322 research outputs found
Fragmentation efficiency of explosive volcanic eruptions: A study of experimentally generated pyroclasts
Products of magma fragmentation can pose a severe threat to health, infrastructure, environment, and aviation. Systematic
evaluation of the mechanisms and the consequences of volcanic fragmentation is very difficult as the adjacent processes cannot be
observed directly and their deposits undergo transport-related sorting. However, enhanced knowledge is required for hazard
assessment and risk mitigation. Laboratory experiments on natural samples allow the precise characterization of the generated
pyroclasts and open the possibility for substantial advances in the quantification of fragmentation processes. They hold the promise
of precise characterization and quantification of fragmentation efficiency and its dependence on changing material properties and
the physical conditions at fragmentation.
We performed a series of rapid decompression experiments on three sets of natural samples from Unzen volcano, Japan. The
analysis comprised grain-size analysis and surface area measurements. The grain-size analysis is performed by dry sieving for
particles larger than 250 Am and wet laser refraction for smaller particles. For all three sets of samples, the grain-size of the most
abundant fraction decreases and the weight fraction of newly generated ash particles (up to 40 wt.%) increases with experimental
pressure/potential energy for fragmentation. This energy can be estimated from the volume of the gas fraction and the applied
pressure. The surface area was determined through Argon adsorption. The fragmentation efficiency is described by the degree of fineparticle
generation. Results show that the fragmentation efficiency and the generated surface correlate positively with the applied
energy
Field-based density measurements as tool to identify preeruption dome structure: set-up and first results from Unzen volcano, Japan
For an improvement in the quality of conduit flow and dome-related explosive eruption models, knowledge of the
preeruption or precollapse density of the rocks involved is necessary. As close investigation is impossible during eruption, the
best substitute comes from quantitative investigation of the eruption deposits. The porosity of volcanic rocks is of primary
importance for the eruptive behaviour and, accordingly, a key-parameter for realistic models of dome stability and conduit flow.
Fortunately, this physical property may be accurately determined via density measurements.
We developed a robust, battery-powered device for rapid and reliable density measurements of dry rock samples in the
field. The density of the samples (sealed in plastic bags at 250 mbar) is determined using the Archimedean principle. We
have tested the device on the deposits of the 1990–1995 eruption of Unzen volcano, Japan. Short setup and operation
times allow up to 60 measurements per day under fieldwork conditions. The rapid accumulation of correspondingly large
data sets has allowed us to acquire the first statistically significant data set of clast density distribution in block-and-ash
flow deposits.
More than 1100 samples with a total weight of 2.2 tons were measured. The data set demonstrates that the deposits of the last
eruptive episode at Unzen display a bimodal density distribution, with peaks at 2.0F0.1 and 2.3F0.1 g/cm3, corresponding to
open porosity values of 20 and 8 vol.%, respectively. We use this data set to link the results of laboratory-based fragmentation
experiments to field studies at recently active lava domes
Fall-experiments on Merapi basaltic andesite and constraints on the generation of pyroclastic surges
International audienceWe have performed fall-experiments with basaltic andesite rock samples from Merapi volcano, using an apparatus designed to analyze samples heated up to 850°C. Relative pressure changes during impact and fragmentation of the samples were measured by a pressure transducer. From 200°C, dynamic pressure waves were formed on impact and fragmentation. Peak and duration of the pressure signal, and degree of fragmentation were found to strongly increase with increasing temperature of rock samples. The pressure waves are most likely generated by sudden heating of air forcing it to expand. We propose that the observed pressure changes are analogues to pyroclastic surges that may be generated on impact and fragmentation of large blocks during passage of a pyroclastic flow over a steep cliff. We infer that rock temperatures of ca. 400°C are sufficient for this process to occur, a temperature common in pyroclastic flows even in distal reaches
The fragmentation threshold of pyroclastic rocks
In response to rapid decompression, porous magma may fragment explosively. This occurs when the melt can no
longer withstand forces exerted upon it due to the overpressure in included bubbles. This occurs at a critical pressure
difference between the bubbles and the surrounding magma. In this study we have investigated this pressure threshold
necessary for the fragmentation of magma. Here we present the first comprehensive, high temperature experimental
quantification of the fragmentation threshold of volcanic rocks varying widely in porosity, permeability, crystallinity, and
