Commission on Explosive Volcanism

Newsletter 97-01, June 1997

1997-2001 CEV Leaders:
Michael Ort
Greg A. Valentine

Newsletter Editor:
Vanessa A. De La Cruz

What's Inside

In this Newsletter...

by Michael Ort and Greg Valentine

At the recent IACEI General Assembly in Puerto Vallartaâ we were voted in as leaders of the Commission on Explosive Volcanism for the next four years, and it is with much pleasure that we send you this first issue of the CEV Newsletter during our term. Before proceeding, allow us to take this opportunity, on behalf of the entire CEV membership, to thank Armin Freundt and Mauro Rosi for their work as CEV leaders for the past four years. They did an outstanding service to our research community in producing the newsletters, promoting workshops, and in organizing numerous symposia and short courses.
Thanks Armin and Mauro!

Now on to this CEV Newsletter issue. We are thankful to a number of people for contributing to this issue. Stephen Straub's article highlights his thoughts on application of granular flow models to the dynamics of dense pyroclastic flows. Giovanni Macedonio and Augusto Neri discuss their opinions on the role of large-scale numerical modeling in the future of volcanology, including its interplay with experimental and field research. Alex Szakacs and Ioan Seghedi write on their perspective as researchers in Eastern Europe.

Paolo Papale, Joan Marti, and Armin Freundt (ex-commission leaders cannot hide!) present summaries of the CEV-sponsored symposia at the Puerto Vallarta meeting. Amanda Clarke, Dannie Hidayar, and Barry Voight summarize the CEV Shortcourse on Physics of Explosive Volcanism that was held before the conference. A recent CEV-sponsored field workshop was held in Romania -- this is summarized in an article by Jaroslav Lexa. We would like to maintain this level of member participation in the newsletter content!

We hope to continue the traditional role of the CEV Newsletter, which includes disseminating information from conferences and workshops, and acting as a "bulletin board" for upcoming events. We also want to encourage members to use the Newsletter as an informal forum for discussion of ideas and research approaches such as the Straub and Macedonio and Neri articles in this issue. This can be a tremendously important use of the newsletter. There are few places where one can discuss ideas that have not yet been fully tested. One can imagine situations where an individual might not have the expertise to test an idea, but by making it known, might forge an alliance with a colleague who can test it. This is not in the tradition of comments and replies, but in the more constructive spirit of developing new ideas and models for poorly understood phenomena.

Other aspects that we would like to see include short reviews and discussions of interesting journal papers and books. A centrally important component of CEV is the field workshop. Please let either one of us know if you have an idea for a field workshop that we can help you promote. Any CEV member can propose a symposium or session in conjunction with any IAVCEI or IUGG conference; again, if you have an idea for such please let us know. We want to particularly emphasize joint symposia with other IAVCEI commissions so that we can continue and strengthen our interdisciplinary tradition. If you have ideas for other CEV activities, please propose them to us. We will then publicize your idea to see if there is interest among our membership.

For descriptions of the Commissions and a schedule of upcoming conferences please visit the IAVCEI home page on the Internet at http://geont1.lanl.gov/HEIKEN/ONE/IAVCEI_HOME_PAGE.HTM.

The updated CEV home page , accessible from the above address, will be available soon. We plan to use the home page to post announcements, full Newsletter issues (beginning with this one), and individual Newsletter contributions immediately upon their submission to us. In this manner, your contribution will be immediately accessible by a large portion of the community, in addition to its "hard-copy" publication in the next Newsletter. In addition, if you have a home page you would like to have us list on the CEV home page, please let Michael know.

Also, please let us know if you have any ideas for improving the Newsletter or the function of the Commission in general. There are a number of issues that might need addressing, but we won't know about them without your action. For example, one concern is the cost of the IAVCEI and commission memberships for potential members from countries with very low salaries. What solutions do you see for this?

Finally, we want to introduce Vanessa De La Cruz, who will be working as publishing editor for the Newsletter as well as maintaining the membership database.

We hope you enjoy this issue of CEV Newsletter!

Upcoming Events

International Volcanological Congress
Cape Town, South Africa
11-17 July 1998

For more info contact:

Secretariat, IVC '98
Department of Geological Sciences
University of Cape Town
Private Bag
Rondebosch 7701
Republic of South Africa
Fax: +27 21 650-3783
Or look us up on the web at: http://www.uct.ac.za/depts/geolsci/ivc98/

Pyroclastic flow mechanics - A Personal Standpoint

by Stephen Straub

The mechanics of pyroclastic flows are incompletely understood. High-concentration pyroclastic flows (HCPFs), and especially the processes that govern them, are the subject of much debate. It is an honor and a pleasure for me to sketch my thoughts on this subject and on the motivation for my approach to learn more about HCPFs. Deposition from HCPFs likely causes massive, poorly sorted pyroclastic flow deposits. This indirect conclusion is drawn from the observation that low-concentration flows produce far better sorted deposits. However, it remains unclear how such high-concentration flows move and how they are deposited.

Initial attempts to explain the motion of HCPFs were taken from hypotheses originally proposed for subaqueous sediment flows. "Sediment support mechanisms" support the weight of the sediment and these are assumed to reduce the internal friction. Laboratory experiments show that an upward moving gas stream can partly fluidize a bed of pyroclastic material. Because gases are present in pyroclastic flows, gas fluidization has become the favorite explanation for HCPF mobility. But what if gas fluidization is not the primary mechanism to support flow motion? Its supposed significant role in the moving flow has never been validated.

HCPFs consist primarily of solid material and the interstitial gases occupy only a minor part of the volume. They are flows of solids transporting a smaller amount of gas. Stresses that arise from the motion are the result of fluxes of mass and momentum of solids and gases. To understand the flow behavior of a mixture (not only the effect of the gases on the solids), we first have to know how stresses in a gas-free sediment flow are generated, how they are transported, and how the sediment flow's kinetic energy is dissipated. Then, we can investigate how this flow behavior is altered by an additional gas fluidization.

