The internal structure of lava flows—insights from AMS measurements II: Hawaiian pahoehoe, toothpaste lava and 'a'

We studied the anisotropy of magnetic susceptibility (AMS) of 22 basaltic flow units, including S-type pahoehoe, P-type pahoehoe, toothpaste lava and 'a' emplaced over different slopes in two Hawaiian islands. Systematic differences occur in several aspects of AMS (mean susceptibility, degree of anisotropy, magnetic fabric and orientation of the principal susceptibilities) among the morphological types that can be related to different modes of lava emplacement. AMS also detects systematic changes in the rate of shear with position in a unit, allowing us to infer local flow direction and some other aspects of the velocity field of each unit. 'A' flows are subject to stronger deformation than pahoehoe, and also their internal parts behave more like a unit. According to AMS, the central part of pahoehoe commonly reveals a different deformation history than the upper and lower extremes, probably resulting from endogenous growth.     

A yield displacement distribution‐based approach for strength assignment to lateral force‐resisting elements having strength dependent stiffness

Recent studies have shown that for many lateral force‐resisting elements (LFRE) stiffness is dependent on strength, and as a result strength assignment to these elements would affect both the strength and stiffness distributions in a structure. Consequently, stiffness distribution cannot be considered known prior to strength assignment. This paper presents a yield displacement distribution‐based strength assignment strategy that does not require the knowledge of stiffness distribution prior to strength assignment. It is shown that structural systems with their center of rigidity (CR) and center of strength (CV) located on the opposite sides of the center of mass (CM) will have small torsional responses under seismic excitation. Copyright © 2003 John Wiley Sons, Ltd.     

Factors controlling the lengths of channel-fed lava flows

Factors which control lava flow length are still not fully understood. The assumption that flow length as mainly influenced by viscosity was contested by Walker (1973) who proposed that the length of a lava flow was dependent on the mean effusion rate, and by Malin (1980) who concluded that flow length was dependent on erupted volume. Our reanalysis of Malin's data shows that, if short duration and tube-fed flows are eliminated, Malin's Hawaiian flow data are consistent with Walker's assertion. However, the length of a flow can vary, for a given effusion rate, by a factor of 7, and by up to 10 for a given volume. Factors other than effusion rate and volume are therefore clearly important in controlling the lengths of lava flows. We establish the relative importance of the other factors by performing a multivariate analysis of data for recent Hawaiian lava flows. In addition to generating empirical equations relating flow length to other variables, we have developed a non-isothermal Bingham flow model. This computes the channel and levee width of a flow and hence permits the advance rates of flows and their maximum cooling-limited lengths for different gradients and effusion rates to be calculated. Changing rheological properties are taken into account using the ratio of yield strength to viscosity; available field measurements show that this varies systematically from the vent to the front of a lava flow. The model gives reasonable agreement with data from the 1983–1986 Pu'u Oo eruptions and the 1984 eruption of Mauna Loa. The method has also been applied to andesitic and rhyolitic lava flows. It predicts that, while the more silicic lava flows advance at generally slower rates than basaltic flows, their maximum flow lengths, for a given effusion rate, will be greater than for basaltic lava flows.     

Generation of block and ash flows during the 1990–1995 eruption of Unzen Volcano, Japan

Processes generating block and ash flows by gravitational dome collapse (Merapi-type pyroclastic flow) were observed in detail during the 1990–1995 eruption of Unzen volcano, Japan. Two different types were identified by analysis of video records and observations during helicopter flights. Most of the block and ash flows erupted during the 1991–1993 exogenous dome growth stage initially involved crack propagation due to cooling and flowage of the dome lava lobes. The mass around the crack became unstable, locally decreasing in tensile strength. Finally, a slab separated from the lobe front, fragmented progressively from the base to the top within a few seconds, and became a block and ash flow. Rock falls immediately followed, in response to local instability of the lobe front. Clasts in these rock falls fragmented and merged with the preceding flow. In contrast, block and ash flows during the endogenous dome growth stage in 1994 were generated due to local bulge of the dome. Unstable lava blocks collapsed and subsequently fragmented to produce block and ash flows.     

