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Pyroclastic Density Currents and the Sedimentation of Ignimbrites

Product Code: M0027
Series: GSL Memoirs
Author/Editor: By M Branney (Leicester University, UK) & P Kokelaar (Liverpool University, UK)
Publication Date: 16 May 2003
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Description

Pyroclastic density currents are awesome volcanic phenomena that can wreak destruction on a regional scale, and can impact global climate. They deposit ignimbrites, which include vast landscape-modifying sheets with volumes exceeding 1000 km3. This book takes stock of our understanding of pyroclastic density currents, and presents a new conceptual framework for investigating how ignimbrites are deposited. It integrates the results of field-based studies, laboratory experiments and numerical modelling, including work on clastic sedimentology and industrial particle transport. Topics covered include the behaviour of particulate currents, mechanisms of clast support and segregation, interpreting ignimbrite lithofacies and architectures, and future research. The new approach focuses on processes and conditions within the lower flow-boundary zone of a current. Superb diagrams explain many new concepts, while the 95 photographs make an explanatory atlas of deposit types.

This is essential reading for workers investigating volcanic hazards, and for anyone wishing to interpret modern or ancient ignimbrites, as well as other catastrophically emplaced sediments.

Type: Book
Ten Digit ISBN: 1-86239-124-6
Thirteen Digit ISBN: 978-1-86239-124-6
Publisher: GSL
Binding: Paperback
Pages: 152
Weight: 0.7 kg

Contents

Preface • Acknowledgements • Chapter 1 Introduction and key concepts Deposits of pyroclastic density currents • The role of ignimbrites in ideas about pyroclastic density currents • Key concepts • current steadiness and uniformity • Lower flow-boundary zones: sites of segregation and variable deposition • Ignimbrite architecture: a record of flow-boundary zone evolution through time and space • Chapter 2 The origin, nature and behaviour of pyroclastic density currents Origin and development of pyroclastic density currents Eruption styles • Current concentration and rheology • Deflation reappraised • The nature of pyroclastic density currents • The leading part of the current • Current velocity • Velocity profiles and turbulence intensity • Plug flow • Current stratification • Partitioning of mass flux in density-stratified currents • The behaviour of pyroclastic density currents • Inertia, buoyancy, runout distance and lofting • Internal waves, hydraulic jumps and granular jumps • Thalwegs (flow axes) and lateral migration • Effects of topography • A new twofold classification of pyroclastic density currents • Chapter 3 Mechanisms of particle support and segregation Significance of current heterogeneity and pyroclast diversity • Fluid turbulence • Support by fluid turbulence • Vertical segregation of clasts in the current during turbulent transport • Segregation at the flow-boundary zone due to turbulence • Support on an interface • Sustained support: rolling and sliding • Intermittent support: saltation • Overpassing: down current segregation at an interface • Granular temperature and dispersive pressure • Clast interactions and current mobility • Segregation in ganular flows • Fluidization • Fluidization and transport • Segregation by fluidization • Hindered settling, fluid escape and sedimentation-fluidization • Segregation by fluid escape and hindered settling • Clast buoyancy • Buoyancy and transport • Segregation by buoyancy • Acoustic mobilization • Acoustic mobilization and transport • Segregation by acoustic mobilization • Support by strength • Quasi-static grain contacts • Cohesion • Segregation associated with strength • Particle interlocking • Chapter 4 Conceptualizing deposition: a flow-boundary zone approach Deposition from steady currents • Direct fallout-dominated flow-boundary zone • Traction-dominated flow-boundary zone • Granular flow-dominated flow-boundary zone • Fluid escape-dominated flow-boundary zone • Gradational types of flow-boundary zone • Selective filtering: flow-boundary zone segregation and overpasing during deposition • Traction carpets • Deposition during unsteadiness • Fluctuating deposition • Sustained gradual changes • Rapid deposition • En masse deposition • Non-uniform deposition • Interpreting ignimbrite lobes and levees • Postdepositional remobilization • Effects of deposition on current behaviour • Chapter 5 Interpreting ignimbrite lithofacies A lithofacies scheme for ignimbrites • Massive lapilli-tuff lithofacies • Description • Interpretation • Origin of fabrics in massive lapilli-tuff lithofacies • Massive to stratified lithic breccia lithofacies • Description • Interpretation • Segregation of blocks from pumice and ask • Interpretation of stratified breccias and breccia lenses • Classifications of ignimbrite breccias • Massive agglomerate lithofacies • Description • Interpretation • Lithofacies with fines-poor (elutriation) pipes, sheets or pods • Description • Interpretation • Vertical grading patterns • Description • Interpretation • Diffuse-stratified and thin-bedded lithofacies • Description • Interpretation • Stratified and cross-stratified tuffs 7bull; Description • Interpretation • Pumice-rich layers, lenses and pods • Description • Interpretation • Massive and parallel-bedded lapilli deposits • Description • Interpretation • Parallel-bedded and parallel-laminated tuffs • Description • Interpretation 7bull; Eutaxitic, rheomorphic and lava-like lithofacies • Description • Interpretation • Origin of poor sorting in ignimbrites • Chapter 6 Ignimbrite architecture: constraints on current dynamics Conceptualizing architecture in a time-geometry framework • Longitudinal architectures • Transverse architecture • Interpreting longitudinal (proximal to distal) lithofacies variations • Longitudinal coarse-tail grading • Downcurrent lithofacies changes from stratified to massive • Distal lithofacies changes from massive to stratified • Interpreting vertical lithofacies variations • Gradational versus sharp lithofacies variations • Bedding and flow-unit boundaries • Repetitious and rhythmic lithofacies successions • Disordered lithofacies succession • Intercalated massive and stratified divisions • Complex longitudinal architecture • Interpreting lithofacies successions at bases of ignimbrites • Stratified bases • Basal pumice lenses • Fines-poor bases • Fine-grained layers at the base of ignimbrites • Some common successions of basal lithofacies • Sheared or loaded substrate • Interpreting lithofacies at the top of ignimbrites • Transverse lithofacies variations • Splay-and-fade stratification • Scour splay-and-fade stratification • Effects of current thalwegs and braiding • Gradations between massive valley-filling ignimbrite and stratified topographic veneers • Radially symmetrical ignimbriete distributions • Ignimbrite fans and asymmetric igimbrite distributions • Interpreting the shape of ignimbrites • A classification of ignimbrite shapes • Significance of aspect ratio • Top surfaces of ignimbrites • Chapter 7 Overview, key implications and future research Overview and key implications • Future research • Definitions of terms used • References • Index •

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