar pattern whose features are similar to those experimentally observed. Two prominent features of the finite-amplitude bar pattern are (1) bar apices are distributed at a radial distance from the inflow consistent with work of Shaw et al. (2018); and (2) that the flow aspect ratio of the interbar areas remain high without provoking further instability. Both features imply that in general expansion acts to reduce bar development relative to an equivalent rectilinear flow. © 2019 John Wiley & Sons,  Ltd.     

Magnetic equilibria resulting from a helically symmetric kink instability

Abstract

This paper explores magnetic equilibria which could result from the kink instability in a cylindrical magnetic flux tube. We examine a variety of cylindrical magnetic equilibria which are susceptible to the kink, and simulate its evolution in a frictional fluid. We assume that the evolution takes place under conditions of helical symmetry, so the problem becomes effectively two-dimensional. The initial cylindrical equilibrium field is specified in terms of its twist function k(r) = B _θ/(rB_z ) and for a variety of k(r) functions we calculate linear growth rates for the kink instability, assuming that it develops under helical symmetry with pitch τ. We find that the growth rate is sensitive to the value of τ.

We simulate nonlinear evolution of the kink using a Lagrangian frictional code which constrains the field to have helical symmetry of a given pitch τ. Ideal MHD is assumed and the plasma pressure is taken to be small in order to mimic conditions in the solar corona. In some cases the flux tube evolves to a new smooth helically symmetric equilibrium which involves a relatively small change in the maximum electric current. In other cases there is evidence of current-sheet formation.     

Two-phase hydrodynamic and sediment transport modeling of wave-generated sheet flow

This numerical investigation was carried out to advance mechanistic understanding of sediment transport under sheet flow conditions. An Euler–Euler coupled two-phase flow model was developed to simulate fluid–sediment oscillatory sheet flow. Since the concentration of sediment particles is high in such flows, the kinematics of the fluid and sediment phases are strongly coupled. This model includes interaction forces, intergranular stresses and turbulent stress closure. Each phase was modeled via the Reynolds-Averaged Navier–Stokes equations, with interphase momentum conservation accounting for the interaction between the phases. The generation and transformation of turbulence was modeled using the two-equation k–ε$k''–''\epsilon$ turbulence model. Concentration and sediment flux profiles were compared with experimental data for sheet flow conditions considering both symmetric and asymmetric oscillatory flows. Sediment and fluid velocity variations, concentration profiles, sediment flux and turbulence parameters of wave-generated sheet flow were studied numerically with a focus on sediment transport characteristics. In all applications, the model predictions compared well with the experimental data. Unlike previous investigations in which the flow is driven by a horizontal pressure gradient, the present model solves the Navier–Stokes equations under propagating waves. The model’s ability to predict sediment transport under oscillatory sheet flow conditions underscores its potential for understanding the evolution of beach morphology.     

An improved method for evaluating the seasonal variability of total suspended sediment flux field in the Yellow and East China Seas

The suspended sediment flux field in the Yellow and East China Seas(YECS) displays its seasonal variability.A new method is introduced in this paper to obtain the flux field via retrieval of ocean color remote sensing data,statistical analysis of historical suspended sediment concentration data,and numerical simulation of three-dimensional(3D) flow velocity.The components of the sediment flux field include(i) surface suspended sediment concentration inverted from ocean color remote sensing data;(ii) vertical distribution of suspended sediment concentration obtained by statistical analysis of historical observation data;and(iii) 3D flow field modeled by a numerical simulation.With the improved method,the 3D suspended sediment flux field in the YECS has been illustrated.By comparison with the suspended sediment flux field solely based on the numerical simulation of a suspended sediment transport model,the suspended sediment flux field obtained by the improved method is found to be more reliable.The 3D suspended sediment flux field from ocean colour remote sensing and in situ observation are more closer to the reality.Furthermore,by quantitatively analyzing the newly obtained suspended sediment flux field,the quantity of sediment erosion and deposition within the different regions can be evaluated.The sediment exchange between the Yellow Sea and the East China Sea can be evident.The mechanism of suspended sediment transport in the YECS can be better understood.In particular,it is suggested that the long-term transport of suspended sediment is controlled mainly by the circulation pattern,especially the current in winter.     

Hydrogen and helium isotope inner radiation belts in the Earth’s magnetosphere

Radial transport theory for inner radiation zone MeV ions has been extended by combining radial diffusive transport and losses due to Coulomb friction with local generation of D, T and ³He ions from nuclear reactions taking place on the inner edge of the inner radiation zone. Based on interactions between high energy trapped protons and upper atmospheric constituents we have included a nuclear reaction yield D, T and ³He flux source that was numerically derived from a nuclear reaction model code originally developed at the Institute of Nuclear Researches in Moscow, Russia. Magnetospheric transport computations have been made covering the L-shell range L=1.0–1.6. The resulting MeV energy D, T and ³He ion flux distributions show a strong influence of the local nuclear source mechanism on the inner zone energetic D, T and ³He ion content.