Invited reviewClassification of debris-covered glaciers and rock glaciers in the Andes of central Chile
Introduction
According to conventional standards, an average annual water availability of 1700 m3/y/person is the threshold for a country to meet all of its hydrological demands. A condition of water scarcity occurs when water availability falls below 1000 m3/y/person; when water availability reaches below 500 m3/y/person, it is said to represent a condition of absolute scarcity (UNDP, 2006). Chile has an impressive per capita annual average of 60,000 m3 of water availability; however, the geographical distribution of natural water availability is highly uneven (Fig. 1). The population in the northern half of the country lives under conditions of water scarcity, where water availability is < 1000 m3/y/person (Dirección General de Aguas (DGA), 1996, Universidad, de Chile, 2010). The northern macro-region (17 to 34° S) covers only 50% of the total area of the country, but it contains about 74% of the total population (17 million people) and 85% of the Gross Domestic Product (GDP). In addition, the northern region is arid, yet contains the most important and most water-dependent economic activities, such as agriculture, mining, and industry (Universidad de Chile, 2010). With a population of 6 million people (about 36% of the total population of Chile), Santiago has an average water availability of 820 m3/y/person.
Since the 1990s, sustained economic growth in Chile has been based on mining and agricultural exports, which has placed increased pressure on natural resources (Universidad de Chile, 2010). Freshwater has been impacted by the rising demand of a growing economy. About 70% of 17 million inhabitants obtain their water supply from the high Andean basins; economic activities are dependent on the same basins (Universidad de Chile, 2010). Recent mining expansion in the Andes has also placed an additional pressure on water resources and communities by increasing competition for this critical resource (Oyarzún and Oyarzún, 2011, Valdés-Pineda et al., 2014). The arid northern and semiarid central regions of the country have also been experiencing water scarcity because of higher demand and prolonged droughts (Programa Chile Sustentable, 2004). The climate of the semiarid zone has a large annual variability of precipitation (> 48%), and it is prone to one drought per decade, lasting between 3 and 6 years. Precipitation during the twentieth century decreased between 40 and 50%, while agriculture and mining have expanded substantially (Ferrando, 2002). Only about 66% of domestic wastewater is treated, whereas the rest is discharged into rivers and the ocean. About 20% of the industries treat their residual waters (Universidad de Chile, 2006). In the northern region, consumptive water use and nonconsumptive water use currently exceed the available natural surface flow; therefore, the unmet demand is satisfied by the overexploitation of aquifers. Available supplies cannot meet the increasing demand, which has resulted in water scarcity and water conflicts (Larraín and Poo, 2010). This problem will become more important in the future because of global climate change: average temperatures are expected to increase, whereas available water is expected to decrease (Universidad de Chile, 2006). Between 1933 and 1992, warming rates at 33° S have been about 2 °C/decade (Rosenblüth et al., 1997). In the northern region, climate models show a projected temperature increase of 1 to 3.0 °C and a decrease in total precipitation of 10 to 25% in the next 90 years (Universidad de Chile, 2006). In central Chile, this would likely result in an increase in future runoff generation from increased melting of snow and glaciers; in the long-term, this will lead to water scarcity and decreased runoff during the summer months (Corripio et al., 2008). Accelerated water use for economic development and a growing population as well as depleted and contaminated water resources associated with climate change are important challenges for water policy in Chile.
The Andes are an important component of the geography of Chile; however, glaciers have only recently been systematically investigated. The majority of glaciological studies have been performed on clean-ice glaciers, often referred to as true glaciers, whereas less is known about debris-covered glaciers and rock glaciers. In the 1950s, Louis Lliboutry (1956) published the first comprehensive glaciological study of the central and Patagonian Andes, which provided a conceptual framework to help establish the discipline in Chile. Lliboutry, 1961, Lliboutry, 1986 mentioned the widespread existence of valley glaciers in the central Andes that had a top debris cover and discussed the evolution into rock glaciers.