Studying high-concentration flow requires a suitable experimental setup. Three reasons why computer simulations are preferred to analog experiments are (1) easier quantification of the flow properties, (2) reproducibility of experiments, and (3) opportunity to simulate even extreme cases. Common modeling methods are based on a set of constitutive equations that defines the kinematics of a flow. The results depend strongly on the assumed rheology introduced by the constitutive equations. My approach is different: high-concentration flows are flows of granular material and, to obtain new information about their flow behavior, they have to be modeled as flows of discrete particles. It is necessary to model detailed, individual particle motions and particle-particle collisions. The data of the individual particles must be integrated over the bulk to determine its flow properties.

I have developed a suitable computer model using molecular dynamics, which is a method common in experimental fluid mechanics. It allows simulation of small-scale discrete particle systems that are representative of larger flow situations. The particle motions and interactions are based on simple Newtonian mechanics. Simplifying assumptions, which is necessary in every computer model, are limited to a minimum, i.e., particles are modeled as inelastic rigid spheres with rough surfaces.

The simulation results are surprising and promising as well as rapidly moving granular flows develop dynamic flow properties that define a separate regime: the rapid granular flow regime. This regime is characterized by particle-particle collisions between adjacent layers of the intensely sheared flow. Shear stress, as the result of momentum transfer between the layers, is lower than one might expect. This is caused by a process of self-organization of the particles that forces the flow into a state of dynamic equilibrium with a low energy dissipation rate and, therefore, extreme flow mobility. In the case of internal or external perturbation, the flow will force itself back into this equilibrium state, the so-called attractor of the dynamic system. Even if the flow is fluidized by gas, it is very likely that the rapid granular flow attractor will control the flow behavior.

The opportunities of a rapid granular flow computer model are manifold: it allows us to investigate shear behavior, the segregation processes, and the development of grading. From the simulation results, it is more than likely that the rapid granular flow regime governs the development of HCPFs. Characteristic features of their deposits can be simulated by simple computer experiments. Finally, the simulation results offer the key on how the deposits have to be investigated to learn more about their formation.

The rich dynamics of granular material are just beginning to be understood. It is a challenge for me to follow this approach that combines research on the basic physics of granular media and volcanic flow processes. In my view, it can be stimulating for both areas of research.

Physical Modeling in Volcanology: Challenges of the year 2000

by Giovanni Macedonio and Augusto Neri

There is no doubt that physical modeling now represents one of the preeminent ways of investigating volcanic processes. This is certainly true if we consider its numerous applications: mantle convection, melt segregation crystallization and mixing, magma chamber processes, magma ascent in conduits, structural deformation of volcanic systems, pyroclastic dispersion in the atmosphere, lava flows, flow in porous media, tsunamis, and others.

Given its critical role, physical modeling may become a major research field in volcanology in the following decades. On the one hand, for many years volcanologists have studied the stratigraphic sequence of a volcano, reconstructing its history and eruptive style. Extremely valuable information on magma chemistry and petrology have allowed recognition of different magma geneses and features. On the other hand, continuous progress in the monitoring techniques of volcanoes and their geophysical and geochemical investigations have increased our knowledge of volcano substructures and eruption precursors. All this information is necessary and represents the basis for a correct understanding of a volcanic system.

However, the various observations must all be more strongly related to each other by a quantitative and objective framework in order to avoid ambiguous and misleading interpretations of volcano dynamics. Physical modeling tries to overcome this problem and allows the integration of different types of data in a consistent scheme by analyzing the volcanic process in terms of physical laws. By using such a method, not only can different aspects of the volcanic system be correlated, but also the dynamics of the process can be followed in space and time. Twenty years ago, the application of simple analytical models allowed the identification of first-order processes in explosive volcanism. The development of complex numerical codes in engineering and physics, as well as the proliferation of inexpensive high-power computers, have allowed further progress in the understanding of magmatic and eruptive processes. The wide variety of phenomena occurring in volcanic systems can now be dealt with using multicomponent, multiphase, and multiscale physical models, in an attempt to describe their nonlinear - and therefore nonintuitive - nature. New processes have been identified and well-known phenomena explained and quantified.

Despite these promising results, new and important steps forward still need to be taken. There appear to be three major guidelines to follow. First, the extremely peculiar and complex nature of volcanic processes needs the formulation and solution of new, sound physical models. This requires the creation of a physical modeling school in volcanology, in which not only advancements in science and technology would be transferred to the volcanological context, but also the specific features of the volcanic system would be accounted for and modeled. A second point concerns the need of experimental research work able to define the appropriate constitutive equations of volcanic mixtures and to calibrate the developed physical models with physical examples. This point is strictly linked to the previous one and requires the creation of new experimental laboratories where fundamental geological problems, such as magma rheology and fragmentation, would be explored. Last but not least, the third point is related to a proper integration of physical modeling with field volcanology, monitoring, and hazard assessment. The linking of physical modeling with ad hoc field studies will allow work with clearly circumscribed geological test-cases, therefore reducing the number of unknown variables. At the same time, the interpretation of monitoring data in the light of quantitative and consistent hypotheses will allow better discrimination between different scenarios and allow improvement in the reliability of predictions. Hazard assessment will eventually benefit from such integration, illustrating another important use of physical modeling.

We think that these three areas of work represent the major challenges volcanology has to accept in the coming years. Several groups, all over the world, have already begun moving in this direction, but the tendency must be widespread and further developed.

We are sure that having Greg and Michael as leaders of the CEV commission will help to make progress in this respect.

