Neogene-Quaternary Continental Margin Volcanism
Metepec, Puebla, México
12-16 January 2004
- Gerardo J. Aguirre-Diaz
- Centro de Geociencas, Universidad Nacional Autónoma de México, Juriquilla, México;
- José Luis Macías and Claus Siebe
- Instituto de Geofisica, Universidad Nacional Autónoma de México, México D.F., México
Grant Heiken, Freeland, Washington, USA
|Conference participants at Tlamacaz mountaineering lodge on the upper slopes of Popocatépetl. Iztaccíhuatl volcano (5,272 m) in the background. Click on photo for larger image.|
The Mexican volcanic belt crosses México from the Pacific to the Gulf of Mexico and is composed of hundreds of Neogene-Quaternary volcanoes, ranging from humble scoria cones to the great composite cones of Popocatépetl and Citlaltépetl (Orizaba). Holocene and Historic eruptions continue to affect the peoples of México; for example, eruptions changed the course of history between Classic and Post-Classic cultures when the valleys of México and Puebla were subjected to tephra fallout and secondary lahars (volcanic mudflows). Understanding magma evolution and eruption dynamics are critical to México in that modern urban agglomerations ranging in size from 2.2 to 20 million people will be affected by future eruptions.
The purpose of this Penrose Conference was to evaluate the present state of knowledge of the source and evolution of magmas that formed the Neogene-Quaternary continental-margin volcanic belt associated with the Mexican portion of the "Ring of Fire." Discussions included the complexities of volcanic styles that promote explosive eruptions, sector collapse of volcanoes, volcaniclastic sedimentation, and related volcanic hazards. The case of México was compared with continental-margin volcanism at other places in the Americas, such as the Andes in Colombia and the Cordillera and Cascades of the western United States.
Working upward from the roots of continental margin volcanic belts, keynote speakers Charles Langmuir and Gerhard Wörner reviewed the latest interpretations of processes at convergent plate margins. Using compositional data mostly from the trans-Mexican volcanic belt lavas and thermal melting models of the subduction process, Langmuir proposed that controls of the degree of melting include wedge geometry and thermal structure, source compositions, and convergence rates. Working with samples from the well-exposed eruption sequences of the central Andes, Wörner concluded that magma genesis through space and time was controlled by crustal heterogeneity and thickening since the Miocene. The posters covered mostly recent petrological studies of volcanic complexes in México, Central America, the Cascades, and New Zealand. Stephen Grand described a newly funded project to image the Mexican convergent margin with a 50+ seismometer array, a work that should tie well into the many petrological transects of the volcanic belt.
Based on the Smithsonian database for the 1926 Mexican volcanoes, Jim Luhr concluded that the three main magmatic suites in México are (1) calc-alkaline magmas of the trans-Mexican volcanic belt and Northern Mexican Extensional Province, (2) lamprophyres of the western trans-Mexican volcanic belt, and (3) intraplate type mafic alkaline rocks of the Northern Mexican Extensional Province and Pacific Islands. Sixteen posters covered a wealth of new work on the petrology of trans-Mexican volcanic belt volcanoes, including the Chichinautzin volcanic field, which rims the southern end of the Valley of México. Xitle, one of the Chichinautzin monogenetic volcanoes, produced a lava flow with a 14C age of 1665 years B.P., which partly buried Cuicuilco pyramid and now underlies the campus of the National University.
|Conference participants inspecting the 14,000 yr B.P. “Tutti Frutti” sequence at Popocatépetl during the field trip. Click on photo for larger image.|
While pungent smells from Popocatépetl occasionally swept the conference complex at Metepec, Fraser Goff and James Gardner gave keynote lectures on the physical role of gases in continental margin volcanism. Given the great depth of Popocatépetl's crater and the possibility of eruptions at any time, in-situ gas sampling has been impossible. Therefore, most of Goff's data were from simultaneous FTIR and COSPEC measurements of eruption plumes. Like most volcanoes at convergent margins, emissions from Popocatépetl are mostly water vapor with high values of Stotal, F, As, and Hg (with respect to Cl). Over a four-year period, the HCl/SO2 ratio has been increasing and HF/SO2 ratios have been decreasing. CO2 fluxes range from 40,000 t/day to >100,000át/day. Plume emissions also contain SiF4. The observed range of gas ratios indicates temperatures of 200 ▒ 20 °C, which may indicate reaction of gases with conduit walls or with ash particles in the cooling plume.