During the 1960s and 1970s, Borde (1966) and Paskoff (1970) noted an abundance of rock glaciers in the western Andes, describing them as a manifestation of permafrost in the high mountain terrain. Paskoff (1970) found that solar insolation played a significant role in the distribution, with rock glaciers occurring more frequently on southern exposures. During this time, Corte, 1976a, Corte, 1976b made the same inference for the eastern Andes and provided qualitative observations regarding the contribution of rock glaciers to streamflow. Marangunic (1976) produced one of the first systematic analyses of the morphology and function of rock glaciers in the Andes of central Chile, measuring horizontal displacements and discharge. These works began to highlight the importance of the contribution of rock glaciers to runoff in the Dry Andes.
Additional systematic studies were conducted during the 1990s. Schrott, 1991, Schrott, 1996 studied the factors that control the distribution of rock glaciers. He also provided estimates of the contribution of melting permafrost to the San Juan River in Argentina during the summer. In Chile, Ferrando (1991) highlighted the importance of rock glaciers in maintaining summer runoff in the semiarid region. These landforms, however, were omitted from national glacier inventories and water balance estimates of river basins because ice was not visible on the surface.
Most recently, the relationship between the morphology of the rock glaciers and the contribution to streamflow was estimated from the internal ice content (Soto et al., 2002, Ferrando, 2003, Ferrando, 2012). Others have studied the geomorphological and hydrological function of rock glaciers at a basin scale (Brenning, 2003, Brenning, 2005). Still others have addressed the impact that mining operations have had on permafrost and rock glaciers in the Andean mountain environment (Brenning, 2008, Brenning and Azócar, 2010).
Besides these efforts by independent researchers, the newly created glaciological unit (Glaciología y Nieves) of the Chilean Water Directorate (hereafter referred by its Spanish acronym DGA) has commissioned two comprehensive studies of rock glaciers and periglacial environments in the semiarid region of the Andes (Instituto de Geografía, Pontificia Universidad Católica de Chile, 2010, Centro de Estudios Avanzados en Zonas Áridas (CEAZA), 2012). Researchers working for the Centro de Estudios Científicos (CECS) and the Centro de Estudios Avanzados en Zonas Áridas (CEAZA) have also studied debris-covered glaciers and rock glaciers (Nicholson et al., 2009, Gascoin et al., 2011, Monnier and Kinnard, 2013). In Chile and Argentina, the institutional organizations that oversee water resources and glaciers have commissioned new glacier inventories to include rock glaciers, although only a few basins have been surveyed (Bottero, 2002, Geoestudios, 2011). Most recently, geophysical and borehole drilling methods have been used to decipher the internal composition of debris-covered glaciers and rock glaciers in the Dry Andes, providing practical data about the value as a water resource (Milana and Maturano, 1999, Croce and Milana, 2002, Monnier and Kinnard, 2013).
Mining in the Andes has increased over the last 30 years; Chile is a leading copper producing country and one of the largest gold producers (Oyarzún and Oyarzún, 2011, Romero et al., 2012). Because of the expansion of open-pit mining, rock glaciers are being removed for access to copper and gold resources. Mining expansion has inadvertently contributed to the scientific knowledge pertaining to rock glaciers in the Dry Andes. Cedomir Marangunic, through his consulting firm, Geoestudios, has conducted many glaciological studies and written many technical reports for the mining industry (Geoestudios, 1998a, Geoestudios, 1998b, Geoestudios, 1999, Geoestudios, 2001, Geoestudios, 2005, Marangunic, 2013). Unfortunately, most of these reports have not been made public, and the results remained unpublished. Current legislation requires that any mining project impacting glaciers shall gather baseline data and assess environmental impacts. The environmental impact assessment (EIA) must be submitted for government approval and be made public. The EIA provides significant information about the dynamics and internal structure of rock glaciers through prospective borehole drillings and the removal of entire rock glaciers for mining or road building by cutting across rock glaciers for mining expansion (Marangunic, 2013). Our intent is to integrate and synthesize these works to build a straightforward classification of glaciers, debris-covered glaciers, and rock glaciers for the Dry Andes and to increase awareness of these landforms as a water resource.