Editor's Remarks

One of the major goals of CEV is to promote collaboration in all areas on the globe where the products of explosive volcanism are present or where there is potential for explosive volcanism. We (Ort and Valentine) requested the following essay as a means to help the community at large to understand the difficulties that some of our colleagues have in carrying out research and international collaboration. The essay, as you will see, is provocative. Whether you agree with some of the statements or not, this article states the perceptions of its authors. These are perceptions that need to be dealt with in order to promote collaboration. Our (Valentine and Ort) suspicion is that these perceptions are shared by many workers in many countries. We hope that this article results in a constructive discussion and mutual understanding between those of us who are fortunate to be in stable research positions in stable countries and those who are in less stable positions and countries.

Eastern European Researchers Need More Support

by Alex Szakacs, and Ioan Seghedi

Most Eastern European countries face severe economic crises that strongly influence their scientific life to varying degrees. Bulgaria, for example, is in an economic collapse. Romania initiated last-minute economic reforms to avoid a similar collapse. The painful reform initiatives both strongly affect already low living standards and keep science at extremely low funding (less than 0.4% of the 1997 budget of Romania). Also, modern research infrastructure is lacking. The average income of research scientists in Hungary is about 8 times less than in Japan or the US, while in Romania this figure is about 10 times less than in Hungary, but still 4 times more than in Bulgaria or Ukraine.

Following the turnover of the communist regimes, and an initially enthusiastic effort to help Eastern European scientists reconnect with the international scientific community, researchers from some of these countries are now experiencing frustration and feelings of isolation due to a more recent reduction in support. Support that is available is unevenly distributed and appears to preclude representation of certain countries at important meetings. For example, organizers of the IAVCEI General Assembly in Puerto Vallarta offered partial support to a number of potential participants from Romania. However, since matching funds were unavailable, this led to non-representation from, and lost opportunity for Romanian volcanologists. A preferable alternative would be full support for one scientist, the choice of a representative being made by the country itself. Apparent preferential support for Former Soviet Union and Chinese scientists at many meetings often results in under- or non-representation of other Eastern European countries. Rather than using a well-established, planned, institutional policy as guidelines to support the involvement of Eastern European scientists in international research projects, the process still relies on personal relationships with Western-world partners or personal research interests of Western scientists.

In an effort to stay involved in the international scientific community, despite harsh domestic economic environments, many Eastern European researchers attempt to bring top-level foreign scientists to their countries to stimulate domestic research activity, as well as to raise interest abroad. Tremendous efforts were made by Ioan Seghedi and Alexandru Szakacs to organize a joint CEV/CVS workshop on volcaniclastic sequences near andesitic stratovolcanoes in Romania in August 1996 (under difficult conditions and a poor local infrastructure). Clearly positive was the fact that IAVCEI generously supported the event through the enthusiastic involvement of Wally Johnson. Less positive was the extremely poor attendance by non-Eastern European countries. Many people attended the workshop at Santorini even though the two workshops were scheduled so that they could be attended successively. The presence of Andre Pouclet and Hiromitsu Yamagishi, the two non-Eastern European volcanologists, was rewarded by excellent discussions on topics of high current interest in volcanology. The presence of other top-level volcanologists, however, would have further increased the scientific level of the workshop. In this respect, we regret their absence and cannot help but wonder if there is a sense of prejudice in the people from more developed countries concerning Eastern European researchers' capabilities? Or, perhaps, the fear of Dracula dissuaded, rather than persuaded, people to come to Romania?

Eastern European countries offer abundant research opportunities for volcanologists from other countries. The Carpatho-Pannonian-Balkanian area of Eastern Europe is the site of Paleogene to Pleistocene volcanism, displaying a wide range of tectonic settings, several types of volcanic activity and edifices, chemical features, and volcaniclastic sequences. These geological offerings have been studied at varying levels of detail in different countries, but cross-border correlations are poor, except for an ongoing cooperative geochronological study by several Eastern European scientists. There is plenty of room for more institutionally-organized research. The few 'outside' volcanologists and igneous petrologists who have visited Eastern European countries to work with local people may help us further by dissipating prejudice and attempting to stimulate more research interest in our area.

1997 General Assembly Reports

Symposium 1, Session 1: Eruption Of Magma From Shallow Chambers

by Paolo Papale

The 1997 IAVCEI General Assembly was held in Puerto Vallarta (Mexico) from 19 to 24 January. The Session 1 "Eruption of Magma from Shallow Chambers" of Symposium 1 "Explosive Volcanism" consisted of 37 oral and 37 poster communications. About 50% of the oral and 25% of the poster communications were devoted to eruption mechanisms by a general point of view or had clearly general purposes, while the remnants consisted of investigations on specific volcanic areas. About 15% of the total consisted of numerical modeling studies.

Takada presented a physical model for the mechanical evolution of magma plumbing systems. His results show that a volcano with fissure eruptions evolves mechanically under a form of self-organization depending on the relations between the relaxed and accumulated stress due to magma supply. The time-dependent fluid-dynamics of magma chambers was investigated by Longo and Macedonio in the case of continuous supply of magma during an eruption, and by Folch and Marti for caldera-forming eruptions. The results include the formation of large vortices (Longo and Macedonio) and insights into the pressure change and the stress concentration around the magma chamber (Folch and Marti).

Papale improved further the modeling of magma ascent along volcanic conduits by considering carbon dioxide as a second volatile species besides water, and showed that the multicomponent nature of the volcanic gas has important consequences on the distribution of the flow parameters along the conduit and at the conduit exit. An application of this model to the well-constrained AD 79 Vesuvius eruption was presented by Arrighi et al., who found that the pressure variations in magma chamber and microlite formation during the eruption may have played a significant role in the eruption dynamics. Lyakhowsky et al. analyzed the kinetics of bubble growth in rhyolitic melts both experimentally and theoretically. A relevant result of their studies is that the thermodynamic equilibrium between exsolved and dissolved gas - which is normally assumed in the modeling of magma ascent dynamics - is a good approximation up to the fragmentation level, whereas above it the kinetic processes may become dominant and the gas exsolution be delayed. The same authors also found a role of iron-bearing mineral species in promoting gas exsolution by heterogeneous bubble nucleation.