chemical composition. We exposed samples to increasing pressure differentials in a high temperature shock tube apparatus
until fragmentation was initiated. Experimentally, we define the fragmentation threshold as the minimum pressure
differential that leads to complete fragmentation of the pressurized porous rock sample. Our results show that the
fragmentation threshold is strongly dependent on porosity; high porosity samples fragment at lower pressure differentials
than low porosity samples. The fragmentation threshold is inversely proportional to the porosity. Of the other factors,
permeability likely affects the fragmentation threshold at high porosity values, whereas chemical composition, crystallinity
and bubble size distribution appear to have minor effects. The relationship for fragmentation threshold presented here can
be used to predict the minimum pressure differential necessary for the initiation or cessation of the explosive fragmentation
of porous magma
Evaluation of game templates to support programming activities in schools
Game creation challenges in schools potentially provide engaging, goal-oriented, and interactive experiences in classes; thereby supporting the transfer of knowledge for learning in a fun and pedagogic manner. A key element of the ongoing European project No One Left Behind (NOLB) is to integrate a game-making teaching framework (GMTF) into the educational app Pocket Code. Pocket Code allows learners to create programs in a visual Lego®-style way to facilitate learning how to code at secondary high schools. The concept of the NOLB GMTF is based on principles of the Universal Design for Learning (UDL) model. Its focus lies on three pillars of learning: the what, how, and why. Thereby, the NOLB GMTF is a common set of concepts, practices, pedagogy, and methods. This framework provides a coherent approach to learning and teaching by integrating leisure oriented gaming methods into multi-discipline curricula. One output of this framework is the integration of game-based methods via game templates that refer to didactical scenarios that include a refined set of genres, assets, rules, challenges, and strategies. These templates allows: 1) teachers to start with a well-structured program, and 2) pupils to add content and adjust the code to integrate their own ideas. During the project game genres such as adventure, action, and quiz, as well as rewards or victory point mechanisms, have been embedded into different subjects, e.g., science, mathematics, and arts. The insights gained during the class hours were used to generate 13 game templates, which are integrated in Create@School (a new version of the Pocket Code app which targets schools). To test the efficiency of these templates, user experience (UX) tests were conducted during classes to compare games created by pupils who used templates and those who started to create a game from scratch. Preliminary results showed that these templates allow learners to focus on subject-relevant problem solving activities rather than on understanding the functionality of the app. This directly leads to more time to express their creativity in different levels and more time for extra tasks
Combination of SAR remote sensing and GIS for monitoring subglacial volcanic activity – recent results from Vatnajökull ice cap (Iceland)
This paper presents latest results from the combined use of SAR (Synthetic Aperture Radar) remote sensing and GIS providing detailed insights into recent volcanic activity under Vatnajökull ice cap (Iceland). Glaciers atop active volcanoes pose a constant potential danger to adjacent inhabited regions and infrastructure. Besides the usual volcanic hazards (lava flows, pyroclastic clouds, tephra falls, etc.), the volcano-ice interaction leads to enormous meltwater torrents (icelandic: jökulhlaup), devastating large areas in the surroundings of the affected glacier. The presented monitoring strategy addresses the three crucial questions: When will an eruption occur, where is the eruption site and which area is endangered by the accompanying jökulhlaup. Therefore, sufficient early-warning and hazard zonation for future subglacial volcanic eruptions becomes possible, as demonstrated for the Bardárbunga volcano under the northern parts of Vatnajökull. Seismic activity revealed unrest at the northern flanks of Bardárbunga caldera at the end of September 2006. The exact location of the corresponding active vent and therefore a potentially eruptive area could be detected by continuous ENVISAT-ASAR monitoring. With this knowledge a precise prediction of peri-glacial regions prone to a devastating outburst flood accompanying a possible future eruption is possible
Entwicklung eines zwangläufigen Schneid- und Fixiersystems für den Einsatz in einem Tapelegekopf
Im Rahmen des Bundesxzellenzclusters MERGE EXC 1075 an der TU Chemnitz erfolgte die Entwicklung einer neuartigen großserientauglichen Technologie zur Herstellung faserverstärkter Thermoplastbauteile. Für die Demonstration der Technologie wurde eine Pilotanlage realisiert, die zum Ablegen des Thermoplasttapes über eine Verlegeeinheit – einen sog. Tapelegekopf – verfügt. Mit Abschluss des Legeprozesses wird das Halbzeug, das aus einer angebremsten Spule abgezogen wird, abgeschnitten und der Prozess wird von neuem begonnen.