The strength and rheology of commercial tungsten carbide cermets used in high-pressure apparatus

Uniaxial compressive stress-strain curves have been measured on a suite of 26 commercial grades of tungsten carbide cermets and three maraging steels of interest for use in high-pressure apparatus. Tests were conducted on cylindrical specimens with a length to diameter ratio of two. Load was applied to the specimens by tungsten carbide anvils padded by extrudable lead disks. Interference fit binding rings of maraging steel were pressed on to the ends of the specimens to inhibit premature corner fractures. Bonded resistance strain gages were used to measure both axial and tangential strains. Deformation was exremely uniform in the central, gauged portion of the specimens. Tests were conducted at a constant engineering strain rate of 1×10^–5s^–1. The composition of the specimens was principally WC/Co with minor amounts of other carbides in some cases. The Co weight fraction ranged from 2 to 15%. Observed compressive strengths ranged from about 4 to just above 8 GPa. Axial strain amplitude at failure varied from 1.5% to 9%. Representative stress-strain curves and a ranking of the grades in terms of yield strength and strain at failure are presented. A power law strain hardening relation and the Ramberg-Osgood stress-strain equation were fit to the data. Fits were very good for both functions to axial strain amplitudes of about 2%. The failure of these established functions is accompanied by an abrupt change in the trend of volumetric strain consistent with the onset of substantial microcrack volume.     

Interpretation of porosity in dacite lava domes as ductile–brittle failure textures

Pores in dacite lavas of three volcanoes in Japan (Unzen, Yakedake and Daisen) have characteristics unlike vesicles. Most of the pores have ragged walls marked by the protrusion of groundmass crystals and phenocrysts. The shapes of these pores range from irregular to planar. Many pores occur in ‘pressure shadows’ adjacent to phenocrysts. Some pores are crack-like and pass around phenocrysts and have blunt terminations. Most of the pores are interpreted as the result of failure of the magma by flow during ductile–brittle transition. This phenomenon, known as cavitation, is well established in hydraulics and the material sciences. Cavitation of the lavas in this study was promoted by steep slope of emplacement, subtle mechanical contrasts between bands, and strain intensification around rigid crystals and lithic fragments. Cavitation in lava can extract volatiles from magma causing premature embrittlement and weakening. Cavitation is inferred to be critical to the initiation of lava-front pyroclastic flows.     

Bed scour by debris flows: experimental investigation of effects of debris‐flow composition

Debris flows can grow greatly in size by entrainment of bed material, enhancing their runout and hazardous impact. Here, we experimentally investigate the effects of debris‐flow composition on the amount and spatial patterns of bed scour and erosion downstream of a fixed to erodible bed transition. The experimental debris flows were observed to entrain bed particles both grain by grain and en masse, and the majority of entrainment was observed to occur during passage of the flow front. The spatial bed scour patterns are highly variable, but large‐scale patterns are largely similar over 22.5–35° channel slopes for debris flows of similar composition. Scour depth is generally largest slightly downstream of the fixed to erodible bed transition, except for clay‐rich debris flows, which cause a relatively uniform scour pattern. The spatial variability in the scour depth decreases with increasing water, gravel (= grain size) and clay fraction. Basal scour depth increases with channel slope, flow velocity, flow depth, discharge and shear stress in our experiments, whereas there is no correlation with grain collisional stress. The strongest correlation is between basal scour and shear stress and discharge. There are substantial differences in the scour caused by different types of debris flows. In general, mean and maximum scour depths become larger with increasing water fraction and grain size, and decrease with increasing clay content. However, the erodibility of coarse‐grained experimental debris flows (gravel fraction = 0.64) is similar on a wide range of channel slopes, flow depths, flow velocities, discharges and shear stresses. This probably relates to the relatively large influence of grain‐collisional stress to the total bed stress in these flows (30–50%). The relative effect of grain‐collisional stress is low in the other experimental debris flows (<5%), causing erosion to be largely controlled by basal shear stress. Copyright © 2016 John Wiley & Sons, Ltd.     

