From Magma to Tephra

Modelling Physical Processes of Explosive Volcanic Eruptions

Edited By

  • A. Freundt, GEOMAR, Forschungszentrum für Marine Geowissenschaften, Wischhofstrasse 1-3, Geb. 4, D-24148 Kiel, Germany.
  • M. Rosi, Università di Pisa, Dipart. di Scienze della Terra, Via S. Maria, 53, I-56126 Pisa, Italy.

Hot magma rising through the Earth's crust releases gases that expand and may come into contact with external water that vaporizes. The magma is then fragmented into an accelerating gas-particle/droplet mixture that is shot into the atmosphere, possibly in an overpressured state, where it may buoyantly rise up into the stratosphere as an ash plume, partially or totally collapse back to the surface, or rapidly expand sideways, or undergo a combination of these processes. Tephra is then deposited on the Earth's surface by pyroclastic fall, flow or surge, or some hybrid mechanism. The combination of processes that operate from the degassing of magma to the emplacement of tephra makes an explosive volcanic eruption, and the physical characterization of these processes is the scope of this book.

In this book we summarize the insights into key aspects of explosive volcanic eruptions gained from physical modelling to date.

The seven chapters are arranged in an order reflecting the sequence from processes acting within the volcanicconduit through dynamics of eruption and transport through the atmosphere to mechanisms of emplacement on the Earth's surface.

Chapter 1 reviews the progress made in understanding how magma vesiculates and fragments, considering results obtained by experiment, theory, and analysis of the vesicle-texture of pumice. Magmatic fragmentation is discussed in terms of brittle failure as tensile strength is exceeded by internal and/or external stresses. The explosive fragmentation of hot magma upon contact to external water is experimentally shown in Chapter 2, emphasizing the need for water-entrapment configurations to cause explosive interaction during which extremely high stresses fracture melt in a brittle fashion. The motion through the conduit of vesiculating magma below the fragmentation level, and of the gas-particle/droplet mixture above fragmentation is investigated in Chapter 3. Pressure evolution along the conduit and exit velocity at the vent are shown tovary with initial magma chamber pressure, magma composition, and composition of the mixed H2O+CO2 volatile phase. Chapter 4 then reviews the processes that control the dynamic evolution of eruption columns during rise into the stratosphere or collapse to form pyroclastic flows, considering supersonic dynamics, influence of the atmosphere, and time-dependent unsteadiness effects. Transport and fallout of pyroclasts from eruption columns with or without cross-wind are the topic ofChapter 5, showing how deposit characteristics can be used to estimate eruption parameters such as discharge rate and column height. The generation, transport and emplacement of pyroclastic flows is discussed in Chapter 6, reviewing the presently much debated transport concepts ranging from grain flow through fluidized flow to suspension currents,and elaborating the suspension-current model thought to be applicable to widespread ignimbrites. Finally, Chapter 7 summarizes observations from nuclear explosions and characteristics of pyroclastic surge deposits as a basis to then theoretically analyze the compressible two-phase flow of both dry and wet pyroclastic surges.

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For research volcanologists, private scientists, professionals, university libraries, government research institutes, graduate students and researchers.


Book information

  • Published: February 2001
  • Imprint: ELSEVIER
  • ISBN: 978-0-444-50708-2


...Those who want a timely summary of much widely disseminated work will find it invaluable... It is a very good addition to the volcanological literature and the editors are to be commended.
(S. Self), Journal of Volcanology and Geothermal Research the reader access to the status of research and modelling in a series of fields of volcanology that literally accompany him/her from the dynamics of magma vesiculation and fragmentation to the deposits of explosive eruptions. ...this is a most valuable book ... advanced students but also (or especially) professional volcanologists and researchers will find in a single book an impressive amount of high standard and updated information about the topics covered by the book, together with a conspicuous number of relevant references to the literature published in the past and especially in the very last years.
(R. Carniel), Volcano Quarterly Online
Cet ouvrage constitue une mise au point et un recueil de données quantitatives fondamentales pour chercheurs et étudiants de haut niveau.
(J.C. Tanguy), Geochronique
Almost twenty years ago, at 08:32 Pacific Daylight Time on the 18th May, 1980, a magnitude 5.1 earthquake triggered an explosive eruption of Mount St. Helens which, in turn, triggered huge interest in the physics of volcanic processes. More recently, the pace has, if anything, accelerated thanks to the motivation provided by further well-studied eruptions, including Pinatubo (Philippines) in 1991, and Soufriere Hills volcano (Montserrat) in 1995–1999. An impressive body of literature has accumulated just in the past decade spanning empirical, theoretical and observational approaches; physical models have been tested, validated and compared using laboratory experiments and remote sensing observations of real eruptions. Much of this recent research has focused on experimental and theoretical treatments of micro-scale processes and scaling issues – for example, how inter-grain interactions influence transport of, and sedimentation from, pyroclastic density currents, and how the mechanisms of bubble nucleation and transport in silicate melts ultimately influence eruption styles. These results greatly flesh out the first-order eruption physics that had been elucidated through research in the 1970?s and 1980?s. In addition, actual eruption deposits have been scrutinised in increasingly detailed and diverse ways supporting the development of techniques for inversion of the tephrastratigraphic record to infer eruption styles.

These developments have enormous implications for volcanic hazards assessment because eruption prediction based solely on empirical pattern recognition (e.g., from seismicity or ground deformation records) is often hampered by the limited periods of monitoring conducted prior to volcanic eruptions. All too often, surveillance networks are only installed after a volcano has shown obvious signs of unrest such that instrumental data are collected once the system is already in an abnormal state. This makes it difficult to judge what &quo
(C. Oppenheimer), Earth-Science Reviews, 49

Table of Contents

Chapter headings and selected papers: Magma Degassing and Fragmentation: Recent Experimental Advances. Abstract. Introduction. Nucleation. Bubble Growth. Accelerating two-phase flows. Brittle failure. Post-fragmentation effects. Non-explosive degassing. Relaxation geospeedometry of volcanic glass. Bibliography. Phreatomagmatic Explosions. Introduction. Explosive phreatomagmatic volcanism. Physics of volcanic MFCI. Diagnosis and monitoring of phreatomagmatic explosions (volcanic MFCI). Volcanic Conduit Dynamics. Introduction. Review of magma ascent models. Magma ascent dynamics in steady state explosive eruptions. Eruption Column Physics. Introduction. Multiphase flow and the multifield approach. Multifield governing equations. Closure of governing equations. Approaches to analysis of eruption column dynamics. Steady state dynamics. Time-dependent dynamics with constant eruption rate. Time-dependent dynamics with transient eruption rate. Influence of the ambient medium. Conclusion. Plinian Eruption Columns: Particle Transport and Fallout. Introduction. Models of particle transport in plinian columns. Clast dispersal in crosswinds. Assessment of eruption parameters. Pyroclastic Flow Transport Mechanisms. Introduction. Characteristics of ignimbrites and pyroclastic flows. Transport processes of sedimenting particulate flows. Pyroclastic flow models. From model flows to ignimbrites: complexities during emplacement. Concluding remarks. Pyroclastic Surges and Compressible Two-Phase Flow. Introduction. Observations. Theory. Conclusions: surges are hazardous but economically significant. Appendix A: origin of the SFT equations. Appendix B: analysis of surge steam condensation. References. Subject index.