Eruptive dynamics evolution during the 800 yr BP Quilotoa eruption

18-22 January 2006

Brandon Browne

Department of Geological Sciences, California State University, Fullerton, USA



A five-day field-intensive workshop on and around Quilotoa caldera near Saucedo, Ecuador (~60 km south of Quito) took place during 18-22 January 2006, immediately preceding the 2006 Cities on Volcanoes 4 Conference. The trip was organized and led by Mauro Rosi (Dipartimento di Scienze della Terra, Pisa, Italy), Andrea Di Muro (Universit Paris VI, Paris, France) and Eduardo Aguilera (ESPE, Sangolqui, Ecuador). Thirty-five participants of various disciplines, nationalities, and dialects assembled at the amusingly eclectic Rumipamba lodge as a “base camp” for investigations the 800 yr BP eruption of Quilotoa. In addition to the extraordinary similarities shared between the 800 yr BP eruption and the 1991 Mt. Pinatubo eruption, the 800 yr BP eruption is also fascinating because of its impact of pre-Incan communities by displacing large populations to the north and triggering widespread social and technological changes. This workshop focused on three themes of the 800 yr BP caldera-forming eruption of Quilotoa: (1) the emplacement of deposits from contemporaneous convective and collapsing eruptive dynamics, (2) the progressive temporal evolution of Plinian dynamics from convective, to transitional, to collapsing, and (3) the relationship between magma ascent dynamics and the chemical and textural heterogeneities of 800 BP eruption products.


Day 1 (18 Jan) involved the arrival of participants to the lodge and evening presentations on the geology, stratigraphy, petrology, and geochemistry of Quilotoa, with an emphasis on relating eruptive processes with deposit facies architecture. Presentations by M. Rosi, A. Di Muro, and C. Principe served as an excellent introduction to the volcanology and eruptive history of Quilotoa volcano, the 800 yr BP eruption stratigraphy, characteristics of the erupted magma, and curious similarities with the 1991 Mt. Pinatubo caldera-forming eruption. These presentations also resulted in interesting discussions on the relationships between eruption behavior, conduit flow, and transport and depositional mechanics of pyroclastic currents, which were carried on throughout the workshop.

Day 2 (19 Jan) field activities concentrated on understanding how the 800 yr BP eruptive stratigraphy changed with decreasing distance to the caldera rim, and relating these changes to a contemporaneously convective and collapsing eruptive column. Morning field stops were located on the northern slopes of Quilotoa volcano in the Rio Toachi valley near the towns of Sigchos and Chucchilán. Brief stops above the Toachi Valley (~15 km from the caldera rim) combined with discussions led by M. Rosi and A. Di Muro afforded an informed and scenic perspective of the 300 m-thick loose pyroclastic flow deposits that accumulated within this and other paleovalleys within approximately 1000 km2 around the volcano. Secondary lahars resulting from remobilization of these pyroclastic flow deposits can be traced up to 200 km from the caldera rim. Continuing towards Quilotoa, field stops emphasized the changes in eruptive stratigraphy from distal to proximal deposits. Whereas distal deposits are characterized by normally graded, clearly defined pumice falls, proximal deposits are characterized by progressively thicker and complexly graded pumice falls interbedded with cross-bedded and normally graded fine-grained ash beds interpreted as pyroclastic surge beds and coignimbrite falls, respectively. As we approached the caldera rim in the afternoon, field stops showed evidence of the emplacement of pyroclastic material from contemporaneously convective and collapsing eruptive columns, as coarse-grained pumice falls became extensively subdivided by thin, laminated, fine-grained surge beds (Figure D2a). We reached the caldera rim around 5pm and were rewarded with a beautiful view of Laguna Quilotoa (Figure D2b). Our arrival also triggered the seemingly spontaneous appearance of dozens of curious (and quite entrepreneurial!) indigenous craftspeople selling a variety of vibrantly colored hats, sweaters, paintings, and masks celebrating their culture and Quilotoa. We arrived back at Rumipamba that evening, somewhat delayed by a spectacular accident involving a jam-packed truck, a hairpin curve, and a steep cliff, we enjoy a delicious meal before staggering off to our rooms to rest up for tomorrow's adventure.