The anisotropy of magnetic susceptibility of lava flows: an experimental approach

Measurements of the anisotropy of magnetic susceptibility (AMS) of natural lavas have shown that AMS varies with depth within a lava flow. We have investigated the reasons for such variation by studying the effects of temperature and strain rate on the AMS of recent lava in the laboratory. Samples of lava from Kilauea were melted and subjected to a range of strain rate and cooling histories. The results show that the degree of anisotropy is a function of both the thermal and shearing history of a sample. High degrees of anisotropy were found only in samples that were deformed at temperatures close to those encountered during eruption and then rapidly quenched. Lavas subjected to similar shear stresses at high temperatures had low degrees of anisotropy if allowed to cool down slowly without further deformation. Additionally, lava subjected to complex shearing yield a lower degree of anisotropy even when high strain rates were imposed on it. These results lead to the conclusion that only the last phase of deformation is detectable using AMS and that high strain rates will not result in high degrees of anisotropy if either deformation ends while lava is still fluid or if the orientation of the maximum shear stress varies with time. The relation between the orientation of the principal susceptibilities and that of shear is less sensitive to variation on shear with time. Consequently, flow directions can be inferred confidently with this type of measurements.     

Runoff and sediment yield from land under various uses in a Mediterranean mountain area: long‐term results from an experimental station

In this study we analyzed runoff and sediment yield from land under various traditional and current land uses in Mediterranean mountain areas, using long‐term data from an experimental station in the Aísa Valley, Central Spanish Pyrenees. Monitoring at this station has provided 20 years of data that can help explain the hydrological and geomorphological changes that have been observed at larger spatial scales, and also the changes that have occurred to some of the most characteristic landscapes of the Mediterranean middle mountains. In spite of the problems associated with the use of small experimental plots, the results obtained are consistent with other studies in the Mediterranean region, and confirm the strong influence of land use changes on runoff generation and sediment yield. The results indicate that: (i) cereal cultivation on steep slopes (both alternating cereal cultivation and fallow on sloping fields and shifting agriculture on the steepest slopes) represents a major problem for soil conservation. This explains the occurrence throughout the Mediterranean mountains of many degraded hillslopes, which show evidence of sheet wash erosion, rilling, gullying and shallow landsliding; (ii) farmland abandonment has led to a marked reduction in runoff and sediment yield as a consequence of rapid plant recolonization, particularly by dense shrubs; (iii) the natural transformation of abandoned fields into grazing meadows has reduced runoff and sediment yield. Land use trends in the Mediterranean mountains are mainly characterized by generalized farmland abandonment and a decrease in livestock pressure. From a hydrological and geomorphological point of view the main consequences have been a reduction in overland flow from the hillslopes, and a reduction in sediment sources, with differences up to one order of magnitude in sediment yield from dense shrub cover and grazing meadow areas compared with areas under shifting agriculture. Copyright © 2012 John Wiley & Sons, Ltd.     

‘frost and salt’ weathering: Further experimental results

A laboratory experiment has been conducted to examine the effects of ‘frost and salt’ weathering (i.e. physical breakdown by the freezing of salt solutions) on a limestone. Results show that the presence of certain salts in solution can inhibit frost damage. These findings are in direct conflict with those presented by Goudie (1974) and, more recently, Williams and Robinson (1981). Comparison of the experimental methods used in each of these three studies suggests that opposing results can be explained in terms of the different experimental procedures which were employed. If salt supply is frequent and plentiful then it seems likely that rock breakdown will be enhanced-this is the case represented by the experiment of Williams and Robinson. Conversely if the salt supply is limited and the amounts of salt remain more or less constant then rock breakdown will be inhibited-the case of the present experimental study. Caution is therefore advocated when attempting to extrapolate laboratory-derived results to infer on the behaviour of rocks under natural conditions. Several environmental situations in which ‘frost and salt’ weathering may be a possibility are dsiscussed, but it is concluded that further field data, especially concerning temperature regimes and salt availability at and below rock surfaces in cold regions, would be necessary before more definite statements could be made about the efficacy of this process.     

Viscosity—The weak link between Darcy's law and Richards' capillary flow

Preferential flow, a term that includes macropore flow, non‐equilibrium flow, and finger flow, stands in well known conflict with Richards' ( 1931 ) capillary flow. Acoustic velocity experiments demonstrate that preferential flow moves independently from, faster than, and before capillary flow during gravity‐driven infiltration. Viscous flow in permeable media is briefly introduced to the point where Richards' ( 1931 ) particular treatment of viscosity turns out as the hydro‐mechanical bifurcation from general laminar flow. Preferential flow is expected during significant infiltration, however, spatio‐temporarily limited according to the viscous‐flow regime. Two ways of delineating capillary flow from viscous flow reveal minimum path widths of preferential flow in the range of about 20 μm.     

