Evidence for lahar-triggering mechanisms in complex stratigraphic sequences: the post-twelfth century eruptive activity of Cotopaxi Volcano, Ecuador

M Pistolesi, R Cioni, M Rosi, Katharine V Cashman, A Rossotti, E Aguilera

Research output: Contribution to journalArticle (Academic Journal)peer-review

27 Citations (Scopus)

Abstract

Cotopaxi volcano is situated in the Eastern
Cordillera of the Ecuadorian Andes and consists of a symmetric
volcanic cone that reaches an altitude of 5,897 m
above sea level; it is capped over its upper 1,000 m by a
permanent glacier. The volcano has erupted frequently in the
past few centuries and, according to the archival records, has
produced dozens of lahars by catastrophic snow and ice
melting during eruptions. In this work, we present a detailed
map and a stratigraphic study of the lahar deposits of the
past 800 years in two different topographic settings. A
thorough knowledge of the tephrostratigraphy of the explosive
activity over the same time period was a first-order prerequisite
for the complete reconstruction and dating of lahar
activity and also allowed us to precisely link lahar units to
eruptive phases of individual eruptions. Results indicate
that, during the thirteenth to seventeenth centuries, highintensity
eruptions (Plinian events or blast-like explosions)
produced large debris flows that transported meter-sized
boulders. A subsequent period of activity that started in
1742 was characterized by several lahar-generating eruptive
episodes that were smaller in scale but with significant
variability in size (the 1877 being the smallest and most
recent). Analysis of events occurring in the eighteenth century
suggests that eruption style affects the volume and
energy of the resulting lahars, with different pyroclastic flow
types causing different mechanisms of water release from
the summit glacier. Lahars produced during this time period
were triggered by: (1) dilute pumice and ash-rich radially
distributed density currents and (2) column collapse-related
radially distributed scoria and lithic-rich pyroclastic-flows.
The former produced lahar deposits that are matrix-rich,
block-poor, and valley-confined, while the high erosive
capacity of the latter produced lahars that are block-rich,
highly energetic, and widespread. The youngest (1853 and
1877) lahars were triggered by (3) confined scoria-flow
lobes that had less capacity to scour and melt the glacier;
resulting flows had lower energy and smaller volumes than
lahars produced by eighteenth-century eruptions. We conclude
that the dynamics of pyroclastic–density–current can
exert a major control on the size and destructive capacity of
lahar at ice-capped volcanoes. Moreover, the total extent of
the glacier at the moment of eruption, which is commonly
considered to exert a major control on lahar formation, may
actually be a second-order factor compared with the way in
which the eruptive products interact with the glacier.
Original languageEnglish
Number of pages18
JournalGeological Society of America Bulletin
Volume75
Publication statusPublished - 2013

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