Valentine reported data on upper-crustal xenoliths erupted from small-volume basaltic volcanoes and assessed the relative importance of various entrainment mechanisms during hydrovolcanic, Strombolian, and effusive processes. His conclusion is that the magma ascent models are justified in neglecting the mass and momentum contribution of entrained materials for purely magmatic, basaltic eruptions, but not for hydrovolcanic eruptions. The extension of such studies to the ascent of highly viscous magma producing very destructive plinian or pyroclastic flow-forming eruptions is of great interest.

Alidibirov and Dingwell presented the results of experimental investigations of the magma fragmentation by rapid decompression. Such a mechanism may be very efficient during rapid depressurization processes like those that led to the May 18, 1980 Mount St. Helen's blast eruption. Moreover, results of the numerical modeling of steady ascent of highly viscous magma along volcanic conduits reveal the existence of a conduit region immediately before fragmentation where large pressure gradients are produced, suggesting that magma fragmentation by rapid decompression may play a role even during sub-steady plinian eruptions.

Zimanowski et al. presented results of experiments on the explosive magma-water interaction, and Buettner et al. compared the interactive particles produced during the experiments with pyroclastic fragments from phreatomagmatic eruptions at Lipari and Volcano and found identical features. These experiments suggest that the magma/water interface area is a critical parameter in determining the explosive intensity, but that a very efficient explosive magma-water interaction may occur even for not previously fragmented magma.

Vergniolle et al. measured the acoustic pressure due to sound waves during explosions at Stromboli, and gained insights into the bubble bursting dynamics and bubble ascent velocity along the volcanic conduit. Stevenson and Dingwell measured the rheological properties of natural rhyolite at volcanologically relevant conditions in terms of temperature, applied strain rates, and crystal/gas bubble contents. Such studies are of fundamental importance in defining the appropriate constitutive equations for use in the physical modeling of volcanic processes.

It is very likely that important progress in the understanding of volcanic activities will be made in the next future by coupling the fluid-dynamics models with the results of thorough studies on the magma properties. In a series of communications, Herzog, Oberhuber, and Graf presented ATHAM, an atmospheric model that simulates the dispersion of gas and particles during volcanic eruptions. The great potential of ATHAM consists in its ability to solve the Navier-Stokes equations describing the gas/pyroclast transport in a very large domain several hundred kilometers horizontally and 50 km in height, and to account for vapor condensation and rain formation. The information that can be obtained on the volcanic column evolution and ash transport and deposition will greatly improve our understanding of large-scale plinian eruptions, especially if integrated with that coming from dispersion models based on a more complete treatment of the multiphase nature of volcanic plumes.

Houghton et al. reported the results of continuous observations of the 1995-96 Mount Ruapehu eruptions, with activities ranging from Surtseyan to Strombolian to phreatomagmatic to sustained drier eruption plumes. Similar observations on active volcanoes are critical not only for the hazard assessment, but because they represent an incomparable occasion to test the conceptual and numerical models of volcanic processes and to gain thorough information from the field and laboratory studies of the eruption products.

In summary, the session was a very rich one, and the scientific interest of the communications was generally high. Many aspects of the volcanic processes from the plumbing system to the pyroclasts dispersion domain were investigated, with different approaches from numerical to experimental to field studies. The increasing tendency toward quantification and interpretation based on physical laws that characterizes the volcanological studies of last years clearly emerged from the communications. Future developments will be probably related to a much closer interaction between scientists having different approaches.

Symposium 1, Session 2: Volcanic Particle Flows

by Armin Freundt

The session on volcanic particle flows comprised 18 posters and 15 oral presentations covering a wide field of topics:

Pyroclastic-Flow Transport and Emplacement Mechanisms
Eruption column dynamics, providing the initial conditions for pyroclastic flows, were numerically investigated by Neri and Papale in terms of volatile content and composition, and Neri and Gidaspow introduced a new 3-D collapse model. Houghton et al. documented natural eruption complexities derived from structure and compositional variations in very near-vent exposures at Novarupta. Pyroclastic flow models that were presented included a numerical simulation of dome-collapse produced flows applied to Soufriere on Montserrat by Hooper et al., a numerical simulation of high-speed grain flows by Straub emphasizing the role of particle-particle interactions, and an analytical model used by Freundt to explain the transport of very hot ash flows generating high-grade ignimbrite.

Concerning high-grade ignimbrites, Sparks suggested that re-dissolution of volatiles (magmatic and entrained air) under the pore pressure during accumulation reduces glass viscosity and is the major control on the degree of welding. An interesting approach to derive dynamic properties of pyroclastic currents by Clarke et al. considered the forces acting on damaged or blown-down trees. Changes in current properties in response to topography were investigated by Bursik et al., focusing on sedimentation behavior over M-scale topographic features, and by De Astis et al. reporting facies changes where pyroclastic surges hit the caldera wall on Vulcano island. Vertical variations in clast fabric led Hughes and Druitt to interpret emplacement of a massive pyroclastic flow unit at Laacher See in terms of progressive aggradation.

Flow separation
Sparks et al. and Calder et al. documented efficient separation of pyroclastic flows into lithic-rich and pumice-rich parts that are controlled in different ways by topography during emplacement. They specifically emphasized the importance of lithic-enrichment in increasing the erosive capacity of the flows. Splitting of block-and-ash flows into block-rich, channelized, and surge-like ash-rich, less topographically controlled, subsystems has been reported by Cole et al. (Soufriere) and Bourdier et al. (Merapi). Takarada et al. interpreted both such facies at Unzen in terms of density-modified grain flow and high-density suspension current, respectively.

Deposits from "mixed" processes
Alvarado et al. showed a lapilli bed from Arenal that has properties indicating a combination of fallout and surge-like jetting. Simultaneous fallout and surges are also invoked by Cole et al. to explain the suppressed cross-bedding in the deposits of Capelinhos 1957-58.