The effect of forest cover on peak flow and sediment discharge—an integrated field and modelling study in central–southern Chile

An integrated field and modelling study was carried out on the 35‐ha La Reina catchment, Chile, to test the hypothesis that the effect of forest cover on flood peaks becomes less important as the size of the hydrological event increases. Meteorological and discharge data were measured at the catchment before and after the pine plantation that covered 80% of the catchment area was logged. Analysis of the measured response of the catchment provides support for the hypothesis but is not conclusive. Therefore, modelling of the catchment using 1000 years of generated rainfall data representative of the current conditions was carried out for the forested and logged states. The simulations show that the absolute difference in discharge between the two cases remains approximately constant as the discharge increases: thus as a percentage of discharge it decreases. This relative convergence appears to become significant at return periods of greater than approximately 10 years. Tests with different hypothetical soil depths for the forested and logged catchments show an absolute convergence in discharge between the two cases for shallow soils and no convergence for deep soils. Sediment transport simulations show that forest cover provides a clear benefit in protecting the soil from erosion. Copyright © 2010 John Wiley & Sons, Ltd.     

The influence of rock mass strength on glacial valley cross-profile morphometry: A case study from the Southern Alps,New Zealand

The erosional morphology in the vicinity of the Main Divide of the Southern Alps, and Fiordland, New Zealand, appears to be a product of the interaction between Alpine Fault-induced tectonic processes, rock mass strength of the uplifted and eroded bedrock, and the processes acting to denude the developing mountain landscape. The magnitude of the effects of glacial erosion on the landscape is directly controlled by the size and physical properties of the glaciers, whilst the form of the trough is a direct consequence of the rock mass strength (RMS) properties of the slope rock. Realistic models of development of the cross-profile shape of glacial valleys must take into consideration the RMS properties of the eroded substrate.     

Volcanic-sedimentary features in the Serra Geral Fm., Paraná Basin,southern Brazil: Examples of dynamic lava-sediment interactions in an arid setting

The formation of volcanic–sedimentary interaction features in extreme arid environments is not a commonly described process. Specifically the occurrence of dynamically mixed sediments and juvenile igneous clasts as peperites, for water has been considered one major important factor in the processes of magma dismantling and mingling with unconsolidated sediment to form such deposits. The study area, located in south Brazil, shows a sequence of lava flows and intertrapic sandstone layers from the Paraná Basin, associated with the formation of clastic dykes, flow striations, peperite and ‘peperite-like’ breccias. Four processes are suggested for the genesis of the peperites: (a) fragmentation of the flow front and base; (b) sand injection; (c) dune collapse; (d) magma cascade downhill. The continued flow of a lava, while its outer crust is already cooling, causes it to break, especially in the front and base, fragments falling in the sand and getting mixed with it, generating the flow front ‘peperite-like’ breccia. The weight of the lava flow associated to shear stress at the base cause sand to be injected inwards the flow, forming injection clastic dykes in the cooled parts and injection peperite in the more plastic portions. The lava flow may partially erode the dune, causing the dune to collapse and forming the collapse ‘peperite-like’ breccia. The shear stress at the base of a flowing lava striates the unconsolidated sand, forming the flow striations. The sand that migrates over a cooled, jointed lava flow may get caught in the cavities and joints, forming the filling clastic dykes. These deposits are analogous to those found in the Etendeka, NW Namibia, and show that sediment–lava interactions in arid settings are widespread throughout the Paraná-Etendeka province during the onset of flood volcanism.     

Assessing the effect of vegetation‐related bank strength on channel morphology and stability in gravel‐bed streams using numerical models

