XII Congreso Geológico Chileno
Santiago, 22-26 Noviembre, 2009
S7_007
Glacier advance at Volcán Michinmahuida; a consequence of
the ongoing eruption of Volcán Chaitén?
Rivera, A.1,2,3, Bown, F. 1,2, Cawkwell, F. 4, Bravo, C. 1, Rada, C. 1, Zenteno, P.1
(1) Centro de Estudios Científicos (CECS), Av. Prat 514, Valdivia, Chile.
(2) Centro de Ingeniería de la Innovación del CECS (CIN), Av. Prat 514, Valdivia, Chile.
(3) Departamento de Geografía, Universidad de Chile, Portugal 84, Santiago, Chile.
(4) Department of Geography, University College Cork, Cork, Ireland.
[email protected]
Introduction
Up to a few years ago, the ice thinning and shrinkage taking place in the glaciers of
Southern Chile was thought to be as a direct response to the climate changes observed by
the meteorological network. However, many of these glaciers are located on top of active
volcanoes, and therefore their behaviour is also influenced by the intensity and type of
eruption and on the basal geothermal influx. For instance, some Strombolian to Plinian
eruptions within the past century have caused glaciers within calderas or on volcanic
flanks to rapidly shrink or even be completely destroyed, but afterwards, some cases of
ice regeneration have been recorded. In contrast, pyroclastic material deposition on
surface may have the effect of reducing the surface ablation, thus preventing rapid
thinning which can occur on bare ice that is directly exposed to the atmosphere [1].
Moreover, the influence of geothermal heat beneath the surface may have a direct impact
on basal ablation and downstream glacier hydrology.
In Southern Chile (41-44ºS) there are several ice capped volcanoes, as well as many
glaciers in non-volcanic environments. Many of these glaciers have shown trends of
retreat since the beginning of the 20th century, with huge ice area losses, irrespective of
underlying volcanic activity [2]. These retreats are associated mainly with a decreasing
trend in precipitation [3], and not to atmospheric temperatures, which have shown only a
weak cooling trend (-0.1ºC/decade) since 1976. No glacier advances have been observed
on the Chilean side of the Andes in this region, however on the Argentinean side, the
glaciers of Monte Tronador (41ºS) experienced a short period of advance between 1970
and 1977 [4], having retreated thereafter.
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XII Congreso Geológico Chileno
Santiago, 22-26 Noviembre, 2009
In order to distinguish between climatic and non-climatic (e.g. volcanic) glacier
behaviours, two glaciated areas located in Southern Chile were studied in detail: a)
Volcán Michinmahuida (42°47’S/72°26’W, 2450 m a.s.l.) - an ice-capped active volcano
with the last recorded eruption in 1835, although more recent geothermal activity could
explain the generation of laharic flows in recent decades [5]; and b) Glaciares
Inexplorados (41º58’S/72º11’W, 2334 m a.s.l) - a non-volcanic mountain range from
which several ice tongues flow, mainly to the west, with significant frontal retreat
recorded since 1961 [2].
Methodology
Aerial photography and visible high resolution satellite imagery (Landsat ETM+ and
ASTER) from several acquisition dates were collected and subsequently orthorectified
with SRTM topographic data. Snow, ice and debris cover were distinguished by
classification of band ratio images (for example using green and MIR wavelengths).
Using Geographical Information System software, glacier frontal and areal variations
over time were derived by comparison of the multi-temporal image data set, allowing
glacier behaviour on decadal time-scales to be mapped. An error assessment for the
position of each glacier front was also performed. Pre- and post-eruptive event images
allow the spatial extent of volcanic ash deposition and albedo changes over ice surfaces
to be evaluated.
Preliminary results and conclusions
Volcán Michinmahuida was covered in 1979 by 93.3 km2 of ice distributed in 9 glacier
basins, which by 2007 had dropped to 81.4 km2, representing a net surface loss of 13%
during the 18 year-period. The longest retreat is recorded at Glaciar Amarillo, the main
glacier flowing southward, which experienced a negative change in excess of 2 km over
the past four decades (Figure 1 and 2C). Superimposed on this long term trend, in
November 2007 the three main tongues, flowing in different directions from the volcanic
summit, were seen to be significantly advancing; a state that was still apparent in June
2008 and January 2009. Overall this advance has produced a mean change in length since
2007 of 234 ± 49 m (Figure 1 and 2).