Land-Sea Transition and Subaqueous Emplacement
Kano interpreted a submarine rhyolitic surge-like tuff to have been emplaced by a highly turbulent turbidity current generated by the entrance of phreatomagmatic surges into the sea. Pumice breccias syn-eruptively emplaced under water by water-supported mass flows were described by Raos and McPhie. Sakamoto et al. described a submarine basaltic lava that traveled 20 km below water and became marginally disintegrated into breccias. Schmincke and Sumita interpreted numerous ash layers drilled in the clastic apron around Gran Canaria as the product of ash turbidites generated in response to hot ash flows entering the sea. They argued that rapid fragmentation of welded ignimbrite masses piled up in shallow near-shore water provided the source of turbidity currents. A systematic comparison of the variations in eruptive style, deposit facies, and volcano edifice structure that arise from variable water supply to eruptions, ranging from infinite supply within a lake through varying supply at emergent volcanoes to restricted supply at subaerial vents, was presented by Godchaux and Bonnichsen for volcanoes in the western Snake River Plain.

Lahars and Debris Avalanches
Urrutia-Fucugauchi et al. determined emplacement temperatures of up to 360¡C for debris avalanches at more than 15 km distance from Colima, Mexico. Houghton et al. reported on very snow- and ice-rich (70-80 vol%) lahars observed during the 1995 Ruapehu eruptions, which contained little liquid water (in part acid crater lake water), traveled on dry snow, and froze upon deposition.

Eruption Evolution
Rosi et al. documented the evolution of an eruption in Ecuador through a growing stage of increasing mass flux in response to vent widening, producing increasingly dispersed plinian fallout, to a waning stage of declining collapse height, producing first surges and then pumiceous pyroclastic flows. Schumacher argued for a gradual increase in water access to the Akdag-Zelve eruption (Anatolia) producing the sequence fallout-surges-ignimbrite, and he described the variably welded ignimbrite facies in detail. Schumacher and Mues-Schumacher showed how volcanic activity of the Cappadocian Volcanic Province changed from early eruptions influenced by external water, producing fallout-surge sequences, to late eruptions producing fallout-ignimbrite sequences.

Variations in grain size and distribution characteristics led Fukushima and Kobayashi to propose a gradual change from phreatomagmatic to magmatic conditions during eruption of the Tsumaya ignimbrite from Aira caldera. Bogoyavlenskaya gave a summary of 40 years of monitored eruptions, and their deposits, at Bezymianny volcano. Itamar et al. reported lapilli breccias inside a quartz-syenite stock that were formed by repeated explosive fragmentation in a fissure vent, interrupted by intervals of hydrothermal alteration variably affecting the breccias. Eyal et al. reconstructed the multi-stage eruptive evolution of basanitic Mount Arod volcano in Israel.

In summary, my impression from this session was that we are rapidly gaining an improved understanding of the physical processes operating in volcanic particle flows, due to both advances in theoretical modeling and monitoring of active events, which both feed back into the interpretation of volcanic deposits. On the other hand, we keep getting reports on volcanic flow deposits that show how complex the natural systems are and how much they deviate from current idealized physical models.

Symposium 1, Session 3: Collapse Calderas and Large Volume Eruptions in Latin America

by Joan Marti

The session on Collapse calderas and large volume eruptions in Latin America was one of three sessions included in the Symposium on Explosive Volcanism at the IAVCEI Puerto Vallarta meeting. Latin America is the site for a large number of collapse calderas, which are related to different tectonic settings, magma compositions and eruption mechanisms. Starting from Mexico to the southern tip of Chile and Argentina, many different types of collapse calderas can be observed, from small-sized calderas related to lava flow-eruptions to the gigantic resurgent collapse calderas of the Central Andes. The session was intended to provide a forum for volcanologists working on collapse calderas from Latin America to discuss all geological and geophysical aspects of these volcanic structures. Special emphasis was given to the tectonic setting, structure, and products, as well as to the unrest and risk assessment of those calderas that represent a potential danger to the population and environment.

Nearly 30 contributions were included in this session. Several of them focused on the description of specific caldera structures, emphasizing particular geological, petrological or geophysical aspects in each case. This allowed the participants to realize the great variety of caldera structures that exist in Latin America and how much work still needs to be done there. Some of the presentations focused on collapse calderas from the Central Andes, analyzing the significance of regional tectonics on the development of such large volcanic depressions. Special mention is needed for the presentation given by S. Desilva, who gave participants a comprehensive and clear view on the origin of ignimbrites in the Central Andes.

H. Sigurdsson exposed submarine evidence of explosive volcanic episodes in Central America from ODP drilling, thus enhancing the importance of such studies in improving our understanding of subaerial volcanism in such a complex setting.

In summary, the session confirmed the importance of the studies on explosive volcanism in general, and of collapse calderas in particular, that are currently being carried out in Latin America

Navier-Stokes Revisited: A Review of "The Physics of Explosive Volcanism"

by Amanda Clarke, Dannie Hidayar, and Barrie Voight

The pleasures of the memorable IAVCEI 1997 General Assembly at Puerto Vallarta were numerous: greeting dear friends, discovering stimulating new colleagues, and trying to judiciously design each day's choreography, choosing carefully from the simultaneous-session program menu involving 11 symposia jam-packed with outstanding speakers and attractive topics. With such a smorgasbord, no choice could be wrong, although one could be wistful about excellent presentations inevitably missed. But, let it be no other way, and all credit to the Organizing Committee and to our Mexican hosts!

Nosotros apreciamos mucho el calor y la amistad que hemos encontrado durante esta conferencia y ademas todo el informacion scientifica adquirido acerca de como manejar las emergencias que resultan de los volcanos!

