In situ formation of welded tuff-like textures in the carapace of a voluminous silicic lava flow,Owyhee County,SW Idaho

 The Badlands rhyolite, on the Owyhee Plateau of southwestern Idaho, can be demonstrated to be a large lava flow on the basis of its geometry of large and small flow lobes, its well-exposed near-vent features, and its response to pre-existing topography. However, samples of the dense upper vitrophyre of the unit reveal a range of annealed fragmental textures, including material which closely resembles the compressed, welded glass shards which are characteristic of ignimbrites. Formation of these tuff-like textures involved processes probably common to emplacement of most silicic lava flow units. Decompression upon extrusion causes inflation of pumice at the surface of the lava flow; some of this pumice is subsequently comminuted, producing loose bubble-wall shards, bits of pumice, chips of dense glass, and fragments of phenocrysts. This debris sifts down around loose blocks and into open fractures deeper in the flow, where it can be reheated, compressed, and annealed to varying degrees. The end result is a dense vitrophyre layer (beneath the true upper, non-welded carapace breccia) which can be extremely texturally heterogeneous, with areas of flow-foliated lava occurring very near lava which in many aspects looks like welded ignimbrite, complete with flattened pumices. Identical textures in other silicic units have been cited by previous workers as evidence that those units erupted as pyroclastic flows which then underwent sufficient rheomorphism to create a flow-foliated rock which otherwise appears to be lava. The textures described herein indicate that lava flows can come to mimic rheomorphic ignimbrites, at least at scales ranging from thin sections to outcrops. Voluminous silicic units with scattered fragmental textures, but with otherwise lava-like features, are probably true effusive lava flows. Received: January 30, 1995 / Accepted: January 22, 1996     

Pyroclastic flows and surges over water: an example from the 1883 Krakatau eruption

Pyroclastic deposits from the 1883 eruption of Krakatau are described from areas northeast of the volcano on the islands of Sebesi, Sebuku, and Lagoendi, and the southeast coast of Sumatra. Massive and poorly stratified units formed predominantly from pyroclastic flows and surges that traveled over the sea for distances up to 80 km. Granulometric and lithologic characteristics of the deposits indicate that they represent the complement of proximal subaerial and submarine pyroclastic flow deposits laid down on and close to the Krakatau islands. The distal deposits exhibit a decrease in sorting coefficient, median grain size, and thickness with increasing distance from Krakatau. Crystal fractionation is consistent with the distal facies being derived from the upper part of gravitationally segregated pyroclastic flows in which the relative amount of crystal enrichment and abundance of dense lithic clasts diminished upwards. The deposits are correlated to a major pyroclastic flow phase that occurred on the morning of 27 August at approximately 10 a.m. Energetic flows spread out away from the volcano at speeds in excess of 100 km/h and traveled up to 80 km from source. The flows retained temperatures high enough to burn victims on the SW coast of Sumatra. Historical accounts from ships in the Sunda Straits constrain the area affected by the flows to a minimum of 4x10³ km². At the distal edge of this area the flows were relatively dilute and turbulent, yet carried enough material to deposit several tens of centimeters of tephra. The great mobility of the Krakatau flows from the 10 a.m. activity may be the result of enhanced runout over the sea. It is proposed that the generation of steam at the flow/seawater interface may have led to a reduction in the sedimentation of particles and consequently a delay in the time before the flows ceased lateral motion and became buoyantly convective. The buoyant distal edge of these ash-and steam-laden clouds lifted off into the atmosphere, leading to cooling, condensation, and mud rain.     

Heat transfer between a thermochemical plume channel and the surrounding mantle in the presence of horizontal mantle flow

Heat and mass transfer processes in the conduit of a thermochemical plume located beneath an oceanic plate far from a mid-ocean ridge (MOR) proceed under conditions of horizontal convective flows penetrating the plume conduit. In the region of a mantle flow approaching the plume conduit (in the frontal part of the conduit), the mantle material heats and melts. The melt moves through the plume conduit at the average velocity of flow v and is crystallized on the opposite side of the conduit (in the frontal part of the conduit). The heat and the chemical dope transferred by the conduit to the mantle flow are carried away by crystallized mantle material at the velocity v.The local coefficients of heat transfer at the boundary of the plume conduit are theoretically determined and the balance of heat fluxes through the side of the plume conduit per linear meter of the conduit height. The total heat generation rate, transmitted by the Hawaiian plume into the upper and lower mantle, is evaluated. With the use of regular patterns of heat transfer in the lower mantle, which is modeled on the horizontal layer, heated from below and cooled from above, the diameter of the plume source, the kinematic viscosity of the melt in the plume conduit, and the velocity of horizontal lower-mantle flows are evaluated and the dependences of the temperature drop, viscosity and Rayleigh number for the lower mantle on the diameter of the plume source are presented.     

