Eruptive dynamics evolution during the 800 yr BP Quilotoa eruption
18-22 January 2006
Department of Geological Sciences, California State University, Fullerton, USA
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
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.
Figure D3a. Thick (up to 300 m) accumulation of dense and massive of pyroclastic flow units in Zumbagua valley.
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.
Figure D4a. Regressive bedforms in the pyroclastic surge deposits at the break-in-slope of the volcano flanks and Zumbagua valley.
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.
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.
Figure D5a. Cloud capped Cotopaxi volcano and apron of Holocene lahar deposits in foreground.
Figure D5b. Hacienda La Cienega. (Photo by
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.
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