Rock glaciers are often interpreted as debris-covered glaciers, despite having different internal ice structures and varying ice contents (Barsch and King, 1975). Genetic and morphological definitions have created confusion and debate; and in some cases, rock glaciers and debris-covered glaciers are often grouped together in a single class (Bodin et al., 2010, Berthling, 2011, Perucca and Angillieri, 2011). In different variants and adaptations, a three-tier classification has been used in the Dry Andes to describe and classify glacial landforms (Corte, 1975, Corte, 1976a, Corte, 1976b, Soto et al., 2002, Brenning, 2003, Brenning, 2005, Brenning, 2008, Brenning, 2010, Ferrando, 2003, Ferrando et al., 2003, Azócar and Brenning, 2008, Bown et al., 2008, Geoestudios, 2008a, Geoestudios, 2008b, Nicholson et al., 2009, Bodin et al., 2010, Brenning and Azócar, 2010, Instituto Argentino de Nivología Glaciología y Ciencias Ambientales (IANIGLA), 2010; Ferrando, 2012).
In this traditional classification, the first category is the uncovered or white glacier (glaciar descubierto or blanco). Ice is clearly visible and contains very little internal/external impurities or debris. The second category, the debris-covered glacier (glaciar cubierto), is a glacier in which the surface is mantled with supraglacial debris that varies in coverage and thickness. Some require that 50% of the ablation zone must be covered by debris (Kirkbride, 2011). Most often the thickness of debris ranges between 0.5 and 2.0 m with increasing thickness toward the snout (Hambrey et al., 2008). Supraglacial debris is delivered by a combination of mechanisms such as avalanching, rockfall, or other mass movements; the rate of delivery of debris is high compared to ice flow (Kirkbride, 2011). In addition, englacial material is often exposed through downwasting. Depressions caused by thermokarst, or collapse features from melting ice, often show massive internal ice structures that are layered with strata containing impurities and debris. The third category is rock glaciers (glaciar de roca, also known as glaciar de escombros, detrítico or rocoso), which have a thicker surface rock layer compared to debris-covered glaciers. Underneath the surface debris, a mixture of ice and rock exists. Typically, rock glaciers have been classified by genetic or taxonomic groups (Giardino and Vitek, 1988). In Argentina and Chile, the most widely used classification is based on a combination of variables such as glacial or periglacial origin, source of debris (talus or moraine), location on a slope, surface relief, form (singular or complex), shape (tongue-shaped or lobate), or size (Corte, 1976a, Corte, 1976b, Corte, 1987).
The Geoestudios consulting firm has proposed a slightly modified version of this classification based on the amount of rock debris present in the glacier by volume (Geoestudios, 2008b). The glaciers of central Chile can be characterized by the following typology:
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a white glacier (glaciar blanco) or a glacier that lacks debris on the surface;
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a glacier with moraines on the surface (glaciar con morrenas en la superficie) or a white glacier in which medial and lateral moraines are exposed in the ablation zone;
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a gray glacier (glaciar gris) or one that has an accumulation of debris or other impurities in the ablation zone. The top debris layer increases in thickness toward the snout of the glacier;
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a transitional glacier (glaciar en transición) or one that is evolving into a rock glacier. Definitive characteristics of this type are the occurrence of small patches of debris on the surface, a transition from white to gray after years of negative mass balance, and the development of a top layer of debris that covers almost the entire glacier; and
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rock glaciers (glaciar de roca), which have a thick top debris layer that covers the entire ablation and accumulation zones.
According to this classification system, rock glaciers and debris-covered glaciers are not distinguished with separate classes. In this category, the thickness of the top rock debris layer ranges from a few centimeters to a few meters. The internal ice-core contains up to 70 to 80% ice and 20 to 30% rock debris. They also have internal pockets of clear ice.
With multiple classification systems available, many different complex forms, and multiple processes of formation that result in similar landforms, confusion exists about what constitutes a glacier, debris-covered glacier, or rock glacier. Rather than developing another classification system, we build upon the existing structure and focus on another important variable: ice content and related water resources.
In the Dry Andes, glaciers must be studied as a water resource. A general lack of knowledge occurs about water resources contained in debris-covered glaciers and rock glaciers in the Andes (Rangecroft et al., 2013). Previously, a range of values for ice content has been reported for rock glaciers. Brenning (2010) reported a range from 40 to 70%; Barsch (1996) indicated that the ice content of rock glaciers ranged from 40 to 60%; Burger et al. (1999) reported values from 50 to 70%. Recent advances in geophysical techniques and borehole drillings, that reveal the complexity of the internal structure, have allowed better estimates of ice content.