Symposium 1 was Explosive Volcanism, and spread out over several days. It featured an outstanding array of strong papers that pushed the research frontier of the topic to a new level. Certainly, the modern understanding of explosive volcanism represents a genuine success story for volcanology. Starting with an explosive thrust a few decades ago by George Walker, Lionel Wilson, and Steve Sparks, growth in the field has been sustained by an increasing flux of new ideas and accelerated computational power. Thus, developments have convected to remarkable heights, and have mushroomed laterally to encompass, under the umbrella of gas dynamics, a variety of applications of interest to field volcanologists and theoreticians alike. However, this accelerated -- one might say superbuoyant-- growth has made it difficult to keep abreast of new developments, especially since the field has diverged as it has grown, with specialty areas rising like phoenix clouds from the ingested ash of spreading ideas. Because of this, we were delighted that the IAVCEI Commission on Explosive Volcanism (CEV) chose to sponsor a 3-day pre-conference short course on The Physics of Explosive Volcanism. The course, attended by 35 participants from 12 nations, was well organized by Armin Freundt and scientist-footballer Gerardo Carrasco-Nunez.

The course featured the following sequence of lectures:

  1. Magmatic Degassing and Fragmentation (Don Dingwell)
  2. Eruption Column Physics (Greg Valentine)
  3. Eruption Columns: Particle Transport and Fallout(Mauro Rosi)
  4. Phreatomagmatic Explosions (Bernd Zimanowski)
  5. Particle Flow Transport Mechanisms (Armin Freundt, Kiel, and Marcus Bursik)
  6. Pyroclastic Surges and Compressible Two-Phase Flow (Ken Wohletz)

The series of lectures was generally well linked, most built on or reinforcing material previously presented. Dingwell's lecture focused on recent developments on the degassing and fragmentation of silicic magmas, beginning with a description of the relevant magma properties (rheology, permeability, conductivity, and solubility, etc.), and continuing with a discussion of bubble growth, accelerating two-phase flows, brittle failure, and post-fragmentation effects. This clear presentation was empirical in outlook, up-to-date, and comprehensive; it summarized the experimental basis for current understanding. The general area of research is extraordinarily active. An indication of this activity is the 100 publications referenced by Dingwell in a summary document, about half of which were published in the past two years.

Greg Valentine, whose contribution on magma ascent dynamics began General Assembly (IAVCEI Abstract Volume, page 1), also began Course Day 2 with a review of fluid dynamics associated with large-scale eruption columns. The first part of the lecture provided a foundation of the fundamental equations for multiphase, multicomponent flows, i.e., flows with a combination of interacting vapor, liquid, and solids. The concept of representative elementary volume (REV), which is a volume large enough to be treated as a continuum, was developed but included the phases in volumetric proportion. Equations were then developed for the conservation of mass, momentum, and internal energy, resulting in a number of equations smaller than unknowns. Additional relations required for closure include equations of state, constitutive equations, and interphase exchange relations.

Valentine presented solutions to these "extended Navier-Stokes equations" that used quasi-analytical approaches and numerical simulations. The former approach characterizes some of the early classics, as well as some modern treatments. Much simplification must be introduced in order to gain solutions, such as steady flow, or single-phase behavior, but significant understanding for isolated subsets of column behavior was presented. Numerical solutions enable relaxation of some of the simplifications noted above.

Valentine then entered into a discussion of column behavior, beginning with vent exit conditions, and including entrainment, buoyancy/collapse thresholds, overpressured jets, pyroclastic flow inception, the role of unsteadiness in column height and coignimbrite plumes, and ambient medium properties. From one perspective, the state of the art is a three-field axisymmetric model of Neri and Macedonio (vapor phase, and two particle populations), building on earlier codes by Dobran with sophisticated vapor-phase treatment.

Valentine's presentation emphasized the more recent work, but placed it in the perspective of pioneering studies. A fruitful approach worth emulating was his use of an "edited" full set of equations to introduce the individual studies under discussion, providing a consistent measure of the assumptions and simplifications involved. The result was a concise, comprehensive, yet user-friendly review of material that easily could have been intimidating.

The next logical step is to consider particle transport in eruption column and fallout. These aspects were presented by Mauro Rosi. Dispersal and fallout in plinian eruptions are among the best understood dynamic processes in volcanology, and Rosi gathered the theoretical treatments of Carey, Sparks, Bursik, Woods and others, to discuss stationary models of transport and sedimentation, and crosswind effects.

The second part of the discussion emphasized the deposits, in the absence and presence of significant wind. Deposits associated with no wind are important, if relatively rare, in that they are suitable for testing model predictions without the complications introduced by wind. Most plinian fall deposits are associated with winds, and modern eruptions with field data and meteorological information, (such as El Chichon and Mount St. Helen's) provide important tests of models. The lecture concluded with a useful discussion of column height, vent discharge predictions, and discrepancies introduced by alternative measures of clast-size data.

Zimanowski's digression on phreatomagmatic explosions followed, emphasizing the Molten Fuel-Coolant-Interaction (MFCI). This topic is complicated and challenging, not fitting neatly into the chain of topics previously discussed, but requiring consideration of a different terminology, and a different approach. MFCI was described in four phases: hydrodynamic mixing, trigger phase, fragmentation, and vaporization/expansion. Detailed emphasis was given to pioneering experiments by Wohletz and recent studies by a German group. Potentially, deductions of volcanological interest might be derived from particle characteristics, but evidently natural processes are still far from being satisfactorily understood.

Column collapse, discussed in Lecture 2, produces pyroclastic flows and surges. The consideration of these topics in Lectures 5 and 6 marked a return to the mainstream short-course theme. Unlike the topic of fallout dynamics, in which the main issues seem to be understood and tractable, the topic of pyroclastic currents intrudes upon contentious ground. Despite the complications and controversies, Bursik and Freundt covered the main points of pyroclastic currents. Using information developed by experiment and theory, Bursik and Freundt also discussed the topics of single-phase density currents, currents with reversing buoyance, and particle-laden flows. The discussion then turned to particle support mechanisms, grain flow, fluidization, hindered settling, and turbulence. The lecturers also presented a consideration of the evolution from initial conditions to depositional conditions, and flow unsteadiness which led to the consideration of flow equations, sedimentation models, and topography, with the approaches being quasi-analytical, or numerical, as previously established. Given the abundance and complexity of material, the time allotted to Bursik and Freundt was insufficient and lecture attendees would have liked this presentation to be longer. Fortunately, their lecture notes are long, inversely proportional to the lecture duration but of matching quality, and may be read with profit.