Bank strength due to vegetation dominates the geometry of small stream channels, but has virtually no effect on the geometry of larger ones. The dependence of bank strength on channel scale affects the form of downstream hydraulic geometry relations and the meandering‐braiding threshold. It is also associated with a lateral migration threshold discharge, below which channels do not migrate appreciably across their floodplains. A rational regime model is used to explore these scale effects: it parameterizes vegetation‐related bank strength using a dimensionless effective cohesion, C_r*. The scale effects are explored primarily using an alluvial state space defined by the dimensionless formative discharge, Q*, and channel slope, S, which is analogous to the Q–S diagrams originally used to explore meandering‐braiding thresholds. The analyses show that the effect of vegetation on both downstream hydraulic geometry and the meandering‐braiding threshold is strongest for the smallest streams in a watershed, but that the effect disappears for Q* > 10⁶. The analysis of the migration threshold suggests that the critical discharge ranges from about 5 m³/s to 50 m³/s, depending on the characteristic rooting depth for the vegetation. The analysis also suggests that, where fires frequently affect riparian forests, channels may alternate between laterally stable gravel plane‐bed channels and laterally active riffle‐pool channels. These channels likely do not exhibit the classic dynamic equilibrium associated with alluvial streams, but instead exhibit a cyclical morphologic evolution, oscillating between laterally stable and laterally unstable end‐members with a frequency determined by the forest fire recurrence interval. Copyright © 2008 John Wiley & Sons, Ltd.     

Modelling artificial channel and land‐use changes and their impact on floods and sediment yield to the Ishikari basin

The potential for flooding and sediment transport is greatly affected by river channel form and changes in land use. Therefore the modelling of channel morphology prior to canalization and of land‐use change is important with respect to the prediction of floods and sediment yield and their consequences. A combination of land‐use transformation maps and soil properties shows certain decision rules for the conversion of forest into arable or vice versa. The model proposed, from this study, was used to simulate possible past and/or future channel and land‐use patterns. Subsequently, the outcome of this simulation was used to assess the risk of flooding, sediment transport and soil‐erosion under different conditions. In this study, channel morphology prior to canalization and land‐use change in the Ishikari basin, Hokkaido, Japan, were analysed by comparing three scenarios using a physical based channel and slope model. The results indicate that pre‐canalization channel morphology has a significant impact on flood peak, but no significant effect on sediment yield. In contrast, land‐use change has a significant effect on soil eroded from hillslopes, but no significant effect on flooding for Ishikari basin. This study also illustrates the challenges that a simple model, such as a physical based channel and slope model, can simulate large‐scale river basin processes using fewer hydrological data resources. Copyright © 2004 John Wiley & Sons, Ltd.     

Overview of the 1990–1995 eruption at Unzen Volcano

Following 198 years of dormancy, a small phreatic eruption started at the summit of Unzen Volcano (Mt. Fugen) in November 1990. A swarm of volcano-tectonic (VT) earthquakes had begun below the western flank of the volcano a year before this eruption, and isolated tremor occurred below the summit shortly before it. The focus of VT events had migrated eastward to the summit and became shallower. Following a period of phreatic activity, phreatomagmatic eruptions began in February 1991, became larger with time, and developed into a dacite dome eruption in May 1991 that lasted approximately 4 years. The emergence of the dome followed inflation, demagnetization and a swarm of high-frequency (HF) earthquakes in the crater area. After the dome appeared, activity of the VT earthquakes and the summit HF events was replaced largely by low-frequency (LF) earthquakes. Magma was discharged nearly continuously through the period of dome growth, and the rate decreased roughly with time. The lava dome grew in an unstable form on the shoulder of Mt. Fugen, with repeating partial collapses. The growth was exogenous when the lava effusion rate was high, and endogenous when low. A total of 13 lobes grew as a result of exogenous growth. Vigorous swarms of LF earthquakes occurred just prior to each lobe extrusion. Endogenous growth was accompanied by strong deformation of the crater floor and HF and LF earthquakes. By repeated exogenous and endogenous growth, a large dome was formed over the crater. Pyroclastic flows frequently descended to the northeast, east, and southeast, and their deposits extensively covered the eastern slope and flank of Mt. Fugen. Major pyroclastic flows took place when the lava effusion rate was high. Small vulcanian explosions were limited in the initial stage of dome growth. One of them occurred following collapse of the dome. The total volume of magma erupted was 2.1×10⁸ m³ (dense-rock-equivalent); about a half of this volume remained as a lava dome at the summit (1.2 km long, 0.8 km wide and 230–540 m high). The eruption finished with extrusion of a spine at the endogenous dome top. Several monitoring results convinced us that the eruption had come to an end: the minimal levels of both seismicity and rockfalls, no discharge of magma, the minimal SO₂ flux, and cessation of subsidence of the western flank of the volcano. The dome started slow deformation and cooling after the halt of magma effusion in February 1995.