With an emphasis on rhyolites, James Gardner reviewed the dynamics of bubble growth in magmas, integrating observation with experiments. The pumice, ash, and gases erupted are all products of bubble nucleation, growth, and degassing. Bubble interactions and partial connectivity can produce coexisting bubbles that differ in size by more that 20 times. During the roundtable discussion, it was noted that there is passive degassing between eruptive phases.
Zimmer and Erzinger's continuous measurements of fumaroles at Merapi, Indonesia, demonstrate the influence of rainfall on gas flux and temperatures; during the dry season, fumarole temperatures are constant. Using data from Mount Pinatubo, Philippines, Hammer concluded that the explosive eruption of dense, microlite-rich, partly-degassed tephra, along with vesicular, microlite-free pumice during pulsatory eruptions reflects variable magma travel times.
Having constructed the volcanoes, the meeting moved on to their destruction via sector collapse, avalanches, and lahars. This session was an excellent introduction to the field trip during the last day of the meeting to deposits left by at least three sector collapses of the Paleo-Popocatépetl cone; the youngest of these deposits is ~23,000 yr B.P. Hummocky terrain and debris avalanche deposits south-southwest of Popocatépetl cover over 300 km2 and have a cumulative volume of >30 km3. The youngest of the avalanche deposits are overlain by blast and pyroclastic flow deposits. Preliminary studies indicate that runout was at least 70 km south of the cone. Sector collapse and debris avalanches have occurred at nearly all large Mexican composite cones.
In the Cascade Range of the United States, the dense forest makes documentation of lahar (volcanic mudflow) deposits extremely difficult. Kevin Scott is now documenting the extent and magnitude of lahar deposits with lidar imagery. This application has enhanced evaluation of lahar hazards, which in the past have been very conservative. He explained that lahar mobility is difficult to demonstrate to disaster planners and has found that comparing it with water floods gets the message across. Lahar warnings are possible. Survivors of the Nevado del Ruiz catastrophe felt ground tremors before the lahar arrived. On that basis, acoustic flow monitors have been installed in the valleys around Mount Rainier to provide at least a 45-minute warning to downstream towns.
In the panel discussion it was agreed that the sector collapse, avalanche, and mudflows that were generated by the 1980 eruption of Mount St. Helens was the turning point in recognizing this widespread hazard. Since 1980, 300 sector collapse deposits have been identified at volcanoes along continental margins; their volumes range from 0.1 to 5 km3 (Lee Siebert). Hot research themes include the transition of sector collapse avalanches to debris flows and why some debris avalanches stop and others liquefy and keep moving; for example, the lahars formed during the last eruption of Cotopaxi reached the sea, 300 km downstream. The sedimentary aftermath of a large-scale eruption may be a hazard larger than the eruption itself. James White described eruptions on the North Island of New Zealand that buried entire drainage systems and created lakes. Subsequent failure of the tephra dams produced high-concentration flows that swept downstream areas now occupied by cities.
México has some of the largest ignimbrite deposits in the world. For example, in the Sierra Madre Occidental, ignimbrite sequences can be thousands of meters thick and have volumes of hundreds or even thousands of km3. Most are associated with Miocene calderas and may have been erupted during the opening of the Gulf of California. Much work needs to be done on this province in spite of the difficulties offered by the terrain and isolation. Mike Branney provided a summary of what is known about massive ignimbrite deposits ("large explosive eruptions" were defined by Branney as those with volumes of >1 km3 to >1000 km3). Most are characterized by high mass flux sustained for hours or days and associated caldera collapse. The pyroclastic density currents produced during these eruptions can be fully dilute or granular-fluid-based, based on conditions near the lower flow boundaries. Complex deposit architectures are affected by the nature of the density current and the complexity of underlying terrain.