The course concluded with a crisp, well-illustrated lecture by Ken Wohletz. The lecture had two parts, the first provided a fine history of the base surge concept, traced from nuclear Test Baker at Bikini in 1946 to the first volcanological field application by R.V. Fisher at Taal. Field characteristics were then reviewed, as well as facies interpretations and lab approaches involving grain size and grain morphology. The second part was a potpourri, involving compressible Navier-Stokes flow modeling, stratified flows, grain transport modes and bedforms, sequential fragmentation/transport analysis (glossed over, but go to the web), vaporization and condensation in surges, and shock waves. All parts provided valuable insights and food for thought.

So, where can those who missed the event find this material, apart from hundreds of articles in the good journals? There is no comprehensive single source, and that underscores the value of the short course. Several good papers on key topics may be found in the recent Scarpa-Tilling monograph on volcano monitoring. But there is certainly a need here for a textbook, despite the fact that any text would be partly obsolete the moment it goes to press, because of the high activity in the various subfields. Nevertheless, a solid text written by a few authors (in preference to many authors, in order to maintain consistency of the themes and mathematical development) would provide a benchmark to aid the Phoenix-like rise of future advances. Candidate authors could certainly include the workshop contributors, to whom all participants owe a round of strong applause.

Note by Greg Valentine: A book based on the shortcourse material is in preparation, and is planned to be published within a year (Armin Freundt and Mauro Rosi, editors).

A Field Workshop Report

Joint CVS/CEV Workshop: Volcaniclastic Sequences Around Andesitic Stratovolcanoes, Romania

by Jaroslav Lexa

This workshop took place in Romania during the week of August 25 through September 1, 1996. To many it may sound strange: a field workshop concerning andesite stratovolcanoes in Romania, almost in the center of Europe. But that is the reality. The Carpathian mountains of central Europe, a Tertiary orogenic arc, host Neogene to Quaternary andesite stratovolcanoes, the youngest of the Pliocene to Quaternary age being the ones in Romania.

Promoters of the workshop have been aware that volcaniclastic sequences around eroded andesite stratovolcanoes are a key to their evolution and structure. They chose well-exposed volcaniclastic sequences along the Mures, Ghurgiu, and Bistrita valleys in the Aclimani and Gurghiu mountain ranges of Eastern Carpathians to examine a complex facies architecture and depositional history of merging peripheral aprons of several coeval volcanoes.

Their intent was to get a group of diverse specialists in front of outcrops to discuss and interpret various aspects of textures and lithology and, in that way, to share experience and learn from each other. Field studies were usually followed by evening discussions. There were 18 participants, both juniors and seniors, from seven European countries and Japan. Dr. Ioan Seghedi and Dr. Alexandru Szakacs (Geological Institute of Romania) were the leaders and principal organizers of the workshop. They were assisted by younger colleagues.

The first day was devoted to a long trip from Bucuresti to Gheorghieni, where the Mures Valley starts. After crossing the beautiful southern Carpathian mountains, we could easily recognize the Harghita volcanic chain. We could not miss the opportunity to see the youngest Carpathian volcano, the Ciomadul Volcano, at the southern end of the Harghita mountain range. This volcano is made up of a group of hornblende andesite extrusive domes with late-stage phreatic craters. Its last eruption took place only 35,000 years ago. A mountain road took us all the way to the bottom of the now-forested crater, which hosts a tranquil St. Anna Lake. Two stops along the road enabled us to observe pumiceous pyroclastic fall, surge, and flow deposits related to the last phreatomagmatic eruption.

The second and third days took us to outcrops in the upper Mures Valley, east and west of the city of Toplita. The Mures Valley in this area follows a former interflow area between the Rusca Tihu volcano to the north and Fancel Lapusna volcano to the south. While the Rusca Tihu volcano is dominated by basaltic andesite lavas, the Fancel Lapusna volcano contains hornblende andesites and explosive pumiceous products during the last stage. This distinction allows the provenance of volcaniclastic material to be recognized, and in that way, we can discriminate between peripheral aprons of the Rusca Tihu volcano from peripheral aprons of the Fancel Lapusna volcano. And, consequently, we can identify both of these from mixed facies of the main channel between the volcanoes. The main channel deposits show textures indicating alternating normal flow, flood flow, hyperconcentrated flow, and debris flow regimes with a relatively high degree of rounding and sorting. The lateral aprons of the volcanoes, dipping degrees to 5 degrees, almost do not contain normal flow deposits, because they are dominated by coarse debris-flow and mud-flow deposits. Aggradation alternated frequently with erosion and channel filling. Aprons of the Rusca Tihu and Fancel Lapusna volcanoes demonstrate how the presence of ash and pumice in the source area (the Fancel Lapusna volcano) changes lithological aspects -- mudflows with ash-dominated matrix are very frequent. Within the group, there was a general consensus concerning criteria to distinguish mud flows and debris flows, however, the hyperconcentrated-flow deposits were a matter of continuous dispute. The problem may be in a rather loose definition that allows for variable views on characteristic textures and other aspects. Another dispute concerned stratified horizons of pumiceous tuffs. There are occasional problems differentiating between subaerial fall and flow deposits on one hand, and shallow subaqueous mass-flow deposits on the other. At one of the visited localities, we observed stratified pumiceous tuffs with textures interpreted as characteristic of subaerial deposition (sorting, grading) interbedded with diatomaceous clays indicating a limnic environment.

