Volcanoes : looking back to their past to predict their future

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Sheet nº 233
January 2006
Volcanoes : looking back to their past
to predict their future
The volcanic arc of the Andes chain was
generated by subduction of the Nazca
oceanic plate under the South American
continental plate. Since 1995, IRD scientists and their colleagues (1) have been
conducting research on volcanoes in
Ecuador with growing interest. Their aim is
the study and management of volcanic
hazard and improve prevention strategies.
Recently they reconstructed the history of
the Cayambe Volcanic Complex and mapped the area. This formation gives the
appearance of being dormant. However, if
its activity resumes this could be violent
and bring 30 000 people under threat.
Knowledge of its past behaviour is the key
to an understanding of its potential future
activity.
he Andean Cordillera is exceptional as a
kind of natural laboratory, useful for understand the structure, evolution and functioning
of a subduction zone between oceanic and continental tectonic plates. The arc arose as a result of
subduction of the Nazca oceanic plate under the
South American continental plate. As the Nazca
plate plunges under the continental plate, descending parts are subjected to increasing pressures
and temperatures. This process leads to the formation of magmas which, rising up again in places
across the continental plate, are the source of the
Andean volcanoes. Since 1995, IRD’s Magmas et
Volcans laboratory (UMR 163) (2) has been
conducting a programme of volcanic research in
Ecuador in conjunction with scientists from the
Institute of Geophysics of National Polytechnic
(Escuela Politecnica Nacional, EPN) of Quito and
from the Jeune Équipe Associée (JEAI) recently set
up there. Interest in volcanic hazard and its management is growing and this team has brought the
subject, and improvement of prevention strategies,
T
©IRD/Michel Monzier
into focus. Reconstruction of the history of a volcanic complex is the foundation for this work; knowledge of the past behaviour of a volcano is the key
to considered prediction of its future activity.
In this context, the latest work published by JEAI
and IRD researchers are the result of several years’
multidisciplinary investigation conducted on the
Cayambe Volcano Complex. Its presentation as a
dome, rising to 5790 m, and glacier-topped, along
with the absence of manifest activity gave sustenance to the idea, held until quite recently, that the
volcano was extinct. Covered by a thick ice cap, it
represents the ideal case of a dormant volcano, but
whose inactivity could be only temporary and
reawakening extremely dangerous. Its activity at
depth is similar to that of active Ecuadorian volcanoes, like Cotopaxi. Recordings have revealed
strong seismic activity beneath the summit.
The researchers have just finalized the first
complete geological map of the Cayambe complex.
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The exhaustive study of the formation began with investigation of lava flows and recent fallout events involving
volcanic rock fragments (pyroclastic fallout), sampling and
mapping of different parts of this volcano. The Cayambe
Volcanic Complex is made up of two large edifices, which
from West to East, consist of:
1) The Viejo Cayambe, a large structure consisting mainly
of lava, highly eroded and aged about 1 million years (Ma);
2) The Nevado Cayambe, a less voluminous structure
formed from about 0.1 Ma B.P., essentially composed of
magma whose chemical composition contains between
63 and 69 % of silica (dacitic) corresponds to a more
explosive form of activity. It is topped by an extensive
complex of domes, partly Holocene, forming the peaks of
this massif; the Angureal cone, aged about 0.4 Ma, corresponds to the transition between the two major formations;
3) The "Cono de la Virgen" is a relatively small satellite
structure, probably Holocene, installed on the lower
eastern slopes of Nevado Cayambe, which poured out
sizeable flows of lava, and which was the site of the last
eruption in 1785-1786.
The first absolute chronological history of this imposing volcanic formation has been obtained, in collaboration with the research unit UMR Géosciences Azur (3),
using isotopic measurements on argon, a rare gas entrapped in the lavas. These 40Ar/39Ar dating measurements
complement data from the study of ash and pumice fallout
on a mountain peat-bog. Carbon 14 dating was thus
carried out at different levels of these deposits. Thus, over
the past 4 millenia, the volcano Cayambe Nevado has
been active in three eruptive periods each about 700
years long, separated by phases of quiescence of about
500 to 600 years. Before this the volcano had long remained inactive.