Für das Schneiden des Materials haben Voruntersuchungen gezeigt, dass konturierte Klingen erforderlich sind, um ein Verlaufen des Bandes zu vermeiden. Darüber hinaus hat sich als zweckmäßig erwiesen eine zusätzliche Fixierung vorzunehmen. Dies verbessert die Schnittqualität und Zuverlässigkeit des Systems.
Aufgrund enger Bauraumrestriktionen wurde entschieden die Schneidbewegung und das Fixieren zwangläufig synchronisiert auf einen Antrieb zurück zu führen. Hierfür wurden zwei ungleichmäßige Rastgetriebe synthetisiert, ausgelegt sowie in weiteren Entwicklungsstufen auskonstruiert, gefertigt und getestet.
Mit der Inbetriebnahme des Gesamtanlagensystems erfolgte ebenfalls der Funktionsnachweis dieses mechanisch zwangläufigen Schneidsystems, das im Rahmen des Vortrags vorgestellt wird.
Diese Arbeiten entstanden im Rahmen des Bundesexzellenzclusters EXC 1075 „Technologiefusion für multifunktionale Leichtbaustrukturen“ und wurde von der Deutschen Forschungsgemeinschaft gefördert. Die Autoren danken für die finanzielle Unterstützung
Spatial Resolution of Double-Sided Silicon Microstrip Detectors for the PAMELA Apparatus
The PAMELA apparatus has been assembled and it is ready to be launched in a
satellite mission to study mainly the antiparticle component of cosmic rays. In
this paper the performances obtained for the silicon microstrip detectors used
in the magnetic spectrometer are presented. This subdetector reconstructs the
curvature of a charged particle in the magnetic field produced by a permanent
magnet and consequently determines momentum and charge sign, thanks to a very
good accuracy in the position measurements (better than 3 um in the bending
coordinate). A complete simulation of the silicon microstrip detectors has been
developed in order to investigate in great detail the sensor's characteristics.
Simulated events have been then compared with data gathered from minimum
ionizing particle (MIP) beams during the last years in order to tune free
parameters of the simulation. Finally some either widely used or original
position finding algorithms, designed for such kind of detectors, have been
applied to events with different incidence angles. As a result of the analysis,
a method of impact point reconstruction can be chosen, depending on both the
particle's incidence angle and the cluster multiplicity, so as to maximize the
capability of the spectrometer in antiparticle tagging.Comment: 28 pages, 18 figures, submitted to Nuclear Instruments and Methods in
Physics Research
The new generation CMB B-mode polarization experiment: POLARBEAR
We describe the Cosmic Microwave Background (CMB) polarization experiment
called Polarbear. This experiment will use the dedicated Huan Tran Telescope
equipped with a powerful 1,200-bolometer array receiver to map the CMB
polarization with unprecedented accuracy. We summarize the experiment, its
goals, and current status
Ultra High Energy Cosmology with POLARBEAR
Observations of the temperature anisotropy of the Cosmic Microwave Background
(CMB) lend support to an inflationary origin of the universe, yet no direct
evidence verifying inflation exists. Many current experiments are focussing on
the CMB's polarization anisotropy, specifically its curl component (called
"B-mode" polarization), which remains undetected. The inflationary paradigm
predicts the existence of a primordial gravitational wave background that
imprints a unique B-mode signature on the CMB's polarization at large angular
scales. The CMB B-mode signal also encodes gravitational lensing information at
smaller angular scales, bearing the imprint of cosmological large scale
structures (LSS) which in turn may elucidate the properties of cosmological
neutrinos. The quest for detection of these signals; each of which is orders of
magnitude smaller than the CMB temperature anisotropy signal, has motivated the
development of background-limited detectors with precise control of systematic
effects. The POLARBEAR experiment is designed to perform a deep search for the
signature of gravitational waves from inflation and to characterize lensing of
the CMB by LSS. POLARBEAR is a 3.5 meter ground-based telescope with 3.8
arcminute angular resolution at 150 GHz. At the heart of the POLARBEAR receiver
is an array featuring 1274 antenna-coupled superconducting transition edge
sensor (TES) bolometers cooled to 0.25 Kelvin. POLARBEAR is designed to reach a
tensor-to-scalar ratio of 0.025 after two years of observation -- more than an
order of magnitude improvement over the current best results, which would test
physics at energies near the GUT scale. POLARBEAR had an engineering run in the
Inyo Mountains of Eastern California in 2010 and will begin observations in the
Atacama Desert in Chile in 2011.Comment: 8 pages, 6 figures, DPF 2011 conference proceeding
- …