稀性泥石流的平均运动速度研究

泥石流的运动速度是泥石流动力学研究中最重要的参数。稀性泥石流是常见也是危害较大的泥石流类型,准确而简洁地计算稀性泥石流的运动速度非常重要。现有的稀性泥石流平均速度经验公式,在使用上和适用地区上还存在一些问题。本文通过分析一系列稀性泥石流观测资料中的体积浓度与稀性泥石流的运动速度和阻力特征的关系,得出了一个新的计算稀性泥石流平均运动速度的经验公式,该公式能适应各种类型的泥石流沟,适用于一般急流的稀性泥石流;对于缓流稀性泥石流,计算值与观测值相比偏大,但很接近;不适用于缓慢稀性泥石流。本文提出的经验公式,使用简洁,计算稳定,与其他方法计算的稀性泥石流平均运动速度很接近。该速度计算经验公式也适用于稀性泥石流堆积扇上游沟道,但对于堆积扇上的速度,计算值偏大,且越往堆积扇的下游,偏差越大。     

TAKING INTO ACCOUNT THE EFFECT OF SECONDARY FLOWS IN DEPTH AVERAGED MORPHODYNAMIC SIMULATIONS

1 INTRODUCTIONThe numerical simulation of morphological changes in rivers, estuaries and coastal areas is because ofits econondcal merit an outstanding scientific task. The comPlexity of the problem is connected with theheterogeneity of the sediment material, an adequate rePresentation of the involved hydrOdynndc stressesand the large comPutational effort fOr long term simulations.Although the aPpllcation of three dimensional models tO engineering problems is increasing and wellvalldate…     

Pyroclast/snow interactions and thermally driven slurry formation. Part 1: Theory for monodisperse grain beds

Received: 15 March 1999 / Accepted: 9 December 1999     

SETTLING PROPERTIES OF COHESIVE AND NON-COHESIVE PARTICULATE MIXTURES IN WATER

1. INTRODUCTIONAs a wide range of size distirbution including usually a certain POrtion of cohesive material is thecommon feature of the sediment constituting hyperconcentrated flows. it is desirable to study the settling properties of mixtures of cohesive and non--cohesjve sediment particles at high concentrations.Past studies on the settling of discrete particles in a suspension of fine cohesjve sediment is scarcein the literature. The Sediment Research Laboratory of Tsinghua Universi…     

Cause of dominance by sheetflood vs. debris-flow processes on two adjoining alluvial fans, Death Valley, California

Two large, adjoining alluvial fans of the Panamint Range piedmont, Death Valley, California, are composed of different facies assemblages deposited by contrasting sedimentary processes. The Anvil Spring fan was built solely by water-flow processes (incised-channel floods and sheetfloods), whereas the neighbouring Warm Spring fan has been constructed principally by debris flows. The boundary between these fans delineates a sharp provincial piedmont contact between sheetflood-dominated fans to the south and debris-flow-dominated fans to the north. Factors such as climate, catchment area, fan area, catchment relief, aspect, vegetation types and density, and neotectonic setting are essentially identical for these two fans. The key difference between them is that their catchments are underlain by dissimilar bedrock types, which weather to yield distinctive sediment suites. Weathering of the granite and andesite of the Anvil fan catchment produces significant volumes of medium to very coarse sand, granules, pebbles, cobbles and boulders, but minimal silt and clay. In contrast, the shale, quartzite and dolomite that dominate bedrock in the Warm Spring catchment weather to yield a wide suite of sedimentary particles spanning from clay to boulders. The abundance of mud, and the unsorted character of the yielded sediment, cause precipitation-induced slope failures in the Warm Spring catchment to transform readily into debris flows. This propensity is due to the low permeability of the colluvial sediment, which causes added water to become trapped quickly and pore pressure to rise rapidly, promoting transformations to debris flows. In contrast, the limited volume of sediment finer than medium sand yielded from the Anvil fan catchment causes the colluvium to have high permeability. This factor prevents the transformation of wet colluvium to a debris flow during hydrologically triggered slope failures, instead maintaining sediment transport as entrained bed load or suspended load in a water flow.     

Risk assessment strategies for the reactivation of earth flows in the Northern Apennines (Italy)

The aim of this paper is to analyze the reactivation mechanism of ancient earth flows, with a view to gleaning information that can subsequently be utilized to formulate a risk-reduction strategy. All considerations made herein are the result of direct experience and observation of actual events which have occurred over the past few decades in the Northern Apennines. Particular attention has been paid to the analysis of the evolution of landslides during actual reactivation, acknowledging a typical, recurring succession of events that precede the failure of the slope. The hazard assessment of these large landslide bodies, which are of slope scale, constitutes a thorny problem, especially in view of the inapplicability of traditional deterministic models such as limit equilibrium stability analysis. Nevertheless, a site-specific assessment of probability of reactivation of these large and ancient earth flows is fundamental to effective land-use planning.     

Application of a BEM time stepping algorithm in understanding complex unsteady propulsion hydrodynamic phenomena

We solve the problem of unsteady potential flow around a system of arbitrarily moving rigid or flexible, lifting or non-lifting bodies, in an infinite fluid free of distributed vorticity. For the solution we use a time stepping algorithm and a potential based formulation of the corresponding free BVP. Nonlinear free shear layer dynamics are included in our modeling. This is a major innovation in treating complex unsteady propulsion problems since no simplifying assumptions (like that of a helicoidal wake) are used regarding the wake model. Bilinear quadrilateral elements are used to describe body and shear layer geometry at each time t. Three types of Kutta conditions can be alternatively applied for the determination of the shed vorticity from lifting bodies. The theoretical and numerical aspects of the method are presented followed by a number of applications, elucidating the qualitative and quantitative physical characteristics of a number of complex unsteady propulsion problems.