The Icelandic ‘rofabard’ soil erosion features

Soil erosion and desertification are severe problems in Iceland. Erosion processes are numerous, and more than one can occur at each site, resulting in many erosional forms. Erosion forms and an erosion severity scale are the basis for a recent national survey of erosion in Iceland. One of the most distinctive erosion forms in Iceland is an erosion escarpment, termed ‘rofabard’ in Icelandic. Rofabards are formed in thick but non‐cohesive Andosols that overlie more cohesive materials such as glacial till or lava. The relatively loose Andosols beneath the root mat are undermined, creating escarpments, or rofabards. The rofabards retreat as a unit, with a fully vegetated and rich ecosystem on top but leaving barren desert in their place. Rofabards are common within a 20 000 km² area. The Agricultural Research Institute and Soil Conservation Service erosion database suggests that erosion associated with rofabards has denuded 15 000 –30 000 km² of land that was previously fully vegetated and had fertile Andosols, but is now mostly desert. Erosion rates associated with rofabards are reported as the loss of vegetated land with Andosol mantle, measured as hectares per square kilometre per year. This measure of erosion has more meaning for Icelandic landscapes than the traditional tonnes per hectare per year. Estimated losses of Andosol cover in rofabard areas for the whole country are currently about 230 ha a⁻¹. This rate is about 10 times lower than the rate needed to cause estimated losses of Andosol mantle in rofabard areas since settlement, 1125 years ago. During peak years of soil erosion, losses were probably several thousand hectares per year, but the erosion rates slowed down as extensive Andosol areas have become barren deserts. Copyright © 2000 John Wiley & Sons, Ltd.     

Experimental and computational synergy for modelling an advanced gas-cooled reactor core under seismic excitation

The advanced gas-cooled reactors (AGRs) are the backbone of the United Kingdom's nuclear generation fleet, producing approximately 17% of the country's electricity. Their safety cases are supported by thorough inspection and monitoring of their graphite cores and extensive theoretical, analytical, and experimental studies. This paper presents a unique, highly innovative and technically challenging earthquake engineering project that has provided vital evidence to underpin the seismic safety assessments of the AGRs. Two modelling approaches, one experimental (a multilayer array physical model), and one numerical (a SOLFEC nonsmooth contact dynamics computational model) have been developed to investigate the seismic behaviour of an aged graphite core. The synergetic relationship between the two approaches is a product of insightful collaborative learning between the University of Bristol and Atkins, with the experiments providing material parameters and validation data and the computer simulations feeding array design and test schedule recommendations to the physical model. The predictive capabilities of the physical and the numerical models are tested by direct comparison and the good agreement between the results has increased the confidence in both. The model's versatility allows a variety of core scenarios to be tested that can explore in detail the AGR core behaviour in seismic conditions.     

How hydrofractures become arrested

Fluids in the earth's crust are commonly transported by hydrofractures, such as dykes and mineral veins, many of which become arrested at various crustal depths. Hydrofractures are commonly arrested – some showing blunt tips – at contacts between soft (low Young's modulus) and stiff (high Young's modulus) layers. For example, many dyke tips are arrested at contacts between soft pyroclastic rocks and stiff basaltic lava flows, and vein tips at contacts between soft marl and stiff limestone. Theoretical models indicate that overpressured, buoyant hydrofractures in homogeneous, isotropic host rocks should normally reach the surface. In layered host rocks, however, abrupt changes in Young's moduli, horizontal discontinuities, and layers with unusually high fracture-perpendicular stresses encourage hydrofracture arrest. It is proposed that for layer-parallel loading, stiff layers favour hydrofracture arrest during active compression but soft layers during extension. It is concluded that for hydrofracture propagation to occur, the stress field along its potential pathway must be essentially homogenous.     

Constrained free space diagrams: a tool for trajectory analysis

Time plays an important role in the analysis of moving object data. For many applications it is not sufficient to only compare objects at exactly the same times, or to consider only the geometry of their trajectories. We show how to leverage between these two approaches by extending a tool from curve analysis, namely the free space diagram. Our approach also allows us to take further attributes of the objects like speed or direction into account. We demonstrate the usefulness of the new tool by applying it to the problem of detecting single file movement. A single file is a set of moving entities, which are following each other, one behind the other. Our algorithm is the first one developed for detecting such movement patterns. For this application, we analyse demonstrate the performance of our tool both theoretically experimentally.     

On the conditions of sheet injections and eruptions in stratovolcanoes

Nearly all eruptions in stratovolcanoes (composite volcanoes, central volcanoes) are supplied with magma through fractures. Consequently, a primary physical condition for an eruption to occur in a stratovolcano is that a magma-driven fracture is able to propagate to the surface. Magma-filled fractures, frozen or fluid, are referred to as sheet intrusions. More specifically, they are named dykes when subvertical, and inclined (or cone) sheets when inclined. Field observations indicate that most sheet intrusions do not reach the surface to feed eruptions but rather become arrested at various crustal depths. For this reason periods of volcanic unrest with sheet injections are much more common than volcanic eruptions. Whether a sheet intrusion becomes arrested or, alternatively, propagates to the surface depends primarily on the stress field in the stratovolcano. A stratovolcano normally consists of layers of contrasting mechanical properties, such as soft (low Youngs modulus) pyroclastic units and stiff (high Youngs modulus) lava flows. We present numerical models indicating that volcanoes composed of such layers commonly develop stress fields encouraging sheet and dyke arrest. The models indicate that a necessary condition for a sheet intrusion to reach the surface and feed a volcanic eruption is that the stress field along the sheet pathway becomes homogenised. We propose that much of the activity in a stratovolcano during a volcanic cycle encourages stress-field homogenisation. Field studies show that the sheet intrusions in individual stratovolcanoes have various dips: some are vertical dykes, others inclined sheets, and still others horizontal sills. Analytical models indicate that the dip of a sheet reaching the surface can have great effects on the magma transport during an eruption. This effect is normally greater for a flat volcano such as a collapse caldera than for a stratovolcano that forms a topographic high. We conclude that the shallower the dip of a sheet intrusion, the less will be its volumetric magma transport to the surface of a stratovolcano.Editorial responsibility: D Dingwell     

Ly+ and ultraviolet emission from high-redshift gamma-ray burst hosts: to what extent do gamma-ray bursts trace star formation?

We report the result of a search for Lyα emission from the host galaxies of the gamma-ray bursts  (GRBs) 030226 ( z = 1.986), 021004 ( z = 2.335)  and  020124 ( z = 3.198)  . We find that the host galaxy of GRB 021004 is an extended (around 8 kpc) strong Lyα emitter with a rest-frame equivalent width (EW) of 68⁺¹²₋₁₁Å, and a star formation rate of  10.6 ± 2.0 M_⊙ yr⁻¹  . We do not detect the hosts of GRB 030226 and GRB 020124, but the upper limits on their Lyα fluxes do not rule out large rest-frame EWs. In the fields of GRB 021004 and GRB 030226 we find seven and five other galaxies, respectively, with excess emission in the narrow-band filter. These galaxies are candidate Lyα-emitting galaxies in the environment of the host galaxies. We have also compiled a list of all   z ≳ 2  GRB hosts, and demonstrate that a scenario where they trace star formation in an unbiased way is compatible with current observational constraints. Fitting the   z = 3  luminosity function (LF) under this assumption results in a characteristic luminosity of   R *= 24.6  and a faint-end slope of  α=−1.55  , consistent with the LF measured for Lyman-break galaxies.