chemical composition of the lavas is linked to a strong
involvement of products of fusion of the oceanic crust
plunging under the continental crust (and not only from
mantle fusion as is usually the case). These results give
clues for understanding the evolution of the eruptive style
of Cayambe whose magmas become increasingly acid, in
other words richer in silica, and more and more viscous
–hence increasingly explosive. The eruptive styles which
characterize the three large recent cycles are therefore
those of a volcanic system with head domes. At least 23
eruptions were identified for a 4000-year period. They
generated pyroclastic lava flows and eruption fallout
forming a turbulent volcanic plume which often rises as far
up as the stratosphere (plinian fallout). This corresponds
to an average of little over one eruption per century. The
time elapsed since the last eruption, in 1785-1786, is
therefore not long enough to affirm that the latest period of
activity has ceased. These results shed new light therefore
on the threat this strongly glaciated volcano, hangs over
populations living at the foot of that mass. In the case of an
event involving the the volcano summit, an ice-flow catastrophe could occur and directly threaten the town of
Cayambe and surrounding villages, along with the fertile
plain situated west of the volcano. A population of over 30
000 would be under threat
Disaster prevention measures have been published
with a map of volcanic hazards threatening Cayambe and
in a book in Spanish, intended for people living around the
volcano in order to advise them how to live with this turbulent neighbour. All these efforts therefore are part and
parcel of a general strategy going from the most detailed
scientific study possible using the most advanced techniques, to the application of the results obtained with a
view to reducing the risks run by the people of Ecuador.
________________________
The researchers also present results of petrological and
geochemical investigations that show the evolution of
magmas during the history of the volcano, resulting mainly
from processes taking place deep under the volcano. The
(1) Researchers of the Institute of Geophysics of the EPN of Quito and
those of the Jeune Équipe Associée (JEAI) recently set up at the
EPN.
Redaction – IRD : Beatrice Le Brun/Sophie Nunziati
Translation : Nicholas Flay
(3) UMR Géosciences Azur; CNRS-IRD-Universities of Nice-Sophia
Antipolis and Paris VI joint research unit.
(2) UMR 163 Laboratoire Magmas et Volcans; IRD-CNRSUniversities Blaise Pascal (Clermont-Ferrand) and Jean Monnet
(Saint Etienne) joint research unit.
For futher information
CONTACTS:
Jean-Philippe EISSEN – UMR 163 Laboratoire Magmas et Volcans, unité mixte de recherche IRD, CNRS, universités Blaise Pascal et
Jean Monnet
Université Blaise Pascal, 5 rue Kessler, 63038 Clermont-Ferrand, France; Tel. : +33 (0)4 73 34 67 53; Email: [email protected]
Pablo SAMANIEGO, Institute of Geophysics, Escuela Politecnica Nacional, Quito, Ecuador and Jeune Equipe Associée at the IRD
(JEAI) – UMR 163 Laboratoire Magmas et Volcans Université Blaise Pascal, 5 rue Kessler, 63038 Clermont-Ferrand, France;
Email: [email protected] / [email protected]
IRD Communication :
Sophie Nunziati (press officer), Tel.: +33 (0)1 48 03 75 19, Email: [email protected]
REFERENCES
Pablo Samaniego, Hervé Martin, Michel Monzier, Claude Robin, Michel Fornari, Jean-Philippe Eissen, Joseph Cotten, Temporal evolution
of magmatism in the Northern volcanic zone of the Andes : the geology and petrology of Cayambe Volcanic complex (Ecuador), Journal of petrology 2005, n°46: 2225-2252;
Samaniego P., Eissen J.-P., Monzier M., Robin C., Alvarado A., Yepes H., 2004 - Los peligros volcánicos asociados con el Cayambe. Serie
Los peligros volcánicos en Ecuador, No. 2. Corporación Editora Nacional, IG-EPN, IRD, 94 pp.
Samaniego P., Monzier M., Eissen J.P., Yepes H., 2003 – Mapa de peligros potentiales del volcán Cayambe. (1/70.000) : Edit. IGM-IG/EPNIRD.
ILLUSTRATIONS
Contact Indigo Base, IRD picture library, Claire Lissalde or Danièle Cavanna, Tel.: +33 (0)1 48 03 78 99, Email : [email protected]
The illustrations can be viewed on: www.ird.fr/us/actualites/fiches/2004/fiche233.htm
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