Within volcanic provinces with large volumes of rhyolite, it is often difficult to distinguish between densely-welded rheomorphic ignimbrites and clastogenic lava flows. Using examples from the Snake River Plain, Bill Bonnichsen described transitions from non-welded ignimbrites to high-grade rheomorphic ignimbrites to clastogenic lava flows and on out to domes having little evidence for fragmentation.
Dealing with a deadly but much smaller-scale eruption type than the large ignimbrites, the session on block and ash flows associated with dome collapse focused on recent eruptions at volcanoes like Colima, México, and Montserrat. Block and ash flows begin with dome collapse and formation of decameter-sized blocks (Marcus Bursik). In the final deposit the same blocks have been fragmented into particles ranging in size from 1 mm to 2 m. High on the dome slopes (~35°), the deposits are 1 to 2 m thick, but in distal regions with slopes of 10° to 20° they are up to 8 m thick. Many are preceded by pyroclastic surges. Much of the mechanical crushing seems to occur at breaks in slope. Bursik also discussed modeling of these flows but emphasized "numerical models are useless without field observation."
The last link in the chain of discussions that began with magma genesis was that of reducing risk at continental-margin volcanoes. The mitigation issues reviewed by Robert Tilling are:
- increasing population growth and air traffic;
- no capability to reliably predict explosive eruptions;
- the fact that most volcanoes are not monitored;
- a low frequency of destructive events; and
- effective communication between scientists, civil authorities, news media, and the population is at risk.
A major quandary concerns monitoring parameters versus time; when do you force local officials to make decisions? There is also the dilemma of acquiring funding for monitoring for infrequent but catastrophic activity. Hazard maps are useful for those who understand how to read them, but don't work well for the public. Public lectures and pamphlets are useful but reach only about 1% of the public. Visualization of processes and videotapes of actual events are more effective, but not if the media will not present them to the public. Methods of reaching the public were discussed during the roundtable, with the suggestion that a system similar to that used by the weather service for tornado warnings be implemented.
Gavilanes noted that mixed signals put the population at risk during eruptions of Volcán de Colima during 1999-2003. There were many conferences in the villages, but conflicting signals from the authorities and the army created confusion and frustration among the residents. He also noted the great need for sociologists to work with volcanologists for effective hazard mitigation.
In slightly less than one week, this dynamic group of 104 workshop participants (including about 30 students) went from the mostly academic pursuit of petrology to the sociological aspects of volcanic disaster mitigation. What was evident is that all of the pieces are available to complete the puzzle, but that the volcanological community has a long way to go to assemble those pieces and to mitigate volcanic hazards. There is hope for increased volcanological research and successful disaster mitigation in México; at this Penrose Conference it was evident that, in addition to the well-respected Mexican volcanological community, there are many talented students at Mexican universities who will carry this work into the future.
For more information on the conference, visit http://tepetl.igeofcu.unam.mx/penrose/index.html. A GSA publication is also in preparation (Volcanic hazards in the México City metropolitan area from eruptions at Popocatépetl, Nevado de Toluca, and Jocotitlán stratovolcanoes and monogenetic scoria cones in the Sierra Chichinautzin volcanic field, by C. Siebe and J.L. Macias).
Additional Sponsoring Organizations:
- International Association of Volcanology and Chemistry of the Earth's Interior
- Coordinación de la Investigación Científica, Universidad Nacional Autónoma de México (UNAM)
- Instituto de Geofísica, UNAM
- Instituto de Geología, UNAM
- Centro de Geociencias (Juriquilla, Querétero), UNAM
- Centro Universitario para la Prevención de Desastres, Benemérita Universidad Autónoma de Puebla
- Gobierno del Estado de Puebla, México
- Volkswagen de México (Puebla)
Gerardo J. Aguirre-Díaz