The fourth and fifth days took us downstream of the Mures Valley towards the city of Reghin. This part of the valley is cut in early products of the Rusca Tihu volcano, overlying, to the west, late Middle Miocene marine sediments. A highly variable complex of breccias, conglomerates, sandstones, and subordinate tuffs shows features characteristic of both terrestrial and shallow marine environment. The complex offers an exceptional opportunity to discuss differences between terrestrial and shallow marine facies, because there are 1) textural differences between fluvial, coarse clastic delta, and beach conglomerates and sandstones, 2) differences between subaerial and submarine mass flow deposits, and 3) differences between subaerial and submarine phreatic tuff deposits. An examination of textures and local successions demonstrated that hyaloclastite breccias hyaloclastites reworked by slumping, mass flows, density currents, and surf activity make up a substantial part of the complex.

Owing to their basaltic andesite to andesite composition, lavas do not form typical pillow lavas. Pseudopillows, however, up to several meters in diameter have been observed along with prevailing angular fragments. Central zones are made up of brecciated lava flows, dykes, and small protrusions along with coarse hyaloclastite breccias, and rare peperite breccias. Proximal facies are represented mostly by debris-flow deposits, while sandy and fine breccia density-current deposits, along with rare breccia-flow deoposits, make up distal facies. Submarine slumping of the central zone accumulations creates deposits similar to debris avalanche deposits, so great care is needed to differentiate among them. Special attention was paid to this problem because real debris-avalanche deposits occur in this area too. The presence of hyaloclastite breccia domains and hyaloclastite fragments, along with phreatic tuffaceous material and tuffaceous sediments, is characteristic of hyaloclastitite-related submarine slump deposits. Variegated altered rock domains, oxidized lava flow blocks, subaerial pyroclastc rocks, and matrix-supported breccias make up typicl debris avalanche deposits. Shattered blocks of debris avalanche deposits are difficult to distinguish from chilled and highly fractured blocks of hyaloclastite slump deoposits.

The fifth afternoon was devoted to a spectacular section of the Rusca Tihu formation at the Rastolita brook dam site. A 300-meter vertical section is exposed in a continuous set of outcrops. The lower part of the section involves phreatic tuffs and reworked hyaclastite material in the form of debris flows and density current deposits indicating a shallow marine environment. Going upward, fall and flow tuffs occur interbedded with breccias, conglomerates and sandstones laid down by debris flows and ephemeral streams on an alluvial fan, indicating transition to terrestrial environment. The uppermost part of the section contains flat and thick lava flows.

We spent the last day of the field workshop at road cuts next to the Bistrita dam, exposing a 300-meter vertical section of the Rusca Tihu formation. Despite some features characteristic of debris-avalance deposits, for example, lithological variability and shattered lava domains, careful examination of outcrops revealed characteristic features of hyaloclastite complex affected by submarine slumping. The succession involves highly shattered and brecciated feeder dykes passing into hyaloclastite breccias with pseudopillows, numerous dykes with chilled margins ¾ feeders to hyaloclastite breccias higher up in the succession, lava flows with intense hyaloclastite disintegration and pseudopillow-like tongues, dense and vesiculated hyaloclastite breccias, reworked hyaloclastite breccias by debris flows and density currents, and variable phreatic tuff admixture.

Being in Romania, the farewell party could not have taken place anywhere else but the famous Dracula Castle overlooking the Calimani volcanoes and hills north of the Bistrita valley. It was a pleasant evening, featuring barbeque and excellent local wines, and a fitting end to a very successful workshop. I found the workshop very interesting and educational. Those who did not take part may regret their loss. Its value/cost ratio was unusually high. We are grateful to Alex Szakacs and Ioan Seghedi for all of their effort and their well-prepared guidebook introducing individual outcrops as well as geologic setting and problems to be discussed. Many thanks go to their colleagues, who made the workshop smooth and pleasant. Finally, we must express our gratitude to the Geological Survey of Romania (Institutul Geologic al Romaniei) and the IAVCEI for their generous support of the workshop.

Author Information

Stephen Straub
Institut fur Geologie, Geophysik und Geoinformatik
Freie Universitaet Berlin
T: +49-30-7792 695

Joan Marti
Inst. of Earth Sci. "Jaume Almera"
Luis Sole Sabaris s/n
08028 Barcelona, Spain
T: -34-3-330 27 16

Jaroslav Lexa
Geological Survey of Slovak Republic

Giovanni Macedonio
Volcanic Simulation Group
CNR-Gruppo Nazionale per la Vulcanologia
via S. Maria 53 I-56126
Pisa, Italy

Amanda Clarke
Penn State University
334 Deike Building
University Park, PA 16802, USA

Dannie Hidayar
Penn State University
334 Deike Building
University Park, PA 16802, USA

Barry Voight
Penn State University
334 Deike Building
University Park, PA 16802, USA

Paolo Papale
Volcanic Simulation Group
CNR-Gruppo Nazionale per la Vulcanologia
via S. Maria 53 I-56126
Pisa, Italy

Alexandru Szakacs
Institutul Geologic al Romaniei
Str. Caransebes 1
78344 Bucuresti 32

Armin Freundt
Wischhofstrasse 1-3, D-24148
Kiel, Germany

Augusto Neri
Volcanic Simulation Group
CNR-Gruppo Nazionale per la Vulcanologia
via S. Maria 53 I-56126
Pisa, Italy

Shanaka Desilva
Indiana State University
Dept. of Geology
Terre Haute, IN 47809

Michael Ort
Northern Arizona University
Dept. of Geology
P.O. Box 4099
Flagstaff, AZ 86011

Greg Valentine
Los Alamos National Laboratory
Mail Stop F665
Los Alamos, NM 87544

Vanessa A. De La Cruz
Los Alamos National Laboratory
Mail Stop F665
Los Alamos, NM 87544

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Dept. of Geology and Center for Environmental Sciences and Education, Northern Arizona University, Flagstaff, AZ 86011 USA

last modified: July 24, 1997

URL: http://vishnu.glg.nau.edu/cev