Fig2a

Figure D2a. A. Di Muro describing 800 BP stratigraphy of coarse-grained pumice falls elaborately interbedded by thin, laminated, fine-grained surge beds on the north flanks of Quilotoa.

Fig2b

Figure D2b.Quilotoa caldera (<3 km) and lake from overlook.

Day 3 (20 Jan) field stops emphasized the transport and depositional characteristics of the 800 BP pyroclastic density currents by exploring the Zumbagua river valley, located on the southern flanks of Quilotoa. We began near Zumbagua village where we observed a sequence of distally emplaced thin fallout beds (analogous to those witnessed during Day 2) capped by fluvio-lacustrine deposits, which likely accumulated soon after the damming of the Zumbagua drainage system by the 800 yr BP deposits. Next, we hiked down into the Zumbagua Valley (Figure D3a) to view the voluminous pyroclastic flow deposits that were emplaced near the end of the fall-and-surge transitional phases observed during much of Day 2. Although these post-Plinian pyroclastic flows propagated all around the caldera, they were channeled preferentially by the N-S trending Zumbagua drainage system, resulting in the accumulation of up to 300 m within an area of only 45 km2. In the main drainage, deposits are massive, dense, and characterized by high aspect ratios (1:49-1:57). Hiking up into tributary channels extending perpendicular to the main flow direction of the drainage, we observed how the massive and dense pyroclastic flow deposits characteristic to the main drainage transform into thinner deposits characterized by improved sorting, waveform bedforms, and intercalation with fall deposits. These deposits have been interpreted by M. Rosi and A. Di Muro as a consequence of increased localized turbulence due to interactions with topographic obstacles. Afternoon field stops concentrated on tracking changes in pyroclastic stratigraphy with topography, culminating at a superb field exposure of large amplitude and wavelength regressive wave forms in the upper fine-grained intra-Plinian surge deposits high atop the eastern ridge of Zumbagua Valley (Figure D3b). These deposits were clearly emplaced by dilute and highly mobile pyroclastic surges, and can be observed up to 13 km from the caldera rim. Our adventure continued on the route home thanks to some extremely muddy back roads. Fortunately, our highly skilled drivers safely avoided drowning several vehicles in mud, delivering us back to Rumipamba around 8pm.

Fig3a

Figure D3a. Thick (up to 300 m) accumulation of dense and massive of pyroclastic flow units in Zumbagua valley.

Fig3b

Figure D3b. A. Di Muro describing large amplitude waveforms in pyroclastic surge beds atop eastern ridge of Zumbagua valley.

Day 4 (21 Jan) field stops included exemplary exposures of the pyroclastic sequence dissected by the Zumbagua drainage system as we continued where we left off at the end of Day 3 in out investigation of evolving depositional facies as a function of topography. Our first field stop was an exposure of the eruptive stratigraphy perched immediately atop the Zumbagua drainage (7 km from the caldera rim), which offered a valuable intermediate example between the massive, dense pyroclastic deposits in the main drainage basin and the waveform deposits from dilute and highly mobile pyroclastic currents high atop the eastern ridge of the Zumbagua valley observed at the end of Day 3. We then moved to an exposure located 3 km from the caldera rim at the break-in-slope between the volcano flanks and Zumbagua valley (Figure D4a). Here, coarse pumice falls are interbedded with laterally continuous stratigraphic units that significantly thicken and display high amplitude regressive waveforms, signifying the importance of the emplacement of deposits from contemporaneous convective and collapsing eruptive dynamics and the influence of topography on the transport and deposition of pyroclastic deposits. The remainder of the day was spent exploring the pyroclastic density current deposits exposed in the numerous channels extending up elevation and perpendicular to the main axis of the Zumbagua drainage (Figure D4b). These outcrops, and the interpretations that arouse from viewing them, initiated some stimulating and quite lively debates (no speaker phones were needed for these!). A number of impromptu presentations (M. Rosi, A. Di Muro, S. Takarada, B. Houghton, R. Cioni, D. Palladino, and J. Marti) ensued where the speakers and audience members had the advantage of genuine outcrops at arms length (as opposed to PowerPoint slides). These impassioned debates resulted in some of the most interesting and insightful discussions of the entire workshop. The exchange of ideas between distinguished researchers and emerging volcanologists brought new energy and fresh perspectives to long standing questions relating to the transport and depositional mechanics of pyroclastic density currents.