Between 1961 and 2007, the main glaciers of the Inexplorado sector (41º58’S/72º11’W,
Río Traidor IGM chart), located within the same climatic region as Michinmahuida in
Chiloé Continental, retreated at a rate very similar to those of Volcán Michinmahuida
(Figure 1). However, between November 2007 and January 2009, the glaciers at
Inexplorado experienced even more rapid retreat, with the ice front receding at rates of up
to -143 ± 9 m a-1.
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XII Congreso Geológico Chileno
Santiago, 22-26 Noviembre, 2009
Glaciers primarily respond to prevailing atmospheric conditions, with a time delay
depending on glacier length, mass balance and ice thickness. On a decadal time scale, the
glaciers of Southern Chile are acknowledged to be responding to the changing climate of
the region in the form of reduced precipitation, however there are no significant climatic
recent anomalies which could explain the ice advances seen on Volcan Michinmahuida.
This suggests that the behaviour of these glaciers is additionally driven by non-climatic
factors, namely volcanic activity associated with the eruption of Volcán Chaitén located a
few kilometres to the west of Volcán Michinmahuida (Figure 2). The volcanic vent,
located in a glacier-free caldera, has generated a large volume of material since the
eruption began in May 2008 [6], creating a thick layer of ash deposits over the region,
including on the glaciers of Volcan Michinmahuida. This deposition of low albedo
material is clearly distinguishable when comparing pre- and post-event ASTER scenes,
and it is possible that the ash layers insulate the underlying ice from solar radiation.
However, as the glacier advance began a few months before the eruption event, the
increased ice flow could also be a response to higher subsurface geothermal heat fluxes
prior to the initial eruption. If heating at the bedrock of these glaciers increased, leading
to the generation of basal meltwater, it is possible that enhanced basal sliding would
occur, resulting in a sudden glacier advance. In spite of the synchronous events (eruption
and glacier advance), much more work is required to determine the causes of present
glacier behaviour.
References
[1] Brock, B., Rivera, A., Casassa, G., Bown, F., Acuña, C. (2007) The surface energy balance of
an active ice-covered volcano: Volcán Villarrica, southern Chile. Annals of Glaciology, vol. 45,
104-114.
[2] Rivera, A., Bown, F., Acuña, C., Clavero, J., Ordenes, F. (2008) Chilean glaciers as indicators
of climate change. Terra Glacialis, 11, 193-207.
[3] Aravena, J.C., and B. Luckman (2008) Spatio-temporal rainfall patterns in Southern South
America. International Journal of Climatology, doi: 10.1002/joc.1761.
[4] Masiokas, M. Villalba, R., Luckman, B., Lascano, M., Delgado, S., Stepanek. P., (2008) 20thcentury glacier recession and regional hydroclimatic changes in northwestern Patagonia. Global
and Planetary Change, vol. 60, 85-100.
[5] González-Ferrán, O. (1995) Volcanes de Chile. Instituto Geográfico Militar, Santiago, Chile,
635 pp.
[6] Watt, F., Pyle, D., Mather, T., Martin, R., Matthews, N. (2009) Fallout and distribution of
volcanic ash over Argentina following the May 2008 explosive eruption of Chaitén, Chile.
Journal of Geophysical Research, vol. 114, B04207, doi:10.1029/2008JB006219.
Acknowledgements:
This research is funded by FONDECYT 1090387.
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XII Congreso Geológico Chileno
Santiago, 22-26 Noviembre, 2009
Figure 1. Frontal variations of Glaciar Amarillo (red) and Glaciares Inexplorados (black).
Figure 2. Glacier frontal variations at Volcán Michinmahuida 2006-2009. ASTER scene
acquired on January 19, 2009, during the eruption of Volcán Chaitén (bottom left).
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