Between 27 and 33° S, rock glaciers are a more significant source of water storage compared to glaciers (Azócar and Brenning, 2010). In central Chile, the water stored in rock glaciers is about a magnitude greater than the more humid Swiss Alps (Brenning, 2005, Bodin et al., 2010). This magnitude is similar to central and southern Asia as well as the southwestern USA (Azócar and Brenning, 2010). Rock glaciers are an important water component of the cryosphere; future warming will affect these landforms by increasing the contribution to streamflow (Trombotto et al., 1999, Nicholson et al., 2009). The contribution to the hydrologic system in the Dry Andes has not been well studied, but it is highly important as reserves of freshwater for summer runoff (Ferrando, 1991, Ferrando, 2003, Ferrando, 2012, Brenning, 2003, Brenning, 2005). A complete inventory that addresses the contribution of debris-covered glaciers and rock glaciers to seasonal streamflow remains to be undertaken.
The classification system presented here permits differentiation of glacier categories according to the ice content to better estimate the contribution to runoff. This approach is more suitable for water planners when preparing sustainable solutions. Moreover, this classification provides important arguments to enhance the public understanding of debris-covered glaciers and rock glaciers as important reservoirs of water that may enhance preservation of these landforms (República Argentina, 2010).
Section snippets
Regional setting: glaciological zones and climatology
Chile is divided into five distinct geographical/glaciological zones with similar climatic conditions: North zone, Semiarid zone, Central zone, South zone, and Austral zone (Centro de Estudios Científicos, 2009) (Fig. 2). Precipitation increases from the North to the Austral zones, from hyperarid to very humid. Temperatures follow the reverse pattern: from hot in the North zone to moderately cold in the Austral zone. The present proposal for classification of debris-covered glaciers and rock
Common characteristics of debris-covered glaciers and rock glaciers in the Andes
Distinctive formation mechanisms have been recognized related to glacial, periglacial, or a combination of these processes (Janke et al., 2013). In a glacial model, rock glaciers are seen as remnants of the last ice age, an expression of a permanently negative glacial mass balance for thousands of years. They often have an ice-cored internal structure. Periglacial rock glaciers develop at the foot of slopes in cold mountain environments by the percolation and accumulation of meltwater that
Borehole drillings
During 1997–1998, 88 boreholes were drilled (using the ODEX Percussion Down-The-Hole Hammer technique and metal casing of 0.15 m), surrounding the open pit copper mine, Andina, located in the Andean highlands of the Blanco River, an upper catchment of the Aconcagua River (32° S) (Marangunic, 2013). The open pit is surrounded to the south and east by a system of debris-covered glaciers, rock glaciers, and ice-rich permafrost—many of which have been disturbed by road construction and mining
Complex debris-covered/rock glacier systems
Although we have divided debris-covered glaciers and rock glaciers into six distinct classes, it is often difficult to assign one class to a single landform because of the complexity of the alpine terrain and interaction of factors such as snowfall, available debris, and temperature that affect glacial and periglacial processes. For example, a complex debris-covered glacier system is illustrated in Fig. 21A. A class 1 semicovered glacier grades into a fully covered class 2 glacier in which some
Conclusions
Debris-covered glaciers and rock glaciers must be recognized as an important water resource. The classification presented here utilizes surface morphology, field observation, and coring data to estimate the ice content of debris-covered glaciers and rock glaciers. For debris-covered glaciers, surface coverage of semi (class 1) and fully covered (class 2) glaciers increases from 25 to 95%, respectively. Debris thickness gradually increases as glaciers become buried (class 3) with more than 3 m of
Acknowledgments
We would like to thank Marco Marquez for his logistical support in Chile. Appreciation is expressed to Catherine Kenrick for providing support to conduct fieldwork in the Parque Andino Juncal. Fieldwork was supported by many grants from the Offices of the Dean, the Provost, International Studies, and Sponsored Research and Programs at the Metropolitan State University of Denver. An ESRI Natural Resource grant provided us with imagery for the Aconcagua basin and ArcGIS© software. Jack Vitek and
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