Fig4a

Figure D4a. Regressive bedforms in the pyroclastic surge deposits at the break-in-slope of the volcano flanks and Zumbagua valley.

Fig4b

Figure D4b. A. Di Muro describing the transformation of pyroclastic density current deposits from the base of the Zumbagua valley to tributary channels extending up-slope perpendicular to the drainage system.

Day 5 (22 Jan) involved the departure of participants from the lodge to Quito for the Cities on Volcanoes 4 Conference, but not without some field activities along the way. Participants visited Cotopaxi National Park, where they were introduced to Cotopaxi volcano's geology, and recent eruptive stratigraphy, including several coarse pumice falls and lahar deposits (Figure D5a) (M. Rosi, M. Pistolesi, and F. Barberi). Participants were driven to an overlook parking area, high on Cotopaxi's north flank for a scenic overview of the regional geology (M. Rosi), some quick pictures, and then quickly descended for lunch. At this point, the group separated. One group headed to Quito to attend the COV4 Ice Breaker and prepare for their conference presentations, while others made their way to Hacienda La Cienega to enjoy a well-deserved and relaxing lunch. La Cienega, built the late 17th Century has served witness to centuries of important events in the history of Ecuador, such as housing activists in the cause for Ecuador's independence from Spain, and scientists like Charles Marie de la Condamine, member of the 18th Century French Geodesic Mission that determined Earth's true size, and Alexander von Humboldt, the renowned German scientist widely credited as one of the founders of modern geology. Participants deeply appreciated this opportunity to end their Quilotoa field workshop in place central to both Ecuadorian history as well as the history of Earth Science.

Fig5a

Figure D5a. Cloud capped Cotopaxi volcano and apron of Holocene lahar deposits in foreground.

Fig5b

Figure D5b. Hacienda La Cienega. (Photo by www.ecuador-images.net)

In short, this was an extremely successful and thought provoking workshop thanks to exceptional organization and enthusiasm by the workshop leaders Mauro Rosi, Andrea Di Muro, and Eduardo Aguilera. Their desire to conduct this workshop in a friendly, informal, and field-intensive environment strongly enhanced the enjoyment of all participants. One of the most successful aspects of the workshop resulted from field trip organizers dedication to conducting each field stop as a special opportunity for discussion and exchange of ideas among the participants. This presented abundant opportunities for discussion between all participants regardless of expertise or experience at outcrops, which generated invaluable exchanges of ideas and resulted in a great deal of new insight from a variety of perspectives to long-standing questions. M. Rosi and A. Di Muro also demonstrated tremendous patience in dealing with the different needs of the participants (e.g. language, knowledge base, experience, contrasting opinions). Lively discussions mostly centered on (1) the transport and deposition of pyroclastic material, (2) the relationship between eruption processes in the conduit and vent with depositional features observed in outcrop, and (3) the nature and mechanics of collapse process at relatively small calderas, like Quilotoa. All participants left the workshop sharing an appreciation for the importance of continuing to synthesize field-based observations with numerical and geophysical modeling efforts.

Group

Photo of 2006 Quilotoa Field Workshop Participants at the end of Day 4 in the Rio Zumbagua drainage. Workshop participants (in alphabetical order): Eduardo Aguilera, Alvaro Amigo, Allison Austin, Daniele Andrinico, Costanza Bonadonna, Brandon Browne, Kate Bull, Rebecca Carey, Kathy Cashman, Corrado Cimarelli, Raffaello Cioni, Licia Costantini, Donatella De Rita, Andrea Di Muro, Mario Gaeta, Bruce Houghton, Patrizia Landi, Nicole Lautze, Anne-Marie Lejeune, Christyanne Melendez, Michael Ort, Danilo Palladino, Laura Pioli, Marco Pistolesi, Claudia Principe, Nancy Riggs, Sheng-Rong Song, Mauro Rosi, Julia Sable, Flavia Salani, Patricia Sruoga, Wendy Stoval, Shinji Takarada, Alain Volentic, Heather Wright, Brian Zimmer, Joan Marti (Photo kindly provided by Laura Pioli)



Last Update: April 2, 2006